11 Solar irradiance and PV power spectrum
The power generated by solar power plants has an intermittent character, mainly because of atmospheric effects such as insolation variability resulting from cloud movement. However, differences exist between thermal andtechnologies and their response to changes in insolation. One example involves generally low PV plant ‘inertia’ when compared with thermal systems which generally have a level of inherent thermal storage. Similarly the reliance of concentrating solar systems on direct irradiance can be contrasted with non-concentrating PV’s acceptance of global irradiance. Further, consideration of the effects of shading and partial shading indicate that the larger the PV plant, the longer it takes for a cloud cover spread to shade the entire field. The following sub-section explores an analytic model developed to establish a relationship between solar irradiance and a PV plant’s output power. This model is capable of estimating the PV plant’s output power behaviour based on historical or forecasted irradiance data and the size of the PV plant.