**1. Introduction**

While energy supplies of various kinds and technologies underscore the economic development of societies, they also result in undesirable side effects [1]. It is common knowledge that the major environmental burden of global concern emanating from the energy sector is the greenhouse effect [2]. One of the prominent greenhouse gases (GHG) is carbon dioxide. It arises from the combustion of fossil fuels, natural gas, and coal in automobiles and power stations, as well as heating in buildings and industrial processes [2]. For instance, around 45–47% of the United Kingdom's (UK) total energy consumption is for heating purposes, and nearly 80% is from fossil fuel sources [3]. Moreover, domestic energy use alone has a share of more than a quarter of national GHG emissions, of which 75% is for space and water heating [4]. Hence, the decarbonization of heat is the major energy challenge that the world faces over the coming decades. One way to achieve 80% emissions reduction in 2050 is by decarbonizing industrial and domestic heat demand and improving resource efficiency [2]. Current choices are around district heating (DH) networks in combination with other technologies to electrify heat or 'green' the gas grid [5]. However, the variability of heat demand, with a predictable winter peak heat load, presents opportunities for thermal energy storage (TES) to manage supply requirements to meet specified demands. Presently, sensible heat storage, latent heat storage, and thermochemical heat storage are the three TES systems being explored by researchers.

**Citation:** Kur, A.; Darkwa, J.; Calautit, J.; Boukhanouf, R.; Worall, M. Solid–Gas Thermochemical Energy Storage Materials and Reactors for Low to High-Temperature Applications: A Concise Review. *Energies* **2023**, *16*, 756. https://doi.org/10.3390/en16020756

Academic Editors: Alon Kuperman and Alessandro Lampasi

Received: 3 December 2022 Revised: 29 December 2022 Accepted: 4 January 2023 Published: 9 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Sensible heat storage (SHS) is the most mature and commercially used type of TES, available as tank thermal storage for hot water and electric storage heaters [3]. This technology is utilized for its material's convectional heat storage at very high temperature differences. Latent heat storage (LHS), on the other hand, is the heat absorbed or released by a substance during a change of phase. Typically, the heat is stored within a very narrow temperature range suitable only for applications requiring very small temperature differences [6]. The phase change materials (PCM) used for this purpose have the merit of achieving higher energy storage density, smaller volume requirements, and lower heat losses compared to sensible heat stores [6,7]. However, PCMs are unsuitable in comparison to SHS, which is more economical for applications requiring larger temperature differences. Thermochemical energy storage (TCES), which operates based on enthalpy change in reversible chemical reactions, is the most promising TES system [8] and has attracted greater interest in recent times.

TCES is recognized to have higher potential for energy stability and efficiency for reasons of high energy density (nearly 1000 kJ/L), smaller storage volume, minimal heat loss, long-term storage [9,10], high exergy efficiency [8], and lower charging temperature [10,11]. With respect to energy density, it is theoretically 5 to 10 times higher than LHS and SHS, respectively, when compared on the same scale basis [12,13]. As a result, the TCES system is more compact and could be effective where space constraints are significant. Additionally, TCES systems can be tuned to operate in a wide range of different temperatures and pressures [14], thus making them suitable for the storage of all grades of waste heat. This offers the possibility of being operated using various heat sources such as solar energy, aiming to operate a sustainable process. Moreover, the effective integration of TCES into district heating (DH) networks can lead to benefits such as increased energy efficiency and reliability, and reduction in energy use, costs, and GHG emissions. Thus, TCES is potentially useful in lowering fossil fuel consumption and related GHG emissions [15].

These attributes have attracted increasing interest in TCES research, albeit still at the experimental stage [3]. Many aspects of the technology are still unknown and yet to be discovered [14]. At present, TCES is hampered by a few problems, some of which include complexity in infrastructure [9] as well as low levels of thermal attainment in practical systems [9,16]. Therefore, a robust approach to operational control and understanding of the system must have a real-time model to predict its dynamics. For this reason, numerical or modelling studies are required for deeper theoretical insights and prediction of the system's behaviour. In addition, research still focuses on finding suitable materials with sufficient energy density, hydrothermal stability, and cyclability at conditions suitable for system operation [17]. Enhancement of materials' properties is crucial for integration into reactor systems. Additionally, it also requires suitable reactor optimization techniques and heat exchanger frameworks [14]. To date, experimental investigations of different reactor configurations have not overcome the dynamic limitations of heat and mass transfer [18]. In all reactors studied, nearly 70% are deployed for solid–gas reactions [19] because they show better values of chemical reaction efficiencies. However, the reaction bed must be further developed for optimum heat and mass transfers.

This paper aims to provide a concise review of the various TCES materials and reactors which could be used for low- to high-temperature applications. The review covers the fundamental concepts of TCES processes and materials, which are presented in Section 2. This includes a succinct discussion of the many experimental and theoretical efforts towards material property enhancement for application in different ranges of temperatures. The main types of solid–gas TCES reactors, together with their merits, challenges, and proposals for system improvement and optimization are discussed in Section 3. An overview of modelling and simulation of TCES systems is presented in Section 4, while the conclusion and recommendations are stated in Section 5.

We present a summary of the most recent reviews in which Desai et al. [20] reviewed sorption and chemical reactions for TCES. Experimental investigations and cyclic studies in low- and medium-temperature applications were discussed. Gbenou et al. [21] highlighted the many research issues hampering TCES implementation. They also discussed reactor prototypes, projects, and limitations as well as suggestions for better analysis of TCES problems. Efforts to link the practical and scientific aspects of TCES problems were the focus of Sadeghi [22]. In addition, findings from cutting-edge research and pertinent aspects of the TCES system consideration were reported. Marie et al. [23] presented advances in TCES with an overview of fluidized beds for low-temperature domestic applications, whilst Kant and Pitchumani [24] focused on open and closed reactors and prototypes for building applications. A perspective on the strengths and weaknesses of TCES materials and systems as well as a discussion on the evolution of TCES research has been provided in the work of Salgado-Pizarro et al. [25]. Gbenou et al. [26] reviewed TCES reactor prototypes and projects for low-temperature applications with analyses of the microscopic and macroscopic aspects of TCES systems.

However, the above reviews covered mainly low- to medium-temperature range applications, whereas, in this review, the coverage is from low- to high-temperature applications. Reviewing the entire spectrum will help to refocus on the broader issues of TCES. This might assist in honing the possibility of combining various techniques ideal for creating composites (hybrids) with novel properties. In addition, far-reaching proposals are made for the improvement of the thermophysical properties of TCES materials as well as the performance enhancement of TCES system integration. A succinct overview of the computational tools and resources available for the modelling and simulation of TCES systems provides motivation for researchers to gain valuable insights and understanding of these systems.

#### **2. Thermochemical Energy Storage**

Generally, thermochemical energy storage (TCES) uses a reversible system in which a source provides heat, for instance, to separate reactants (*AB*) into products (*A* and *B*). The products are stored separately at ambient temperatures, thereby eliminating the cost of insulation in storage containers. The separation route is endothermic and is the charging process. When heat release is required, the products (*A* and *B*) are recombined and preheated to an activation temperature to produce the reactant (*AB*) through a reversible exothermic route. The reversible exothermic path is the discharging process expressed as:

$$AB + \Delta H \leftrightharpoons A + B,\tag{1}$$

where Δ*H* is the molar heat (enthalpy) of the reaction. Essentially, a thermochemical energy storage cycle involves three main processes [27]: (i) charging, which is the endothermic reaction requiring a heat resource to dissociate the reactant AB; (ii) storing, where the products *A* and *B* are both stored separately; and (iii) discharging, which is the exothermic reaction where the products A and B are combined again to release heat energy.

The stored thermal energy (*Q*) depends on the molar reaction enthalpy (Δ*H*) and the number of moles (n) of one of the products [12], as in the equation:

$$Q = n\Delta H \tag{2}$$

Based on the storage mechanism, TCES processes are divided into sorption processes and reversible chemical reactions [9,13,28]. Figure 1 shows the classification chart for TCES systems. However, there appears to be a thin boundary for clear distinction between terminologies such as chemical storage, thermochemical storage, and sorption processes [27]. Often, a sorption process is considered a chemical reaction in which the chemical bonds are weak [29].

**Figure 1.** Classification of thermochemical energy storage systems [13].

#### *2.1. Sorption Processes and Materials in Thermochemical Energy Storage*

Sorption is described as a phenomenon by which a vapor or gas (sorbate) is captured by a denser substance (solid or liquid), called sorbent [27]. The reverse process, called desorption, requires heat to unbind the sorbate from the sorbent. Therefore, in a sorption process, heat storage is accomplished via a chemical potential when the force binding the sorbent and the sorbate is broken [30]. Essentially, absorption and adsorption are the two categories of sorption processes. Although dissimilar, they do, however, involve the physical transfer of a volume of mass or energy [27]. Absorption, in simple terms, is the process of one material (absorbent) retaining another material (absorbate). This takes place within the molecular enclave of the sorbent, resulting in alteration in its structure and morphology [17]. Examples of absorption materials for water include MgSO4, LiCl, LiBr, CaCl2, MgCl2, KOH, and NaOH [29].

Unlike absorption, adsorption involves a very thin layer of atoms or molecules on the adsorbent surface without altering its structure [17]. Here, there is an accumulation of energy or matter (of adsorbate) onto a surface (of adsorbent) [27]. According to Srivastava and Eames [31], adsorption is a phenomenal occurrence at the periphery of two phases where both weak intermolecular and strong chemical bonds act between the molecules. Adsorption either proceeds as a physical process, physical adsorption (physisorption), or a chemical process, chemical adsorption (chemisorption), based on the type of bond between adsorbent and adsorbate. Generally, physisorption occurs whenever an adsorbate is brought into contact with the surface of the adsorbent and involves weak intermolecular forces (Van der Waals forces) [30]. Chemisorption, on the other hand, is due to strong chemical bonds (hydrogen bonds, charge-transfer interactions, covalent bonds) [32] in the same manner as in other chemical compounds. Both types of adsorptions involve the evolution of heat. The reason that chemical forces are stronger than physical forces is that the heat of chemisorption is larger than that of physisorption [33]. Though these are different processes, they often take place simultaneously at different sites or locations of the adsorbent [30,32]. Similarly, at the mesoscale, it is difficult to distinguish between absorption and adsorption [33] as both may occur simultaneously. In that case, the term sorption is generally used for both processes [31,34].

Materials for sorption processes are usually solids, liquids, and composite sorbents with solid/gas and liquid/gas systems as working pairs [13]. Moreover, reversible adsorption of vapours onto porous solid surfaces is a potential option for TCES, particularly for space heating applications [32]. According to Yu et al. [30], the most studied adsorbents are silica gels and zeolites using water as a working fluid. In particular, zeolite 13X is the commonest and most widely studied TCES material because of its hydrothermal and mechanical stability and corrosion behaviour [13], although aluminophosphates (AlPOs), silico-aluminophosphates (SAPOs), and metal-organic frameworks (MOFs) have recently been reviewed by Makhanya et al. [35] as promising materials for heat storage. Composite materials formed by a combination of salt hydrate and a porous additive with high thermal conductivity have also been studied [16]. Figure 2 shows a summary of typical sorption reactions and materials.

**Figure 2.** Sorption reactions and materials [10].

Sorption processes are essentially attractive for low-temperature applications due to their attribute of high kinetics at low temperatures [13]. Therefore, sorption reactions are typically not suitable for high-temperature applications [29].

#### *2.2. Chemical Reactions and Materials in Thermochemical Energy Storage*

TES based on chemical reactions is justifiably advantageous for seasonal storage [12]. These reaction systems store energy in the form of chemical potential, and the energy per mole required to break up chemical bonds is more than any other thermal storage system. These reactions are characterized by changes in the molecular composition of the reactants involved [13], and usually take place at temperatures above 400 ◦C [36]. High energy storage density and reversibility are key requirements for TCES materials, as it is challenging to find a suitable reversible reaction for a system. This is significant because the type of reaction has immense implications on the reactor design and system integration [37]. The difficult task for a reaction choice is the requirement for efficient heat and mass (HAM) transfer to and from the storage volume. This requirement, according to Aydin et al. [10] can be a limiting factor for the overall storage volume, unlike SHS and LHS, which allow higher volumes to be utilized. This volume limitation due to HAM transfer characteristics is the key area for current research in TCES systems.

In the literature [8,29,38], TCES reactions are classified into three categories, namely solid–gas, liquid–gas, and gas–gas reactions with regard to the nature of the reactants and products. However, for temperatures over 300 ◦C, only solid–gas and, in some cases, liquid–gas reactions remain practicable [29]. Furthermore, solid–gas reactions have been widely studied as a very promising heat storage method [39]. The interest in these reactions is due to their wide range of equilibrium temperatures and self-separation of reactants. Chemical reactions, including chemical sorption processes, premised on solid–gas systems are an encouraging method for the storage and conversion of heat energy for heating or cooling purposes [40]. While the sorption processes are used to store low (<100 ◦C) and medium (100–400 ◦C) grade heat with enthalpies in the range of 20–70 kJ/mol [40], chemical reactions are utilized for the storage of medium (100–400 ◦C) and high (>400 ◦C) grade heat and the enthalpies are in the range 80–180 kJ/mol [13,36].

Different kinds of solid–gas reactions are employed for TCES. These are categorized depending on the composition of the solid reactant as the most prominent [38]. The reactions include those based on hydrates, hydrides, hydroxides, carbonates, and oxides. It might be important that TCES materials be flanked by an appropriate reaction temperature and enthalpy for the application. For this reason, Bauer [29] characterized the solid–gas reactions according to reaction temperatures:

