Photovoltaic thermal collectors, typically abbreviated as PVT collectors and also known as hybrid solar collectors, photovoltaic thermal solar collectors, PV/T collectors or solar cogeneration systems, are power generation technologies that convert solar radiation into usable thermal and electrical energy. PVT collectors combine photovoltaic solar cells (often arranged in solar panels), which convert sunlight into electricity, with a solar thermal collector, which transfers the otherwise unused waste heat from the PV module to a heat transfer fluid. By combining electricity and heat generation within the same component, these technologies can reach a higher overall efficiency than solar photovoltaic (PV) or solar thermal (T) alone. Research has gone into developing a diverse range of PVT technologies since the 1970s. The different PVT collector technologies differ in their collector design and heat transfer fluid and address applications ranging from low temperature heat below ambient up to high temperature heat above 100 °C.
PVT markets PVT collectors generate solar heat and electricity basically free of direct CO2 emissions and are therefore a green technology that can supply renewable electricity and heat to buildings and industrial processes. Heat is the largest energy end-use. In 2015, the provision of heating for use in buildings, industrial purposes and other applications accounted for approximately 52% (205 EJ) of the total energy consumed. Of this, over half was used in the industry and around 46% in the building sector. While 72% of the heat was provided by the direct combustion of fossil fuels, only 7% was from modern renewables such as solar thermal, biofuel or geothermal energy. The low-grade heat market up to 150 °C is estimated to be 26.8% of the worldwide final energy demand, which is currently serviced by fossil fuels (gas, oil, and coal), electricity and renewable heat. This is the sum of industry demand 7.1% (25.5 EJ) and building demand 19.7% (49.0 EJ residential and 13.6 EJ commercial). The electricity demand in buildings and industry is expected to grow further due to ongoing electrification and sector coupling. For a significant reduction of greenhouse gas emissions, it is essential that the major share of electricity is sourced from renewable energy sources, such as wind power, solar energy, biomass and water power. The market for renewable heat and electricity is therefore vast, illustrating the market potential of PVT collectors. The report "Solar Heat Worldwide" assessed the global market of PVT collectors in 2019. According to the authors, the total area of installed collectors was 1.16 million square meters. Uncovered water collectors had the largest market share (55%), followed by air collectors (43%) and covered water collectors (2%). The country with the largest installed capacity was France (42%), followed by South Korea (24%), China (11%) and Germany (10%).
PVT collector technology PVT collectors combine the generation of solar electricity and heat in a single component, and thus achieve a higher overall efficiency and better utilization of the solar spectrum than conventional PV modules.
Photovoltaic cells typically reach an electrical efficiency between 15% and 20%, while the largest share of the solar spectrum (65% – 70%) is converted into heat, increasing the temperature of PV modules. PVT collectors, on the contrary, are engineered to transfer heat from the PV cells to a fluid, thereby cooling the cells and thus improving their efficiency. In this way, this excess heat is made useful and can be utilized to heat water or as a low temperature source for heat pumps, for example. Thus, PVT collectors make better use of the solar spectrum. Most photovoltaic cells (e.g. silicon based) suffer from a drop in efficiency with increased cell temperatures. Each Kelvin of increased cell temperature reduces the efficiency by 0.2–0.5%. Therefore, heat removal from the PV cells can lower their temperature and thus increase the cells' efficiency. Improved PV cell lifetimes are another benefit of lower operation temperatures. This is an effective method to maximize total system efficiency and reliability, but causes the thermal component to under-perform as compared to that achievable with a pure solar thermal collector. That is to say, the maximum operating temperatures for most PVT system are limited to less than the maximum cell temperature (typically below 100 °C). Nevertheless, two or more units of heat energy are still generated for each unit of electrical energy, depending on cell efficiency and system design.
Types of PVT collectors There are a multitude of technical possibilities to combine PV cells and solar thermal collectors. A number of PVT collectors are available as commercial products, which can be divided into the following categories according to their basic design and heat transfer fluid:
PVT liquid collector PVT air collector In addition to the classification by heat transfer fluid, PVT collectors can also be categorized according to the presence of a secondary glazing to reduce heat losses and the presence of a device to concentrate solar irradiation:
Uncovered PVT collector (WISC PVT) Covered PVT collector Concentrating PVT collector (CPVT) Moreover, PVT collectors can be classified according to their design, such as cell technology, type of fluid, heat exchanger material and geometry, type of contact between fluid and PV module, fixation of heat exchanger, or level of building integration (building integrated PVT (BIPVT) collectors). The design and type of PVT collectors always implies a certain adaption to operating temperatures, applications, and giving priority to either heat or electricity generation. For instance, operating the PVT collector at low temperatures leads to a cooling effect of PV cells compared to PV modules and therefore results in an increase of electric power. However, the heat also has to be utilized at low temperatures. The maximum operating temperatures for most PV modules are limited to less than the maximum certified operation temperatures (typically 85 °C). Nevertheless, two or more units of thermal energy are generated for each unit of electrical energy, depending on cell efficiency and system design.
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