Assessment and development of diffuse irradiance models for application within the UK

The ratio of direct to diffuse radiation directly impacts on solar system design. Currently no diffuse fraction models have been specifically developed for the UK; development of existing global models could lead to increased accuracy within Loughborough and the UK. Data from Loughborough was used to test existing global models and linear square regression used to optimise model coefficients. Two successfully optimised models were combined with sensitivity testing to develop a much simplified Loughborough and UK specific di ffuse fraction model; this model reduced error over global models by 75% at the Loughborough test site and 25% at a second test site in Lerwick. It has been demon strated that the model developed within this study should be used within the UK for diffuse fraction prediction. The methodology used has been shown to be beneficial in characterising sites; this may enable enhanced short term prediction of the diffuse fra ction at measurement sites, depending on local environmental factors such as humidity and solar elevation angle.