Performance of a hybrid solar collector system in days with stable and less stable radiative regime

The aim of this work is to analyse the performance of a solar energy collector system for water and air heating in real working conditions. Two coupled mathematical models have been developed. One of them describes the thermal behaviour of the Hybrid Solar Collector (HSC) and the second one describes the simultaneous operation of the HSC and of a fully mixed water storage tank. The dependence of the performance of the HSC system on the following three parameters has been studied: (1) water and air mass flow rate; (2) water pipe diameter and air channel height; (3) water storage tank volume. The mathematical models were used to evaluate the HSC system performance during 29 different days, covering all four seasons. A higher water flow rate generally enhances the thermal efficiency of the HSC system, but the enhancement became significantly smaller at higher air flow rates. Positive but small values are recommended for the difference between the fluid temperature at solar collectors exit and the water temperature in the tank. The thermal efficiency of the HSC system is higher on nearly clear sky and decreases in case the amount of clouds on the sky increases.