H. I. Villafan-Vidales*, C. A. Arancibia-Bulnes, U. Dehesa-Carrasco

Centro de Investigation en Energia, Universidad National Autonoma de Mexico, A. P. 34, Temixco 62580,

Morelos, Mexico.

Corresponding Author, hivv@,cie. unam. mx
Abstract

The radiative heat transfer in a solar thermochemical reactor, for the thermal reduction of cerium oxide, was simulated with the Monte Carlo method. The model takes into account, in a detailed manner, the directional characteristics and the power distribution of the radiative energy that enters to the reactor. It is considered that the reactor contains a gas/particle suspension directly exposed to concentrated solar radiation. The suspension is treated as a non-isothermal, non-gray, absorbing, emitting, and anisotropically scattering medium. The optical properties of cerium oxide were obtained from Mie-scattering theory. From the simulations, the aperture radius and the particle concentration were optimized to match the characteristics of the concentrator.

Keywords: Thermochemical reactor, Solar concentrator, Cerium oxide, Radiative transfer.

1. Introduction

Water-splitting thermochemical cycles constitute one of the possibilities for hydrogen production. Greenhouse gas emission can be avoided in these cycles, if solar radiation is used as energy source. Some of them are based on two-step metal-oxide redox pairs like the ZnO/Zn cycle, which has been studied extensively in connection with solar energy utilization [1]. In a recent publication, Abanades and Flamant [2] have demonstrated experimentally a new cycle based on cerium oxide. This cycle consists of the endothermic reduction of Ce2O3 at high temperature (2300 K), where solar energy is used as a source of heat; and of the subsequent steam hydrolysis (700-800 K) of the resulting cerium sesquioxide to produce hydrogen, as in the equations below

2CeO2 (s) ^ Ce2O3 (s) + A O2 (AH = 198 kJ/mol at 2300 K) (1)

Ce2O3(s) + H2O (g) ^ CeO2(s) + H2 (g) (AH = -125kJ/mol at 700 K) (2)

One of the main advantages of this process is that the reduced oxide remains in the condensed phase, while oxygen is released and transferred to the surrounding gas, and the reverse reaction is not carried out during the material quenching. The opposite happens for instance in the reduction of zinc oxide, where zinc and oxygen are produced simultaneously and can recombine if cooling is not efficient [2].

Solar thermochemical reactors are used in the endothermic step of the above cycles. These reactors sometimes feature cavities containing directly irradiated reacting particles. In particular, as in the case of CeO2, cavity receivers may help to compensate for the very low solar absorption of the material particles (scattering albedo of 0.75), although initial demonstrations have been carried out in an all glass reactor. To propose a reactor design that deals with this problem it is necessary to carry out a radiative modelling.

In this work, radiative heat transfer is analyzed within a cylindrical cavity solar reactor by the Monte Carlo method [3]. The reactor is assumed to contain a gas fluidized suspension of radiatively participating CeO2 particles. The model takes into account the radiative characteristics of the particles (absorption and scattering cross sections) as predicted from the Mie scattering theory, and by using the optical properties of CeO2 measured over the UV, visible, and infrared spectrum. The opening of the cavity is assumed subject to concentrated solar radiation from a paraboloidal concentrator.