The influence of sunshape on the DLR solar furnace beam

The circumsolar ratio CSR is often used as a simple description of the relevant solar energy feature of the solar brightness distribution, known as sunshape. The variation of the CSR can affect the performance of solar concentrators. DLR has conducted sunshape studies since the beginning of the design phase of the DLR Solar Furnace in 1992. In addition, in 1996 a mobile sunshape measurement system was developed with which many CSR measurements have been performed since then. These investigations are still going on for statistical purposes in parallel with the operation of the solar furnace. This work shows new and unique data of simultaneous sunshape records and solar furnace beam diameter measurements. For narrow sun conditions with a CSR <1% the solar furnace beam has a diameter of less than 13 cm. Atmospheric conditions with a higher forward scattering can cause the CSR to exceed 10%, sometimes even above 40%. In the latter case the focal beam measurement showed a beam diameter of more than 16 cm. The increase of the focal diameter turned out to be linear with respect to CSR variation. The measured sunshapes were introduced into the ray-tracing model OPTEC, built for solar furnace and heliostat field simulation. The model results for the DLR Solar Furnace flux maps were compared to the measurements. The model does not show the same linear trend for increasing CSR, but the deviation is not too pronounced. The influence of CSR on the focal spot size leads to a nonlinear decrease with direct normal irradiance of the power collected by a predesigned receiver aperture. Depending on the aperture size, an additional decrease of the order of 10-15% will be encountered under higher CSR conditions. (C) 1999 Elsevier Science Ltd. All rights reserved.