Survey on chemical storage applications for CSP

Solar energy is an abundant and unlimited resource, but its discontinuous nature represents nowadays a barrier to a large diffusion of this technology. Currently, energy dispatchability, that is, the ability to modulate the production of electricity according to the consumption, is a major challenge for the success of solar technology and, as a consequence, one of the active field in research. To date, significant progress has been made in the development of sensible heat thermal storage systems, that ensure the damping of solar fluctuations due to adverse weather conditions and allow the use of the accumulated heat overnight. These systems are for short term periods as they guarantee only a few hours of compensation. A bigger challenge is represented by the realization of long-term storage systems that would allow both to solve the problem of dispatchability of the solar energy and to maximize the capacity factor, aligning the seasonal production capacities of the concentrating solar power (CSP).
Chemical storage is a technology where the solar heat is used to carry out a reaction, or a series of reaction (charging phase); then, the obtained product is reacted when the solar radiation is not available, by an exothermic process, so making usable the solar heat previously stored (discharging phase). In this respect two main methods are applied: in a former one a reversible process is used, in general employing inorganic oxides, hydroxides or salts: the endothermic reaction is performed during the charging phase, and the exothermic one to discharge the energy stored. In another configuration a “solar fuel” is produced by means of the solar heat: hydrogen (by reforming, pyrolysis or thermochemical cycles), organic fuels, and so on. In that case the discharging phase is the combustion of the fuels. In principle, the strong point of the methods is the possibility of a seasonal storage. A chemical can be produced during the summer and used over the rest of the year. The present report describes and discusses the state of the art of this technology