Optimization of solar thermal fraction in PVT systems

Over the last years there has been a growing interest in hybrid Photovoltaic-Thermal (PVT) collectors for their applications in building integration. The hybrid systems integrate the features of the photovoltaic and the solar thermal (water or air) systems in one combined product/system. The PV electricity production in a hybrid system could be significantly different from the one of a standard PV module because, mainly, cells temperatures change according to the amount of heat removed by the absorber of the PVT system and, moreover, to the insulation level of the PVT system. This last factor is related to many parameters, among which it is possible to identify water flow rate and temperature, which are directly related to PVT plant configuration and size as a function of users heat demand. Starting from these considerations, the aim of this paper is to calculate the optimal value of solar fraction f for hybrid PVT systems, under energetic end economic point of views, and to find a correlation between the percentage of heat demand covered by the PVT system and photovoltaic cells temperature. In fact, changes in solar fraction imply different average cells operating temperatures and consequently, variation in total energy efficiency. For this purpose, simulations of liquid-based PVT systems for domestic application have been performed through TRNSYS energy simulation tools, carrying out subsequently a detailed energetic and economic analysis.