Photovoltaic/Thermal Solar Collectors and Their Potential in

Final Report EFP project 1713/00-0014

Photovoltaic/Thermal Solar Collectors and Their Potential in Denmark

Miroslav Bosanac, Bent Sørensen*, Ivan Katic, Henrik Sørensen**, Bruno Nielsen**, Jamal Badran

Danish Technological Institute, Solar Energy Centre Gregersensvej, DK 2730 Taastrup, tel. +4572202486, fax +4572202500, email: [email protected]

*Novator Advanced Technology Consulting Østre Alle 43D, DK 3250 Gilleleje, Tel. +45 48361540, fax +4548361541, email: [email protected]

**Esbensen Consulting Engineers Ltd. Vesterbrogade 124B, DK-1620 Copenhagen V, Tel +45 33267304, Fax +45 33267301, email: [email protected]

Copenhagen, 21 May 2003

EFP report Combined photovoltaic/thermal solar collectors and their potential in Denmark Table of Contents:

1. Introduction to solar co-generation.................................................................................. 4 2. Literature Study................................................................................................................. 5 2.1 Definition of PV/T collector designs ...................................................................................6 2.1.1 Water PV/T collectors ................................................................................................6 2.1.2 Air PV/T collectors ........................................................................................................7 3. Market survey of PV/T solar collectors ............................................................................ 8 3.1 Introduction ......................................................................................................................8 3.2 Overview of the survey.....................................................................................................9 3.3 Manufacturers of Commercial PV/T Collectors ................................................................10 3.3.1 Water PV/T collectors ..............................................................................................10 3.3.2 Air PV/T collectors...................................................................................................12 3.3.3 Conclusion................................................................................................................16 3.4 Existing Building Projects with PV/T Collectors ................................................................16 3.5 Research and governmental support .................................................................................20 3.6 Design Considerations .....................................................................................................22 3.6.1 General recommendations .........................................................................................22 3.7 Conclusions for chapter 3 ................................................................................................23 3.8 References.....................................................................................................................24 4. Initial calculations ............................................................................................................ 28 4.1. Spreadsheet model for PV/T collectors...........................................................................28 4.1.1 Performance in terms of energy and exergy ...............................................................28 5. Simulations ....................................................................................................................... 33 5.1 Collector parameters and characteristics...........................................................................33 6. Simulation of the PV/T Performance with TRNSYS ........................................................ 35 6.1. PV/T Collector for Water Heating ..................................................................................35 6.1.1. Theoretical Model of the PV/T - Collector.................................................................36 6.1.2. Simulation Program..................................................................................................42 6.1.3. PV/T Collector Efficiency Curves as a Function of Design Parameters.......................45 6.1.4. Annual Performance of Water-Heating PV/T Collector .............................................51 6.1.5. Summary................................................................................................................57 6.1.6. Verification of the Computer Program Accuracy......................................................58 6.2 PV/T - Wall....................................................................................................................60 6.2.1. Theoretical considerations ........................................................................................61 6.2.2. Simulation Program..................................................................................................62 6.2.3. System Annual Yield................................................................................................62 Nomenclature ...................................................................................................................72 References ..........................................................................................................................74 7. Complete system modelling using NSES ........................................................................ 75 7.1 Model description............................................................................................................75 7.1.1 Reference year data .................................................................................................75 7.1.2 The solar collector ....................................................................................................77 7.1.3 Heat Exhange...........................................................................................................83 7.1.4 Flat-plate collector with heat storage ..........................................................................84 7.1.5 Solar heat-producing systems.....................................................................................87 7.1.6 Heat pump system....................................................................................................91 7.2 Results of simulations ......................................................................................................92

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EFP report Combined photovoltaic/thermal solar collectors and their potential in Denmark 7.2.1 Software verification.................................................................................................92 7.2.2. Small PVT system simulation....................................................................................94 7.2.3. Medium-size PVT system simulation.........................................................................97 7.2.4. Large PVT system simulation................................................................................. 101 7.2.5. PVT system with heat pump................................................................................... 102 7.3 References................................................................................................................... 110 8. Conclusions ................................................................................................................... 111 8.1. Water-Heating PV/T collector ...................................................................................... 111 8.2. Air PV/T collector (PV/T – Wall) ................................................................................. 113 9. Recommendations for future work ................................................................................ 114

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1. Introduction to solar co-generation In recent years building integration of photovoltaic modules has become more and more popular in the industrialised part of the world, where national support programmes has accelerated the dissemination of grid connected PV systems. The installation of a building integrated PV (BIPV) systems has certain advantages compared to a traditional PV system mounted in a separate structure. Function as a rain screen, sun shading device and a visually attractive cladding of the building are the most popular “added values” the house owner gets from the BIPV system. Traditional (silicon) photovoltaic modules will produce more electricity the cooler they are. Typically power increases with 0,2-0,5% per °C decrease of temperature, but when a PV module is integrated in a facade or roof, it will normally get warmer than a module mounted in free air. It is thus logical to remove the excessive heat from the module, giving the reason for the growing interest in “solar co-generation” or photovoltaic/thermal (PV/T) collectors. If the surplus heat from the PV module can not only be removed, but be used to fulfil thermal energy needs of the building, an extra added value could be achieved. The status of commercial PV modules is that only 10-15% of the incident solar energy is transformed to electricity. The potential heat production from a given surface is thus much higher than the electrical performance, but it is an open question if this heat can be used in a sensible way. There seem to be several obstacles: -

Most buildings need heating in winter when the solar gain is at its lowest The heat is needed at a higher temperature than the surrounding air, leading to increased module temperature unless a heat pump is used. For heating of domestic hot water, a heat exchanger is needed. The collectors could become very hot and thereby damaged if circulation of cooling media is blocked. The construction may be too complex and thus expensive compared to separate PV and thermal collectors.

The obvious advantages are: -

The total area used to extract a given amount of electricity and heat may be smaller than for two separate systems The materials used for a PV/T plant, and thus the total energy and economy balance, may be better than for separate units. The roof or facade will have a more uniform look.

In Denmark there already exist a handful of BIPV demonstration projects, where PV/T solutions have been integrated on an experimental basis. It is not yet proven that any of these demonstration projects will benefit from that, or they have been better off with separate systems. An urgent need therefore exist to establish guidelines and calculation methods for future BIPV projects. It is the purpose of this project to evaluate the feasibility of using combined PV/T collectors in typical Danish building installations.

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2. Literature Study Several reports and papers exist within the field of PV/T collectors. The majority of the published papers and reports are concerning theoretical developments of the efficiency of PV/T collectors. In the following only a few of these reports and papers are considered. Norwegian PhD project [1]. In this project an experimental model of a PV/T collector was constructed and measured. The model consisted of a conventional solar thermal collector with PV cells pasted to the absorber. One of the main conclusions from this study is that the thermal bonding between PV cells and absorber plate is very important for the thermal efficiency. The electrical efficiency is reduced when the flow rate of the cooling fluid is low, because the PV cells near the outlet then became relatively warm, and thus having a lower electrical output. Report from IEA workshop [2]. Several authors have contributed to this report, which also describe experimental results. The main conclusion is that most PV/T collectors have same thermal characteristics than ordinary nonselective thermal solar collectors. In this report the evaluation of PV/T collectors is also discussed, and the exergy method is suggested as one of the ways to compare different constructions. The authors of this report have adopted the method. PV-Hyphen project This is a EU Joule project that has carried out extensive studies of PV/T systems, in particular PV modules with ventilated air behind the PV cells for cooling the PV cells. The project involved practical measurements as well as theoretical work and simulations with the ESP-r program developed for passive solar heating. Eight different (office) buildings at different locations in Europe were selected for a total simulation of energy usage. The conclusions were that in southern/mid Europe, and in buildings with a modest fresh air demand, the savings are negligible. In Northern Europe, and for buildings with a high and constant fresh air demand, the savings are up to 10% of the annual heating demand. The effect on the electricity production caused by cooling the PV cells is very small. The above mentioned references and others concerning PV/T collectors are listed in chapter 3.8

EFP report Combined photovoltaic/thermal solar collectors and their potential in Denmark

2.1 Definition of PV/T collector designs PV/T collectors being considered in this report are collectors, which can provide both electrical and thermal energy. These hybrid collectors are divided in two groups: Water PV/T collectors. One example is a conventional flat plate solar heat collector with integrated PV cells on the absorber, to produce both thermal and electrical energy. Air PV/T collectors. These can be façade or roof integrated PV cells with ventilation air passed behind or in front of the PV cells. Furthermore, water PV/T collectors can be divided into groups according to temperature levels of the heat transfer fluid. This range from low-temperature applications for e.g. swimming pool and heat pump applications to medium-temperature applications around 55°C for e.g. domestic hot water applications. In the present study focus has been on the medium-temperature applications. In the study of F. Leenders et al. [21-22] further information can be found regarding the benefits and potential markets for low-temperature applications.

2.1.1 Water PV/T collectors For these systems, water is used as heat transfer fluid. The PV cells are pasted either directly on the absorber or interior on a coverplate with a dielectric material. This means that the only contact between the PV cells and the absorber or the coverplate is a high thermal contact. The heat transfer fluid runs inside the ducts on the absorber and collects heat from the absorber. If the PV cells are pasted to the absorber, heat is also extracted from the PV cells resulting in a higher electrical efficiency of the PV cells. Useful thermal energy is extracted to one end of the ducts where it can be utilised. The ducts can be coupled either in series or in parallel, which effects the efficiency of the system. The heat transfer fluid can be circulated by either a pump (a pumped system) or by the difference in specific gravity of the heat transfer fluid (a gravity system). Coverplate PV cells Absorber Ducts for heat transfer

Figure 2.1: A typical water PV/T collector

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EFP report Combined photovoltaic/thermal solar collectors and their potential in Denmark

2.1.2 Air PV/T collectors The other type of PV/T collector is an air-based system. Instead of water, air is used as heat transfer fluid. The PV cells are either pasted to the interior of the coverplate or to an absorber or the PV cells are acting as an absorber or coverplate itself. The air can be circulated by either natural ventilation or forced ventilation.

Coverplate Airflow

PV Cells Absorber

Figure 2.2: Example of an air PV/T collector

This kind of PV/T collector have been tried out in practice in several different projects and a few have been commercialised, e.g. the Canadian “PV SOALRWALL”. The PV SOLARWALL consists of a perforated metal absorber in front of the exterior wall of a building with an airgab in between. In front of the perforated metal absorber a PV module is mounted is such a way that the cooler ambient air is allowed to pass behind the PV module. Heat generated by the PV module and the metal absorber will be transferred to the air and thereby cooling the PV module causing a higher electrical efficiency for the PV module. See also chapter 3.

Figure 2.3: Canadian SOLARWALL with PV cover, see also chapter 3.3.2 [2]

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3. Market survey of PV/T solar collectors 3.1 Introduction A questionnaire was circulated to manufacturers, designers and research institutes in the field of PV/T collectors. Purpose of this questionnaire was to collect the basic information about the systems and products available or planned for the building market the coming years. Especially information characterising the energetically characteristics, weight and dimensions relevant when considering building integration and key data to be used for further analysis of the systems were asked: -

Weight per m2 and per kW for air and water based systems Price per m2 and per kW for air and water based systems Area per kW for air and water based systems Range of temperatures supplied for air and water based systems Thermal output power range for air and water based systems Electrical output power range for air and water based systems Typical efficiencies (thermal and electrical) for air and water based systems

Generally, a positive response has been achieved, but especially on the “one-off” project (rather than product based) building integrated systems, limited information has been made available. However several manufacturers have expressed their interest in further monitoring of their systems both in realised projects and laboratory experiments.

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3.2 Overview of the survey During the survey 32 examples of PV/T projects or products were found. Of these, 8 appear to be commercially available products. The remainder, appear to be either one-off designs for specific building projects or else products under development. Most PV/T systems concentrate typically on preheating of ventilation air. This might be due to the fact that a cavity behind the PV panels is almost always available. Thus, it seems logical to use this cavity against little costs for thermal heat gains. Table 1 shows a categorical system breakdown. The total given in the table 1 is greater than the total number of products found because some products can be mounted in more than one manner. Heat transfer Mounting Number medium location-method identified Air Roof integrated 12 Facade integrated 10 Separate module 9 Water Roof integrated 5 Facade integrated 3 Separate module 9 Table 1: Breakdown of PV/T systems identified

Commercial available Products 3 4 3 2 2 3

As mentioned earlier, there are currently only 8 commercial available products. Some companies have announced that they soon would have a PV/T collector in production. However, they are having a problem maintaining the long time stability of the PV cells as they are integrated on an absorber. Heat transfer Manufacturer Nationality medium Air Millennium Electric Israel (formerly Chromagen Solar Energy Systems) Aidt Miljø A/S Denmark Conserval Engineering Canada Grammer KG Germany Phototronics Solar-technik, Putzbrunn Germany part of ASE in Germany. Water Millennium Electric Israel (formerly Chromagen Solar Energy Systems) ICEC AG Switzerland Sekisui Chemical Co., Ltd Japan Table 2: Breakdown of commercially available PV/T systems identified by manufacturers

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The systems located were constructed or first made available to the market in the period 1991 to 2001. The year 1998 seemed to be a particularly busy year with a number of activities being undertaken/initiated in the USA under the PV BONUS II program, in Germany and Switzerland. The manufacturers/designers were located in 12 different countries as listed below. Canada Denmark Germany Israel Italy Austria

2 3 4 1 1 1

Japan Netherlands Spain Switzerland USA Great Britain

2 2 1 6 8 1

3.3 Manufacturers of Commercial PV/T Collectors 3.3.1 Water PV/T collectors In Israel PV/T collectors have been developed by the company "Millennium Electric" (formerly known as Chromagen) since 1991. The commercially available PV/T collector is a flat plate solar heat collector with PV cells integrated on the absorber. Chromagen first tested their PV/T collector in several locations in Israel, finding that the PV/T collector could supply apartment's electricity and hot water demand. The system was developed in such a way that additional generated electricity could be sold to the local Electricity Company. The average cost for the PV/T collector, named "Multi Solar System", are about USD 940/m2 collector. The system can be grid-connected or standalone. It is sold with a minimum of 2 modules with a total collector area of 4.64 m2. The daily thermal and electrical output is about 1.5 kWh/m2 heat and 0.4 – 0.8 kW/m2 electricity in Israeli climatic conditions. The product is marketed commercially and about 20 systems have been sold in Israel [11] and 20 systems have been sold world-wide. 362 systems are contracted for USA in 2002.

Multi Solar System in Singapore In Switzerland, ICEC AG has http://www.icec.ch/products.html

developed

a

PV/T

collector

“HYSOLAR”,

see

In Asia, Sekisui Chemical Co., Ltd in Japan have developed a PV/T collector for domestic hot water. The hybrid collector converts about 10% of the solar energy into electricity and 30% into hot

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water on an annual average. It was commercialised in 1999 and further development is taking place so the thermal energy from the PV/T collector can be used for space heating.

Sekisui Chemical Co., Ltd

Sekisui Chemical Co., Ltd See also http://www.sekisui.co.jp/general/english/eco/report2001_e.htm Two German companies, “SolarWerk” and “SolarWatt” have developed a similar kind of PV/T Collector. Both systems are flat plate solar heat collectors with PV cells integrated on the absorber. “SolarWerk” developed their system in co-operation with the Institut fur Solarenergiforschung (ISFH) GmbH in Hameln, Germany. ISFH participated in the development and in the experiments on the PV/T Collector. The contact person for the various performance tests on the PV/T Collector done at ISFH is Roland Sillmann. The product developed is named “Spectrum” with a collector area of 2.2 m2. It can be installed either as a stand-alone or as grid-connected system. [Information received from SolarWerk Homepage at http://www.solarwerk.de/spectrum.htm

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SolarWerk “Spectrum” The other German Company “SolarWatt” developed a product named “MultiSolar” which is also a flat plate solar heat collector with PV cells integrated on the absorber, [13]. Both companies have however problems maintaining long time stability of the PV cells as they are integrated on an absorber. This seems to be a common problem for companies interested in commercialising PV/T systems where the PV cells are integrated on the absorber.

3.3.2 Air PV/T collectors The Danish company “Aidt Miljø A/S”, has developed a solar air collector with integrated PV cells and fan. This product preheats ventilation air, but the main purpose of the product is actually to provide dehumidification of the air in cabins, garages, allotment houses, mobile homes etc. The PV cells supply a fan in the top of the collector with electricity. The fan draws in outdoor air through a perforated aluminium plate on the whole backside of the collector, the air is warmed up in the collector and is blown into the room. Two sizes are available: 0.35 m2 and 0.7 m2 with capacities of 25 m3/h and 50 m3/h respectively.

EFP report Combined photovoltaic/thermal solar collectors and their potential in Denmark

Aidt Miljø A/S “SolarVenti”j

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“SolarVenti” installed on a cabin

More information can be found on the homepage of the company www.aidt.dk. The Canadian company “Conserval Engineering Inc.”, has developed a PV solar wall product where PV panels can be mounted onto a perforated absorber. This building integrated PV/T Collector can be used as a facade or roof element and is named “PV SolarWall”. The PV panels are mounted in such way that cool ambient air is allowed to pass behind the PV panels in a uniform way. Heat generated from the PV cells will be transferred to the air, which can then be used for heating ventilation air. The PV Solar Wall is a variation of their standard SolarWall, which collects thermal energy using the same perforated absorber plate without the PV panels. [14 and Information received from Conserval Engineering at their homepage www.solarwall.comm.html#12c.]

Conserval Engineering Inc. “PV SolarWall”

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Phototronics Solartechnik of Putzbrunn, part of ASE, have developed an a-Si commercial curtain-wall module about 1m by 0.6 m with 6% efficiency. These modules have among others been used on the PV facade at the Bavarian Environment Ministry. This product is designed for vertical facades of commercial and residential buildings and four basic product approaches are offered. One of these is a combined PV/Thermal panel incorporating semi-transparent PV panels for view windows within an insulated glass sandwich with warm air heat recovery. More at http://www.aseinternational.com/english/start_e.html

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Grammer Solar & Bau in Germany currently have a commercial available PV/T product where the PV panel is cooled by air for preheating ventilation air. The product is made available in 4 different module sizes ranging from 50 kWp to 250 kWp per module, http://www.grammersolar.de/solarthermie/pvkollektor.htm.

Grammer Solar & Bau in Germany

Grammer Solar & Bau in Germany

Grammer Solar & Bau in Germany

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More information can be found on the homepage of the company www.grammer-solar.de, or http://www.twinsolar.de/index1.htm.

3.3.3 Conclusion Air-cooled PV panels for electrical and thermal gain is currently the most commonly used PV/T system. It is most suitable for façade type applications (second skin, Conserval and Phototronics Solartechnik). These BIPV systems add a little thermal to the PV. Otherwise, thermal systems are available that adds a little PV to their thermal gains e.g. PV powered thermal collectors produced by Grammer Solar & Bau in Germany. In between are products being developed, e.g. Solarwerk, that both optimise the use of PV and the thermal system. These systems have PV cells integrated on the absorber in a conventional solar thermal collector. There are a number of companies working in developing commercial liquid PV/T collectors but currently only three companies Millennium Electric, Sekisui Chemical Co., Ltd, and ICEC AG have succeeded. There seems to be a problem in maintaining the long time stability of the solar cells as they are integrated on the absorber. These systems need to be developed further to serve the large consumer market of solar powered energy (heat and electricity).

3.4 Existing Building Projects with PV/T Collectors A “PV SolarWall”, made by the Canadian Company “Conserval Engineering”, has been installed at the West Prep School in Toronto, Canada. The 15 m2 SolarWall, with two 60-Watt UNISOLAR PV panels, was installed to improve the indoor air quality in the classrooms. The heat generated from the conventional SolarWall and from the PV panels is transferred to cooler ambient air, to provide fresh air to the classrooms and thereby improve the indoor air quality and reduce the heating costs. The electricity generated from the two 60 Watt PV panels is used for running two ventilation fans which provide between 0 and 680 m3 of air per hour [14]. In 1993, the Japanese government invested more than 1200 million yen on PV demonstrations. An area of interest was the development of PV/T Collectors for buildings in Japan. One prototype was developed for residential applications and was tested on a test house in Japan. The PV/T system consists of PV cells backed by a thermal absorber and it produces daily about 3.2 kWp of electricity and 25 kW of thermal energy. [Information received from www.nrel.gov/ncpv/documents/japan.html]. Also in Japan, Sekisui Chemical Co., Ltd has installed PV/T collectors for domestic hot water in several residential homes. The hybrid collector converts about 10% of the solar energy into electricity and 30% into hot water on an annual average.

EFP report Combined photovoltaic/thermal solar collectors and their potential in Denmark

Sekisui Chemical Co. Ltd.

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Sekisui Chemical Co. Ltd.

Richard Komp and Terry Reeser of SunWatt Corporation [9] have constructed and operated an air PV/T Collector for a passive solar house in Louisville , Kentucky. It is built into the roof structure of an attached sunspace and uses natural convection to extract excess heat from finned module assemblies, and delivers that heat into the house during the winter. During the summer, the heat is exhausted from clerestory windows, creating a draft of cooler air into the lower part of the building. The electrical output is delivered through a battery to low voltage lights and other appliances in the house. Solar Design Associates, Inc was involved in a roof-integrated array of PV/T Collectors at the Montana State University Research Centre that delivers both thermal and electrical energy to the building. An array of PV cells was also roof integrated at a custom solar residence on the coast of Maine, where the arrays are passively cooled from behind by ambient cool air. The heated air is then used for ventilation purposes. [Information received from http://www.solardesign.com/prodev.html. Innovative Design from North Carolina has developed a integrated PV system that use waste heat from a PV array to heat up water [10]. A roof-integrated example has been installed on an Applebee’s restaurant that uses 32 amorphous PV modules. Eight of the modules are connected to a fan that circulates air through a series of passages underneath the 32 modules. About 7 % of the total area of the system is clear glass between the PV cells, facing a black-painted highabsorbing metal pan. As insolation increases and the temperature goes up, a fan switches on to circulate the heat away from the PV modules towards a heat exchanger. The heat flows through a closed loop and is thereby not wasted as it is in conventional PV modules. Since there is a big demand for hot water in the building, all the hot water produced from the solar energy system is used. As it is a roof-integrated system, the costs for conventional roof finishing were saved. In 1994 in Ispra, Italy, the ELSA building had its 25 m high south facade covered by an area of 505 m2 of amorphous PV cells with an electrical output rating of 21 kWp. The PV cells were mounted onto an insulated wall with an air gap behind the PV cells. The heated air behind the PV cells is used for ventilation purposes [15].

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Similarly PV/ T energy systems are installed at the Library of Mataro in Spain and at the Yellow House in Aalborg, Denmark. At the Library of Mataro in Spain, PV cells are mounted onto the facade and on skylights on the roof. In chambers behind the PV cells air is heated and is then used for preheating the water in the conventional gas-fired heating system of the building [16]. The Yellow House in Aalborg, Denmark has 5 different groups of PV installations in the facade for demonstration purposes. [17-21]

Yellow House in Aalborg, Denmark During the last 10 years, Atlantis Energy Systems, Ltd. from Switzerland, have made several PV/T systems around the world. One system, installed in 1991 at the factory building “Aerni” in Arisdorf, Switzerland, has a ventilated PV facade and ventilated PV skylights with a total electrical output of 62 kW and a thermal output of 115 kW. Atlantis Energy Systems Ltd. also developed a PV/T shingle roof, two of them are named “Brig” and “Rigi” where PV panels are installed on the roof with ventilation air passed behind the PV cells. Both systems have been in operation since 1993 without interruption and have shown good results [21]. At the City Archives in Rotterdam, Netherlands some 1840 m2 of PV cells are installed on the roof. Beneath the PV cells heat is generated. During the summer this heat is stored in the ground to provide heating in the winter. During the winter, cool air is stored in the ground to provide cooling during the summer. [Information received from www.dubo-centrum.nl and 22] Several systems of the type “Multi Solar System” from "Millennium Electric" (formerly known as Chromagen) with a daily output of 2 to 4 kWh electricity and 6,000 kcal of hot water have been installed in residential homes in Klil, a small mountainous community in northern Galilee, Israel, isolated from the national grid. The systems have been operating and monitored since 1991 and have been providing the total energy demands of the typical family household. [11]

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Multi Solar System in Klil, Israel In Denmark, several projects using the PV-Vent concept have been realised. Fresh air for low energy ventilation systems with heat recovery passes behind PV modules integrated in the building facade or roof, thereby both preheating the ventilation air and cooling the PV modules. The ventilation systems are directly supplied with DC electricity and a so-called “PV-mixer” measures the PV electricity and when there is not sufficient PV electricity, supplementary electricity is supplied from the grid. One of the projects was realised in a housing block with 27 flats in Lundebjerg near Copenhagen. [53]

Lundebjerg in Skovlunde, Denmark

Solar Chimney, Lundebjerg, Denmark

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3.5 Research and governmental support The DOE initiated the Building Opportunities in the United States for PV (PV Bonus) program in 1993 to develop cutting-edge solar products for the building industry. Program objectives were to develop technologies and foster business arrangements for products cost-effectively integrating PV or PV/hybrid technology into buildings. An important factor is that these products must be installed without the need for specialised training. The program was conducted through two competitive solicitations, termed PV Bonus I and PV Bonus II. Twenty-two partnerships were initiated under PV Bonus and from these, five new products were developed. Products included solar roofing shingles and a factory-built modular home integrating PV. PV Bonus II began with 16 partnerships. Seven were selected for additional work, resulting in five commercially available products. Products range from an “enabling” PV application to dual-purpose or hybrid products, and products for unique applications. PowerLight Corporation is completing final tests for a PV/T product named PowerRoll TM, a combined PV/thermal hybrid system for medium-temperature hot water applications. The product combines the USSC flexible triple-junction module adhered to a heat-transfer backing material. Lessons learned in this work are the challenge of combined testing to meet solar concentrator standards, PV module standards and UL requirements. Technical lessons learned included materials selection, such as an adhesive meeting safety codes and surviving outdoor exposure and operating at elevated temperatures. [23].

PowerLight Corporation “PowerRoll” The team of Solar Design Associates (SDA), United Solar Systems Corp. (USSC), and SunEarth Inc. is developing a hybrid PV/thermal product called PhotoTherm. The product is a unitised combination of a water thermal collector and the USSC triple-junction a-Si thin-film module. PhotoTherm resembles a traditional solar thermal design, except the PV module replaces the top surface of the absorber plate. The current Phototherm product is designed for installation on an existing roof. The partners gained experience in defining a solar product capable of higher temperature operation and selecting materials to lower product cost. Because the hybrid product will operate as a PV module and a source for hot water, qualification tests had to be defined. The product must also meet requirements for safety (UL), PV modules and solar thermal products and building codes.

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Solar Design Associates, United Solar Systems Corp. And SunEarth Inc. “PhotoTherm” IT Power's Test and Training Centre, located near to the office in Eversley, Great Britain, is currently working on Photovoltaic/Thermal system development - concept assessment, system design & testing, see http://www.itpower.co.uk/services/r&d.htm At North Carolina Solar Center, the Solar Engineering Specialists Rob Stevens and Shawn Fitzpatrick are conducting an exciting patentable PV/T research project. Stevens and Fitzpatrick hope to develop a practical way to use heat from the backside of a PV panel to provide water and space heating and possibly drive an air conditioning system. “One of the key features of this design is that the panels will be roof integrated, or incorporated into the actual structure of the roof,” said Stevens. More information can be found at http://www.engr.ncsu.edu/news/news_articles/solar.center.html

North Carolina Solar Center

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3.6 Design Considerations The following chapter summarises the “state-of-the-art” regarding recommendations for designing BIPV solutions with PV/Thermal solar collectors. The recommendations are provided by IEA PVPS Task7 and other external companies and scientists working in the field of designing solar thermal systems and PV systems.

3.6.1 General recommendations In choosing what type of PV/T system is most suitable for a particular project, the project demands need to be considered, e.g.: 1. 2. 3. 4. 5.

Temperature and characteristics of thermal load Thermal load (kW) Electrical load (kW) Suitable mounting locations Building constraints, e.g. weight bearing capacity, aesthetics

Some general notes: - System is sized for the thermal load, as this is more usually the constraint. Particularly with grid connected buildings where the electrical load can be considered infinite - The current PV/T building integrated systems are mostly air based. Some water-based systems are under development. - PV/T water based systems are most suitable for low temperature applications (swimming pool, heat pump combinations). - Water based systems are normally designed as separate modules, in part because they have evolved from solar water heating systems and also presumably because there are less concerns regarding the built quality of factory built modules and hence the risk of water leakage. - Domestic water heating loads normally require relative high temperatures (in some cases min. 60°C to avoid Legionellosis) and usually water based systems in order to provide heat exchange with storage. - Medium temperature demands normally use water based systems, due to the practical limitations in capacity-flow of air-based systems. - Process heating (industrial applications) – no references known so far. Will usually require a combination with storage to achieve a constant energy flow and usually relatively high temperatures. Some design ideas have been developed for preheating elements for burners, moisture control of bio-fuel but not realised so far. - (Systems or projects associated with thermal storage for the long-term storage of heat could also be discussed).

The current technical level of the commercial PV/T systems still need to be verified tested and monitored. Still many issues regarding the combination of materials, the dependency on temperature level on the overall yield and the optimum combination of heat and electricity production for various climates and applications needs to be determined.

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3.7 Conclusions for chapter 3 The status of commercial PV modules is that only 10-15% of the incident solar energy is transformed to electricity. The potential heat production from a given surface is thus much higher than the electrical performance, but it is an open question whether this heat can be used in a sensible way. Combining PV and a solar thermal collector for tap water heating ends up with a temperature compromise. PV needs to have a low temperature to maintain a high efficiency, whereas a solar thermal collector requires a high temperature. With the current technologies, the PV/T combination has a lower efficiency than two separate systems and, due to the initial development stage, the PV/T combination is also more expensive. However, advantages are foreseen in aesthetics, future (production & installation) cost reductions and market / consumer requirements. Based on the survey carried out and discussions with various building designers listed, there seem to be several more obstacles: Based on the survey carried out and discussions with various building designers listed, there seem to be several obstacles: -

Most buildings (e.g. in Western European climates) need heating in winter when the solar gain is at its’ lowest The heat is needed at a higher temperature than the surrounding air, leading to increased module temperature unless a heat pump is used. For heating of domestic hot water, a heat exchanger is needed for safety and health regulations The collectors could become very hot and thereby could get damaged if circulation of cooling media is blocked. The construction may be too complex and thus expensive compared to separate PV and thermal collectors.

A number of obvious advantages exist, and if the obstacles are overcome, there seems to be a large potential for PV/Thermal Solar collectors in the future: However, much R&D work is still required, and knowledge transfer is needed etc. -

-

The total area used to extract a given amount of electricity and heat may be smaller than for two separate systems The materials used for a PV/T plant, and thus the total energy and economy balance, may be better than for separate units. The roof or facade will have a more uniform look, providing the potential for pre-fabricated systems developed for various types of roofs. When using integrated elements a potential saving in installation costs compared to separate systems can turn out to be a very important factor for the future development of the market for PV/Thermal solar collectors. A careful design with utilisation of low temperature levels for e.g. for swimming pools and in combination with heatpumps.

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3.8 References [1]

B. Sandnes. Oslo Universitet.

[2]

Photovoltaic/Thermal Solar Systems. IEA workshop report. Amersfoort, 17-18 September 1999.

[3]

M. Mattei, C. Cristofari and A. Louche, Modelling a Hybrid PV/T Collector, 2nd World Conference on Solar Energy Conversion, July 1998.

[4]

PV Power and Heat Production: An added Value. Bent Sørensen, Roskilde University. 16th European Photovoltaic Conference, Glasgow 2000.

[5]

Technology Review on PV/Thermal concepts. F.Leenders et al. 16th European Photovoltaic Conference, Glasgow 2000.

[6]

Solar Photovoltaic/thermal Co-generation Collector. B.J.Huang et al. Dept. of Mechanical Engineering, National Taiwan University, Taipei, Taiwan.

[7]

Cooling of Building Integrated Photovoltaics by Ventilation Air. M.Sandberg, Laboratory of Ventilation and Air Quality, KTH Dept. of Built Environment, Gävle.

[8]

“Hybrid Collectors, Theoretical Developments and Performance Evaluation of PhotoVoltaic Thermal Collectors”, Master Thesis by Bruno Nielsen, Esbensen Consulting Engineering Ltd. Copenhagen.

[9]

Richard Komp and Terry Reeser, "Design, Construction and Operation Of A Site Built PV/Hot Air Hybrid Energy System", Article obtained from Richard Komp at SunWatt Corporation at +1 207-497-2204, 1998.

[10]

K. Sheinkopf, "PV System With Thermal Heat Recovery", Article of the work of the IEA, obtained from www.caddet-re.org/re/html/body_298art2.htm

[11]

Dr. A. Elazari, "Multi Solar System, Field Experience In Israel", Several articles obtained from Dr. Elazari at Chromagen in Tel Aviv, fax: 972 - 3 - 525 - 6305.

[12]

"Building Opportunities In The U.S For Photovoltaics (PV:BONUS), Two", Article obtained from Robert J. Hassett, U.S. Department of Energy, Tel: +1 202 586 8163.

[13]

"MULTISOLAR", Multisolar is a registered trademark of KRUSE Gmbh Technik.

[14]

"Hybrid Solar Collectors for Portable School Classrooms", Article obtained from Per Drewes, Ontario Hydro Technologies.

[15]

”Thermal and Power Modeling of the Photovoltaic Facade on the ELSA Building, ISPRA", “Thermal Aspects of PV Integration in Buildings”, “Analysis of Fluid Flow and Heat Transfer within the PhotoVoltaic Facade on the ELSA Building, JRC ISPRA”, 3 papers from the 13th. European Photovoltaic Solar Energy Conference, October 1995.

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[16]

"The Library of Mataro description and first results of monitoring", Article by Dr. Antoni Lloret, Labotatoire des Interfaces et des Couches Minces Ecole Polytechnique, 91128 Palaiseauu cedex, France.

[17]

"The Yellow House- An innovative solar renovation of multi storey housing" O. B. Jørgensen, Esbensen Consulting Engineers Ltd. 1997, proceedings from 7th. International Conference on Solar Energy at High Latitudes – North Sun `97, Espoo-Otaniemi, Finland

[18]

"Integration of Solar Energy in future renovation of multi storey housing - The Yellow House" O. B. Jørgensen, Esbensen Consulting Engineers Ltd. 1997, proceedings from EuroSun `96, Freiburg, Germany

[19]

"Monitored results from the Yellow House" O. B. Jørgensen and L. T. Nielsen, Esbensen Consulting Engineers Ltd. 2000, paper presented at EuroSun 2000 in Copenhagen, Denmark

[20]

"The Yellow House-Final Report" Esbensen Consulting Engineers Ltd and SBS Byfornyelse, 2002, Copenhagen, Denmark.

[21] "The Importance of Hybrid PV-Building Integration", Paper by M. Posnansky, Atlantis Energy Ltd. 3012 Bern, Switzerland, Tel: +41 031 300 3220 Fax: +41 031 300 3230. [22] "City Archives Rotterdam", Paper received by email from Siard Hovenkamp at [email protected] [23]

“Building Integrated PV and PV/Hybrid Products – The PV:BONUS Experience”, Presented at the NCPV Program Review Meeting, Lakewood, Colorado 14-17- October 2001.

[24]

“Technology reveiw on PV/Thermal concepts”, Paper by F. Leenders, Ecofys, NL, B.G.C. vand der Ree, TNO Bouw, NL: and W.G.J. vand er Helden, TUE University of Technology Eindhoven. Presented at the EuroSun Conference Copenhagen 19.-22. June 2000.

[25]

“Photovoltaic/Thermal Solar Systems”, Report on a joint workshop of the IEA Solar Heating & Cooling Programme, IEA Photovoltaic Power Systems Programme, Amersfoort, the Netherlands, 17-18. September 1999. Edited by ir. F. Leenders and drs B.G.C. van der Ree, Ecofys NL.

[26]

L. W. Florschuetz, "Extension of The Hottel-Whillier Model to The Analysis of Combined Photovoltaic/Thermal Flat Plate Collectors", Solar Energy, Vol. 22 pp. 361 - 366, 1979.

[27]

Susan D. Hendrie, "Evaluation of Combined Photovoltaic/Thermal Collectors", Proceedings of the International Solar Energy Society, Vol. III pp. 1865 - 1869, 1979.

[28]

D. J. Mbewe et al., "A Model of Silicon Solar Cells for Concentrator Photovoltaic And Photovoltaic/Thermal System Design", Solar Energy, Vol. 35, No. 3, pp. 247 - 258, 1994.

[29]

C. H. Cox and P. Raghuraman, "Design Consideration for Flat Plate Photovoltaic/Thermal Collectors", Solar Energy, Vol. 35, No.7, pp. 227 - 241, 1985.

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[30]

A. Braunstein and A. Kornfeld, "On The Development of The Solar Photovoltaic And Thermal (PVT) Collector", IEEE Transaction on Energy Conversion, Vol. EC-1, No.4, pp. 31 - 34, December 1986.

[31]

Ram Kumar Agarwel and H. P. Garg, "Study of a Photovoltaic-Thermal SystemThermosyphonic Solar Water Heater Combined with Solar Cells", Energy Conversion And Management , Vol. 35, No. 7, pp. 605 - 620, 1994.

[32]

Ram Kumar Agarwel, H. P. Garg, and J. C. Joshi "Experimental Study On A Hybrid Photovoltaic-Thermal Solar Water Heater And Its Performance Predictions", Energy Conversion And Management , Vol. 35, No. 7, pp. 621 - 633, 1994.

[33]

Jai Prakash "Transient Analysis Of A Photovoltaic-Thermal Solar Collector For CoGeneration Of Electricity And Hot Air/Water", Energy Conversion And Management, Vol. 35, No. 11, pp. 967 - 972, 1994.

[34]

Takumi Takashima, et al., "New Proposal For Photovoltaic/Thermal Solar Energy Utilization Method", Solar Energy, Vol. 52, No. 3, pp. 241 - 245, 1994.

[35]

Ram Kumar Agarwel and H. P. Garg, "Some Aspects Of A PV/T Collector/Forced Circulation Flat Plate Solar Water Heater With Solar Cells", Energy Conversion And Management , Vol. 36, No. 2, pp. 87 - 89, 1995.

[36]

Takumi Takashima, et al., "On The Consideration Of Total Efficiency Of Photovoltaic/Thermal Panel", T. IEEE Japan, Vol. 115-B, No. 4, pp. 430 - 435, 1995.

[37]

Trond Bergene and Ole Martin Loevvik, "Model Calculations On A Flat Plate Solar Heat Collector With Integrated Solar Cells", Solar Energy, Vol. 55, No. 6, pp. 453 - 462, 1995.

[38]

K. Sopian et al., "An Investigation Into The Performance Of A Double Pass Photovoltaic Thermal Solar Collector", AES , Vol. 35, pp. 89 - 94, ASME 1996.

[39]

K. Sopian et al., "Performance Of A Hybrid Photovoltaic Thermal Solar Collector", AES , Vol. 36, pp. 341 - 346, ASME 1996.

[40]

Gouri Datta and H. P. Garg , "Theoretical And Experimental Studies On A Solar Photovoltaic Thermal (PV/T) Liquid Heating System With Thermosyphonic Flow", World Renewable Energy Congress, Vol. III, pp. 1815 - 1819, Denver, Colorado 1996.

[41]

K. Sopian and K. S. Yigit et al., "Performance Analysis Of Photovoltaic Thermal Air Heaters", Energy Conversion And Management, Vol. 37, No. 11, pp. 1657 - 1670, 1996.

[42]

H. P. Garg and R. S. Adhikari, "Conventional Hybrid Photovoltaic/Thermal (PV/T) Air Heating Collectors: Steady State Simulation", Renewable Energy, Vol. 11, No. 3, pp. 363 385, 1997.

[43]

K. Sopian et al., "Research and Development Of Hybrid Photovoltaic Thermal Solar Air Heaters", International Journal Of Global Energy Issues, Vol. 9, Nos. 4-6, pp. 382 - 392, 1997.

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[44]

C. Choudhury and H. P. Garg , "Performance Of A Two-Pass Photovoltaic/Thermal Air Heater", World Renewable Energy Congress, Vol. III, pp. 1803 - 1806, Denver, Colorado 1996.

[45]

S. Gasner and L. Wen, "Evaluation Of Unglazed Flat Plate Photovoltaic-Thermal Collectors In Residential Heat-Pump Applications", Solar Engineering, ASME, pp. 302 - 308, 1982.

[46]

G. V. Tsiklauri et al, "Combined Photovoltaic And Thermal Modules With Hydrogen Accumulation For Solar Power Plants", ECE Energy Series, Vol. 11, pp. 194 - 199, 1993.

[47]

Mark M. Koltun, "The First Russian 1 MW Combined Photovoltaic And Solar Thermal Power Plant: Basic Ideas Advantages Of The Project", Renewable Energy, Vol. 5, part I, pp. 179 - 181, 1994.

[48]

M. A. K. Lodh, "Hybrid Systems Of Solar Photovoltaic, Thermal And Hydrogen: A Future Trend", International J. Hydrogen Energy, Vol. 20, No. 6, pp. 471 - 484, 1995.

[49]

Douwe de Vries, "Design of a photovoltaic/thermal combi-panel", PhD-thesis, at the University of Technology in Eindhoven.

[50]

"Entwicklung eines PV-Hybrid-Kollektors", 8. Symposium Thermische Solarenergie, Ostbayer-isches Technologie Transfer Institut, Regensburg, 1998, pp 77 – 82.

[51]

"PV-Hybrid and Thermo-Electric-Collectors", Gunther Rockendorf and Roland Sillmann, Institut für Solarenergieforschung Gmbh, Hamln/Emmerthal, ISFH. Tel. +49 5151/999-521

[52]

"Photovoltaic cogeneration in the built environment", M.D. Brazilian, F. Leenders, B.G.C. van der Ree, D/. Prasad, ":, Solar Energy, Vol. 71, No. 1, pp 57-69

[53]

"PV-Vent Low Cost Energy Efficient PV-Ventilation in Retrofit Housing", Non-NuclearEnergy Programme Joule III, Paper by Mr. Peder Vejsig Pedersen and Ms. Anne Rasmussen, Cenergia Energy Consultants obtained from www.ecobuilding.dk.

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4. Initial calculations Whether a PV/T collector is better than a separate PV module and a solar collector or not, depends on several factors: -

Is space a problem, or is the total collector area unimportant? Does the load profile fit to the combined production of heat and electricity? Does the installation become simpler or more difficult than in separate systems? Does the production costs increase due to a more complex construction?

In order to get a first estimate of the characteristics of PV/T the traditional and well-documented set of equations for solar thermal collectors are used. The only difference to a PV/T collector is that a part of the collected energy is extracted as electricity instead of heat. If the formulas are corrected for this and secondary effects such as radiation heat transfer inside the solar cells are neglected, they are valid for the PV/T collector.

4.1. Spreadsheet model for PV/T collectors A model in Excel was made for the two mentioned PV/T collector types. Two cases were investigated for the first type, namely: a) Standard PV module with ideal rear surface cooling, PV cells are acting as the absorber b) As for a) but with thick (15 mm) acrylic coverplate for improvement of U-value c) As for b) but with an airgab between coverplate and PV cells for additional improvement of U-value The results are calculated at standard conditions used for characterisation of thermal collectors, namely G=800 W/m², V=5 m/s, and Ta=293 K

4.1.1 Performance in terms of energy and exergy When the various types of PV/T collectors are to be evaluated, a principal question arises – what is the value of the electricity versus the heat form the collector? The consumer’s rate of electricity and heat is to a large extent a politically determined value, and if those rates were used the results would not be universally valid. We have therefore chosen to present two key figures for each collector: 1) The total energy yield per year for the Danish Test Reference Year (TRY). The results are calculated from the 1st. law of thermodynamics, known as the energy efficiency. 2) The total exergy per year, which is the part of the energy that could theoretically be converted to work in an ideal Carnot process. The results are calculated from the 2nd. law of thermodynamics, known as the exergy efficiency.

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If economy is calculated on basis of case 1, the cost of electricity and heat is thus the same, while for case 2 the electricity is given a much higher value than the heat. The reality will be somewhere in between these two extremes.

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The energy efficiency is calculated as ηthermal =

Qthermal G

η power =

Q power

ηtotal =

G

Q thermal + Q power G

While the exergy efficiency is calculated as ε thermal =

Qthermal ⋅ (1 − 293K /( 293K + ( T − Ta ) G ε power = η power =

ε total =

Q power G

Q thermal ⋅ (1 − 293K /( 293K + ( T − Ta ) Q power + G G

Where Q power = η el ,ref ⋅ ( 1 − β ref ⋅ ( T − Ta )) ⋅ G

Qthermal = ( η0 − U L ⋅

( T − Ta ) ) ⋅G G

The constants and the calculated variables for use in the above expressions are found in table 4.1

G η0 UL ηel , ref β ref

T

No coverplate

15 mm acrylic coverplate directly on PV cells

800 W/m2 0,6101 14,8192 12,5 %

800 W/m2 0,6236 8,3618 12,5 %

15 mm acrylic coverplate with an airgab between coverplate and PV cells 800 W/m2 0,6100 7,2165 12,5 %

0,005 0,005 0,005 293 K 293 K 293 K Table 4.1. Constants and variables for use in efficiency expressions

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Exergy Efficiency of PV/T collector

Energy Efficiency of PV/T collector 0,16

1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0

0,14 0,12 0,1 0,08 0,06 0,04 0,02 0

0

10

20

30

40

50

60

70

80

0

10

20

30

T-Ta Thermal

40

50

60

70

80

T-Ta

Electrical

Total

Thermal

Electrical

Total

Figure 4.1 Efficiency of PV/T collector without coverplate Table 4.1 and the curves in Figure 4.1 show that the high heat loss coefficient due to the low thermal resistance of glass results in a low thermal performance at temperature above 20K over ambient temperature.

Exergy Efficiency of PV/T collector

Energy Efficiency of PV/T collector 0,16

1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0

0,14 0,12 0,1 0,08 0,06 0,04 0,02 0

0

10

20

30

40

50

60

70

80

0

10

20

30

T-Ta Thermal

Electrical

40

50

60

70

T-Ta

Total

Thermal

Electrical

Total

Figure 4.2: Efficiency of PV/T collector with 15 mm acrylic coverplate directly on PV cells

In case b) the thermal performance is improved for temperatures above 10K over ambient temperature. The electrical output is almost the same for the two, because acrylic has very good optical transmission properties so the thickness is not a serious drawback.

80

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Exergy Efficiency of PV/T collector

Energy Efficiency of PV/T collector 0,16

1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0

0,14 0,12 0,1 0,08 0,06 0,04 0,02 0

0

10

20

30

40

50

60

70

80

0

10

20

30

T-Ta Thermal

Electrical

40

50

60

70

80

T-Ta

Total

Thermal

Electrical

Total

Figure 4.3: Efficiency of PV/T collector with 15 mm acrylic coverplate and airgab between coverplate and PV cells

If an air gap is introduced between absorber/PV cells and the coverplate, the thermal performance is slightly improved, but not to the level of modern solar collectors with a selective coating. All simulations were made with a back insulation of 30 mm mineral wool and an ideal heat transfer from solar cells to the collector fluid. It is clear that the practical performance depends on the system the collectors are a part of. Assuming that the PV part is grid connected, and all produced power therefore is useful, it is mainly the temperature level and thermal storage size that determines the actual yield.

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5. Simulations 5.1 Collector parameters and characteristics After the initial calculations - presented in chapter 4 of this report - showed a great potential of PV/T collectors the next step is to identify the most promising design of PV/T collectors regarding main fields of application: •

Water PV/T collectors for hot water.

•

Air PV/T collectors for warm air.

The detailed mathematical models have been set for the above PV/T units and the following userfriendly computer programs have been developed: (i) (ii)

Subroutines within TRNSYS Program for performance simulation of the PV/water-heating and the PV/air-heating components (developed at TI) Visual Basic program for detailed simulations developed at NOVATOR Advanced Technology Consulting

In order to identify optimal design of the PV/T components multi-parametric analyses were carried out. Following parameters have been taken into consideration: • • • • • • •

Absorptance of absorber (PV cells front surface) Emittance of the absorber (PV cells front surface) Quality of the thermal contact between the PV cells and the absorber Finn efficiency factor Back insulation thickness Temperature coefficient of PV cells Efficiency of PV cells

The results of this analysis are presented in chapter 6. After identification of optimal design, the effect of these parameters on system performance (i.e. efficiency curve) for two basic types of collectors (selective and non-selective absorber) has been analysed. The conclusions on the results extracted from annual energy simulations are also outlined in chapter 6 of this report. With respect to air PV collectors, two different assumptions are made: •

The PV cells are situated on the coverplate acting as an absorber and fully absorb incident radiation,

or • The PV cells are situated on the coverplate acting as an absorber and are partly transparent to incoming radiation.

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In the analysis of energy yield we consider the three most important energy yields of the PV/T collector as follows: • Electrical energy extracted all over a typical year • Availability of thermal energy for space heating delivered by PV/T collector • Availability of thermal energy for water heating delivered by PV/T collector – using air/water heat exchanger A multi-parameter analysis has been carried out using two reference constructions of air PV/T collectors. The first reference assumes a small light transparency for the solar spectrum and the second reference assumes a light transparency of 80%. The first strategy was to determine the airflow rate, which leads to maximum annual yield for a given PV/T collector design. After determination of an optimum airflow rate for a particular system, the next step is determination of optimal design parameters, which will lead to a maximum energy yield. In chapter 6, the focus is on the PV/T collector itself and therefore, the analysis has been based on energy contribution assuming constant temperature levels for inlet fluid in water PV/T collectors. For air PV/T collectors (once-through-flow units) the profile of inlet air temperature is equal to ambient air temperature according to Danish Reference Year. Finally, computation of typical system yields under real load consumption has been carried out. These results are presented in chapter 7.

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6. Simulation of the PV/T Performance with TRNSYS 6.1. PV/T Collector for Water Heating The basic structure of a hybrid thermal and photovoltaic collector (PV/T) is shown in Figure 6.1. The PV cells are in thermal contact with the absorber. A fraction of the solar energy incident on the collector surface is transformed to electricity while the remaining part of the solar energy is transformed into thermal energy in the same manner as for a conventional solar thermal collector.

Coverplate

Frame

Sealfilm PhotoVoltaic Cells Absorber Fluid Circuit Insulation Back wall

Power point Fluid connection

Figure 6.1: Construction of a PV/T Collector for Water heating [1]

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6.1.1. Theoretical Model of the PV/T - Collector Collector thermal performance is usually described by the useful heat output as a function of input radiation and collector operating temperature relative to its surroundings. When the equation of the performance curve is known for a specific solar collector, the system designer has the information needed to employ any of several recognized computational techniques to predict the daily, seasonal, or annual energy output of the collector under the anticipated use conditions of the system being analyzed. The analytical derivation of this efficiency is briefly reviewed in the next section expression ([2] Hotel, Whillier and Bliss). The General PV/T Collector Performance Equation In order to characterize the performance of a PV/T solar collector properly, an energy balance must be performed which considers all the energy flows to and from the collector. For a typical glazed flat-plate solar collector with PV cells as absorber, this balance may be expressed as q useful = q solar − (q loss + q heatcapacity ) − q el

where quseful qsolar qloss qel qheatcapacity

= the rate at which useful energy is delivered = the rate of solar radiation absorbed = the rate of energy loss from the collector to the environment caused by convection, infrared radiation, and conduction = the rate of ele ctrical energy extracted from the collector = the rate of energy storage within the collector

The qsolar term is a function of the optical properties of the collector coverplate and absorber surface (PV-cells). The qloss term depends upon how well the collector is thermally isolated from its surroundings. qheatcapacity is a function of mass and type of collector’s materials. The General PV/T-Collector Performance Equation with Heat Capacity A modified multi-node collector model [3 Bosanac et al. 1993] has been used for this analysis. The model has the following features: -

The collector is modeled with distributed capacities in flow direction. A linear dependence of the heat loss coefficient on the surrounding air speed as well as on the temperature difference between collector and ambience is assumed. Incident angle modifiers for beam (as a function of incident angle) and diffuse irradiance are used.

In this section the main features of the model are briefly described. Each node of a flat-plate collector is characterized by: C n dTn = An F ′[( τα )e G − U L ( Tn − Ta )] − qel − qu ,n dt

where G is the equivalent normal irradiance taking into account irradiance components multiplied by respective incident angle modifiers:

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G = K beamGbeam + K diff Gdiff + K albGalb

UL is the overall heat loss coefficient: U L = U o + U v v + U T ( Tn − Ta )

qu,n is the rate of energy gained by the collector node: q u ,n = mc C p ( Tn − Tn−1 )

(τα)e is the transmittance-absorptance-product at normal incidence. An incidence angle modifier for beam irradiance is defined by the modified Ambrosetti equation ([4] Ambrosetti 1983): θ  K beam( θ ) = 1 - tan1/r   2

The incident angle modifier for diffuse irradiance assuming isotropic distribution is used as derived in ([3] Bosanac et al.1993). The incident angle modifiers for diffuse irradiance and for albedo are assumed to be equal. They are both derived in ([3] Bosanac et al.1993) as a function of the parameter, r. Hence, the following parameters fully characterize the presented model: • • • • • •

The optical efficiency of collector array, F ′( τα )e . The overall heat loss coefficient if Tn = Ta and v = 0, Uo. The coefficient characterizing wind dependence of overall heat loss, Uv. The coefficient characterizing temperature dependence of overall heat loss, UT . The total thermal capacity of collector array, C. The incident angle modifier coefficient, r.

As the temperature of the collector mass increases or decreases during the day, energy is stored or released from the collector thermal mass. In the most cases, collector heat capacity effects can be neglected, yielding in quasi-steady-state conditions and simplified form of equation: q useful = qsolar − q loss − qel

The collector thermal output, quseful, may be represented graphically by a second order curve as shown in Figure 6.2 The ordinate intercept of the curve equals the maximum output, which is achieved when the collector is delivering energy at the ambient temperature. The unavailable portion of the energy falling on the collector is that which is reflected from the coverplate or absorber, or absorbed by the coverplate.

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Figure 6.2: Efficiency curve of the collector (tm = mean fluid temperature and ta = ambient temperature)

As useful heat is removed from the collector at higher temperatures the efficiency decreases, since losses from the absorber increase in proportion to its temperature above its surroundings. Radiation accounts for a significantly greater proportion of the losses at elevated operating temperature from some collectors than the others. A second or higher order efficiency equation is often used to describe the performance of collectors in which radiation causes significant heat loss at higher operating temperatures. Energy into the Collector The net rate of solar radiation absorbed by a collector, qsolar, is a function of the radiation on the coverplate and the optical and radiative properties of the materials constituting the coverplate and absorber. Since no real glazing material is perfectly transparent, one part of the radiation coming on the cover is absorbed and the other part is reflected by the glazing material; only a fraction is transmitted through the cover. The transmitted fraction is partly absorbed by the absorber and partly reflected back toward the cover; this reflected radiation is again partly transmitted through the cover, partly absorbed by it, and partly reflected back to the absorber. The result of this multiple absorption, reflection, and transmission is only a fraction of that radiation is ultimately absorbed by the collector. The parameter that quantifies the capability of the collector to absorb solar radiation is called the effective transmittance-absorptance product, (τα)e. The description "effective" is important while the energy absorbed is primarily a function of τ, the transmittance of the glazing(s), and α, the absorptance of the absorber plate surface, the complex interactions discussed above modify the product (τα)e, in complex ways, particularly in collectors which have two or more glazing layers. Using (τα)e, the net rate at which incoming solar energy absorbed by a collector may be expressed as q solar = GAa (τα )e

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where G is the total incident radiation per unit area, measured in the aperture plane of the collector, and Aa is the collector absorber area. The dependence of the collector performance on the physical properties of the cover and absorbers is more complex than suggested by the above discussion, since these material properties also influence the radiative heat losses from the absorber plate. In this respect the wavelength dependence of the radiative properties of the cover and absorber materials can be used advantageously in solar collector design. The ideal glazing material (at least from the thermal performance point of view) should be transparent to solar radiation in order to maximize the transmitted fraction of the incident insulation, but opaque in the infrared region where the radiative heat losses from absorber plate occur. Glass is one material, which exhibits these characteristics quite well; glass is virtually opaque to radiation of wavelengths higher than 3 µm, but highly transparent to solar radiation, while many plastic materials, particularly films, are transparent to both solar and infrared radiation. Similarly, absorber plate materials are available which have high absorptance in the solar spectrum to maximize the useful absorbed radiation, and low infrared emittance, in order to minimize radiative heat losses. Materials that exhibit such behavior are called selective. A selective absorber is defined here as a material achieving an absorptance in the solar spectrum of at least 0.85 while having an infrared emittance no greater than 0.6. Collectors using selective absorber surfaces will generally have lower heat loss (qloss) than those using conventional nonselective absorbers, particularly at higher collector operating temperature. In the case of the PV/T collector, PV cells act as non-selective absorber having coefficient of emittance of approximately 90%. Heat Losses from the Collector Thermal losses from solar collectors occur in three ways: conduction, convection and radiation. Heat losses due to conduction are usually negligible, unless poor collector design or construction results in the collector case or mounting structure coming into direct thermal contact with the absorber or inlet and outlet piping. Convective losses are a linear function of the temperature difference between the collector glazing and ambient air. These losses can be substantial due to the effects of wind on the outer glazing. Within the collector, convection also transfers heat to the glazing(s) from the absorber. Radiative losses are relatively small at conventional domestic water or space heating temperatures. However, since these losses are a function of the difference between the fourth power of the absorber absolute temperature and the sky absolute temperature, which is usually several degrees lower than the ambient air temperature, radiative losses may become significant at higher operating temperatures. Although convective and radiative losses occur from all of the exposed surfaces of the collectors, it is a common practice to express the overall heat loss from the collector as a function of the absorber area, Aa. This is because the major radiative and convective losses from a well-insulated collector occur primarily through the glazing. For both experimental and analytical purposes, it is common to combine the convection and radiation heat transfer terms to yield a single heat loss coefficient based upon the temperature difference between the average collector absorber plate temperature and the ambient temperature. When this is done, the radiation heat transfer term must be linearized. Thus; U L = F´ ( U convection+U radiation)

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and Qloss = U L Aa ( T p - T a )

where Uconvection and Uradiation are the overall convection heat transfer coefficient and the linearized overall radiation heat transfer coefficient, respectively. Tp is the average absorber temperature.

Useful energy from the collector By substituting the expressions developed previously for qsolar and qloss, the equation describing the quasi-steady performance of a solar collector becomes: q useful = An F ′[( τα )e G − U L ( Tn − Ta )] − q el

Electrical energy extracted from collector may be approximated by a linear function of irradiance and temperature difference Tn-Ta. q el = An E0 [(τα )e G − Et ( Tn − Ta )]

where Eo is the efficiency of PV cells and Et is their temperature coefficient. So we can write: q useful = An [( F ′( τα )e − Eo )G − ( F ′U L − Et )( Tn − Ta )]

Let BL = F ′U L − Et and ηo,e = F ′( τα )e − E o , then the above equation can be written as q useful = An [ηo ,eG − B L ( Tn − Ta )]

The new parameters BL and ηo,e represents the effective heat loss coefficient and the effective optical efficiency, respectively.

These equations are the basic equation used in developing analytical models to describe collector performance. The useful energy delivered to storage or load by a collector can also be determined experimentally by measuring the inlet and outlet collector temperatures, the properties of the heat transfer fluid, and the mass flow rate of that fluid through the collector; thus Quseful = m& C p ( T o - T i )

where m& = fluid flow-rate through the absorber Cp = specific heat capacity of the fluid

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To = outlet fluid temperature Ti = inlet fluid temperature

Collector Efficiency Thermal and electrical efficiency of the PV/T collector should be measured separately. Thermal efficiency is measured according to the ISO 9806-1 Standard and Electrical efficiency by measurements of I-U curves by capacitive load. The thermal efficiency of a flat-plate solar collector is defined as the ratio of the useful heat delivered by the collector to the total solar radiation intercepted by the colle ctor: η=

actual usefulenergy collected solar energy intercepted by the collector

By substitution, the collector efficiency becomes: η = ( τα )e - U L ( T p - T a )/G

The difficulty in using this equation is that the average temperature of the absorber plate, Tp, is usually unknown. The system designer does, however, know or can estimate with reasonable accuracy the temperature of the fluid entering the collector, Ti, since the fluid temperature either approximates that which comes from storage or the supply water main. Therefore, in order to provide a more useful expression for collector efficiency, Whillier rewrote this equation substituting Ti for Tp, and introducing the collector heat removal factor, FR, to compensate for the reduced heat losses. Hence η = F R [( τα )e - U ( T i - T a )/G]

FR represents the ratio of the actual useful energy gain to the maximum possible useful energy gain. The maximum possible useful energy gain in a solar collector occurs when the whole collector is at the inlet fluid temperature Ti. In such an ideal case, heat losses to the surroundings are at a minimum. Obviously, FR cannot exceed unity; water collectors commonly have FR values between 0.7 and 0.9. Analogously, if we apply mean collector fluid temperature the efficiency becomes: η = F ′[( τα )e - U L ( T m - T a )/G]

where Tm = Ti+

To-Ti 2

If F′ , FR, U, and (τα)e were constant, the collector efficiency would be a linear function of the reduced temperature difference T*.

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η =η0 − F ′U LT *

where T* =

Tm − Ta G

In practice this it not exactly true, because UL varies with fluid and air temperature, (τα)e is influenced by the relative proportion of beam, diffuse, and ground-reflected radiation, and FR is weekly dependent on UL. Hence, a higher order correlation between η and T* is more realistic. η =η 0 − a1T * − a 2GT *

2

where a1 and a2 are fitting constants. The ordinate intercept, ηo, represents a measure of the ability of the collector to absorb solar energy and to transfer it to the collector fluid. If electrical energy is extracted from collector, this ability is reduced to ηo,e in accordance with the first law of thermodynamics. The slope of the equation, F′U L , represents a measure of the ability of the collector to prevent heat losses to the surroundings. If electrical energy is extracted from the collector, this ability is improved to BL.

6.1.2. Simulation Program The performance of a PV/T collector depends on design parameters and on various weather and operating conditions, e.g. irradiance, ambient temperature, wind velocity, inlet fluid temperature, etc. Therefore it is necessary to develop simplified theoretical model in order to carry out multiparameter analysis. A module for the TRNSYS simulation program was developed in order to describe the behavior of this collector and compute the electrical and thermal yields of the system. The developed program is accompanied with user friendly interface to help those without any experience to perform the required computations. The user can simply vary the design parameters to see the response of the system under study, accordingly. The basic principle of the simulation program built as a subroutine in the TRNSYS program is shown in Fig 6.3. The iterative procedure for computation of plate and cover temperature is repeated for case without and with electrical load. The user friendly screen output is shown in figure 6.4.

EFP report Combined photovoltaic/thermal solar collectors and their potential in Denmark

Co mp utat ion of Collec tor Efficiency an d Plate T emp era ture w/o PV L o ad with estimated p late and c over temp era tures

T p-Tp'