energies Review Recent Advances in Thermochemical Energy Storage via Solid–Gas Reversible Reactions at High Temperature Laurie André 1 and Stéphane Abanades 2,* 1 Institut de Chimie Moléculaire de l’Université de Bourgogne, UMR 6302, CNRS, Univ. Bourgogne Franche-Comté, 9, Avenue Alain Savary, 21000 Dijon, France; [email protected] 2 Processes, Materials and Solar Energy Laboratory, PROMES-CNRS, 7 Rue du Four Solaire, 66120 Font-Romeu, France * Correspondence: [email protected]; Tel.: +33-(0)4-68-30-77-30 Received: 12 October 2020; Accepted: 6 November 2020; Published: 10 November 2020 Abstract: 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 (MOx/MOx 1), non-stoichiometric perovskites − (ABO3/ABO3 δ), alkaline earth metal carbonates and hydroxides (MCO3/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 (H2O or CO2). 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, CaCO3/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. Keywords: thermochemical energy storage; solid-gas reaction; redox systems; carbonate; hydroxide; perovskite; concentrated solar power 1. Introduction The enthalpy of solid-gas chemical reactions stored in chemical materials can be used to generate heat when necessary via endothermal/exothermal reversible reactions. The stored and released heat can be used for example to run power cycles or more generally in industrial processes operating at high temperatures and thus requiring high amounts of energy that are usually provided by fossil fuel combustion. Thus, thermochemical energy storage (TCES) has potential to lower fossil fuel Energies 2020, 13, 5859; doi:10.3390/en13225859 www.mdpi.com/journal/energies Energies 2020, 13, x FOR PEER REVIEW 2 of 24 Energies 2020, 13, 5859 2 of 23 consumption and related greenhouse gas emissions [1]. A high potential also exists in the combinationconsumption of and TCES related systems greenhouse with gasrenewable emissions ener [1gy]. A systems. high potential Thermal also energy exists instorage the combination is indeed particularlyof TCES systems suitable with for renewablebeing combined energy with systems. concentr Thermalated solar energy energy storage that relies is indeed on an intermittent particularly resource,suitable for with being the aim combined to operate with the concentrated process contin solaruously energy (day that and relies night on as an well intermittent as stable operation resource, withduring the fluctuating aim to operate solar theenergy process input) continuously (Figure 1). Indeed, (day and solar night energy as well is variable as stable and operation can fluctuate during a lotfluctuating in nature solar due energyto clouds input) and (Figureweather1 ).conditions, Indeed, solar thus energy requiring is variable a storage and system can fluctuate for smooth a lot and in stablenature dueoperation to clouds under and fluctuating weather conditions, solar irradiation thus requiring conditions. a storage TCES system is forthus smooth attractive and stablesince continuousoperation under operation fluctuating allows solara strong irradiation increase conditions. in the capacity TCES factor is thus of the attractive solar plant, since while continuous it can furtheroperation contribute allows a strong to eliminating increase in the transient capacity factoreffects of thedue solar to plant,start-up/shutdown while it can further periods contribute and unstable/variableto eliminating transient solar econditions.ffects due to The start-up possible/shutdown envisioned periods applications and unstable are/ variablepertaining solar to conditions.electricity Theproduction possible by envisioned concentrated applications solar power are (CSP) pertaining plants to or electricity more generally production high temperature by concentrated chemical solar powerprocesses (CSP) requiring plants an or moreexternal generally energy highinput temperature as the process chemical heat supply processes (e.g., requiringcement and an concrete external production,energy input minerals as the process calcination, heat supply metallurgical (e.g., cement processes, and concrete fuel production production, processes minerals or calcination, chemical industrialmetallurgical processes). processes, Most fuel indu productionstrial energy-intensive processes or chemical processes industrial require processes). a high temperature Most industrial heat sourceenergy-intensive generally provided processes by require fossil a fuel high burning. temperature In such heat high source temperature generally processes, provided bythe fossil required fuel highburning. temperature In such high heat temperaturefor running processes,power cycles the or required driving high endothermal temperature reactions heat for can running be generated power withcycles solar or driving concentrating endothermal systems reactions (parabolic can be dish generated, trough, with linear solar concentratingFresnel systems systems or solar (parabolic tower receiversdish, trough, with linear heliostat Fresnel field). systems This or is solar the towercase of receivers CSP plants with heliostatfor electricity field). Thisgeneration is the case and of solar CSP plantsthermochemical for electricity processes generation for fuels and solar(syngas thermochemical production via processes reforming, for gasification fuels (syngas of production carbonaceous via feedstocks, H2O and CO2 splitting via thermochemical cycles, etc.) or chemical commodity reforming, gasification of carbonaceous feedstocks, H2O and CO2 splitting via thermochemical cycles, productionetc.) or chemical (cement, commodity metals, productionetc.). Thus, (cement, the inte metals,rest in etc.). TCES Thus, integration the interest in insuch TCES processes integration for continuousin such processes operation for continuous is constantly operation growing. is constantlyAnother possible growing. application Another possibleis the utilization application of TCES is the forutilization the recovery of TCES and for storage the recovery of waste and storageheat of ofva wasterious heatenergy of various and industrial energy and processes industrial at processesdifferent temperatureat different temperature levels in levelsorder into order increase to increase proces processs efficiencies efficiencies or to or toproduce produce additional additional extra heat/electricity.heat/electricity. Figure 1. Scheme of the solar power plant main components integrating buffer thermal energy Figurestorage 1. system. Scheme of the solar power plant main components integrating buffer thermal energy storage system. The TCES integration within a solar power plant implies the utilization of a heat transfer fluid (HTF)The (Figure TCES2). integration During on-sun within hours, a solar the power HTF flows plant inside implies the solarthe utilization receiver and of a is heat used transfer to store fluid heat (HTF)in the TCES(Figure system 2). During (heat charge,on-sun endothermal).hours, the HTF The flows TCES inside system the can solar be receiver combined and with is used or integrated to store heatinto thein the solar TCES receiver system (direct (heat storage) charge, or endothermal). separated (indirect The storage).TCES system The exitingcan be HTF combined is then with used or to integratedrun the turbine into the of the solar power receiver block. (direct During storage) off-sun or hours,separated the HTF(indirect directly storage). flows The through exiting the HTF TCES is thensystem used for to heat run recovery the turbine (discharge of the power step, exothermal)block. During in off-sun order to hours, increase the itsHT temperatureF directly flows and through provide theheat TCES to the system downstream for heat process, recovery thereby (discharge enabling step, continuous exothermal) operation. in order