COMBINED PHOTOVOLTAIC AND SOLAR-THERMAL SYSTEMS: OVERCOMING BARRIERS TO MARKET ACCEPTANCE

Transcription

1 IGEC-1 Proceedings of the International Green Energy Conference June 2005, Waterloo, Ontario, Canada Paper No. xxx COMBINED PHOTOVOLTAIC AND SOLAR-THERMAL SYSTEMS: OVERCOMING BARRIERS TO MARKET ACCEPTANCE Michael R. Collins Department of Mechanical Engineering, University of Waterloo 200 University Ave. W., Waterloo, Ontario, Canada, N2L 3G1 ABSTRACT In 1997, the International Energy Association's (IEA) Photovoltaic Power Systems Program (PVSP) initiated IEA Task 7 to evaluate the technical status of combined Photovoltaic and Solar-Thermal systems (PV/T), and to formulate a roadmap for future development. Because the Task was initiated by the PVSP, however, individuals from the Solar Heating and Cooling Program (SHCP) were not invited to participate, and the Task Group lacked any significant expertise with solar-thermal When the Task submitted its final report in 2002, it consisted of an accounting of existing systems and a list of the perceived market barriers. Without input from the SHCP, however, no move could be made to actually address those barriers. IEA Task 7, however, did recognize that the participation of the SHCP was needed, and in 1999 made an effort to initiate some discussion between the PVSP and the SHCP. The result was IEA Task 35 PV/T Systems, which met for the first time in January of The new group intends to reevaluate the findings of Task 7, and to develop the means by which these market barriers can be overcome. The current discussion will provide an overview of existing and potential PV/T systems and their technical status. Further, it will report on the methodology established by the Task 35 work group to overcome the aforementioned market barriers. INTRODUCTION Combined Photovoltaic and Solar-Thermal Systems (PV/T Systems) join Photovoltaic (PV) technologies and solar thermal technologies into one system with both electrical and thermal energy output. Typically, the systems are solar-thermal collectors with PV panels integrated in the collector-surface. In comparison to stand-alone PV or solar-thermal systems, the overall efficiency of the PV/T system can be higher for a specific collector-area. The thermal system cools the PV, leading to increased electrical efficiency and better electrical production (approximately 1% increase in efficiency per 4 o C drop in temperature). In general, the thermal aspects of the system do not perform as well (10 to 15% of the input energy is used by the PV components). PV/T systems have a number of perceived advantages to stand-alone PV or solar-thermal The increased efficiency and dual nature of the systems make them an ideal solution for situations where installation space is limited. Homeowners who are currently forced to decide between meeting thermal or electrical needs can now do both. The financial benefit of the combined system is also significant. Joining the long payback of PV systems with the relatively short payback of solar-thermal systems, results in a payback that is more palatable to the building owner than a PV-only installation. Types of Systems Originally, the International Energy Association (IEA) Task 7 identified two classifications of PV/T systems; air and water, named after the fluid used by the thermal system (IEA PVPS, 2002). Unfortunately, those classifications do not accurately describe the numerous system types currently under development. It is far more convenient to group these systems based on a more formal descriptor of the thermal collector type. These are: Unglazed Water Collectors (Figure 1): typically consist of an uncovered solar absorber surface attached to piping. Fluid is circulated through the pipes to remove heat. In this situation, the PV would take the place of the solar absorber surface. These are generally low-cost and low efficiency thermal collectors that have found widespread usage as solar pool heating systems in cooler climates. In warmer climates, they are suitable for use in producing domestic hot water. Figure 1: An unglazed PV/T water heating system (Leenders, 2000) Glazed Water Collectors (Figure 2): are similar to unglazed water collectors except that the solar absorber surface is placed beneath one or more pieces of glass. In doing so, the thermal efficiency of the system is significantly improved. As with the unglazed collector, the PV would replace the solar absorber. Glazed water collectors are typically medium cost and good efficiency collectors that are used for heating domestic water in both warm and cool climates.

2 Figure 3: An unglazed air collector (Conserval Engineering Inc., 2005) Figure 2: A glazed PV/T water heating system (Sekisui, 2005) Unglazed Air Collectors (Figure 3): consist of an uncovered perforated solar absorber through which air is drawn. In doing so, the air is heated. In difference to the unglazed water heating systems, the PV cannot replace the solar absorber surface because of the perforations. Instead, PV panels are attached above the absorber surface, and the heated air behind the PV is drawn into the system. These are typically low-cost and high efficiency collectors that have found widespread usage as HVAC solar preheat Glazed Air Collectors (Figure 4): are similar to glazed water collectors with the exception of the working fluid. In difference to the unglazed air collectors, glazed air collectors are generally closed loop. Here, the PV would replace the solar absorber. These are typically medium cost collectors that are generally used for domestic air heating. Air-Flow Windows (Figure 5): are window elements through which building intake air is drawn. The PV would be placed in the cavity between two panes of glass, and the air would be drawn over it. In doing so, the PV will be cooled (and become more efficient), and the air will be preheated for HVAC use. These systems have the potential to turn building facades into high efficiency electrical and thermal energy producers, thereby reducing energy requirements for commercial buildings. Figure 4: A glazed air collector (Grammer, Solar & Bau, 2005). Concentrating Collectors (Figure 6): are collectors which focus light onto a PV cell, thereby increasing solar input and electrical output. In this case, the thermal aspects are introduced to prevent the PV from being damaged. Water or air is circulated behind the PV to keep it cool, and the heated fluid is used for some other purpose. These are the only PV/T systems, where a thermal collection system is added to PV as opposed to the PV being added to a thermal system.

3 EXHAUST FAN II OUTER GLAZING T(H2) H2 MOTORISED BLIND ROOM AIR INTAKE FAN PRE-HEATED FRESH AIR INTERIOR GLAZING manufacturers of photovoltaic systems and solar thermal systems, building clients, financing bodies, and marketing consultants with experience in the field of renewable energy (Collins, 2005). A total of 22 participants took part in the meeting, representing Australia, Canada, China, Denmark, Germany, Greece, Israel, Netherlands, Spain, Sweden, and Switzerland. A number of Countries (Italy, France, and the USA) did show an interest in the task, but were unable to send representatives. AIR CAVITY PHOTOVOLTAIC (PV) SOLAR PANEL FRESH AIR H1 GLAZING OR SPANDREL Overall, the group demographic was well balanced. Eight companies were represented, including both PV and solar-thermal manufacturers, and consulting companies. Of those companies, most had either attempted, or were trying to attempt to develop some form of PV/T product. Seven research institutes were also represented by researchers in various stages of developing PV/T products. Most of the meeting participants had an excellent level of PV/T expertise. The objectives of the Task is to catalyse the development and market introduction of high quality and commercial competitive PV/T systems, and to increase general understanding and contribute to internationally accepted standards on performance, testing, monitoring and commercial characteristics of PV/T systems in the building sector. To do this, the Task group intends to perform market studies, develop simulation and performance tools, build and test prototype systems, develop full scale demonstration projects, and actively disseminate the knowledge gained (IEA SHCP, 2004). None of these objectives were part of the Task 7 work group. Figure 5: An air-flow window (Athienitis et al., 2005) PV/T SYSTEMS TECHNOLOGICAL STATUS It is noted that the preceding discussion was intended to inform the reader as to what PV/T technologies might exist. The systems presented do not reflect all of the possible configurations of PV/T In fact, based on the results of a survey conducted under IEA Task 7, only 7 products are currently available to the public: 4 air heating systems, 2 water heating, and one combined air and water heating (IEA PVPS, 2002). Of the remaining technologies, most are under development at various research institutions in North America and Europe. Figure 6: Concentrating collectors (Australian National University, 2005) A number of systems have been or are being developed that show excellent promise. These included concentrating collectors with water and air heating (Australian National University, 2005), air-flow windows (Athienitis et al., 2005, ASE, 2005), unglazed air heating systems (Conserval Engineering Inc., 2005), and glazed air heating systems (Grammer, Solar & Bau, 2005). Unglazed water heating systems (Millennium Electric T.O.U., 2005) have also been successfully developed for domestic water heating in warm climates. In cooler climates, the thermal aspects of an unglazed collector has only limited applicability as pool heating systems, and the addition of PV would only make the system economically unfeasible. Task 35 The IEA-Task 35: PV/T kickoff meeting was held on January 27-28, 2005, in Copenhagen, Denmark. The task group included experts assigned from universities, technological test institutes, engineers and architects in the field of building integrated solar applications, Not all of the systems show the same level of promise. Many attempts at developing glazed water heating systems (Sekisui, 2005, Millennium Electric T.O.U., 2005) have been made with limited success. Those systems have a number of significant and difficult to resolve issues

4 which suggest that a useful PV/T glazed water system will be many years away. In particular: Using PV cells as the absorber would negate any benefits gained from selective absorber coatings. It is necessary to optimize the optical properties of the PV absorber for the combination of systems, which is not optimal for the PV or thermal system independently. The efficiency of both systems is reduced as a result. The glazing on the thermal system reduces input to the PV by 15% or more depending on type of glass and solar incident angle. Furthermore, typical absorber temperatures in glazed water systems would likely not provide enough cooling to the PV for any significant benefit to be realized as in the case of air-flow windows. The efficiency of the PV system would therefore be reduced further. Manufacturers of these systems are experiencing significant difficulty with thermal expansion issues when attaching the cells directly to the copper absorber. The differences in thermal expansion is significant enough to break any bonds and/or crack the PV cells (IEA PVPS, 2002). Under stagnation (i.e., when the thermal collector is not operating and in full sun), typical temperatures in a glazed heater would either destroy the cells, or significantly reduce their lifespan. Stagnation in solar hot water systems is a frequent occurrence, and is the likely result of any system malfunction. Residential system installation was perceived to be so complex, that they were not easily marketed. Solar water systems have a projected useful life cycle well short of the PV life span. Adhering PV to a glazed solar water heater was perceived to be the equivalent of "throwing away the PV", unless the reliability issues of the thermal systems could be extended. The only perceived benefit of a glazed PV/T water heating system would be in areas where installation space is limited. In addition to the technical barriers present in the development of PV/T systems, are the barriers to efficient design and performance prediction. That is, it is difficult to size the system or components because tools useful for predicting performance simply do not exist. What is most needed technologically, are design tools and the demonstration projects and research which support their validity. These same tools will also be useful for optimizing the design of PV/T One final technical issue involves the rating and testing of PV/T There are, for example, no standard ways to commission a system. Current designs adhere to the criteria used for both thermal and PV Further, there are no standard guidelines that dictate design parameters. E.g., when integrating PV with solar hot water, what must be done to prevent the fluid system from interfering with the electrical system? Finally, there is no standard way of rating PV/T output. Do you rate the system as kwhp PV and kwhp/m 2 Thermal as is currently done, or do you standardize to one measurement or the other? To overcome market barriers to widespread introduction of PV/T systems, IEA Task 35 will follow the "roadmap" that was formulated by the Task 7 work group. In doing so, Task 35 has been subdivided into 5 Subtasks. The details of each will be discussed in turn. The objectives and anticipated results are from the IEA Task 35 Draft Annex (IEA SHCP, 2004). Subtask A Objective: To investigate and identify the critical design parameters and commercial performance criteria, which determine the targets and conditions for successful new components and systems, developed and evaluated in Subtask C and D. 1) A market survey on potential markets for PV/T 2) An article on the outcome and recommendations of focus group interviews on the potential and requirements for PV/T 3) A report on commercial parameters defining the market for PV/T 4) A recommendation on the marketing mix (product, price, place and promotion) for PV/T 5) An article on a detailed road-map for the successful development of commercial PV/T Discussion: The Subtask was a result of the European "PV-Catapult Project", which was intended to prepare for the large scale integration of PV/T to the European market. In devising their plan, the Catapult group found that there were significant barriers to introducing the technology. Of note, these barriers include: A) the need for a standard way of describing the technology (PV/T encompasses many technologies), B) the need for an understanding of present and potential markets, C) the need for an action plan, and D) other factors such as a lack of consumer/installer knowledge, price, the lack of any track record, installation problems, and the lack of any design tools. The following points were put forward by the Subtask for further consideration: (a) A list of existing systems and components needed to be compiled. (b) Market and energy needs in a given region need to be understood. If a given region needs the system to produce mostly electricity, then designers should be able to tune the system to meet that requirement. A geographically segmented market survey is required. (c) The incentive system for different countries needs to be clarified. PV/T systems may be able to take advantage of grants for PV and solar-thermal systems at the same time. Further, key "decision makers" have to be identified. (d) Standardized reporting methods (kwp PV x kwp Thermal was suggested), simulation and sizing tools, and demonstration products need to be created. (e) Regional certification issues need to be identified and addressed. (f) The environmental impact, life cycle analysis, and energy payback of the systems needs to be accessed to assist in the marketing of the products. TASK 35 - METHODOLOGY

5 It was clear that Subtask A is intended to drive the activities of the remaining Subtasks. By the time that it is able to collect any useful data, however, the IEA Task 35 mandate will be well into its three-year mandate, and the research Subtasks will already be in progress. It is likely that the research Subtasks (B and C) will have to be expanded past the three-year mandate of the Task if they are to respond to the needs of Subtask A. Still, Subtask A will be an interesting and useful exercise. Subtask B Objective: To provide the necessary understanding of the energy transfer processes in PV/T systems in order to define, model and predict the energy performance of the systems separately and in a whole building context. 1) A report on heat transfer models in PV/T 2) A report on recommended standards for characterisation and monitoring of PV/T The format of the document will set up to be useful for further elaboration by international standardisation bodies as well as experts working in the laboratory or on full scale monitoring (with Subtask B). 3) The implementation of performance models for PV/T systems in selected leading solar thermal systems simulation tools. 4) A report on comparison between the simulated and monitored performance of PV/T systems (with Subtask D). Discussion: Essentially, the group will need to (a) Identify the forms that a PV/T system might take (i.e., glazed and unglazed air and water, air-flow windows, and concentrating collectors), and determine which parts of these systems can be examine using conventional models. (b) Compile a listing of what input and output data is required. The required input data will be critical in correlating the performance of prototypes (with Subtask C) and demonstration projects (with Subtask D). The output format has the same concern as listed in point d) of Subtask A. (c) Develop optical, thermal, and electrical models of system components, when they do not already exist. (d) Using the results of c), develop dynamic system models of full systems, and a standardized methodology for developing new models. (e) Validate the results of point d) using the results of Subtasks C and D. (f) Develop models for determining the life-cycle performance and control strategies of PV/T The progression of this task seems very clear, even in the absence of data from Subtask A. The group had a good idea of the types of systems that could be developed, and was well familiar with the current state of both PV and solar-thermal modelling. Furthermore, it was largely thought that few new-models would have to be developed; rather the group would be able to adapt existing models for optical, thermal, and electrical characteristics. The only exception would be with air-flow windows, which a number of the participants were already working on. There is also excellent potential for new research to be identified. Of the Subtasks, this one will produce many of the significant contributions of Task 35. Subtask C Objective: To develop, test and evaluate PV/T system components and concepts based on the findings in Subtask A and the experiences from products and components already on the market. 1) A database of PV/T system components publicly available on-line. 2) Working document(s) on specific R&D-activities performed in collaboration with manufacturers of PV/T system components. 3) A report on recommended control strategies for PV/T 4) A report on reliability and durability of PV/T systems (with Subtask D). Discussion: The group will initially work in parallel with the modelling activities of Subtask B, but will eventually perform further investigations of PV/T In particular, they will (a) Identify the forms that a PV/T system might take (i.e., glazed/unglazed air and water, air-flow windows, and concentrating collectors) (with Subtask B). (b) Develop full-scale prototypes. The prototypes will actively be used to support the activities of Subtask B in a validation capacity. (c) Perform side-by-side comparisons of PV/T to separate PV and Thermal The testing will include both new and existing (d) Assist manufacturers in overcoming design issues (e.g. the development of new materials, aesthetics, optimization, control strategies etc.). (e) Develop certification standards Subtask C, like Subtask B, was very clear in its progression, even in the absence of data from Subtask A. The group was well familiar with the current state of both PV and Solar-Thermal research. Like Subtask B, there is significant potential for new research to be identified. Of the Subtasks, this one will produce many of the significant contributions of Task 35. Subtask D Objective: To gain the knowledge from full-scale demonstrations of PV/T systems in order to verify and identify the potential for improvement of energy performance, expectations to reliability, durability and economical feasibility. 1) A database on PV/T system projects publicly available on-line. 2) A report and article on the experience gained by all stakeholders involved in selected PV/T 3) Monitoring of selected PV/T system projects (with Subtask B). 4) A report on a workshop and a design review with design teams on initiated and realised demonstration projects Discussion: The Subtask is intended to: (a) Initiate demonstration projects which showcase the technology and promote the understanding and visibility of PV/T The systems must show progress and help the consumer to feel safe with the technology. (b) Monitor some systems in conjunction with Subtask B's requirements. The Subgroup identified existing installations which could be monitored now.

6 The Subtask is directly related to the objectives of Subtask A. The success of this Subtask is largely dependent upon what opportunities arise during the Task mandate. Subtask E Objective: To provide efficient and targeted information of the task results to all stakeholders of the Task, and to make this information available to the target audiences through various media and formats according to the preferences of the target audience. 1) A continuously updated task website with a public part for general dissemination and a restricted part for task experts, where all communication and documents will be available to the participants. 2) A dissemination plan for the task and each Subtask, to be revised at each semi-annual meeting. 3) A collection of templates and graphics for preparation of reports, presentations, poster, exhibitions etc. All material will be available to task participants on the restricted part of the task website. 4) Online information leaflets, newsletters and pressreleases in support of task-dissemination activities. 5) Proceedings from national workshops. Discussion: There was no discussion of this Subtask. It is expected that all Task 35 members do their best to participate in Subtask E. CONCLUSIONS The technical barriers to PV/T production, and a strategy for improving their marketability have been discussed. IEA Task 35 could have a significant effect on PV and Solar Thermal manufacturers, and on the marketability of PV/T systems in North America, Europe, and Asia. Acknowledgement Natural Resources Canada Varennes is gratefully acknowledged for supporting Canada's participation in this IEA Task. NOMENCLATURE PV photovoltaic PV/T combined PV and solar-thermal systems PVPS photovoltaic power systems program SHCP solar heating and cooling program REFERENCES ASE, Athienitis, A., Liao, L., Charron, R., Collins, M. Poissant, Y., and Park, K.W., "Experimental and Numerical Results for a Building-Integrated Photovoltaics Test Facility ", Accepted for Publication in IEEE, 2005 Australian National University, solar.anu.edu.au, Collins, M. R Meeting Report for IEA-Task 35: PV/T Systems. Meeting Report Submitted to CANMET Energy Technology Centre, NRCan, Varennes, Quebec, Canada Conserval Engineering Inc, Grammer, Solar and Bau, Accessed on February 24, IEA PVPS., 2002., PV/Thermal Solar Energy Systems: Status of the Technology and Roadmap for Future Development. IEA Report PVPS T7-10 IEA SHCP IEA SHCP Task 35: PV/Thermal Solar Systems Draft Annex. IEA Report. Leenders, F., 2000., Technology Review on PV/Thermal Concepts, Presented at Eurosun2000, Copenhagen, Denmark, June 19-22, Millennium Electric T.O.U., Sekisui,