Advancements in hybrid photovoltaic-thermal systems: performance evaluations and applications

Due to European Directives (2010/31/UE on buildings energy performance, 2009/28/CE on the use of renewable energy, 2012/27/UE on the energy efficiency) the electric and thermal energy needs of new and retrofitted buildings are faced by increasing percentages of renewable energy. Solar energy and heat pumps are the most promising technologies mainly in residential buildings as they have reached great maturity. Anyway, in most cases solar energy utilizations systems are thermal (which convert solar energy to thermal energy) and photovoltaic (which convert solar energy to electricity) used as separated collectors. Commercial photovoltaic modules have nowadays an efficiency around 15 % - 18 %. It means that the most relevant part of solar radiation is lost. Such a remark gets more importance if the active surface is located in an urban environment, where the availability of surfaces exposed to the sun is scarce if compared to the buildings thermal loads. PhotoVoltaic / Thermal cogeneration (PV/T) aims to utilize the same area both for producing electricity and heat. As solar cells are sensitive to temperature (their efficiency lowers when temperature increases), heat is beneficially collected but it cannot be available at high temperatures. Many researches on performances and characteristics of different hybrid photovoltaic–thermal technologies and systems have been carried out during the last years to face this problem; among these designs, systems utilizing air, liquid, heat pipes, phase change materials, and thermoelectric devices to aid cooling of PV cells. This paper provides a description of the applications of the photovoltaic–thermal systems, such as building integrated PV/T, concentrating PV/T systems and photovoltaic–thermal heat pump systems. Several factors affecting the performances and characteristics of the photovoltaic–thermal systems are also summarized.