Relating solar resource and its variability to weather and climate across the northwestern United States

The spatiotemporal variability of the solar resource has implications for every stage in the development and operation of a photovoltaic power plant from siting and design through grid integration and day-to-day operation. The irregular and relatively unpredictable portion of this variability is primarily caused by the passage of clouds and large-scale weather systems. In this study, we examine the variability of solar irradiance at five locations in the northwestern US at temporal scales ranging from 5 min to 9 days in the context of local and regional weather patterns. The total solar resource in this area exceeded expectations, with values as high as 4.5 kWh/m2/day. The magnitude of the seasonal cycle of the solar resource at these locations was found to be greater than in other regions of the US due to the strong seasonality of cloudiness. Seasonal differences were also seen in the variability of measured solar irradiance down to sub-hourly time scales. Site-to-site differences in the magnitude of irradiance fluctuations could be traced to local meteorological differences, but also depended on temporal scale, reflecting the contrasting influences of overall cloudiness and the smaller scale spatial characteristics of cloud structure. The degree to which combining irradiance time series from pairs or groups of diverse locations smoothed temporal irradiance variability mainly depended on the magnitude of the variability of the original data at short time scales but on the separation of the sites for daily averages. This distinction can be traced to the negligible/appreciable correlation of sub-hourly/daily irradiance fluctuations measured at these widely separated (250-850 km) sites, which again reflects differences between the spatial and temporal extent of weather systems as opposed to their underlying cloud structure. Each of these results demonstrates how knowledge of local and regional meteorology can provide insights into the character of the solar resource at a particular location.