Spatial and temporal variability of incoming solar irradiance at a measurement site in Hawai'i

Widespread deployment of photovoltaic (PV) energy systems is currently limited by insufficient knowledge of the temporal variability of incoming solar irradiance across the scales of an individual utility-size (1 MW to 100s of MW) PV plant to a distributed network of such plants. This variability, which is a function of meteorological conditions, is extremely important to high penetration of PV on the power grid. Here we explore this variability using data from an extended measurement site on Oahu Island.

Global horizontal irradiance has been measured by 17 instruments covering an area of approximately 1 km square beginning in March of 2010. Temporal statistics have been computed for each individual instrument and for various instrument groupings covering areas typical of photovoltaic arrays. Differences between the irradiance time series have also been computed as a function of instrument separation. Since it is not practical to install instrument arrays at a large number of locations, a main goal of this work is to determine the extent to which measurements at a given site can be used to represent a larger area. Results of analysis addressing this question under weather conditions typical of the lee side of a Hawaian island will be presented. Such a location represents an extreme in spatial and temporal variability of surface radiation because of the high occurrence of cumulus clouds. It is expected that lessons learned from such a setting can be employed in other regions where variability is lower.