Measurements and Model Evaluation of Direct Normal Irradiance in Central Spain
Abstract
The availability of hourly irradiance data is very important in the assessment of solar energy resources. This quantity is estimated for hourly global solar irradiance recorded in most radiometric stations. Measurements of global, diffuse, direct normal irradiance (DNI) and meteorological variables have been recorded and evaluated in an rural area northwest Spain. Daily solar radiation data show a large annual cycle with a maximum in June when the influences of summer solstice occur. Maximum hourly and daily direct normal irradiance occurs in June 2011, 1006.5 W m-2 and 43.70 MJ m-2, respectively. Daily clearness index varies between 0.79 from June to August and 0.2 in winter. It has been observed that DNI tends to show much variability than global solar irradiance in December and January months as consequence of cloudy weather associated with low-pressure systems. The most representative statistical DNI indices (mean, median, standard deviation, maximum, minimum, quartile, percentiles coefficient of quartile variation) has been evaluated. In addition four models have been selected for predicting hourly direct normal solar irradiance from global solar irradiance. Model validation has been performed by comparison with experimental data by means of two statistic estimators, root mean square error and mean bias deviation.
Keywords
Direct normal irradiance, Clearness index, Dni modeling, Model validation