Sky Temperature Estimation and Measurement for Longwave Radiation Calculation

Abstract

Building Performance Simulation (BPS) tools rely on different methods to estimate the downwelling longwave radiation incident on surfaces, which can be expressed as an equivalent “effective sky temperature”. The longwave radiation is calculated using regressions based on the temperature, humidity, and cloud cover. This paper assesses the regressions implemented in 3 popular BPS tools (EnergyPlus, TRNSYS, and ESP-r) using measured data for the downwelling longwave radiation obtained from the SURFRAD network for 2 stations in Illinois and Colorado. Results show that the 3 regressions deliver acceptable estimations of the sky temperature. Differences are relatively small, with CVRMSD values in the order of 10 %. The impact on simulation results is also relatively small but not insignificant compared to other factors, especially in the context of inter-model validation exercises. The effective sky temperature is also required to model other energy systems such as unglazed solar collectors. A simulation of a PV/T collector shows that the useful thermal energy output changes by up to 11 % depending on the regression used.