Comparison between measurements and Meliss simulations of the direct solar irradiance for a winter month in Brasov

Abstract

When aiming to increase the solar share in the energy demand of a building in the cold season, one solution is to increase the surface of solar thermal collectors but this could negatively affect the system during summer by overheating. Thus, an accurate estimation model during clear sky winter days is needed. The paper analyzes the measured values of the direct solar irradiance in the months of November from 2013 to2016 and, based on the daily received direct solar energy and the variability of the direct solar irradiance, the days are classified in four categories: clear sky days, partially clear sky days, partially cloudy days and cloudy days. In the entire monitoring period of four years (2013-2016) resulted 11 clear sky days for which, the measured values of the direct solar irradiance are comparatively analysed with the direct solar irradiance simulated with Meliss clear sky model. Further on, a statistical analysis is performed for the time interval 8:00-16:00 to evaluate absolute, relative and root mean square errors between the measured and simulated values. The results show that the simulation model underestimates, in nine out of the eleven clear sky days, the solar direct irradiance in the central part of the day. The measurements were performed in the Renewable Energy Systems and Recycling (RESREC) Research Centre located in the R&D Institute of the Transilvania University of Brasov, Romania.