Thermochemical energy storage chemistries and reactor concepts for integration in solar thermal power plants

The ability and flexibility in controlling the power output of Concentrated Solar Power (CSP) plants via thermal energy storage (TES) is considered their major competitive edge vs. alternative solar energy exploitation technologies like photovoltaics. Among the three types of implementing TES based on the nature of heat to be stored, sensible, latent and thermochemical heat i.e. heat produced through the use of reversible endothermic chemical reactions, the third, TCS, exhibits advantages like higher storage energy densities, indefinitely long storage duration, suitability for large scale implementation and hybridization with sensible energy storage. The presentation will summarize our current activities on such TCS approaches implemented through gas-solid, reversible, non-catalytic, decomposition reactions with significant heat effects, namely those of metal oxides, hydroxides and carbonates with emphasis on the first case. These reaction schemes share commonalities and requirements, like e.g. use of abundant and inexpensive raw materials, handling of the gaseous reaction products, efficient incorporation of the exothermic reaction within the normal operation of a CSP plant, as well as solid materials structural stability issues due to the repeated, long-term, cyclic reactions inducing thermochemical stresses. Furthermore, they involve not only gas-solid chemical reactions but heat exchange at high temperatures; therefore a major technical challenge lies in the proper design and operation of gas-solid thermochemical reactors operating simultaneously and efficiently as heat exchangers. Such reactors/heat exchangers can employ the solid material as either moving “loose” particle/granule streams or as moving as well as non-moving porous structures, either directly exposed to solar irradiation or confined within containers decoupled from the solar receiver and being heated by the heat transfer fluid from it. In this perspective, approaches to tackle these issues by both structured reactors/heat exchangers employing porous ceramics as well as of non-structured, rotary kiln-type will be presented and discussed