This paper presents an unsteady two-dimensional analysis of a flat-plate solar collector subjected to time varying insolation with considerable diffuse components as a better characterization of practical solar collectors. The analysis considers the thermal masses of the absorber, tube, glazing, and the working fluid in the system, as well as the different optical and thermodynamic properties of beam and diffuse radiation. Using two sets of insolation data, one for a near clear day and the other for an overcast day, an exergetic optimization of the system was carried out and flow rates which maximize the total exergy output (extracted and accumulated) were determined for flow update periods of once every 30 min, 1 h, and the entire daylight period. The instantaneous optimum flow rates were found to follow the insolation pattern. On a daily basis, the optimum exergetic efficiencies and optimum flow rates were almost independent of the choice of the interval of fluid update, but were about 30% and 10% respectively, higher for the clear day than for the overcast day.
Solar Energy 01/1993; 50(1-50):85-95. DOI:10.1016/0038-092X(93)90010-L