Final Report Summary - POLYSOL (Development of a modular, all-POLYmer SOLar thermal collector for domestic hot water preparation and space heating)
Executive Summary:
Solar thermal systems are increasingly common and are, to the general public, probably the most visible Renewable Energy System. Moreover, compared to biomass they have distinct user benefits when it comes to domestic heating or hot water provision as they do not need physical feedstock. The project focus is to develop a novel modular, all-polymer glazed solar thermal collector that can directly substitute a conventional metallic solar thermal collector for domestic heating and hot water applications. The following summarises the work carried out over the first nine month period.
In order to select the best materials to be tested a literature review was carried out. The main objective of the preliminary trials was to evaluate the effects of the doping nano-reinforcements on the thermal conductivity values using melt blending due to its cost effectiveness, fast production and environmental benefits. Ten formulations were developed and thermal conductivity measurements were carried out on all the samples and a polymer, doping material and concentration identified.
Polycarbonate formulations were produced by extrusion, to obtain pellets of three different formulations.
The trials showed that:
Carbon based nanoreinforcements show better thermal conductivity enhancement than ceramic ones.
The obtained results show that using Expanded Graphite we can increase three times neat PET thermal conductivity. The increase of PC doped expanded graphite thermal conductivity is not as high as the PET one.
A suitable material has been formulated for the absorber that meets the specifications for thermal and mechanical behaviour and a required collector efficiency factor F’ > 0.95.
The results of adhesion tests were different for different substrate materials. The adhesion is good on the doped PC with 10% EG (rep) however it was apparent that surface roughness plays it a big part in the adhesion of the coating to the polymer.
The conclusions were that:
The solar absorbance and thermal emittance of the selective coating depend on the type of the substrate as well as of its surface (preparation and roughness).
The thickness of the IR reflective layer has positive influence on the thermal emittance up to and over 80°C. But, at the same time it shows negative influence on the solar absorbance, i.e. decreases the solar absorbance.
For the PC samples with selective coatings give similar results to those obtained with the PET samples, hence M16 is appropriate.
This material was formulated for the extrusion of the absorber profile as a single extrusion.
To allow heat transfer to the absorber a five layer PVD selective coating was developed on three different types of substrates (glass microscope slides, aluminium sheets and doped polymer samples) and specular reflectance spectral measurements carried out. The selective coatings for the polymer based absorber profiles were investigated and five layers each with a specific role/function were specified over the absorber profile and a deposition procedure was developed for the absorber and the manifold. Lab validation of the temperature dependent emissivity of the developed selective coating was measured using IR thermal emittance spectra at different temperatures. The coated absorbers can perform their role (can be mounted) at different weather / exploitation conditions. In regions with high solar irradiation and high ambient temperatures, emissivity can be controlled (increased) without changing the absorbance with simply designing a proper selective coating on collector absorbers (95% absorbance and 30% emittance). And, contrary on this, if the solar radiation in particular region is low and low ambient temperatures exist, a coating with low emissivity values and max absorbance on the absorbers can be made to harness as much as possible the poor solar irradiation in those regions.(95% absorbance and min emittance of 5%).
Three different geometries of conceptual absorbers and heat transfer channels were modelled. Each had different volume flows of the heat transfer fluid and thermal conductivity of the absorber materials. All 3 designs fulfilled the requirements regarding pressure loss (dp < 70 mbar at 30 l/(h m²)) and performance (F’>0.95) using a polymer with a thermal conductivity of 0.6 W/(m K).
Four solar thermal systems (two systems for domestic hot water and two systems for domestic hot water and space heating) have been implemented in TRNSYS. Simulations have been carried out for all 4 systems and for three locations (Athens, Stockholm and Würzburg) for a wide variation of collector area and storage volume. The fractional energy savings were calculated for each system and will be used as a benchmark for further investigations
Five different polymers were selected and referenced against ABS to determine the most appropriate for the casing of the solar panel. A range of mechanical tests were carried out at ambient temperature. Aging trials were also carried out on the samples at 85°C and 120°C for up to 500 hrs at relative humidity and UV light between 0 – 250hrs. The tests indicated that two of the polymers could be used as a potential alternative to ABS. Both polymers had superior mechanical properties and UV resistance compared to ABS. Creep tests were also carried out over 98 days under constant load and at ambient temperature an elevated temperature of 65°C to determine the creep. Based on the results the same two polymers showed the best results in terms of elongation.
Two concept casing designs ‘Stepped’ and ‘E’ shaped profiles were designed to provide speed and simplicity to the assembly and production of the all plastic solar panel casing. The ‘Stepped’ shaped profile was chosen as the best concept.
A mechanical simulation of the preferred absorber geometry (tubes) was carried out and it identified that both the stresses and deformations were far below the critic admissible values.
The casing profile was approved using FEA to determine the minimum wall thickness and the integrated solar thermal collector comprises a 1 sq. mtr area of collector surface integrated into the insulated casing. For all the infield trials two collectors will be joined together in the different locations to produce a 2 sq. mtr collection area.
For the absorber simulations were carried out to determine the minimum wall thickness that could be achieved and due to reasons related to the mechanical stability, the polymer used and production techniques the final geometry was changed quite significantly compared to the ones investigated previously. The main changes were:
1. Reduction in wall thickness from 2 mm to 1 mm to compensate for the low conductivity of 0.4 W/(m K)
2. Parallel surfaces to improve the coating and ease integration into the manifold
3. Reduction of inner diameter for higher mechanical resistance
The final absorber design geometry theoretical has a cocollectorecollector efficiency factor values of about 0.97 so the desired value of 0.95 has been achieved or rather exceeded for all flow rates.
Following the extrusion guidelines proposed Plásticos Alai produced two different parts of a solar collector by extrusion:
i) The casing of the solar collector
ii) The absorbers
The extrusion of the PolySol absorber and casing were carried out using bespoke dies at Plasticos Alai using their production lines.
The absorbers which were extruded were coated using a multilayer PVD selective coating procedure to achieve the correct absorbance and emittance values and these absorber profiles were then integrated into the final collector design.
A quantity of tests of coated vs. uncoated samples of polymer absorbers were made using the Optosol instrument as it offered fast and efficient measurements. For each of the samples, uncoated and later coated ones (after each coating), a set of measurements to obtain absorbance and emittance of samples were made. The measurements of each sample were repeatable for at least three different spots on the same sample. The absorbance of the final coating was measured using the Optosol instrument and was confirmed as 94.9% and emittance of ~17 %. The most important fact of the last coating is that it has shown protective behavior on rigorous tests, such as temperature shocking, humidity tests. The selective coating over the absorber under these in-house testing remained undisturbed with the same optical properties that didn’t change before and after the test examination.
Further validation about proper use and test measurement of Optosol was confirmed by validating our results with additional complimentary measurements on the selected samples with FHG. Comparison of FHG and Plasma measurements for the same samples differed for only 1 to 1.5% which is within the tolerance limit of the instrument.
The optical properties measured by the Optosol instrument are absorbance of 94.9 % and emittance value of 17.5%. These properties are considered acceptable by Plasma, FHG and USTUTT
Testing was also carried out on the extruded profiles and the test was at least similar to the original test results if not better in some instances. The extruded profiles were also aged. The conditions were: UV exposure along with 85°C temp and 40% RH for 125h, 250h and 500h, the ageing trials showed no degradation over time and visual analysis of the aged samples also showed no degradation over time.
The overall conclusion is that from the test results the doped polycarbonate is an acceptable material for producing the profiles for the PolySol Solar Thermal Collector whilst also being a cheaper material than the original Crastin material and easier to manufacture.
Testing of polycarbonate against previous materials was carried out and all mechanical testing was carried out as in D1.5:
Three tests were carried out:
1) Flexural test or 3 point bend test: Standard used is BS 2782 Method335 A
2) Tensile Test: Standard used is BS 2782 Method335 A320A
3) Charpy Test:
The overall conclusion is that from the test results the doped polycarbonate is an acceptable material for producing the profiles for the PolySol Solar Thermal Collector.
The casing profiles and the coated absorbers were sent to Camel Solar who integrated 2m2 collectors which was composed of 2 panels of 1m2 each where each 1m2 consists of 5 individual 20cm wide panels that was fitted together via a dovetail. The collector casing was produced in 1m2 sections which could easily be fitted to the absorber profiles.
The collector was assembled after Camel Solar’s internal tests have showed that this collector fulfils the requirements according to the project.
Polymer collector prototype
The collectors were sent to the following locations/partners for further in-field tests:
1 unit remained in Macedonia
1 unit was sent to ERA (Spain)
2 units were sent to ITW (Germany)
1 unit was sent to Ezinc (Turkey)
1 half unit was sent to MaTRI (England)
An installation procedure for the collectors was developed. Different environments and/or typical solutions of fixing collectors as tiles roofs, plane roofs, sandwich, etc., was taken into account. A simple way of installation, standard elements and security was considered and distributed to all in field trial partners. Following the installation procedure a field test regime suitable for the in-field validation on the prototype collectors that will be applied in four sites was designed:
This designed test was composed by a hydraulic installation and a control-datalogging system that will allow us to validate the models and simulations carried on.
The in-field test consists in an analysis of the representative values that can explain the behaviour of a real installation in a real environment with a simulated energy consumption system. The main objective is to acquire all the representative data (temperatures, solar radiation, flow, energy, etc.) in a simple database that will be analyzed after the test period. This analysis will compare the real performance values with the models used in the project.
A Market Analysis report was prepared that sought to sharpen understanding of the true opportunity for the PolySol technology in the European solar thermal market for households. It examines aspects of the industry and market which have an influence on the prospects for the innovation. Many of the conclusions can be applied to commercial as well as residential premises and 15 conclusions were made. A preliminary plan for the use of dissemination and exploitation has also been prepared.
A state of art regarding patenting in respect to the PolySol project took place. In the view of the patent researcher there are no patents which block the patenting of the PolySol inventive steps.
Project Context and Objectives:
Copper and aluminium are good thermal conductors which makes them very suitable for solar thermal applications. Furthermore they can resist the high operating temperatures of solar thermal collectors during stagnation. As a result copper/ aluminium solar collectors are efficient at converting solar irradiation into usable thermal energy. In order to increase their efficiency further, they can be coated with so-called selective coatings. The function of this coating is to absorb nearly 100% of the short wave radiation originating from the sun in order to convert this energy into heat and to emit only a few percent of the long wave radiation from the hot absorber surface. The main problem with commodity (i.e. PE,PP) and standard engineering plastics (i.e. PET, PPE, PC, ABS) for this application is their poor thermal conductivity (three orders of magnitude lower than copper/ aluminium) and relatively low heat deflection temperature. (<80°C). As a result polymeric absorbers are generally inefficient and potentially warp/ soften when exposed to higher temperatures (>80°C) and pressures. Due to these limitations polymer solar collectors are mainly used for low temperature, low pressure applications (30°C) such a swimming pool heating or product drying.
The project focus is to develop a novel modular, all-polymer glazed solar thermal collector that can directly substitute a conventional metallic solar thermal collector for domestic heating and hot water applications. In order to achieve this we propose the following innovations a): a novel selective physical vapour deposition (PVD) coating with a high thermal emittance (temperature dependent) over 80°C. The coating will increase absorption of incident solar radiation to levels comparable to PVD coating metallic collectors (i.e. 95%) whilst simultaneously serving as in-built overheating protection to avoid polymer softening; b): a novel, modular polymer composite absorber profile capable of operating at pressures up to 6 bars. The decrease in thermal conductivity due to increased absorber thickness (required to withstand high operating pressure) will be overcome by selectively dispersing nano-dopants within the polymer(s), effectively creating a nano-composite layer; c) a co-extrusion process capable of producing a modular absorber with selectively doped sections (only top absorber surface); d) a novel, low cost collector casing composed of up to 85% recycled mixed waste polymers. The recycled polymer casing will contain a blown core of mixed waste polymer which will partially serve as thermal insulation thereby reducing the need for typical insulation material (i.e. Rockwool, polyurethane foam) by 50%. The main project integrated objectives are:
• Demonstrate a glazed, all-polymer solar thermal collector suitable for space heating and hot water preparation that achieves similar performance compared to a state-of-the-art metallic flat plate collector (thermal energy output) with an aperture area no larger than 150% that of the comparable metallic collector and a total production cost of <€90 per m2
• To achieve an overall collector weight saving of up to 30% corrected for thermal output performance (in terms of overall system size)
• To achieve a cost-reduction of at least 25% on a per kWhth basis compared to metallic collectors
Major benefits of using virgin and recycled polymers for glazed solar thermal collectors include:
- Cost benefits due to material cost savings and manufacturing technology economies of scale (up to 50% reduction i.e. copper costs ~€8per kg versus €2-3 per kg for typical commodity polymers
- Light weight (up to 50% weight reduction per m2)
- Complex integrated structures can be manufactured in a single step, via i.e. extrusion
- Colour coordination with rest of building and integrated design (i.e. aesthetic appeal)
- Reduced use of expensive metallic raw materials (especially copper which is not possible to replace in electronics, electricity generation and transmission etcetc.)
- Easier and low cost maintenance/ replacement parts
- Reduction of CO2 emissions in manufacture (i.e. displacing aluminium/ copper)
- Reduction of waste due to increased use of recycled waste polymers
For the PolySol project there are three scientific and five technical objectives to be met:
Scientific
1. Determine heat transfer rates through polymer absorber body as a function of polymer material, absorber thickness and dopant type and concentration
2. CFD model of conceptual absorber and fluid channel designs
3. Increase the understanding of temperature dependant thermal emittance of PVD coatings when applied to polymers
Technical
1. Develop a PVD coating process that can be applied to polymeric components and has good temperature dependent thermal emittance properties
2. Experimentally validate thermal performance of selected PVD coating and doped polymer samples to prove their temperature stability and durability
3. Design a modular polymeric solar absorber profile with suitable thermal conductivity, mechanical characteristics and durability
4. Develop a polymer co-extrusion process which enables diffusion of selected dopants in the upper part of the absorber
5. Design a functional prototype collector casing based on virgin polymer skin layers and recycled polymer core
All scientific objectives have been met however for the technical objectives 1, 2, 4 and 5 have been met whereas 3 has been partially met however the thermal conductivity and durability characteristics have not currently been carried out and a project extension has been requested for these sub tasks.
Summary of Recommendations from Previous Reviews
At the 1st review period a number of recommendations were made:
The consortium need to carry out three critical activities promptly so that the project can move forward: - 1 - establish whether an efficiency of 0.6 can be achieved with PC, either via expanded graphite or an alternative nano-filler. 2 – establish whether collector design changes can be employed to overcome the material shortfall at present. 3 - examine the current IPR position in the area to establish that the consortium is not infringing on any existing IPR and also to identify possible areas where IPR generated within this project can be protected.
All of the above were carried out and reported to the project officer and we were given permission for the project to proceed.
Project Results:
Work Package 1: Preparatory Research, and experimental validation
Task 1.1 Determine conductive heat transfer coefficient of selected polymer composites
In order to select the best materials to be tested in this task, different issues were analysed and a literature review was carried out.
According to the literature review and the requirements that a solar collector has to fulfil, it was decided in agreement with PLASMA to select a polymer PET matrix and as nano-reinforcements graphite, boron nitrate and carbon nanofibres to improve PET thermal conductivity.
The main objective of the preliminary trials was to evaluate the effects of the doping nano-reinforcements on the thermal conductivity values. The preparation method chosen was Melt blending due to its cost effectiveness, fast production and environmental benefits, being a solvent-free process. Indeed, melt blending use high temperature and high shear forces to disperse nano-reinforcements in a thermoplastic polymer matrix, using conventional equipment for industrial polymer processing.
TECNALIA in agreement with PLASMA produced a number of formulations and all the doped formulations were produced using a COPERION ZSK 26 MEGACOMPOUNDER twin screw extruder.
The configuration of the extruder is mainly oriented to the production of nano-compounds. The advantage of the modular design principle of the extruder is the individual adaptation of the process section to every application so that optimum product quality and maximum throughput are achieved. Different processing zones can be set up alternately as required for conveying, plasticizing, mixing and shearing, homogenizing, degassing and pressure build-up. The process section of this extruder itself consists of several barrels in which the screws co-rotate. The closely intermeshing, self-wiping screws prevent stagnant zones along the whole length of the process section. The effect: a constantly high conveying effect and optimum self-cleaning.
Thermal conductivity measurements were carried out using P670 Thermal Transport System (TTO) device that measures thermal conductivity, of the ability of a material to conduct heat, by monitoring the temperature drop along the sample as a known amount of heat passes through the sample. The TTO measures the thermoelectric Seebeck effect as an electrical voltage drop that accompanies a temperature drop across certain materials. The TTO system can perform these two measurements simultaneously by monitoring both the temperature and voltage drop across a sample as a heat pulse is applied to one end.
All the samples were produced, in a first step by extrusion and in a second one by injection, and were characterized in the same conditions in order to obtain thermal conductivity comparable values.
The main conclusions from this work task are:
- Carbon based nano-reinforcements show better thermal conductivity enhancement than ceramic ones.
- The results show that using Expanded Graphite we can increase neat PET thermal conductivity by three times
Following on from the unsuccessful trials using PET. PC formulations were produced by extrusion, to obtain pellets and by injection to obtain testing specimens with a thermal conductivity of up to 0.33 W/(mK).
The trials showed that:
• Carbon based nanoreinforcements show better thermal conductivity enhancement than ceramic ones.
• The obtained results show that using Expanded Graphite the neat PET thermal conductivity can be increased by the factor of 3. The increase of PC doped expanded graphite thermal conductivity is not as high as the PET one.
Further investigations, increasing nano-reinforcement contents, improving processing parameters and, if possible combining different type of nano-reinforcements looking for a synergistic effect, will be carried out in WP4 with both PET and PC matrices.
A suitable material has been formulated for the absorber that meets the specifications for thermal and mechanical behaviour and a required collector efficiency factor F’ > 0.95 i.e. M16: Polycarbonate +10% Expanded Graphite
Task 1.2 Investigation of high conversion efficiency PVD coatings for polymer composite absorbers
The selective PVD coatings on the polymer absorbers are different compared to those ones on the metals (Cu or Al absorbers). First of all, metals are IR reflectors by themselves, so to achieve high absorption and low emission on Cu or Al absorbers it is necessary to place only absorptive and antireflective layers. Also, the increasing of the metallic absorber’s temperature is not limited because of the high melting temperature of these metals.
The direct PVD deposition on polymeric absorbers will allow better heat transfer from the top surface towards the polymeric material. The selective coating will also protect the polymeric absorber from UV irradiation and degradation over the longer period of time.
PVD selective coatings from Plasma on polymeric absorbers
In order to achieve high absorption and small emission values at working temperature as well as high emission on temperature >/= 80°C, it is necessary to modify the deposited layers by the standard PVD process for metals (such as those for deposition on Cu or Al absorbers). At the same time, the polymeric structure should be kept undisturbed and persistent over wider temperature ranges.
• First interface layer is a good adhesive layer with multifunction purpose.
• Second layer is an infrared reflector with high thermal conductivity and selective infrared reflectance/transmittance.
• Third layer is a diffusion barrier.
• Fourth and Fifth layers are IR transparent layers with high UV and VIS absorbance, respectively. These layers are characterized with metal concentration gradient. The role of these two IR transparent layers is to absorb the light from UV, VIS and near IR region of the solar spectrum and transmit far IR part of the solar spectrum.
• Sixth layer is the antireflective layer. It possesses high transmission and low reflectivity. The role of this layer is to transmit as much as possible of the incident solar radiation with a minimum reflectance through the whole incident solar spectrum.
Thickness of each of the layers mentioned above in this section depends on the optical characteristic of each layer. The thickness of particular layers is going to be higher than thickness of the particular layers on metal substrate in order to obtain higher heat losses on temperature over 80°C.
Finally, the role of tailored PVD selective coating(s) will contribute to a good coefficient of efficiency at working temperature ~ 80°C and low efficiency on temperature higher than 80°C. With good design of PVD selective coating(s), the stagnation temperature of polymer absorbers should be lower than the stagnation temperature of metal absorbers.
Four types of different composition based on PET as well as four new compositions based on polycarbonate (PC samples) were examined and tested.
Before designing of PVD coat(s), it was necessary to have good optical characterisation of the substrate materials (samples) that affect the final deposited coating(s) later. Optical screening of all samples received by Technalia in terms of transmittance, reflectance and absorbance are made by using JASCO V-630 UV-Vis-NIR Spectrophotometer and Perkin Elmer ATR 100 Spectrum Spectrometer.
Based on optical measurements of PET samples the following conclusions are given:
a) Highly enhanced absorbances of doped PET samples compared to neat undoped PET samples are noticed (this is important for increased absorbed solar irradiation from the absorber);
b) Graphite dopants showed to be more effective than BN dopant in PET matrix;
c) Graphite (expanded) compared to Graphite KS is more efficient dopant in PET matrix (better absorbance is achieved);
d) Dopant percolation threshold for Graphite KS is already achieved at 6% dopant loading (or Absorbance of 6% and 10% -loaded graphite KS in PET samples showed the same results. It means that there is no need to increase the dopant loading to higher concentration).
The measurements of specular optical characteristics of PET samples doped with 15% expanded graphite are not performed due to the scattering effects (dominant diffuse reflection) from their surface as a result of their roughness.
Specification of PVD coating suitable for polymer absorber
Coating procedure
The coating procedure consists of the following steps:
1) surface preparation
a. plasma etching
b. chemical modification
2) PVD coating
The selective coating was developed on three types of substrates: glass substrate, aluminum sheets and doped PET and PC samples. The first two substrates are used for comparison purposes.
Each layer within the multilayered selective coating was developed by using different deposition rates aiming to achieve the best and appropriate optical characteristics. First, all thin films were deposited on microscope glass slides in order to measure their specular transmittance and reflectance characteristics.
Conclusion
1. Selective coating–substrate tandem - The solar absorbance and thermal emittance of the selective coating depend on the type of the substrate as well as of its surface (preparation and roughness).
2. The thickness of the IR reflective layer has positive influence on the thermal emittance up to and over 80°C. But, at the same time it shows negative influence on the solar absorbance, i.e. decreases the solar absorbance. This can be seen From reflectance spectra of coating III and IV comparing to the reflectance spectra of the coating I and II (which thickness of the IR reflective layer is lower than thickness of III and IV) (Figure 28).
3. For PC samples with selective coating generally the similar results are obtained like those for PET samples.
For more detailed information refer to D1.2
Task 1.3 CFD model of absorber profile
In the beginning three absorber geometries were investigated. The first absorber type is built of cylindrical tubes. This type is compared with the second geometry possessing tubes with an octagonal cross section. To achieve a plane surface on top for optical reasons and to reduce the required polymer material, semi-circle tubes were investigated as a third option.
Due to the fact that concrete property data for the polymer was not available until the time of the simulation, assumptions were made as. The thermal conductivity was set to 2 W/(m K), as this is the least value expected for the improved polymer investigated in work package WP1.1. To get a comparison towards a “common” polymer, simulations with a value of 0.2 W/(m K) were also performed.
Pressure drop, temperature distribution and thermal performance were simulated. Because of reasons related to the mechanical stability, the polymer used and production techniques the final geometry was changed quite significant compared to the ones investigated previously. The main changes were:
1. Reduction in wall thickness from 2 mm to 1 mm to compensate the low conductivity of 0.4 W/(m K)
2. Plan parallel surfaces to improve the coating
3. Reduction of inner diameter for higher mechanical resistance
Although the thermal conductivity of the polymer material used for the absorber does not meet the requirement of 2 < < 5 W/(m K) the requirements related to collector efficiency factor (F’ > 0.95) and pressure drop (dp < 70 mbar at a flow rate of 30 l/(m² h) could be reached.
Task 1.4 TRNSYS modelling of the complete solar thermal system.
The evaluation of an all polymeric solar thermal collector suggests the investigation of its behaviour (i.e. its thermal performance) in solar thermal systems. An investigation using annual simulations of solar thermal systems including such a collector reflects not only its maximum potential but the performance throughout an entire year. In order to evaluate this annual performance 4 different system concept models were implemented in TRNSYS including a generic conventional (metal based) solar thermal collector. Annual system simulations were performed for this generic collector and first results are presented. The system models therefore allow quick implementation of different solar thermal collector behaviour into the existing system configurations.
Solar thermal systems
In order to get best representative results of the collector´s thermal performance the 4 chosen solar thermal systems represent standard systems that are commonly sold on the European market. Two systems are systems for domestic hot water preparation (DHW) and two are systems for domestic hot water preparation and space heating, so called solar thermal combisystems (SCS). All four systems are investigated in 4 different heat store volume sizes and a wide range of collector areas (see section Boundary conditions). The systems separated into DHW and SCS are described in more detail below.
Solar thermal systems for domestic hot water preparation (DHW)
The first system for domestic hot water preparation (DHW1). In this system the heat is stored within the tapping water itself. The pressurized heat store is heated either by the auxiliary boiler or by the solar collector. In both heating loops an internal heat exchanger is used. The tap water temperature is limited by mixing the outlet of the hot water store with cold water.
The second DHW system (DHW2). This system includes a pressureless heat store and is designed as a drain back system. The heat is not stored within the tapping water itself but within the water running through the solar collector. The hot water preparation is performed via an external heat exchanger. The heat is delivered to this heat exchanger from the heat store via a controlled pump controlling the tap temperature. The heat store can be heated by the auxiliary heater via an internal heat exchanger as in DHW1. The heat store can also be charged directly by the pressureless solar collector circuit.
Within the drain back system the collector fluid is drained back into a so called drain back tank whenever the pump of the collector loop stops.
Solar thermal systems for domestic hot water preparation and space heating (SCS)
The first solar thermal combisystem (SCS1) is a pressurized tank-in-tank system. The DHW preparation is performed via the internal tank which is heated indirectly, either by the solar loop or the auxiliary heating loop. The solar charge is performed using an internal heat exchanger separating the heat store fluid from the solar collector fluid. The heat store is integrated into the space heating loop according to the return flow increase principle. Within this control strategy the space heating return flow is lead through the heat store in case the store is warmer than the return flow. This way the space heating flow is preheated by solar if possible and afterwards heated to its set flow temperature by the auxiliary heater if necessary. The pressurized store enables a direct integration of the return flow increase without heat exchanger.
The second solar combisystem includes a pressureless heat store tank and works as a drain back system. The heat is stored within the solar collector fluid (water). All other circuits are connected to the heat store via internal or external heat exchangers.
System Simulations
All four systems are investigated in 4 different heat store volume sizes and a wide range of collector areas in order to predict the thermal performance for a wide range of operating conditions. This wide range of operating conditions includes three different locations throughout Europe. In order to determine the system performance in the different locations, reference conditions in terms of weather data and heating loads are used for the simulations. Three different locations (Athens, Greece, Stockholm, Sweden and Würzburg, Germany) are defined to cover most of the European climatic conditions. For those locations corresponding weather data and load profiles for domestic hot water and space heating are specified in the European standard series CEN/TS 12977 [1].
The results from the simulation study show the largest difference to occur between the different locations concerning the fractional energy savings. This is due to large differences in the boundary conditions (i.e. irradiation and heat demand). As expected the positive effect on the fractional energy savings decreases with growing collector areas as well as with growing heat store volumes. This effect is larger for the investigated DHW system regimes as for the SCS regimes. As the system efficiency decreases with larger collector areas and smaller heat store volumes both fractional energy savings and system efficiency should be considered for dimensioning. But an all polymeric solar thermal collector which is modular and inexpensive can potentially reduce the disadvantage of lower system efficiency and make higher fractional energy savings more appealing. This will help to reduce overall greenhouse gas emissions.
Task 1.5 Investigate collector casing designs as a function of mixed waste polymer concentration.
The aim of this sub task was to establish the basic specifications of the material that can potentially be used for the casing of the solar panel.
To conduct this work, 5 different polymers were selected which included HDPE, Polystyrene, HIPS, ASA and PBT. ABS is used as a reference. For each of the above 6 types of polymers, flexural test, tensile test and Impact (Charpy) test were conducted. Samples were made by Injection moulding under similar conditions.
After conducting the tests it was clear that ASA and PBT (Kibilac and Crastin) can be used as a potential alternative to ABS. Both ASA and PBT had superior mechanical properties and UV resistance compared to ABS. These tests were done at ambient temperature.
After 2530 hours (98 days), ABS, ASA, PBT and HIPS were tested under constant load and at ambient temperature to determine the creep. ABS showed a maximum elongation of 2.59% followed by HIPS with 2.01%, ASA with and 1.76% and least in the case of PBT with 1.65%. At the elevated temperature of 65°C after 98 days, both ABS and ASA elongated to 2.80% and HIPS and PBT showed similar values of 2.02%. Based on the results at ambient temperature and at 65°C, ASA and PBT showed best results in terms of elongation.
Ageing trials was carried out on Crastin (PBT) and Kibilac (ASA), ABS and HIPS on 2mm thick, 10cm x 15cm injection moulded plaques.
Ageing Conditions:
a) Heat at 85°C; (h85)
b) Heat at 120°C; (h120)
c) Heat at 85°C + rel.humidity 85%; (h85d85) for 0h, 125h, 250h, 500h
In the above cases samples are exposed to UV for 0h, 125h, 250h and then the UV-Lamp is switched off.
During ageing of the samples, it was observed that at 120°C, ASA plaques started to deform but retained the shape and its flexural properties at 85°C even after 500 hours of exposure both in absence and presence of relative humidity.
Flexural properties of ABS, HIPS and PBT remained the same throughout the ageing process. It was also important to know that ABS inherently does not possess a good UV resistance compared to ASA. ASA on the other hand has much better performance than ABS but can retain its flexural properties best up to 85-90°C range.
The tensile properties of ASA and HIPS show a decrease during ageing at 120°C but retained the properties at 85°C throughout 500 hours exposure. It was also seen that during ageing PBT became brittle. Brittleness was determined from Elongation at break for all the four polymers (ASA/ABS/PBT and HIPS).
The values of elongation at break for ABS, ASA and PBT decreased during the ageing at 120°C over the entire period of time. PBT on the other hand showed a decrease in elongation at break both at 85°C and 120°C. Elongation at break for ASA and ABS however decreased only during ageing at 120°C and not at 85°C. The drop in the values of elongation in break indicated that the polymer became brittle where increase in value indicated that the polymer became ductile. ASA showed the most stable values (compared to ABS).
Crastin (PBT) emerged as the most suitable material compared to ABS as a reference, based on its mechanical properties and UV stability, especially as the absorber (being installed in the inside of the casing) has a maximum specified stagnation temperature of 120°C. A creep test on all materials was conducted for the period of 98 days both at ambient temperature and at 65°C. ASA and PBT showed the best creep performance.
Post-ageing mechanical test results (flexural and tensile) also suggested that PBT and ASA are better both than ABS and HIPS. PBT retained its mechanical properties at 120°C and ASA on the other hand retained it mechanical properties at 85°C. Besides this technical aspect, the cost factor also plays an important role in further deciding the casing material. However, Kibilac (ASA) could be considered if cost was a major issue and the solar collector was being used in cooler climates
Work Package 2: Develop selective coating for polymeric absorber profile
T2.1. Specify prototype selective coating suitable for selected polymer composite
Based on the gained knowledge for selective coating from T1.2 that will be suitable for the polymer materials (started first with PET and later switch to PC composition). Coating of the actual polymer based absorber profiles were specified. This investigation involved an appropriate bonding layer between coatings and the base substrate material, specifying the characteristics of the coating and thickness of the specific layers within the multilayered coating. Each of the layers with a specific role/function were specified over the absorber profile.
The first or interface layer (adhesive) is the layer that needed to establish the connection (adhering) between the substrate polymeric material and the next multilayered coating. Usually, it can be made by plasma activation step- etching in PVD chamber with different types of working gasses.
The second layer is the IR reflector layer. It is a PVD deposited Cu layer that reflect the IR radiation from the fluid in the absorber channels.
The third and fourth layers are cermet TiOxNy layers (high metal fraction and low metal fraction), followed by the fifth one that is TiO2. The last TiO2 layer possesses antireflective (AR) properties, and actually this layer is a good base for placing the final protective / AR layer.
The double CERMET absorbing layers (actually the third and fourth layers) are metal-dielectric composite layer transparent in the thermal IR region (3- 25) µm, while it is strongly absorbing in the solar region (0.3 -2.5) µm because of interband transitions in the metal and the small particle resonance. Thermal emittance strongly increases as the thickness of the cermet increases due to NIR absorption.
By reducing the thickness of cermet layer and increasing the IR reflector layer; one can reduce the emittance value as well.
Finally, the antireflective and protective layer achieved higher transmission and low reflectivity values for optimal optical performance as well as a good protection from weather and environmental conditions.
T2.2. Develop process for coating application
This task involved the development of a coating process/methodology suitable for 20cm (wide) by 100cm (long) absorber profiles. Special holders were designed and produced for these profiles and the software required for PVD process control was developed.
Furthermore, targets for PVD sputtering suitable for the proposed absorber profile were designed and fabricated.
The absorber samples received by Plasma had a high gloss surface finishing. This meant that from this type of surface one could expect that reflectivity of the final selective coating to be higher, which is not desirable due to the low expected absorptance values, especially as higher absorbance values and lower reflectance are targeted. This meant that we had to consider how to decrease the gloss of the surface and transfer it to matt surface, starting from the first process step: cleaning and activation of the surface. Initial measurement of optical characteristics of polymer profiles are as follows: absorbance of 89.1 % and emittance of ~94.9 %.
The procedure of creating the selective coating over these polymer absorber profiles is as follows:
First a suitable activation process of these absorber profiles was developed. This is an important step, actually a process where an adherence between the absorber’s surface and next layers of the selective coating is creating. Several different processes and methodologies were considered, such as acid etching, polishing with abrasive, chemical treatment, the best and most promising activation process was established. It consists of plasma surface modification process (cleaning/etching step) in PVD chamber with Ar and O2 as working gasses. Ar gas is highly effective and ablative gas for cleaning and etching of polymers; its inertness with the substrate material is usually desirable, while the role of the other gas O2 is to make a chemical modification (fictionalization) of the substrate surface. Plasma treatments affect only the near-surface molecular layer of the substrate material.
Inherently polymers have very low surface energy. They are hydrophobic and covering by coating, painting or layer deposition over them usually is not possible. The process of plasma etching of the polymer absorber surface contributes to surface modification and increase the surface energy as well as to make it less hydrophobic or in certain way hydrophilic. Usually, the process of etching happens on the outermost surface layers.
At the same time, this procedure is useful for cleaning the surface of the absorbers from different kind of surface contaminants with energetic electron beams in PVD chambers. Namely, first the contaminants of the surface are cut off and taken away with the vacuum. Next, the polymer surface is going through modification or replacement of the very thin polymer surface layer that is mainly consists of weaker polymer groups like C-H groups, and creating new chemical functional groups. The new fictionalization of the surface is due to the fact of gas dissociation in the chamber, reacting with the surface and creating of carbonyl, hydroxyl, carboxyl groups etc…that are highly reactive. In this way, the surface properties are altered; it means that the surface has different chemical activity, possess higher surface energy, the wetting properties are enhanced and the most important is that the adhesion strength of the surface is improved.
After surface preparation and activation, the surface gloss of profiles was decreased, and the following deposition process was Cu coating.
Many trials and examinations followed in terms of time of Cu deposition, current and potential of the deposition process, etc. Finally, it was established that deposition of 15 min of Cu produced approx. ~150 nm thick layer and at the same time a good adhesion and uniformity of deposited coating was achieved with the first adhesive layer. This layer was measured with the above mentioned characteristics and had an absorbance of ~34% and emittance over 10 %.
It was noticed that the thickness of IR Cu layer didn’t influence the surface roughness of the base polymer material, it means that reflectivity of the final coating was still high that consequently lower the absorbance values. The next chronological task was to find a better solution instead of pure PVD Cu layer to develop additional IR Cu layer that was thicker and can reflect IR radiation from the fluid towards the absorber. Plasma team developed additional Cu/Ni layers that showed better IR reflectivity and higher absorptive properties.
NiO over the Ni layer was the next layer that showed to be a good base for deposition of the next cermet TiOxNy layers. In this way improved composition from the IR layer towards the final protective layer was obtained.
With pure TiO2 AR layer the absorbance reached over 93% and emittance of ~12%. The in-house test of this coating showed that this layer was not good enough to protect the coating from the weather conditions as a final one. The test consists of temperature shocking of the coating in terms of heating the sample up to 1000 C and immediately cooling in cold water of 100 C. The cycling of 20 times temperature shocks of the coatings showed that the final TiO2 coating was not good enough to survive these conditions and protect underneath layers within the selective coating stack. Namely, after these tests optical measurements showed decrease of ~4% in absorbance.TiO2 didn’t protect the water penetration through this layer towards the cermet TiOxNy layer, so additional protective layer was more than needed to make this protection.
Several different protective layers were considered, performed and examined. Most of them were commercial chemicals (solution as well as sol gel coatings) from different vendors, but finally we decided to go with a chemical - sol gel composition based on two-components that contains inorganic chemical. The thickness of the layer of 11-12 microns was enough to show good protective and AR properties. The sol gel composition showed the best performance among other examined protective solutions due to the fact that it increased the total absorbance of the selective coated absorber profiles, resulting in an increase of the absorbance was up to 94.9% because of its AR properties and emittance of ~17 %. The most important property of the last coating is the proven protective behavior after rigorous testing, such as temperature shocking, humidity tests. The selective coating over the absorber under these in-house testing remained undisturbed with the same optical properties that didn’t change before and after the test examination. Finally, the last coating offers improved mechanical and corrosion protection as well as low-soiling effect. The last effect is important due to the facts that activities related to cleaning and maintaining costs and time are decreased.
Variety of samples were developed and examined on polymer composite plates and absorber profiles as well as on different substrates such as glass, Al and Cu, only for comparison purposes. Firstly the process parameters on current substrates that we already had and worked before to establish the deposition technology on polymer substrates had to be established. The conclusions on these issues were that deposition parameters for all variety of substrates are different. This meant that a lot of trials and parameters changes were followed before a final set of deposition parameters could be established.
With the above explanation for the deposited layers a max absorbance on polymer composite material was developed. Even though the surface to be coated had a variable surface finish which was really unsuitable for PVD coating, a coating that had controlled optical properties, especially controlled emittance value was established. Namely, the controlled emittance of the coated absorbers will allow use of them under different weather and exploitation conditions. In regions with high solar irradiation and high ambient temperatures, emissivity can be controlled (increased) without changing the absorbance with simply designing a proper selective coating on collector absorbers. And, contrary to this, if the solar radiation in particular region is low and low ambient temperatures exist, a coating with low emissivity values and max absorbance on the absorbers and harness as much as possible of the poor solar irradiation in those regions can be produced.
Additional work performed by Plasma was coating of connecting tubes for the collectors. The connecting tubes are made of PC polymer that is transparent with different surface roughness, smoother surface than previous absorbers, so it was a really big challenge to find way how to coat them.
The idea was that in this way we will have a better absorption area of the collector as well as increased value of efficiency of the collector. Finally coated tubes were obtained with ~ 90% absorption and thermal emittance of around 15%.
T 2.3. Coating of absorber prototype
This task involved reproduction of the coatings with best selective properties as specified in T2.1 (explained in the previous paragraph 2.1.). The coating is applied to a prototype absorber profile produced, via extrusion, in WP3 (T3.2) by use of PVD chamber.
According to the procedure that has just been explained above in Section T 2.2. coating layers were applied over the actual sized absorber profiles .
First, starting with plasma cleaning and activation of these profiles in PVD chamber, an interface adhesive layer is formed. This is a very important step in the chain of forming a multilayered and functional coating; this layer is the first one in that stack of multilayered composition. Properly established first interface layer enables a good base of ‘growing’ the next layers within the selective coating. As explained earlier this step was done in the PVD chamber under Ar and O2 working gasses. The first one is the effective gas that makes clean plasma treatment-cleaning of the surface from surface contaminants (dust or organic compounds) and at the same time destroying the very outer thin molecular layer of the polymer substrate material (breaking down of the weak covalent bonds within the polymer surface) and opening the structure for further fictionalization. It means that O2 working gas plays a role to dissociate on the surface of the polymer material and replacing the polymer groups by establishing new chemical groups. There are a number of functional groups such as carbonyl, hydroxyl etc. They change the surface in few ways, like in terms of: 1) energy level the surface is changed from low to high; 2) the novel established chemical groups make the surface with improved wetting properties – very important fact for good adhering (bonding) to the next layers within the selective coating; 3) enhanced adhesion strength and permanency of the multilayered coated absorbers.
Second IR reflective layer is based on PVD Cu deposited layer, followed by additional Cu-Ni-NiO layers.
Third and fourth cermet layers are actually set of low- and high-metal fraction of TiOxNy composition layers.
The next layer is AR layer is TiO2 deposited layer with thickness of nearly 100nm.
Because this layer didn’t protect the multilayered structure over the absorber profiles from weather exploitation conditions of application as well as didn’t protect on tests such as humidity attacks (water immersing), temperature shocks etc…This layer needs further protection from all mentioned exploitation and weather conditions from the outer side.
After several considerations of protective layers one final coating that showed the best performance was selected. Namely, the last coated layer based on sol gel solution with inorganic components (trade secreted chemical) that is applied through silk printing method. The thickness of ~11 microns is enough thick layer that performed the best characteristics in terms of: 1) improved absorbance (transmittance is increased for 2-3 %), 2) good AR, 3) low soiling properties, that reduce the requirements for cleaning as well as maintenance costs and consequently reduce the time for those activities, 4) protective for the whole stack of selective coating because survive in-house tests of water immersion and temperature shocks without disturbing the optical characteristics before and after test examinations. 5) Finally, it offers a good mechanical and corrosion protection.
T 2.4. Validate selective coating performance characteristics
This work involved lab validation of the temperature dependent emissivity of the developed selective coating. The IR thermal emittance spectra at different temperatures is measured by use of FTIR-Fourier transform infrared spectroscopy.
Summary
The selective coating for polymer absorber profiles is the one explained above. The above explained deposited layers that are developed through this project allows max absorbance of solar irradiation on polymer composite material. At the same time, on the surface of polymer collectors’ absorber, the controlled emittance can be achieved. Actually, the coated absorbers can perform their role (can be mounted) at different weather / exploitation conditions. In regions with high solar irradiation and high ambient temperatures, emissivity can be controlled (increased) without changing the absorbance with simply designing a proper selective coating on collector absorbers (95% absorbance and 30% emittance). And, contrary on this, if the solar radiation in particular region is low and low ambient temperatures exist, a coating with low emissivity values and max absorbance on the absorbers can be made to harness as much as possible the poor solar irradiation in those regions.(95% absorbance and min emittance of 5%).
Work Package 3: Design polymer absorber profile, collector casing and transparent cover
Task 3.1 Geometrical design of solar absorber profile and integral heat transfer channels.
The only variation from the above specification in Annex 1 is the Thermal Conductivity. Thermal simulations carried out in Task 1.3 proved that a lower Thermal Conductivity value of 0.4 W/mK would achieve an efficiency of the absorber of approximately 95%. This is an important issue because it is not needed to increase thermal conductivity until 2 W/mK values, as it was initially planned in the project proposal, becoming a more achievable objective with regards to thermal properties increase of polymeric materials.
Three different absorber geometries were modelled with different volume flows of the heat transfer fluid and thermal conductivity of the absorber materials. All three designs fulfilled the requirements regarding pressure loss (dp < 70 mbar at 30 l/(h m²)) and performance (F’>0.95) using a polymer with a thermal conductivity of 0.6 W/(m K).
Both ends of the absorber have been considered fixed in their side faces, since that delivers the most critical boundary conditions regarding stresses.
Mechanical simulations of the absorber were carried out with the following conclusions:
• At ambient temperature performance, and for an internal pressure of 6 bar, the maximum deformations and stresses in the absorber are far below the admissible limits. A Young modulus of 2300 MPa and a maximum allowable stress (yeld stress) of 51.9 MPa have been considered.
• However, at working conditions of 6 bar internal pressure and 120ºC the material modulus for PET (Polyethylene terephthalate) the material chosen for the absorber drops to nearly zero past its Tg, which is around 80-90ºC (see Figure 4). Once this temperature is surpassed, the material tends to flow, the modulus dropping dramatically. This needs to be taken into account, since the simulation would not be valid for those working conditions.
• The design can be considered as valid for working conditions were temperature rises up to 60-70ºC.
As a consequence of above, the base material for the absorber was changed from PET to PC (Poly Carbonate), due to the low Tg value of PET, which was below the operating temperature of the absorber (120ºC). The Tg of PC is around 150ºC with a Young Modulus of around 1700 MPa at 120ºC (against 2450 MPa at room temperature) with a Yield stress around 70 MPa.
The model was simplified to contain a group of two pipes, in order to have a first estimation of deformation and stress results. The model was then gradually expanded and more pipes added, in order to check whether the number of pipes does have a significant effect on the results. The conclusion is that the stress values do not change significantly.
A 0.75 mm wall thickness is considered as the minimum value that fulfils the acceptance criterion described for design concept 4.
To enhance the heat transport from the absorber sheet to the fluid channels the thickness of the absorber sheet was set to the thickness of the tube channels. This change resulted in three different absorber designs being investigated.
The maximum stress and deformation values are far below the admissible ones. The stress level is below 7 MPa (10% of the material yield stress), which arbitrarily can be accepted as a security threshold for a good creep behaviour.
Because of reasons related to the mechanical stability, the polymer used and production techniques the final geometry was changed quite significantly compared to the ones investigated previously. The main changes were:
1. Reduction in wall thickness from 2 mm to 1 mm to compensate for the low conductivity of 0.4 W/(m K)
2. Parallel surfaces to improve the coating and ease integration into the manifold
3. Reduction of inner diameter for higher mechanical resistance
As in the previous investigations, only one tube was used for the simulation. The length of the tube was reduced to 1.0 meter for this design compared to the 1.8 meter of the previous geometries. That is due to the fact that it was decided that the prototype absorber will be built with a length of 1 meter.
Six cross sections of the tube, starting from the inlet and going to the outlet in steps of 200 mm from the left to the right, are shown in horizontal direction. Three different flow rates are presented as indicated on the left hand side.
As expected the pressure drop is lower compared to design 1, 2 or 3 because of the shorter tube. The collector efficiency factor shows values of about 0.97 so the desired value of 0.95 has been achieved or rather exceeded for all flow rates.
Task 3.2 Manufacture absorber profile prototypes, validate thermal conductivity.
The consortium took the decision to manufacture the absorber using single extrusion as opposed to co-extrusion as proposed in the DoW. The new absorber geometry (Design 10) is significantly thinner than original envisaged hence the material content is also significantly reduced therefore the expense and complexities are not required to minimise the material use.
For the extrusion small beads of doped PC containing 10% expanded graphite will be gravity fed from a top mounted hopper into the barrel of the extruder then comes into contact with the screw. The rotating screw (normally turning at up to 120 rpm) forces the plastic beads forward into the barrel which will be heated to approximately 200 °C (392 °F).
After passing through the breaker plate molten plastic enters the die which has been designed specifically to ensure that Design 10 can be produced.
The extrusion of the PolySol absorber and casing will be carried out on extrusion lines using custom made tooling and calibrators. The tooling and calibrators are used to both form and cool the extrudate into the required shape as previously described.
The profile will still be warm when it exits the calibrator so specially designed pads will be produced and fitted to haul offs to draw the profile at a consistent speed, this will prevent any stress. Also allowing additional cooling to take place
An absorber configuration was chosen after extensive mechanical and CFD simulation which meets the specification set out in Annex 1.
The material developed for producing the absorber is polycarbonate dosed with 10% graphite. The detailed doping procedure is described in D4.1.
The absorber was extruded using a single bespoke die by Plasticos Alai.
Task 3.3 Perform stress, mechanical impact, internal pressure and accelerated aging tests
Due to confusion in Annex 1 this was split into two tasks 1) Lab validation of the absorber prototype and 2) Lab validated casing prototype
Lab validated absorber prototype
The absorbers were extruded by Plasticos Alai using dosed polycarbonate produced by Tecnalia and described in Deliverables 4.1 4.2 and 4.3
The absorbers were coated using a multilayer PVD selective coating on polymer absorber and described in Deliverable 2.2 which was validated as to its suitability and described in Deliverable 3.
For each collector unit of 1m x 1m 5 absorber profiles were required. However the actual collector will be compose of 2 units to give us a collection surface of 2 sq m2
A process has been developed for producing the multilayer PVD absorbers to the correct absorbance and emittance values and these absorber profiles will be integrated into the final collector design. The absorbers were produced using the coating layers as described in D2.2
A quantity of tests of coated vs. uncoated samples of polymer absorbers were made using the Optosol instrument as it offered fast and efficient measurements. For each of the samples, uncoated and later coated ones (after each coating), a set of measurements to obtain absorbance and emittance of samples were made. The measurements of each sample were repeatable for at least three different spots on the same sample.
The absorbance of the final coating was measured using the Optosol instrument and was confirmed as 94.9% because of its AR properties (Fig. 2 a) and emittance of ~17 %. The most important fact of the last coating is that it has shown protective behavior on rigorous tests, such as temperature shocking, humidity tests. The selective coating over the absorber under these in-house testing remained undisturbed with the same optical properties that didn’t change before and after the test examination.
Further validation about proper use and test measurement of Optosol was confirmed by validating our results with additional complimentary measurements on the selected samples with FHG. Comparison of FHG and Plasma measurements for the same samples differed for only 1 to 1.5% which is within the tolerance limit of the instrument.
The optical properties measured by the Optosol instrument are absorbance of 94.9 % and emittance value of 17.5%. These properties are considered acceptable by Plasma, FHG and USTUTT
Lab validated casing prototype
In Deliverable No 1.5 seven different materials were tested for performance and Crastin was chosen as the material of choice. Although Crastin was chosen as the material as suitable for extrusion to be used for the extruded profile it was discovered that the die manufacturer in Spain could not use this material. As a consequence doped polycarbonate was used and was tested against the original materials by injection moulding test pieces and was found to be compatible in terms of performance when mechanical tested.
Testing was also carried out on the extruded profiles and the test was at least similar to the original test results if not better in some instances. The extruded profiles were also aged. The conditions were: UV exposure along with 85°C temp and 40% RH for 125h, 250h and 500h, the ageing trials showed no degradation over time and visual analysis of the aged samples also showed no degradation over time.
The overall conclusion is that from the test results the doped polycarbonate is an acceptable material for producing the profiles for the PolySol Solar Thermal Collector whilst also being a cheaper material than the original Crastin material and easier to manufacture.
Testing of polycarbonate against previous materials was carried out as in D1.5 and all the results for the polycarbonate were positive when compared to the other materials. These results were sent to the partners for comments and were accepted as acceptable by all.
Samples of the profiles were cut up and sent to FHG for ageing trials as per WP1. The size of sample was smaller than the standard classification however the results can be used as a comparison as the sample size was used on all the tests. The replacement polycarbonate material on testing was compatible in terms of performance with the other tested materials when produced by injection moulding. The ageing trials showed no degradation over time.
Task 3.4 Design recycled polymer collector casing and cover
Two concept casing designs were designed to provide speed and simplicity to the assembly and production of the all plastic solar panel casing. It is designed without the need for any mechanical fixings and is intended that the casing will be bonded together using a UV resistant adhesive.
1. ‘Stepped’ shaped profile design. This arrangement had steps providing a resting position for each component of the assembly as well as easy location in to position
2. ‘E’ shaped profile design. Each ‘branch’ of the profile is intended to provide both resting position and simple assembly location by effectively sliding or slotting the component between the branches until they are located correctly
The consortium decided that the ‘Stepped’ shaped profile design is the preferred option. The casing design was finalised with consultation from other partners and only slight modifications of size were made to accommodate the absorber
Work Package 4: Development of co-extrusion and doping process
Task 4.1 Set-up of nano-reinforcement adding into thermoplastic matrices
CFD simulations were carried out by USTUTT - ITW (T1.3 Thermofluid model of conceptual PolySol absorber and heat transfer channels) to determine whether the Polycarbonate +10% Expanded Graphite (M16) which could only achieve a maximum thermal conductivity value of 0.4 (W/mK) would be suitable to achieve the required collector efficiency factor F’ > 0.95 on the 4 different absorber designs. These simulations confirmed that collector efficiency factor F’ > 0.95 could be reached, hence Polycarbonate +10% Expanded Graphite was chosen as the material for the extrusion of the absorber. According to both, thermal and mechanical behaviour and the fact that the required collector efficiency could be achieved the Consortium agrees that the material selected for the production of the absorber would be Polycarbonate +10% Expanded Graphite.
Task 4.2 Fabrication and characterization of specimens for material selection
Planar samples were made by injection moulding for material selection and physical testing process. This task is a continuation of work carried out in Period 1 and after mechanical simulations material Polycarbonate +10% Expanded Graphite was chosen.
Task 4.3 Selection and adjustment of co-extrusion technology
Single screw extruders and twin screw extruders are the most widely used extruders. The screw that is used to push the resin out of the die is the important component of a screw extruder. In the earlier days rubber screw were used but the rubber screw was not able to give enough amount of shear into the polymer. Therefore, screws were designed that would start deeper in the feed and gradually taper shallower in the metering section to apply more work on the polymer as it was going from the feed to the discharge.
For POLYSOL project a twin screw extruder has been used to prepare nano-reinforced plastic compounds.
Task 4.4 Manufacturing of demonstrator prototypes
Following the extrusion guidelines proposed by TECNALIA and, in order to test their industrial technical viability, PLÁSTICOS ALAI has been produced two different parts of a solar collector by extrusion:
i) The casing of the solar collector, in pure PC
ii) The absorbers, in 10% Expanded Graphite PC.
At PLASTICOS ALAI facilities, line 6 with the dies manufactured by ARVITEC for the production of the casing and the absorbers parts at PLASTICOS ALAI facilities.
Work Package 5: Integration, prototype assembly and lab validation
T5.1 Integrate absorber, collector casing, cover and fittings.
ALAI produced the composite polymer absorber and casing components. The absorbers were coated at Plasma and Camel Solar integrated all components.
The first POLYSOL prototype collectors were sent to the following locations/partners for further in-field tests:
1 prototype (2 collectors or it is 1 unit) is remaining in Macedonia and will be tested here;
2 collectors (1 unit) was sent to ERA (Spain)
4 collectors (2 units) was sent to ITW (Germany)
2 collectors (1 unit) was sent to Ezinc (Turkey)
1 collector was sent to MaTRI (England)
T5.2 Controlled tests in laboratory environment.
The thermal performance of the collector will be determined according to EN 12975-2 by applying the steady-state test method using a solar simulator. In addition to the thermal performance basic durability and reliability tests such as a stagnation test in order to investigate the collector behaviour at high temperatures and an internal thermal shock test will be performed.
Thermal performance test was performed under steady state conditions and artificial light in solar simulator. Irradiance was set to 835 W/m2 and ambient temperature to 20 °C during all measurements. Mass flow rate was about 72 l/h. Inlet temperature was adjusted on different levels in 5 steps: about 20 °C, 38 °C, 57 °C, 78 °C and 93 °C.
The collector used for thermal performance test showed leakage during the test at a temperature of more than 40 °C and water was flowing inside the collector. This leakage was small however it can be assumed that it had a negative impact on the heat losses of the collector. Nevertheless the results are considered to be satisfying.
A second specimen was installed outdoors for exposure test. The test lasted 66 days and 30 days of this period were valid days according to EN 12975-2 with irradiation more than 14 MJ/m2 per day. The mean ambient temperature was 11.8 °C. Irradiation during this time was 871 MJ/m2. 207 l/m2 rain was recorded. After one month the first clearly apparent discolouration of header tubes were detected as shown in Figure 5. During further exposure the effect became more and more visible and after two months many vertical discoloured strips appeared on absorber surface. Nearly the whole time of the exposure test a certain area of the glazing showed water and other condensation.
Measurements for the determination of the stagnation temperature were performed outdoors under clear sky conditions around noon. Absorber temperature was measured at the backside of absorber using a NiCr-Ni thermocouple. Location was half of width and 2/3 of height.
The test of high temperature resistance was performed during determination of stagnation temperature. Radiation induced discolouration has already been reported earlier. Final inspection later shows a damage of connection between absorber panel and header tubes.
Cold water (about 9°C) was spread twice lasting for 15 minutes on the front side of the collector during sunny weather conditions. Irradiance was about 1000 W/m², ambient temperature higher than 20°C. No problem was observed.
The mechanical load test was performed with positive and negative pressure to the collector cover. Load was raised in steps of 500 Pa alternating positive and negative pressure up to 3000 Pa. The construction of the collector and the connection between transparent cover and collector frame is very stable. No problem was observed.
After the tests both collectors were dismantled and inspected according to EN 12975-1 [1] in detail. The collector used for durability and reliability test shows major failures. The absorber coating became very much lighter during exposure and cracking was detected. The upper header tube changed color from blue to gold. Welding connection between absorber and pipes broke during exposure, although it never was stressed by water under pressure. In addition insulation of recycling rubber foam discoloured induced by high temperature during exposition. It is supposed that out gassing happens because condensate film on inner side of the glazing was detected.
The collector used for thermal performance test showed problems, too. Due to the small leakage condensed water on the inside of the glazing was apparent on the whole area after the test. The absorber surface was discoloured from dark grey to light silver during weeks after the test. Three leakages were detected between upper header pipe and absorber panel. The reason for these leakages is a bad welding connection between the components. Insulation of recycling rubber foam was very much wet caused by the leakages.
The thermal performance test showed good result although the heat losses could have been better in the case the insulation material has not been soaked with water.
Within the durability and reliability test, however, the collector showed major deficits, mainly related to the absorber and the insulation material.
Due to the fact that an increasing emissivity of the absorber coating could not be achieved absorber temperatures up to 160 °C could be observed. For this temperature the chosen polymeric material (poly carbonate) is not suitable. Also the insulation material seems not to be stable at the temperatures it was exposed during the tests. The basic concept of the collector however is suited for a polymeric collector in case appropriate polymers are used for the absorber and the thermal insulation. The frame and casing of the collector are strong and suitable. However mechanical load test should be repeated on the full-size collector to the different strength of the frame being twice as long.
For the solar thermal collector, generally the absorber material and design are acceptable at recommended stagnation temperature of 120°C; however the material at the higher stagnation temperatures i.e. 160°C is unsuitable as warping starts to occur. It is thought that using the dosing material as a nano addition and not a micro addition may resolve this problem. The major problem that was discovered that caused the leakages to occur was the incompatibility between the manifold (polycarbonate) and the absorber (graphite dosed polycarbonate) where it was thought that thermal incompatibility resulted in splitting and warpage causing a break in the bond between the absorber and the manifold. Ideally the absorber and manifold must be made from the same material however the manifold would have to be injection moulded which would increase the cost.
The discolouration that was observed with the PVD coating (developed for a temperature of 120°C) occurred at the elevated temperature of 160°C. This can be overcome as Plasma has developed new protective layers which are believed to be stable at this temperature.
T5.3 Correlation of test results and modelling data.
The results from the simulation study show the largest difference to occur between the different locations concerning the fractional energy savings. This is due to large differences in the boundary conditions (i.e. irradiation and heat demand). As expected the positive effect on the fractional energy savings decreases with growing collector areas as well as with growing heat store volumes. Exceptions to this observation comprise only systems with very small ratios of collector area and heat store volume which are not relevant in practice. This effect is larger for the investigated DHW system regimes as for the SCS regimes. As the system efficiency decreases with larger collector areas and smaller heat store volumes both fractional energy savings and system efficiency should be considered for dimensioning. The dependency of the thermal performance on the system hydraulics is rather small compared with the effects mentioned above.
The comparison between a conventional flat plate and the all-polymer solar thermal collector shows the expected difference in the fractional energy savings. The conventional flat plate collector shows higher figures in fractional energy savings as well as in system efficiencies for all investigated systems and locations. This difference depends significantly on the heat store volumes as well as on the total collector areas though.
The difference in the fractional energy savings depend strongly on the location and the system configuration. Thus a general ratio between the area of a conventional and an all-polymer solar collector leading to the same fractional energy savings cannot be identified. However, using the presented graphs it is possible to determine this ratio for each location and system configuration.
T5.4 Finalise overall collector design and dimensioning.
Based on the results of the performed lab tests we will be able to finalise the overall design of the collector and its subcomponents. This also includes the overall dimension(s) required to satisfy hot water and or space heating demand as a function of a specific range of usage profiles (and solar irradiation available in specific geographic locations around Europe).
For commercialisation we would be recommending a modular system twice the size of the tested PolySol prototype resulting in an aperture area of 2.08 m². Taking this into account we would propose the following estimate for the overall collector design and dimensioning:
• For hot water preparation only: 3-4 collectors resulting in an aperture area of 6.24 m² - 8.32 m² depending on the location, store volume and desired fractional energy savings
• For hot water preparation and space heating: 5 - 20 collectors resulting in an aperture of 10.4 m² - 41.6 m² depending on the location, store volume and desired fractional energy savings
Although this is a broad range for the hot water preparation and space heating this is still typical of the range for conventional collectors.
T5.5 Develop installation process
The main objective of this task is to develop an installation procedure for PolySol which can subsequently by trialled in WP6 where up to four individual collector prototypes will be installed. ERA worked closely with the other participants to ensure that an installation process was developed that is suitable for mass market application
The methodology of installation of the PolySol Solar Thermal Collectors is as follows:
Different environments or typical solutions of fixing collectors as tiles roofs, plane roofs, sandwich, etc., will be taken into account. A simple way of installation, standard elements and security was considered to design the installation methodology of the whole system.
Mechanical installation of the Solar Thermal Collector. This procedure of installation will be considered from the base that the under-structure is already installed. This structure will give the inclination of the Solar Thermal Collector for each installation in particular and, of course, must accomplish all applicable regulations regarding wind loads, snow loads, strength and flexibility.
The installers may use standard solutions available in their zones and local warehouses to build up the mentioned under-structures.
Typical places to install a Solar Thermal Collector are tiled, plane and sandwich roofs.
Installation of the PolySol
The under-structure will be attached to two horizontal (or vertical) flat mounting rails. This solution will be the same for all kind of installations. The two rails must be separated a minimum of 50cm and a maximum of 80cm between them for horizontal or vertical installation. There must be at least 30cm room in the plane of the solar collector under the down rail and 30cm respect the upper rail.
The first collector will be installed with the back face in the mounting rails and fixed with the 4 fixing nuts in each sides of the collector. When the installation is finished fix the end joints and complete the piping installation. If the array has more collectors install the joint in the fixed collector.
Install the second collector sliding it through the previously installed joint. Support the collector while fix the 4 fixing nuts. Alternatively repeat above until the whole array is installed.
Hydraulic installation.
Joint Fittings
For the PolySol we have used a brass D40 press fitting joint. The pipes must be installed opposite. The inlet must be inserted in one side in the down pipe and the outlet in the opposite side in the upper one. The other two pipes must be closed with a tap or similar.
Hydraulic schemes
As PolySol is a solar thermal collector with similar hydraulic characteristics as other collectors typical schemes can be used. As shown in the Deliverable 1.4 will be shown four typical solutions:
Deviations & Corrective Actions:
None, information has been supplied to the partners in case the requested extension is approved.
Work Package 6: In-field trials
Work Package Objectives: Demonstrate a glazed, all-polymer collector for domestic heating and hot water preparation that achieves similar performance compared to a state-of-the-art metallic flat plate collector (thermal energy output) with an aperture area no larger than 150% that of the comparable metallic collector and a total production cost of <€100 per m2, To achieve an overall collector weight saving of up to 30% corrected for thermal output performance (in terms of overall collector size), To achieve a cost-reduction of at least 25% per kWhth produced solar thermal heat compared to collectors with metallic absorbers.
Please refer to the attached PDF for report with figures and tables
T6.1 Design field testing regime.
With the collaboration of ITW a field test regime suitable for the in-field validation on the prototype collectors that will be applied in four sites was designed:
- Germany (ITW)
- Macedonia (Camel Solar)
- Turkey (EZinc)
- Spain (ERA)
This designed test is composed by a hydraulic installation and a control-datalogging system that will allow us to validate the models and simulations carried on.
The in-field test consists in an analysis of the representative values that can explain the behaviour of a real installation in a real environment with a simulated energy consumption system. The main objective is to acquire all the representative data (temperatures, solar radiation, flow, energy, etc.) in a simple database that will be analyzed after the test period. This analysis will compare the real performance values with the models used in the project.
Hydraulic scheme.
The basic scheme for each site is composed by two collectors of 1m2 to achieve a solar field of 2m2 surface.
Thermal load.
To obtain a simple, cheap and effective cooling system standard radiators will be used for evacuating heat and simulate energy consumption. This system can be installed outdoors and near the rest of the equipment in order to reduce space and piping.
The power needed to be dissipated in the in-field trials will be 2kW.
Operation
The control will start the pump when a radiation measured by CS10 of at least 200W/m2 will fall on the collector. The 3 way mixing valve will be closing the load path until the temperature within the 3 way valve (approx. the temperature measured by sensor S1) is lower than 80ºC. If that temperature rises up to 80ºC the 3 way mixer valve will open to the thermal load to that extent that the outlet temperature of the 3 way valve will stay at 80°C.
Installation methodology this was sent to the partners
Task 6.2 Install 4 prototypes in different locations and validate installation methodology.
At the end of 2012 integrated collectors were sent to ERA in Spain, Ezinc in Turkey, MatRI in the UK, USTUTT-ITW in Germany and 1 was retained by Camel Solar in Macedonia.
The collectors were installed as described in Task 5.5 Develop installation process and ERA provided the organisations with the monitoring equipment. Unfortunately all partners realised there was a serious problem with the collectors after about two weeks of operation. Leaks were observed when the collectors were operating at full pressure (this has been described in Task 5.2 Controlled tests in laboratory environment).
A telephone conference was quickly organised and the problems were discussed. As a result it was decided that Camel Solar would manufacture one more collector using different adhesives.
Camel Solar and Plasma designed a new manufacture and assembly protocol for the PolySol collectors.
As mentioned above the main problems that appeared were the problems of connections (welding) of absorbers and the welding of the manifolds to the absorbers within the Polysol collector. Camel Solar and Plasma, both, started to look for better and better solutions for connections of collector's parts.
It is thought that the main problems of assembling of PolySol collector are:
Thermal dilatation of absorbers (made of PC composite material) , and thermal dilatation of manifolds (pure PC material),
Welding between: absorbers (PC composite) and welding between absorbers and manifolds (made of two different materials: PC composite and neat PC),
Difference in thermal dilatations of absorbers and welding rod (made of PC), different thermal dilatation between absorbers, welding rods and manifolds in three directions.
Specific shape of the welding area between pure PC tube and PC composite absorbers. This is one of the critical issues to solve during connecting collector's parts. The bonding area between manifold and absorbers due to their particular (specific) shapes does not provide useful or appropriate conditions for their adhering.
Advice to try to solve the problem was sought from The Welding Institute TWI (England) and alternative adhesives and solvents were purchased i.e. Weld-On brand of solvent cemented adhesives (#55 and # 58, as the most appropriate ones recommended by the producers Figure 34), and a solvent for PC material like, dimethylene chloride (DMC).
Two different approaches were tested in order to solve the integration problems with the Polysol collector:
Chemical bonding: solvents and cements
The last choice of adhesives showed good results for bonding together neat PC tubes and PC composite absorbers (frontal bonding)
In-house internal tests were conducted at Camel Solar with most of the prepared test specimens that were adhered with above mentioned methods, DMC and Weld-On. After breaking during clamping, the joint area showed good behaviour and it was very difficult to destroy joint area (parts are well stuck together).
Also, for initial test experiments two small collectors were made that were bonded with:first on bonded with solution of DMC solvent and PC chips dissolved in it, and the other one bonded with Weld On adhesives.
These tests helped us in further selection of adhesives or welding, and behaviour of materials under pressure tests.
After pressure testing of the integrated collectors it was noticed that the adhesives and solvents had not given very good results. This is thought to be because the shape of the manifolds and absorbers do not provide enough surface area for good adhesion between parts of the collector which is a key factor to enable a good resistance to high pressure. As a result we returned to the previous method of ‘Welding Polysol components with hot-air gun and PC-welding rod’
As mentioned in the last report, and the earlier experience of welding with PC rods this was repeated again with some modifications; namely deeper grooves, surface polishing before welding between absorbers and manifolds were made and some welding parameters were changed.
With this approach better results were achieved. Testing of the collector was carried out which consisted of heating the assembly to 500 C under a pressure of 3 bars. The initial results indicated that the quality of assembled product is better but thermal cycle of heating to 80 0C and cooling down to room temperature was not tested and the efficiency of the collector was measured at temperatures below 50 0C and a lower pressure, e.g. less than 3 bars.
After testing up to 3 bars of pressure the first crack was observed on a clear PC tube: water spray was shown in the area that was not welded, but on a clean and clear surface.
The cracks were repaired by grinding and then with air gun welding.
After repairing the manifolds the next step was assembling of the connected absorbers plate within the collector case and finally attaching the glass cover.
This collector was then used for the in field trials in Macedonia
For a more detailed explanation please refer to the attached PDF
Deviations & Corrective Actions:
We experienced failure of the collectors due to leaking on the initial trials hence a decision was taken to manufacture a new collector.
Task 6.3 Conduct in-field trials
This task will be on-going for 5 – 6 months over which all prototype PolySol systems will be thoroughly tested. ERA, STU and USTUTT-ITW (with support of the other partners) will be inspecting the systems on a 2 week basis (or more often in case of problems). Data loggers will enable real-time collection of performance data.
Due to the problems of having to manufacture a completely new collector it was decided by the consortium to install it at the test site at the Camel Solar test facility, Skopje in Macedonian. A video was also taken and uploaded onto the PolySol website as per D6.2 - Video of Operational Test Sites)
Due to the delay in having to manufacture a new collector the in field testing could only be carried out over 2 month’s duration. During that period of time (Sept 2013 – October 31st 2013) this collector collected 231.1 kWh over 60 days.
Due to the problems discovered operating the PolySol collector it was decided to operate at a pressure of 3 bars and exposure to solar radiation to around 40ºC to stop potential leaking and crashing of the welding areas especially on the pure PC tubes. This collector did not exhibit any of the leaking problems associated with the original prototypes.
Performance testing of the PolySol collector
Measurment equipment for the PolySol collector was installed.
In order to keep the temperature of water up to 45º C and perform non-pressurised circulation through the polymeric collector, cool water was kept in the non-pressurised storage.
Measurements were taken using the Technische Alternative Model: UVR1611 controller and it was observed that the polymer collector has shown much better efficiency than Cu-based collector made by Camel Solar up to temperatures of 45ºC.
Results were stored every minute during the working day. A video was also taken and uploaded onto the PolySol website as per D6.2 - Video of Operational Test Sites.
Summary
In some cases the efficiency at 45 or 50ºC was very close to metallic. The efficiency of every collector depends from absorber area and some additional components: glass, isolation, geometry and material of the tubes. After some analyses we concluded that in absorber area we have to put also approximately ½ of the surface of the connecting tubes.
It was due to the fact that we calculated absorber area only of absorbers is not real but actually the absorber area is bigger in the case for PolySol collector because of the area of connecting tube; however the connecting tube does not have the selective coatings, hence only ½ of the surface of the connecting tubes has been calculated. Using this calculation the efficiency of the PolySol collector is close to a metallic collector, i.e. around 5% lower than the metallic, which gives higher than expected results.
This collector has a good absorber shape, good heat transfer through composite material and channels. This collector will have very big advantage compared to metallic one in some areas such as: direct flow circulation of swimming water through the collector without heat exchanger- this is due to the resistance of polymers against swimming water. With this application the price of solar system will be several times cheaper than the price of metallic systems; it will be saved money on heat exchanger, pump station, controller a big advantage of using a PolySol collector for swimming pools is the working temperature also, because temperature in the pool is never higher than 35ºC. At this temperature, the collector will be good.
For a more detailed explanation please refer to the attached PDF
Deviations & Corrective Actions:
We experienced failure of the collectors due to leaking on the initial trials hence a decision was taken to manufacture a new collector and just carryout 1 set of in field testing at Camel Solar test facility, Skopje in Macedonian. These trials gave us results that show the PolySol solar thermal collector is viable after further development.
Task 6.4 Analysis trial data and correlate with lab and modelling results
Trial data will be analysed on a continuous basis and compared with lab test results from WP5. We will also correlate data with the models developed in WP1 (see T5.3 for the approach).
The ambient temperature was not recorded during the infield trials but was taken from a nearby weather station as 30 min mean values.
During the infield trials outlet temperature was kept below 45 °C to limit the risk of leakage due to temperature stress.
The measured specific collector output shows the same magnitude like the hemispherical irradiance and about 50% more than the calculated specific collector output. Also a strong fluctuating, due to the fluctuating mass flow can be observed. Due to the high difference in the measured and calculated specific collector output no validation of the infield trials measurements could be performed. As reason for the high difference a malfunction of the mess flow measurement is suspected.
FOR A MORE DETAILED EXPLANATION PLEASE REFER TO THE ATTACHED PDF
Potential Impact:
The projects overall aim is to develop an all-polymer solar thermal collector suitable for domestic hot water and/ or heating contribution with the following end-user benefits: i) user cost saving of at least 25% compared to metallic collectors (on a per kWh thermal basis); ii) weight saving of approximately 30% compared to existing metallic collectors (also on a per kWh thermal basis); iii) a footprint no more than 150% compared to existing metallic collectors; iv) modular design enabling improved integration within existing and new building designs.
PolySol will therefore result in benefits for the whole supply chain ranging from end-users (improved return on investment and reduced payback period); to installers (i.e. lower weight makes installation easier); architects (improved building integration); producers (improved economies of scale in polymer component production, reduced assembly cost, increased sales potential) and finally local, national and European governments (increased market penetration of RES in order to achieve EC targets of 20% market penetration by 2020; reduced need for subsidies to attain desired RES market growth).
The EC and Member States have made a clear commitment to achieve RES market penetration targets by 2020. As heating accounts for close to 50% of overall EU final energy demand it seems obvious that it will be important to increase market penetration of RES that provide heat. Of these solar thermal is an important, but so far underutilised option (others are biomass and geothermal).
This is especially true in a domestic setting where solar thermal is often the most practical option considering that biomass requires manual feedstock replenishment. A medium growth scenario is most likely considering the growth rates achieved over the past decade. Based on this scenario and an average price of €750 per m2 (cost of installed component excluding auxiliary system) we therefore anticipate a market potential of 22 million m2 of new capacity in 2020 which will be worth €16.5 billion in the same year. This would result in at least 250,000 solar thermal related jobs by that time in Europe alone.
Impact In theory, PolySol should be able to compete with any existing metallic collector and for any domestic heating and hot water application. However, in reality there will be specific market sectors that will suit PolySol and others that will not:
- PolySol will be most relevant to households with sufficient roof space to handle >4-6m2. As well as the associated thermal storage. Hence, for very small dwellings with significant space constraints, metallic collectors will be more effective
- PolySol will be most suitable for households in Central or Southern European countries where solar energy is more abundant compared to Northern Europe where evacuated tubes or CPC collectors are probably a better solution
- Due to the cost savings achieved PolySol will be well suited to solar combi-systems which require large surface areas of collector space. These are systems that provide a contribution to both space heating and hot water provision
These assumptions may limit the overall market approx. 60% of the total market potential for solar thermal systems. However, due to the significantly improved cost/benefit ratio of PolySol it is anticipated that the project can increase the overall available potential market by 30%, i.e. consumers who would previously be uninterested in solar thermal systems due to the high investment required.
Although it is predicted that PolySol will benefit the whole supply chain including other stakeholders such as local, national and European governments to date none of these benefits have been acheived.
The predicted socio-economic impact and the wider societal implications of the project are as follows: First of all, making solar thermal systems more financially attractive will encourage market penetration which will help achieve ambitious RES targets. Not only will this reduce fossil fuel consumption and associated GHG emissions, it will also reduce the need for government subsidies to support market penetration. The project benefits are therefore not confined to the PolySol consortium but include significant wider economic and environmental benefits. Based on our expected market penetration of 289,600 collectors within the first decade of commercialisation we aim to achieve a financial return (profit) of close to €80 million for the PolySol consortium and our licensees.
The following is a summary of the benefits to Europe as a whole.
Environmental impact: Overall household heating and hot water needs amount to approximately 30% of total European energy consumption. CO2 emissions are a function of a wide range of factors which vary significantly. However, on average each kWh of combusted gas will result in approximately 0.2kg of CO2 emissions whereas a kWh of generated electricity yields emissions of 0.43kg of CO2 and heating oil in 0.24kg of CO2. If we assume that 50% of the PolySol systems displace gas; 30% displace electric heating and the remaining 20% heating oil then by 2022 we will be reducing annual CO2 emissions by 168,460 tonnes (per annum).
However, this is just the saving achieved due to increased market penetration of solar thermal systems and does not take into consideration the savings achieved by displacing carbon intensive aluminium. Every kilogram of aluminium results in 12kg of CO2 compared to around 8kg for an average polymer and just 2kg for recycled polymer waste. If we assume that we can displace 5kg of aluminium in each collector and replace it with a combination of virgin and waste polymer than we could basically halve the carbon footprint per collector (aluminium component only).
Employment: According to ESTIF every 80kW (thermal) of newly installed capacity. Based on our sales forecasts PolySol could contribute to over 6,500 new and retained EU-based jobs within manufacturing, assembly and installation over 10 years post project.
Exports: PolySol will be an alternative to metallic solar thermal collectors that are produced in
Europe as well as in the Far East. We will maintain production within the EU, which means that we can displace imports of collectors from non-EU countries. Simultaneously, we aim to start exporting EU-made PolySol collectors to non-EU countries. In addition, we also aim to sell licenses to non-European manufacturer but will ensure that these products are not exported back to the EU.
Energy cost and security of supply: it is evident that energy prices are likely to increase significantly over the next 10-20 years. Moreover, these increases are expected to be above average inflation which means that they will drive up the prices of a wide range of products and services. As a result energy costs will increase as a proportion of average net income. This is clearly undesirable. PolySol will be a means of reducing fossil energy consumption thereby reducing the overall impact of increasing energy prices. Reducing fossil energy consumption also reduces Europe's excessive reliance on imported gas and oil.
List of Websites:
www.fp7-polysol.eu
Solar thermal systems are increasingly common and are, to the general public, probably the most visible Renewable Energy System. Moreover, compared to biomass they have distinct user benefits when it comes to domestic heating or hot water provision as they do not need physical feedstock. The project focus is to develop a novel modular, all-polymer glazed solar thermal collector that can directly substitute a conventional metallic solar thermal collector for domestic heating and hot water applications. The following summarises the work carried out over the first nine month period.
In order to select the best materials to be tested a literature review was carried out. The main objective of the preliminary trials was to evaluate the effects of the doping nano-reinforcements on the thermal conductivity values using melt blending due to its cost effectiveness, fast production and environmental benefits. Ten formulations were developed and thermal conductivity measurements were carried out on all the samples and a polymer, doping material and concentration identified.
Polycarbonate formulations were produced by extrusion, to obtain pellets of three different formulations.
The trials showed that:
Carbon based nanoreinforcements show better thermal conductivity enhancement than ceramic ones.
The obtained results show that using Expanded Graphite we can increase three times neat PET thermal conductivity. The increase of PC doped expanded graphite thermal conductivity is not as high as the PET one.
A suitable material has been formulated for the absorber that meets the specifications for thermal and mechanical behaviour and a required collector efficiency factor F’ > 0.95.
The results of adhesion tests were different for different substrate materials. The adhesion is good on the doped PC with 10% EG (rep) however it was apparent that surface roughness plays it a big part in the adhesion of the coating to the polymer.
The conclusions were that:
The solar absorbance and thermal emittance of the selective coating depend on the type of the substrate as well as of its surface (preparation and roughness).
The thickness of the IR reflective layer has positive influence on the thermal emittance up to and over 80°C. But, at the same time it shows negative influence on the solar absorbance, i.e. decreases the solar absorbance.
For the PC samples with selective coatings give similar results to those obtained with the PET samples, hence M16 is appropriate.
This material was formulated for the extrusion of the absorber profile as a single extrusion.
To allow heat transfer to the absorber a five layer PVD selective coating was developed on three different types of substrates (glass microscope slides, aluminium sheets and doped polymer samples) and specular reflectance spectral measurements carried out. The selective coatings for the polymer based absorber profiles were investigated and five layers each with a specific role/function were specified over the absorber profile and a deposition procedure was developed for the absorber and the manifold. Lab validation of the temperature dependent emissivity of the developed selective coating was measured using IR thermal emittance spectra at different temperatures. The coated absorbers can perform their role (can be mounted) at different weather / exploitation conditions. In regions with high solar irradiation and high ambient temperatures, emissivity can be controlled (increased) without changing the absorbance with simply designing a proper selective coating on collector absorbers (95% absorbance and 30% emittance). And, contrary on this, if the solar radiation in particular region is low and low ambient temperatures exist, a coating with low emissivity values and max absorbance on the absorbers can be made to harness as much as possible the poor solar irradiation in those regions.(95% absorbance and min emittance of 5%).
Three different geometries of conceptual absorbers and heat transfer channels were modelled. Each had different volume flows of the heat transfer fluid and thermal conductivity of the absorber materials. All 3 designs fulfilled the requirements regarding pressure loss (dp < 70 mbar at 30 l/(h m²)) and performance (F’>0.95) using a polymer with a thermal conductivity of 0.6 W/(m K).
Four solar thermal systems (two systems for domestic hot water and two systems for domestic hot water and space heating) have been implemented in TRNSYS. Simulations have been carried out for all 4 systems and for three locations (Athens, Stockholm and Würzburg) for a wide variation of collector area and storage volume. The fractional energy savings were calculated for each system and will be used as a benchmark for further investigations
Five different polymers were selected and referenced against ABS to determine the most appropriate for the casing of the solar panel. A range of mechanical tests were carried out at ambient temperature. Aging trials were also carried out on the samples at 85°C and 120°C for up to 500 hrs at relative humidity and UV light between 0 – 250hrs. The tests indicated that two of the polymers could be used as a potential alternative to ABS. Both polymers had superior mechanical properties and UV resistance compared to ABS. Creep tests were also carried out over 98 days under constant load and at ambient temperature an elevated temperature of 65°C to determine the creep. Based on the results the same two polymers showed the best results in terms of elongation.
Two concept casing designs ‘Stepped’ and ‘E’ shaped profiles were designed to provide speed and simplicity to the assembly and production of the all plastic solar panel casing. The ‘Stepped’ shaped profile was chosen as the best concept.
A mechanical simulation of the preferred absorber geometry (tubes) was carried out and it identified that both the stresses and deformations were far below the critic admissible values.
The casing profile was approved using FEA to determine the minimum wall thickness and the integrated solar thermal collector comprises a 1 sq. mtr area of collector surface integrated into the insulated casing. For all the infield trials two collectors will be joined together in the different locations to produce a 2 sq. mtr collection area.
For the absorber simulations were carried out to determine the minimum wall thickness that could be achieved and due to reasons related to the mechanical stability, the polymer used and production techniques the final geometry was changed quite significantly compared to the ones investigated previously. The main changes were:
1. Reduction in wall thickness from 2 mm to 1 mm to compensate for the low conductivity of 0.4 W/(m K)
2. Parallel surfaces to improve the coating and ease integration into the manifold
3. Reduction of inner diameter for higher mechanical resistance
The final absorber design geometry theoretical has a cocollectorecollector efficiency factor values of about 0.97 so the desired value of 0.95 has been achieved or rather exceeded for all flow rates.
Following the extrusion guidelines proposed Plásticos Alai produced two different parts of a solar collector by extrusion:
i) The casing of the solar collector
ii) The absorbers
The extrusion of the PolySol absorber and casing were carried out using bespoke dies at Plasticos Alai using their production lines.
The absorbers which were extruded were coated using a multilayer PVD selective coating procedure to achieve the correct absorbance and emittance values and these absorber profiles were then integrated into the final collector design.
A quantity of tests of coated vs. uncoated samples of polymer absorbers were made using the Optosol instrument as it offered fast and efficient measurements. For each of the samples, uncoated and later coated ones (after each coating), a set of measurements to obtain absorbance and emittance of samples were made. The measurements of each sample were repeatable for at least three different spots on the same sample. The absorbance of the final coating was measured using the Optosol instrument and was confirmed as 94.9% and emittance of ~17 %. The most important fact of the last coating is that it has shown protective behavior on rigorous tests, such as temperature shocking, humidity tests. The selective coating over the absorber under these in-house testing remained undisturbed with the same optical properties that didn’t change before and after the test examination.
Further validation about proper use and test measurement of Optosol was confirmed by validating our results with additional complimentary measurements on the selected samples with FHG. Comparison of FHG and Plasma measurements for the same samples differed for only 1 to 1.5% which is within the tolerance limit of the instrument.
The optical properties measured by the Optosol instrument are absorbance of 94.9 % and emittance value of 17.5%. These properties are considered acceptable by Plasma, FHG and USTUTT
Testing was also carried out on the extruded profiles and the test was at least similar to the original test results if not better in some instances. The extruded profiles were also aged. The conditions were: UV exposure along with 85°C temp and 40% RH for 125h, 250h and 500h, the ageing trials showed no degradation over time and visual analysis of the aged samples also showed no degradation over time.
The overall conclusion is that from the test results the doped polycarbonate is an acceptable material for producing the profiles for the PolySol Solar Thermal Collector whilst also being a cheaper material than the original Crastin material and easier to manufacture.
Testing of polycarbonate against previous materials was carried out and all mechanical testing was carried out as in D1.5:
Three tests were carried out:
1) Flexural test or 3 point bend test: Standard used is BS 2782 Method335 A
2) Tensile Test: Standard used is BS 2782 Method335 A320A
3) Charpy Test:
The overall conclusion is that from the test results the doped polycarbonate is an acceptable material for producing the profiles for the PolySol Solar Thermal Collector.
The casing profiles and the coated absorbers were sent to Camel Solar who integrated 2m2 collectors which was composed of 2 panels of 1m2 each where each 1m2 consists of 5 individual 20cm wide panels that was fitted together via a dovetail. The collector casing was produced in 1m2 sections which could easily be fitted to the absorber profiles.
The collector was assembled after Camel Solar’s internal tests have showed that this collector fulfils the requirements according to the project.
Polymer collector prototype
The collectors were sent to the following locations/partners for further in-field tests:
1 unit remained in Macedonia
1 unit was sent to ERA (Spain)
2 units were sent to ITW (Germany)
1 unit was sent to Ezinc (Turkey)
1 half unit was sent to MaTRI (England)
An installation procedure for the collectors was developed. Different environments and/or typical solutions of fixing collectors as tiles roofs, plane roofs, sandwich, etc., was taken into account. A simple way of installation, standard elements and security was considered and distributed to all in field trial partners. Following the installation procedure a field test regime suitable for the in-field validation on the prototype collectors that will be applied in four sites was designed:
This designed test was composed by a hydraulic installation and a control-datalogging system that will allow us to validate the models and simulations carried on.
The in-field test consists in an analysis of the representative values that can explain the behaviour of a real installation in a real environment with a simulated energy consumption system. The main objective is to acquire all the representative data (temperatures, solar radiation, flow, energy, etc.) in a simple database that will be analyzed after the test period. This analysis will compare the real performance values with the models used in the project.
A Market Analysis report was prepared that sought to sharpen understanding of the true opportunity for the PolySol technology in the European solar thermal market for households. It examines aspects of the industry and market which have an influence on the prospects for the innovation. Many of the conclusions can be applied to commercial as well as residential premises and 15 conclusions were made. A preliminary plan for the use of dissemination and exploitation has also been prepared.
A state of art regarding patenting in respect to the PolySol project took place. In the view of the patent researcher there are no patents which block the patenting of the PolySol inventive steps.
Project Context and Objectives:
Copper and aluminium are good thermal conductors which makes them very suitable for solar thermal applications. Furthermore they can resist the high operating temperatures of solar thermal collectors during stagnation. As a result copper/ aluminium solar collectors are efficient at converting solar irradiation into usable thermal energy. In order to increase their efficiency further, they can be coated with so-called selective coatings. The function of this coating is to absorb nearly 100% of the short wave radiation originating from the sun in order to convert this energy into heat and to emit only a few percent of the long wave radiation from the hot absorber surface. The main problem with commodity (i.e. PE,PP) and standard engineering plastics (i.e. PET, PPE, PC, ABS) for this application is their poor thermal conductivity (three orders of magnitude lower than copper/ aluminium) and relatively low heat deflection temperature. (<80°C). As a result polymeric absorbers are generally inefficient and potentially warp/ soften when exposed to higher temperatures (>80°C) and pressures. Due to these limitations polymer solar collectors are mainly used for low temperature, low pressure applications (30°C) such a swimming pool heating or product drying.
The project focus is to develop a novel modular, all-polymer glazed solar thermal collector that can directly substitute a conventional metallic solar thermal collector for domestic heating and hot water applications. In order to achieve this we propose the following innovations a): a novel selective physical vapour deposition (PVD) coating with a high thermal emittance (temperature dependent) over 80°C. The coating will increase absorption of incident solar radiation to levels comparable to PVD coating metallic collectors (i.e. 95%) whilst simultaneously serving as in-built overheating protection to avoid polymer softening; b): a novel, modular polymer composite absorber profile capable of operating at pressures up to 6 bars. The decrease in thermal conductivity due to increased absorber thickness (required to withstand high operating pressure) will be overcome by selectively dispersing nano-dopants within the polymer(s), effectively creating a nano-composite layer; c) a co-extrusion process capable of producing a modular absorber with selectively doped sections (only top absorber surface); d) a novel, low cost collector casing composed of up to 85% recycled mixed waste polymers. The recycled polymer casing will contain a blown core of mixed waste polymer which will partially serve as thermal insulation thereby reducing the need for typical insulation material (i.e. Rockwool, polyurethane foam) by 50%. The main project integrated objectives are:
• Demonstrate a glazed, all-polymer solar thermal collector suitable for space heating and hot water preparation that achieves similar performance compared to a state-of-the-art metallic flat plate collector (thermal energy output) with an aperture area no larger than 150% that of the comparable metallic collector and a total production cost of <€90 per m2
• To achieve an overall collector weight saving of up to 30% corrected for thermal output performance (in terms of overall system size)
• To achieve a cost-reduction of at least 25% on a per kWhth basis compared to metallic collectors
Major benefits of using virgin and recycled polymers for glazed solar thermal collectors include:
- Cost benefits due to material cost savings and manufacturing technology economies of scale (up to 50% reduction i.e. copper costs ~€8per kg versus €2-3 per kg for typical commodity polymers
- Light weight (up to 50% weight reduction per m2)
- Complex integrated structures can be manufactured in a single step, via i.e. extrusion
- Colour coordination with rest of building and integrated design (i.e. aesthetic appeal)
- Reduced use of expensive metallic raw materials (especially copper which is not possible to replace in electronics, electricity generation and transmission etcetc.)
- Easier and low cost maintenance/ replacement parts
- Reduction of CO2 emissions in manufacture (i.e. displacing aluminium/ copper)
- Reduction of waste due to increased use of recycled waste polymers
For the PolySol project there are three scientific and five technical objectives to be met:
Scientific
1. Determine heat transfer rates through polymer absorber body as a function of polymer material, absorber thickness and dopant type and concentration
2. CFD model of conceptual absorber and fluid channel designs
3. Increase the understanding of temperature dependant thermal emittance of PVD coatings when applied to polymers
Technical
1. Develop a PVD coating process that can be applied to polymeric components and has good temperature dependent thermal emittance properties
2. Experimentally validate thermal performance of selected PVD coating and doped polymer samples to prove their temperature stability and durability
3. Design a modular polymeric solar absorber profile with suitable thermal conductivity, mechanical characteristics and durability
4. Develop a polymer co-extrusion process which enables diffusion of selected dopants in the upper part of the absorber
5. Design a functional prototype collector casing based on virgin polymer skin layers and recycled polymer core
All scientific objectives have been met however for the technical objectives 1, 2, 4 and 5 have been met whereas 3 has been partially met however the thermal conductivity and durability characteristics have not currently been carried out and a project extension has been requested for these sub tasks.
Summary of Recommendations from Previous Reviews
At the 1st review period a number of recommendations were made:
The consortium need to carry out three critical activities promptly so that the project can move forward: - 1 - establish whether an efficiency of 0.6 can be achieved with PC, either via expanded graphite or an alternative nano-filler. 2 – establish whether collector design changes can be employed to overcome the material shortfall at present. 3 - examine the current IPR position in the area to establish that the consortium is not infringing on any existing IPR and also to identify possible areas where IPR generated within this project can be protected.
All of the above were carried out and reported to the project officer and we were given permission for the project to proceed.
Project Results:
Work Package 1: Preparatory Research, and experimental validation
Task 1.1 Determine conductive heat transfer coefficient of selected polymer composites
In order to select the best materials to be tested in this task, different issues were analysed and a literature review was carried out.
According to the literature review and the requirements that a solar collector has to fulfil, it was decided in agreement with PLASMA to select a polymer PET matrix and as nano-reinforcements graphite, boron nitrate and carbon nanofibres to improve PET thermal conductivity.
The main objective of the preliminary trials was to evaluate the effects of the doping nano-reinforcements on the thermal conductivity values. The preparation method chosen was Melt blending due to its cost effectiveness, fast production and environmental benefits, being a solvent-free process. Indeed, melt blending use high temperature and high shear forces to disperse nano-reinforcements in a thermoplastic polymer matrix, using conventional equipment for industrial polymer processing.
TECNALIA in agreement with PLASMA produced a number of formulations and all the doped formulations were produced using a COPERION ZSK 26 MEGACOMPOUNDER twin screw extruder.
The configuration of the extruder is mainly oriented to the production of nano-compounds. The advantage of the modular design principle of the extruder is the individual adaptation of the process section to every application so that optimum product quality and maximum throughput are achieved. Different processing zones can be set up alternately as required for conveying, plasticizing, mixing and shearing, homogenizing, degassing and pressure build-up. The process section of this extruder itself consists of several barrels in which the screws co-rotate. The closely intermeshing, self-wiping screws prevent stagnant zones along the whole length of the process section. The effect: a constantly high conveying effect and optimum self-cleaning.
Thermal conductivity measurements were carried out using P670 Thermal Transport System (TTO) device that measures thermal conductivity, of the ability of a material to conduct heat, by monitoring the temperature drop along the sample as a known amount of heat passes through the sample. The TTO measures the thermoelectric Seebeck effect as an electrical voltage drop that accompanies a temperature drop across certain materials. The TTO system can perform these two measurements simultaneously by monitoring both the temperature and voltage drop across a sample as a heat pulse is applied to one end.
All the samples were produced, in a first step by extrusion and in a second one by injection, and were characterized in the same conditions in order to obtain thermal conductivity comparable values.
The main conclusions from this work task are:
- Carbon based nano-reinforcements show better thermal conductivity enhancement than ceramic ones.
- The results show that using Expanded Graphite we can increase neat PET thermal conductivity by three times
Following on from the unsuccessful trials using PET. PC formulations were produced by extrusion, to obtain pellets and by injection to obtain testing specimens with a thermal conductivity of up to 0.33 W/(mK).
The trials showed that:
• Carbon based nanoreinforcements show better thermal conductivity enhancement than ceramic ones.
• The obtained results show that using Expanded Graphite the neat PET thermal conductivity can be increased by the factor of 3. The increase of PC doped expanded graphite thermal conductivity is not as high as the PET one.
Further investigations, increasing nano-reinforcement contents, improving processing parameters and, if possible combining different type of nano-reinforcements looking for a synergistic effect, will be carried out in WP4 with both PET and PC matrices.
A suitable material has been formulated for the absorber that meets the specifications for thermal and mechanical behaviour and a required collector efficiency factor F’ > 0.95 i.e. M16: Polycarbonate +10% Expanded Graphite
Task 1.2 Investigation of high conversion efficiency PVD coatings for polymer composite absorbers
The selective PVD coatings on the polymer absorbers are different compared to those ones on the metals (Cu or Al absorbers). First of all, metals are IR reflectors by themselves, so to achieve high absorption and low emission on Cu or Al absorbers it is necessary to place only absorptive and antireflective layers. Also, the increasing of the metallic absorber’s temperature is not limited because of the high melting temperature of these metals.
The direct PVD deposition on polymeric absorbers will allow better heat transfer from the top surface towards the polymeric material. The selective coating will also protect the polymeric absorber from UV irradiation and degradation over the longer period of time.
PVD selective coatings from Plasma on polymeric absorbers
In order to achieve high absorption and small emission values at working temperature as well as high emission on temperature >/= 80°C, it is necessary to modify the deposited layers by the standard PVD process for metals (such as those for deposition on Cu or Al absorbers). At the same time, the polymeric structure should be kept undisturbed and persistent over wider temperature ranges.
• First interface layer is a good adhesive layer with multifunction purpose.
• Second layer is an infrared reflector with high thermal conductivity and selective infrared reflectance/transmittance.
• Third layer is a diffusion barrier.
• Fourth and Fifth layers are IR transparent layers with high UV and VIS absorbance, respectively. These layers are characterized with metal concentration gradient. The role of these two IR transparent layers is to absorb the light from UV, VIS and near IR region of the solar spectrum and transmit far IR part of the solar spectrum.
• Sixth layer is the antireflective layer. It possesses high transmission and low reflectivity. The role of this layer is to transmit as much as possible of the incident solar radiation with a minimum reflectance through the whole incident solar spectrum.
Thickness of each of the layers mentioned above in this section depends on the optical characteristic of each layer. The thickness of particular layers is going to be higher than thickness of the particular layers on metal substrate in order to obtain higher heat losses on temperature over 80°C.
Finally, the role of tailored PVD selective coating(s) will contribute to a good coefficient of efficiency at working temperature ~ 80°C and low efficiency on temperature higher than 80°C. With good design of PVD selective coating(s), the stagnation temperature of polymer absorbers should be lower than the stagnation temperature of metal absorbers.
Four types of different composition based on PET as well as four new compositions based on polycarbonate (PC samples) were examined and tested.
Before designing of PVD coat(s), it was necessary to have good optical characterisation of the substrate materials (samples) that affect the final deposited coating(s) later. Optical screening of all samples received by Technalia in terms of transmittance, reflectance and absorbance are made by using JASCO V-630 UV-Vis-NIR Spectrophotometer and Perkin Elmer ATR 100 Spectrum Spectrometer.
Based on optical measurements of PET samples the following conclusions are given:
a) Highly enhanced absorbances of doped PET samples compared to neat undoped PET samples are noticed (this is important for increased absorbed solar irradiation from the absorber);
b) Graphite dopants showed to be more effective than BN dopant in PET matrix;
c) Graphite (expanded) compared to Graphite KS is more efficient dopant in PET matrix (better absorbance is achieved);
d) Dopant percolation threshold for Graphite KS is already achieved at 6% dopant loading (or Absorbance of 6% and 10% -loaded graphite KS in PET samples showed the same results. It means that there is no need to increase the dopant loading to higher concentration).
The measurements of specular optical characteristics of PET samples doped with 15% expanded graphite are not performed due to the scattering effects (dominant diffuse reflection) from their surface as a result of their roughness.
Specification of PVD coating suitable for polymer absorber
Coating procedure
The coating procedure consists of the following steps:
1) surface preparation
a. plasma etching
b. chemical modification
2) PVD coating
The selective coating was developed on three types of substrates: glass substrate, aluminum sheets and doped PET and PC samples. The first two substrates are used for comparison purposes.
Each layer within the multilayered selective coating was developed by using different deposition rates aiming to achieve the best and appropriate optical characteristics. First, all thin films were deposited on microscope glass slides in order to measure their specular transmittance and reflectance characteristics.
Conclusion
1. Selective coating–substrate tandem - The solar absorbance and thermal emittance of the selective coating depend on the type of the substrate as well as of its surface (preparation and roughness).
2. The thickness of the IR reflective layer has positive influence on the thermal emittance up to and over 80°C. But, at the same time it shows negative influence on the solar absorbance, i.e. decreases the solar absorbance. This can be seen From reflectance spectra of coating III and IV comparing to the reflectance spectra of the coating I and II (which thickness of the IR reflective layer is lower than thickness of III and IV) (Figure 28).
3. For PC samples with selective coating generally the similar results are obtained like those for PET samples.
For more detailed information refer to D1.2
Task 1.3 CFD model of absorber profile
In the beginning three absorber geometries were investigated. The first absorber type is built of cylindrical tubes. This type is compared with the second geometry possessing tubes with an octagonal cross section. To achieve a plane surface on top for optical reasons and to reduce the required polymer material, semi-circle tubes were investigated as a third option.
Due to the fact that concrete property data for the polymer was not available until the time of the simulation, assumptions were made as. The thermal conductivity was set to 2 W/(m K), as this is the least value expected for the improved polymer investigated in work package WP1.1. To get a comparison towards a “common” polymer, simulations with a value of 0.2 W/(m K) were also performed.
Pressure drop, temperature distribution and thermal performance were simulated. Because of reasons related to the mechanical stability, the polymer used and production techniques the final geometry was changed quite significant compared to the ones investigated previously. The main changes were:
1. Reduction in wall thickness from 2 mm to 1 mm to compensate the low conductivity of 0.4 W/(m K)
2. Plan parallel surfaces to improve the coating
3. Reduction of inner diameter for higher mechanical resistance
Although the thermal conductivity of the polymer material used for the absorber does not meet the requirement of 2 < < 5 W/(m K) the requirements related to collector efficiency factor (F’ > 0.95) and pressure drop (dp < 70 mbar at a flow rate of 30 l/(m² h) could be reached.
Task 1.4 TRNSYS modelling of the complete solar thermal system.
The evaluation of an all polymeric solar thermal collector suggests the investigation of its behaviour (i.e. its thermal performance) in solar thermal systems. An investigation using annual simulations of solar thermal systems including such a collector reflects not only its maximum potential but the performance throughout an entire year. In order to evaluate this annual performance 4 different system concept models were implemented in TRNSYS including a generic conventional (metal based) solar thermal collector. Annual system simulations were performed for this generic collector and first results are presented. The system models therefore allow quick implementation of different solar thermal collector behaviour into the existing system configurations.
Solar thermal systems
In order to get best representative results of the collector´s thermal performance the 4 chosen solar thermal systems represent standard systems that are commonly sold on the European market. Two systems are systems for domestic hot water preparation (DHW) and two are systems for domestic hot water preparation and space heating, so called solar thermal combisystems (SCS). All four systems are investigated in 4 different heat store volume sizes and a wide range of collector areas (see section Boundary conditions). The systems separated into DHW and SCS are described in more detail below.
Solar thermal systems for domestic hot water preparation (DHW)
The first system for domestic hot water preparation (DHW1). In this system the heat is stored within the tapping water itself. The pressurized heat store is heated either by the auxiliary boiler or by the solar collector. In both heating loops an internal heat exchanger is used. The tap water temperature is limited by mixing the outlet of the hot water store with cold water.
The second DHW system (DHW2). This system includes a pressureless heat store and is designed as a drain back system. The heat is not stored within the tapping water itself but within the water running through the solar collector. The hot water preparation is performed via an external heat exchanger. The heat is delivered to this heat exchanger from the heat store via a controlled pump controlling the tap temperature. The heat store can be heated by the auxiliary heater via an internal heat exchanger as in DHW1. The heat store can also be charged directly by the pressureless solar collector circuit.
Within the drain back system the collector fluid is drained back into a so called drain back tank whenever the pump of the collector loop stops.
Solar thermal systems for domestic hot water preparation and space heating (SCS)
The first solar thermal combisystem (SCS1) is a pressurized tank-in-tank system. The DHW preparation is performed via the internal tank which is heated indirectly, either by the solar loop or the auxiliary heating loop. The solar charge is performed using an internal heat exchanger separating the heat store fluid from the solar collector fluid. The heat store is integrated into the space heating loop according to the return flow increase principle. Within this control strategy the space heating return flow is lead through the heat store in case the store is warmer than the return flow. This way the space heating flow is preheated by solar if possible and afterwards heated to its set flow temperature by the auxiliary heater if necessary. The pressurized store enables a direct integration of the return flow increase without heat exchanger.
The second solar combisystem includes a pressureless heat store tank and works as a drain back system. The heat is stored within the solar collector fluid (water). All other circuits are connected to the heat store via internal or external heat exchangers.
System Simulations
All four systems are investigated in 4 different heat store volume sizes and a wide range of collector areas in order to predict the thermal performance for a wide range of operating conditions. This wide range of operating conditions includes three different locations throughout Europe. In order to determine the system performance in the different locations, reference conditions in terms of weather data and heating loads are used for the simulations. Three different locations (Athens, Greece, Stockholm, Sweden and Würzburg, Germany) are defined to cover most of the European climatic conditions. For those locations corresponding weather data and load profiles for domestic hot water and space heating are specified in the European standard series CEN/TS 12977 [1].
The results from the simulation study show the largest difference to occur between the different locations concerning the fractional energy savings. This is due to large differences in the boundary conditions (i.e. irradiation and heat demand). As expected the positive effect on the fractional energy savings decreases with growing collector areas as well as with growing heat store volumes. This effect is larger for the investigated DHW system regimes as for the SCS regimes. As the system efficiency decreases with larger collector areas and smaller heat store volumes both fractional energy savings and system efficiency should be considered for dimensioning. But an all polymeric solar thermal collector which is modular and inexpensive can potentially reduce the disadvantage of lower system efficiency and make higher fractional energy savings more appealing. This will help to reduce overall greenhouse gas emissions.
Task 1.5 Investigate collector casing designs as a function of mixed waste polymer concentration.
The aim of this sub task was to establish the basic specifications of the material that can potentially be used for the casing of the solar panel.
To conduct this work, 5 different polymers were selected which included HDPE, Polystyrene, HIPS, ASA and PBT. ABS is used as a reference. For each of the above 6 types of polymers, flexural test, tensile test and Impact (Charpy) test were conducted. Samples were made by Injection moulding under similar conditions.
After conducting the tests it was clear that ASA and PBT (Kibilac and Crastin) can be used as a potential alternative to ABS. Both ASA and PBT had superior mechanical properties and UV resistance compared to ABS. These tests were done at ambient temperature.
After 2530 hours (98 days), ABS, ASA, PBT and HIPS were tested under constant load and at ambient temperature to determine the creep. ABS showed a maximum elongation of 2.59% followed by HIPS with 2.01%, ASA with and 1.76% and least in the case of PBT with 1.65%. At the elevated temperature of 65°C after 98 days, both ABS and ASA elongated to 2.80% and HIPS and PBT showed similar values of 2.02%. Based on the results at ambient temperature and at 65°C, ASA and PBT showed best results in terms of elongation.
Ageing trials was carried out on Crastin (PBT) and Kibilac (ASA), ABS and HIPS on 2mm thick, 10cm x 15cm injection moulded plaques.
Ageing Conditions:
a) Heat at 85°C; (h85)
b) Heat at 120°C; (h120)
c) Heat at 85°C + rel.humidity 85%; (h85d85) for 0h, 125h, 250h, 500h
In the above cases samples are exposed to UV for 0h, 125h, 250h and then the UV-Lamp is switched off.
During ageing of the samples, it was observed that at 120°C, ASA plaques started to deform but retained the shape and its flexural properties at 85°C even after 500 hours of exposure both in absence and presence of relative humidity.
Flexural properties of ABS, HIPS and PBT remained the same throughout the ageing process. It was also important to know that ABS inherently does not possess a good UV resistance compared to ASA. ASA on the other hand has much better performance than ABS but can retain its flexural properties best up to 85-90°C range.
The tensile properties of ASA and HIPS show a decrease during ageing at 120°C but retained the properties at 85°C throughout 500 hours exposure. It was also seen that during ageing PBT became brittle. Brittleness was determined from Elongation at break for all the four polymers (ASA/ABS/PBT and HIPS).
The values of elongation at break for ABS, ASA and PBT decreased during the ageing at 120°C over the entire period of time. PBT on the other hand showed a decrease in elongation at break both at 85°C and 120°C. Elongation at break for ASA and ABS however decreased only during ageing at 120°C and not at 85°C. The drop in the values of elongation in break indicated that the polymer became brittle where increase in value indicated that the polymer became ductile. ASA showed the most stable values (compared to ABS).
Crastin (PBT) emerged as the most suitable material compared to ABS as a reference, based on its mechanical properties and UV stability, especially as the absorber (being installed in the inside of the casing) has a maximum specified stagnation temperature of 120°C. A creep test on all materials was conducted for the period of 98 days both at ambient temperature and at 65°C. ASA and PBT showed the best creep performance.
Post-ageing mechanical test results (flexural and tensile) also suggested that PBT and ASA are better both than ABS and HIPS. PBT retained its mechanical properties at 120°C and ASA on the other hand retained it mechanical properties at 85°C. Besides this technical aspect, the cost factor also plays an important role in further deciding the casing material. However, Kibilac (ASA) could be considered if cost was a major issue and the solar collector was being used in cooler climates
Work Package 2: Develop selective coating for polymeric absorber profile
T2.1. Specify prototype selective coating suitable for selected polymer composite
Based on the gained knowledge for selective coating from T1.2 that will be suitable for the polymer materials (started first with PET and later switch to PC composition). Coating of the actual polymer based absorber profiles were specified. This investigation involved an appropriate bonding layer between coatings and the base substrate material, specifying the characteristics of the coating and thickness of the specific layers within the multilayered coating. Each of the layers with a specific role/function were specified over the absorber profile.
The first or interface layer (adhesive) is the layer that needed to establish the connection (adhering) between the substrate polymeric material and the next multilayered coating. Usually, it can be made by plasma activation step- etching in PVD chamber with different types of working gasses.
The second layer is the IR reflector layer. It is a PVD deposited Cu layer that reflect the IR radiation from the fluid in the absorber channels.
The third and fourth layers are cermet TiOxNy layers (high metal fraction and low metal fraction), followed by the fifth one that is TiO2. The last TiO2 layer possesses antireflective (AR) properties, and actually this layer is a good base for placing the final protective / AR layer.
The double CERMET absorbing layers (actually the third and fourth layers) are metal-dielectric composite layer transparent in the thermal IR region (3- 25) µm, while it is strongly absorbing in the solar region (0.3 -2.5) µm because of interband transitions in the metal and the small particle resonance. Thermal emittance strongly increases as the thickness of the cermet increases due to NIR absorption.
By reducing the thickness of cermet layer and increasing the IR reflector layer; one can reduce the emittance value as well.
Finally, the antireflective and protective layer achieved higher transmission and low reflectivity values for optimal optical performance as well as a good protection from weather and environmental conditions.
T2.2. Develop process for coating application
This task involved the development of a coating process/methodology suitable for 20cm (wide) by 100cm (long) absorber profiles. Special holders were designed and produced for these profiles and the software required for PVD process control was developed.
Furthermore, targets for PVD sputtering suitable for the proposed absorber profile were designed and fabricated.
The absorber samples received by Plasma had a high gloss surface finishing. This meant that from this type of surface one could expect that reflectivity of the final selective coating to be higher, which is not desirable due to the low expected absorptance values, especially as higher absorbance values and lower reflectance are targeted. This meant that we had to consider how to decrease the gloss of the surface and transfer it to matt surface, starting from the first process step: cleaning and activation of the surface. Initial measurement of optical characteristics of polymer profiles are as follows: absorbance of 89.1 % and emittance of ~94.9 %.
The procedure of creating the selective coating over these polymer absorber profiles is as follows:
First a suitable activation process of these absorber profiles was developed. This is an important step, actually a process where an adherence between the absorber’s surface and next layers of the selective coating is creating. Several different processes and methodologies were considered, such as acid etching, polishing with abrasive, chemical treatment, the best and most promising activation process was established. It consists of plasma surface modification process (cleaning/etching step) in PVD chamber with Ar and O2 as working gasses. Ar gas is highly effective and ablative gas for cleaning and etching of polymers; its inertness with the substrate material is usually desirable, while the role of the other gas O2 is to make a chemical modification (fictionalization) of the substrate surface. Plasma treatments affect only the near-surface molecular layer of the substrate material.
Inherently polymers have very low surface energy. They are hydrophobic and covering by coating, painting or layer deposition over them usually is not possible. The process of plasma etching of the polymer absorber surface contributes to surface modification and increase the surface energy as well as to make it less hydrophobic or in certain way hydrophilic. Usually, the process of etching happens on the outermost surface layers.
At the same time, this procedure is useful for cleaning the surface of the absorbers from different kind of surface contaminants with energetic electron beams in PVD chambers. Namely, first the contaminants of the surface are cut off and taken away with the vacuum. Next, the polymer surface is going through modification or replacement of the very thin polymer surface layer that is mainly consists of weaker polymer groups like C-H groups, and creating new chemical functional groups. The new fictionalization of the surface is due to the fact of gas dissociation in the chamber, reacting with the surface and creating of carbonyl, hydroxyl, carboxyl groups etc…that are highly reactive. In this way, the surface properties are altered; it means that the surface has different chemical activity, possess higher surface energy, the wetting properties are enhanced and the most important is that the adhesion strength of the surface is improved.
After surface preparation and activation, the surface gloss of profiles was decreased, and the following deposition process was Cu coating.
Many trials and examinations followed in terms of time of Cu deposition, current and potential of the deposition process, etc. Finally, it was established that deposition of 15 min of Cu produced approx. ~150 nm thick layer and at the same time a good adhesion and uniformity of deposited coating was achieved with the first adhesive layer. This layer was measured with the above mentioned characteristics and had an absorbance of ~34% and emittance over 10 %.
It was noticed that the thickness of IR Cu layer didn’t influence the surface roughness of the base polymer material, it means that reflectivity of the final coating was still high that consequently lower the absorbance values. The next chronological task was to find a better solution instead of pure PVD Cu layer to develop additional IR Cu layer that was thicker and can reflect IR radiation from the fluid towards the absorber. Plasma team developed additional Cu/Ni layers that showed better IR reflectivity and higher absorptive properties.
NiO over the Ni layer was the next layer that showed to be a good base for deposition of the next cermet TiOxNy layers. In this way improved composition from the IR layer towards the final protective layer was obtained.
With pure TiO2 AR layer the absorbance reached over 93% and emittance of ~12%. The in-house test of this coating showed that this layer was not good enough to protect the coating from the weather conditions as a final one. The test consists of temperature shocking of the coating in terms of heating the sample up to 1000 C and immediately cooling in cold water of 100 C. The cycling of 20 times temperature shocks of the coatings showed that the final TiO2 coating was not good enough to survive these conditions and protect underneath layers within the selective coating stack. Namely, after these tests optical measurements showed decrease of ~4% in absorbance.TiO2 didn’t protect the water penetration through this layer towards the cermet TiOxNy layer, so additional protective layer was more than needed to make this protection.
Several different protective layers were considered, performed and examined. Most of them were commercial chemicals (solution as well as sol gel coatings) from different vendors, but finally we decided to go with a chemical - sol gel composition based on two-components that contains inorganic chemical. The thickness of the layer of 11-12 microns was enough to show good protective and AR properties. The sol gel composition showed the best performance among other examined protective solutions due to the fact that it increased the total absorbance of the selective coated absorber profiles, resulting in an increase of the absorbance was up to 94.9% because of its AR properties and emittance of ~17 %. The most important property of the last coating is the proven protective behavior after rigorous testing, such as temperature shocking, humidity tests. The selective coating over the absorber under these in-house testing remained undisturbed with the same optical properties that didn’t change before and after the test examination. Finally, the last coating offers improved mechanical and corrosion protection as well as low-soiling effect. The last effect is important due to the facts that activities related to cleaning and maintaining costs and time are decreased.
Variety of samples were developed and examined on polymer composite plates and absorber profiles as well as on different substrates such as glass, Al and Cu, only for comparison purposes. Firstly the process parameters on current substrates that we already had and worked before to establish the deposition technology on polymer substrates had to be established. The conclusions on these issues were that deposition parameters for all variety of substrates are different. This meant that a lot of trials and parameters changes were followed before a final set of deposition parameters could be established.
With the above explanation for the deposited layers a max absorbance on polymer composite material was developed. Even though the surface to be coated had a variable surface finish which was really unsuitable for PVD coating, a coating that had controlled optical properties, especially controlled emittance value was established. Namely, the controlled emittance of the coated absorbers will allow use of them under different weather and exploitation conditions. In regions with high solar irradiation and high ambient temperatures, emissivity can be controlled (increased) without changing the absorbance with simply designing a proper selective coating on collector absorbers. And, contrary to this, if the solar radiation in particular region is low and low ambient temperatures exist, a coating with low emissivity values and max absorbance on the absorbers and harness as much as possible of the poor solar irradiation in those regions can be produced.
Additional work performed by Plasma was coating of connecting tubes for the collectors. The connecting tubes are made of PC polymer that is transparent with different surface roughness, smoother surface than previous absorbers, so it was a really big challenge to find way how to coat them.
The idea was that in this way we will have a better absorption area of the collector as well as increased value of efficiency of the collector. Finally coated tubes were obtained with ~ 90% absorption and thermal emittance of around 15%.
T 2.3. Coating of absorber prototype
This task involved reproduction of the coatings with best selective properties as specified in T2.1 (explained in the previous paragraph 2.1.). The coating is applied to a prototype absorber profile produced, via extrusion, in WP3 (T3.2) by use of PVD chamber.
According to the procedure that has just been explained above in Section T 2.2. coating layers were applied over the actual sized absorber profiles .
First, starting with plasma cleaning and activation of these profiles in PVD chamber, an interface adhesive layer is formed. This is a very important step in the chain of forming a multilayered and functional coating; this layer is the first one in that stack of multilayered composition. Properly established first interface layer enables a good base of ‘growing’ the next layers within the selective coating. As explained earlier this step was done in the PVD chamber under Ar and O2 working gasses. The first one is the effective gas that makes clean plasma treatment-cleaning of the surface from surface contaminants (dust or organic compounds) and at the same time destroying the very outer thin molecular layer of the polymer substrate material (breaking down of the weak covalent bonds within the polymer surface) and opening the structure for further fictionalization. It means that O2 working gas plays a role to dissociate on the surface of the polymer material and replacing the polymer groups by establishing new chemical groups. There are a number of functional groups such as carbonyl, hydroxyl etc. They change the surface in few ways, like in terms of: 1) energy level the surface is changed from low to high; 2) the novel established chemical groups make the surface with improved wetting properties – very important fact for good adhering (bonding) to the next layers within the selective coating; 3) enhanced adhesion strength and permanency of the multilayered coated absorbers.
Second IR reflective layer is based on PVD Cu deposited layer, followed by additional Cu-Ni-NiO layers.
Third and fourth cermet layers are actually set of low- and high-metal fraction of TiOxNy composition layers.
The next layer is AR layer is TiO2 deposited layer with thickness of nearly 100nm.
Because this layer didn’t protect the multilayered structure over the absorber profiles from weather exploitation conditions of application as well as didn’t protect on tests such as humidity attacks (water immersing), temperature shocks etc…This layer needs further protection from all mentioned exploitation and weather conditions from the outer side.
After several considerations of protective layers one final coating that showed the best performance was selected. Namely, the last coated layer based on sol gel solution with inorganic components (trade secreted chemical) that is applied through silk printing method. The thickness of ~11 microns is enough thick layer that performed the best characteristics in terms of: 1) improved absorbance (transmittance is increased for 2-3 %), 2) good AR, 3) low soiling properties, that reduce the requirements for cleaning as well as maintenance costs and consequently reduce the time for those activities, 4) protective for the whole stack of selective coating because survive in-house tests of water immersion and temperature shocks without disturbing the optical characteristics before and after test examinations. 5) Finally, it offers a good mechanical and corrosion protection.
T 2.4. Validate selective coating performance characteristics
This work involved lab validation of the temperature dependent emissivity of the developed selective coating. The IR thermal emittance spectra at different temperatures is measured by use of FTIR-Fourier transform infrared spectroscopy.
Summary
The selective coating for polymer absorber profiles is the one explained above. The above explained deposited layers that are developed through this project allows max absorbance of solar irradiation on polymer composite material. At the same time, on the surface of polymer collectors’ absorber, the controlled emittance can be achieved. Actually, the coated absorbers can perform their role (can be mounted) at different weather / exploitation conditions. In regions with high solar irradiation and high ambient temperatures, emissivity can be controlled (increased) without changing the absorbance with simply designing a proper selective coating on collector absorbers (95% absorbance and 30% emittance). And, contrary on this, if the solar radiation in particular region is low and low ambient temperatures exist, a coating with low emissivity values and max absorbance on the absorbers can be made to harness as much as possible the poor solar irradiation in those regions.(95% absorbance and min emittance of 5%).
Work Package 3: Design polymer absorber profile, collector casing and transparent cover
Task 3.1 Geometrical design of solar absorber profile and integral heat transfer channels.
The only variation from the above specification in Annex 1 is the Thermal Conductivity. Thermal simulations carried out in Task 1.3 proved that a lower Thermal Conductivity value of 0.4 W/mK would achieve an efficiency of the absorber of approximately 95%. This is an important issue because it is not needed to increase thermal conductivity until 2 W/mK values, as it was initially planned in the project proposal, becoming a more achievable objective with regards to thermal properties increase of polymeric materials.
Three different absorber geometries were modelled with different volume flows of the heat transfer fluid and thermal conductivity of the absorber materials. All three designs fulfilled the requirements regarding pressure loss (dp < 70 mbar at 30 l/(h m²)) and performance (F’>0.95) using a polymer with a thermal conductivity of 0.6 W/(m K).
Both ends of the absorber have been considered fixed in their side faces, since that delivers the most critical boundary conditions regarding stresses.
Mechanical simulations of the absorber were carried out with the following conclusions:
• At ambient temperature performance, and for an internal pressure of 6 bar, the maximum deformations and stresses in the absorber are far below the admissible limits. A Young modulus of 2300 MPa and a maximum allowable stress (yeld stress) of 51.9 MPa have been considered.
• However, at working conditions of 6 bar internal pressure and 120ºC the material modulus for PET (Polyethylene terephthalate) the material chosen for the absorber drops to nearly zero past its Tg, which is around 80-90ºC (see Figure 4). Once this temperature is surpassed, the material tends to flow, the modulus dropping dramatically. This needs to be taken into account, since the simulation would not be valid for those working conditions.
• The design can be considered as valid for working conditions were temperature rises up to 60-70ºC.
As a consequence of above, the base material for the absorber was changed from PET to PC (Poly Carbonate), due to the low Tg value of PET, which was below the operating temperature of the absorber (120ºC). The Tg of PC is around 150ºC with a Young Modulus of around 1700 MPa at 120ºC (against 2450 MPa at room temperature) with a Yield stress around 70 MPa.
The model was simplified to contain a group of two pipes, in order to have a first estimation of deformation and stress results. The model was then gradually expanded and more pipes added, in order to check whether the number of pipes does have a significant effect on the results. The conclusion is that the stress values do not change significantly.
A 0.75 mm wall thickness is considered as the minimum value that fulfils the acceptance criterion described for design concept 4.
To enhance the heat transport from the absorber sheet to the fluid channels the thickness of the absorber sheet was set to the thickness of the tube channels. This change resulted in three different absorber designs being investigated.
The maximum stress and deformation values are far below the admissible ones. The stress level is below 7 MPa (10% of the material yield stress), which arbitrarily can be accepted as a security threshold for a good creep behaviour.
Because of reasons related to the mechanical stability, the polymer used and production techniques the final geometry was changed quite significantly compared to the ones investigated previously. The main changes were:
1. Reduction in wall thickness from 2 mm to 1 mm to compensate for the low conductivity of 0.4 W/(m K)
2. Parallel surfaces to improve the coating and ease integration into the manifold
3. Reduction of inner diameter for higher mechanical resistance
As in the previous investigations, only one tube was used for the simulation. The length of the tube was reduced to 1.0 meter for this design compared to the 1.8 meter of the previous geometries. That is due to the fact that it was decided that the prototype absorber will be built with a length of 1 meter.
Six cross sections of the tube, starting from the inlet and going to the outlet in steps of 200 mm from the left to the right, are shown in horizontal direction. Three different flow rates are presented as indicated on the left hand side.
As expected the pressure drop is lower compared to design 1, 2 or 3 because of the shorter tube. The collector efficiency factor shows values of about 0.97 so the desired value of 0.95 has been achieved or rather exceeded for all flow rates.
Task 3.2 Manufacture absorber profile prototypes, validate thermal conductivity.
The consortium took the decision to manufacture the absorber using single extrusion as opposed to co-extrusion as proposed in the DoW. The new absorber geometry (Design 10) is significantly thinner than original envisaged hence the material content is also significantly reduced therefore the expense and complexities are not required to minimise the material use.
For the extrusion small beads of doped PC containing 10% expanded graphite will be gravity fed from a top mounted hopper into the barrel of the extruder then comes into contact with the screw. The rotating screw (normally turning at up to 120 rpm) forces the plastic beads forward into the barrel which will be heated to approximately 200 °C (392 °F).
After passing through the breaker plate molten plastic enters the die which has been designed specifically to ensure that Design 10 can be produced.
The extrusion of the PolySol absorber and casing will be carried out on extrusion lines using custom made tooling and calibrators. The tooling and calibrators are used to both form and cool the extrudate into the required shape as previously described.
The profile will still be warm when it exits the calibrator so specially designed pads will be produced and fitted to haul offs to draw the profile at a consistent speed, this will prevent any stress. Also allowing additional cooling to take place
An absorber configuration was chosen after extensive mechanical and CFD simulation which meets the specification set out in Annex 1.
The material developed for producing the absorber is polycarbonate dosed with 10% graphite. The detailed doping procedure is described in D4.1.
The absorber was extruded using a single bespoke die by Plasticos Alai.
Task 3.3 Perform stress, mechanical impact, internal pressure and accelerated aging tests
Due to confusion in Annex 1 this was split into two tasks 1) Lab validation of the absorber prototype and 2) Lab validated casing prototype
Lab validated absorber prototype
The absorbers were extruded by Plasticos Alai using dosed polycarbonate produced by Tecnalia and described in Deliverables 4.1 4.2 and 4.3
The absorbers were coated using a multilayer PVD selective coating on polymer absorber and described in Deliverable 2.2 which was validated as to its suitability and described in Deliverable 3.
For each collector unit of 1m x 1m 5 absorber profiles were required. However the actual collector will be compose of 2 units to give us a collection surface of 2 sq m2
A process has been developed for producing the multilayer PVD absorbers to the correct absorbance and emittance values and these absorber profiles will be integrated into the final collector design. The absorbers were produced using the coating layers as described in D2.2
A quantity of tests of coated vs. uncoated samples of polymer absorbers were made using the Optosol instrument as it offered fast and efficient measurements. For each of the samples, uncoated and later coated ones (after each coating), a set of measurements to obtain absorbance and emittance of samples were made. The measurements of each sample were repeatable for at least three different spots on the same sample.
The absorbance of the final coating was measured using the Optosol instrument and was confirmed as 94.9% because of its AR properties (Fig. 2 a) and emittance of ~17 %. The most important fact of the last coating is that it has shown protective behavior on rigorous tests, such as temperature shocking, humidity tests. The selective coating over the absorber under these in-house testing remained undisturbed with the same optical properties that didn’t change before and after the test examination.
Further validation about proper use and test measurement of Optosol was confirmed by validating our results with additional complimentary measurements on the selected samples with FHG. Comparison of FHG and Plasma measurements for the same samples differed for only 1 to 1.5% which is within the tolerance limit of the instrument.
The optical properties measured by the Optosol instrument are absorbance of 94.9 % and emittance value of 17.5%. These properties are considered acceptable by Plasma, FHG and USTUTT
Lab validated casing prototype
In Deliverable No 1.5 seven different materials were tested for performance and Crastin was chosen as the material of choice. Although Crastin was chosen as the material as suitable for extrusion to be used for the extruded profile it was discovered that the die manufacturer in Spain could not use this material. As a consequence doped polycarbonate was used and was tested against the original materials by injection moulding test pieces and was found to be compatible in terms of performance when mechanical tested.
Testing was also carried out on the extruded profiles and the test was at least similar to the original test results if not better in some instances. The extruded profiles were also aged. The conditions were: UV exposure along with 85°C temp and 40% RH for 125h, 250h and 500h, the ageing trials showed no degradation over time and visual analysis of the aged samples also showed no degradation over time.
The overall conclusion is that from the test results the doped polycarbonate is an acceptable material for producing the profiles for the PolySol Solar Thermal Collector whilst also being a cheaper material than the original Crastin material and easier to manufacture.
Testing of polycarbonate against previous materials was carried out as in D1.5 and all the results for the polycarbonate were positive when compared to the other materials. These results were sent to the partners for comments and were accepted as acceptable by all.
Samples of the profiles were cut up and sent to FHG for ageing trials as per WP1. The size of sample was smaller than the standard classification however the results can be used as a comparison as the sample size was used on all the tests. The replacement polycarbonate material on testing was compatible in terms of performance with the other tested materials when produced by injection moulding. The ageing trials showed no degradation over time.
Task 3.4 Design recycled polymer collector casing and cover
Two concept casing designs were designed to provide speed and simplicity to the assembly and production of the all plastic solar panel casing. It is designed without the need for any mechanical fixings and is intended that the casing will be bonded together using a UV resistant adhesive.
1. ‘Stepped’ shaped profile design. This arrangement had steps providing a resting position for each component of the assembly as well as easy location in to position
2. ‘E’ shaped profile design. Each ‘branch’ of the profile is intended to provide both resting position and simple assembly location by effectively sliding or slotting the component between the branches until they are located correctly
The consortium decided that the ‘Stepped’ shaped profile design is the preferred option. The casing design was finalised with consultation from other partners and only slight modifications of size were made to accommodate the absorber
Work Package 4: Development of co-extrusion and doping process
Task 4.1 Set-up of nano-reinforcement adding into thermoplastic matrices
CFD simulations were carried out by USTUTT - ITW (T1.3 Thermofluid model of conceptual PolySol absorber and heat transfer channels) to determine whether the Polycarbonate +10% Expanded Graphite (M16) which could only achieve a maximum thermal conductivity value of 0.4 (W/mK) would be suitable to achieve the required collector efficiency factor F’ > 0.95 on the 4 different absorber designs. These simulations confirmed that collector efficiency factor F’ > 0.95 could be reached, hence Polycarbonate +10% Expanded Graphite was chosen as the material for the extrusion of the absorber. According to both, thermal and mechanical behaviour and the fact that the required collector efficiency could be achieved the Consortium agrees that the material selected for the production of the absorber would be Polycarbonate +10% Expanded Graphite.
Task 4.2 Fabrication and characterization of specimens for material selection
Planar samples were made by injection moulding for material selection and physical testing process. This task is a continuation of work carried out in Period 1 and after mechanical simulations material Polycarbonate +10% Expanded Graphite was chosen.
Task 4.3 Selection and adjustment of co-extrusion technology
Single screw extruders and twin screw extruders are the most widely used extruders. The screw that is used to push the resin out of the die is the important component of a screw extruder. In the earlier days rubber screw were used but the rubber screw was not able to give enough amount of shear into the polymer. Therefore, screws were designed that would start deeper in the feed and gradually taper shallower in the metering section to apply more work on the polymer as it was going from the feed to the discharge.
For POLYSOL project a twin screw extruder has been used to prepare nano-reinforced plastic compounds.
Task 4.4 Manufacturing of demonstrator prototypes
Following the extrusion guidelines proposed by TECNALIA and, in order to test their industrial technical viability, PLÁSTICOS ALAI has been produced two different parts of a solar collector by extrusion:
i) The casing of the solar collector, in pure PC
ii) The absorbers, in 10% Expanded Graphite PC.
At PLASTICOS ALAI facilities, line 6 with the dies manufactured by ARVITEC for the production of the casing and the absorbers parts at PLASTICOS ALAI facilities.
Work Package 5: Integration, prototype assembly and lab validation
T5.1 Integrate absorber, collector casing, cover and fittings.
ALAI produced the composite polymer absorber and casing components. The absorbers were coated at Plasma and Camel Solar integrated all components.
The first POLYSOL prototype collectors were sent to the following locations/partners for further in-field tests:
1 prototype (2 collectors or it is 1 unit) is remaining in Macedonia and will be tested here;
2 collectors (1 unit) was sent to ERA (Spain)
4 collectors (2 units) was sent to ITW (Germany)
2 collectors (1 unit) was sent to Ezinc (Turkey)
1 collector was sent to MaTRI (England)
T5.2 Controlled tests in laboratory environment.
The thermal performance of the collector will be determined according to EN 12975-2 by applying the steady-state test method using a solar simulator. In addition to the thermal performance basic durability and reliability tests such as a stagnation test in order to investigate the collector behaviour at high temperatures and an internal thermal shock test will be performed.
Thermal performance test was performed under steady state conditions and artificial light in solar simulator. Irradiance was set to 835 W/m2 and ambient temperature to 20 °C during all measurements. Mass flow rate was about 72 l/h. Inlet temperature was adjusted on different levels in 5 steps: about 20 °C, 38 °C, 57 °C, 78 °C and 93 °C.
The collector used for thermal performance test showed leakage during the test at a temperature of more than 40 °C and water was flowing inside the collector. This leakage was small however it can be assumed that it had a negative impact on the heat losses of the collector. Nevertheless the results are considered to be satisfying.
A second specimen was installed outdoors for exposure test. The test lasted 66 days and 30 days of this period were valid days according to EN 12975-2 with irradiation more than 14 MJ/m2 per day. The mean ambient temperature was 11.8 °C. Irradiation during this time was 871 MJ/m2. 207 l/m2 rain was recorded. After one month the first clearly apparent discolouration of header tubes were detected as shown in Figure 5. During further exposure the effect became more and more visible and after two months many vertical discoloured strips appeared on absorber surface. Nearly the whole time of the exposure test a certain area of the glazing showed water and other condensation.
Measurements for the determination of the stagnation temperature were performed outdoors under clear sky conditions around noon. Absorber temperature was measured at the backside of absorber using a NiCr-Ni thermocouple. Location was half of width and 2/3 of height.
The test of high temperature resistance was performed during determination of stagnation temperature. Radiation induced discolouration has already been reported earlier. Final inspection later shows a damage of connection between absorber panel and header tubes.
Cold water (about 9°C) was spread twice lasting for 15 minutes on the front side of the collector during sunny weather conditions. Irradiance was about 1000 W/m², ambient temperature higher than 20°C. No problem was observed.
The mechanical load test was performed with positive and negative pressure to the collector cover. Load was raised in steps of 500 Pa alternating positive and negative pressure up to 3000 Pa. The construction of the collector and the connection between transparent cover and collector frame is very stable. No problem was observed.
After the tests both collectors were dismantled and inspected according to EN 12975-1 [1] in detail. The collector used for durability and reliability test shows major failures. The absorber coating became very much lighter during exposure and cracking was detected. The upper header tube changed color from blue to gold. Welding connection between absorber and pipes broke during exposure, although it never was stressed by water under pressure. In addition insulation of recycling rubber foam discoloured induced by high temperature during exposition. It is supposed that out gassing happens because condensate film on inner side of the glazing was detected.
The collector used for thermal performance test showed problems, too. Due to the small leakage condensed water on the inside of the glazing was apparent on the whole area after the test. The absorber surface was discoloured from dark grey to light silver during weeks after the test. Three leakages were detected between upper header pipe and absorber panel. The reason for these leakages is a bad welding connection between the components. Insulation of recycling rubber foam was very much wet caused by the leakages.
The thermal performance test showed good result although the heat losses could have been better in the case the insulation material has not been soaked with water.
Within the durability and reliability test, however, the collector showed major deficits, mainly related to the absorber and the insulation material.
Due to the fact that an increasing emissivity of the absorber coating could not be achieved absorber temperatures up to 160 °C could be observed. For this temperature the chosen polymeric material (poly carbonate) is not suitable. Also the insulation material seems not to be stable at the temperatures it was exposed during the tests. The basic concept of the collector however is suited for a polymeric collector in case appropriate polymers are used for the absorber and the thermal insulation. The frame and casing of the collector are strong and suitable. However mechanical load test should be repeated on the full-size collector to the different strength of the frame being twice as long.
For the solar thermal collector, generally the absorber material and design are acceptable at recommended stagnation temperature of 120°C; however the material at the higher stagnation temperatures i.e. 160°C is unsuitable as warping starts to occur. It is thought that using the dosing material as a nano addition and not a micro addition may resolve this problem. The major problem that was discovered that caused the leakages to occur was the incompatibility between the manifold (polycarbonate) and the absorber (graphite dosed polycarbonate) where it was thought that thermal incompatibility resulted in splitting and warpage causing a break in the bond between the absorber and the manifold. Ideally the absorber and manifold must be made from the same material however the manifold would have to be injection moulded which would increase the cost.
The discolouration that was observed with the PVD coating (developed for a temperature of 120°C) occurred at the elevated temperature of 160°C. This can be overcome as Plasma has developed new protective layers which are believed to be stable at this temperature.
T5.3 Correlation of test results and modelling data.
The results from the simulation study show the largest difference to occur between the different locations concerning the fractional energy savings. This is due to large differences in the boundary conditions (i.e. irradiation and heat demand). As expected the positive effect on the fractional energy savings decreases with growing collector areas as well as with growing heat store volumes. Exceptions to this observation comprise only systems with very small ratios of collector area and heat store volume which are not relevant in practice. This effect is larger for the investigated DHW system regimes as for the SCS regimes. As the system efficiency decreases with larger collector areas and smaller heat store volumes both fractional energy savings and system efficiency should be considered for dimensioning. The dependency of the thermal performance on the system hydraulics is rather small compared with the effects mentioned above.
The comparison between a conventional flat plate and the all-polymer solar thermal collector shows the expected difference in the fractional energy savings. The conventional flat plate collector shows higher figures in fractional energy savings as well as in system efficiencies for all investigated systems and locations. This difference depends significantly on the heat store volumes as well as on the total collector areas though.
The difference in the fractional energy savings depend strongly on the location and the system configuration. Thus a general ratio between the area of a conventional and an all-polymer solar collector leading to the same fractional energy savings cannot be identified. However, using the presented graphs it is possible to determine this ratio for each location and system configuration.
T5.4 Finalise overall collector design and dimensioning.
Based on the results of the performed lab tests we will be able to finalise the overall design of the collector and its subcomponents. This also includes the overall dimension(s) required to satisfy hot water and or space heating demand as a function of a specific range of usage profiles (and solar irradiation available in specific geographic locations around Europe).
For commercialisation we would be recommending a modular system twice the size of the tested PolySol prototype resulting in an aperture area of 2.08 m². Taking this into account we would propose the following estimate for the overall collector design and dimensioning:
• For hot water preparation only: 3-4 collectors resulting in an aperture area of 6.24 m² - 8.32 m² depending on the location, store volume and desired fractional energy savings
• For hot water preparation and space heating: 5 - 20 collectors resulting in an aperture of 10.4 m² - 41.6 m² depending on the location, store volume and desired fractional energy savings
Although this is a broad range for the hot water preparation and space heating this is still typical of the range for conventional collectors.
T5.5 Develop installation process
The main objective of this task is to develop an installation procedure for PolySol which can subsequently by trialled in WP6 where up to four individual collector prototypes will be installed. ERA worked closely with the other participants to ensure that an installation process was developed that is suitable for mass market application
The methodology of installation of the PolySol Solar Thermal Collectors is as follows:
Different environments or typical solutions of fixing collectors as tiles roofs, plane roofs, sandwich, etc., will be taken into account. A simple way of installation, standard elements and security was considered to design the installation methodology of the whole system.
Mechanical installation of the Solar Thermal Collector. This procedure of installation will be considered from the base that the under-structure is already installed. This structure will give the inclination of the Solar Thermal Collector for each installation in particular and, of course, must accomplish all applicable regulations regarding wind loads, snow loads, strength and flexibility.
The installers may use standard solutions available in their zones and local warehouses to build up the mentioned under-structures.
Typical places to install a Solar Thermal Collector are tiled, plane and sandwich roofs.
Installation of the PolySol
The under-structure will be attached to two horizontal (or vertical) flat mounting rails. This solution will be the same for all kind of installations. The two rails must be separated a minimum of 50cm and a maximum of 80cm between them for horizontal or vertical installation. There must be at least 30cm room in the plane of the solar collector under the down rail and 30cm respect the upper rail.
The first collector will be installed with the back face in the mounting rails and fixed with the 4 fixing nuts in each sides of the collector. When the installation is finished fix the end joints and complete the piping installation. If the array has more collectors install the joint in the fixed collector.
Install the second collector sliding it through the previously installed joint. Support the collector while fix the 4 fixing nuts. Alternatively repeat above until the whole array is installed.
Hydraulic installation.
Joint Fittings
For the PolySol we have used a brass D40 press fitting joint. The pipes must be installed opposite. The inlet must be inserted in one side in the down pipe and the outlet in the opposite side in the upper one. The other two pipes must be closed with a tap or similar.
Hydraulic schemes
As PolySol is a solar thermal collector with similar hydraulic characteristics as other collectors typical schemes can be used. As shown in the Deliverable 1.4 will be shown four typical solutions:
Deviations & Corrective Actions:
None, information has been supplied to the partners in case the requested extension is approved.
Work Package 6: In-field trials
Work Package Objectives: Demonstrate a glazed, all-polymer collector for domestic heating and hot water preparation that achieves similar performance compared to a state-of-the-art metallic flat plate collector (thermal energy output) with an aperture area no larger than 150% that of the comparable metallic collector and a total production cost of <€100 per m2, To achieve an overall collector weight saving of up to 30% corrected for thermal output performance (in terms of overall collector size), To achieve a cost-reduction of at least 25% per kWhth produced solar thermal heat compared to collectors with metallic absorbers.
Please refer to the attached PDF for report with figures and tables
T6.1 Design field testing regime.
With the collaboration of ITW a field test regime suitable for the in-field validation on the prototype collectors that will be applied in four sites was designed:
- Germany (ITW)
- Macedonia (Camel Solar)
- Turkey (EZinc)
- Spain (ERA)
This designed test is composed by a hydraulic installation and a control-datalogging system that will allow us to validate the models and simulations carried on.
The in-field test consists in an analysis of the representative values that can explain the behaviour of a real installation in a real environment with a simulated energy consumption system. The main objective is to acquire all the representative data (temperatures, solar radiation, flow, energy, etc.) in a simple database that will be analyzed after the test period. This analysis will compare the real performance values with the models used in the project.
Hydraulic scheme.
The basic scheme for each site is composed by two collectors of 1m2 to achieve a solar field of 2m2 surface.
Thermal load.
To obtain a simple, cheap and effective cooling system standard radiators will be used for evacuating heat and simulate energy consumption. This system can be installed outdoors and near the rest of the equipment in order to reduce space and piping.
The power needed to be dissipated in the in-field trials will be 2kW.
Operation
The control will start the pump when a radiation measured by CS10 of at least 200W/m2 will fall on the collector. The 3 way mixing valve will be closing the load path until the temperature within the 3 way valve (approx. the temperature measured by sensor S1) is lower than 80ºC. If that temperature rises up to 80ºC the 3 way mixer valve will open to the thermal load to that extent that the outlet temperature of the 3 way valve will stay at 80°C.
Installation methodology this was sent to the partners
Task 6.2 Install 4 prototypes in different locations and validate installation methodology.
At the end of 2012 integrated collectors were sent to ERA in Spain, Ezinc in Turkey, MatRI in the UK, USTUTT-ITW in Germany and 1 was retained by Camel Solar in Macedonia.
The collectors were installed as described in Task 5.5 Develop installation process and ERA provided the organisations with the monitoring equipment. Unfortunately all partners realised there was a serious problem with the collectors after about two weeks of operation. Leaks were observed when the collectors were operating at full pressure (this has been described in Task 5.2 Controlled tests in laboratory environment).
A telephone conference was quickly organised and the problems were discussed. As a result it was decided that Camel Solar would manufacture one more collector using different adhesives.
Camel Solar and Plasma designed a new manufacture and assembly protocol for the PolySol collectors.
As mentioned above the main problems that appeared were the problems of connections (welding) of absorbers and the welding of the manifolds to the absorbers within the Polysol collector. Camel Solar and Plasma, both, started to look for better and better solutions for connections of collector's parts.
It is thought that the main problems of assembling of PolySol collector are:
Thermal dilatation of absorbers (made of PC composite material) , and thermal dilatation of manifolds (pure PC material),
Welding between: absorbers (PC composite) and welding between absorbers and manifolds (made of two different materials: PC composite and neat PC),
Difference in thermal dilatations of absorbers and welding rod (made of PC), different thermal dilatation between absorbers, welding rods and manifolds in three directions.
Specific shape of the welding area between pure PC tube and PC composite absorbers. This is one of the critical issues to solve during connecting collector's parts. The bonding area between manifold and absorbers due to their particular (specific) shapes does not provide useful or appropriate conditions for their adhering.
Advice to try to solve the problem was sought from The Welding Institute TWI (England) and alternative adhesives and solvents were purchased i.e. Weld-On brand of solvent cemented adhesives (#55 and # 58, as the most appropriate ones recommended by the producers Figure 34), and a solvent for PC material like, dimethylene chloride (DMC).
Two different approaches were tested in order to solve the integration problems with the Polysol collector:
Chemical bonding: solvents and cements
The last choice of adhesives showed good results for bonding together neat PC tubes and PC composite absorbers (frontal bonding)
In-house internal tests were conducted at Camel Solar with most of the prepared test specimens that were adhered with above mentioned methods, DMC and Weld-On. After breaking during clamping, the joint area showed good behaviour and it was very difficult to destroy joint area (parts are well stuck together).
Also, for initial test experiments two small collectors were made that were bonded with:first on bonded with solution of DMC solvent and PC chips dissolved in it, and the other one bonded with Weld On adhesives.
These tests helped us in further selection of adhesives or welding, and behaviour of materials under pressure tests.
After pressure testing of the integrated collectors it was noticed that the adhesives and solvents had not given very good results. This is thought to be because the shape of the manifolds and absorbers do not provide enough surface area for good adhesion between parts of the collector which is a key factor to enable a good resistance to high pressure. As a result we returned to the previous method of ‘Welding Polysol components with hot-air gun and PC-welding rod’
As mentioned in the last report, and the earlier experience of welding with PC rods this was repeated again with some modifications; namely deeper grooves, surface polishing before welding between absorbers and manifolds were made and some welding parameters were changed.
With this approach better results were achieved. Testing of the collector was carried out which consisted of heating the assembly to 500 C under a pressure of 3 bars. The initial results indicated that the quality of assembled product is better but thermal cycle of heating to 80 0C and cooling down to room temperature was not tested and the efficiency of the collector was measured at temperatures below 50 0C and a lower pressure, e.g. less than 3 bars.
After testing up to 3 bars of pressure the first crack was observed on a clear PC tube: water spray was shown in the area that was not welded, but on a clean and clear surface.
The cracks were repaired by grinding and then with air gun welding.
After repairing the manifolds the next step was assembling of the connected absorbers plate within the collector case and finally attaching the glass cover.
This collector was then used for the in field trials in Macedonia
For a more detailed explanation please refer to the attached PDF
Deviations & Corrective Actions:
We experienced failure of the collectors due to leaking on the initial trials hence a decision was taken to manufacture a new collector.
Task 6.3 Conduct in-field trials
This task will be on-going for 5 – 6 months over which all prototype PolySol systems will be thoroughly tested. ERA, STU and USTUTT-ITW (with support of the other partners) will be inspecting the systems on a 2 week basis (or more often in case of problems). Data loggers will enable real-time collection of performance data.
Due to the problems of having to manufacture a completely new collector it was decided by the consortium to install it at the test site at the Camel Solar test facility, Skopje in Macedonian. A video was also taken and uploaded onto the PolySol website as per D6.2 - Video of Operational Test Sites)
Due to the delay in having to manufacture a new collector the in field testing could only be carried out over 2 month’s duration. During that period of time (Sept 2013 – October 31st 2013) this collector collected 231.1 kWh over 60 days.
Due to the problems discovered operating the PolySol collector it was decided to operate at a pressure of 3 bars and exposure to solar radiation to around 40ºC to stop potential leaking and crashing of the welding areas especially on the pure PC tubes. This collector did not exhibit any of the leaking problems associated with the original prototypes.
Performance testing of the PolySol collector
Measurment equipment for the PolySol collector was installed.
In order to keep the temperature of water up to 45º C and perform non-pressurised circulation through the polymeric collector, cool water was kept in the non-pressurised storage.
Measurements were taken using the Technische Alternative Model: UVR1611 controller and it was observed that the polymer collector has shown much better efficiency than Cu-based collector made by Camel Solar up to temperatures of 45ºC.
Results were stored every minute during the working day. A video was also taken and uploaded onto the PolySol website as per D6.2 - Video of Operational Test Sites.
Summary
In some cases the efficiency at 45 or 50ºC was very close to metallic. The efficiency of every collector depends from absorber area and some additional components: glass, isolation, geometry and material of the tubes. After some analyses we concluded that in absorber area we have to put also approximately ½ of the surface of the connecting tubes.
It was due to the fact that we calculated absorber area only of absorbers is not real but actually the absorber area is bigger in the case for PolySol collector because of the area of connecting tube; however the connecting tube does not have the selective coatings, hence only ½ of the surface of the connecting tubes has been calculated. Using this calculation the efficiency of the PolySol collector is close to a metallic collector, i.e. around 5% lower than the metallic, which gives higher than expected results.
This collector has a good absorber shape, good heat transfer through composite material and channels. This collector will have very big advantage compared to metallic one in some areas such as: direct flow circulation of swimming water through the collector without heat exchanger- this is due to the resistance of polymers against swimming water. With this application the price of solar system will be several times cheaper than the price of metallic systems; it will be saved money on heat exchanger, pump station, controller a big advantage of using a PolySol collector for swimming pools is the working temperature also, because temperature in the pool is never higher than 35ºC. At this temperature, the collector will be good.
For a more detailed explanation please refer to the attached PDF
Deviations & Corrective Actions:
We experienced failure of the collectors due to leaking on the initial trials hence a decision was taken to manufacture a new collector and just carryout 1 set of in field testing at Camel Solar test facility, Skopje in Macedonian. These trials gave us results that show the PolySol solar thermal collector is viable after further development.
Task 6.4 Analysis trial data and correlate with lab and modelling results
Trial data will be analysed on a continuous basis and compared with lab test results from WP5. We will also correlate data with the models developed in WP1 (see T5.3 for the approach).
The ambient temperature was not recorded during the infield trials but was taken from a nearby weather station as 30 min mean values.
During the infield trials outlet temperature was kept below 45 °C to limit the risk of leakage due to temperature stress.
The measured specific collector output shows the same magnitude like the hemispherical irradiance and about 50% more than the calculated specific collector output. Also a strong fluctuating, due to the fluctuating mass flow can be observed. Due to the high difference in the measured and calculated specific collector output no validation of the infield trials measurements could be performed. As reason for the high difference a malfunction of the mess flow measurement is suspected.
FOR A MORE DETAILED EXPLANATION PLEASE REFER TO THE ATTACHED PDF
Potential Impact:
The projects overall aim is to develop an all-polymer solar thermal collector suitable for domestic hot water and/ or heating contribution with the following end-user benefits: i) user cost saving of at least 25% compared to metallic collectors (on a per kWh thermal basis); ii) weight saving of approximately 30% compared to existing metallic collectors (also on a per kWh thermal basis); iii) a footprint no more than 150% compared to existing metallic collectors; iv) modular design enabling improved integration within existing and new building designs.
PolySol will therefore result in benefits for the whole supply chain ranging from end-users (improved return on investment and reduced payback period); to installers (i.e. lower weight makes installation easier); architects (improved building integration); producers (improved economies of scale in polymer component production, reduced assembly cost, increased sales potential) and finally local, national and European governments (increased market penetration of RES in order to achieve EC targets of 20% market penetration by 2020; reduced need for subsidies to attain desired RES market growth).
The EC and Member States have made a clear commitment to achieve RES market penetration targets by 2020. As heating accounts for close to 50% of overall EU final energy demand it seems obvious that it will be important to increase market penetration of RES that provide heat. Of these solar thermal is an important, but so far underutilised option (others are biomass and geothermal).
This is especially true in a domestic setting where solar thermal is often the most practical option considering that biomass requires manual feedstock replenishment. A medium growth scenario is most likely considering the growth rates achieved over the past decade. Based on this scenario and an average price of €750 per m2 (cost of installed component excluding auxiliary system) we therefore anticipate a market potential of 22 million m2 of new capacity in 2020 which will be worth €16.5 billion in the same year. This would result in at least 250,000 solar thermal related jobs by that time in Europe alone.
Impact In theory, PolySol should be able to compete with any existing metallic collector and for any domestic heating and hot water application. However, in reality there will be specific market sectors that will suit PolySol and others that will not:
- PolySol will be most relevant to households with sufficient roof space to handle >4-6m2. As well as the associated thermal storage. Hence, for very small dwellings with significant space constraints, metallic collectors will be more effective
- PolySol will be most suitable for households in Central or Southern European countries where solar energy is more abundant compared to Northern Europe where evacuated tubes or CPC collectors are probably a better solution
- Due to the cost savings achieved PolySol will be well suited to solar combi-systems which require large surface areas of collector space. These are systems that provide a contribution to both space heating and hot water provision
These assumptions may limit the overall market approx. 60% of the total market potential for solar thermal systems. However, due to the significantly improved cost/benefit ratio of PolySol it is anticipated that the project can increase the overall available potential market by 30%, i.e. consumers who would previously be uninterested in solar thermal systems due to the high investment required.
Although it is predicted that PolySol will benefit the whole supply chain including other stakeholders such as local, national and European governments to date none of these benefits have been acheived.
The predicted socio-economic impact and the wider societal implications of the project are as follows: First of all, making solar thermal systems more financially attractive will encourage market penetration which will help achieve ambitious RES targets. Not only will this reduce fossil fuel consumption and associated GHG emissions, it will also reduce the need for government subsidies to support market penetration. The project benefits are therefore not confined to the PolySol consortium but include significant wider economic and environmental benefits. Based on our expected market penetration of 289,600 collectors within the first decade of commercialisation we aim to achieve a financial return (profit) of close to €80 million for the PolySol consortium and our licensees.
The following is a summary of the benefits to Europe as a whole.
Environmental impact: Overall household heating and hot water needs amount to approximately 30% of total European energy consumption. CO2 emissions are a function of a wide range of factors which vary significantly. However, on average each kWh of combusted gas will result in approximately 0.2kg of CO2 emissions whereas a kWh of generated electricity yields emissions of 0.43kg of CO2 and heating oil in 0.24kg of CO2. If we assume that 50% of the PolySol systems displace gas; 30% displace electric heating and the remaining 20% heating oil then by 2022 we will be reducing annual CO2 emissions by 168,460 tonnes (per annum).
However, this is just the saving achieved due to increased market penetration of solar thermal systems and does not take into consideration the savings achieved by displacing carbon intensive aluminium. Every kilogram of aluminium results in 12kg of CO2 compared to around 8kg for an average polymer and just 2kg for recycled polymer waste. If we assume that we can displace 5kg of aluminium in each collector and replace it with a combination of virgin and waste polymer than we could basically halve the carbon footprint per collector (aluminium component only).
Employment: According to ESTIF every 80kW (thermal) of newly installed capacity. Based on our sales forecasts PolySol could contribute to over 6,500 new and retained EU-based jobs within manufacturing, assembly and installation over 10 years post project.
Exports: PolySol will be an alternative to metallic solar thermal collectors that are produced in
Europe as well as in the Far East. We will maintain production within the EU, which means that we can displace imports of collectors from non-EU countries. Simultaneously, we aim to start exporting EU-made PolySol collectors to non-EU countries. In addition, we also aim to sell licenses to non-European manufacturer but will ensure that these products are not exported back to the EU.
Energy cost and security of supply: it is evident that energy prices are likely to increase significantly over the next 10-20 years. Moreover, these increases are expected to be above average inflation which means that they will drive up the prices of a wide range of products and services. As a result energy costs will increase as a proportion of average net income. This is clearly undesirable. PolySol will be a means of reducing fossil energy consumption thereby reducing the overall impact of increasing energy prices. Reducing fossil energy consumption also reduces Europe's excessive reliance on imported gas and oil.
List of Websites:
www.fp7-polysol.eu
