Measuring and Modelling the BRDF of Solar Receiver Paints

We describe the angular distribution of the reflected radiation for a coating used on central receivers: Our aim is to provide a model which is able to describe the bidirectional reflectance distribution function (BRDF) obtained with a 3-dimensional scanning photo-goniometer. Two different reflection models from computer graphics were tested for their ability to fit the measurements, and we found that the Cook-Torrance model and a modified version of the He-Torrance model are suitable choices. The first model is simpler but partly empirical, the latter gives a good description of the data, using a set of 6 physically interpretable parameters. The deeper understanding of the dependence of reflectance on angles and surface properties can help to improve numerical receiver simulations and thus optimization of receiver geometry and plant design and operation, for instance applying camera-based flux monitoring and control. We expect that our findings can serve as a sound starting point for the physical modelling of other absorber coatings currently being developed.