Increasing the temporal resolution of direct normal solar irradiance series in different climatic zones

A precise knowledge of the high-frequency variation of the incoming Direct Normal solar Irradiance (DNI) is required for an accurate design and operation of Concentrating Solar Thermal Power (CSTP) plants. The time resolution of the numerical weather prediction models or satellite derived solar irradiance data are typically limited to 1-h (at best 15-min). Unfortunately, this resolution is not sufficient in the design and performance of a CSTP plant, which shows a nonlinear response to DNI governed by various thermal inertias due to their complex response characteristics. In this study, a new methodology has been developed to increase the temporal resolution of DNI series from 1-h to 1-min. This methodology is based upon the nondimensionalization of the daily DNI curve by the clear-sky envelope approach and uses the solar radiation data obtained in the one-year measurement campaign to characterize the DNI high-frequency dynamics at a given site. The evaluation of the method with 2 years of measured data in different climatic zones has resulted in KSI(%) (Kolmogorov-Smirnov test Integral parameter) and normalized root mean square deviation values below 23% and 22% respectively for each month, with negligible bias. Indicators of overall performance show an excellent agreement between measured and modeled 1-min DNI data for each month: average values for Nash-Sutcliffe efficiency, Willmott index of agreement and Legates coefficient of efficiency are found to be 0.90, 0.97 and 1.0, respectively.