High-frequency irradiance fluctuations and geographic smoothing

Using six San Diego solar resource stations, clear-sky indices at 1-s resolution were computed for one site and for the average of six sites separated by less than 3 km to estimate the smoothing of aggregated power output due to geographic dispersion in a distribution feeder. Ramp rate (RR) analysis was conducted on the 1-s timeseries, including moving averages to simulate a large PV plant with energy storage. Annual maximum RRs of up to 60% per second were observed, and the largest 1-s ramp rates were enhanced over 40% by cloud reflection. However, 5% per second ramps never occurred for a simulated 10 MW power plant. Applying a wavelet transform to both the clear-sky index at one site and the average of six sites showed a strong reduction in variability at timescales shorter than 5-min, with a lesser decrease at longer timescales. Comparing these variability reductions to the Hoff and Perez (2010) model, good agreement was observed at high dispersion factors (short timescales), but our analysis shows larger reductions in variability than the model at smaller dispersion factors (long timescales).

► Fluctuations in solar irradiance at various time and spatial scales were analyzed.
► Irradiance was decomposed using wavelets at timescales from 1-s to 1-h.
► Over 3 km distances ramp rates are uncorrelated at timescales less than 5 min.
► Geographic dispersion dramatically reduces aggregate short-term ramp rates.