Abstract
A generalized analytical model was developed to determine the optimum tilt angle of flat-plate solar collectors for any location worldwide. The developed analytical model was found to be directly related to the latitude φ of the location under consideration, the declination angle δ of the earth axis, and the day hour angle at sunset at any day throughout the year. The optimum tilt angle β opt was mathematically expressed by β opt = φ – tan−1[{tan δ(πω s /180)}/sin ω s ]. This model was applied to Riyadh, Kingdom of Saudi Arabia. The results were compared with those calculated by NASA, and showed very good agreement.