Solar Hydrogen – Status and Trends

Abstract

A promising way to use heat from carbon-free sources like concentrated solar is to drive chemical reactions for the production of hydrogen for different sectors like the transport sector. Recent developments offer attractive medium- to long-term prospects for the provision of hydrogen and synthetic fuels. The direct use of solar heat to split H2O to H2 and O2, or CO2 to CO and O2, is possible. However, these single-step processes require operating temperatures above 2000°C, placing high demands on materials and process conditions. To reduce these extremely high operating temperatures and to eliminate the need for high temperature gas separation, present research activities focus on alternative thermo-chemical cycles. The most prominent examples are cycles from the so-called sulphur family, the hybrid sulphur and the sulphur-iodine process, cycles applying metal – metal oxide pairs or multi-valent metal oxides, and low-temperature cycles like the hybrid Cu-Cl cycle. The present analysis gives an overview on recent developments and the state-of-the art of those cycles, has a look on the most important performance parameters involved, and gives an outlook on further potential and necessary developments. In addition we look at the solar upgrading of hydrocarbons, high temperature electrolysis and on solar recycling processes.