Porous materials as open volumetric solar receivers: Experimental determination of thermophysical and heat transfer properties

Porous solids like extruded monoliths with parallel channels and thin walls made from various oxide and non-oxide ceramics, ceramic foams and metal structures have been tested in the past with the objective of applying them as open volumetric receivers in concentrated solar radiation. In this application, ambient air flows through the solid, which is heated by concentrated solar radiation. A heat exchanger then transfers the energy to a conventional steam turbine process. In all cases, to obtain high efficiencies, high absorptivity in the visible and near infrared range has to be combined with a high porosity to create large surfaces for convective heat transfer from the solid absorber to the fluid. However, it can be shown that especially high performance absorbers tend to be sensitive to inhomogeneous flux distributions, which may cause local overheating of the material. In various tests with specific kinds of materials, flow instabilities occurred, which partly leads to hot spots and a sudden destruction of the receiver. To achieve both high efficiencies and reliable operation, an optimised combination of geometrical as well as thermal conductivity and heat transfer parameters has to be selected. A precise knowledge of these quantities for a number of various materials is necessary to estimate the limits for stable flow conditions on the basis of complex numerical simulation programs. Finally, efficiency and performance tests with candidate materials have been carried out. In this paper, the experimental work on a variety of porous materials is reported. The paper will report on methodology and results of thermal conductivity, convective heat transfer coefficient and efficiency measurements of these monolithic materials. It will also present an experimental set-up designed to investigate how the properties of the porous materials affect flow stability. Based on these results, a recommendation for the design of volumetric absorbers will be given.