Design Parameters Influence on Flux Distribution through Prismatic Channels of Volumetric Absorbers

Marcelino Sanchez, Maria J. Marcos, Manuel Romero, CIEMAT, High Solar Concentration Systems, Avenida Complutense 22, 28040 Madrid, Spain

Claudio Estrada, CIE-UNAM, Aptdo. Postal 34, CP 62580 Temixco, Morelos, Mexico

The application of monolithic volumetric receivers to solar processes is increasing due to their optimal optical-thermal properties. At high temperature, the volumetric absorber design means that the effective area for solar absorption is many times larger than the receiver aperture area, this effect leads to a minimization of radiation losses. For thermal and photochemical applications monolithic catalysts have significant advantages in lowering pressure drop and improving both chemical and photon contact surfaces. For those and other applications the precise calculation of photons penetrability as a function of monolithic parameters such as pitch length, wall thickness, and material reflectance, has to be performed in order to choose the optimal configuration. Photon trap efficiency and thermal performance is not at all trivial for this 3D structures where contradictory effects may appear regarding “volumetricity”. The authors analyze the influence and the relationship between that parameters. A sensitivity analysis of photons penetrability is performed by means of ray tracing techniques and discussed in connection to three-dimensional CFD simulation of heat transfer efficiency and thermal losses for a single channel. The results obtained suggest that many experimental experiences with volumetric receivers reported until now are noticeable away from a theoretical highly volumetric structure and that operational limits still remain unexplored offering enough room for design improvements.

Introduction

Porous volumetric absorbers have a large potential for use in receivers of solar thermal power plants, high-temperature catalysis or hot air supply in industrial process heat. Early solar prototypes of several-hundred kW were developed by Sanders in the USA in late seventies [1] and Sulzer in Europe in middle eighties [2]. Since then, the more extensive operational experience in the field has been collected at the Plataforma Solar de Almeria where more than 20 different absorbers and receivers have been tested within the range between 200 to 3,000 kW [3]. The list of materials and porous structures tested includes high-temperature metallic alloys and high thermal conductivity ceramics composing wire mesh absorbers, fibres, foams, fins and monolithic channels [4].

In general the volumetric concept offers experimental absorber efficiencies higher than 80%. In spite of the high temperature at the working fluid, above 700°C, the high efficiency is due to the photon trap effect of the porous medium. Theoretically, the porous material of the absorbing surface and the gradual absorption through the porous matrix acts as a black body and leads to much lower temperature gradients between the wall temperature (solid) and the thermal fluid. Solar flux is high at the cold air inlet (front surface) and it is low at the hot air outlet. This means that both air and wall temperatures are higher at the inner portion of the absorber, leading to lower IR radiation losses.

One of the options offering substantial benefits as volumetric absorbers are monolithic matrixes made of prismatic channels. Indeed monolithic structures are commonly used in photo-catalytic applications because of their outstanding properties as photon/heat exchangers [5]. Monolithic solar absorbers made of metallic foils or ceramic honeycomb

have significant advantages in lowering pressure drop and improving both chemical and photon contact surfaces. These makes them especially useful for receiver designs with large flowrates and treatment capacities, and subsequently for scaling-up the volumetric receiver technology. In addition, prismatic channels allow to compartmentalize air flow patterns in individual ducts so that high degrees of “volumetricity” can be achieved but controlling fluid-dynamics by means of appropriate modular designs [6].

A good design of a monolithic volumetric absorber must necessarily be associated to a balance between photon penetrability, absorber porosity, radiation losses and profile of the heat transfer to the fluid. Even though experimental results have demonstrated the feasibility of producing hot air at temperatures above 700°C with absorber thermal efficiencies higher than 80%, it is also evident that current designs can be significantly improved in terms of solar radiation penetration, heat transfer efficiency a nd minimization of radiation losses. Then, the real operational limits of volumetric receivers still remain unexplored in practice. The present work analyzes the effect of several, sometimes contradictory, parameters like channel dimensions, solar concentrator/receiver aperture view angle, material reflectivity and specularity, on photons penetrability and the corresponding effect in terms of thermal behaviour.