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Back contact modules in PhotoVoltaic Thermal (PVT) applications

Zande, M.R. van de (2015) Back contact modules in PhotoVoltaic Thermal (PVT) applications.

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Abstract:This report describes the development of hybrid systems that consist of a collector which generates both electricity and heat from solar radiation. These systems are called PhotoVoltaic Thermal or PVT and the PVT collector is 'a device in which a PV module is used as a thermal absorber'. Essentially the working principle of a PVT is the same as for a PV module and a Solar Thermal (hence ST) collector. To understand PVT and indicate the performance one must first understand the working principle, build up and performance indicators of both PV modules and ST collectors. Both PV modules as ST collectors operate on solar radiation. For PV modules the incident radiation is absorbed in a semiconducting material which has the intrinsic property to generate a current when illumi- nated. The amount depends on the material and the doping. Because of the photovoltaic effect a potential is built up and the electrical power can be extracted. The module also absorbs radiation above the required band gap. The surplus is turned into heat which rises the temperature of the module when it is limited to dissipate. Under high irradiance levels and high ambient temperature PV modules can reach temperatures above 80 degrees. Because of a negative temperature coefficient the power output is decreased when the temperature is increased. All PV cell technologies have a negative temperature coefficient but the coefficient for wafer-based crystalline silicon is the highest at Beta = -0,45%/K. This could make thermal management or active cooling of these PV modules a fruitful measure. Common ST collectors absorb a large portion of the incident radiation with a metallic absorber sheet. Often this plate has a selective coating which reduces the emission to the ambient in the infrared spectrum. The absorbed radiation increases the temperature of the plate and with a heat removal construction the heat is transferred to a heat transfer fluid. This can be either air or water but most often a mixture of water-glycol is used to prevent freezing and/or boiling. A glazed collector is insulated on the sides and has an additional cover with an airgap above the plate. The airgap greatly reduces heat losses due to convection and radiation at the front. The induced greenhouse effect results in higher operating temperatures and higher fluid outlet temperatures. Unglazed collectors do not have an additional cover and have higher overall heat losses. PVT collectors are split into flat-plate collector, concentrators and ventilated PV facades. This report focuses on flat-plate glazed and unglazed PVT collectors. For PVT collectors the effective transmittance-absorptance product is compensated with the transmittance of the PV glass and encapsulant times the module efficiency at the operating temperature. This means the thermal efficiency is lower for PVT collectors as part of the absorbed radiation is converted into electricity. Glazed PVT collectors can provide large fractions of the domestic hot water (DHW) and space heating (SH) demand. Because of the high overall heat loss unglazed collectors should be used in a solar assisted heat pump system for a similar thermal demand or be used for active cooling. Due to lower operating temperatures the electrical efficiencies for unglazed PVT collector can be much higher. As PVT collectors generate both thermal and electrical power the overall efficiency is high. To more fairly compare the electrical energy with the thermal energy a primary energy savings (PES) coefficient is used which represents the thermal efficiency in a power plant. New developments in PVT include the direct lamination of the PV laminate with the absorber plate, selective coatings, alternative high transparent covers, polysiloxane gel as encapsulant material, ow disruptors in collector, higher contact area with fluid in channel box absorber and finally cost-effective roll bond absorber (add-ons). In addition a market overview was made of PVT. There are 25 manufacturers indicated of which the vast majority is in Europe. Most products are unglazed and still has the traditional sheet-and-tube absorber. Thermal efficiencies range from 45-72% at zero reduced temperature. Electrical efficiencies of these products are between 12%-16% (STC) depending on the glazing. Normally the thermal efficiencies are not given for MPP operation but for 'heat only' modus and are therefore overestimates. Since mid 2013 the Solar Keymark for ST collectors now includes MPP operations for testing PVT collectors. There is no international standard so PVT collectors have to comply with both IEC61215 and 61730 for PV and EN12975 for ST collectors. Several highlights from the market are given. Because of the complexity, unawareness, lack of specialized installers and costs a low number of PVT system have been installed so far when compared with PV modules and ST collectors. The potential market for PVT system are buildings with limited roof area and high energy demands. Because of the trends towards (nearly) net zero housing PVT research (again) received addition funds and should be considered as PVT collectors have the highest yield per unit area. The report furthermore describes the investigation of the potential for back contact modules in PVT ap- plications. The potential market, thermal model, validation, possible improvements and conclusion are not included here due to confidentiality.
Item Type:Internship Report (Master)
ECN, the Netherlands
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Sustainable Energy Technology MSc (60443)
Keywords:Solar, Photovoltaic, Photovoltaic-Thermal, PVT, Back contact, Thermal model, Market overview, Heat transfer coefficent
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