In a PV-T collector, a thermal absorber and the PV cells are integratedinto a single module. The sunlight that is not converted to electricity, andnot lost to the surroundings, is transferred to the rear thermal absorber andto the fluid. As a result, both heat and electricity are generated from thesame panel. The same considerations apply to PV-T regarding the varietyof module designs as for thermal collectors. In a conventional PV module,90% of the sunlight is absorbed, but only ~15% of this is converted to usefulelectricity . The rest is partially stored as thermal energy (which is why PVmodules can reach temperatures up to ~80 C during operation on hot days)and ultimately lost to the environment. In a PV-T module, a useful fractionof the heat loss is instead transferred to a fluid stream at the back of thesolar cell resulting in a thermal output that can be used in various applications.The efficiency of silicon solar cells drops by 0.4% per C temperaturerise from 25 C . If heat is required at low temperature (e.g., for heat pumpsor pool heating at ~40 C) the PV module is cooled down by the fluid sothat the module itself operates at a higher efficiency, making the electricaloutput of the PV-T system higher than that of an equivalent conventionalPV system. This means that on a sunny day, when the module temperaturereaches 80 C, a PV module with an 18% peak efficiency at the standardtemperature of 25 C will operate with an efficiency of about 14% if no coolingtakes place. A PV-T module increases the longevity of the PV cells asthese are operated at lower temperatures, especially in applications such asswimming pools, hotels, heat pumps and underfloor heating, which requirea large amount of low-temperature heat. This benefit arises from the factthe solar cells in PV-T collectors suffer lower temperature stresses, which areknown to give rise to major causes for PV system failures due to cell breakage,encapsulation discoloration and delamination. Moreover, PV-T systemsenable ‘self-consumption’ – the generation and use of electricity on site. .PV-T systems can be integrated with heat pumps or cooling systems andthe electricity generated in excess could be stored in the integrated thermalstore, or in the ground to be reutilised by ground-source heat pumps111-115. Careful planning of the energy use (demand-side management) mayalso be important in the effective operation of the system. Even though PVTsystems combine the benefits of solar thermal and PV technologies, only asmall number of manufacturers are producing PV-T systems and the marketremains small.