Recent Advances in Thermochemical Energy Storage via Solid–Gas Reversible Reactions at High Temperature

The exploitation of solar energy, an unlimited and renewable energy resource, is of prime interest to support the replacement of fossil fuels by renewable energy alternatives. Solar energy can be used via concentrated solar power (CSP) combined with thermochemical energy storage (TCES) for the conversion and storage of concentrated solar energy via reversible solid–gas reactions, thus enabling round the clock operation and continuous production. Research is on-going on efficient and economically attractive TCES systems at high temperatures with long-term durability and performance stability. Indeed, the cycling stability with reduced or no loss in capacity over many cycles of heat charge and discharge of the material is pursued. The main thermochemical systems currently investigated are encompassing metal oxide redox pairs (MO x /MO x−1 ), non-stoichiometric perovskites (ABO 3 /ABO 3−δ ), alkaline earth metal carbonates and hydroxides (MCO 3 /MO, M(OH) 2 /MO with M = Ca, Sr, Ba). The metal oxides/perovskites can operate in open loop with air as the heat transfer fluid, while carbonates and hydroxides generally require closed loop operation with storage of the fluid (H 2 O or CO 2 ). Alternative sources of natural components are also attracting interest, such as abundant and low-cost ore minerals or recycling waste. For example, limestone and dolomite are being studied to provide for one of the most promising systems, CaCO 3 /CaO. Systems based on hydroxides are also progressing, although most of the recent works focused on Ca(OH) 2 /CaO. Mixed metal oxides and perovskites are also largely developed and attractive materials, thanks to the possible tuning of both their operating temperature and energy storage capacity. The shape of the material and its stabilization are critical to adapt the material for their integration in reactors, such as packed bed and fluidized bed reactors, and assure a smooth transition for commercial use and development. The recent advances in TCES systems since 2016 are reviewed, and their integration in solar processes for continuous operation is particularly emphasized.