Attaining Optimum Tilts of Flat Solar Surfaces Utilizing Measured Solar Data: Case Study for Ilam, Iran

The efficiency of the flat solar surfaces, such as thermal collectors and photovoltaic (PV) panels, depends on the received beam and diffuse radiation components and the angle of the beam incidence. In this study, based on the long term measured data tilt angle optimization of the flat solar surfaces in the city of Ilam, Iran, for the daily, monthly, seasonal and annual fixed adjustments have been performed. Study of characteristics of solar radiation showed that Ilam has 220 days in the year with horizontal global radiation in the range of 15-35 MJ/m2 and 182 sunny or very sunny days having daily clearness index higher than 0.6. The optimum tilt for the fixed daily adjustment throughout the year varies between zero and 61° and the optimum tilt for the fixed monthly adjustment varies between 0° in June and July up to 60.1° in December. The optimum tilt for the fixed annual adjustment is 26°, which is close to latitude of Ilam (33.38°). Furthermore, the optimum tilt for the fixed seasonal adjustment in winter and autumn are 12.3 and 20° higher than the latitude of Ilam, respectively; whereas in spring and summer they are 30.3 and 21.1° less than the latitude, respectively.