Facts and prospects on solar thermochemical processes applying redox materials

Abstract

Thermochemical redox reactions play in essential role in a number of processes for solar energy conversion to store energy or to use it for the production of commodities and fuels. Such reactions can be used to enhance the availability of solar energy in terms of energy transport, of energy demand/supply management and of potential energy related applications. Suitable sequences of endothermal and exothermal thermochemical reaction enable the production of fuels like hydrogen or syngas and other fuels by water- and CO2-splitting as well as the storage of solar energy by breaking and forming chemical bonds in suitable reversible reactions. The energy source for the endothermal steps is concentrated solar radiation. The concentrated solar energy is converted into storable and transportable chemicals and fuels. If the oxidation step of the cycle is carried out with air instead of water/CO2 the sequence of redox reactions can be applied for air separation opening pathways to solar oxygen and nitrogen production and from there further on to ammonia and fertilizer production. One of the major barriers to technological success of many of those processes is the identification of suitable active redox materials exhibiting satisfactory durability, reactivity and efficiencies. Besides materials aspects also process engineering issues needs to be overcome. Challenges are to couple an intermittent energy source to a chemical process and to efficiently recover high temperature heat. The most promising processes and the main applications are being described and discussed with respect to further development and future potential. Technical approaches and development progress in terms of solving them are addressed and assessed.