Model for measuring concentration ratio distribution of a dish concentrator using moonlight as a precursor for solar tower flux mapping

Abstract

Measuring solar flux distribution (also called flux mapping) of a large receiver is quite challenging. Lunar flux mapping measures the illuminance distribution on the receiver aperture and the direct normal lunar illuminance during moonlight concentration experiments to determine the concentration ratio distribution (CRD). This paper presents a new lunar flux mapping model to extend the applicability to parts of the lunar cycle where the moon is not full. A dish concentrator with a similar concentration ratio to a tower concentrator was built in Beijing and used for lunar flux mapping experiments. A general method of backward ray tracing with effective sun/moon shapes for simulation of CRD is developed. The moonshape image and the normalized error image are convolved in two dimensions using the Fast Fourier Transform to give the effective moon shape image. Several optical simulations and moonlight concentration measurements on the concentrator show good similarity in the effects of changes in light source shape between solar and lunar CRD images. This model recognizes the potential of a solar concentrator to enhance the similarity between the solar CRD and a lunar CRD and that the residual differences can be compensated to some extent by using the smoothing filtering of the lunar CRD image to approximate the expected solar CRD image. The cosine similarity between lunar and solar CRDs is a function of the cosine similarity between the corresponding light source shapes, which can be derived from the dish concentrator and shows promise for application to a large solar tower system.

Keywords

• solar tower concentrator
• moonlight concentration
• flux mapping
• solar dish
• concentration ratio distribution
• smoothing filter
• cosine similarity