Differences between along-wind and cross-wind solar irradiance variability on small spatial scales

In this paper we investigate the relationship between the downwelling solar irradiances measured at pairs of stations in a unique dense radiometer array on Oahu Island. The nearly constant wind direction at this site allows us to examine irradiance variability in the along-wind and cross-wind directions separately. Over 13 days dominated by broken clouds, the correlation between the ramp time series from radiometer pairs is found to depend on the separation and relative orientation of the stations as well as the time interval over which the ramps are computed. In general, correlations are highest over short distances and increase with the averaging time. The degree of smoothing achieved by combining the ramps from a pair of stations depends on the correlation of their ramp time series, which in turn depends on their separation and the orientation of the axis between them. For 60-s ramps, correlation drops more rapidly with distance for along-wind station pairs than for cross-wind pairs, so that greater smoothing is achieved for stations oriented in the along-wind direction. A mechanism for this alignment-dependent behavior is proposed and supported by additional analysis. The results of this study highlight the importance of accounting for local meteorological conditions when designing individual or distributed photovoltaic systems. (C) 2012 Elsevier Ltd. All rights reserved.