Abstract
Accurate solar radiation estimates in Alpine areas represent a challenging task, because of the strong variability arising from orographic effects and mountain weather phenomena. These factors, together with the scarcity of observations in elevated areas, often cause large modelling uncertainties. In the present paper, estimates of hourly mean diffuse fraction values from global radiation data, provided by a number (13) of decomposition models (chosen among the most widely tested in the literature), are evaluated and compared with observations collected near the city of Bolzano, in the Adige Valley (Italian Alps). In addition, the physical factors influencing diffuse fraction values in such a complex orographic context are explored. The average accuracy of the models were found to be around 27% and 14% for diffuse and beam radiation respectively, the largest errors being observed under clear sky and partly cloudy conditions, respectively. The best performances were provided by the more complex models, i.e., those including a predictor specifically explaining the radiation components’ variability associated with scattered clouds. Yet, these models return non-negligible biases. In contrast, the local calibration of a single-equation logistical model with five predictors allows perfectly unbiased estimates, as accurate as those of the best-performing models (20% and 12% for diffuse and beam radiation, respectively), but at much smaller computational costs.
Highlights
The accurate knowledge of solar radiation locally available at the Earth’s surface is essential for the proper sizing, design, and dynamic simulation of solar energy systems [1]
While July, August, and September are characterized by the lowest daily diffuse fractions, the months of November, December, and January show in general the highest values, partly due to the lower solar elevation and the more relevant associated orographic shadows
When the error statistics are evaluated in terms of hourly irradiances rather than of a-dimensional coefficients, diffuse horizontal irradiation (DHI) displays higher errors under clear-sky conditions than under partly cloudy conditions (RMSE = 37%), while relative errors for direct normal irradiation (DNI) are comparable with those calculated for kb
Summary
The accurate knowledge of solar radiation locally available at the Earth’s surface is essential for the proper sizing, design, and dynamic simulation of solar energy systems [1]. The solar radiation models predicting beam and diffuse components from more commonly available quantities are called “decomposition models”. They predict beam and diffuse radiation from global radiation measurements only, but they might include additional predictors (e.g., astronomical and geometrical parameters or other meteorological quantities). These models were developed following the pioneering work of [11], and are generally based on empirically-determined correlations between the clearness index kt (the ratio between observed global radiation and extraterrestrial radiation) and the diffuse fraction kd (the ratio between observed diffuse radiation and global radiation), or, less often, the beam transmittance kb (the ratio between observed beam radiation and extraterrestrial radiation). In addition to the above empirically-inferred models, a quasi-physical model (the DISC model) was developed by [20]
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