Analytical models for the computation and optimization of single and double glazing flat plate solar collectors with normal and small air gap spacing

► Analytical models of the top heat loss of flat plate solar collectors were developed. ► The models are simple and require no iteration to solve. ► The models are for normal and very small air gap spacing lengths. ► The models accurately model parameters with discrepancies of less than 9%. ► The models are used to optimize design, the performance is improved by 14%.