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].
2.2.1. Dehydration/Hydration of Metal Salt Hydrates
TCES materials for low-temperature applications have attracted remarkable attention. Salt hydrates and composite sorbents based on salt hydrates belong to this category. They have become preferred materials for TCES in building applications [
41] due to their high energy density and low turning temperatures [
42]. The low turning temperatures are suitable for integration with sources such as solar energy or low-grade waste heat and make them fit for residential space heating applications [
42]. Much literature is available on high-potential salts, and this includes chlorides—LiCl [
43], CaCl
2 [
44,
45,
46], and MgCl
2 [
47]; bromides—SrBr
2 [
48,
49] and LiBr [
43,
50]; and sulphates—MgSO
4 [
46,
47,
51], Al
2(SO
4)
3 [
46,
52], and CuSO
4 [
53]. Furthermore, other promising hydrates such as Na
2S and K
2CO
3 were studied by de Jong et al. [
54] and Gaeini et al. [
55], respectively.
TCES materials must fulfil common conditions such as low cost, being non-poisonous, and non-corrosive, in addition to having sufficient energy density and suitable turning temperatures. These requirements are fulfilled by many salt hydrates [
46]. However, numerous salts proposed for low-grade thermal energy storage have failed [
42]. In a typical case, for instance, van Essen et al. [
46] conducted a theoretical study of four salt hydrates, namely MgSO
4.7H
2O, Al
2(SO
4)
3.18H
2O, CaCl
2.2H
2O, and MgCl
2.6H
2O, using a thermogravimetry and differential scanning calorimetry (TG-DSC) apparatus. Based on the measured temperature lift under practical conditions, MgCl
2 was considered the most promising with a high theoretical energy density of 2.8 GJ/m
3. However, both hygroscopic chlorides under investigation tended to form a gel-like material (due to melting or formation of solution) during the hydration experiments, which prohibited further water uptake. Similarly, Donkers et al. [
53] studied the cyclability of CuCl
2, CuSO
4, MgCl
2, and MgSO
4 in hydration/dehydration reactions. They observed the effect of fracturing to be greater in hydrates with larger volumetric changes. In conclusion, CuCl
2 was adjudged the most promising heat storage material.
A systematic evaluation of 125 salt hydrates was performed by N’Tsoukpoe et al. [
52] using criteria such as safety, theoretical calculations, and thermogravimetry analysis (TGA). Out of 45 preselected salt hydrates, SrBr
2.6H
2O and LaCl
3.7H
2O appeared to be the most promising. However, the expected efficiency and net energy storage density (including water storage) remained low. Similarly, a review of 563 reactions was carried out [
56] to evaluate the theoretical suitability of salt hydrates as seasonal heat storage materials. Up to 25 salt hydrates were identified. By considering cost, chemical stability, reaction kinetics, and safety, K
2CO
3 was determined to be the most promising candidate, but low energy density was noticed.
Table 1 gives the theoretical and experimental energy density, reaction temperature, and water vapour pressure of some salt hydrates (extracted from [
57]). It is, however, noteworthy that in all these comparative investigations, the difference in behaviour is attributable to the intrinsic properties (crystal structure and thermodynamics) of the materials. Therefore, a general kinetic model of the sorption process in salt hydrates will require specific information on the material properties. Thus, besides the high potential shown by some salt hydrates, several associated issues are still obvious. These include poor hydrothermal stability, slow thermodynamics, high corrosivity, and toxicity [
41]. Such attributes make it difficult for monomer salt hydrates to be used for TCES without modification of their properties. For this reason, researchers have experimented with composite materials. Fopah Lele et al. [
48] evaluated four salt hydrates (CaCl
2, MgCl
2, SrBr
2, and MgSO
4) and host matrices (activated carbon, expanded natural graphite, and silica gel). The results on both systems for only salts gave thermal conductivity in the range of 0.3–1.3 W/mK with a measurement uncertainty of less than 14%. Zhao et al. [
58] mixed SrBr
2 and expanded natural graphite treated with sulphuric acid. The composite with 10 wt% of SrBr
2 proved satisfactory with good mass transfer performance and no degradation in water uptake.
Salt mixtures appear promising, but the general technical issue reported is mass transport within the matrix’s pores due to deliquescence, overhydration (with possible leakage or pore blockage), and a low-temperature lift [
41]. A proposal for pairing suitable salt hydrates according to different matrix materials, reactor analyses, and structural optimization methods for the enhancement of HAM transfer has recently been published [
59]. Again, selecting a suitable binary salt mixture may increase the performance of each material and avoid its unique individual shortcomings. A double salt hydrate, Na
2Zn(SO
4)
2·4H
2O, has been reported as having exhibited suitable stability at the first ten hydration/dehydration cycles, with an excellent energy storage density of 4.7 GJ/m
3 and theoretical efficiency up to 77.4% [
60]. It might be necessary that the influence of material characterization and reaction parameters are considered to determine the optimum mixing pair and ratio, as well as optimize system controls under different operating conditions [
41]. In spite of this, N’Tsoukpoe and Kuznik [
34] assert that the performance achieved with salt-hydrate systems is not competitive and that the performance or advantages of the TCES materials have probably been overestimated.
2.2.2. Dehydrogenation/Hydrogenation of Metal Hydride
Metal hydrides (MHs) are compounds formed by the reversible reaction of hydrogen and metal or metal alloy, and this reversible absorption of hydrogen gas is exothermic [
39]. The utilization of MHs for TCES is encouraged due to high energy efficiency, high volumetric energy density, and cost [
61]. It also offers flexibility in its wide range of operating temperatures. On the other hand, one of the main disadvantages of MH systems is the need for hydrogen storage [
37]. This means that the MH system can be a closed system with an intermediate hydrogen storage subsystem. It is suggested that by coupling a high temperature with a low-temperature metal hydride system, a self-regulating reversible metal hydride energy storage system can be established [
62].
Lithium hydride (LiH), calcium hydride (CaH
2), and magnesium hydride (MgH
2) systems have been studied for their TCES potentials. However, more attention has been paid to MgH
2 [
9]. It has a working temperature between 200–500 °C and decomposes into Mg metal, releasing hydrogen with a reaction enthalpy of 75 kJ/mol and a heat storage capacity of around 0.8 kWh/kg [
39,
63]. The hydrogen gas can be stored in a reservoir under the equilibrium pressure of MgH
2. For instance, the MgH
2/Mg equilibrium pressure of 10 bar at 350 °C and 20 bar at 400 °C is shown in
Figure 3. According to Felderhoff et al. [
39], if the pressure is lower than the equilibrium pressure at a given temperature (coloured area in
Figure 3), MgH
2 decomposes until the pressure inside the system reaches the equilibrium pressure. At pressures higher than the equilibrium pressure, Mg metal can be hydrogenated (white area in
Figure 3).
Chen et al. [
9] noted that the MgH
2/Mg pair suffers from poor reversibility. Its cyclic stability drops by 75% after 500 cycles [
8] which is a limiting factor in large-scale application. Additionally, its high thermodynamic stability and sluggish sorption kinetics are the major obstacles to its extensive application [
64].
Table 2 shows the thermodynamic properties of MgH
2 [
65].
Strategies employed to overcome these issues include the addition of nanostructures, alloying, and MgH
2-based composites. The catalytic addition of different transition metals or transition metal oxides can greatly accelerate the hydrogenation/dehydrogenation kinetics [
39]. Khan et al. [
66] investigated two nanostructured MgH
2 and cobalt (Co) powders. The hydrogen storage properties of the 2MgH
2-Co powder and 2MgH
2-Co compressed pellet were analysed. Fast hydrogenation was observed in the de-hydride 2MgH
2-Co compressed pellet, with about 2.75 wt% absorbed in less than 1 min at 300 °C, and a maximum hydrogen storage capacity of 4.43 wt%. The hydrogen absorption activation energy of the 2MgH
2-Co compressed pellet was also lower than in the 2MgH
2-Co powder. Banrejee et al. [
67] prepared nanocrystalline magnesium and compared it with micro-crystalline magnesium. The developed nanocrystalline Mg exhibited improved properties with a higher hydrogen storage capacity of 6.24 wt% at 300 °C. Prolonged ball milling led to faster hydrogenation kinetics (up to 90% of the saturation value in 15.5 min at 250 °C) and a substantial decrease in the activation barrier. Nanostructuring has also been studied [
68,
69] with remarkable improvements in Mg-based storage properties. However, nanostructuring could result in poor thermal conductivity [
9]. Additionally, the drawback in powder materials is usually due to their tendency towards coarsening and sintering during dehydrogenation/hydrogenation cycles. This is susceptible in Mg, which has a relatively low melting temperature and, thus, displays significant atomic mobility at the cycle operating temperatures [
66].
Alloying is an effective and easy-to-handle method of improving the sorption property of MgH
2/Mg [
9]. Intermetallic compounds of transition metals are among the catalytic materials that can facilitate the thermal storage processes in MgH
2 [
69]. Usually, intermetallic hydrides are composites of a hydride-forming element at high temperatures and a non-hydride-forming element, such as Mg
2NiH
4 and Mg
2FeH
6. Research efforts aimed at reducing the reaction temperatures of these composite hydrides have been achieved through the addition or substitution of existing elements [
70]. There seems to be a consensus that increasing the number of 3D elements would improve the kinetics by decreasing the activation energy of hydrogen desorption [
71]. For instance, the enthalpy change associated with the formation of Mg
2FeH
6 at 500 °C was measured to be 77.4 kJ/mol H
2, lower than the reported values of 98 kJ/mol H
2 [
62]. Zhang et al. [
64] also reported a decrease in the hydrogen desorption enthalpy and initial dehydrogenation temperature of MgH
2 through incorporation of either Ti or Ni. Sulaiman et al. [
72] reported that a 5 wt% K
2NiF
6-doped MgH
2 sample started desorbing around 260℃, which was a reduction of about 95 °C and 157 °C compared with the as-milled and as-received MgH
2. Additionally, the de/absorption kinetics were also improved significantly compared to the un-doped MgH
2. In another approach, Majid et al. [
73] selected TiFe
0.8Mn
0.2, graphite, and Fe as additives. Compared to pure milled MgH
2 powder, they found that the dehydrogenation peak temperatures were decreased by 90, 160, and 165 °C for Mg-TiFe
0.8Mn
0.2-graphite, Mg-Fe-graphite, and Mg-TiFe
0.8Mn
0.2-Fe-graphite composites, respectively. The co-addition of TiFe
0.8Mn
0.2, graphite, and Fe exhibited synergistic effects in improving the hydrogen desorption properties of MgH
2.
The roles of Ti-based catalysis and its consequent hydrogen storage effects on MgH
2 were reviewed by Zhou et al. [
65]. They concluded that the doping technique via Ti-based catalysis is a viable approach to enhancing the reaction of Mg-based materials. A comprehensive compilation of Ti-based catalysis of MgH
2 systems, corresponding synthesis approaches, and kinetic behaviours is presented in their review. On the other hand, Kumar et al. [
74] performed calculations based on the first principles to investigate the dehydrogenation kinetics, considering doping at various layers of MgH
2 (110) surface with Ca, Al, Ga, Sc, Ni, Ti, and V. Doping at the first and second layers of MgH
2 (110) had a significant role in lowering the H
2 desorption (from surface) barrier energy. The screening approach found Al and Sc to be the best possible dopants at lowering desorption temperature while preserving similar gravimetric density and bulk modulus to a pure MgH
2 system. By extending frontiers, Jain et al. [
75] conducted an investigation on the role of alkaline metal fluoride (MgF
2) as a catalyst in the hydrogen-storage behaviour of MgH
2. For 5 mol% MgF
2 admixed into MgH
2 powder, hydrogenation measurements at 335 °C showed 92% of absorbed theoretical capacity in less than 20 min (compared to 70% by pure MgH
2). Sorption studies further point to the possibility of complete absorption at low temperatures down to 145 °C. Again, cyclic measurements made at 310 °C revealed an inconsequential loss in the total storage capacity. These results implied that the sensitivity of the material to atmospheric conditions is low, and it is easy to handle. Thus, it can be employed in applications where operation at relatively high temperatures is insignificant.
A variety of dopants for MgH
2 has been reported and the respective Mg-based hydride materials have been enhanced. Despite improvement in the material properties, thermodynamic tuning remains a major challenge [
76]. Present approaches have been successful in addressing it, to some extent, but much is still desired for practical application.
2.2.3. Dehydration/Hydration of Metal Hydroxides
Thermochemical heat storage with metal hydroxides results from a reversible reaction of water (steam) and metal oxides at high temperatures (~500 °C) and near-atmospheric pressures [
8]. The alkaline earth metal hydroxides such as Mg(OH)
2, Ca(OH)
2, Sr(OH)
2, or Ba(OH)
2 have been considered as storage materials [
39]. The initial candidate hydroxide–oxide pairs are Mg(OH)
2/MgO, Ca(OH)
2/CaO, Sr(OH)
2/SrO, and Ba(OH)
2/BaO. The theoretical turning temperatures and thermodynamic data of these hydroxides are presented in
Table 3. The reactions are in the range of 70–1005 °C, though most of the reactions are too low for high-temperature application [
11], usually occurring at medium temperatures of 250 ˂ T ˂ 450 °C. The steam partial pressure and the temperature drive the hydration/dehydration reactions [
12].
Figure 4 shows some of the couples which could be used for TCES application.
With reference to the high reaction enthalpies and energy storage densities (
Table 3), mainly the hydroxides Ca(OH)
2 and Mg(OH)
2 have been extensively studied theoretically and experimentally [
9,
39]. However, the Ca(OH)
2/CaO system is more attractive [
10]. It is the most-explored hydroxide system for thermochemical energy storage, prompting tests in both lab-scale reactors and TGA [
14]. One reason is that the hydration of MgO is very slow in superheated steam and the rate of reaction drops with the rise in temperature [
8,
39].
Figure 5 shows the decomposition of the Mg(OH)
2 system at a relatively low temperature of around 330 °C [
9]. The reaction enthalpy of Mg(OH)
2 degenerates with temperature up to 500 °C, unlike the enthalpy of Ca(OH)
2. In addition, CaO has a higher heat of adsorption over a short period [
77] than MgO, as well as being much cheaper [
8]. In view of these attractive attributes, a plethora of research efforts has been conducted for the potential application of Ca(OH)
2/CaO as a long-term thermal energy storage system.
Schaube et al. [
78] investigated a 10 mg sample in a Ca(OH)
2/CaO reversible system, and full conversion and cycling stability were reported over 100 cycles at a water partial pressure of 1 bar (even at 0.956 bar), with an equilibrium temperature of 505 °C and enthalpy of 104.4 kJ/mol. However, little success was achieved in the cycling stability of a 60 g sample of the reaction system as degradation was reported over 25 cycles. Agglomeration was also observed [
79]. Additionally, Criado et al. [
80] investigated Ca(OH)
2/CaO hydration/dehydration reaction and obtained higher rates than those reported in the literature at temperatures in the range of 400–560 °C and partial steam pressures between 0 and 100 kPa. However, particle attrition was observed for large particle sizes of the material. In another work, Dai et al. [
81] investigated the cycling stability of the Ca(OH)
2/CaO system for 20 successive dehydration/hydration cycles. Existing problems relating to agglomeration, sintering, poor thermal conductivity, and irregularity in the rate of heat release were raised. There is a consensus that major problems encountered in Ca(OH)
2 reactors relate to particle agglomeration and sintering, poor heat transfer characteristics, and low permeability of the packed bed. Though the issue of permeability has been addressed in optimized reactors, other problems still exist [
39]. As a result, much of the research has been around material enhancement through additives or composites, as well as reactor optimizations.
Criado et al. [
82] synthesized composite materials using sodium silicate (Na₂Si
3O
7) to bind Ca(OH)
2/CaO particles for fluidized/fixed-bed application. The mechanical properties of the resulting CaO/Ca-silicate composites over hundreds of hydration/dehydration cycles were investigated. The results confirmed the primary role of Ca(OH)
2 anisotropic expansion as the main cause of the reduction in the crushing strength of the pellets. Another study by Funayama et al. [
83] relates to a composite material using SiC/Si foam. The performance of the ~63 g of composite material with a packed-bed reactor was evaluated. The rate of heat output per volume under maximum hydration pressure of the composite was 1.3 kW/L-bed for the first 5 min, which is 1.4 times higher than previously reported for a bed of Ca(OH)
2 pellets. The composite material with pore size 400 µm of the CaO/Ca(OH)
2 samples maintained high reactivity and bulk volume during cycle reactions. This study was extended by using a ceramic honeycomb support composed of SiC/Si [
84]. A volumetric energy density of 0.76 MJ/L-bed was obtained, and a heat output rate 1.8 times higher than the previously reported value for the pure Ca(OH)
2 pellet bed was achieved. In addition, the material also sustained high reactivity during the reaction cycles. Mixtures of expanded graphite (EG) with Ca(OH)
2 were also investigated by Kariya et al. [
85] with the aim of enhancing the heat transfer and reactivity of the hydroxide. The results indicated that the maximum mean heat output of a sample mixture containing 11 wt% EG was twice as high as the heat of pure Ca(OH)
2. The decreasing effect of EG in the hydration reaction in the repetitive cycles was due to particle pulverization.
The doping of Ca(OH)
2 by hexagonal boron nitride (HBN) was approached by Huang et al. [
86]. Analysis showed improvement in both thermal conductivity and dehydration enthalpy of the material. It also revealed a 15 wt% as optimal mass content of HBN-doped composite with improved activity after 10 dehydration/rehydration cycles. In addition, a 67% rehydration conversion and energy density greater than 1000 kJ/kg were achieved. Doping of Ca(OH)
2 with potassium nitrate, KNO
3, has also been reported by Shkatulov et al. [
87]. With a 5 wt% KNO
3 addition to Ca(OH)
2, the dehydration temperature of the material was reduced and the reaction rates increased, but the material lost its dehydration heat by 7%. Wang et al. [
88] obtained a similar result with a 10 wt% KNO
3 addition and the doped Ca(OH)
2 further showed good cycling stability in the nitrogen atmosphere, but failed in air. Gollsch et al. [
89] modified Ca(OH)
2 powder with nanostructured flow agents to improve the powder’s flowability. The additives consisted of nanostructured Si and/or Al
2O
3. Additives of weight fractions 6–12% improved the flowability of the powder. However, after cycling, the flowability of the mixtures decreased, while that of the pure powder increased. Analysis showed a correlation between growth in particle size and increased flowability. Additionally, the formation of phases in the additives led to a decrease in absolute heat release of up to 50%, although some of the side products seemingly added to the measured heat release by hydrating exothermally.
In tackling the problem of low conductivity and cohesive nature of powder bulk material, Mejia et al. [
90] investigated ceramic-encapsulated CaO granules and Ca(OH)
2 granules coated with Al
2O
3 nanostructured particles. The results showed that both encapsulated materials did not change their shape after six-fold cycling. However, the Al
2O
3-coated sample exhibited volume expansion during hydration. There was a reduction in the reaction activity of the ceramic sample, whereas the performance of the Al
2O
3 sample was almost the same as the unmodified Ca(OH)
2 particles. Afflerbach et al. [
91] investigated an encapsulated sample and good mechanical stability of the material was attained, followed by a considerably improved thermal conductivity. Again, over ten reaction cycles were attained in a lab-scale reactor. Additionally, issues concerning the poor flowability of the storage material and poor heat and mass transport with a strong agglomeration tendency could be overcome by persistent particle size stabilization. Of course, much is still required in terms of material enhancement, but research efforts so far show the Ca(OH)
2/CaO system to have higher prospects for long-term TCES application.
2.2.4. Decarbonation/Carbonation of Metal Carbonates
Decarbonation/carbonation reactions of metal carbonates have also proven to be attractive high-temperature heat storage systems. In this case, heat is used to perform the endothermic breakdown of carbonate, and the products are CO
2 and metal oxide. The interest in carbonates is due to their relatively high operating temperatures (typically over 800 °C), high volumetric density, low operating pressure, non-toxicity, abundance, and cheapness [
8,
13]. The decomposition of CaCO
3, SrCO
3, BaCO
3, MgCO
3, and PbCO
3 has been studied [
14]. Alas, the controversy about the high refractoriness of MgCO
3/MgO and the toxicity of PbCO
3/PbO [
9] is a drawback for further research in these materials. Additionally, the carbonation reaction of BaO into BaCO
3 was hindered by the melting of the material during the decomposition step [
92]. Therefore, among the carbonates, CaCO
3 is considered the most promising heat storage material [
9,
39] and the focus will be on the CaCO
3/CaO system. It has been reported that after 40 high-temperature carbonation/decarbonation cycles with CaO, the carbonation (adsorptive) reaction significantly decreased because of the decrease in pore volume in the material [
9]. This loss in porosity is caused by a decrease in the surface area of CaO due to the sintering of the particles [
14,
39], thereby inhibiting CO
2 access to the active sites within the material. Several techniques have been developed to minimize this loss in adsorption capacity. To increase the active surface area and stability of the pore structure, the use of additives, reduction in the particle size, and the synthesis of novel materials with the microporous structure were proposed [
93].
To this end, Lu and Wu [
94] doped nano CaO with Li
2SO
4 and showed that the Li
2SO
4-nano CaO adsorbent maintained a 51% conversion after 11 cycles, compared to pure nano CaO maintaining 27.3% under the same conditions. The superior performance of the Li
2SO
4-nano CaO adsorbent was attributed to pore enlargement and increase in macro-pore proportion through the Li
2SO
4 addition. Moreover, there were increased reaction rates and a lowering of the decomposition temperature by 15 °C in comparison with the pure material. In another work [
95], different MgO concentrations were added to the CaO material. The additions with 5 and 10 wt% of MgO exhibited high CO
2 adsorption and retention capacity over multiple cycles. In particular, the CaO with 10 wt% MgO exhibited steady adsorption capacity over 30 cycles. Similarly, Wang [
96] recently synthesized a porous MgO-stabilized nano CaO powder and realized highly effective long-term conversion because of its resistance to pore-plugging and sintering. Benitez-Guerrero [
97] reported the synthesis of porous CaO/SiO
2 composites through a bio-template route using calcium nitrate, Ca(NO
3)
2, and rice husk as support. The morphology and composition of the biomorphic material improved the CaO multicycle activity, as it served to enhance CaO and inhibited pore-plugging effects. The influence of SiO
2 on CaO/CaCO
3 was also studied by Chen et al. [
98]. The optimal 5 wt% SiO
2-doped CaCO
3 was demonstrated to enhance the reactivity and heat capacity, and led to a 28% enhancement of the reversibility, owing to the rise in grain boundary migration resistance.
Table 4 shows the cycling stability achievements of some dopants, adapted from [
98].
Binary metallic elements and oxides have been experimented with recently. For instance, composites of CaO doped with Mn and Fe were reported to enhance the cycling stability of the TCES material [
99]. A synergy between the small grain size and the reinforced skeletal structures prevented agglomeration of the composites, thereby enhancing their cycling stability. On the other hand, Sun et al. [
100] reported that 5 wt% Al
2O
3 and 5 wt% CeO
2 co-doped on CaO showed the highest and most stable energy storage capacity under the carbonation pressure of 1.3 MPa during 30 cycles. In addition, the synthetic material possessed strong basicity and provided a large surface area and pore volume during the multicycle energy storage. Again, Raganati et al. [
101] experimented with the application of an acoustic perturbation method that remarkably enhanced the carbonation performance of fine limestone particles. Indeed, it prevented agglomeration, which affects carbonation from both the gaseous (CO
2) and solid (CaCO
3) sides of the reaction, thus enhancing the fluidization quality, reactants’ contact, and mass transfer coefficients. More information on in situ data of CaCO
3 doping samples, measurement parameters, and results can be accessed in the work of Moller et al. [
102].
Calcium carbonate has the most economic advantage of being widespread, cheap, and having high gravimetric energy density (3029 kJ/kg). The high operating temperatures make this TCES system suitable for various applications such as integration with a solar furnace, and calcium-looping technology. However, this system is stable only up to 20 cycles without any degradation in the absorption capacity. The cycling stability and reversibility must, however, be improved up to 1000 cycles to make this system practical [
8].
2.2.5. Deoxygenation/Oxygenation of Metal Oxides
Suitable transition metal oxides undergo a reduction reaction at high temperatures, through which thermal energy is absorbed. The reversible re-oxidation takes place below specific equilibrium temperatures and hence thermal energy will be delivered [
103]. Thus, the reversible reduction/oxidation (redox) reactions of metal oxides show high potential as TCES materials. In comparison to the other TCES options, redox systems have the advantage of using air as both the heat transfer fluid (HTF) and the reactant. This eliminates the necessity for a different heat exchanger or gas storage needs. For this reason, TCES based on metal oxide redox reactions permits working with an open system [
14]. In this case, it is important to investigate these systems in consistency with the control of oxygen partial pressure (pO
2) [
14]. With lowering partial pressures of the reactive gas, the reduction temperature also decreases, as represented in the Van ’t Hoff diagram in
Figure 6.
By comparing metal oxide systems, it was revealed that only cobalt oxide (Co
3O
4), iron oxide (Fe
2O
3), copper oxide (CuO), and manganese oxide (Mn
3O
4) showed befitting reaction temperatures, enthalpies, cycling stabilities, and material costs [
103]. In another work, Silakhori et al. [
104] assessed the redox reactions of CuO/Cu
2O, Co
3O
4/CoO, Mn
2O
3/Mn
3O
4, and Pb
3O
4/PbO using TGA. The results showed that CuO/Cu
2O and Co
3O
4/CoO were highly reversible under isothermal pressure-swing cycles, while Mn
2O
3/Mn
3O
4 exhibited slight signs of sintering, and Pb
3O
4 was unreactive up to 550 °C. The free Gibbs energy (∆G°) was determined for several oxides and PbO
2/PbO, PbO
2/Pb
3O
4, Pb
3O
4/PbO, CuO/Cu
2O, and Sb
2O
5/Sb
2O
3 were confirmed to show thermal storage attributes based on negative ∆G°. Among these, CuO/Cu
2O displayed higher total enthalpy of 404.67 kJ/mol. However, the occurrence of phase transition was observed at temperatures near 1200 °C, and the molten state is prone to corrosiveness. Deutsch et al. [
105] carried out kinetic investigations of the CuO/Cu
2O reaction cycle under isothermal and isokinetic conditions and used simultaneous thermal analysis (STA) and a lab-scale fixed-bed reactor. The outcome of the reaction resulted in substantial discrepancies between both analyses. In STA, outstanding stability of the reaction over 20 cycles was shown with some sintering occurring, whereas heavy sintering occurred in the reactor, which hampered the reaction as well as increased the reaction time three times higher than previously reported values in the literature. Alonso et al. [
106] tested the suitability of CuO/Cu
2O in an argon atmosphere and the results indicated the reduction of CuO led to nearly 80% conversion. The reduction in air atmosphere was not favourable because of stronger coalescing particles that hindered the redox reactions. The synthesis of porous CuO-based granules with yttria-stabilized zirconia (YSZ) was also reported [
107]. The synthesized granules exhibited high conversion over 100 consecutive cycles in air between 950 and 1050 °C. Stable cycling performances were also obtained in the reactor for 30 consecutive isobaric and isothermal operation modes.
Cobalt and manganese oxides have also been considered promising redox systems for TCES. The CoO/Co
3O
4 system has the potential to be the most suitable pure metal oxide system for TCES due to its fast reaction kinetics and complete reaction reversibility. However, cobalt oxide is also considered potentially toxic and would not be cost-effective for large-scale storage [
108]. On the other hand, the Mn
2O
3/Mn
3O
4 redox couple is favoured in terms of minimal cost and toxicity in comparison to its alternatives and has been suggested as an appropriate material for TCES. However, several contentions have been singled out with respect to its capability of full energy storage sustainability with the required number of cycles necessary for this application [
109]. In view of this, Bielsa et al. [
109] studied several variables such as temperature and heating/cooling rates. A suitable choice of these variables was shown to enhance the heat storage capacity by 1.46 times in a 10-cycle test. The weight-change curves during the TGA are shown in
Figure 7, although several levels of sintering were observed, proving the major drawback of this material.
Andre et al. [
110] studied the impact of Fe, addition which decreased the redox activity and energy storage capacity of Co
3O
4. However, the cycling stability of Mn
2O
3 was significantly improved with added Fe amounts above 20 mol% while the energy storage capacity was unchanged. Similarly, a mixed oxide of Co-Cu-O with low amounts (<10 mol%) of Cu showed very good cycling stability and higher reaction enthalpy than the others (Mn-Cu-O and Co-Mn-O systems) [
108]. Neises et al. [
111] performed 30 cycles on a 5 wt% Al
2O
3-doped Co
3O
4 without any material degradation but yielding only a 50% conversion. This was attributed to the insufficient stirring and mixing of the metal oxide particle bed inside the reactor. Notwithstanding, about 400 kJ/Kg energy density was achieved per cycle. The graphical presentation of oxygen absorbed per mol of the doped Co
3O
4 during reduction is shown in
Figure 8.
In contrast, Carrillo et al. [
37] obtained results that indicated that operation with pure oxides (Mn
2O
3 and Co
3O
4) was more effective for TCES application compared to their mixed oxides. Although the values of heat released and absorbed for Mn
2O
3 were far from those obtained with Co
3O
4, its excellent cycling stability, low toxicity, and low cost make it an interesting candidate for heat storage applications. Relatively, the Cu
2O/CuO system has more prospects for TCES application than the CoO/Co
3O
4 system if the reaction is optimized in the reactor. In one experiment [
112], isothermal runs at different oxygen partial pressures were carried out with TGA, and defined fractions of CuO samples were analysed. The results revealed that the oxygen partial pressure affects the kinetics, and the reparameterization of the pressure term influences the kinetic analysis of the oxidation reaction. It was concluded that the models described for various parameters provide indispensable prerequisites for the redox reactor designs [
112].
So far, this section has presented focused discussions on the various aspects of TCES materials at low, medium, and high temperatures. It is, nevertheless, exigent to present a comparison of some techno-economic parameters of these materials, as summarized in
Table 5. The materials in the table are representative candidates according to each temperature domain and could help in making preliminary considerations for suitable technology.
Ferchaud et al. [
114] demonstrated that the heat stored (0.84 GJ/m
3) and released (0.71 GJ/m
3) in MgCl
2.6H
2O was, respectively, 79% and 87% in the storage cycle. The small difference in the charging/discharging heat was attributed to the textural changes in the material. In another TGA/DSC test, the MgCl
2.6H
2O material showed a 40% lower heat output after 25 cycles and a further 10% drop at the 28th cycle, compared to the first [
115]. An open sorption system using MgCl
2.6H
2O has also been tested, but the lower heat-recovery efficiency resulted in a power loss of nearly 70% [
116]. It is common knowledge that the actual energy density obtained from experimental prototype tests differs significantly from the theoretical energy density [
59].
For MgH
2, a multicycle system has been reported [
117], where cycle 1 represented the activation phase at around 300 °C. The desorption kinetics showed a slight improvement from cycle 4, and stabilized in cycles 6–8 with 90% storage efficiency. However, performance degradation of the system was noticed after 10 cycles with a storage capacity reduction of 50% after 20 cycles. However, a solar-heated MgH
2 material at 420 °C showed the metal hydride to be thermally cycled more than 20 times with a minimal loss in hydrogen capacity [
118]. A coupled TGA study of a modified MgH
2 powder also reported a cyclic conversion of 98.4% after 30 full cycles, with a calculated degradation rate of 0.00043 wt% per cycle [
119].
In TGA experiments, a remarkable 100% conversion efficiency and cycling stability of Ca(OH)
2 has been proven over 100 cycles up to 95.6 kPa vapour pressure [
78]. However, only 32 charge/discharge cycles could achieve 100% conversion efficiency at 100 kPa due to material structural failure [
80]. Yan and Zhao [
120] analysed the charging/discharging characteristics of Ca(OH)
2 and showed that heat storage efficiency increased with temperature (47% at 510 °C and 65% at 540 °C). However, higher heat-release efficiency could be achieved by reducing the temperature and increasing the vapour pressure. In particular, the conversion was 31.7%, 60.9%, and 72.8% under vapor pressures of 180 kPa, 240 kPa, and 320 kPa, respectively.
Generally, pure CaCO
3 does not show complete reversibility during decarbonation/carbonation cycles due to pore-plugging effects [
9]. The best performance reported a 40-cycle high-temperature carbonation/decarbonation run. The initial cycle was always 100%, but the cyclic conversion progressively decreased due to the loss of pore volume [
9]. A recent study on the multicyclic stability of different CaCO
3 minerals showed a limitation after 20 conversion cycles [
97]. A comparative study of a sulphate-modified CaCO
3 found a 51% cyclic conversion over 11 decarbonation/carbonation cycles, which was higher than the 27.3% of pure CaCO
3 [
121]. Thus, the addition of inert materials is seen as a viable option for improving the cycling stability of the CaCO
3. This is proven by CaO multicycle conversion data for different pretreated samples over 20 carbonation/decarbonation cycles carried out under calcium-looping-concentrated solar power (CaL−CSP) storage conditions.
For Co
3O
4, TGA results show rapid full thermal reduction and complete weight recovery for CoO oxidation to Co
3O
4 [
11]. Hutchings et al. [
122] reported 100 cycles of Co
3O
4 between 870 °C and 955 °C with 99% conversion efficiency and no evidence of degradation under these conditions. Again, Co
3O
4 powders exhibited long-term (30 cycles) performance with complete and reproducible cyclic redox performance within the temperature range of 800–1000 °C [
123]. It has been observed that TGA results generally show higher oxidation fractions with lower cooling rates. Neises et al. [
111] tested the solar-heated redox reaction of Co
3O
4 for 30 cycles in an integrated system and achieved a storage density of 400 kJ/kg per cycle. The system was only limited by insufficient mixing of the material.