High Spatial Resolution Solar Atlas in Provence-Alpes-Côte d'Azur

Abstract

Considering the spatial resolution of satellite-based databases of surface solar irradiation such as HelioClim-3, the geographic scale of solar mapping that can be derived is approx. 1 / 5 000 000. This geographic scale is suitable for continent or country solar mapping. Nevertheless, customers' feedback notably states that there is a need of spatial resolution improvement. In this context of spatial resolution improvement of solar resource estimation, the project solar atlas in Provence-Alpes-Cote d'Azur (PACA), finalized in December 2010, aims at increasing spatial resolution of HelioClim-3 to derive 250 m resolution solar maps. The solar atlas accounts for the global, direct and diffuse on inclined plane or in normal incidence irradiations and their temporal variations intra and inter-annual, on a monthly basis, since February 2004. The Digital Elevation Model SRTM-V4 is used to get topographic data with a spatial resolution of approximately 100 m and a localization accuracy of typically 10 m. This topographic data is used to estimate high resolution terrain effects on HelioClim-3 derived solar radiation fields, taking into account orographic effects: * Effects of optical path length variations due to terrain elevation; * Shadow effects on direct and circumsolar diffuse radiation computed from local horizon estimation; * Horizon effects on isotropic diffuse radiation with the sky view factor estimation. These orographic phenomena are indeed the source of significant local spatial variations in solar radiation values and potentially discriminating in choosing the location, the calculation of profitability or the sizing of photovoltaic or thermodynamic solar power systems. Solar mapping with a geographic scale of 1 / 3 000 000 or more does not represent properly these local phenomena. In order to establish an accurate mapping at 250 m with associated uncertainty using HelioClim-3 data, procedures of calibration has been established with meteorological ground stations. The global horizontal irradiation is calibrated thanks to the Meteo France meteorological network comprising, in PACA, about 30 automatic ground stations delivering at least daily global irradiation on horizontal plane. A strict data quality control has been applied to these ground data to guarantee their quality as reference ground-based daily irradiation data. The diffuse and direct irradiations are calibrated thanks to three specific ground stations dedicated expressly to the project, delivering one year of simultaneous 10-min global and diffuse irradiations on the horizontal plan and direct irradiation on normal incidence. Statistical analysis of direct and cross-validation estimation errors after calibration with the pyranometric ground stations has been made to establish the uncertainties associated with the solar atlas. The root mean square error of monthly irradiation estimation is about 5% (c. 5 kWh / m2) for global, direct and diffuse components on horizontal plane, without significant bias error. The accuracy of monthly direct normal irradiation is about 10% (c. 15 kWh/m2). Data from the solar atlas in PACA is available for free at www.atlas-solaire.fr. In particular, a Web Map Service (WMS), based on the Open Geographical Consortium standards, has been set up to deliver maps from the solar atlas in an interoperable and standard way.