A mathematical model for determining the optimal reflector position of a double exposure flat-plate solar collector

A double exposure, flat-plate solar collector (DEFPC) can absorb solar irradiation from both its upper and lower absorber surfaces (LAS). Absorption from the LAS is accomplished using a flat-plate reflector placed below and parallel to the collector. This paper presents a mathematical model for determining the optimum reflector position of the DEFPC in the condition where the LAS is fully irradiated. Compared to other models, this model enables the calculation of the instantaneously irradiated area of the LAS for arbitrary finite dimensions of the reflector and the collector, their arbitrary mutual positions and at any position of the sun in the sky. The optimum reflector positions were obtained by simulating the model in FORTRAN for the spring (autumn) equinox and the winter and summer solstices. The simulations were performed for the optimal yearly position of the collector at 44° N Latitude (Kragujevac, Serbia) and for equal dimensions of the collector and the reflector whose minimum dimensions allow the full irradiation of the LAS. The model was experimentally verified, and the range of the reflector movement during a single year, as well as the optimal reflector dimensions for minimum movement, was determined.