A Theoretical Framework for Utilizing Long-Term Measurements of Radiation and Clouds for Solar Energy Research

Abstract:

One of the greatest challenges facing solar energy research is accurately predicting global horizontal irradiance (GHI) for photovoltaic (PV) modules and direct normal irradiance (DNI) for concentrated solar power (CSP) plants at the surface with a high temporal resolution. Addressing this solar energy challenge is tied ultimately to the quantitative relationship between the direct, diffuse and total radiation reaching the surface and clouds in the atmosphere, which remains elusive. Here we will first introduce a theoretical framework that relates DNI and GHI to cloud fraction and cloud albedo through two dimensionless numbers: the relative cloud radiative forcing for the direct radiation (defined as the difference between the clear sky and all sky direct downwelling radiative fluxes normalized by the clear sky direct downwelling radiative fluxes) is primarily determined by cloud fraction; the ratio of the relative cloud radiative forcing for the total downwelling radiation to the relative cloud radiative forcing for the direct radiation is primarily determined by cloud albedo. We then use decade-long measurements of partitioned radiation and cloud properties at the ARM sites to validate the theoretical relationships between the two dimensionless radiation-based parameters and cloud properties. We will also explore the potentials and challenges of using the relationships between cloud properties and radiation partition at the surface for solar energy research, opening new avenues to utilizing ARM measurements.