Sorption thermal storage for solar energy

Sorption technologies, which are considered mainly for solar cooling and heat pumping before, have gained a lot of interests for heat storage of solar energy in recent years, due to their high energy densities and long-term preservation ability for thermal energy. The aim of this review is to provide an insight into the basic knowledge and the current state of the art of research on sorption thermal storage technologies. The first section is concerned with the terminology and classification for sorption processes to give a clear scope of discussion in this paper. Sorption thermal storage is suggested to cover four technologies: liquid absorption, solid adsorption, chemical reaction and composite materials. Then the storage mechanisms and descriptions of basic closed and open cycles are given. The progress of sorption materials, cycles, and systems are also reviewed. Besides the well-known sorbents like silica gels and zeolites, some new materials, including aluminophosphates (AlPOs), silico-aluminophosphates (SAPOs) and metal-organic frameworks (MOFs), are proposed for heat storage. As energy density is a key criterion, emphais is given to the comparison of storage densities and charging tempertures for different materials. Ongoing research and development studies show that the challenges of the technology focus on the aspects of different types of sorption materials, the configurations of absorption cycles and advanced adsorption reactors. Booming progress illustrates that sorption thermal storage is a realistic and sustainable option for storing solar energy, especially for long-term applications. To bring the sorption storage solution into market, more intensive studies in fields of evaluation of advanced materials and development of efficient and compact prototypes are still required.