Comparison of Solar Power Output Forecasting Performance of the Total Sky Imager and the University of California, San Diego Sky Imager

The performance of the Total Sky Imager (TSI) and University of California, San Diego Sky Imager (USI) instruments has been evaluated using correlation and dispersion metrics applied to clear sky index. The TSI was located at a 48MW PV plant in Nevada, USA, and the USI was located at the UCSD campus in San Diego, USA. Distributed pyranometer measurements over several square kilometers were available at both locations to provide spatial averages of irradiance. Clear sky index was forecast for a 15 minute horizon at 30 sec. intervals, and the correlation and Euclidean dispersion were evaluated. A persistence forecast was generated for each location to provide a reference for comparison. For the aggregate dataset, the forecast performance of the USI exceeded that of persistence beyond the 10 minute forecast horizon, whereas the TSI did not show skill over persistence. For the period of study, the USI gave correlations of clear sky index between 0.70 and 0.82, whereas the TSI provided correlations between 0.55 and 0.69. Overall, the mean value and the trend of the correlation results were more consistent for the USI than the TSI on the days evaluated. It is concluded that the USI is a more reliable solar resource assessment instrument for the wide range of atmospheric conditions evaluated.