METHODS TO ESTIMATE TILTED SURFACES GLOBAL AND SPECTRAL IRRADIANCE IN BANGKOK

Abstract

Since 2006, we have been measuring irradiation, spectral irradiance, and other meteorological data for Bangkok, Thailand. This report describes two research results were obtained through research using those meteorological data. The first study was intended to find a method for presuming the amount of global solar radiation of the slope, tilted south by 14°, that can be expected to yield the maximum global irradiance in metropolitan Bangkok. First, we examined six previously reported models to separate horizontal global irradiance into direct and diffusion elements. Four other models were examined to estimate irradiance on tilted surfaces by direct and diffusion elements. The surface was tilted at a 14° angle to the south. The prediction accuracy of the combination of two models was evaluated using the root mean square error (RMSE) values. Results showed the most appropriate combination of those models: a combination of U, W, and Z models with the isotropic sky irradiance model yielded the smallest RMSE value. The second investigation developed a method to estimate spectral irradiance using global irradiation values. Theoretically, distribution of the solar spectrum irradiance is determined by various factors such as airmass, atmospheric pressure, water vapor, solar altitude, turbidity of atmosphere, and the earth's albedo. For variables in this study, we used amounts of the horizontal global irradiation, airmass, rainfall, and precipitable water processed with the sigmoid function. We developed a method for estimating the amount of global irradiance on the tilted surface under optimum condition, and developed a precise estimation method for spectrum irradiance on a horizontal plane for metropolitan Bangkok.