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Improving the Performance of Concentrating PV by Exploiting the Excess Heat through a Low Temperature Supercritical Organic Rankine Cycle

Final Report Summary - CPV/RANKINE (Improving the Performance of Concentrating PV by Exploiting the Excess Heat through a Low Temperature Supercritical Organic Rankine Cycle)

Executive Summary:
In concentrating photovoltaic systems (CPV) the incident solar radiation is multiplied by a factor equal to the concentration ratio. The electricity production per cell area is increasing almost linearly with the concentration ratio. Concentration on PV cells surface results in an almost proportional temperature increase, deteriorating the performance of the cells. To cope with this effect, in some configurations the cells are cooled and the rejected heat is then used for other purposes, like for domestic hot water production, resulting in a CPV/Thermal (CPV/T) configuration.
On the other hand, Organic Rankine Cycle (ORC) is one the most efficient technologies to convert heat to power, especially in cases the available thermal source is of low temperature. The innovative concept of the CPV/Rankine project deals with the conversion of that CPV/T heat to additional electricity through the Supercritical ORC (SCORC) process. SCORC is selected because it enjoys higher efficiency at low temperature in comparison to a similar Subcritical ORC.
During the project effort was given on adapting the solar collector for operation at higher temperature than usual. Such task was succeeded by Lumicum, which enabled the production of heat at temperature up to 95 oC. Various tests have been implemented, both indoors and outdoors, in order to resolve some issues with micro-cracks and deteriorated electrical/thermal efficiency. Finally, the adapted collector was available and was further tested. In total 100 m2 of these collectors have been manufactured and shipped to Greece for the combined installation.
In the meantime, UGENT and DECO reached an effective design of the engine heat exchanger, with the appropriate input from the collectors and SCORC engine side. These boundary conditions have been reached after detailed optimization studies. The helical coil design has been examined and finally manufactured by DECO and shipped to Greece, to be included in the SCORC engine.
In Athens, HEN and AUA continued their work with the SCORC engine. This engine has been designed and manufactured. Great effort was given on the conversion of the scroll expander, which required much more work than originally planned. The engine has been then installed at the laboratory and tested under various conditions, which include various HTF temperatures, flow rates, and heat input, covering a very large range of possible operating conditions. The final outcomes of these tests are actually the performance maps of both the engine and its main components, with the aim to optimize the operation. For this task, a dedicated control unit has been developed that adjusts the pump and expander speeds within a very large range. During the tests, the thermal efficiency reached even 6%, while supercritical operation was difficult to be maintained. Moreover, the supercritical heat exchanger performed extremely well at all conditions and for both subcritical and supercritical operation, with a reduced pinch point and pressure drops.
After all laboratory tests have been implemented, the collectors have been installed at the AUA and the electrical and thermal circuits have been designed and constructed. The SCORC engine has been then installed inside the control building and connected with the thermal part of the collectors (max. heat production of 41 kW). Also, the condenser during the laboratory tests has been replaced and an evaporative condenser has been used at the final site. The collectors are equipped with single-axis tracking, in order to follow the movement of the sun, and because of the irradiation concentration by 10 times, it is very important to fine adjust the HTF temperature by regulating the pumps. The net capacity of this combined system is 13 kW and has been tested at the field.
The tests showed that such combined system can perform really well, increasing the annual electric productivity of CPV/T collectors. Especially in cases where heat is not required (or not required for some period), such configuration can produce more electricity in comparison to a standard CPV/T system. Nevertheless, CHP operation is also feasible, which is one of the aims of the SMEs and will be examined after the project end, when the combined system will continue to operate and more tests to be implemented.

Project Context and Objectives:
The main concept of this project is to use concentrating PV/thermal collectors to produce electricity from the PV cells, while the recovered heat is provided in a heat-to-power engine for additional electricity generation. Such configuration ensures that all heat is used, which is an important aspect incase there is no thermal consumer (in that case heat should be dissipated, in order to protect the collectors from overheating).
The heat engine selected is an organic Rankine cycle (ORC), which is the ideal engine for such low temperature heat recovery. These engines can have a thermal efficiency of up to 5% for temperature below 100 oC. A unique feature that has been followed is to use a supercritical design, which according to the theory ensures a higher productivity, due to better thermal matching and higher thermal efficiency. The efficiency increase could justify the additional cost for operating at higher pressures than usual, although the technology exists and is available in the refrigeration and air-conditioning industry.
The research starts by developing numerical tools that can adequately simulate each component. Such tools have to some extent optimization capabilities. Especially for the supercritical heat exchanger and SCORC engine, a very detailed analysis has been implemented, using the built-in optimization tools of the commercial software used. Appropriate subroutines have been made, which allowed the detailed analysis of such components. On the other hand, the solar collectors are based on an existing commercial product and such similar study has been also conducted, but focusing on some specific aspects. These tools have been made according to a research methodology concluded during the beginning of the project.
Each component and process has been examined and effort has been given on simulating them not only at design conditions, but mainly at off-design, since the whole system will be powered by solar energy. Once all components have been thoroughly examined, these tools have been merged and combined under a single software environment (EES – Engineering Equation Solver). Various cases have been examined, simulating standard cases, in order to evaluate this complex tool, as well as the system operation during some representative days and during a whole year. This analysis has been implemented with the weather data of Athens (Greece), where the final system will be installed.
After all simulation studies have been finalized, the project focused on the design of the SCORC engine with all of its components and of the adapted solar collectors. The design work was based on the simulation studies that have been implemented and led to the final design of the supercritical heat exchanger, the SCORC engine and the solar collector. The control of each component and of the combined system has been also developed at this stage, which is directly relevant to the design and operation.
The design is one of the main objectives of the project. This was the outcome of the research and such knowledge has been transferred to the SMEs. This design process can help them produce high-performance products that meet the end-user expectations.
Then, the supercritical heat exchanger has been constructed by DECO, according to the final design of UGENT and then shipped to Greece. It has been included in the SCORC engine frame, where all the other components have been added. Especially the expansion machine (an original scroll compressor) has been converted, and sustained many modifications.
The SCORC engine when finalized has been moved to the National Technical University of Athens (NTUA) laboratory, where it has been connected with the laboratory infrastructure. The electric heater to simulate the solar collectors has been installed and its piping connected with the engine. Moreover, all electrical connections have been implemented, as well as mounting all instrumentation at key locations. One of the most important objectives was actually to construct such small-scale engine (net capacity of around 3 kW), while another one concerns its detailed testing at the lab and operation at supercritical conditions. All these relevant targets have been reached, proving that such small engines can be built and operate at such non-conventional conditions.
Once the tests have been finalized, the SCORC engine has been disconnected from the piping and moved to its final installation site at AUA. In the meantime, the solar collectors have been already manufactured and shipped to Greece. They have been installed at the AUA field, while a small control building has been constructed. In this building the SCORC engine has been moved and connected with the piping of the collectors.
The combined system has been finalized, control unit calibrated, charged with organic fluid and heat transfer fluid and then it has been tested during various days, in order to identify its performance and durability. Various configurations have been made, including an electric heater and an air-fan, in order to be able to operate the collectors alone or the SCORC engine alone. This has been followed, in order to allow the SMEs to test each technology separately.
The final research work deals with the drafting of the initial plan of the production process and how these can be adjusted, in order to include the manufacturing and future commercialization of the technologies developed. Moreover, an economic analysis has been conducted, in order to calculate the specific energy cost (euros/kWh) and evaluate the system from the economic point of view.
Finally, dissemination and exploitation work has been accomplished. An international workshop has been implemented towards the project end and a video developed, which has been also uploaded in the project website. Various presentations in workshops and conferences have been accomplished by the RTD partners, while an exploitation and dissemination plan has been recorded, which can be regarded as an action plan for benefiting from the technologies produced, which can have various applications.

Project Results:

The RTD work has been initiated in WP2, which included the development of the numerical tools for the investigation of each process and the system as a whole. The software that has been selected and applied is appropriate for simulating each process, while their effective coupling can be easily implemented in the EES environment. The numerical methodology has been presented for each process (CPV/T, supercritical heat exchanger and SCORC engine), with special focus on the operational temperature, which is the key parameter. This can be adjusted during operation, by effectively controlling the different pumps.
These tools have been then applied for the simulation and optimization of the SCORC engine, the CPV/T collector and the supercritical heat exchanger, as well as of the combined system, using common boundary conditions (e.g. flow rates, fluid temperature, etc.). Each process has been extensively studied, in order to evaluate its performance even at variable operating conditions. At this stage, the supercritical heat exchanger type has been also selected (helical-coil heat exchanger). Then, the integration of these three processes has been made under the EES environment, in order to simulate the combined system, since these processes are highly coupled.
The heat transfer fluid temperature at the adapted collectors has been decided to be 95 oC, with the use of alternative materials and PV cells in the receiver. The expansion machine of the SCORC engine is a hermetic scroll compressor in reverse operation, requiring conversion for efficient operation and the selected organic fluid is R-404a.
The option of using a thermal storage unit has been also investigated, leading to the conclusion that its use will not be favoured. Then, the integrated system simulation has been implemented, where the system optimization has been also considered as the weather conditions change. The main focus is to maximize the electric power produced from the system at all operating conditions. Moreover, the realistic operation of the system has been investigated during a summer/winter day and a whole year, using the weather data of Athens, Greece. The performance of the integrated system has been simulated under these conditions, where it has been revealed that the SCORC engine can significantly contribute to the total power produced, by effectively recovering the low-temperature heat of the solar collectors.
The results showed that the use of a bottoming cycle in such configuration is an effective way of increasing the productivity of such units, even if the solar collectors operate at higher temperature, which decreases their thermal efficiency. Overall, a significant performance increase has been observed, which fully justifies the CPV/Rankine project concept.

Apart from that, in WP3 there has been intensive research work for the adaptation/development of the CPV/T collector. Proper solutions have been found, by using alternative materials and receiver designs, in order to operate at elevated temperatures (up to 95-100 oC), making the SCORC engine more efficient, by supplying it with heat of higher temperature through the supercritical heat exchanger. The design of the adapted CPV/T collector has been successfully accomplished, according to the operating conditions and project boundaries that have been defined in WP2. Many of the analyses and considerations anticipated in WP3 have been utilized to obtain product modifications or data to be delivered to the next work packages in the project. One of the design aspects that was kept the same after analyses is the optical shape, which did not need to be modified to adapt the X10 to the combined system. However, a supporting wire was installed within the existing shape to improve the performance. The main objective has been reached; the collector has new receiver design capable of reaching 100 °C and is ready for implementation into the project integrated installation. Several production problems have been solved (new manufacturing methods have been followed), including delamination of reflective film and micro-cracks in the solar cells. Moreover, the tracking system has been re-designed to reduce costs by approximately 25%, aiding in the cost-effectiveness of the integrated system.
The collector has been tested and evaluated both indoors and outdoors, using new and re-designed testing equipment exclusively for the needs of this project. The testing equipment includes a medium temperature thermal rig capable of testing solar collectors up to 200 °C. There were indications on lower electric production than expected during the outdoor test. This will be closer analyzed in the demonstration field, since the quality and efficiency of the solar cells are a key element in the total efficiency of the system.
The detailed experimental results of the adapted CPV/T unit have been implemented, revealing its performance capability. The adaptation of the X10 CPVT system for the SCORC engine has been successfully concluded, according to the operating conditions and project boundaries that were defined in WP2. Many of the analyses and considerations have been utilized to obtain product modifications or data for further elaboration. Overall, a new receiver has been developed and it has withstood the main tests of IEC61208 conducted in the laboratory, while the quality and efficiency of the solar cells are a key element in the total efficiency of the system.

WP4 is running in parallel with WP3 and the relevant work conducted deals with the final design of the supercritical heat exchanger and the SCORC engine. This heat exchanger has been investigated, concluding to its type (helical coil heat exchanger) and design, according to the operating conditions. It has been then manufactured and delivered to Greece, in order to be included in the SCORC engine. The SCORC engine design has been also finalized. A contractor has been selected for the engine construction, and the whole work has been conducted jointly with HEN and AUA.
The SCORC engine has been constructed based on the design parameters obtained during WP2. However, many issues have arisen during the construction phase, which however have been overcome, in order to have available a supercritical ORC engine with a power capacity of 3 kW. Significant research work was devoted to the conversion of the expansion machine, since a new casing was necessary, in order for the flanges to sustain the high pressures. Such task required high-precision manufacturing work. After the conversion of the hermetic scroll compressor to operate as expander, all components and instrumentation have been mounted on the common structure, and connected with the appropriate piping circuit. The SCORC engine has been then installed at the NTUA laboratory for detailed testing. The heat input has been supplied from an electric heater (max. 48 kWth) with temperature up to 100 oC. Many different tests have been conducted, in order to monitor and evaluate the engine and its components. The lower HTF temperature examined was 65 oC, in order to identify the lowest possible operation with power output, while performance maps have been concluded, which concern the operating conditions, where the thermal efficiency is maximized. Inverters have been used to drive the SCORC engine pump and expander. The latter one has been connected with an electric brake and resistors, in order to dissipate the power produced and be able to conduct detailed measurements.
The maximum thermal efficiency reached was around 6%, which is within the initial targets, while supercritical operation was noticed. Moreover, supercritical operation was difficult to be achieved and only when the organic fluid flow was chocked or the cooling water flow rate was decreased, could the engine operate at supercritical conditions. At such conditions, the expander speed was low (around 15 Hz), leading to a low expansion efficiency, not because of the off-design pressure ratio, but due to the low electric efficiency of the asynchronous generator. If the expansion efficiency is increased (mainly having to do with the frequency operation and the increase of the electrical efficiency), the thermal efficiency at supercritical conditions seems superior to the one at subcritical ones.

Then, WP5 included the installation of the solar field and its connection with the SCORC engine. Focus has been given on the electrical and thermal connections, as well as on all the secondary components, such as the fan, heat transfer fluid, pump, foundations, etc. The control of the integrated system has been also developed. Focus is given on the tracking mechanism of the solar collectors and the thermal coupling of both main components (collectors with SCORC engine). This coupling is actually adjusted using variable speed pumps, in order to maximize the total electricity generation. The main control aspects have been already tested during WP3 and WP4, ensuring that their coupling is efficient and that few issues require attention and tuning. The control can be adjustable, in order to maximize the power output or even regulate the HTF temperature for other applications, such as CHP or for thermal storage.
The combined system has been then installed at the AUA field. The installation of the combined system included the installation of the collectors, the control building, the SCORC engine and its connection with the piping circuit. The additional air-fan has been used to protect the collectors in case the SCORC engine is not operating (CHP operation can be examined as well with such configuration). These tasks included many minor activities that had to be timely scheduled, due to the project duration restriction. The final outcome is a hybrid system, capable of producing more electricity than a conventional system using only PV cells for electricity generation, and most importantly using a limited land surface.
This combined concept has been tested, focusing on the transient operation of the SCORC engine and its control unit. Its performance has been identified, while focus has been given on the system durability and the functionality of the components of the SCORC engine at transient operating conditions. This aspect is of high priority for the SMEs, in order to be able to adjust to the market needs reliable components and products.

Work package 6 includes demonstration activities, which are focused on the SCORC engine. Monitoring of the engine has been accomplished mainly during the laboratory tests, when it has been operated for many hours per day and for some weeks. Such work helped the consortium to identify any aspects that could be fine tuned and to evaluate the engine’s durability. One aspect of the engine examined is its control unit and its ability to track the optimized operating condition, as the heat input changes. Some modifications have been implemented, in order to accomplish this. These modifications deal with the more precise HTF temperature control, keeping the target temperature within a smaller range. Overall, the operation of the control unit ensured the maximization of the net electricity generation by the SCORC engine.
The similar control unit has been used in the combined system as well. Similar tests have been implemented and the same conclusions have been reached. This updated configuration ensures a higher productivity of the combined system throughout the year (especially in areas with high direct insolation), making it even more attractive for various end-users, such as for the residential or commercial sector.

In order to examine the functionality of the control unit, mainly the SCORC engine has been long-term monitored. This has been implemented at the laboratory, where the conditions were favoured for such aspect. This has been followed, in order to be sure that the engine’s integration and coupling with the solar field would lead to a reliable energy system. From this monitoring no issue has been observed with the main components, such as the pump, condenser, power electronics, etc. Even when operating the engine at its load limit or even slightly higher, no unsteady operation was observed. Moreover, all measurement instruments have been correctly calibrated and operated very well.
The combined system testing will continue even after the project end, examining its performance at various conditions and unsteady operation. Focus will be given on the SCORC engine, investigating alternative applications of such small-scale heat-to-power engines with capacity of few kWs.

Next in WP7, an economic analysis of the combined system has been implemented, in order to identify the energy production cost. This depends on the installation location and the available direct solar irradiation. Different cases and scenarios have been considered, while the costs for the prototype system as well as a cost estimation of its commercial version have been recorded and analyzed. Such economic analysis can help the SMEs to identify some possible cost reductions, as well as to examine the economics of each component separately (such as the collectors and the SCORC engine).
Then, the production planning of each component and of the combined system has been examined by the SMEs, which have identified how to incorporate these new products and exploit the project results. In case they make it to serial (mass) production, the hardware costs are decreased, making such technologies very effective and with a low pay-back-period.
Moreover, each SME can find and promote their products to other markets and applications as well, in order to increase their chances of success. For example HEN can combine the SCORC engine with waste heat recovery applications, SICA to apply the adapted collector for production of electricity and cooling, and DECO to include its heat exchanger for applications with CO2, requiring very high pressures.

Potential Impact:
The impact of the project has mostly to do with the participating SMEs. The impact can be divided into four main activities, which correspond to the main results of the project.
Specifically, the CPV/T collector has been adapted to operate at higher temperature (up to 100 oC), while effort is given to reach even higher temperatures. This is implemented, in order to provide thermal energy of adequate temperature to the secondary circuit (SCORC engine) to produce electric energy with acceptable efficiency. The adapted solar collector can have a variety of applications, since it can produce heat of low and/or medium temperature, although for the latter case future upgrades in the receiver design, materials and PV cells are required. It can be also used in combined heat and power (CHP) systems or even for cooling production and seawater desalination, when properly combined with the appropriate existing commercial systems. Therefore, the impact for SICA is high, since it is already dealing with solar installations. Such collector can enlarge its product range and combine it with various applications (combined as well as with heat-to-power engines).
The developed supercritical heat exchanger was the main task of DECO. The type of this heat exchanger has been decided to be a helical coil type. This key component has been designed with capacity of 43 kWth, but similar studies can be implemented in order to up/down-scale it, in order to be used in similar heat-to-power engines or other applications. The main advantages of this heat exchanger are the very low pinch-point temperature difference (5 K at design conditions) and its operation at elevated pressure (around 40 bar or even more), enabling it to be used in alternative applications as well, including ORC technology, and CO2 applications (requiring high-pressure).
The main core of the project and with the higher impact is the development of a small-scale supercritical ORC (SCORC) engine. Such engine was not available at commercial level, especially at such low-temperature regime. During the project an ORC engine ideal for supercritical operation has been developed, with the aim for flexible operation and keeping the costs low. Such tasks are important especially for HEN. The designed SCORC engine has a capacity of around 3 kW and significant research work has been devoted to the heat transfer process from the solar collectors to the organic fluid selected and to the expansion machine. Such heat-to-power engine can be combined with other heat sources as well, such as biomass combustion, waste heat recovery, geothermal energy, requiring at each case some design modifications and different control logic. Its main and most important advantage is its higher efficiency in comparison to a conventional subcritical ORC engine, which ensures the productivity increase of the combined process.
The combination of the above three technologies into one integrated system concerns all three SMEs. The SCORC engine has been coupled with the adapted collectors (10 collectors in total). The CPV/T field produces electricity from the PV cells and the extracted heat is transferred through the supercritical heat exchanger to the SCORC engine for additional electric energy generation. Such system has been installed and tested at the AUA premises, showing adequate performance, even at unsteady conditions. It can be an alternative small-scale system for power production, supplied by solar energy, ideal for the residential and commercial sector, and with a variety of other applications, such as co-generation or even poly-generation for the production of heat, power, desalinated water, and cooling.
Finally, it has been decided to share equally among the SMEs the IPR relevant to the combined system, while the IPR relevant to the adapted collector belongs to SICA, the IPR relevant to the supercritical heat exchanger belongs to DECO, and the IPR relevant to the SCORC engine (without the heat exchanger) belongs to HEN.

Dissemination activities:
A large number of dissemination activities have been implemented during the project. Various poster presentations have been implemented, as well as oral presentations in conferences. Moreover, the project concept has been presented in workshops, while the project coordinator has participated in two exhibitions relevant to solar PV technologies and promoted the project mainly among the solar industry.
An international workshop has been also organized at AUA close to the project end, where the major activities have been presented. This event attracted interesting presentations from people relevant to solar technologies, and ORC engines.
In order to reach a wide public, a project video has been also developed and uploaded in different media. This video describes the main concept of the project.
Finally, the project website informs its visitors for the project progress and the activities that have been implemented and is the main dissemination tool.

Exploitation of results:
The developed technologies examined by the RTD performers are of high interest to the SMEs. They have clearly identified the benefits of the integrated system and its increased performance in comparison to a conventional small-scale CPV/T unit. Therefore, they can be in the position of promoting it to the market and share the IPR accordingly. But further development and demonstration activities should be conducted and investigated over a long term, in order to have rigid conclusions about its cost-effectiveness and performance at real conditions. Some aspects that are of interest to the SMEs are highlighted next:
- Supercritical heat exchanger: the construction and testing of such component can provide insight on the proper engineering of such heat exchangers to be used in supercritical ORC engines. Up till now there is a lot of uncertainty in the heat transfer correlations used for the design of supercritical heat exchangers. Data collected from the prototype will help in reducing the uncertainty on heat transfer correlations. This will result in smaller, more compact and cheaper heat exchangers, which strengthens the market position of the SME involved. The technology and know how can also be integrated in other high-pressure applications, such as in CO2 applications (requiring pressures over 30-40 bar), and boilers. The SME participant DECO is focusing on these aspects and wants to enter other promising markets as well.

- SCORC engine: the SME participant HEN is dealing up to now with subcritical ORC engines. The operation of such unit at supercritical conditions makes this small engineering company to be the front-runner in such applications. The SCORC engine has been assembled, constructed and tested. From the detailed testing and evaluation that have been conducted, such configuration shows some very promising aspects, while its operation at CHP mode should be promoted, being ideal for this kind of application.

- Adapted CPV/T collector operating at higher temperature: the design and testing of such collector was the main target of Lumicum. The calculated results have shown that the collector can operate at such elevated temperature with adequate electric/thermal efficiency, without substantially diminishing its performance. The SME participant SICA is responsible for the adaptation of such collector, which could be used in a variety of applications, even without being coupled to a SCORC engine. Some interesting ones with high potential are: 1) Preheating of feed-water for power plants: A lot of industries use oil/gas/electricity to heat the incoming feed water to the steam generation units, which can be accomplished by the developed collectors (in that case the installation sizes needed to replace this preheating is often in the range of 10.000-100.000 m2), 2) Heating of water for desalination through membranes: Here the electricity is used to drive the pumps that increase the water pressure, while the heat is used to increase the flow through the membranes. The temperature conditions are usually very suitable for CPV/T collectors, since the water is heated from around 10-20 up to 30-50 oC.

Apart from the aforementioned aspects, there are some further issues that will be elaborated by the participating SMEs in the future, in order to benefit from the research conducted to the greatest extent. Such aspects are briefly mentioned next.
- The use of a small thermal storage/buffer is not considered in the developed system. Nevertheless, such option can be investigated by the SMEs and included in their future exploitation plans, since it can be considered in future studies and installations after the project end, according to the special needs of each application.
- The use of an alternative organic fluid, instead of R-404a, with lower GWP. The replacement fluid (R-407f) already exists, but it still has high cost. Except from that, other fluids can be considered as well in future applications, which however should be carefully evaluated, in order for the SCORC engine to show similar high performance.
- The operation of the integrated system at CHP mode. According to the application and the energy needs (for heat or electricity), the system has high flexibility on the energy flow management. Such detailed study can be investigated by the participating SMEs in the future, since a fraction of the produced heat can be used for heating purposes as well (perhaps during the winter only), decreasing accordingly the electricity generated by the SCORC engine.

Finally, standards for each technology should be examined and followed, in order to be in full compatibility with international regulations and safety aspects, such as the “CE” marking.

List of Websites:

The website of the CPV/Rankine project is: This website will continue to be live even after the project end. It will be also updated frequently and includes all latest publications, events and news.

The contact person for the project is Dr. Dimitris Manolakos. His full contact details are given next:
Dr. Dimitris Manolakos, Lecturer
Agricultural University of Athens (AUA),
Iera Odos 75, 118 55 Athens, Greece
Tel. +30 210 5294036, +30 6948595967
Fax. +30 210 5294032
Renewable Energy Systems – RES – Group: