Solar high-temperature direct water splitting — a review of experiments in France

Abstract

Direct thermal decomposition of water vapour by solar energy is potentially an attractive means of producing hydrogen. A coordinated program on this topic has been conducted in four laboratories of the Centre National de la Recherche Scientifique (CNRS) during the past decade. The purpose of this paper is to review the results of this program and to propose new orientations for further research. Experiments described in this review illustrate various different approaches to solve the two major problems of thermal water splitting, i.e. the energy transfer to water molecules and the separation of the products. The methods used to transfer energy to water molecules are the following: - Bulk heating of water vapour in a furnace cavity. - Direct contact of cold vapour with a hot solid plate. - Direct contact of liquid water with a hot solid plate (in this latter case a vapour film takes place at the liquid-solid interface). Product separation has been performed either within the hot gas or at low temperature. - High temperature separation has been carried out by extracting oxygen across a semi-permeable membrane made of a solid electrolyte material (the ternary system ZrO 2 CeO 2 Y 2O 3) in a temperature range of 1800 to 2300 K. - Low-temperature separation by conventional techniques was made possible by using the “quenching method” that consists in cooling down the mixture of reacting gases in a time short enough to prevent significant recombination. The quenching effect provides a convenient way to stabilize the composition of the dissociated vapour. Efficient quenching has been obtained by various techniques: cooling by using auxiliary jets of cold gas, auto-cooling of the hot products by the jet of cold stream, or auto-cooling by direct contact with liquid water. In the experiments that are discussed here the hydrogen production was significant, but the overall efficiency never reached figures above few per cent. There is a need of further work to develop a better understanding of some basic mechanisms such as the dissociation of molecules impinging on a high-temperature solid target, and to investigate new efficient high-temperature separation techniques.