A critical review of high-temperature reversible thermochemical energy storage systems

Thermal energy storage promises to be cheaper, with significantly lesser environmental encroachment, compared to electrical energy storage in batteries. Among all thermal energy storage systems, thermochemical energy storage is the most promising due to its high energy density, high exergetic efficiency, and high operating temperature. This paper presents a review of thermal energy storage systems that are suitable for concentrating solar thermal power plant. The review here mainly focuses on experimental studies carried out on solid-gas, gas-gas, and sulfuric acid/sulfur-based thermochemical energy storage systems that operate above 300 degrees C. Also, various reactor designs and storage concepts for solid-gas and gas-gas reaction systems and cyclic studies for the case of a solid-gas system are addressed. Finally, a comparative summary is presented, preparing the path for the future course of research and development. The review reveals that the carbonate systems are an economically viable option as future thermal energy storage system if their cyclic stability and reversibility are improved. A new metal hydride thermal energy storage system is proposed that would provide cooling and heat upgradation as an additional output. For gas-gas reactions, the major challenges are the development of efficient solar receiver and optimization of the catalyst use.