Thermochemical storage for long‐term low‐temperature applications: a review on current research at material and prototype scales

Thermochemical heat storage has the potential to store large amount of energy from renewables and other intermittent distributed sources, ideally without losses typical of sensible heat storage. owever, in order to have a commercially attractive system able to compete with conventional storage technologies, research at material, reactor, and ultimately at system level is still required. The aim of this work is to investigate the current state of the art research at the abovementioned scales, which can then be used to investigate the performances of long-term low-temperature thermochemical storage systems integrated in the energy grid. Regarding materials, focus is on pure salt hydrates, adsorbents, and composites for solid/gas reactions. Concerning reactors and systems, a review on existing prototypes based on solid/gas sorption reactions for low-temperature thermal storage is performed. At material level, pure salt hydrates have hydrothermal instabilities, kinetics, and corrosiveness issues. Composites are largely investigated because of their potential to overcome issues of pure salts but have various implementation problems. Amongst them, reduced mass transport within matrix pores, salt overhydration, and possible leaking of active material are still to be solved. Especially for open systems, the choice of zeolites rather than pure salts as active materials is prominent due to their better stability. However, high material costs and desorption temperatures coupled with lower energy densities decrease their commercial attractiveness. Beside research at material and reactor levels to overcome technological challenges, integration of thermochemical storage at grid level has to be investigated to assess its techno-economic feasibility based on performance and interactions with production and consumption technologies. In order to investigate a thermochemical storage incorporated in an energy grid, the most promising materials and reactors characteristics are evidenced with the aim to realize a system for that purpose, in which system reliability and cyclability will have priority over high energy density