Norihiro Kawasaki

Modeling of Solar Spectral Irradiance Data from Cloudless to Overcast Skies

Solar spectral models applicable to partly cloudy and overcast skies as well as cloudless skies are developed by the analyses of standing spectral observation data in Japan. To characterize solar spectral irradiance in detail by decomposing a global spectrum into a direct and diffuse spectrum, not only global spectra but also diffuse spectra have been observed. The average photon energies of the diffuse spectra were changed largely by the cloud cover in contrast to the little dependence in the direct spectra, which confirms the importance of separate observations. Spectral models describing the effects of clouds on the direct and diffuse spectra were derived; these models were demonstrated by regression analyses of the observations. Additionally, we have revealed a relationship associating Rayleigh scattering intensity in diffuse irradiance with the diffuse broadband irradiance and the solar altitude, which permits us to estimate the model parameter from the available parameters.

Spatial interpolation of the solar irradiance; A study from the smoothing effect of irradiance fluctuations

In the near future, short time fluctuations of the power generation from grid-connected photovoltaic (PV) power generation systems will become an important issue in the utilities. The smoothing effect that the fluctuations of the power generation from scatteringly installed PV power generation systems cancel out is well known. In this study, we have demonstrated the smoothing effect for short time fluctuations from analyses of the solar irradiance in an area of an about 10km square. Additionally, the spatial interpolation of the solar irradiance is also discussed. A good agreement between the measurements and the predictions of the spatial interpolation is obtained in fluctuation cycles longer than 8 minutes.