Theoretical and numerical investigation of flow stability in porous materials applied as volumetric solar receivers

Abstract

This article reports results of a theoretical analysis as well as a numerical study investigating the occurrence of flow instabilities in porous materials applied as volumetric solar receivers. After a short introduction into the technology of volumetric solar receivers, which are aimed to supply heat for a steam turbine process to generate electricity, the general requirements of materials applied as solar volumetric receivers are reviewed. Finally, the main methods and results of the two studies are reported. In the theoretical analysis it is 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. The numerical study has been performed to investigate the occurrence of unstable 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/m 2 a parameter chart could be generated, showing the possible occurrence of ''unstable'' or ''stable'' thermal and fluid mechanical behaviour. These numerical results are beneficial for the design of optimized materials for volumetric receivers.

The use of graphite foams for simultaneous collection and storage of concentrated solar energy

“…The thermal diffusivity of this low density foam is 1.2x10 -4 m 2 .s -1 which is very close to that of pure copper, however the density of the material is 25 times less. These properties make them ideal candidates for use in solar receiver design [28]. The first objective of this investigation is to assess the performance of pitch derived foams as heat transfer medium for use in concentrated solar designs.…”

Open Volumetric Air Receiver Based Solar Convective Aluminum Heat Treatment Furnace System

“…The porosity of the designed receiver is 52% that compares well with Solair 200 [16]. Such receivers are usually made of material with high thermal conductivity, like, Silicon-carbide (~100-300 W/mK) in temperature range from 200-600ᵒC [17,18,19]. In the designed OVAR brass is used with comparable thermal conductivity for design evaluation only.…”

Heat transfer simulation in a thermochemical solar reactor based on a volumetric porous receiver

“…For a gas phase reaction, the study made by Valdés-Parada et al [17] is another example of optimization of the reactor size by means of this kind of technique. In the field of volumetric solar receivers or catalytic monoliths, the heat transfer to the reactor volume and the temperature homogenization are targeted [18][19][20][21][22][23][24].…”