Design of a water PVT system integrated into prefabricate concrete

Abstract

Hybrid photovoltaic/thermal collectors (PV/T) are devices that convert solar energy simultaneously into thermal and electric energy. As a general rule, they consist of common PV panels refrigerated by means of a suitable coolant fluid, that is collected and made available for appropriate uses. Thus, one can improve the electric efficiency of the PV cells while recovering low-grade heat; hence, PVT collectors are expected to show very interesting energy performance. In this study, a preliminary analysis was carried out to determine the size of the cross section, the depth of the water duct, the thick and the material of the absorber-plate maintaining the overall performance at a satisfactory level. By means of a thermo-fluid dynamic modeling software Comsol Multiphysics it was also possible to determine the temperature distribution throughout the PVT collector . In addition, this work shows the results of an experimental campaign, carried out on a prototype of PVT collector. The prototype consists of two panels, where a stainless steel absorber plate is cooled by means of a water flow circulating into square channels. On the top side of the absorber plate, polycrystalline PV cells are applied by lamination. The prototype has been tested during the summer 2013 in the premises of a factory situated near Catania. In order to get controlled operating conditions, the panels have been fed with water at constant temperature, thanks to an appropriate control system. The main parameters acquired during the measurement campaign were: absorber plate temperature, inlet and outlet fluid temperature, available solar radiation and outdoor air temperature. Now, from the examination of the experimental data, and from the simulations carried out with a mathematical model implemented on Mathcad, it is possible to identify too high temperatures on the absorber plate, as well as a quite low efficiency for the heat recovery. This is due to a series of constructive faults, and mainly to the bad contact between the absorber plate and the channels where the coolant flows. For this reason, the mathematical model has been modified, in order to introduce an additional thermal resistance. A parametric analysis has allowed to tune the model and to identify the value of this thermal resistance that minimizes the discrepancy between simulated and experimental results. Starting from the results of this investigation, it was possible to construct a new PVT panel lacking any construction fault, and whose performance are very promising and significantly better than the prototype tested so far. Finally, in this dissertation, the Second Law analysis of a real PVT and an improved PVT collectors are presented to study a crucial problem for the optimal exploitation of this technology: since the electricity production from PV cell is favoured by low temperatures, whereas the usability of the thermal energy gets higher at high temperatures.