A cloud shading direct solar radiation model



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Texas Tech University


The Crosbyton Solar Power Project (CSPP) was initiated to study the feasibility of using hemispheric bowl solar collectors for power generation. A non-spectral direct solar radiation (DSR) model was developed to aid in determining whether there exists a preferred spacing of these solar collectors based solely on meteorological considerations. The DSR model is applicable to the Northern Hemisphere and, with a few adjustments, to the Southern Hemisphere. The DSR model considers the reduction of direct insolation through the atmosphere due to Rayleigh scattering, uniformly mixed gases, ozone, precipitable water, and aerosols. It incorporates geographical information along with temperature, dew point, barometric pressure, and visibility data, updated every 15 minutes.

This clear sky DSR model was verified against actual direct insolation data. Propagating cloud field algorithms were added to this model to yield the cloud shading DSR model. A variety of cloud patterns typically observed in the West Texas area were chosen to be modelled. Specified parameters are speed and direction of propagation, cloud base height, size, and transmissivity- Six solar collector array patterns were tested. Each array consisted of four 100-foot radius collectors, spaced differently in each array. For a given cloud pattern, a cloud shading DSR model run was made for each collector array.

The model output includes, for each collector, the amount of time shaded and the average direct insolation for the day, and, for the array, the percentage of time any number of collectors were simultaneously shaded. The majority of model runs were for 60% cloud coverage. Additional runs for 15% coverage led to extrapolation of results for all non-clear and non-overcast sky conditions. It was concluded that the closest possible spacing of the collectors, in any pattern, was desirable. This coincided with engineering considerations. The use of the clear sky DSR model in an early warning network is discussed.