General formula for estimation of monthly average daily global solar radiation in China

Solar radiation and sunshine duration data from 69 meteorological stations in China was used to develop the formula for estimating the monthly average daily global radiation on a horizontal surface. Several models and correlations that embrace such variables as the fraction of sunshine duration, the latitude and the altitude have been selected, tested and compared to decide which model is recommended on the basis of statistical error tests. In this study, using the relative duration of sunshine as a single independent variable, the most accurate equation that is expressed in a third order form was obtained for each station. The results show that the coefficients are site dependent. Subsequently, taking the radiation data and geographical parameters at all 69 stations as reference, a countrywide general equation was established. Compared with the equation using the relative duration of sunshine as a single independent variable, the general equation shows relatively lower accuracy, but estimating the global solar radiation by this equation has proved sufficiently reliable. This equation can be used to estimate the monthly average daily global radiation in areas where the radiation data is missing or not available, so that a geographical distribution of global solar radiation across the country may be obtained.