Improved methodology to evaluate clear-sky direct normal irradiance with a multi-pyranometer array
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2015 Elsevier Ltd An improved methodology to estimate the clear-sky direct normal irradiance based on an anisotropic clear-sky model using a Multi-Pyranometer Array (MPA) is analyzed in this paper. The MPA is a static platform with four sensors on surfaces at different tilt and azimuth angles, which measured solar irradiance can be used to estimate the normal incident component without the tracking devices that involve more detailed installation and maintenance. The array's sensors are of the photovoltaic type, which require both the spectral and angle-of-incidence corrections. Based on the general expressions for the solar radiation on each of the MPA sloped surfaces and using a hybrid model that includes an anisotropic sky diffuse component, in this work an improved procedure is presented that estimate the normal direct solar irradiance by grouping two expressions of a couple of MPA sensors and analytically look for a satisfactory solution; one set for the horizontal and south sensors, and another for the east and west sensors. The solution for the east and west sensors is expanded to two new solutions by mirroring their readings according to the solar noon. A final switching scheme based on the expressions denominator size provides the satisfactory direct normal irradiance estimation. The analysis of the results for the analyzed days showed that the root mean square error (RMSE) decreased by around 1443% in comparison with previous results reported in the literature. The proposed methodology is a promising new approach for MPA.
