DNI and Slant Path Transmittance for the Solar Resource of Tower Thermal Solar Plants: the Validation of the ASoRA Method and Impact in Exploiting a Global Data Set

The ASoRA method consists in estimating the solar resource in tower concentrated solar plants by exploiting global data sets. The solar resource parameters are not only the Direct Normal Irradiance (DNI) but also the slant path transmittance (Tsp). The column to surface-level conversion process was validated by comparing the retrieved aerosol extinction coefficient (AEC) to independent measurements performed by a ground-based nephelometer. The Aerosol Robotic NETwork (AERONET) provided column-integrated aerosol extinction properties. The European Centre for Medium-Range Weather Forecasts Reanalysis Interim boundary layer height was used to reproduce the seasonal dependence of the aerosol vertical profile. At the daily resolution, ASoRA reproduced observed AEC with a relative root mean square difference (rRMSD) of 33% and a correlation coefficient of 0.90. The spectral extrapolation from monochromatic measurements to the solar broadband was validated by comparing computations and observations of DNI. ASoRA was also able to reproduce the hourly variability of DNI observed by a pyrheliometer with a RMSD of 38 W/m(2), a rRMSD of 4.6%, and a correlation coefficient larger than 0.98. Despite larger uncertainties caused by using Modern-Era Retrospective analysis for Research and Applications (version 2) instead of AERONET, the annual average of Tsp was underestimated by only 0.01, and the annual average of the irradiance loss in the slant path by only 4 W/m(2). Largest impact on Tsp comes from the parameterisation of the aerosol vertical profile, and the annual average of Tsp was 0.945 at Ouarzazate (Morocco) in 2014, with an estimated uncertainty of 0.030.