Final Report Summary - RENEWX (Development and demonstration of compact, multi-source heat eXchanger technologies for renewable energy applications)
Executive Summary:
The objective of RenewX was to develop and demonstrate novel heat exchanger technologies and manufacturing processes that will enable increased market penetration of European-made air source type heat pump systems. This will be achieved by i) increased efficiency and smaller footprint of the evaporator (compact heat exchanger design); ii) ability to utilise secondary or even tertiary energy sources to complement the air side (i.e. renewables such as solar thermal). The latter will enable significant increases in Seasonal Performance Factors (SPF) which will improve the return on investment and hence attractiveness of air based heat pump systems.
There are over 200 million households in Europe whose combined domestic heating needs account for around 30% of Europe’s energy demand. Reducing this will be essential if Europe is to achieve ambitious emission reduction targets (i.e. 2020 targets) and decrease our excessive reliance on imported energy. The clear solution to these two problems is to increase the amount of renewable energy used, an ambition at the heart of much recent EU legislation, of which new Heat Pump systems are a key enabling technology. Europe's refrigeration, air conditioning and heat pump industry has an annual turnover of around €30 billion, employing around 200,000 people. Within this industry our SME Associations (PIMEC, PSPC and SolMa) represent over 1,000 SMEs in the heat pump manufacture, solar thermal system manufacture, design and installation sectors. Led by these associations the RenewX consortium consists of European SME associations, SME end users and some of Europe’s leading research associations. Together, we are seeking to develop new platform technologies relevant to our industries and markets across Europe.
Novel heat exchanger designs and technologies for improving heat exchange e.g. linear vortex generators (LVGs) were identified and modelled at the start of the project. Two types of compact heat exchanger were down-selected for further investigation, an aluminium tube-fin array with enhanced heat transfer and a stainless steel micro-channel version. Manufacturing processes for the heat exchangers were reviewed and selected for fabrication of prototype assemblies. Three candidate manufacturing processes were investigated; mechanical assembly, brazing and diffusion bonding. The manufacturing processes were also modelled using relevant software to allow an optimisation of resources to be achieved for specific manufacturing volumes.
Whilst the prototype heat exchanger units were being manufactured and tested, a test rig was built to simulate real world performance under a range of operating conditions. This included the design and manufacture of bespoke refrigerant and water circuit conditioning and measurement rigs, a wind tunnel and the associated computer control and data acquisition systems for detailed monitoring. Two conventional air source based heat pumps were selected and integrated with a solar thermal system (six panels) and a heated water system (e.g. simulation low temperature waste heat) using the RenewX design. One of the air source heat pumps was modified to accommodate the RenewX heat exchanger design. The integrated system, designed to provide sufficient heating for an domestic property built after 1995 with a floor space of 150 m2 and occupied by an average European household (2.3 people), has been installed in the Re/genT laboratories together with a unmodified air source heat pump of equal type. It therefore provided a direct comparison of the systems under equal operating conditions.
In parallel to the prototype manufacture and demonstration of a multi-source system, an online interactive portal was developed to provide a knowledge centre on heat exchanger design for the SME-AGs and their membership. The SME-AGs in the project have a selection of new technologies which could be exploited independently as well as a central repository of information for potential exploitation with their members. In addition, the project has identified opportunities for further enhancement of heat exchange that it is proposed will be investigated in a follow-on project.
Project Context and Objectives:
There are over 200 million households in Europe whose combined domestic heating needs account for around 30% of Europe’s energy demand. Reducing this will be essential if Europe is to achieve ambitious emission reduction targets (i.e. 2020 targets) and decrease our excessive reliance on imported energy. The clear solution to these two problems is to increase the amount of renewable energy used, an ambition at the heart of much recent EU legislation, of which new Heat Pump systems are a key enabling technology.
Europe's refrigeration, air conditioning and heat pump industry has an annual turnover of around €30 billion, employing around 200,000 people. Within this industry our SME Associations (PIMEC, PSPC, and SolMa) represent over 1,000 SMEs in the heat pump manufacture, solar thermal system manufacture, design and installation sectors. Led by these associations the RenewX consortium consists of European SME associations, SME end users and some of Europe’s leading research associations. Together, it was proposed to develop new platform technologies relevant to our industries and markets across Europe.
In order to support the achievement of the Renewable Energy Directive (2009/28/EC), ratified and agreed by Member States in 2009, European governments have already committed to significantly reduce the carbon footprint of new and existing homes, and some have introduced subsidies for renewable energy systems. Mandatory national targets currently being developed into Renewable Energy Action Plans by individual Member States require that by 2015 where appropriate new and renovated buildings must contain a degree of renewable energy technology. An example of this is Germany’s ‘Erneubare-Energien-Waermegesetz’ which mandates that all new homes from January 2009 will have to produce at least 10% of their required heating and hot water demand by renewable energy systems. It is estimated that by 2020 this could save €50 billion in heating costs in Germany alone. Other Member States have similar legislation.
The objective of RenewX was to develop and demonstrate novel heat exchanger technologies and manufacturing processes that will enable increased market penetration of European-made air source type heat pump systems. An example of a heat pump cycle can be seen in Figure 1. The thermal cycle of the heat pump consists of the evaporator (heat exchanger), compressor, condenser (or gas cooler - heat exchanger) and a throttle valve. This was to be achieved by i) increased efficiency and smaller footprint of the evaporator (compact heat exchanger design); ii) ability to utilise secondary or even tertiary energy sources to complement the air side (i.e. renewables such as solar thermal). The latter will enable significant increases in Seasonal Performance Factors (SPF) which will improve the return on investment and hence attractiveness of air based heat pump systems.
The overall project objectives were:
1. To develop and demonstrate novel heat exchanger technologies and manufacturing processes.
Heat exchangers are available in a variety of forms and span all industries. The most fundamental definition considers a heat exchanger to be an interface which facilitates the transfer of energy from one fluid (typically gas or liquid) to another. The elements of a heat exchanger design depend on compromises in the balancing of several factors, such as: size, thermal performance, cost, pressure, energy consumption, blockage risk and reliability In practice heat exchangers can have many forms, use a variety of materials, and an equally wide range of manufacturing methods can be valid in their production.
There are a broad range of heat exchanger technologies currently being manufactured. The types of heat exchangers relevant to heat pump applications include fin and tube heat exchangers, plate-fin heat exchangers, wavy-fin heat exchangers, polymer heat exchangers, compact heat exchangers, printed circuit heat exchangers (PCHX), spiral heat exchangers (SHX), and ceramic heat exchangers (CRHX).
Enhancement techniques can be separated into two categories: passive and active. Passive methods require no direct application of external power. Instead, passive techniques employ special surface geometries or fluid additives which cause heat transfer enhancement. On the other hand, active schemes such as electromagnetic fields and surface vibration do require external power for operation. The majority of commercially interesting enhancement techniques are passive ones. Active techniques have attracted little commercial interest because of the costs involved, and the problems that are associated with vibration or acoustic noise.
Within the RenewX project, manufacturing methods were considered for all forms of heat exchangers, but with a focus on the types used in RenewX. The focus in the review was metal heat exchangers. Manufacturing methods are grouped into component manufacturing (formation of heat exchanger parts) and assembly technologies (where components are joined together).
2. Increased efficiency and smaller footprint of the evaporator (compact heat exchanger design)
Increased efficiency and therefore a decreased footprint of the evaporator can be achieved through improved heat transfer e.g. via improving the heat transfer of surfaces. In general, enhanced heat transfer surfaces can be used for three purposes:
• to make heat exchangers more compact in order to reduce their overall volume, and possibly their cost,
• to reduce the pumping power required for a given heat transfer process,
• to increase the overall UA value of the heat exchanger. A higher UA value can be exploited in either of two ways:
a) to obtain an increased heat exchange rate for fixed fluid inlet temperatures,
b) to reduce the mean temperature difference for the heat exchange; this increases the thermodynamic process efficiency, which can result in a saving of operating costs.
There are a number of basic ways of improving the gas-side heat transfer using special surface geometries, which provide enhancement by establishing a higher hA per unit base surface area. These include:
1. Increase the effective heat transfer surface area (A) per unit volume without appreciably changing the heat transfer coefficient (h). Plain fin surfaces enhance heat transfer in this manner.
2. Increase h without appreciably changing A. This is accomplished by using a special channel shape, such as a wavy or corrugated channel, which provides mixing due to secondary flows and boundary-layer separation within the channel. Vortex generators also increase h without a significant area increase by creating longitudinally spiraling vortices exchange fluid between the wall and core regions of the flow, resulting in increased heat transfer.
3. Increase both h and A. Interrupted fins (i. e. offset strip and louvered fins) act in this way. These surfaces increase the effective surface area, and enhance heat transfer through repeated growth and destruction of the boundary layers.
3. Investigation and design of several methodologies that achieve efficient incorporation of other energy sources (i.e. solar, waste heat).
The range of heat sources is considerable, and there are many options for combining them, but this is strongly influenced by practicality and cost. The source options range from ambient air and forced ventilation extract air to the ground (vertical or horizontal coils or, in some instances, air tunnels), and an increasing variety of solar collector types, ranging from flat plate solar water heaters in isolation, thermal plus photovoltaic (PV) and thermal plus PV plus storage. The selection of the heat source depends upon the climate, location (of the house) and the costs of energy and heat pump installation. In Northern Europe solar collectors with PV may be seen as a secondary heat input compared to, for example, ground, where the continuity of reasonable temperatures throughout the heating season is more favourable. For systems where solar collectors can be used to preheat domestic hot water, the collector output may bypass the heat pump (be it air or ground source) and be directed to the thermal store.
There are various studies that discuss the ‘coupling’ of heat sources. Normally there are two but in some cases three and even four. It is interesting and potentially important to note that a key element in most, if not all, of the combinations (and some single source heat pumps) is the thermal store. Storage becomes increasingly challenging as multiple sources serving (probably) multiple sinks (space heating and domestic hot water) increase in popularity. While there is evidence that the energy demand of homes will decrease as the number of low-energy buildings increases, particularly for space heating, the majority of current housing stock is not classed as ‘low energy’. Therefore, examples of low energy homes with small heat pumps, to meet demands of e.g. 9 kW with compressor drives of around 2.5 kW, are few at present and will take many years to dominate the heat pump market.
During the project, the options for combining sources were considered and the relative merits and limitations presented in tabulated form. The direction of research to optimise the combination of sources, with particular respect to the evaporator of the heat pump, was investigated. This included options for single and multiple-source domestic heat pumps, based upon concepts already out in the field (in most instances), and explored the possibility that innovative heat exchangers might be employed to use more than one heat source in heat pump evaporators. In some instances the concepts might be extended to include the compressor, condenser and other components.
Project Results:
The main S&T results/foregrounds can be described as follows:
1. Enhanced heat exchange using LVGs
The first design used in the RenewX project was a tube-fin heat exchanger. Tube-fin heat exchangers are the most commonly used type of heat exchanger, and are often used in air conditioning systems. They take the form of sheets termed fins (which are typically aluminium) threaded onto tubes (which are typically copper). Water or refrigerant passes through the tubes, and heat is transferred between the air and the tube contents. The objectives of the RenewX project included increasing the efficiency (and reducing the footprint) of the evaporator and develop and demonstrate novel heat exchanger technologies. As a result, it was determined that the best route to achieving this in the RenewX design would be to investigate the used of turbulator designs (eg Figure 2) for improving heat transfer and the associated methods of assembly.
During the project a number of different turbulator designs were considered and modelled to assess effectiveness under different operating conditions, Figure 3. The heat transfer coefficient was the primary reference value as improving it will improve efficiency of the heat exchanger design. The effect of air side pressure drop along the channels and Reynolds number on the heat transfer co-efficient were predicted.
It was determined that for a fixed pressure drop, the heat transfer coefficient was increased by up to 50% with turbulator enhanced channel compared to the plain one. This was valid for the whole range of pressure drop represented. Furthermore, the addition of a turbulator to an air channel can enhance the heat transfer by 75% (when the air Reynolds number (Re) equals 3000) compared to a plain air channel, Figure 4. In other words, a fixed heat transfer coefficient can be achieved with lower air inlet velocity (meanings lower Re). This has an impact on the dimensions of the fan; it means that the addition of an LVG can reduce the fan duty significantly.
Once a turbulator enhanced design had been down selected, it was necessary to investigate methods for manufacturing prototype heat exchanger units. An all-aluminium construction was selected for the heat exchanger to reduce costs which in turn focussed the selection of manufacturing methods. Options for dip brazing of the prototype units were considered as this technique allows for the formation of multiple brazed joints simultaneously. However, during the dip brazing process there is a requirement for venting of entrapped gases which requires re-work after brazing (welding of vent holes) introducing another manufacturing step. In addition, large scale manufacturing by this method would be relatively complicated.
In parallel to dip brazing, the possibility of using a mechanical assembly method was also explored as it was felt that the performance enhancement provided by the turbulator design could offset reduced heat transfer of a mechanical joint compared to a brazed joint. Following the manufacture of a number of test pieces, including the turbulator design within the fins, it was decided to proceed with mechanical assembly of the fins and tube and adhesive bonding of the ‘u’-bend tubes. An example of a prototype assembly with a turbulator design can be seen in Figure 5.
Specific test rigs were designed and manufactured for testing of both baseline and turbulator enhanced heat exchangers. A number of technical challenges were highlighted from the prototype testing and included issues with refrigerant distribution, header design and fin pitch versus turbulator size. This information was fed back to the design and a second set of heat exchangers were manufactured for testing and demonstration activities. It was found in the case of the evaporator, that there was an increase in the heat transfer coefficient of 18.2%. The lower than expected performance of the turbulator design on the heat exchangers compared to the modelling predictions has been rationalised. Three potential sources of the discrepancy in performance were identified and will be explored in further work.
In addition to a turbulator enhanced heat exchanger, a micro-channel compact heat exchanger was designed within the RenewX project. Plate heat exchangers distribute the heat through a series of stacked plates. In a simple version, tubes can be passed through the corners of the plates. Channels can also be created in the plates themselves. Micro-channel heat exchangers use channels with dimensions less than 1mm to direct fluids and exchange heat. A RenewX design using 0.3mm thick plates containing multiple channels measuring 0.1mm deep, 0.3mm wide and with a 1mm pitch was developed. Prototype units were manufactured from 316L stainless steel and refrigerant R134a was selected for the trials.
The micro-channel heat exchanger units were manufactured by diffusion bonding with process development undertaken in the project. A number of design iterations were undertaken to refine the design of the manifold plates and to assist with process development. A theoretical algorithm was developed which enabled the evaluation of thermal hydraulics issues in the micro-channel units. Application of the algorithm to measured data enabled good consistency to be attained between model prediction and experimental results.
2. Design of multisource heat exchanger
A successful design was developed and demonstrated for a multisource heat exchanger system based on the RenewX enhanced designs. The source options range from ambient air and forced ventilation extract air to the ground, as well as solar collector types, e.g. flat plate solar water heaters in isolation, thermal plus photovoltaic (PV) and thermal plus PV plus storage. For systems where solar collectors can be used to preheat domestic hot water, the collector output may bypass the heat pump (be it air or ground source) and be directed to the thermal store.
Based on the results of the literature survey and the overall objectives of RenewX (to improve the performance of air-source heat pumps), the following heat source combinations were chosen for further investigation and multi-source heat exchanger design:
• Fresh air and solar thermal
• Fresh air and forced ventilation extract air
The fresh air and solar thermal, and fresh air and forced ventilation systems were based on the finned-tube-type heat exchangers developed within the project, including the turbulator enhancement technique. The combination of all three sources (fresh air, forced ventilation extract air and solar) is also possible and was investigated.
For a fresh air and forced ventilation extract air multi-source heat exchanger design is was found that COP gains are possible for all seasonal conditions and ventilation air mass flow rates. The effect was larger in winter due to the higher difference between the two air temperatures, and shows a COP increase of 13% for the typical ventilation air mass flow rate of 0.249 kg/s. The effect is reduced to 9.5% in spring/autumn, and 5% in summer. Overall, the system shows potential, particularly in regards to COP gains. It also has the advantage of not requiring any further heat exchangers or equipment (assuming a forced ventilation system is in place).
In the case of the solar-air system, two heat exchangers were designed: one as an air preheater (HEX-1) and one as an evaporator (HEX-2). Both heat exchangers are of similar design, featuring aluminium tubes and fins with turbulator enhancement. In order to investigate the potential advantages of the solar-air dual source system, a steady-state thermodynamic model was used to generate results in three key areas:
• Heat pump coefficient of performance (COP). This is a measure of the overall heat pump performance and is directly correlated to the economic and environmental performance of the system.
• Frosting. This is a key issue in the performance of air-source evaporators. Frosting is greatly detrimental to heat pump performance as it often takes a significant amount of energy to defrost the evaporator (via reverse cycling). The effects of frosting are more prominent in compact heat exchangers as narrow channels can become rapidly blocked.
• Condensation. Condensation from the air stream (due to cooling below the dew point) provides an extra heat transfer resistance and can also lead to channel blockage in compact units.
It was shown that significant COP gains can be made by utilising a fresh air and solar thermal dual source system. The most significant gains are observed at lower air temperatures when the temperature difference between the air and the solar water is maximised which creates a larger log mean temperature difference across HEX-1, Figure 6. This therefore allows a greater amount of heat to be transferred to the air, meaning the air-side temperature increase is larger, which leads to higher COP gains. However, even at an air temperature of 15oC, up to a 20% increase in COP is observed which represents a significant gain. In addition, it was also found that the frost-free operation range is significantly expanded by implementation of the dual-source solar-air system. At a collector temperature of 35oC (achievable on clear days in winter), the evaporator can operate frost-free at ambient temperatures below freezing which shows significant performance improvement compared to traditional single air-source evaporators.
Following the successful investigation of the two dual-source systems, it was decided to combine the approaches into a tri-source system utilising all three of the suggested heat sources. This would be advantageous as both additional heat sources may be intermittent in nature (solar dependant on solar gain and storage efficiency, while the ventilation air mass flow rate may vary by season or time of day). The model variables were the fresh air condition, the ventilation mass flow rate and the solar water temperature:
• 3 fresh air conditions based on UK lowland conditions:
o Winter: 4.4 oC and 85% relative humidity
o Summer: 15 oC and 76% relative humidity
o Spring/Autumn: 9.9 oC and 75% relative humidity
• 5 ventilation air mass flow rates: 0 (single source system condition), 0.125 0.187 0.249 0.312 kg/s. 0.249 was considered the typical condition and is based on 1.5 air volume changes per hour in a typical sized UK house. This was chosen as a variable as it will not be constant throughout the course of a day or different seasons.
• Solar water temperature varying between 15 and 40oC.
For a tri-source system it was found that COP increases of up to 40% in winter, 33% in spring/autumn and 26% in summer were achievable and that the integration of the three sources together is worthwhile. In all cases, a COP increase of 10% should be possible as this was observed for all cases of vice-versa maximum and minimum input from the two sources (for example, when the solar temperature was at the minimum value of 15oC and the ventilation extract mass flow rate is at the maximum value of 0.312 kg/s). Hence, significant COP gains should be shown throughout all seasons and throughout a typical day providing the scheduling of the forced ventilation system is designed to counter-balance troughs in the solar storage temperature.
Condensation from the air stream was also studied for the tri-source system. In all cases it was shown to be entirely prevented. Hence, the tri-source system has an advantage over the two dual-source systems in that it completely eradicates the problem of condensation in the evaporator. In addition, the frost free operation range was expanded as the ventilation air mass flow rate was increased. Even at the minimum ventilation mass flow rate, frost-free operation can be achieved at an ambient below 0oC for a solar collector temperature of around 25oC. This is comfortably achievable in winter.
For an example in the UK, based on a thermal energy consumption of 15,000 kWh/year and a 30% increase in COP, the estimated decrease in electricity demand would be approximately 23% with a cost saving of £165 per year and reduced carbon emissions of 724 kgCO2eq/year.
A dual source solar-air source was demonstrated within the project, Figure 7. The integrated system, designed to provide sufficient heating for a domestic property built after 1995 with a floor space of 150 m2 and occupied by an average European household (2.3 people), was installed in the Re/genT laboratories, together with a unmodified air source heat pump of equal type, thereby providing a direct comparison of the systems under identical operating conditions. Measurements, conducted in Helmond in The Netherlands during December 2015 and February 2016, showed a 7 to 11% reduction in energy consumption for the RenewX design compared to the reference system for operation with Solar heat and a 15 to 17% reduction in energy consumption for operation with low temperature waste heat (20°C). The low temperature waste heat system performed better due to seasonal factors such as cloud cover. The concept of using multiple heat sources to increase the performance of the air source heat pumps was successfully demonstrated.
3. Manufacturing processes
A range of different manufacturing processes for both the heat exchanger units and components were considered within the project. Methods that were considered for component manufacturing included additive and subtractive manufacturing as well as casting, forming and moulding. Assembly/joining technologies that were considered for the manufacture of heat exchanger units included mechanical assembly using sealants and seals, fusion welding, brazing (e.g. dip, vacuum etc..) and diffusion bonding.
A mechanical forming technique (punching) was selected for the manufacture of the turbulator design based on simplicity and cost. The thin nature of the aluminium fins and the turbulator design would allow cost effective scaling of this technology suitable for mass production. With suitable integration into a manufacturing line, it should be possible to form the turbulator feature on an automated basis. In addition, the design should be compatible with adding multiple turbulators to the fins, which in certain cases, should further improve heat transfer.
A mechanical assembly approach was used for manufacturing the all-aluminium heat exchangers as this was a very low cost and scalable process. Simul8 software was used to investigate options for optimised manufacturing as a function of the different process steps and resource availability.
The micro-channel heat exchangers were fabricated by diffusion bonding in vacuum. The process parameters were developed on an iterative basis by varying the diffusion bonding temperature, time and applied load. Some re-design was required in order to better optimise the manifold plate structure. Photochemically etched shims of 316L stainless steel, Figure 8, were manufactured by ACE Ltd and were used to form the main core of the heat exchanger units.
4. Online information portal
In order to capture the extensive knowledge generated within the project for the SME-AGs and to provide a possible source of on-going support and/or revenue, it was agreed to develop an interactive online information portal. The content of the portal was discussed by the consortium and the following points were agreed:
• A separate website was developed by TWI to host the content.
• The content will be developed by all partners.
• The portal can be entered via the RenewX website, but it will also be independent.
• At the end of the project, the website will be transferred to PIMEC.
• The proposed domain name: www.heatexdesign.com.
• Content to include:
o Design tools – e.g. datasheets and calculators.
o Aspects of results from technical work packages in project.
o Technical documents such as white papers, images and videos.
• Content will be predominantly public, but with password protected access to the design tools.
• SME associations in the project will receive free access to the design tools, and non-members can pay a fee to download.
From this brief, TWI and the project partners developed an online portal www.heatexdesign.com and to ensure maximum exposure, similar named domain names (heatexdesign.eu etc) were also purchased. A specific logo was created for the second project website, Figure 9.
Three design tools were developed for the portal:
o Tube-fin heat exchanger designer (Designing with turbulators)
The concept of this tool is to allow the user to quickly estimate the heat transfer coefficient of a tube-fin heat exchanger. Calculations are provided in a web based spreadsheet. A user guide is also available for download to help the user select appropriate constant.
o RenewX benefits calculator
This Java based tool is used to calculate the benefits of the RenewX dual source solution. Outputs include:
o Coefficient of Performance (COP) by season and overall.
o Key performance indicator of heat pumps.
o Electricity cost per season and year.
o CO2 emissions per season and year.
o Average mass flow rate of condensate from air stream by season.
o Likelihood of frosting by season.
o Micro-channel heat exchanger design
In addition to the online information portal, a design manual was developed for tube-fin heat exchanger design. The design manual provides a step by step process guide for developing a complete design. The design manual provides guidance for two situations. The first considers a case where a solar collector and air source heat pump (ASHP) are used together in a building and the second is a calculation guide for the design of a micro-channel heat exchanger.
In the first case, the solar collector has a water loop, which outputs hot water to a storage system. Onto this loop is placed a tube-fin heat exchanger, transferring heat to the air. The evaporator in the ASHP also has a tube-fin heat exchanger, which takes heat from the air, pre-heated by the solar collector. This heat is then transferred to the refrigerant on the ASHP loop.
Potential Impact:
There are over 200 million households in Europe whose combined domestic heating needs account for around 30% of Europe’s energy demand. Reducing this will be essential if Europe is to achieve ambitious emission reduction targets (i.e. 2020 targets) and decrease an excessive reliance on imported energy. The clear solution to these two problems is to increase the amount of renewable energy used, an ambition at the heart of much recent EU legislation, of which new Heat Pump systems are a key enabling technology.
Europe's refrigeration, air conditioning and heat pump industry has an annual turnover of around €30 billion, employing around 200,000 people. Within this industry our SME Associations (PIMEC, PSPC and SolMa) represent over 1,000 SMEs in the heat pump manufacture, solar thermal system manufacture, design and installation sectors. Led by these associations the RenewX consortium consists of European SME associations, SMEs and some of Europe’s leading research associations. Together, we propose to develop new platform technologies relevant to our industries and markets across Europe.
The European Heat Pump market has doubled over the last 5 years. However, this growth has been rather unstable. The most important indicator is the energy price ratio, which compares major energy sources such as gas or oil. In most markets competition with gas is very close due to the relative high Seasonal Performance Factor (SPF) compensating for the lower price of gas on a per kWh basis (usually around 1/3 of the price of electricity per kWh). The volatility of electricity and gas prices makes it difficult to predict future price ratios on a per kWh basis. In addition, initial investment cost is a major factor. Currently, ASHP’s (Air Source Heat Pumps) are approximately 1.5 times more expensive than a comparable highly efficient gas condensing boiler. Typical payback periods are usually 6-8 years, therefore, technologies that offer improved performance and/or reduced cost are critical to increasing the market uptake of ASHPs. This also reduces the reliance on, and effect of, government subsidies, that can significantly alter market dynamics.
In addition to unpredictable economic conditions, a sharp increase in low-cost imports from Asia has also had a significant effect on market dynamics. It is estimated that over 30 million Chinese households have installed solar collector water heating systems. These systems are > 80% cheaper than a comparable system in Europe. The same applies to the heat pump market, where Chinese imports are now sold for ~€1,500 in the UK, compared to similar European systems at €3,500. These systems are often poor quality imitations of European technology which has an adverse effect on the reputation of our ASHPs. The significant uptake of solar thermal collectors and corresponding decrease in cost presents a significant opportunity for efficiency gains via multi-source systems.
In order to support the achievement of the Renewable Energy Directive (2009/28/EC), ratified and agreed by Member States in 2009, European governments have already committed to significantly reduce the carbon footprint of new and existing homes, and some have introduced subsidies for renewable energy systems. Mandatory national targets currently being developed into Renewable Energy Action Plans by individual Member States require that by 2015 where appropriate new and renovated buildings must contain a degree of renewable energy technology. An example of this is Germany’s ‘Erneubare-Energien-Waermegesetz’ which mandates that all new homes from January 2009 will have to produce at least 10% of their required heating and hot water demand by renewable energy systems. It is estimated that by 2020 this could save €50 billion in heating costs in Germany alone. Other Member States have similar legislation.
The major challenges to further expansion of the European heat pump industry are competition from gas-based alternatives as well as competition from lower cost imports. The only way to mitigate these challenges and turn them into opportunities is by a process of continuous innovation to achieve major efficiency improvements and cost reductions. Although significant incremental improvements have been made (e.g. modulating compressors, alternative refrigerants) the basic design and operation of the major components have not changed for decades. In Ground Source Heat Pumps (GSHP) a lot of work has been done to improve the efficiency of compact plate heat exchangers. More recently a range of manufacturers have started to offer hybrid heat pumps which can be combined with solar thermal systems. These range from completely separate systems to systems with direct coupling of solar thermal energy to the evaporator. Systems such as the latter are not yet available across the whole market. Such combinations offer significant potential for an increase in the COP (Coefficient of Performance) or SPF of the heat pump system. However, the main challenge is to effectively utilize more than one energy source which adds cost and complexity, especially with regards to the heat exchanger design.
The existing design of simple finned tube heat exchangers (evaporator) is relatively basic and can easily be produced in bulk. However, they are not as efficient as they could be due to the poor thermal and mechanical contact between their main components and require large fans to achieve the air flow required to produce energy. Their design is optimized for heat transfer between the ambient air and the refrigerant circulated through the system. In contrast to the plate heat exchangers used for GSHPs, only limited efforts have been made to improve the basic finned tube design.
The objective of RenewX was to develop and demonstrate novel heat exchanger technologies and manufacturing processes that will enable increased market penetration of European-made air source type heat pump systems. This will be achieved by i) increased efficiency and smaller footprint of the evaporator (compact heat exchanger design); ii) ability to utilise secondary or even tertiary energy sources to complement the air side (i.e. renewables such as solar thermal). The latter will enable significant increases in Seasonal Performance Factors (SPF) which will improve the return on investment and hence attractiveness of air based heat pump systems.
A number of results have been generated at the successful completion of the RenewX project for exploitation by the relevant consortium partners. These are:
• Development of a method for improving the heat transfer coefficient by up to 18% with significant scope for further improvement through identified design modifications.
• Achieved a 7 to 11% reduction in energy consumption for the RenewX design compared to the reference system for operation with Solar heat.
• Achieved a 15 to 17% reduction in energy consumption for operation with low temperature waste heat (20 °C).
• Creation of an online information portal for assisting with heat exchanger design has been created for use by the SME-AG partners and their respective members.
The potential impact of the project could be to increase air based heat pump sales by European manufacturers by at least 50,000 across Europe between 2017-2027 adding €150 million of value to Europe’s economy. In addition, 50% of the current total waste heat generated in the EU is classified as low to moderate grade heat (e.g. waste water or solar). The opportunity to harvest some of this low grade energy using RenewX based technologies provides the potential to save a proportion of the 180GW of power (low to moderate grade heat) that is wasted per year in the EU. Correspondingly, this offers the potential to also contribute to reducing a proportion of the 94M tonnes of CO2 (equivalent to ~135Mtoe fossil fuels) generated per annum related to wasted low and moderate grade heat. In order to achieve the potential impacts, it will require the SME-AGs to actively promote the project results to their members and work together on exploitation and licencing.
The expected final result of the RenewX project was the development of new compact heat exchanger technologies that will improve the competitive position of the SME members of the consortium associations as well as to provide direct benefit to the associations themselves. This has been achieved by
• Improving efficiency of air based heat pump systems by improved design and combining them with alternative energy sources, thereby increasing return on investment whilst decreasing payback periods
• Reducing the footprint of air based heat pump systems, enabling potential integration in buildings
The main business activities of two of the three SME association partners are within industrial sectors relevant to the proposed technology development, i.e. solar thermal collectors and heat pumps. The third association, PIMEC, has a remit of assisting Spanish SMEs in improving and developing new business opportunities. This provides all of the SME associations with an opportunity to assist their member organisations in improving their business position. Combined, the SME associations have access to over 1,000 SMEs in the renewable energy community.
The networks and industrial contacts of the four RTD partners have been used to actively disseminate the projects results. These networks reach a broad industrial audience as, by their nature, the contacts of the participating associations are relatively narrow. For example, TWI has disseminated project results to its current Industrial membership whilst UNEW is the co-ordinator of HEXAG (the Heat Exchanger Action Group). There is currently significant interest in compact heat exchangers across a number of industrial sectors outside of renewable energy, e.g. oil and gas and aerospace, due to the potential space and weight savings associated with smaller unit sizes.
The project partners have identified opportunities for improving the design and performance of both the turbulator design and the micro-channel heat exchanger and are discussing a proposal for Horizon 2020 funding. This should provide additional improvements in performance of the ASHP heat exchanger improving both efficiency and reducing the footprint.
List of Websites:
www.renewx.eu
Online information portal
www.heatexdesign.com
Contact:
Dr Nick Ludford
TWI Ltd
Granta Park,
Great Abington
Cambridge,
CB21 6AL
The objective of RenewX was to develop and demonstrate novel heat exchanger technologies and manufacturing processes that will enable increased market penetration of European-made air source type heat pump systems. This will be achieved by i) increased efficiency and smaller footprint of the evaporator (compact heat exchanger design); ii) ability to utilise secondary or even tertiary energy sources to complement the air side (i.e. renewables such as solar thermal). The latter will enable significant increases in Seasonal Performance Factors (SPF) which will improve the return on investment and hence attractiveness of air based heat pump systems.
There are over 200 million households in Europe whose combined domestic heating needs account for around 30% of Europe’s energy demand. Reducing this will be essential if Europe is to achieve ambitious emission reduction targets (i.e. 2020 targets) and decrease our excessive reliance on imported energy. The clear solution to these two problems is to increase the amount of renewable energy used, an ambition at the heart of much recent EU legislation, of which new Heat Pump systems are a key enabling technology. Europe's refrigeration, air conditioning and heat pump industry has an annual turnover of around €30 billion, employing around 200,000 people. Within this industry our SME Associations (PIMEC, PSPC and SolMa) represent over 1,000 SMEs in the heat pump manufacture, solar thermal system manufacture, design and installation sectors. Led by these associations the RenewX consortium consists of European SME associations, SME end users and some of Europe’s leading research associations. Together, we are seeking to develop new platform technologies relevant to our industries and markets across Europe.
Novel heat exchanger designs and technologies for improving heat exchange e.g. linear vortex generators (LVGs) were identified and modelled at the start of the project. Two types of compact heat exchanger were down-selected for further investigation, an aluminium tube-fin array with enhanced heat transfer and a stainless steel micro-channel version. Manufacturing processes for the heat exchangers were reviewed and selected for fabrication of prototype assemblies. Three candidate manufacturing processes were investigated; mechanical assembly, brazing and diffusion bonding. The manufacturing processes were also modelled using relevant software to allow an optimisation of resources to be achieved for specific manufacturing volumes.
Whilst the prototype heat exchanger units were being manufactured and tested, a test rig was built to simulate real world performance under a range of operating conditions. This included the design and manufacture of bespoke refrigerant and water circuit conditioning and measurement rigs, a wind tunnel and the associated computer control and data acquisition systems for detailed monitoring. Two conventional air source based heat pumps were selected and integrated with a solar thermal system (six panels) and a heated water system (e.g. simulation low temperature waste heat) using the RenewX design. One of the air source heat pumps was modified to accommodate the RenewX heat exchanger design. The integrated system, designed to provide sufficient heating for an domestic property built after 1995 with a floor space of 150 m2 and occupied by an average European household (2.3 people), has been installed in the Re/genT laboratories together with a unmodified air source heat pump of equal type. It therefore provided a direct comparison of the systems under equal operating conditions.
In parallel to the prototype manufacture and demonstration of a multi-source system, an online interactive portal was developed to provide a knowledge centre on heat exchanger design for the SME-AGs and their membership. The SME-AGs in the project have a selection of new technologies which could be exploited independently as well as a central repository of information for potential exploitation with their members. In addition, the project has identified opportunities for further enhancement of heat exchange that it is proposed will be investigated in a follow-on project.
Project Context and Objectives:
There are over 200 million households in Europe whose combined domestic heating needs account for around 30% of Europe’s energy demand. Reducing this will be essential if Europe is to achieve ambitious emission reduction targets (i.e. 2020 targets) and decrease our excessive reliance on imported energy. The clear solution to these two problems is to increase the amount of renewable energy used, an ambition at the heart of much recent EU legislation, of which new Heat Pump systems are a key enabling technology.
Europe's refrigeration, air conditioning and heat pump industry has an annual turnover of around €30 billion, employing around 200,000 people. Within this industry our SME Associations (PIMEC, PSPC, and SolMa) represent over 1,000 SMEs in the heat pump manufacture, solar thermal system manufacture, design and installation sectors. Led by these associations the RenewX consortium consists of European SME associations, SME end users and some of Europe’s leading research associations. Together, it was proposed to develop new platform technologies relevant to our industries and markets across Europe.
In order to support the achievement of the Renewable Energy Directive (2009/28/EC), ratified and agreed by Member States in 2009, European governments have already committed to significantly reduce the carbon footprint of new and existing homes, and some have introduced subsidies for renewable energy systems. Mandatory national targets currently being developed into Renewable Energy Action Plans by individual Member States require that by 2015 where appropriate new and renovated buildings must contain a degree of renewable energy technology. An example of this is Germany’s ‘Erneubare-Energien-Waermegesetz’ which mandates that all new homes from January 2009 will have to produce at least 10% of their required heating and hot water demand by renewable energy systems. It is estimated that by 2020 this could save €50 billion in heating costs in Germany alone. Other Member States have similar legislation.
The objective of RenewX was to develop and demonstrate novel heat exchanger technologies and manufacturing processes that will enable increased market penetration of European-made air source type heat pump systems. An example of a heat pump cycle can be seen in Figure 1. The thermal cycle of the heat pump consists of the evaporator (heat exchanger), compressor, condenser (or gas cooler - heat exchanger) and a throttle valve. This was to be achieved by i) increased efficiency and smaller footprint of the evaporator (compact heat exchanger design); ii) ability to utilise secondary or even tertiary energy sources to complement the air side (i.e. renewables such as solar thermal). The latter will enable significant increases in Seasonal Performance Factors (SPF) which will improve the return on investment and hence attractiveness of air based heat pump systems.
The overall project objectives were:
1. To develop and demonstrate novel heat exchanger technologies and manufacturing processes.
Heat exchangers are available in a variety of forms and span all industries. The most fundamental definition considers a heat exchanger to be an interface which facilitates the transfer of energy from one fluid (typically gas or liquid) to another. The elements of a heat exchanger design depend on compromises in the balancing of several factors, such as: size, thermal performance, cost, pressure, energy consumption, blockage risk and reliability In practice heat exchangers can have many forms, use a variety of materials, and an equally wide range of manufacturing methods can be valid in their production.
There are a broad range of heat exchanger technologies currently being manufactured. The types of heat exchangers relevant to heat pump applications include fin and tube heat exchangers, plate-fin heat exchangers, wavy-fin heat exchangers, polymer heat exchangers, compact heat exchangers, printed circuit heat exchangers (PCHX), spiral heat exchangers (SHX), and ceramic heat exchangers (CRHX).
Enhancement techniques can be separated into two categories: passive and active. Passive methods require no direct application of external power. Instead, passive techniques employ special surface geometries or fluid additives which cause heat transfer enhancement. On the other hand, active schemes such as electromagnetic fields and surface vibration do require external power for operation. The majority of commercially interesting enhancement techniques are passive ones. Active techniques have attracted little commercial interest because of the costs involved, and the problems that are associated with vibration or acoustic noise.
Within the RenewX project, manufacturing methods were considered for all forms of heat exchangers, but with a focus on the types used in RenewX. The focus in the review was metal heat exchangers. Manufacturing methods are grouped into component manufacturing (formation of heat exchanger parts) and assembly technologies (where components are joined together).
2. Increased efficiency and smaller footprint of the evaporator (compact heat exchanger design)
Increased efficiency and therefore a decreased footprint of the evaporator can be achieved through improved heat transfer e.g. via improving the heat transfer of surfaces. In general, enhanced heat transfer surfaces can be used for three purposes:
• to make heat exchangers more compact in order to reduce their overall volume, and possibly their cost,
• to reduce the pumping power required for a given heat transfer process,
• to increase the overall UA value of the heat exchanger. A higher UA value can be exploited in either of two ways:
a) to obtain an increased heat exchange rate for fixed fluid inlet temperatures,
b) to reduce the mean temperature difference for the heat exchange; this increases the thermodynamic process efficiency, which can result in a saving of operating costs.
There are a number of basic ways of improving the gas-side heat transfer using special surface geometries, which provide enhancement by establishing a higher hA per unit base surface area. These include:
1. Increase the effective heat transfer surface area (A) per unit volume without appreciably changing the heat transfer coefficient (h). Plain fin surfaces enhance heat transfer in this manner.
2. Increase h without appreciably changing A. This is accomplished by using a special channel shape, such as a wavy or corrugated channel, which provides mixing due to secondary flows and boundary-layer separation within the channel. Vortex generators also increase h without a significant area increase by creating longitudinally spiraling vortices exchange fluid between the wall and core regions of the flow, resulting in increased heat transfer.
3. Increase both h and A. Interrupted fins (i. e. offset strip and louvered fins) act in this way. These surfaces increase the effective surface area, and enhance heat transfer through repeated growth and destruction of the boundary layers.
3. Investigation and design of several methodologies that achieve efficient incorporation of other energy sources (i.e. solar, waste heat).
The range of heat sources is considerable, and there are many options for combining them, but this is strongly influenced by practicality and cost. The source options range from ambient air and forced ventilation extract air to the ground (vertical or horizontal coils or, in some instances, air tunnels), and an increasing variety of solar collector types, ranging from flat plate solar water heaters in isolation, thermal plus photovoltaic (PV) and thermal plus PV plus storage. The selection of the heat source depends upon the climate, location (of the house) and the costs of energy and heat pump installation. In Northern Europe solar collectors with PV may be seen as a secondary heat input compared to, for example, ground, where the continuity of reasonable temperatures throughout the heating season is more favourable. For systems where solar collectors can be used to preheat domestic hot water, the collector output may bypass the heat pump (be it air or ground source) and be directed to the thermal store.
There are various studies that discuss the ‘coupling’ of heat sources. Normally there are two but in some cases three and even four. It is interesting and potentially important to note that a key element in most, if not all, of the combinations (and some single source heat pumps) is the thermal store. Storage becomes increasingly challenging as multiple sources serving (probably) multiple sinks (space heating and domestic hot water) increase in popularity. While there is evidence that the energy demand of homes will decrease as the number of low-energy buildings increases, particularly for space heating, the majority of current housing stock is not classed as ‘low energy’. Therefore, examples of low energy homes with small heat pumps, to meet demands of e.g. 9 kW with compressor drives of around 2.5 kW, are few at present and will take many years to dominate the heat pump market.
During the project, the options for combining sources were considered and the relative merits and limitations presented in tabulated form. The direction of research to optimise the combination of sources, with particular respect to the evaporator of the heat pump, was investigated. This included options for single and multiple-source domestic heat pumps, based upon concepts already out in the field (in most instances), and explored the possibility that innovative heat exchangers might be employed to use more than one heat source in heat pump evaporators. In some instances the concepts might be extended to include the compressor, condenser and other components.
Project Results:
The main S&T results/foregrounds can be described as follows:
1. Enhanced heat exchange using LVGs
The first design used in the RenewX project was a tube-fin heat exchanger. Tube-fin heat exchangers are the most commonly used type of heat exchanger, and are often used in air conditioning systems. They take the form of sheets termed fins (which are typically aluminium) threaded onto tubes (which are typically copper). Water or refrigerant passes through the tubes, and heat is transferred between the air and the tube contents. The objectives of the RenewX project included increasing the efficiency (and reducing the footprint) of the evaporator and develop and demonstrate novel heat exchanger technologies. As a result, it was determined that the best route to achieving this in the RenewX design would be to investigate the used of turbulator designs (eg Figure 2) for improving heat transfer and the associated methods of assembly.
During the project a number of different turbulator designs were considered and modelled to assess effectiveness under different operating conditions, Figure 3. The heat transfer coefficient was the primary reference value as improving it will improve efficiency of the heat exchanger design. The effect of air side pressure drop along the channels and Reynolds number on the heat transfer co-efficient were predicted.
It was determined that for a fixed pressure drop, the heat transfer coefficient was increased by up to 50% with turbulator enhanced channel compared to the plain one. This was valid for the whole range of pressure drop represented. Furthermore, the addition of a turbulator to an air channel can enhance the heat transfer by 75% (when the air Reynolds number (Re) equals 3000) compared to a plain air channel, Figure 4. In other words, a fixed heat transfer coefficient can be achieved with lower air inlet velocity (meanings lower Re). This has an impact on the dimensions of the fan; it means that the addition of an LVG can reduce the fan duty significantly.
Once a turbulator enhanced design had been down selected, it was necessary to investigate methods for manufacturing prototype heat exchanger units. An all-aluminium construction was selected for the heat exchanger to reduce costs which in turn focussed the selection of manufacturing methods. Options for dip brazing of the prototype units were considered as this technique allows for the formation of multiple brazed joints simultaneously. However, during the dip brazing process there is a requirement for venting of entrapped gases which requires re-work after brazing (welding of vent holes) introducing another manufacturing step. In addition, large scale manufacturing by this method would be relatively complicated.
In parallel to dip brazing, the possibility of using a mechanical assembly method was also explored as it was felt that the performance enhancement provided by the turbulator design could offset reduced heat transfer of a mechanical joint compared to a brazed joint. Following the manufacture of a number of test pieces, including the turbulator design within the fins, it was decided to proceed with mechanical assembly of the fins and tube and adhesive bonding of the ‘u’-bend tubes. An example of a prototype assembly with a turbulator design can be seen in Figure 5.
Specific test rigs were designed and manufactured for testing of both baseline and turbulator enhanced heat exchangers. A number of technical challenges were highlighted from the prototype testing and included issues with refrigerant distribution, header design and fin pitch versus turbulator size. This information was fed back to the design and a second set of heat exchangers were manufactured for testing and demonstration activities. It was found in the case of the evaporator, that there was an increase in the heat transfer coefficient of 18.2%. The lower than expected performance of the turbulator design on the heat exchangers compared to the modelling predictions has been rationalised. Three potential sources of the discrepancy in performance were identified and will be explored in further work.
In addition to a turbulator enhanced heat exchanger, a micro-channel compact heat exchanger was designed within the RenewX project. Plate heat exchangers distribute the heat through a series of stacked plates. In a simple version, tubes can be passed through the corners of the plates. Channels can also be created in the plates themselves. Micro-channel heat exchangers use channels with dimensions less than 1mm to direct fluids and exchange heat. A RenewX design using 0.3mm thick plates containing multiple channels measuring 0.1mm deep, 0.3mm wide and with a 1mm pitch was developed. Prototype units were manufactured from 316L stainless steel and refrigerant R134a was selected for the trials.
The micro-channel heat exchanger units were manufactured by diffusion bonding with process development undertaken in the project. A number of design iterations were undertaken to refine the design of the manifold plates and to assist with process development. A theoretical algorithm was developed which enabled the evaluation of thermal hydraulics issues in the micro-channel units. Application of the algorithm to measured data enabled good consistency to be attained between model prediction and experimental results.
2. Design of multisource heat exchanger
A successful design was developed and demonstrated for a multisource heat exchanger system based on the RenewX enhanced designs. The source options range from ambient air and forced ventilation extract air to the ground, as well as solar collector types, e.g. flat plate solar water heaters in isolation, thermal plus photovoltaic (PV) and thermal plus PV plus storage. For systems where solar collectors can be used to preheat domestic hot water, the collector output may bypass the heat pump (be it air or ground source) and be directed to the thermal store.
Based on the results of the literature survey and the overall objectives of RenewX (to improve the performance of air-source heat pumps), the following heat source combinations were chosen for further investigation and multi-source heat exchanger design:
• Fresh air and solar thermal
• Fresh air and forced ventilation extract air
The fresh air and solar thermal, and fresh air and forced ventilation systems were based on the finned-tube-type heat exchangers developed within the project, including the turbulator enhancement technique. The combination of all three sources (fresh air, forced ventilation extract air and solar) is also possible and was investigated.
For a fresh air and forced ventilation extract air multi-source heat exchanger design is was found that COP gains are possible for all seasonal conditions and ventilation air mass flow rates. The effect was larger in winter due to the higher difference between the two air temperatures, and shows a COP increase of 13% for the typical ventilation air mass flow rate of 0.249 kg/s. The effect is reduced to 9.5% in spring/autumn, and 5% in summer. Overall, the system shows potential, particularly in regards to COP gains. It also has the advantage of not requiring any further heat exchangers or equipment (assuming a forced ventilation system is in place).
In the case of the solar-air system, two heat exchangers were designed: one as an air preheater (HEX-1) and one as an evaporator (HEX-2). Both heat exchangers are of similar design, featuring aluminium tubes and fins with turbulator enhancement. In order to investigate the potential advantages of the solar-air dual source system, a steady-state thermodynamic model was used to generate results in three key areas:
• Heat pump coefficient of performance (COP). This is a measure of the overall heat pump performance and is directly correlated to the economic and environmental performance of the system.
• Frosting. This is a key issue in the performance of air-source evaporators. Frosting is greatly detrimental to heat pump performance as it often takes a significant amount of energy to defrost the evaporator (via reverse cycling). The effects of frosting are more prominent in compact heat exchangers as narrow channels can become rapidly blocked.
• Condensation. Condensation from the air stream (due to cooling below the dew point) provides an extra heat transfer resistance and can also lead to channel blockage in compact units.
It was shown that significant COP gains can be made by utilising a fresh air and solar thermal dual source system. The most significant gains are observed at lower air temperatures when the temperature difference between the air and the solar water is maximised which creates a larger log mean temperature difference across HEX-1, Figure 6. This therefore allows a greater amount of heat to be transferred to the air, meaning the air-side temperature increase is larger, which leads to higher COP gains. However, even at an air temperature of 15oC, up to a 20% increase in COP is observed which represents a significant gain. In addition, it was also found that the frost-free operation range is significantly expanded by implementation of the dual-source solar-air system. At a collector temperature of 35oC (achievable on clear days in winter), the evaporator can operate frost-free at ambient temperatures below freezing which shows significant performance improvement compared to traditional single air-source evaporators.
Following the successful investigation of the two dual-source systems, it was decided to combine the approaches into a tri-source system utilising all three of the suggested heat sources. This would be advantageous as both additional heat sources may be intermittent in nature (solar dependant on solar gain and storage efficiency, while the ventilation air mass flow rate may vary by season or time of day). The model variables were the fresh air condition, the ventilation mass flow rate and the solar water temperature:
• 3 fresh air conditions based on UK lowland conditions:
o Winter: 4.4 oC and 85% relative humidity
o Summer: 15 oC and 76% relative humidity
o Spring/Autumn: 9.9 oC and 75% relative humidity
• 5 ventilation air mass flow rates: 0 (single source system condition), 0.125 0.187 0.249 0.312 kg/s. 0.249 was considered the typical condition and is based on 1.5 air volume changes per hour in a typical sized UK house. This was chosen as a variable as it will not be constant throughout the course of a day or different seasons.
• Solar water temperature varying between 15 and 40oC.
For a tri-source system it was found that COP increases of up to 40% in winter, 33% in spring/autumn and 26% in summer were achievable and that the integration of the three sources together is worthwhile. In all cases, a COP increase of 10% should be possible as this was observed for all cases of vice-versa maximum and minimum input from the two sources (for example, when the solar temperature was at the minimum value of 15oC and the ventilation extract mass flow rate is at the maximum value of 0.312 kg/s). Hence, significant COP gains should be shown throughout all seasons and throughout a typical day providing the scheduling of the forced ventilation system is designed to counter-balance troughs in the solar storage temperature.
Condensation from the air stream was also studied for the tri-source system. In all cases it was shown to be entirely prevented. Hence, the tri-source system has an advantage over the two dual-source systems in that it completely eradicates the problem of condensation in the evaporator. In addition, the frost free operation range was expanded as the ventilation air mass flow rate was increased. Even at the minimum ventilation mass flow rate, frost-free operation can be achieved at an ambient below 0oC for a solar collector temperature of around 25oC. This is comfortably achievable in winter.
For an example in the UK, based on a thermal energy consumption of 15,000 kWh/year and a 30% increase in COP, the estimated decrease in electricity demand would be approximately 23% with a cost saving of £165 per year and reduced carbon emissions of 724 kgCO2eq/year.
A dual source solar-air source was demonstrated within the project, Figure 7. The integrated system, designed to provide sufficient heating for a domestic property built after 1995 with a floor space of 150 m2 and occupied by an average European household (2.3 people), was installed in the Re/genT laboratories, together with a unmodified air source heat pump of equal type, thereby providing a direct comparison of the systems under identical operating conditions. Measurements, conducted in Helmond in The Netherlands during December 2015 and February 2016, showed a 7 to 11% reduction in energy consumption for the RenewX design compared to the reference system for operation with Solar heat and a 15 to 17% reduction in energy consumption for operation with low temperature waste heat (20°C). The low temperature waste heat system performed better due to seasonal factors such as cloud cover. The concept of using multiple heat sources to increase the performance of the air source heat pumps was successfully demonstrated.
3. Manufacturing processes
A range of different manufacturing processes for both the heat exchanger units and components were considered within the project. Methods that were considered for component manufacturing included additive and subtractive manufacturing as well as casting, forming and moulding. Assembly/joining technologies that were considered for the manufacture of heat exchanger units included mechanical assembly using sealants and seals, fusion welding, brazing (e.g. dip, vacuum etc..) and diffusion bonding.
A mechanical forming technique (punching) was selected for the manufacture of the turbulator design based on simplicity and cost. The thin nature of the aluminium fins and the turbulator design would allow cost effective scaling of this technology suitable for mass production. With suitable integration into a manufacturing line, it should be possible to form the turbulator feature on an automated basis. In addition, the design should be compatible with adding multiple turbulators to the fins, which in certain cases, should further improve heat transfer.
A mechanical assembly approach was used for manufacturing the all-aluminium heat exchangers as this was a very low cost and scalable process. Simul8 software was used to investigate options for optimised manufacturing as a function of the different process steps and resource availability.
The micro-channel heat exchangers were fabricated by diffusion bonding in vacuum. The process parameters were developed on an iterative basis by varying the diffusion bonding temperature, time and applied load. Some re-design was required in order to better optimise the manifold plate structure. Photochemically etched shims of 316L stainless steel, Figure 8, were manufactured by ACE Ltd and were used to form the main core of the heat exchanger units.
4. Online information portal
In order to capture the extensive knowledge generated within the project for the SME-AGs and to provide a possible source of on-going support and/or revenue, it was agreed to develop an interactive online information portal. The content of the portal was discussed by the consortium and the following points were agreed:
• A separate website was developed by TWI to host the content.
• The content will be developed by all partners.
• The portal can be entered via the RenewX website, but it will also be independent.
• At the end of the project, the website will be transferred to PIMEC.
• The proposed domain name: www.heatexdesign.com.
• Content to include:
o Design tools – e.g. datasheets and calculators.
o Aspects of results from technical work packages in project.
o Technical documents such as white papers, images and videos.
• Content will be predominantly public, but with password protected access to the design tools.
• SME associations in the project will receive free access to the design tools, and non-members can pay a fee to download.
From this brief, TWI and the project partners developed an online portal www.heatexdesign.com and to ensure maximum exposure, similar named domain names (heatexdesign.eu etc) were also purchased. A specific logo was created for the second project website, Figure 9.
Three design tools were developed for the portal:
o Tube-fin heat exchanger designer (Designing with turbulators)
The concept of this tool is to allow the user to quickly estimate the heat transfer coefficient of a tube-fin heat exchanger. Calculations are provided in a web based spreadsheet. A user guide is also available for download to help the user select appropriate constant.
o RenewX benefits calculator
This Java based tool is used to calculate the benefits of the RenewX dual source solution. Outputs include:
o Coefficient of Performance (COP) by season and overall.
o Key performance indicator of heat pumps.
o Electricity cost per season and year.
o CO2 emissions per season and year.
o Average mass flow rate of condensate from air stream by season.
o Likelihood of frosting by season.
o Micro-channel heat exchanger design
In addition to the online information portal, a design manual was developed for tube-fin heat exchanger design. The design manual provides a step by step process guide for developing a complete design. The design manual provides guidance for two situations. The first considers a case where a solar collector and air source heat pump (ASHP) are used together in a building and the second is a calculation guide for the design of a micro-channel heat exchanger.
In the first case, the solar collector has a water loop, which outputs hot water to a storage system. Onto this loop is placed a tube-fin heat exchanger, transferring heat to the air. The evaporator in the ASHP also has a tube-fin heat exchanger, which takes heat from the air, pre-heated by the solar collector. This heat is then transferred to the refrigerant on the ASHP loop.
Potential Impact:
There are over 200 million households in Europe whose combined domestic heating needs account for around 30% of Europe’s energy demand. Reducing this will be essential if Europe is to achieve ambitious emission reduction targets (i.e. 2020 targets) and decrease an excessive reliance on imported energy. The clear solution to these two problems is to increase the amount of renewable energy used, an ambition at the heart of much recent EU legislation, of which new Heat Pump systems are a key enabling technology.
Europe's refrigeration, air conditioning and heat pump industry has an annual turnover of around €30 billion, employing around 200,000 people. Within this industry our SME Associations (PIMEC, PSPC and SolMa) represent over 1,000 SMEs in the heat pump manufacture, solar thermal system manufacture, design and installation sectors. Led by these associations the RenewX consortium consists of European SME associations, SMEs and some of Europe’s leading research associations. Together, we propose to develop new platform technologies relevant to our industries and markets across Europe.
The European Heat Pump market has doubled over the last 5 years. However, this growth has been rather unstable. The most important indicator is the energy price ratio, which compares major energy sources such as gas or oil. In most markets competition with gas is very close due to the relative high Seasonal Performance Factor (SPF) compensating for the lower price of gas on a per kWh basis (usually around 1/3 of the price of electricity per kWh). The volatility of electricity and gas prices makes it difficult to predict future price ratios on a per kWh basis. In addition, initial investment cost is a major factor. Currently, ASHP’s (Air Source Heat Pumps) are approximately 1.5 times more expensive than a comparable highly efficient gas condensing boiler. Typical payback periods are usually 6-8 years, therefore, technologies that offer improved performance and/or reduced cost are critical to increasing the market uptake of ASHPs. This also reduces the reliance on, and effect of, government subsidies, that can significantly alter market dynamics.
In addition to unpredictable economic conditions, a sharp increase in low-cost imports from Asia has also had a significant effect on market dynamics. It is estimated that over 30 million Chinese households have installed solar collector water heating systems. These systems are > 80% cheaper than a comparable system in Europe. The same applies to the heat pump market, where Chinese imports are now sold for ~€1,500 in the UK, compared to similar European systems at €3,500. These systems are often poor quality imitations of European technology which has an adverse effect on the reputation of our ASHPs. The significant uptake of solar thermal collectors and corresponding decrease in cost presents a significant opportunity for efficiency gains via multi-source systems.
In order to support the achievement of the Renewable Energy Directive (2009/28/EC), ratified and agreed by Member States in 2009, European governments have already committed to significantly reduce the carbon footprint of new and existing homes, and some have introduced subsidies for renewable energy systems. Mandatory national targets currently being developed into Renewable Energy Action Plans by individual Member States require that by 2015 where appropriate new and renovated buildings must contain a degree of renewable energy technology. An example of this is Germany’s ‘Erneubare-Energien-Waermegesetz’ which mandates that all new homes from January 2009 will have to produce at least 10% of their required heating and hot water demand by renewable energy systems. It is estimated that by 2020 this could save €50 billion in heating costs in Germany alone. Other Member States have similar legislation.
The major challenges to further expansion of the European heat pump industry are competition from gas-based alternatives as well as competition from lower cost imports. The only way to mitigate these challenges and turn them into opportunities is by a process of continuous innovation to achieve major efficiency improvements and cost reductions. Although significant incremental improvements have been made (e.g. modulating compressors, alternative refrigerants) the basic design and operation of the major components have not changed for decades. In Ground Source Heat Pumps (GSHP) a lot of work has been done to improve the efficiency of compact plate heat exchangers. More recently a range of manufacturers have started to offer hybrid heat pumps which can be combined with solar thermal systems. These range from completely separate systems to systems with direct coupling of solar thermal energy to the evaporator. Systems such as the latter are not yet available across the whole market. Such combinations offer significant potential for an increase in the COP (Coefficient of Performance) or SPF of the heat pump system. However, the main challenge is to effectively utilize more than one energy source which adds cost and complexity, especially with regards to the heat exchanger design.
The existing design of simple finned tube heat exchangers (evaporator) is relatively basic and can easily be produced in bulk. However, they are not as efficient as they could be due to the poor thermal and mechanical contact between their main components and require large fans to achieve the air flow required to produce energy. Their design is optimized for heat transfer between the ambient air and the refrigerant circulated through the system. In contrast to the plate heat exchangers used for GSHPs, only limited efforts have been made to improve the basic finned tube design.
The objective of RenewX was to develop and demonstrate novel heat exchanger technologies and manufacturing processes that will enable increased market penetration of European-made air source type heat pump systems. This will be achieved by i) increased efficiency and smaller footprint of the evaporator (compact heat exchanger design); ii) ability to utilise secondary or even tertiary energy sources to complement the air side (i.e. renewables such as solar thermal). The latter will enable significant increases in Seasonal Performance Factors (SPF) which will improve the return on investment and hence attractiveness of air based heat pump systems.
A number of results have been generated at the successful completion of the RenewX project for exploitation by the relevant consortium partners. These are:
• Development of a method for improving the heat transfer coefficient by up to 18% with significant scope for further improvement through identified design modifications.
• Achieved a 7 to 11% reduction in energy consumption for the RenewX design compared to the reference system for operation with Solar heat.
• Achieved a 15 to 17% reduction in energy consumption for operation with low temperature waste heat (20 °C).
• Creation of an online information portal for assisting with heat exchanger design has been created for use by the SME-AG partners and their respective members.
The potential impact of the project could be to increase air based heat pump sales by European manufacturers by at least 50,000 across Europe between 2017-2027 adding €150 million of value to Europe’s economy. In addition, 50% of the current total waste heat generated in the EU is classified as low to moderate grade heat (e.g. waste water or solar). The opportunity to harvest some of this low grade energy using RenewX based technologies provides the potential to save a proportion of the 180GW of power (low to moderate grade heat) that is wasted per year in the EU. Correspondingly, this offers the potential to also contribute to reducing a proportion of the 94M tonnes of CO2 (equivalent to ~135Mtoe fossil fuels) generated per annum related to wasted low and moderate grade heat. In order to achieve the potential impacts, it will require the SME-AGs to actively promote the project results to their members and work together on exploitation and licencing.
The expected final result of the RenewX project was the development of new compact heat exchanger technologies that will improve the competitive position of the SME members of the consortium associations as well as to provide direct benefit to the associations themselves. This has been achieved by
• Improving efficiency of air based heat pump systems by improved design and combining them with alternative energy sources, thereby increasing return on investment whilst decreasing payback periods
• Reducing the footprint of air based heat pump systems, enabling potential integration in buildings
The main business activities of two of the three SME association partners are within industrial sectors relevant to the proposed technology development, i.e. solar thermal collectors and heat pumps. The third association, PIMEC, has a remit of assisting Spanish SMEs in improving and developing new business opportunities. This provides all of the SME associations with an opportunity to assist their member organisations in improving their business position. Combined, the SME associations have access to over 1,000 SMEs in the renewable energy community.
The networks and industrial contacts of the four RTD partners have been used to actively disseminate the projects results. These networks reach a broad industrial audience as, by their nature, the contacts of the participating associations are relatively narrow. For example, TWI has disseminated project results to its current Industrial membership whilst UNEW is the co-ordinator of HEXAG (the Heat Exchanger Action Group). There is currently significant interest in compact heat exchangers across a number of industrial sectors outside of renewable energy, e.g. oil and gas and aerospace, due to the potential space and weight savings associated with smaller unit sizes.
The project partners have identified opportunities for improving the design and performance of both the turbulator design and the micro-channel heat exchanger and are discussing a proposal for Horizon 2020 funding. This should provide additional improvements in performance of the ASHP heat exchanger improving both efficiency and reducing the footprint.
List of Websites:
www.renewx.eu
Online information portal
www.heatexdesign.com
Contact:
Dr Nick Ludford
TWI Ltd
Granta Park,
Great Abington
Cambridge,
CB21 6AL
