Optimization study of volumetric receivers for high temperature solar plants

Nowadays, due to the growing green energy demand, one of the most promising renewable source is represented by solar radiation. Concentrated solar power (CSP) could be an excellent choice, also as alternative technology for centralized energy production at large scale. Within CSP technologies, Central Receiver Towers (CRT) are one of the most interesting applications. CRT consists of a solar tower plant, mirroring surfaces (heliostat) that concentrate sunlight on a receiver and an associated thermodynamic cycle. The solar receiver is a key element of the overall technology and converts the solar radiation into heat, delivering it to a forced flow of air. Two different typologies are investigated in this paper: a porous ceramic absorber and a hierarchical receiver patented by FBK, an independent research institute in Trento, Italy. The purpose of this work is to develop an optimization tool that may generate practical guidelines for the receiver design; this task is performed by studying the potential maximum efficiency as a function of geometry (shape and dimensions) and material properties. The core of this tool is a simplified mono-dimensional model, representing both types of receiver considered in local thermal non equilibrium. The S4 discrete-ordinates approximation is employed, requiring less computational effort and ensuring at the same time accurate simulations. Examining the porous receiver, the optimization results indicate that, with a linear variable porosity, the conversion efficiency increases, for a selected pore diameter and a given thermal conductivity. The optimization of the hierarchical receiver combines four layers, each one coupled with the following one in terms of radiative and conductive heat transfer. The results suggest that the performance is substantially influenced by the behaviour of the first layer. The efficiency can be improved by a proper balance, a better optical efficiency and an enhanced convective transport.