Measurement of DNI Angular Distribution with a Sunshape Profiling Irradiometer

Abstract

A low cost, field-deployable instrument for retrieving the radial profile of solar radiance across the sun´s disk and trough the circumsolar region is described. A prototype of the sunshape Profiling Irradiometer (SPI) has been built by modifying a stepping.motor-based Rotating Shadowband Irradiometer (RSI) with straghtforward changes to the control and data collection program and mechanical modifications that are novel but simple. A slit, rather than circular, reciever aperture is disposed on the bank axis such that penumbra crosses the aperture with a very small angualar displacement of the shadowband. because a single detectzor with diffusing fore-optic is used to generate the sunshape profiles, calibrationa nd soliling problems commonly associated with high resolution cameras are avoided. The circumsolar ratio (CSR) of the generated SPI trajectory is estimatedby finding the best match of the normalized measured profile to the family of simulated profiles using a single-parameter model. Results of CSRs obtained by this inverse method are compared to teh broadband CSR given by the sun aureole measurement system and sun photometer. Applications of SPI include optimizing concetration rations of CSP plants based on local CSR distiribution. adjusting normal-incidence- and IAM-test results for the direct normal irradiation (DNI) within a concentrator´s acceptance angele, and inferring aerosol distributions for climate studies or as input to remote sensing models.