Theoretical and Numerical Investigation of Flow Stability in Porous Materials Applied as Volumetric Solar Receivers

Kurzfassung

Hot spots have been experimentally observed in porous materials in several applications as volumetric solar receivers. In this application, cold ambient air flows through a solid, which is heated by concentrated solar radiation. After that the hot porous material transfers the heat to the cold air flow. The heated air then transfers its heat to an air-water heat exchanger. The evaporated water drives a conventional steam turbine process. To obtain high efficiencies, a high absorptivity in the visible and near infrared range has to be combined with a high porosity of the volumetric solar receiver to create large surfaces for convective heat transfer from the solid absorber to the fluid. For the durability of the receiver materials it is of key importance to avoid the occurrence of hot spots. Following studies showed that these hot spots have been caused by flow instabilities, which generally may occur, if cold air flows through a heated porous material. This phenomenon is due to the temperature dependent viscosity properties of air. In a theoretical analysis it could be shown that heat conductivity as well as permeability properties of the porous materials have significant influence on the probability of the occurrence of flow instabilities. A numerical study has been performed to investigate the occurrence of instable flow in heated ceramic foam materials. In the simulations a constant heat flow of radiation, that is absorbed in a defined volume, and constant permeability coefficients are assumed. Boundary conditions similar to those of the 10MW Solucar Solar project have been chosen. In a three dimensional, heterogeneous two phase heat transfer model it was possible to simulate local overheating of the porous structure. The parameters heat conductivity, turbulent permeability coefficient and radial dispersion coefficient have been varied systematically. Consequently, for a heat flux density of 1 MW/m2 a parameter chart could be generated, showing the possible occurrence of “instable” or “stable” thermal and fluid mechanical behaviour. These numerical results are beneficial for the design of optimized materials for volumetric receivers. In order to be able to make use of these results, a precise knowledge of the mentioned quantities heat conductivity, turbulent permeability coefficient and radial dispersion coefficient is necessary. Experimental methods to determine these quantities are described or reviewed.