Influence of selected solar positions for shading device calculations in building energy performance simulations

Control strategies of building shading devices can achieve important savings in the air conditioning and lighting energy consumption of a building. Calculation procedures of the different solar control devices (overhangs, side fins, etc.) are of special interest in annual energy performance simulation programs, which have to obtain solar gains in hourly time-steps and, more recently, in five minute time-steps. A common approach used to overcome this problem in the past was to select a representative day of a certain period of time and maintain the same results of shading calculations for the rest of the period. Nowadays, the new generation computers have drastically reduced computational costs, but some programs still use the former approach. The present paper assesses the error made by the use of it. Different overhang and side fin typologies and orientations, location latitudes and simulation time-steps have been analysed. 25% of the studied cases have shown annual average relative errors of 5% or greater in the incident solar radiation over the glazing surface. Common configurations, such as south oriented overhangs, show daily deviations of incident solar energy up to 0.48 kW h/m2, with a relative error of 17%, having hourly errors as high as 26%.