The Integration of Thermal Energy Storage Within Metal Hydride Systems: A Comprehensive Review

Abstract

Hydrogen storage technologies are key enablers for the development of low-emission, sustainable energy supply chains, primarily due to the versatility of hydrogen as a clean energy carrier. Hydrogen can be utilized in both stationary and mobile power applications, and as a low-environmental-impact energy source for various industrial sectors, provided it is produced from renewable resources. However, efficient hydrogen storage remains a significant technical challenge. Conventional storage methods, such as compressed and liquefied hydrogen, suffer from energy losses and limited gravimetric and volumetric energy densities, highlighting the need for innovative storage solutions. One promising approach is hydrogen storage in metal hydrides, which offers advantages such as high storage capacities and flexibility in the temperature and pressure conditions required for hydrogen uptake and release, depending on the chosen material. However, these systems necessitate the careful management of the heat generated and absorbed during hydrogen absorption and desorption processes. Thermal energy storage (TES) systems provide a means to enhance the energy efficiency and cost-effectiveness of metal hydride-based storage by effectively coupling thermal management with hydrogen storage processes. This review introduces metal hydride materials for hydrogen storage, focusing on their thermophysical, thermodynamic, and kinetic properties. Additionally, it explores TES materials, including sensible, latent, and thermochemical energy storage options, with emphasis on those that operate at temperatures compatible with widely studied hydride systems. A detailed analysis of notable metal hydride–TES coupled systems from the literature is provided. Finally, the review assesses potential future developments in the field, offering guidance for researchers and engineers in advancing innovative and efficient hydrogen energy systems.

1. Introduction

One of the main challenges we currently face as a modern, technology-dependent society is guaranteeing universal access to our energy supplies, and ensuring their reliability as the global population continues to increase. The issue of climate change and the urgency to mitigate our overall carbon footprint have also imposed new restrictions on how we can continue to expand the energy infrastructure worldwide, raising the bar for our required efforts.

The world’s overall demand for electricity grew by 2.2% in 2023, mainly driven by emerging economies such as India and China. However, demand is expected to rise faster over the next two years, averaging 3.4% growth annually [1]. Projections indicate that energy consumption in the industrial sector will grow between 9% and 62%, and between 8% and 41% in the transportation sector from 2022 to 2050 [2]. Until recently, the majority of our newly installed energy infrastructure was reliant on fossil fuels such as petrol, coal, and natural gas, but zero-carbon technologies (including their storage) are quickly picking up the pace. They are expected to account for up to 81% to 95% of the new global generating capacity installed in the period between 2022 and 2050, with hydrogen emerging as an extremely attractive candidate energy carrier molecule. Hydrogen, when combusted or combined with oxygen in an electrochemical reaction, results only in water as a by-product, avoiding the production of any greenhouse gases during its consumption. On top of that, provided that the hydrogen is produced from renewable energy sources, it can become a completely non-polluting energy carrier during its entire life cycle. Molecular hydrogen is also the lightest known element and has the highest gravimetric energy density of all known substances, with a lower heating value (LHV) of approximately $120 \mathrm{kJ}·{\mathrm{g}}^{-1}$, three times higher than gasoline [3].

Despite its benefits, hydrogen still faces major bottlenecks impeding its widespread usage, with its transportation and storage presenting critical opportunities for improvement [4]. Hydrogen can currently be stored both through physical means, e.g., as a compressed gas, in liquefied form, and by cryo/cold compression, or it can be stored in materials and chemical bonds (e.g., adsorbents, ammonia, liquid organic carriers, and metal hydrides) through physisorption and chemisorption [5]. Even with the wide array of available options, most still present significant challenges to overcome. Physical storage methods require high-energy inputs, wasting a considerable part of the hydrogen’s energy potential. Hydrogen compression to 750 bar uses approximately 15% of its lower heating value, and cooling hydrogen down to −253 °C required for storage as a liquid consumes between 20 and 50% of its LHV [6]. For material-based storage options, the main drawbacks are the need for separation and storage of carrier molecules, the need for additional carrier molecule cracking or hydrogen extraction processes, and the potential health and environmental safety concerns during events of leakages [6,7]. It is in this context that metal hydrides (MHs) emerge as a promising option for energy storage in electronics, vehicles, and renewable energy systems. Metal hydrides have been known to store hydrogen at larger densities than that of liquid hydrogen [8], are considered reusable solutions (as they can store and release hydrogen multiple times), and their properties can be tailored to meet the needs of various applications with different capacity and temporal requirements [9,10]. Additionally, their ability to release hydrogen only under specific pressure/temperature conditions and their high thermodynamic stability makes them a safer alternative than other known solutions [10]. For metal hydrides to become a commercially viable solution, however, there is still much work to be done, particularly in terms of heat management [9]. The reaction for the storage (hydride formation) and release (hydride dissociation) of hydrogen in a metal hydride is known to produce or consume heat, respectively. The heat involved in these reactions can range from less than 10 $\mathrm{kJ}·{(\mathrm{mol} {\mathrm{H}}_2)}^{-1}$ to over 200 $\mathrm{kJ}·{(\mathrm{mol} {\mathrm{H}}_2)}^{-1}$, depending on the chemical nature and composition of the MH [11]. Both the removal and provision of heat from and to the MH can pose technical challenges and impact the economics of a MH hydrogen storage system. In particular, identifying a viable source to provide the heat for hydride dissociation, or finding a way to reduce the amount of heat demanded by the hydride, has thus presented itself as a major problem to be solved. Various strategies have been devised, including the use of waste heat from fuel cells [12,13,14] as well as the improvement of metal hydride properties themselves. The latter can be achieved through the use of preparation methods such as mechanical alloying [15], destabilization of the metal hydride alloy [16], and catalysts usage to overcome the reaction’s energy barriers [17], or the addition of high thermal conductivity materials [9].

Another promising approach to reduce the amount of external heat required to release hydrogen from metal hydrides is to couple them with thermal energy storage (TES) materials. The basic idea behind this concept is to store the heat produced by the MH during the hydrogen absorption process in a TES material. Later, this heat can be reused by the MH to release its stored hydrogen. By applying this concept, a MH–TES storage system with high thermal efficiency, good economic performance, and sustainability can be developed. The focus of this review article is therefore to explore the current developments in the field of utilizing metal hydride hydrogen storage systems in combination with thermal energy storage systems. The article is structured in the following way: first, an overview of the main working principles of metal hydride and thermal energy storage materials are presented; second, the technical concepts of facilitating heat exchange in metal hydride reactor systems are presented; third, the current developments in coupling metal hydride systems with TES systems are reviewed; and fourth, some future developments are proposed, which are necessary to improve the technical maturity of MH–TES systems. This review provides a reference point that can aid engineers and scientists to design MH–TES systems and provide valuable information for future developments in hydrogen storage technologies.

2. Metal Hydrides

2.1. Overview

The term ‘metal hydride’ refers to any kind of compound formed between metal and hydrogen atoms, in which these are chemically bound [9]. Materials of the metal hydride family have many modern technological applications, including hydrogen purification, isotope separation, acting as heat pumps, hydrogen compression, and—most importantly—hydrogen storage [18]. The fact that metal hydrides can be used for hydrogen storage is due to their ability to reversibly absorb and release H₂ gas in what are known as ‘hydriding’ and ‘dehydriding’ reactions, respectively [9]. These reactions are governed by the following general equation:

$$M_{(s)}+{\displaystyle \frac{x}{2}} H_{2 (g)} \iff M{H}_{x (s)}+Q$$

in which M is a metal-hydride-forming compound and Q is the amount of heat liberated during the reaction [15]. The conditions under which hydrating or dehydrating reactions are favored depend not only on the system’s temperature, but also on the H₂-partial pressure at any given time [19]. Generally, the hydriding reaction proceeds exothermically, while the dehydriding reaction is endothermic. The absolute value of the heat liberated/required during reaction depends on the composition of the hydride-forming material [6,15,20].

2.2. Classification

Metal hydrides can currently be classified into one of three main families: the binary (or elemental) hydrides, the ternary (or intermetallic) hydrides, and the complex hydrides [10,21].

Binary metal hydrides (AH_x) are the chemically simplest form a metal hydride can adopt, as hydrogen bonds to only one element [6]. It is known that most of the 91 naturally occurring elements are able to hydride under the right conditions [11], but not all of them do so in the same way. As a result of its particular electronic configuration, hydrogen (1s¹) is able to behave differently with individual elements, forming ionic hydrides with alkaline metals, covalent compounds with group 4a and 5a metals [18], and metallic bonds with transition metals, including the rare earth and actinide series [8] (Figure 1). While most of these compounds form hydrides under conditions that are too extreme to be of interest for practical applications [11], MgH₂ has been identified as a worthy candidate because of its favorable hydrogen-release properties and good reversibility at moderate conditions (with a hydrogen storage capacity of 7.6 wt.%) [22].

Ternary metal hydrides (A_nB_mH_x), on the other hand, are the result of reacting hydrogen with an alloy of two distinct metals (A and B). Because of the fact that upon absorption, hydrogen atoms occupy interstitial sites on the parent lattice, these ternary compounds are often referred to as ‘interstitial hydrides’ (along with a few binary MHs that behave the same) [18]. Alloys offer great benefits over binary hydrides in terms of versatility, stemming from the possibility of combining different metal behaviors and thermodynamic properties within a single compound, and thus opening the door for selective design [11]. Their biggest advantage so far is that they have demonstrated good hydrogen absorption rates and kinetics at near ambient conditions, while eliminating the need for thermal activation in most cases [6,10]. As a general rule, ternary hydrides are mixtures of stable (A) and unstable (B) hydride-forming metals, resulting in more complex structural configurations than binary hydrides [8]. They can be broadly classified according to their stoichiometry in one of five groups: AB, AB₂, A₂B, AB₅, and BCC solid solutions [8,25]. Other additional configurations (e.g., AB₃, A₂B₇, and A₆B₂₃) have been known to exhibit reversible hydrogen absorption behavior; however, they have either never really been considered as promising materials for practical applications, or only until very recently (as is the case for AB₃ and A₂B₇ alloys) [11,26]. Some of the most well-known and studied examples of ternary metal hydrides are TiFe (AB-type) and LaNi₅ (AB₅-type), due to their remarkable hydriding properties at room temperature [27].

The complex metal hydride group stands out from the rest, as in these cases hydrogen atoms covalently bond with transition metals to form anionic coordination compounds, that are then stabilized by certain Group IA or IIA elements [11]. These materials can be divided into transition-metal complex hydrides (such as Mg₂FeH₆ and Mg₂NiH₄ Mg₂CoH₅), borohydrides (like LiBH₄), alanates (like NaAlH₄), and amides (like LiNH₂ and Mg(NH₂)₂) [11,28]. Complex metal hydrides have been known to possess the highest theoretical hydrogen absorption capacities among all existing metal hydrides (18.5 wt.% in the case of lithium borohydride LiBH₄ [29]). However, high thermodynamic stabilities, kinetic barriers, and multi-step formation reactions with potentially irreversible side reactions limit their practical applications [20,30,31,32]. Additionally, their formation and decomposition reactions require a certain degree of metal atom diffusion, resulting in slow kinetics compared to interstitial hydrides [11]. Among the transition-metal complex hydrides, Mg₂FeH₆ is one of the most studied due to its very high volumetric hydrogen storage capacity ($150 \mathrm{g}·{\mathrm{L}}^{-1}$) and good cycling properties, although the system’s reversibility has been shown to depend strongly on the experimental conditions [33]. Additionally, the slow diffusion rate of Fe still leads to high temperature and pressure conditions being required for hydride formation [34]. LiBH₄ is another example of a widely studied complex metal hydride. However, because of the high stability of LiH [35], the desorption reaction of LiBH₄ will yield LiH and B, reducing its practical reversible capacity to around 13.8 wt.% [35,36]. Even then, the rehydrogenation of LiH + B still requires very harsh conditions (155 bar and 600 °C) [37].

One approach to improve the reversibility of some of these complex metal hydrides are the so-called reactive hydride composites (RHCs), mixtures of complex metal hydrides with more traditional hydride forms (binary/ternary), that can react together exothermically to release hydrogen and form a new, more stable, compound [38]. For example, through the mixture of LiBH₄ and MgH₂ in the stoichiometric proportion of 2:1, the so-called Li-RHC can be produced [36]. While the absorption enthalpies for LiBH₄ and MgH₂ are, respectively, 67 and 76 $\mathrm{kJ}·{(\mathrm{mol} {\mathrm{H}}_2)}^{-1}$, it has been found that the theoretical value for this composite is reduced to 46 kJ/mol H₂ [39]. Said reduction is caused by the formation of MgB₂ upon dehydrogenation, which is much more stable than the products of the individual decomposition reactions of MgH₂ and LiBH₄ (Mg and B, respectively) [40]. While this composite has a high theoretical gravimetric capacity of 11.4 wt.%, the involved reactions are rather complex. The absorption reaction proceeds as a one-step reaction [41], but desorption proceeds in two steps, with the fast decomposition of MgH₂ followed up by a slow reaction for the formation of MgB₂ and LiH as H₂ is released [42].

2.3. Thermodynamics and Kinetics

To evaluate the potential coupling of metal hydrides with TES materials, it is first necessary to understand the thermodynamic and kinetic phenomena that govern their ability to store hydrogen. All chemical compounds store energy in their bonds, and the amount of energy stored can change whenever said compound undergoes a structural reconfiguration. For hydrogen uptake by metals, the net energy change between the reactants and the products over the course of the reaction is generally negative (exothermic reaction), meaning that the system releases excess energy and a more stable compound is formed [43]. This behavior is illustrated in Figure 2.

As shown in Figure 2, metal hydriding occurs via the following sequence of steps: physisorption of molecular hydrogen, dissociation of the hydrogen molecule and coordination with metal atoms (chemisorption), hydrogen penetration and diffusion into the host lattice, and finally, hydride nucleation and phase progression. These steps will be briefly explained, as they will provide us with valuable insights on how MH performance can be improved for practical applications.

Following the pressure-induced mass transport of hydrogen towards the metal, the H₂ molecules adhere to the metal surface as a result of Van der Waals forces, in a process known as physisorption [15,45]. At the surface, hydrogen is then chemisorbed into the metal: the molecules split into individual hydrogen atoms at specific dissociation sites, and consequently coordinate with the exposed metal atoms [45]. These sites have been found to be various physical defects in the metal surface, and in the case of alloys, clusters of the least stable-hydride forming metal (B) [15,45,46]. Once individual hydrogen atoms have been made available, those with high energies penetrate into the metal, to occupy octahedral and tetrahedral interstitial sites and form a solid-solution phase, otherwise known as the α-phase (or low-concentration phase). Hydrogenation then proceeds with further diffusion of hydrogen atoms into the metal’s bulk structure, a process that can occur at rates comparable to those for ions in aqueous solutions, one of the many reasons why metal hydrides are so effective at storing hydrogen. By the time enough hydrogen has diffused, and the concentration of hydrogen reaches its saturation point, a homogeneous higher concentration phase (commonly known as β-phase) begins to nucleate. This new phase is what is ultimately, and formally, known as the ‘metal hydride’ [45,47,48]. The nucleation of the metal hydride phase usually follows the hydrogen concentration gradient in metals, preferring zones with the lowest activation energy and paths of higher diffusion, such as the metal surface and grain boundaries [49]. Dehydriding reactions can be thought to proceed through these mechanisms in the reversed order [50].

Pressure-composition-isotherms (PCIs) are recorded to describe the thermodynamics of hydrogen sorption, as shown in Figure 3a.

Three main regions are easily identifiable on the ideal PCI graph and separated by a dotted curve (the biphasic zone, inside the dotted curve; the α-phase, to the left of the biphasic zone; and the β-phase, to the right of the biphasic zone). For each of the isotherm (solid) lines, the following pattern is observed from left to right, according to the correct sequence of steps for hydride formation. In the low hydrogen concentration region (α-phase), hydrogen concentration in the metal (C_H) can be found to increase along with the square root of the partial pressure of hydrogen (P_H2), a relationship that can be described by Sievert’s Law (Equation (1)), where K_H is Sievert’s constant [47,51]:

$$C_H=K_H\sqrt{P_{H_2}}$$

(1)

By the time the dotted phase boundary curve is reached, the system is at its saturation point, and the β-phase begins to nucleate. Here, the system attains its equilibrium pressure (P_eq), and a pressure plateau appears. Hydrogen concentration can now increase with no changes in the partial pressure of hydrogen. The width of the biphasic zone is therefore referred to as the ‘reversible hydrogen storage capacity’, which is also visible in Figure 3c. While the system is in equilibrium, the pressure values are, however, still dependent on temperature conditions. Said relationship can be expressed through the Van’t Hoff equation (Equation (2)):

$$ln\left({\displaystyle \frac{P_{eq}}{P_0}}\right)={\displaystyle \frac{\Delta H}{R}}\left({\displaystyle \frac{1}{T}}\right)+{\displaystyle \frac{\Delta S}{R}}$$

(2)

where P₀ is the reference pressure, T is the system temperature, ΔH and ΔS are the net enthalpy and entropy changes in the reaction, respectively, and R is the ideal gas constant [52]. This relationship can be found in graphical form in Figure 3b. Figure 3c shows a schematic representation of a real PCT curve. The presence of deviations from an ideally flat plateau become evident in the form of a plateau slope and hysteresis. Plateau slopes are generally attributed to compositional inhomogeneities, and the relaxation of the metal matrix that results from lattice expansion during metal hydriding [8]. Hysteresis, on the other hand, mainly refers to the difference in equilibrium pressures between absorption and desorption plateaus [53], with hydrogenation occurring at a lower equilibrium pressure. There are, however, further implications of hysteresis: differences in the saturation point of the α-phase during absorption and desorption become present (solvus hysteresis), as well as differences in the formation and decomposition temperatures of metal hydrides (thermal hysteresis) [54]. All of these hysteresis-related behaviors have been attributed to the creation of additional energy barriers by either the plastic deformation of the metal lattice (Flanagan–Clewley theory) [55], or by the elastic strains exerted on it (Schwarz–Khachaturyan theory) [56] during hydrogen absorption.

The kinetic behavior of gas–solid reactions, which include MH–H₂ reactions, can be described by three main components, as shown in Equation (3):

$${\displaystyle \frac{d\alpha }{dt}}=K(T)\times F(P)\times G(\alpha )$$

(3)

in which alpha (α) is the reacted fraction, K(T) is the temperature dependency, F(P) is the pressure dependency, and G(α) is associated with the morphological changes experienced by the metal during the course of the reaction [50]. Temperature has a very important impact on the kinetic behavior of hydration/dehydration reactions, and its influence on the overall reaction rate can be described through the Arrhenius equation (Equation (4)):

$$K(T)=A·exp\left({\displaystyle \frac{-E_a}{R·T}}\right)$$

(4)

where A is the pre-exponential factor, E_a is the activation energy, and R is the ideal gas constant [50]. The pressure dependency F(P) term is associated with the driving force of the chemical reaction (in this case, pressure), and is typically represented by an empirical mathematical relationship between the system’s pressure and the hydride’s equilibrium pressure P_eq at the operating temperature (T) [41,42]. These expressions usually differ for hydriding and dehydriding reactions. Finally, it is worth mentioning that the morphological changes in the metal hydride during hydrogen absorption/desorption can be accounted for by the assumptions made in specific mathematical models for gas–solid reactions, whose integral form can be used to derive an expression for G(α). Some models that have been developed and proved experimentally throughout the years include nucleation and growth models, geometrical contracting models, diffusion models, and autocatalytic models. A few of them are shown in Figure 4. These, along with their variations have been thoroughly reviewed and described in the works of J. A. Puszkiel [50], Wang and Suda [57], and Pang and Li [58].

Another aspect that should be mentioned briefly is that both hydriding and dehydriding reactions, like all multi-step chemical reactions, have what is considered a ‘rate-limiting step’. This term refers to the slowest step in a sequence of chemical reactions and the overall reaction cannot occur faster than the speed of its rate-limiting step [59]. For the absorption/desorption of hydrogen in metals, it is widely agreed that the rate-limiting step can be any of the following, depending on the specific metal and the experimental conditions: a surface related process such as H₂ dissociation and penetration, the diffusion of H₂ within the metal, or a phase transformation step (metal-metal hydride or between hydride phases) [49]. The rate-limiting step is considered within the G(α) term in the overall reaction rate expression [48], as it is part of the assumptions made for the different mathematical gas–solid models mentioned in the last paragraph. Having said this, the determination of the rate-limiting step for specific metal hydrides therefore plays an important role in selecting specific methods, and modifying process conditions, to improve the overall reaction kinetics.

One of the most common methods to tailor reaction kinetics in metal hydrides includes the addition of catalysts. Oxides and halides of multivalent transition metals have been extensively studied and proven as viable options [17]. Catalysts have been found to facilitate hydrogen dissociation at the gas–metal interface, reduce the energy barrier for hydrogen diffusion into the bulk metal, and act as nucleation centers for both the hydrogenated and dehydrogenated metal phases [60,61]. Some typical heterogeneous catalysts used to enhance the kinetics of binary and complex hydrides are Ti, Co, Fe, V, Nb, as well as their oxides and halides, while metals such as Cr, Mn, Fe and Ni are preferred for interstitial MHs [17,62]. These additives significantly increase the hydrogenation and dehydrogenation rates in MH. For example, Zhang et al. found that adding 10 wt.% Mn₃O₄ nanoparticles to MgH₂ can decrease the apparent activation energy for H₂ absorption from 72.5 to 34.4 $\mathrm{kJ}·{(\mathrm{mol} {\mathrm{H}}_2)}^{-1}$ and reduce the temperature required to desorb 6 wt.% H₂ by approximately 100 °C [63]. Furthermore, Ali et al. showed that upon doping LiAlH₄ with 10 wt.% MgFe₂O₄, 4 wt.% H₂ could be successfully desorbed after 40 min at 90 °C and 1 bar(a), compared to no desorption at all in the same conditions for pristine LiAlH₄ [64]. Similarly to catalyst addition, partial metal atom substitution in intermetallic hydride matrices has also been proven not only to speed up reaction kinetics, but to also lower pressure plateaus and improve cyclability [10,65]. Substitution can either be conducted with a single element or multiple elements, with alkali-earth metals, transition metals, p-block elements, rare-earth metals and even non-metals having been studied as candidates [65].

While understanding the reaction kinetics of gas–solid interactions in hydride systems plays a pivotal role in overcoming some material limitations, it is well known that as the characteristic dimensions of hydride-based H₂ storage systems increase, the hydrogen release rate will depend much more on the heat transfer than on the intrinsic kinetics of the hydride [66]. Therefore, considering the high amount of heat associated with gas–solid reactions in hydride materials and the typical low heat conductivity of metal hydrides, managing the heat release or consumption during hydriding/dehydriding reactions is a critical factor to consider if adequate reaction rates are to be maintained. To provide a reference point on the thermodynamics of selected metal hydriding and dehydriding reactions,

Table 1. Absorption and desorption enthalpies and entropies for select metal hydrides.

Material	Absorption		Desorption		Other	References
	Enthalpy [$\mathbf{k}\mathbf{J} {\mathbf{m}\mathbf{o}\mathbf{l}}^{-1}{\mathbf{H}}_2$]	Entropy [$\mathbf{J} {\mathbf{m}\mathbf{o}\mathbf{l}}^{-1}{\mathbf{H}}_2{\mathbf{K}}^{-1}$]	Enthalpy [$\mathbf{k}\mathbf{J} {\mathbf{m}\mathbf{o}\mathbf{l}}^{-1}{\mathbf{H}}_2$]	Entropy [$\mathbf{J} {\mathbf{m}\mathbf{o}\mathbf{l}}^{-1}{\mathbf{H}}_2{\mathbf{K}}^{-1}$]	Hysteresis [$\oslash$]
MgH2	−74.6	−134.8	74.6 *	134.8 *	N/A	[67,68]
TiFe (L)	−24.3	−100	27.4	103	N/A	[69]
TiFe (U)	N/A	N/A	N/A	N/A	N/A	[69]
TiFe0.85Mn0.05 (L)	−27.8	−99	30.6	103	N/A	[69]
TiFe0.85Mn0.05 (U)	−32.5	−121	35.2	126	N/A	[69]
ZrNi	−68.2	−125.4	73.6	127.6	N/A	[70]
TiMn1.5	N/A	N/A	28.7	114	0.93	[11]
TiCr1.8	N/A	N/A	20.2	111	0.11	[11]
Hydralloy C5	−22.69	−97.2	27.83	109.90	N/A	[71]
Mg2Ni	−57.47	−94.94	61.26	99.26	N/A	[72]
LaNi5	−28.4	−102.3	28.3	100.2	N/A	[73]
CaNi5	N/A	N/A	31.9	101	0.16	[11]
MmNi5	N/A	N/A	21.1	97	1.65	[11]
CeNi5	−17.0	−105.8	22.2	111.0	N/A	[74]
NaAlH4 (Step 1)	−35.1	−118.1	38.4	126.3	N/A	[75,76]
NaAlH4 (Step 2)	−46.1	−123.8	47.6	126.1	N/A	[75]
NaBH4	N/A	N/A	108	133	N/A	[77]
Mg2FeH6	−66	−124	67 (Step 1)	123 (Step 1)	N/A	[78]
			80 (Step 2)	137 (Step 2)
Li3N (LiNH2-LiH)	−66.1	−118	66.6	120	N/A	[79,80]
Li-RHC	−34	−70	76 (Step 1)	110 (Step 1)	N/A	[41,42,81]
			61 (Step 2)	107 (Step 2)

L = lower plateau; U = upper plateau. * Values correspond to MgH₂ absorption, but were presented here like this since it is known that the hysteresis for MgH₂ is negligible [68].

presents a summary of prominent metal hydride materials and reactive hydride composites.

If heat management strategies are not sufficiently well designed, the temperature changes resulting from heat accumulation/extraction could shift the equilibrium pressure sufficiently to considerably alter hydrogen absorption/release rates [8], or even invert the direction of the process completely [82]. Hence, due to the importance of heat and mass transport for the performance of hydride-based H₂ storage systems, the next section is dedicated to covering the physical and thermophysical properties of these materials.

2.4. Physical and Thermophysical Properties

Some of the most important properties for the practical application of MHs as hydrogen storage materials are their volumetric and gravimetric energy storage densities. As previously mentioned, every metal hydride can only store a limited amount of hydrogen. Gravimetric and volumetric storage densities refer to the hydrogen stored per unit mass and volume, respectively.For convenience purposes, when comparing hydrides to other energy storage technologies, these capacities are often converted to energy densities using the LHV of hydrogen, and consequently become known as gravimetric energy density or volumetric energy density, respectively [83]. Excelling both in terms of gravimetric and volumetric energy densities has been identified as a critical factor to competitively introduce metal hydrides, especially in mobile applications [6]. In terms of stationary storage applications, volumetric energy densities tend to be of much greater importance than gravimetric densities, given the need to minimize costs associated with land use [27]. Having said this, it is worth mentioning that the volumetric energy densities of metal hydrides can be further improved, with metal-hydride powder compaction into pellets already being a proven and commonly used method [84,85,86,87]. Additionally, Safyari et al. have recently identified that exposing Mg–Ni based alloys to small amounts of oxygen on the second absorption cycle leads to improved storage capacities (and therefore volumetric energy densities), but more research is needed on the topic [88].

The effective heat conductivity of the MH bed is another property upon which hydrogen uptake and release depends, given that it can limit the rate at which heat is transferred within the hydride, and therefore limit the rate at which the absorption and desorption reactions occur [66,82,89]. The effective heat conductivity is not only a function of the individual thermal conductivities of both the metal and the interstitial hydrogen atoms, but also of pressure, temperature, reacted hydrogen fraction, porosity, and particle size [90,91]. Reference values for the effective thermal conductivities of some common metal hydride powders are provided in

Table 2. Effective thermal conductivities of some selected metal hydride powders under hydrogen atmosphere.

Material	Effective Thermal Conductivity [W/(m·K)]	Pressure [bar]	Temperature [°C]	Reference
Mg	0.64–1.24	0.97–29.61	25–410	[92]
Mg2Ni	0.35–0.75	1–50	35–200	[93]
LaNi4.7Al0.3	0.878	1	20	[94]
TiMn1.5	0.2–1.3	1–50	21	[95]
NaAlH4	0.46–0.55	1	25	[96]

.

Increasing either the pressure or the temperature of the system has been found to increase the effective heat conductivity of the hydride bed [97]. Regarding the reacted hydrogen fraction, it has been observed that the effective heat conductivity will diminish as the reacted fraction increases; this happens because adding more hydrogen atoms into the metal lattice blocks the ability of free electrons to conduct heat. Similarly, a smaller particle size in the hydride bed leads to larger surface areas, and therefore an increase in thermal contact resistance, which brings down the heat conductivity. However, smaller powder particles may agglomerate and sinter to form a denser structure during hydriding/dehydriding cycles, leading to a rise in thermal conductivity throughout the hydride’s lifetime [82]. The agglomeration of metal hydride particles can also be used to explain the relationship between porosity and conductivity. As the particles agglomerate, the reactor bed becomes less porous, and there is less room for hydrogen gas (whose heat conductivity is smaller than that of the bulk metal) in a given volume of hydride [98]. Therefore, in less porous beds, the effective thermal conductivity’s value approaches that of the bulk metal, which is greater. It is relevant to mention that several strategies have been identified to avoid these changes that occur during cycling, or to generally increase the effective thermal conductivity of the MH bed. These consist of the addition of high-conductivity materials, mechanical compaction, or the combination of both. Some of the more widely applied high-conductivity materials include carbon-based materials (graphite powder, carbon nanotubes, carbon bars, and expanded graphite) [9], metal foams and powders (aluminum, nickel, copper) [82,99], or copper mesh [9]. None of these materials will react with the metal hydride powders, and they will fill the void space in the hydride bed, increasing contact between particles to create more effective heat conduction channels [9,82].

Microstructural aspects of metal hydrides such as their crystallinity must also be taken into account for hydride-based H₂ storage systems, as they can provide crucial insights on how to maximize their storage capacities and for the selection of adequate preparation methods for a specific metal hydride. It is known that most conventional metal hydrides are crystalline phases; however, materials with varying degrees of crystallinity have been investigated as potential hydrogen storage candidates. Amorphous metal hydrides have been reported to possess much larger hydrogenation capacities, faster kinetics, and lower dehydrogenation temperatures as a consequence of the increased availability of interstitial sites resulting from their disordered structure [100]. However, the absence of a pressure plateau, poor structural stability, and reversibility are still challenges that need to be overcome [11,100]. In a similar manner, quasicrystals have been shown to possess a greater number of interstitial sites than crystalline metal alloys, increasing their capacity to store H₂; TiZr-based quasicrystals have been identified as good candidates for H₂ storage due to their low thermodynamic stability [101,102].

Another important behavior that must be considered for practical applications of metal hydrides is the lattice expansion of metals resulting from the occupation of interstitial sites by hydrogen and the formation of the β-phase. Upon repeated hydrogenation and dehydrogenation, the constant expansion–contraction of the metal hydride lattice (up to an additional 25% of the original size) can lead to cracking in metal hydrides and their subsequent pulverization, a phenomenon also known as decrepitation [8,9]. If not properly managed, decrepitation can result in the metal hydride powder sinking to the bottom of the reactor and, in combination with the expansion–contraction cycles, induce enough stress on the reactor walls to be able to deform or even destroy them [11,103]. Various techniques have been long known to mitigate the consequences of volume expansion. Some relevant examples are strengthening container walls, lubricating the reactor bed with non-volatile oils such as silicon, adding aluminum powder or fiber, or partially substituting the metal matrixes with other metallic atoms [103,104,105].

2.5. Hydride Based H₂ Storage Systems

Metal hydrides present several advantages in comparison with more traditional H₂ storage methods. To begin with, hydrides are able to store hydrogen at a very wide range of temperatures and pressures (including ambient conditions), whereas other alternatives have very specific requirements to successfully store it. For instance, some methods such as liquified hydrogen storage and the use of some solid adsorbents (organic polymer networks and metal–organic frameworks) require cryogenic temperatures of −253 °C and ca. −196 °C [106,107], respectively), while gaseous hydrogen storage involves elevated pressures that typically range from 350 to 750 bar, and cryo-compression needs the combination of both high pressures and very low temperatures [106]. Besides the fact that maintaining these temperatures and pressures translates to increased operational costs, the use of specialized tanks (with reinforced walls, extreme insulation, and lightweight materials) is usually necessary to account for such harsh storage conditions [106,108], meaning a larger capital investment is required. Metal hydrides also excel in terms of volumetric hydrogen storage densities, outcompeting both compressed and liquified hydrogen [10]. Their performance is only rivaled by some chemical hydrogen carriers, such as liquid organic hydrogen carriers (LOHCs) or ammonia [109], but it has been found that metal hydrides can be more energy efficient than both of these options [110]. Regarding safety, metal hydrides are advantageous over other H₂ storage alternatives mainly because of the endothermic nature of their desorption reactions, and their high bond stability [106]. Having an endothermic hydrogen release reaction allows for the careful control of H₂ gas exposure to the tank surroundings, as it is only released when heat is supplied. Additionally, a high bonding energy means that desorption will not occur without external heat supply. Therefore, spontaneous H₂ leaks that might lead to accidents, and even tank depletion (as is the case in passive boil-off for liquid hydrogen storage [111]) can be avoided. The possibility of operating metal hydride storage tanks at near-ambient pressure conditions (depending on the selected metal hydride) also eliminates the risk of explosions or jet fire in the case of a tank rupture, as sometimes occurs with gaseous hydrogen tanks [112]. Finally, metal hydrides are promising options for H₂ storage because of their compositional and structural diversity. By choosing specific alloys, preparation methods and additives, their properties can be tailored to suit very particular operating requirements.

Despite their advantages, metal hydrides still present some drawbacks, such as their high weights, high material costs, poor reversibility, poor reaction kinetics, and the requirement for high desorption temperatures [106]. Nonetheless, the benefits of using hydrides for H₂ storage outweigh their disadvantages, as can be seen by their gradual incorporation into existing energy systems.

The potential of MH reactors for energy systems has already been demonstrated by real world applications, mainly in the transportation and energy storage sectors. The German and Italian navies have implemented hydride-based power systems in submarines [6], and pilot studies with mini-passenger trains [113,114], forklifts [115], and canal boats [116] have been successfully carried out. Road-bound applications, however, are still very scarce as no metal hydride has managed to accomplish all the targets set by the DOE [117] for on-board H₂ storage yet. In terms of stationary energy storage, off-grid hydrogen energy storage systems for buildings, hotels, and small communities [118], as well as auxiliary power units [119,120], have been tested at a pilot scale. Furthermore, the use of metal hydrides in heating/cooling systems and as thermal energy storage systems has received a lot of attention lately, with prototypes for heat transformers, heat pumps, refrigeration systems, steam production systems, and CSP energy storage having already been built and tested [121]. Despite the wide variety of current implementations, most of the other lesser-known applications of hydrides that might benefit from thermal energy storage coupling (such as hydrogen purification) are still in the experimental phase. In-depth reviews of the current developments can be found in the scientific literature [6,122,123]. The following paragraphs will address some key design aspects of MH H₂ storage systems, for consideration when coupling with TES solutions.

The first crucial factor is the geometry of the hydride reactor. A proper reactor design allows for the control of heat and mass transfer in a hydride bed, and thus of the metal hydride’s performance in practical applications [9,124]. Reactor geometries can be mainly divided into the following categories, as shown in Figure 5: tubular, tank, planar, disk, and spherical.

Although there are some existing studies on planar reactors with a rectangular geometry [125,126], these are not widely adopted. Their planar shape does not allow for a uniform distribution of mechanical stresses generated by the gas pressure, thus making their use with high operating pressures not viable [127]. Cylindrical tubular and tank-type reactors, on the other hand, are the most widely employed configurations, given that they facilitate the equal distribution of heat and mass transfer within the bed, tolerate high operating pressures/stresses, and are relatively easy to manufacture [124,127,128]. Tubular reactors typically employ a ‘central artery’ to feed hydrogen into the hydride bed, while tank reactors have a larger volume, and involve multiple arteries or surrounding outer filters for hydrogen diffusion [9,127,129]. It is worth mentioning that despite both tubular- and tank-type reactors commonly involving circular cross-sections, they are not limited to it; elliptical configurations have been proposed for the former [130], while the latter can come in cubic shapes [9,129]. Additionally, one advantage that tubular reactors present is that the individual units can be treated as modules, and bundled up whenever there is a demand for more hydrogen storage capacity [131,132]. Disk reactors are composed of a flat metal hydride bed, with hydrogen flowing axially in and out of the reactor, and a large surface area enables efficient heat exchange and therefore remarkable hydrogen uptake/release rates [124,129]. Recent progress has shed light on their true potential, outperforming even multi-tubular reactors, particularly in modular annulus configurations [133,134,135].

Another aspect to consider in terms of reactor geometry is their orientation. It has been documented that horizontal reactors are less prone to stress accumulation than their vertically oriented counterparts, with particle agglomeration in the latter reaching densities so high that they were able to deform thermocouples inside the reactor [103,136]. These aspects must be taken into account when contemplating the integration of internal heat exchangers.

When designing H₂ storage systems, taking heat management strategies into consideration is extremely important. As aforementioned, if heat is not extracted/supplied at the appropriate rate from the metal hydride bed, temperature changes in the powder bed might bring the system very close to its equilibrium conditions, dramatically decreasing the hydrogen release/uptake rates. An illustrative example of the demand placed upon heat management technologies is the case of Nb₂O₅-doped MgH₂ reactors. These metal hydride beds can absorb over 5 wt.% H₂ in 30 s; if translated to a tank containing 5 kg H₂, then 500 kW of heat would need to be extracted from the reactor to successfully store said amount of hydrogen [8,137]. The most common solutions to the problem of heat accumulation, in terms of reactor design, are reducing the distance through which heat must travel to escape, increasing the heat transfer area, and improving the heat transfer coefficient [128]. Internal heat exchangers can satisfy all three requirements, while external apparatuses such as jackets are only able to contribute to the improvement of the last two. However, combining both is also a possible alternative. Developments in internal heat exchangers have mostly focused on tube shape/placement/number, and fin design. Cooling and heating tubes can be straight, U-shaped, helical or coiled, while alternatives such as embossed plate or annular heat exchangers can also be installed [124,128]. An illustration of these common types of heat-exchange tubes can be found in Figure 6.

Past studies have established the superiority of helical tubes over straight and finned tubes, even highlighting the improvements that can be made by fine tuning the pitch-to-diameter ratio [138,139]. Simulations have similarly shown that doubled U-shaped tubes allow for reduced charging times compared to straight cooling tubes [140]. In terms of the number of tubes and tube passes, a large collection of studies have confirmed that increasing both of these values within a reactor has positive effects on reaction kinetics and temperature distribution [141,142,143,144,145,146]. However, this behavior is observed only up to the point at which a certain number of tubes are added [141,142] and an optimization-based approach should also be considered, so as to keep the system weight within acceptable levels and avoid complicated manufacturing. Tube placement within the reactor is an additional factor to consider for adequate heat management. Krokos et al. studied five different configurations of nine cooling tubes in a cylindrical reactor, and found that charging and cooling times were reduced to a minimum when the tubes were equally spread out in the metal hydride bed [132]. Liu et al. studied configurations of two, three, and four pipes (arranged either in the corners of a square, or in the vertices of an equilateral triangle), and found that the least time to achieve 90% saturation of the hydride bed was obtained with the four tubes arranged as a square [141]. Additionally, they identified an overall decreasing trend in saturation time as the tubes are placed farther away from the center, but when they are placed too close to the reactor wall, the reaction rate slightly increases. Raju et al. also reported a positive correlation between H₂ uptake and tube separation; although, an increasing tube diameter for each of the cases analyzed must be considered [147].

Fin design has proved to be another aspect of great relevance for heat removal, mainly by increasing the heat-transfer area, as well as providing better performance in terms of local thermal conductivity and natural convection, for internal and external fins, respectively [148]. Typically, internal fins, which are placed inside the metal hydride bed, can be found in either transverse/radial or longitudinal form, with respect to the orientation of the heat exchange tube. It has been reported by several authors that both types of fins exhibit very similar heat-transfer performances overall, with the longitudinal variation exhibiting slightly better performance [149,150]. A wide variety of less conventional fin designs are also evaluated in the literature, such as conical fins [151], perforated disk-shaped fins [152], pin-shaped fins [153], fan or quadratically arranged fins [154], honeycomb patterned fins [155], tree-shaped fins designed by genetic algorithms [156], or more complex 3-D printed patterns [124]. Regarding other fin parameters, it has been found that an increase in fin length [157,158,159], and fin thickness [152,157] both improve hydrogen uptake/release rates, but none are more influential than fin number in that aspect [126,152,160]. A crucial consideration to be made when employing fins is that they can consume a considerable portion of the metal reactor volume, therefore leading to a decrease in its volumetric and gravimetric energy density. Even optimized designs can result in 29% of the reactor internal volume being occupied by fins [161].

External fins can be utilized to facilitate heat transfer between the reactor outside wall and a cooling medium. It has been found that the effectiveness of external fins is greatly dependent on the cooling fluid, whereas internal fins are not [162]. Using water jackets by themselves has been found to outperform both external fins [163] and straight cooling tubes [164], emerging as a good option. Nonetheless, they still underperform compared to helical cooling tubes [164,165], and adding fins to the jacket has in some occasions proved to be one of the best options for heat extraction from the metal hydride bed [157,164].

3. Thermal Energy Storage (TES)

3.1. Types of TES and Working Principle

TES technologies are one of the most prevalent energy storage systems (ESSs) and are, depending on the material, able to be used both as short- and long-term energy storage media [166]. The three main categories of TES systems are sensible, latent, and thermochemical energy storage, which are grouped based on the main physical phenomena that govern the ability of the TES materials to store heat [166,167].

Sensible heat storage systems are the simplest of the three types of TES technologies, and refer to the storage of heat associated with a change in the temperature of a given material, without involving a phase change [168]. Latent heat storage systems, aside from the energy stored through a change in temperature, can also store additional energy due to the presence of a phase transition, which usually results in these materials having a higher specific energy storage capacity than sensible heat storage materials [166]. Finally, thermochemical energy storage (TCS) systems make use of chemical interactions in sorption-based and reaction-based materials as a means of storing and liberating heat whenever it is required [169,170].

The materials discussed in this work are presented schematically under the appropriate classification in Figure 7. The materials were chosen based on their perceived relevance and potential for coupling with hydrogen storage systems. Also, a summary of the most important characteristics of each of these storage methods is described in

Table 3. Summary of main characteristics of sensible, latent and thermochemical thermal energy storage materials.

TES Aspect	Sensible	Latent	Thermochemical
Complexity	Lowest	Intermediary	Highest
Technology Maturity	Mature	New Technology	Under development/mostly laboratory scale
Type of Materials	Rock, Pebble, Sand, Gravel, Water, Thermal Oil, Salt, Salt Eutectics, etc.	Alkenes (Paraffins), Alcohols, Esters, Fatty Acids, Salt, Salt Eutectics, Metals, Metallic Alloys, etc.	Adsorption materials (e.g., Silica Gel/H2O, Zeolite/H2O), Absorption materials (e.g., LiBr/H2O, LiCl/H2O) and reaction-type materials (e.g., Hydroxides, Metal Oxides, Carbonates, Hydride Materials, Salt Hydrites)
Gravimetric Energy Storage Density [kWh/kg]	~0.02–0.03	~ 0.05–0.10	~0.5–1.0
Volumetric Energy Storage Density [kWh/m3]	~50	~100	~500
Energy Losses Over Time	Yes	Yes	No losses associated with long-term storage (except heating and cooling)
Operation Temperature Range	From room temperature to over 1000 °C	From room temperature to around 1000 °C	From room temperature to around 1500 °C
Comments	Outlet temperature variable	Narrower outlet temperature (in relation to sensible TES)	Operation temperature is pressure-dependent (adjustable to some extent)

. In the following sections, the selected materials are described in more detail.

Some of the most important parameters for the selection of an appropriate material for TES are listed below [171]:

• Capacity: defines the energy stored in the system and varies with the size, storage technology and storage medium;
• Gravimetric and/or volumetric energy density: defines the amount of energy that the material can store per unit of mass or per unit of volume, respectively;
• Power: defines how quickly the stored energy in the system can be charged or discharged;
• Efficiency: defined as the ratio between the energy made available to use, and the energy needed to charge the system. This value quantifies the losses during the charge/discharge cycles;
• Storage period: defines how long the energy can be stored;
• Charge and discharge time: defines how long a system takes to charge or discharge;
• Cost: can be defined in terms of the cost of stored energy (€/kWh) or power (€/kW) and depends on the specific configuration of the implemented system, i.e., its CAPEX and OPEX values;
• Environmental impact: refers to considerations regarding the production of the material and the operation of the plant, or the implementation of the TES technology, that should be taken into account;
• Mechanical and chemical stability: this parameter affects not only the associated costs but also has implications on the environmental impact due to the potentially lower lifetime of the material/plant;
• System complexity: this parameter can influence both CAPEX and OPEX as well as the flexibility and reliability of a given TES technology.

3.1.1. Sensible Heat Storage

As previously mentioned, sensible heat TES refers to the storage of heat associated with a change in temperature without the occurrence of a phase change [167]. The amount of thermal energy Q_Sensible that a given system is able to store can be determined by Equation (5):

$$Q_{sensible}={\int }_{T_i}^{T_f}m·c_p dT=m·c_p·(T_f-T_i)$$

(5)

in which m is the mass of the material, c_p is its specific heat capacity, and T_i and T_f refer to the initial and final temperatures, respectively. This technology for storing heat is the most mature and most utilized heat-storage technology [172], with applications in use and in development that include industrial heating [173], industrial heat recovery [174], district heating [175], and concentrated solar plants [171], to name a few.

While sensible heat storage materials have comparatively low gravimetric and volumetric energy densities, they present very interesting properties, such as their low complexity (compared to the two other TES technologies), their cost-effectiveness [171] and the possibility of using highly available materials, like rocks, bricks, sand, soil, and water [172]. Additionally, these materials exhibit a very wide operation temperature range, which goes from room temperature to temperatures that can exceed 1000 °C [176]. A main benefit of sensible heat storage materials over the other two types is that the materials tend to present very good reversibility, with little to no limitations in terms of their life cycle [167]. Sensible TES materials can be found in both liquid or solid form, with solid TES materials not being prone to leakages and usually having wider operation temperature ranges, which makes them eligible for higher temperature applications. Liquid materials, on the other hand, such as water, might cause corrosion and can have unfavorable properties, like having a high vapor pressure at the desired operation temperature range [166].

It is also worth mentioning that solid and liquid materials for sensible heat storage differ in the strategies available for the heat exchange between the heat storage medium and the system that is providing/extracting heat, be that during charging or discharging. In the case of solid materials, some sort of fluid needs to be used as a heat exchange medium to transfer the heat from the storage material to the process in which it is required. In the case of liquids, the heat storage material can function both as the heat storage medium and the heat transfer fluid (HTF), provided that some sort of circulation (e.g., a pump) is used to allow the heat exchange between the heat storage medium and the process requiring this heat. As such, not only does the heat storage capacity play an important role in defining a system’s properties, but also the heat transfer and mass transport properties, and the design of the particular system [166]. In the next subsections, the different options for sensible heat storage materials will be discussed more in detail.

3.1.2. Rocks, Pebbles, Sand, and Gravel

Solid sensible heat storage materials include rocks [177], pebbles [178], sands, gravel, and other similar materials. These are arranged as a packed bed inside a container to be used as fillers in single tank thermocline thermal energy storage systems. Earth materials are comparatively cheap, abundant, non-toxic, non-flammable, and require the use of some heat transfer fluid for its charge and discharge cycles. This heat transfer fluid is typically put directly into contact with the heat storage medium, which in this case, doubles up as the heat transfer surface [168]. Thermal oil and water/steam are the most common heat transfer fluids [172].

Still, not every rock can be used for higher temperature operations. According to Tiskatine et al. [179], the durability of rocks depends on its chemical and mineralogical composition, the grain structure, the nature of the contacts in the rocks, as well as the crystallographic texture of the rock-forming minerals. The authors also discuss some degradation mechanisms as influencing factors, among which are the cracking of quartz crystals caused by their thermal expansion or the loss of mass in marble and limestone rocks resulting from the decomposition of CaCO₃ with production of CO₂ [179]. In another study [180], Tiskatine et al. conclude, based on several criteria discussed, that among 50 candidate rocks the most promising candidates for high-temperature thermal energy storage are dolerite, granodiorite, hornfels, gabbro, and quartzitic sandstone. Chemical changes associated with the thermal cycling and the selection of the HTF can induce changes that can be deleterious for the operation of TES systems, with consequences such as dust formation, micro-cracks, disintegration, mass loss, and decreases in specific heat capacity [181].

The specific heat capacity of sensible heat storage systems, as mentioned earlier, is one of the most important properties to consider during material selection. Studies have reported mean values near 2300 $\mathrm{k}\mathrm{J}·{({\mathrm{m}}^3·\mathrm{K})}^{-1}$ for this property in rocks in general [180], which, combined with their low price, make this technology a very competitive alternative. A second important property to be considered is the thermal conductivity of the material, given that it strongly influences the charge/discharge dynamics of a system [180]. An evaluation from the data available in Zoth and Haenel [182] carried out by Tiskatine et al. [180] has found, for a selected group of rock materials, adequately high values of thermal conductivity varying from 2.4 to 4.3 $\mathrm{W}·{(\mathrm{m}·\mathrm{K})}^{-1}$ at 25 °C and decreasing to 1.4 to 2.4 $\mathrm{W}·{(\mathrm{m}·\mathrm{K})}^{-1}$ for a temperature of 500 °C.

3.1.3. Concrete and Ceramic Materials

Concrete blocks have also been considered as sensible heat storage materials [168]. These materials possess good mechanical properties, have a relatively low cost, are comparatively easy to produce, and are non-toxic and non-flammable.

Improvements to the concrete composition have been achieved by Xu et al. [183] by using an admixture of silane-coated silica fume to increase the specific gravimetric heat capacity by 50% and the thermal conductivity by 38%, to 1.05 $\mathrm{k}\mathrm{J}·{(\mathrm{k}\mathrm{g}·\mathrm{K})}^{-1}$ and 0.719 $\mathrm{W}·{(\mathrm{m}·\mathrm{K})}^{-1}$, respectively. Experimental studies have shown that a high-temperature concrete with a specific gravimetric heat capacity of 0.916 $\mathrm{k}\mathrm{J}·{(\mathrm{k}\mathrm{g}·\mathrm{K})}^{-1}$ and a thermal conductivity of 1.0 $\mathrm{W}·{(\mathrm{m}·\mathrm{K})}^{-1}$ at 350 °C was able to reach temperatures over 300 °C in an experimental setup designed to demonstrate the technology on a 350 kWh test unit equipped with an oil-based heat exchanger [184]. Investigations by Alonso et al. [185] have shown that a calcium aluminate cement blended with an admixture of blast furnace slag is suitable for applications with temperatures up to 550 °C. While the concrete investigated had initially a heat conductivity of 2.05 $\mathrm{W}·{(\mathrm{m}·\mathrm{K})}^{-1}$, because of the heat cycling that leads to cracking, this value decreased to 1.16 $\mathrm{W}·{(\mathrm{m}·\mathrm{K})}^{-1}$. The authors suggest the use of other materials and design techniques to improve the heat conductivity and mitigate degradation of other properties, while pointing out the importance of heat conductivity to concretes as TES materials. Kunwar et al. [186] produced concrete cylindrical blocks with drillings in different positions and different sizes to investigate the heat transfer performance of the material with air as a heat transfer fluid. In terms of heat storage capacity, the studied samples were reported to have a volumetric heat capacity of 2531 $\mathrm{k}\mathrm{J}·{({\mathrm{m}}^3·\mathrm{K})}^{-1}$, which is close to values found for some rock materials [180].

3.1.4. Solid Waste Materials

Another alternative for solid-state heat storage systems is to use waste/inertized materials. This approach is attractive due to its low costs and sustainability. Examples of inertized products are fly ashes from municipal waste [187], industrial by-products like potash [188], by-products from the mining and metallurgy industry [189,190], post-industrial ceramic [191], recycled Nylon fiber from textile industry [192], and waste glass [193].

Investigations using material prepared from demolition waste have shown promising results as sensible heat storage materials. Using a material developed in a previous study [194], Kocak et al. [195] have made numerical and experimental investigations on a lab-scale packed-bed prototype for sensible TES that can operate at temperatures between 130 and 180 °C. The authors claim that, compared to a very common sensible heat storage medium (Therminol 66), their material could store up to 45% more heat, and that in comparison to other solid materials for sensible TES, demolition wastes can be up to tenfold cheaper. As such, the use of waste material from demolition might be an alternative to make TES more economical while simultaneously conserving natural resources.

In a recent study, Zhang et al. [196] studied the development of a sensible TES material prepared from steel slag, MgO, and refractory clay. This material presents a decent heat conductivity of 1.137 $\mathrm{W}·{(\mathrm{m}·\mathrm{K})}^{-1}$, with a maximum temperature of 1000 °C and a heat capacity of 1.29 $\mathrm{k}\mathrm{J}·{(\mathrm{k}\mathrm{g}·\mathrm{K})}^{-1}$. On top of that, the authors claim that the costs per unit of energy stored are very competitive compared to other, commonly used, high-temperature sensible heat storage materials.

3.1.5. Water

Water, the most used liquid sensible heat storage material [166], has the advantage of being able to act not only as the heat storage material of the system, but also as the heat transfer fluid [168]. Alongside its non-toxicity, abundance and low cost, it has a rather high gravimetric specific heat capacity (4.184 $\mathrm{k}\mathrm{J}·{(\mathrm{k}\mathrm{g}·\mathrm{K})}^{-1}$). Water can be used in a solid, liquid, or gaseous state and is an excellent candidate for home space heating, cold storage of food products, and hot water supply applications due to its non-flammable and non-toxic properties [168]. Another attractive aspect of storing heat using water is the good scalability and the low level of complexity for its operation (compared to latent and thermochemical energy storage) [175]. One of the limitations of this technology is the high vapor pressure that the material has. In cases in which higher temperatures are required, pressurization of the system is necessary in order to keep the water in the liquid state [166,175].

3.1.6. Thermal Oils

This class of materials is represented by organic fluids which possess good heat transfer capabilities. Thermal oils, much like water, can work simultaneously as thermal energy storage medium and heat transfer fluid. These materials present advantages over water because they can remain in the liquid state at much higher temperatures (usually around 250 °C under atmospheric pressure). Some materials have been reported to operate at even broader temperature ranges, like between 12 °C and 400 °C [168]. Since they have a wide temperature range, this means that they can generally have a higher sensible energy storage capacity. On top of that, their low vapor pressure allows for cost savings in the installations due to reduced pressure built up during higher temperature operations, resulting in lower container and piping costs. For example, at 374 °C, the DOWTHERM thermal oil has a vapor pressure of only 7.6 bar, while water under the same conditions would have a vapor pressure of 221 bar. On top of that, since these thermal oils are liquid at room temperature, they do not require an anti-freeze mechanism like molten salts and metallic alloys do [168].

Despite their benefits, thermal oils, in general, have lower specific heat capacities (around 2 $\mathrm{k}\mathrm{J}·{(\mathrm{k}\mathrm{g}·\mathrm{K})}^{-1}$) and have very modest heat conductivity values (in the range of 0.1 $\mathrm{W}·{(\mathrm{m}·\mathrm{K})}^{-1}$) compared to water [168]. The heat conductivity, however, can be improved by introducing additives like graphene, graphite, and metal oxides. By adding up to 0.20 mg graphene per mL of heat transfer oil, an increase of up to around 25% in this property could be obtained [197]. The observed changes in the kinematic viscosity of the samples to which the additives were added were minor under the investigated temperatures, resulting in negligible changes in the liquid’s properties [197]. Still, these developments should be scrutinized under the perspective of an application as a thermal energy storage material, since the time scales and operation conditions under which these materials might have to work might affect the stability of the additive nanoparticles.

While vegetable oils have the potential for cost reduction, there are concerns about their rate of degradation. According to Kenda et al. [198], exposure to oxygen, moisture, and higher temperatures affect the degradation of these materials. Furthermore, the presence of fatty acids, unsaturated components, and light transition metals with two or more valence states are some of the aspects that influence the degradation rate.

3.1.7. Salts and Salt Eutectics (as Sensible Heat Storage)

For applications of sensible TES technologies in which temperatures exceed 400 °C, molten salts are the preferred storage medium and heat transfer fluid. The use of molten salts is becoming increasingly widespread in applications like middle-sized concentrated power (CSP) plants and in generation III and III+ nuclear reactors. Properties like their high specific volumetric heat capacity, high boiling points, very high thermal stability and their very low vapor pressure even at elevated temperatures make them well-suited candidates for these types of applications. Molten salt materials are also cheap, easily available, non-toxic and non-flammable [168]. Commercially available systems can reach up to 565 °C [199].

While these materials present many interesting properties as TES materials, they have important drawbacks. The fact that their melting points are usually over 200 °C calls for the use of some anti-freezing system to avoid solidification. One way to reduce the solidification temperature of these salts is by combining one or more salts in a eutectic mixture. For example, pure KNO₃ has a melting temperature of 334 °C and pure NaNO₃ has a melting temperature of 307 °C. Their binary mixture with a proportion of 60% NaNO₃ and 40% KNO₃ (known as “Solar Salt”) has a melting point of 220 °C [168]. The addition of Li-based salts like LiNO₂ and LiNO₃ is deemed as an interesting alternative to further lower the melting point of salt compositions and increase the heat storage capacity. Both these aspects also help to reduce the cost of the stored energy (by reducing costs associated with keeping the salt mixture in liquid state) [200]. Another candidate that is being used as a sensible heat storage material is the HITEC™ salt, which is composed of 53 wt.% KNO₃, 40 wt.% NaNO₂, and 7 wt.% NaNO₃ [199]. In relation to Solar Salt, this material possesses better thermophysical properties and a lower melting point.

An important drawback of molten salts as sensible heat storage materials is that they have high viscosities compared to other thermal fluids like oil and water. These poor flow properties increase costs associated with the circulation of the fluid. On top of that, molten salts have very poor heat transfer properties, with heat conductivities ranging from 0.5 to 0.6 $\mathrm{W}·{(\mathrm{m}·\mathrm{K})}^{-1}$, depending on the selected salt [168].

In the subsubsection Salts and Salt Eutectics (as latent heat storage) the application of this class of materials as latent heat storage materials is discussed.

3.1.8. Latent Heat Storage

Latent TES materials, on top of storing sensible heat energy as temperature, can additionally store energy whenever the material undergoes a phase change. For their applications as a TES medium, the transition between solid and liquid phases is most explored [166]. These materials are called phase change materials (PCMs) and can be categorized as organic and inorganic PCMs, depending on their composition.

The storage of heat Q for a latent heat storage material with a melting temperature T_m (with T_i < T_m < T_f), starting from solid state at some initial temperature T_i until reaching a final temperature T_f can be described in Equation (6) as follows:

$$Q_{sensible}+Q_{latent}={\int }_{Ti}^{Tm}{m}_{sol}·c_{p, sol} dT+m·\Delta h_{fus}+{\int }_{Tm}^{Tf}{m}_{liq}·c_{p, liq} dT$$

(6)

where m is the mass of the latent TES material, c_p is the specific heat capacity at constant pressure, Δh_fus is the enthalpy of fusion of the solid phase given in kJ/kg, and the underscripts “sol” and “liq” refer to the solid and liquid phases. On the right-hand side of Equation (7), the first term refers to the amount of sensible heat stored in the solid phase, the second term is associated with the heat involved in the phase change and the third term is the amount of sensible heat stored in the liquid state.

The use of latent TES has several advantages over the sensible TES technology. The operation approaches an isothermal regime in the vicinity of the phase change temperature, which can be beneficial for the process. On top of that, the gravimetric and volumetric heat storage capacities are significantly higher compared to the sensible heat storage [166]. However, besides the additional complexity for operation, some materials lack an acceptable long-term stability, tend to have low thermal conductivity, and present some undercooling during phase transition [201].

The difference between sensible and latent TES materials is illustrated in Figure 8. Assume two different materials, both solid at an initial temperature T_i. These materials, which have similar specific heat capacity at constant pressure c_p, experience an increase in temperature, causing both of them to store an amount of energy that is proportional to their respective c_p values. This happens until a temperature T_m is reached. While an increase in the stored energy for the sensible heat storage material is only possible with an increase in temperature, the latent heat storage material undergoes the phase change, storing energy as the transformation to liquid state progresses. The latent energy storage material keeps a nearly constant temperature until the complete conversion of the solid phase into liquid phase is completed, after which a further increase in the stored energy can only occur with an increase in the temperature until reaching the final temperature T_f.

As mentioned before, the high amount of energy that is stored in a very narrow temperature range in the vicinity of T_m is advantageous for many processes, since it gives more predictability and might simplify the control mechanisms to operate the system efficiently.

For a particular application in a system, the selection of a suitable latent TES material depends on its phase change temperature, its latent heat of fusion, the associated volume changes during phase change, and the degradation of its thermophysical characteristics with repeated cycling [166].

Latent TES systems rely on dedicated heat transfer fluids to transfer the heat from the PCMs to the relevant process under consideration. These PCMs are stored in tanks/vessels, and the system performance is very commonly limited by heat transfer between the PCMs and the heat exchange fluid [202]. Because of this fact, several techniques for the enhancement of heat exchange have been investigated and developed. Said techniques can be divided into passive and active improvement techniques.

Passive systems make use of various strategies to improve the heat transfer between the PCM and the heat transfer fluid without the need of additional energy (external power). Some of these techniques are nanoparticle dispersion, and adding heat pipes, porous matrices or conductive foams/surfaces, and extended fins [203] within the PCM storage tank/vessel [202].

When some source of external power is applied to improve the heat transfer, then it is said that an active heat transfer enhancement technique is being used. Examples for active heat exchange enhancement are mechanical aids such as pumps, vibration, jet impingement, injection, and external fields [202]. While active enhancement can improve the performance of the system in terms of power (or time to charge/discharge), it comes at the cost of considerable energy expenses and an increase in the system’s complexity. These two aspects obviously impact the costs associated with the operation and construction of such a system. Because of this, passive techniques are currently more prevalent [202]. Among the passive techniques for heat conductivity enhancement, according to Al-Salami et al. [203], fins are very promising due to their minor volume occupancy, low cost, reliability, ease of manufacturing, and simplicity. In an assessment conducted by Zhang et al. [200], the latent energy storage materials considered had values of heat conductivity that ranged from 0.2 to 0.8 $\mathrm{W}·{(\mathrm{m}·\mathrm{K})}^{-1}$. These values are significantly lower than the values typically found for sensible TES materials, which further underlines the significance of heat transfer enhancement.

There are two important categories of materials used in latent heat storage systems, organic and inorganic materials. Organic materials include alkanes, fatty acids, esters, alcohols, and glycols. Due to their higher relevance and potential, only alkanes and fatty acids are covered in this review. For a brief summary of some of the properties of esters, alcohols and glycols in the context of latent heat storage materials, the reader is referred to the literature [168,203]. Inorganic materials include salts and salt eutectics, salt hydrates, metals, and alloys. A brief summary of examples and the properties of these materials is provided in the following subsections.

3.1.9. Alkanes

Alkanes are a family of organic chemical compounds with the general formula C_nH_2n+2. These compounds are often called paraffins, although the term “paraffin” usually refers to a mixture of alkenes [168]. Due to their interesting properties such as high enthalpy of fusion, low vapor pressure, chemical inertness, low cost, minimal tendency to separate in different phases, non-corrosivity, and non-toxicity, these materials are suitable for waste heat recovery systems and other low- to medium-temperature applications [166,204]. At room temperature, alkenes with values of n (n = number of C atoms in the molecule) between 5 and 17 are liquid, while molecules with higher n-values are waxy, solid materials [204]. The latent heat storage capacity and the melting temperature varies with the number of atoms in the molecule, with longer/heavier molecules presenting higher values for their latent heat storage capacity and higher melting points. These paraffin mixtures have melting temperatures that typically range from −5 to 100 °C [168]. In general, the values for the latent heat associated with the solid–liquid phase transition lie between 150 and 250 $\mathrm{k}\mathrm{J}·{\mathrm{k}\mathrm{g}}^{-1}$ [204].

Some of the main drawbacks of these PCMs are their low heat conductivity and high volume change during the phase transition of ca. 10% [168]. According to a summary made by Zalba et al. [205], mixtures of alkenes with different n values such as the ones designated in that work as “paraffin C₁₆-C₂₈”, “paraffin C₂₀-C₂₃”, and “paraffin C₂₂-C₄₅” show, respectively, melting points of ca. 43 °C, 49 °C and 59 °C. Still, all these materials exhibit the same, very low thermal conductivity of 0.21 $\mathrm{W}·{(\mathrm{m}·\mathrm{K})}^{-1}$. Some works have tried to improve the thermal conductivity of these materials, by adding, for example, 0.06 wt.% natural graphite to an alkene material, leading to an increase in the heat conductivity to 0.38 $\mathrm{W}·{(\mathrm{m}·\mathrm{K})}^{-1}$ [204].

3.1.10. Fatty Acids

Fatty acids are organic compounds that contain a carboxylic functional group (COOH) on its aliphatic chain (i.e., do not have aromatic rings). These aliphatic chains can be saturated (i.e., only simple bonds) or unsaturated (i.e., with double or triple bonds) and have no heteroatoms in the carbon chain. Fatty acids have advantages similar to the alkenes/paraffins, as they have relatively low cost, good chemical stability, and have hardly any phase separation [168]. They also suffer from very similar drawbacks as alkanes, such as low thermal conductivities and large volume changes associated with their phase transition. Most of the fatty acids considered for latent heat storage applications are saturated fatty acids, since they present higher melting points in relation to unsaturated ones [168]. Examples of saturated fatty acids considered for heat storage applications are caprylic acid (n = 8; T_m = 16.7 °C), lauric acid (n = 12; T_m = 44.2 °C), and stearic acid (n = 18; T_m = 69.9 °C) [206]. Some of these fatty acids, like lauric acid, are naturally occurring in agricultural crops, so that they can, in principle, be extracted from low-cost, renewable sources [168] and from non-edible sources of fatty acids, like waste oils from agricultural and food processing facilities, used cooked oils or genetically modified oils that are not approved for human consumption [204].

Similarly to what has been observed for the alkanes/paraffins, attention has been given to improving the thermophysical properties of these materials, with special emphasis on thermal conductivity by means of, for example, using carbon nanotubes, as reported by Sari et al. [207].

3.1.11. Salts and Salt Eutectics (as Latent Heat Storage)

Salts have melting temperatures that make these materials attractive both as sensible and latent TES materials, depending on the operation temperatures required by the specific applications. In the subsubsection Salts and Salt Eutectics (as sensible heat storage), the application of salts in the liquid state as sensible heat storage materials was described. Here, the focus is given to salts with comparatively higher melting points, so that they can be used as latent heat storage materials in applications at higher temperatures.

There are several types of inorganic salts, which can be divided into nitrates, hydroxides, chlorides, sulfates, and fluorides. For latent heat storage applications, salts and salt eutectics with melting temperature of at least 250 °C are usually considered [168]. Nitrate salts are currently the most used in CSP plants and typically operate at temperatures of up to 565 °C [171]. Hydroxide salts have melting points between 250 °C and 600 °C, while chlorides, carbonates, sulfates, and fluorides have melting points that surpass 600 °C [168]. While the thermodynamic and thermophysical properties of pure salts are predetermined, a mixture of different salts can be used to form a eutectic mixture, in which these properties can be tuned to a degree, especially the crystallization temperature. In this mixture, the solidification and melting points are reduced and become lower than the melting points of any of the individual components. These compositions of two or more components melt and freeze congruently [200], i.e., they solidify at the same temperature. Because the phases involved solidify simultaneously, macroscopic phase separation is hindered.

According to Ong et al. [199], most commercially available salt mixtures that could work for latent TES have only a useful temperature range for applications such as hot water supply and building heating/cooling systems, although there has been some interest in finding suitable compositions and experimental efforts to determine properties relevant for high-temperature applications.

3.1.12. Metals and Alloys

Some alloys and metallic materials exhibit favorable properties for latent TES, namely, low melting points (sometimes close to or even below ambient temperatures) and very high boiling points, with negligible vapor pressure values even at elevated temperatures [168]. With the exception of cast iron/steel (which were considered as a sensible heat storage materials) [172], metallic materials and their alloys have been investigated as latent TES materials [168,208,209].

According to Costa et al. [208], metals and alloys considered for latent heat storage have a wide range of melting points, ranging from as low as −38.9 °C for pure Hg, up to 1414 °C, for pure Si. These materials present comparatively very high thermal conductivities, which is a desirable property that affects the dynamic response of the system. For instance, liquid Na exhibits a thermal conductivity of 65 $\mathrm{W}·{(\mathrm{m}·\mathrm{K})}^{-1}$ while the value for liquid Pb–Bi eutectic alloy (LBE) is 15 $\mathrm{W}·{(\mathrm{m}·\mathrm{K})}^{-1}$ [168]. Because of their high thermal conductivities, these materials do not require further additives to improve this property.

For low-temperature conditions, Ga and Ga-based alloys are the most used and investigated materials, and can be useful in applications such as thermal management. For intermediate temperature applications in the range 40 to 300 °C, Pb and Cd are the most attractive candidates based on their melting points [208]. However, the high toxicity of these elements raises concerns related to the environment and human health. For temperatures higher than 300 °C, several binary and ternary alloys have been considered, including (but not limited to) Al-, Cu-, Mg-, Si-, and Zn-based alloys. The particular properties of these alloys has been extensively covered in the work of Costa et al. [208].

These materials have been comparatively less studied due to being heavy and costly [208]. On top of that, their corrosiveness at high temperatures is a technical challenge and potential leakages can pose severe harm to the environment. A method called encapsulation can mitigate these particular issues. Different techniques have been developed for different metal alloys, including encapsulation with ceramics and other metals [208]. Furthermore, the necessary prevention of leakages or corrosion (with methods like encapsulation) reduces the storage density and can significantly increase the costs [208].

3.1.13. Thermochemical Heat Storage

Thermochemical heat storage (TCS) systems use chemical reactions to store and release thermal energy. The energy storage process of TCS materials comprises three phases, namely, charging, storage and discharging. During charging, energy in the form of heat is provided to the TCS material, which then undergoes an endothermic reaction. That means that in order to charge the material, a certain amount of energy is provided as heat to the reactant compounds and used to break the chemical bonds that hold them together. During the storage phase, the products of the first reaction are kept separate from one another until the stored energy is required. The discharge process is then carried out by combining the products of the charging reaction, and releasing heat as the reaction proceeds [169,170]. The amount of stored energy Q can be described by Equation (7):

$$Q_{sensible}+Q_{thermochemical}={\int }_{Ti}^{Tr}{m}_{S1}·c_{p, S1} dT+m_{S1}·\Delta h_{reaction}+{\int }_{Tr}^{Tf}{m}_{S2}·c_{p, S2} dT$$

(7)

in which m is the mass, c_p is the specific heat capacity, T_r is the temperature at which the reaction occurs at that specific pressure, Δh_reaction is the amount of energy in $\mathrm{kJ}·{\mathrm{kg}}^{-1}$ of reactant, and the underscripts S1 and S2 refer to the reactants (1) and the products (2), respectively.

There are two main classes of materials within the thermochemical heat storage group, namely, the sorption-type and reaction-type materials [169,201]. These sorption-type materials, in turn, can be divided into adsorption- and absorption-type. This schematic division is represented in Figure 9.

During adsorption, one of the reactants accumulates in, or attaches itself to, the exposed layer of the other material involved. Said material, capable of adsorbing (or ‘receiving’ the attaching molecules) is called an adsorbent; the material which is adsorbed (or attaches itself to the interface) is called an absorbate. Depending on the cohesive forces between the pair, a further subdivision is conducted. When the interaction is mainly caused by Van der Waals intermolecular forces, it is said that physisorption is occurring. If these forces are covalent, as the ones that are found in some chemical bonds, the phenomenon is called chemisorption. Generally, chemisorption tends to present a higher activation energy, which is related to a more significant dependence of the reaction rates on the system’s temperature [201]. In adsorption, only a thin film from the solid phase takes part in the process, so that the molecules/atoms underneath the material’s surface are not disturbed [170]. Examples of materials (adsorbent/adsorbate) that can store energy through sorption reactions include the silica gel/H₂O and zeolites/H₂O [201]. Most materials in this category work with H₂O as the volatile reactant and a solid material as the reactant pair.

While the process of adsorption is mainly related to the material’s surface, absorption on the other hand, is a process in which the absorbate penetrates the bulk material, causing a change in composition, which occurs as a solution is formed. Usually, the absorbent is either solid or liquid and the absorbate is either liquid or gaseous [201].

The most relevant properties of sorption-type materials in the context of energy storage are the heat of reaction, thermal conductivity, thermal and chemical stability, toxicity, and corrosiveness [170].

Reaction-type TCS employ proper chemical reactions as a means of storing heat. A schematic representation of a reaction-type thermochemical heat storage material is presented in Figure 10. Here, some chemical A receives an amount of heat, causing its decomposition into B and C components. These components are kept separate, until the energy that is effectively stored in their chemical bonds is made available again, by combining B and C under adequate temperature and pressure conditions. Comparatively, these reactions can thermodynamically occur only at higher temperatures. While sorption-type materials tend to operate at temperatures below 200 °C [166], reaction-type materials can vary from room temperature to temperatures as high as 1400 °C [169].

A direct comparison of the gravimetric storage capacities from sorption-type and reaction-type materials reveal that most of the sorption-type systems have gravimetric energy storage densities around 2880 $\mathrm{kJ}·{\mathrm{kg}}^{-1}$, with some candidates reaching as high as 4320 $\mathrm{kJ}·{\mathrm{kg}}^{-1}$, like the LiCl/H₂O pair [201]. Reaction-type TCS has some candidates with very high gravimetric energy storage densities, such as MgH₂/H₂ (2880 $\mathrm{kJ}·{\mathrm{kg}}^{-1}$) and CaH₂/H₂ (<4430 $\mathrm{kJ}·{\mathrm{kg}}^{-1}$) to name a few examples [169].

Some advantages of TCS are the long storage duration with low thermal losses (if the reaction products are stored separately), and the high gravimetric and volumetric heat storage densities. Furthermore, minimal thermal losses are experienced if the heat storage process is performed at room temperature [169,170]. This is, of course, of particular interest for long-term/seasonal TES. Also, unlike latent TES materials, the temperature of the heat storage/release process can be adjusted (within a material-specific operating window) by changing the system state (typically, a change in the partial pressure of some reactant in gaseous form) [210]. This allows highly flexible operation of a TCS system.

To define the operation windows and screen for useful materials for each application, an initial approach is to consider the equilibrium conditions for each TCS system. The equilibrium conditions of these systems can be better understood by making use of the Clausius–Clapeyron or the Van’t Hoff equations (which, although coming from different formulations, yield the same expressions for the case of TCS) [211,212,213]. The Van’t Hoff equation was previously defined as Equation (2) in chapter “Metal Hydrides”. Specifically for reaction-based TCS materials, the reaction kinetics can be described in a very similar way as described in the Metal Hydrides chapter under the Section 2.3. Examples of such an approach can be seen for MgO/Mg(OH)₂ [214,215] and CaO/Ca(OH)₂ [216,217,218,219].

3.1.14. Hydroxide Systems

The two main hydroxide systems which have been investigated for heat storage applications are the MgO/Mg(OH)₂ and CaO/Ca(OH)₂ systems. The CaO/Ca(OH)₂ system is able to store/release heat at temperatures between 400 and 600 °C, depending on the pressure conditions, making it an interesting option for applications such as CSPs [216,218,220]. It has a reaction enthalpy of −104 kJ/mol, good cyclability and presents a high material volumetric energy of 195 kWh/m³ [216,217,218,219,221].

Working at a lower temperature range, around 300 and 400 °C, the MgO/Mg(OH)₂ system also shows promise as a TES material. This material has a high gravimetric storage density of 1389 $\mathrm{kJ}·{\mathrm{kg}}^{-1}$; however, it suffers from very low conversion rates between Mg(OH)₂ to MgO. Consequently, in its pristine state, storage densities are only limited to around 270 $\mathrm{kJ}·{\mathrm{kg}}^{-1}$ [222]. This phenomena originates in the high activation energy associated with the hydration/dehydration cycles, as well as in the low thermal conductivity, and a tendency to agglomerate larger particles with the number of cycles, which leads to a decrease in porosity [222]. Strategies to counteract this low conversion include changing the material’s microstructure and improving thermal conductivity. This has been achieved mainly through the use of additives such as C-based [223,224], Fe-based [222], Li-based [225] additives, among others. In particular, Tian et al. [222] investigated the cycling properties of pristine MgO and Fe-doped MgO particles. The addition of Fe led to the formation of a layered structure and enabled the material to reach a TES capacity of approximately 50 (i.e., around 700 $\mathrm{kJ}·{\mathrm{kg}}^{-1}$) of its theoretical heat storage capacity even after 10 cycles. While promising, it remains to be seen whether the materials being investigated can maintain their layered structure after hundreds or even thousands of cycles.

Another strategy to improve the performance is through compression to produce pellets. According to Myagmarjav et al. [226], the addition of LiBr and expanded graphite to Mg(OH)₂ and a combination of compressing and drying of the obtained pellets, resulted in an increase in the thermal conductivity (measured in the perpendicular direction) from 0.28 $\mathrm{W}·{(\mathrm{m}·\mathrm{K})}^{-1}$ for pure Mg(OH)₂ pellets to 1.91 $\mathrm{W}·{(\mathrm{m}·\mathrm{K})}^{-1}$ for the obtained composite material.

3.1.15. Salt Hydrates

This class of materials is represented by mixtures between inorganic salts and water, which form a crystalline solid with general formula M_pX_q·m(H₂O) [227]. During the thermal charging phase, these materials undergo dehydration reactions, in which one or more molecules of water are separated for every molecule of salt, along with some compound with general formula M_pX_q·n(H₂O), in which m > n. When the stored energy is required, the material can react with water molecules once again (hydration). While these compounds are already used as latent TES materials [168], several authors have investigated them as TCS materials as well [170,227]. These materials have high energy densities and, on top of that, water is abundant, non-toxic, and inexpensive [227]. However, problems with reversibility are one of the major issues these materials face.

During dehydration, if the salt product is completely soluble in the water used for crystallization, then it is said that there is a congruent melting. If there is only partial solubility of the product salt, then it is said that there is an incongruent melting. Due to the difference in density between the salt product and water, a phase separation can occur, making subsequent reactions impossible. This phenomena can progressively decrease the heat storage capacity of these materials [168].

Due to their potential as materials for energy storage, investigations related to the development of new compositions/materials have been undertaken. As an example of these efforts, Kiyabu et al. [227] have used high-throughput density functional theory (DFT) calculations to look for suitable candidates for TCS applications. The authors claim to have found several hydrates which surpass the U.S. Department of Energy’s system energy density target (at least 95 kWh/m³ or 342 $\mathrm{M}\mathrm{J}·{\mathrm{m}}^{-3}$).

Salt hydrates are generally divided as inorganic, nitrate, carbonate, chloride, sulfate, and fluoride salts. Several systems have been investigated so far. For applications in the range between 50 °C and 100 °C, salt hydrates like CaCl₂, MgSO₄, SrBr₂, Na₂S, and MgCl₂, are some of the most relevant [168,170].

3.1.16. Hydride Materials

In the Metal Hydrides chapter, the basic concepts and the development of these materials is reviewed as a means of storing hydrogen. In this section, hydride materials are considered for their ability to serve as TES materials. According to Sunku et al. [228], MHs have been investigated primarily for high-temperature applications, and the main candidates for those have been MgH₂, TiH₂, and CaH₂. Other interesting systems are complex metal hydrides, due to their thermodynamic properties. Carrillo et al. [229] notes that MHs have an outstanding energy storage density with an operation temperature that ranges from 250 °C to > 1000 °C. To put it into perspective, a sensible TES system based on molten salt exhibits a storage density of ca. 150 $\mathrm{kJ}·{\mathrm{kg}}^{-1}$ per 100 °C of temperature change, while MgH₂ exhibits approximately 2811 $\mathrm{kJ}·{\mathrm{kg}}^{-1}$ of heat storage capacity [229,230].

Among the materials investigated, MgH₂ is certainly one of the most studied. The abundance of Mg sources, its comparatively low cost, high H₂-gravimetric capacity, and high heat storage capacity make it a very good candidate for this kind of application [228]. Mg-based materials also have the advantage of containing no critical and/or hazardous components [229]. Compared to some other hydrides, pristine MgH₂ possesses a decent cyclability, but the material will experience a decrease in capacity over repeated cycling. This, however, can be improved by the use of additives [231,232]. Other Mg-based materials have been investigated for this application, such as Mg₂Ni/Mg₂NiH₆ [233,234] and Mg₂Fe/Mg₂FeH₆ [33] systems.

Metal hydrides have demonstrated their cyclic stability up of at least 2000 cycles; however, cost concerns [235,236,237] and the slow reaction kinetics remain the biggest hurdles for the deployment of these materials as a heat storage technology [228]. Their low thermal conductivity, problems related to sintering, and low tolerance towards gas phase impurities further impair their performance, particularly for scaled-up applications [230,238].

4. H₂-Storage Systems Coupled with TES

Up to this day, various strategies have been employed to enhance the performance of metal hydride hydrogen storage systems. As was previously mentioned in the Section 2, these include improving the intrinsic properties of the MH materials, optimizing the manufacturing processes, and the design of advanced tank configurations, which stand among some of the latest efforts found in the recent literature. Nonetheless, in most cases, these developments have not sufficiently improved the performance of MH reactors to make their scalability possible, for a couple of reasons. The most prominent example is associated with the comparatively low effective heat conductivity of the powder beds, which still demands more effort in the integration of different systems in innovative designs and configurations, as improvements in kinetic behavior are typically outweighed by heat management limitations [66]. Another frequently observed constraint is the desorption temperature of some MH candidates which present high H₂-storage capacities, like MgH₂, Mg₂Ni, NaAlH₄, and Li-RHC, but are not able to operate at near-room temperature and pressure conditions [10,42,72]. As has been previously mentioned in this review, a common solution to said problem is the addition of substitutional metal additives to modify the hydride’s properties (like ΔH and ΔS), but this comes at the expense of cost. Some good candidates for H₂-storage also incorporate these expensive metals as part of their basic composition, or use them as a catalysts/additives to enhance kinetic behavior. Examples of these components and catalysts include precious metals such as Pd, Ru, and Pt [239], and more common metals like Co and V, which can still cost around three times the price of widely available materials like Mn [240]. However, all of these different hydrides share the same limitation: alone, they would require external energy to maintain their desorption process, making their overall efficiency very limited. It is in this context that the first advantage of coupling MH with TES systems comes in. Temporarily storing the heat of absorption in TES materials opens the door for the use of inexpensive high-temperature metal hydrides such as MgH₂ in stationary applications, as little to no additional heat has to be supplied [241]. Similarly, the coupling of MH storage tanks with TES solutions allows these systems to be installed independently of the existing heat-related operations that occur in their surroundings. Hence, H₂ could now be stored in locations that are physically distant from engines, turbines, fuel cells, or any other equipment that needed to be previously considered as a supply of waste heat for desorption. This allows for a more versatile system design and increases the number of potential applications for the use of H₂ as an energy carrier. Furthermore, by using TES materials as heat sinks/reservoirs, heat transfer in and out of the hydride bed is facilitated, leading to faster hydrogenation/dehydrogenation times [242], and thus potentially reducing the need to incorporate expensive metal catalysts to improve kinetics.

Having stated these advantages, the integration of metal hydride (MH) hydrogen storage systems with thermal energy storage (TES) is reviewed in the following section. Although MH–TES systems are still in the development stage, some applications scenarios highlight the potential of these systems, especially in industries where both hydrogen and heat are produced or consumed. Some fields that would benefit from MH–TES are the following:

• Power Generation: Surplus electricity produced from both renewable or non-renewable sources during low-demand periods could be converted to H₂ via an electrolyzer and stored in MH–TES systems for posterior grid load balancing in periods of peak demand. Waste heat from combined heat and power (CHP) plants could also be stored, using systems based on metal hydride pairs [243]. The incorporation of a TES system not only minimizes the need of additional energy inputs, but also allows for the subsequent re-conversion of hydrogen into electricity in times of the day where the processes that would otherwise provide heat for H₂ desorption are not operating. The application of MH–TES solutions is particularly attractive nowadays due to the ongoing switch to H₂ gas turbines.
• Industrial Processes and Waste Heat Recovery: Industries that produce or consume substantial amounts of either heat or hydrogen, such as the cement, petrochemical, glass, or steel industries, could integrate MH–TES systems and benefit from them in several ways. Waste heat from industrial unit operations could be captured by installed MH–TES systems and stored for later use in different processes. Alternatively, the heat generated during hydrogen absorption reactions can be leveraged through the use of TES materials and repurposed in other industrial operations. Processes that require molecular hydrogen could also directly obtain it from nearby MH tanks. On top of that, the incorporation of TES material might simultaneously guarantee a steady supply of H₂ for industrial operations, and benefit the system’s overall energy efficiency, since TES materials can be used to control the temperature of the MH vessel, and therefore the rate of release of H₂ can be adjusted to meet the required inputs.
• District Heating and Cooling: MH–TES systems could be employed in district heating networks. Waste thermal energy generated by district power plants or by day-to-day residential activities can be used to operate MH heat pumps based on coupled MH pairs, and subsequently, either cool or provide heating for homes. This heat-pump concept has already been studied by Kumar et al. [244]. The benefit of including TES materials in these devices, similarly to other applications, would be to provide a more careful control of the H₂ release and uptake by each of the MH beds, and thus allow for the fine tuning of at home heating/cooling.

Table 4. Overview on numerical and experimental studies on metal hydrides coupled with thermal energy storage.

TES Method	Metal Hydride	TES Material	Heat Exchange Strategies	Operating Temperature [°C]	Operation Pressure in Metal Hydride [bar]	H2-Capacity [kg]	Key Findings	Study Type	Year	Reference
Sensible	LaNi5	Concrete	Embedded cooling tubes with water as HTF	20–100	30 (abs)/0.1–0.9 (des)	0.5 kg H2	The system stores 37% of the heat generated during hydrogen absorption. Optimized system with eight cooling tubes. Sensible TES reduces the need for external heating.	Numerical	2019	[245]
Sensible	LaNi5	Water	Helical tube embedded in both reactors	20–97	4–20 (abs)/0.01–1.00 (des)	2.5 kg powder/0.0375 kg H2	Integration with thermocline storage achieved 81.7% storage efficiency during absorption and delivered 60% of required heat during desorption with over 70% discharging efficiency.	Numerical	2023	[246]
Latent	MgH2+4 at.-% Ti-V-Cr	Mg-Zn-Al Alloy	Cylindrical tank with internal heat exchanger	300–360	10 (abs)/1 (des)	10 kg material, 0.63 g H2	The PCM effectively stored the heat of reaction during hydrogen absorption and provided it during desorption, resulting in a daily storage efficiency of 69%.	Experimental	2013	[241]
Latent	MgH2	Mg69Zn28Al3 (at.-%)	Cylindrical, annular reactor with metal hydride bed in the core and the PCM surrounding the metal hydride in the annular gap	300–370	8 (abs)	10 kg material, 0.63 g H2	A numerical model was proposed which agrees well with the experimental results of a MgH2 tank. It was shown that neglecting the natural convection in the fully melted PCM provides good results while simplifying the calculations.	Numerical	2013	[247]
Latent	MgH2	Mg69Zn28Al3 (at.-%)	Metal hydride and PCM were separated by steel wall.	300–360	10 (abs)/1 (des)	-	Two technical tank configurations were analyzed numerically. The spherical tank was found to have a higher system performance than a cylindrical tank.	Numerical	2015	[248]
Latent	Mg2Ni	NaNO3	Cylindrical tubes filled with PCM concentrically inside the MH bed	344 (abs)/270 (des)	15 (abs)/2 (des)	-	Integration of PCM reduced filling time by 58.1%. Aluminum foam enhances thermal conductivity, reducing charging/discharging times.	Numerical	2016	[249]
Latent	LaNi5	LiNO3–3 H2O	Heat exchange through steel wall between metal hydride tank and PCM. The PCM is located in four cylindrical tubes within the LaNi5 hydride bed.	20–70	10 (abs)/1 (des)	0.309 kg of LaNi5 alloy~0.004 kg H2 (est.)	Up to 80% of the hydrogen can be released utilizing the heat from the PCM.	Numerical	2016	[250]
Latent	LaNi5	Paraffin RT35	Heat exchange through steel wall between metal hydride tank and PCM.	20–70	8 (abs)	0.270 kg powder/~0.004 kg H2 (est.)	The influence of thermal conductivity, the mass and latent heat of the PCM on the system performance was studied. The high thermal conductivity of the PCM is beneficial to achieve a high capacity of the metal hydride storage system. The incorporation of metal foams further improves the system performance, with copper foam showing better performance than aluminum foam.	Numerical	2019	[251]
Latent	Mg2Ni	NaNO3	Cylindrical sandwich bed packed with PCM	307	12 (abs)/3 (des)	-	A new MH-PCM configuration reduced the time for hydrogenation by 81.5% and dehydrogenation by 73% compared to conventional systems.	Numerical	2020	[242]
Latent	MgH2	NaNO3	Two reactor configurations: 1. Annular reactor with MgH2 in the central part and the PCM located in the annular gap. Heat exchange through the steel wall. 2. Sandwiched MH-PCM units with steel brackets separating the units.	280–372	10 (abs)/1 bar (des)	-	The configuration of the PCM affects the heat transfer. The sandwiched configuration exhibits improved performances. Absorption and desorption times are 78% and 59% shorter, respectively, for the sandwiched configuration.	Numerical	2021	[252]
Latent	LaNi5	LiNO3·3 H2O	The application of metal foam was investigated. PCM channels within the metal hydride bed were investigated.	20–70	10 (abs)/1 (des)	0.4813 kg of alloy; ~0.006 kg H2 (est.)	PCM channels within hydride bed show superior performance compared to alternative structures of MH-PCM contacting.	Numerical	2021	[253]
Latent	MmNiMnCo (AB5 alloy)	RT35HC (Organic PCM)	Integrated PCM unit for MH thermal management	35–40	10 (abs)/3 (des)	-	PCM-based thermal storage system captured heat released during hydrogen absorption and utilized it during hydrogen desorption, improving overall system efficiency.	Numerical	2022	[254]
Latent	MmNiMnCo (AB5 alloy)	RT35HC (Organic PCM)	Integrated PCM unit for MH thermal management	35–40	10 (abs)/3 (des)	0.072 kg	With the introduction of the PCM, hydrogen flows could be maintained significantly longer than without the PCM. Mixing Cu foam significantly enhanced the H2 charge and discharge performance.	Experimental	2022	[255]
Latent	LaNi5	Paraffin + Metal Foams (Cu, Al, Ni, Ti)	Tube-and-shell heat exchanger	22–56	50 (abs)	-	Metal foams enhance PCM thermal conductivity and melting rate. The Cu-metal foam showed the best thermal performance. Economic analysis pointed out as necessary to choose best metal foam.	Numerical	2022	[256]
Latent	LaNi5	Paraffin C13-C24 pure and with 5 wt.% metal oxide (Al2O3, MgO, SiO2, SnO2) nanoparticle additives	Heat exchange through steel wall between metal hydride tank and PCM. The PCM is located in four cylindrical tubes within the LaNi5 hydride bed.	20–60	50 (abs)	-	Mixing metal oxide nanoparticles with PCM was studied on a large-scale metal hydride storage reactor system. No effect was found of including 5 wt.% metal oxide additives on the overall thermal and hydrogen storage performance of the reactor.	Numerical	2022	[257]
Latent	LaNi5	Paraffin RT35	Heat exchange through steel wall between metal hydride tank and PCM. The PCM is located in four cylindrical tubes within the LaNi5 hydride bed.	20–70	6–12 (abs)	-	Natural convection in the PCM increases the average hydrogen storage by 13%. Optimizing the thermal conductivity of the PCM is important to increase the system thermal performance. Higher inlet pressure improves storage rate but increases compression costs. Optimal inlet pressure for the system found to be approximately 10 bar.	Numerical	2024	[258]
Latent	Mg2FeH6/LaNi5	C2H3O2Na·3 H2O	Heat exchange between the LaNi5 LTMH bed and the coupled PCM outer jacket occurs through the LTMH tank wall.	46.85–61.85	5.6–9.7 (abs and des)	0.0499 kg H2	The proposed system couples Mg2FeH6 as a heat storage medium receiving waste heat and providing H2 to the low-temperature MH (LaNi5), which is coupled with a latent TES for thermal management. The temperature rise in the LaNi5 powder bed during H2 absorption is more pronounced at the center of the tank. Due to the melting of the PCM, its temperature rise is smaller than the one experienced by the MH powder bed itself. Desorption of H2 occurs faster at the MH/PCM interface. Heat is absorbed faster by the PCM in the form of latent heat, than sensible heat. Authors reported an efficiency of around 87% for the proposed system.	Numerical	2024	[259]
Thermochemical	MgH2 + 5 wt.% expanded natural graphite (ENG)	Mg(OH)2 + ENG (Pellet, block and powder + Al foam)	Integrated thermochemical storage	305–370	10 (abs)/2 (des)	-	The system reduces the mass of the heat storage media by a factor of 4 compared to PCM-based systems. Offers flexibility in operating pressure conditions, enabling shorter hydrogen absorption times.	Numerical	2016	Bhouri et al. [260]
Thermochemical	MgH2	Mg(OH)2/MgO	Adiabatic storage reactor	280–330	1 (des)	-	Mg/MgH2–MgO/Mg(OH)2 system investigated numerically. Feasibility of H2-desorption with MgO hydration in the system demonstrated. Compatible reaction times were identified.	Numerical	2019	Lutz et al. [210]
Thermochemical	Mg90Ni10	Mg(OH)2/MgO	Adiabatic storage reactor	370 (abs)/300 (des)	9 (abs)/1 (des)	0.012 kg H2	Absorption under 9 bar H2 and 60 mbar H2O; H2 release and CaO hydration at 10 bar H2O and 300 °C demonstrated. High volumetric capacity at low pressures.	Numerical and Experimental	2020	Lutz et al. [261]
Thermochemical	LaNi5	Na2HPO3·7 H2O	Metal hydride contained in a steel tank surrounded by PCM directly.	20–70	6–10 (abs)	0.078 kg powder; ~0.001 kg H2 (est.)	A 3D model of a metal hydride reactor with PCM was presented. The effects of operating pressure and PCM properties were investigated. A higher operating pressure is beneficial in achieving a higher reaction fraction, e.g., the reacted fraction at 2400 s increases from 53% at 6 bar to 77% at 10 bar. Reported metal hydride reactor gravimetric capacity of 0.58 wt.%. Electrical power of FC coupled system 8.2 kJ.	Numerical and Experimental	2020	Yao et al. [262]
Thermochemical	MgH2	Mg(OH)2/MgO	A heat exchanger with fins was modeled. The heat exchanger fins were optimized as part of the study.	300–370	10 (abs)	-	The heat of reaction of the Mg/MgH2 system is stored using the thermochemical dehydrogenation of Mg(OH)2. Through topology optimization of the heat exchanger and the metal hydride and TES material, the charging performance can be improved.	Numerical	2022	Shi et al. [263]
Thermochemical	MgH2	Mg(OH)2/MgO	No HT fluid, TCS material around MH	300–360	10 (abs)	-	Absorption takes place at 10 bar and desorption is studied at 5 bar backpressures. The thermal management can be optimized by adjusting the water vapor pressure. An increase in water pressure decreases the dehydrogenation time of the MgH2 material.	Numerical	2022	Shi et al. [264]
Thermochemical	LaNi5	LiNO3·3 H2O	Double-walled cylinder with MH/PCM (no thermal losses)	20–120	6 (abs)/0.068 (des)	0.0324 kg H2	Development of a non-dimensional methodology to identify MH-PCM/TCS materials pairings based on charge/discharge and equivalent power.	Numerical	2024	Maggini et al. [265]

offers an overview of the developments on MH hydrogen storage coupled with TES. The results of some key publications are then presented in more detail. Table 4 and the subsequent discussions are subdivided according to the TES method (sensible, latent, and thermochemical). Significantly more numerical studies were performed than experimental studies. Therefore, all experimental studies will be discussed in detail, while only selected numerical studies will be discussed.

4.1. Metal Hydride: Sensible Thermal Energy Storage

While sensible TES systems are the most used in current heat storage applications [168], they are not frequently considered as candidates for coupling with MH systems. There are two important reasons for this. First, the comparatively low storage density of sensible TES significantly increases the size of the whole hydrogen storage system, and thus reduces the volumetric and gravimetric H₂ densities. Second, the non-isothermal behavior during the thermal charging/discharging process is not optimal for MH-based H₂ storage systems, which ideally require a constant temperature of the heat sink and source during the hydrogenation and dehydrogenation reactions, respectively. Latent and thermochemical TES systems exhibit higher storage densities and store/release heat more isothermally than sensible TES systems.

Two numerical studies where sensible heat storage materials and hydride systems were coupled are presented in more detail in this section. Tiwari et al. [245] studied the heat exchange between a LaNi₅-containing H₂ storage tank and a concrete-based TES system, using a 3D numerical model developed in COMSOL Multiphysics 5.2 software. The proposed system is shown in Figure 11 and consists of two cylindrical tanks containing the metal hydride and the TES material. The heat exchange was facilitated by water as a heat transfer fluid, flowing through embedded cooling tubes.

The influence of the dimensions of the hydride tank on the reaction rates was studied. The highest reaction rates and lowest H₂ charging times were found for tank dimensions of $\mathrm{D}·\mathrm{L}=(0.15 \mathrm{m})·(0.7 \mathrm{m})$. The number and arrangement of the cooling tubes was studied, and it was found that eight tubes homogeneously distributed in the hydride bed were the optimal number (the maximum number of tubes studied was twelve). The velocity of the heat transfer fluid was also evaluated. Lower velocities allowed for higher heat transfer rates to the sensible storage system, but reduced cooling efficiency in the metal hydride tank. A velocity of 0.1 m/s was considered optimal for balancing these effects. The study found that, with the optimal hydride tank design, a maximum of 37% of the heat generated during hydrogen absorption could be stored in the concrete-based system. This limit of 37% was attributed to the low thermal conductivity of concrete [245].

In a later study [246], the same authors numerically investigated a LaNi₅ hydride storage tank in combination with a water-based sensible TES system. The system design is shown in Figure 12.

Here, the metal hydride reactor was equipped with helical heat exchanger tubes, through which the heat exchange fluid (water) could flow. During the H₂ absorption process, the water temperature increased as it flowed through the metal hydride bed and reached a maximum increment of 32 K. A total of 81.7% of the heat emitted during absorption was successfully stored by the TES system (at an H₂ supply pressure of 16 bar). During the subsequent desorption process, 60% of the heat required to discharge the metal hydride reactor could be supplied by the TES system. The authors mention that the system’s efficiency can be further improved by optimizing the helical tube design and operating parameters inside the metal hydride storage reactor [246].

4.2. Metal Hydride: Latent Thermal Energy Storage

The earliest studies of coupling latent TES systems with metal hydrides were published in 2013 by Garrier et al. [241] and Marty et al. [247]. In these studies, the possibility of reducing the amount of external heat required for H₂ desorption from MgH₂ was explored. MgH₂ requires high-temperature heat (>300 °C) to release its hydrogen. Such heat is classified as high-grade heat, which is thermodynamically and economically valuable. Thus, there is strong motivation to reduce the amount of externally supplied heat for H₂ desorption from MgH₂ by storing and re-using the heat from absorption.

Garrier et al. [241] built a 10 kg MgH₂/0.63 kg H₂ prototype reactor, which employed a metallic Mg–Zn alloy (Mg₆₉Zn₂₈Al₃) as PCMs for latent TES. Compacted disks of MgH₂ mixed with a 4 at.% Ti–V–Cr alloy were inserted into a cylindrical reactor. This reactor was surrounded by an external cylindrical tube containing pure Mg–Zn alloy. The heat released during Mg hydrogenation flowed through the tank wall (which separated the two compartments) and melted the Mg–Zn alloy. The heat released during the solidification of the alloy was then used for hydrogen desorption.

Figure 13 shows the temperature and pressure conditions of the metal hydride and Mg–Zn alloy PCM during the absorption and desorption processes. The red curve represents the equilibrium conditions of the Mg/MgH₂ system. At any point below that curve, the equilibrium shifts towards the formation Mg and H₂. Above that curve, the equilibrium favors the formation of the hydride, MgH₂. In order to maintain a temperature gradient between the metal hydride and the PCM, the temperature and pressure conditions in the MgH₂ reactor were very different during absorption and desorption. The PCM melted and solidified at ca. 340 °C. Absorption took place at around 375 °C and 10 bar, and desorption took place at around 310 °C and 2 bar.

Heat was exchanged between hydride and PCM through the steel wall, which separated the two materials. No additional heat transfer enhancement methods were employed, such as fins or a heat transfer fluid.

Considering a 24 h charge/discharge cycle, the daily efficiency of the reported tank was about 69%. The authors propose better insulation and a scale-up of the system to further improve this efficiency. They further note that the efficiency of the first prototype was already much better than the efficiency of a conventional MgH₂ tank, which had a daily efficiency of 37% [241].

To investigate the influence of the tank shape on the system performance, Mellouli et al. [248] numerically studied different geometrical configurations of MgH₂ tanks coupled with Mg₆₉Zn₂₈Al₃ PCM to compare their heat exchange efficiency. Cylindrical and spherical tank shapes were studied and compared with each other in regard to the time required to absorb and desorb H₂. It was found that a spherical tank shape is superior in terms of the hydrogen charging time. A 22% improvement was achieved by spherical tank, in terms of the time required to reach 80% of its hydrogen storage capacity, compared to the cylindrical tank.

Ye et al. [252] numerically investigated two different reactor types for the coupling of MgH₂ with a NaNO₃ TES material. The first reactor configuration had an annular reactor-type design similar to the reactor reported by Garrier et al. [241]. The second reactor had a cylindrical, sandwich-type design, in which circular metal hydride and PCM disks were alternately layered on top each other as shown in Figure 14.

It was found that the hydrogen absorption and desorption time of the sandwiched MH–PCM unis were 78% and 59% shorter, respectively, than in the case of annular reactor design. This behavior was attributed to the enhanced heat transfer between the hydride and PCM disks in the case of the sandwich reactor. For the same hydrogen storage capacity, the heat transfer area of the sandwich reactor was 2.5 times larger than in the annular reactor. Thermal resistances between the hydride and PCM were also considerably lower [252].

The studies by Garrier et al. [241], Mellouli et al. [248] and Ye et al. [252] on MgH₂ coupled with latent TES materials highlight the importance of the tank design for its performance.

The Mg₂Ni/Mg₂NiH₄ material is another high-temperature hydride system, which operates at similar temperature conditions as Mg/MgH₂ [266]. No experimental studies have been reported for the coupling of this system with a TES material yet. However, in a numerical study by Mellouli et al. [249], Mg₂Ni/Mg₂NiH₄ was investigated with sodium nitrate (NaNO₃) as latent TES material. Mg₂Ni/Mg₂NiH₄ is able to absorb and desorb hydrogen at temperatures around 344 °C and 272 °C and pressures of 15 bar and 2 bar for absorption and desorption, respectively. These temperature ranges make the selection of NaNO₃ adequate, since it melts at a temperature of 307 °C [267]. An adequate temperature gradient is provided for both processes.

In their design, aluminum foam was incorporated to improve the heat exchange in both the metal hydride reactor as well as the PCM compartment. Four configurations were investigated, as shown in Figure 15.

• Design 1: A basic configuration with a disk of Mg₂Ni alloy surrounded by PCM stacked on a central filter tube.
• Design 2: Similarly to Design 1 but with 48 encapsulated spherical shells filled with PCM, arranged concentrically inside the MH bed.
• Design 3: Each spherical shell in Design 2 is replaced by a hexagonal tube filled with PCM.
• Design 4: Each hexagonal tube in Design 3 is replaced by a cylindrical tube filled with PCM.

Mellouli et al. found that Design 4 was the most efficient option. This conclusion was rationalized based on the observation that cylindrical tubes filled with PCM inside the hydride bed favored a more effective heat exchange than the spherical shells or the hexagonal tubes. The geometry of the cylindrical tubes provides a larger surface area for heat exchange, resulting in more efficient thermal management. Design 4 showed a 58.1% decrease in filling time, relative to the basic configuration (Design 1) [249]. It was also found that the aluminum foam enhances the effective thermal conductivity of the hydride bed as well as that of the PCM medium. This improvement is critical, as it results in reduced heat accumulation in the hydride bed, which accelerates the heat transfer between metal hydride and TES material. It was also mentioned that a balance must be found between the enhancement of the heat transfer characteristics of the hydride and PCM beds and an acceptable hydrogen storage capacity, which is reduced by the addition of the foam [249].

In a later numerical study conducted by Alqahtani et al. [242], the heat transfer efficiency of two different cylindrical reactor configurations that also employed Mg₂Ni/Mg₂NiH₄, with NaNO₃ as a PCM, was analyzed. The following designs were investigated, as shown in Figure 16:

• Design 1: A metal hydride reactor was surrounded by a single cylindrical jacket packed with the PCM. The heat exchange took place at the interface between the hydride reactor and the PCM jacket.
• Design 2: A metal hydride reactor was enclosed by PCM in a cylindrical sandwich bed, which increased the heat transfer area by adding more interfaces between the MH reactor and the PCM.

It was found that Design 2 showed a dramatic improvement in terms of the hydrogenation and dehydrogenation rates compared to Design 1. The hydrogenation rate improved by 81.5%, and the dehydrogenation by 73%. This was attributed to a greater heat transfer rate in Design 2, which was achieved as a result of the increase in heat transfer area [242].

In addition to high-temperature hydrides such as MgH₂ and Mg₂NiH₄, the coupling of low-temperature hydrides such as LaNi₅ and other AB₅-type alloys with latent TES materials has been investigated (see Table 4 for a comprehensive list). For this class of hydrides, the number of numerical studies also greatly exceeds the number of experimental studies.

Nguyen et al. [255] experimentally studied the application of metal foam as a means to improve heat transfer between a LaNi₅ hydrogen storage canister and paraffin wax (RT28HC) latent TES material, as shown in Figure 17.

An 800-NL hydride canister was tested for its ability to charge and discharge hydrogen at flow rates of 1.1 slpm, 1.5 slpm, and 2 slpm. Without the PCM, the canister could only absorb or release hydrogen for a short duration, until a maximum of 12% of the total storage capacity was reached. Introducing RT28HC paraffin wax PCM allowed the system to maintain hydrogen flows for longer periods, achieving up to 26% and 29% of the total storage capacity during charging and discharging at 1.1 slpm. The study also found that the PCM’s thermal conductivity significantly affects the MH system’s performance. Pure PCM with low thermal conductivity (0.2 W/(m·K)) limited the effectiveness of the thermal management solution. Enhancing the PCM with copper foam increased its effective thermal conductivity to about 4.3 W/(m·K), improving its thermal and charging/discharging performance. The copper foam-embedded PCM maintained the hydride canister’s temperature around 28 °C, which is ideal for the PCM’s functionality. The enhanced system achieved much higher storage utilization, reaching 62.5% and 84% of the canister’s theoretical capacity at 1.1 slpm and 1.5 slpm over 450 min [255].

In a numerical study by Chibani et al. [256], different metal foams were investigated to enhance heat transfer between LaNi₅ hydrogen storage material and a paraffin PCM. The setup design is shown in Figure 18. It consists of a cylindrical tank filled with LaNi₅ metal hydride and a central hydrogen feed tube surrounded by four PCM tubes which were arranged in a rhombus pattern. These tubes were embedded with metal foams (aluminum, copper, nickel, and titanium) to enhance heat transfer.

4.3. Metal Hydride: Thermochemical Thermal Energy Storage

This section deals with the reports on the coupling of metal hydrides with thermochemical TES systems. Thermochemical TES systems exhibit the highest theoretical thermal storage capacities of the three TES methods. This attribute is attractive for their coupling with hydrogen storage systems, where the system gravimetric and volumetric hydrogen densities are important parameters.

In this section, a sequence of three consecutive studies is presented, which all originated from the same laboratory at the German Aerospace Center (DLR) in Germany. These studies properly illustrate the sequence of steps needed to transition from a feasibility study to a numerical study, and finally to a working prototype. In the first study, performed by Bhouri et al. [260] in 2016, a feasibility analysis of a solid-state hydrogen storage reactor is presented. MgH₂ was used as the metal hydride, and Mg(OH)₂ was used as the thermochemical TES material. An advantage of thermochemical TES materials is their ability to store and release heat energy at different temperature and pressure conditions. Besides their high storage density, this behavior makes them attractive for coupling with metal hydrides. By adjusting the pressure of the heat storage and release reactions, it is possible to perfectly match the operating conditions of the thermochemical TES system to the optimal operating conditions of the metal hydride.

Bhouri et al. analytically investigated a novel reactor concept, in which the heat released during hydrogen absorption in Mg was stored in the Mg(OH)₂ bed by driving its dehydration reaction, and vice versa. Figure 19 shows the operating principle.

The study concluded that the use of Mg(OH)₂ could reduce the overall mass of the heat storage system by a factor of four compared to traditional PCMs. This, in turn, resulted in an increase in the system’s gravimetric H₂ storage capacity compared to the utilization of PCMs as TES solutions. Garrier et al. reported a system gravimetric H₂ capacity as 0.315 wt.% when utilizing a Mg–Zn–Al PCM [241]. The concept proposed by Bhouri et al. exhibited a H₂ capacity of 1.5 wt.% [260]. Additionally, the TES material costs of the thermochemical TES system were also reported to be considerably cheaper compared to the PCM costs (by a factor of around 12), thus demonstrating the high potential of this concept. During the analysis, the thickness of the metal hydride and thermochemical material layers was optimized, and additives were used to enhance the thermal conductivity of Mg(OH)₂, emphasizing their importance [260].

The first analytical feasibility study by Bhouri et al. [260] was followed up by two numerical studies performed by Bhouri et al. [268] and by Lutz et al. [210], in which the H₂ absorption and desorption processes were investigated, respectively. The same material combination was used as in the initial feasibility study [260]. In this review, the results of the H₂ desorption study [210] are presented in detail. In said study, the results were obtained by running a 2D numerical simulation. The reactor was designed with a double-walled cylindrical geometry, separating the metal hydride bed from the thermochemical bed. The basic design of the reactor is shown in Figure 20.

It was found that the hydrogen release process was completed in 132 min, which the authors of the study suggest would be suitable for stationary applications. It was also found that the efficiency of the system is highly dependent on the thermal conductivity of the reactive beds, with the MgO bed being particularly sensitive. A concept was proposed to utilize the waste heat of a high-temperature PEM fuel cell in combination with a superheater to generate water vapor at the desired temperature and pressure level of 350 °C and 10 bar, which was found to be adequate for the hydration reaction. It was also found that adjusting the water vapor pressure for higher temperatures during the MgO hydration could enhance heat transfer between the TES and metal hydride compartments. This resulted in a faster dehydrogenation [210].

On the basis of the three previous studies [210,260,268], Lutz et al. [261] performed an experimental study on a H₂ storage reactor prototype including peripheral components, combining MgH₂ in the form of a 90% Mg/10% Ni alloy (Mg₉₀Ni₁₀) as the metal hydride and Mg(OH)₂ as the thermochemical TES material. The authors did not provide an explanation for their switch from pure Mg, which was utilized in the previous studies, to the Mg–Ni alloy. The experimental setup (Figure 21) consisted of three main components:

• Water vapor infrastructure, where water vapor was supplied or withdrawn using a tube bundle that acted as an evaporator/condenser, regulated by thermal oils. This allowed the control of the water vapor pressure in the reactor, and the measurement of the reacted fraction of the thermochemical system via liquid level changes and pressure sensors.
• The hydrogen infrastructure, where hydrogen was supplied or withdrawn from the reactor using volume flow controllers.
• The reactor, which consisted of a double-walled tube with the metal hydride in the form of 300 mm diameter pellets located in the inner tube, and Mg(OH)₂ powder in the outer tube for heat absorption. The reactor included thermocouples for temperature monitoring and was designed to exclude mass transfer limitations.

The study demonstrated that hydrogen storage was achieved at pressures as low as 9 bar, storing hydrogen at 20.8 g H₂/L based on the reactive materials alone. It was found that the optimal operating temperature for the magnesium oxide hydration was around 300 °C, achieved at a water vapor pressure of 9.75 bar. This temperature was sufficient for the dehydrogenation of the magnesium hydride. Thermal losses and poor kinetics of the Mg(OH)₂/MgO system prevented full conversion during simultaneous thermochemical cooling and heating. However, the proposed reactor concept allows for high-capacity hydrogen storage at low pressures, making it a promising option for stationary storage applications.

5. Future Developments

Throughout this review, metal hydrides have been presented as a promising means to store hydrogen, and consequently contribute to the integration of hydrogen-based technologies into the global energy infrastructure. However, further research is still necessary to successfully optimize the properties of metal hydride beds themselves and make H₂ storage in metal hydrides industrially viable. Addressing the thermal management challenges inherent in metal hydrides remains especially critical for achieving efficient, cost-effective, and reliable hydrogen storage. For this reason, coupling MH systems with thermal management solutions such as TES emerges as a favorable approach, offering a synergistic solution which maximizes energy efficiency by reusing stored thermal energy. In the following sections, key topics related to the improvement of MH–TES hydrogen storage systems, such as material optimization, heat transfer improvements, and cost reduction strategies are discussed, highlighting the importance of heat integration alongside other metal hydride improving strategies.

5.1. Material and System Development

Despite the progress made in coupling metal hydride systems with thermal energy storage solutions (MH–TES), these technologies still have a low degree of maturity. Research and development are key to the success of hydrogen storage in metal hydrides, with material optimization and the design of efficient systems that can adequately manage heat transfer being crucial aspects to improve upon. The latter is particularly important, as the efficiency of MH systems depends on the ability to store and reuse heat during the hydrogen absorption/desorption processes.

Regarding developments in the properties of hydride materials, some widely known compounds such as MgH₂ have been the subject of ample investigation, with existing works focusing on its various alloys, system configurations, and enhancement additives. This, however, is not the case of complex metal hydrides (for example), whose performance still needs to be improved, despite presenting some favorable properties. Complex metal hydrides show great promise as hydrogen storage materials for stationary applications, mainly due to their high H₂ capacity and potentially low raw material cost, but their relatively high operating temperatures of over 100 °C makes effective thermal management imperative. Here, the application of TES materials could play an important role in the future; however, there have been no studies on complex hydrides coupled with TES systems to date.

As has been mentioned throughout this review, a significant challenge in improving these systems rests in the poor thermal conductivity of both metal hydrides and TES storage materials. In metal hydrides, this deficiency leads to uneven temperature distributions during hydrogen absorption/desorption, which complicates the heat exchange process and results in operational inefficiencies. Meanwhile, poor thermal conductivity in TES materials has been shown to be a limiting factor, and reduce system performance [261]. Adding metal foams has been suggested as a solution to improve thermal conductivity of PCMs at low temperatures. However, for high-temperature applications, the potential reactivity of metals with TES materials could become an issue. In this regard, encapsulating PCMs with ceramic materials for operation at high temperatures has been explored as a potential solution [269].

Cyclic stability has also presented itself as an issue in metal/hydrogen systems which must be solved, particularly in the case of complex hydrides and Mg-based systems [118]. It has been long known that many metal hydrides tend to deactivate or passivate in the presence of impurities in the gas phase [270] or when brought in contact with ambient air [19]. These unfortunate behaviors greatly reduce the potential of metal hydrides for practical applications, as significant efforts to purify H₂ gas would be required before exposing it to the metal hydride, and the implementation of costly safety measures during the operation and maintenance of metal hydride systems becomes necessary. More research is required to reduce the sensitivity of metal hydrides when they are in contact with gasses other than H₂.

While all the above-mentioned developments might greatly contribute to the competitiveness of MH and MH–TES technologies, significant challenges lie in how these coupled systems are designed. Heat management plays a crucial role in their effectiveness. It is thus important not only to take into consideration strategies that might minimize heat losses to the environment, but also ways to improve the heat exchange between MH and TES during operation. In these systems, having heat losses means that additional energy inputs will need to be factored in to compensate for the lost heat, which directly translates into higher operational costs and, somewhat indirectly, into higher capital expenditures, because of the additional measures that need to be devised to mitigate these losses. Due to the importance of these two aspects, more research efforts should be dedicated to investigating optimum design strategies/geometries for coupling MH–TES systems in different use-case scenarios, while minimizing environmental heat losses whenever possible.

5.2. Economic Feasibility

Building a commercial-scale MH–TES system requires numerous materials and technical components. The costs of hydride materials remain high, especially for complex metal hydrides. Thermochemical materials can be expensive as they often contain rare metals. Catalysts and thermal additives are also costly, but necessary, as many MHs depend on them to guarantee reversible storage behavior, and for property enhancements. Besides elevated investment costs, the operating costs of MH systems can also be considerable. Inflated operating costs could potentially be offset by integrating TES materials, as they allow for the recovery and reuse of heat during the absorption/desorption cycles. This reduces or could even eliminate the need for additional energy inputs, improving the overall energtic and economic efficiency of the system. Durability is also crucial for economic viability, with systems needing to cycle around 11,000 times over 30 years with minimal degradation [11,271].

A practical aspect to consider is the integration of metal hydride hydrogen storage systems into existing infrastructures with established heat exchange systems. By leveraging preexisting heat exchange systems, the additional costs of TES implementation may be reduced. This approach could be especially relevant to ensure viability in industrial applications, where heat exchange networks are already in place.

Efficient synthetic routes are critical to process large volumes of materials quickly and cost-effectively, and only through research will overall production prices be eventually reduced. For instance, Wang et al. proposed a new direct synthesis of complex metal hydrides (NaAlH₄ and LiAlH₄) through hydrogenation of their respective metal hydrides with aluminum in the presence of a catalyst and a liquid complexing agent. This process produces NaAlH₄ and LiAlH₄ in high yield at ambient temperature and near ambient pressure conditions, a breakthrough that will lead to reductions in production costs [30]. Low-cost synthesis routes like this one will encourage researchers and engineers to integrate complex hydrides in large-scale developments.

Another important aspect rests in the sustainability of these systems. Recycling metal hydrides after usage and incorporating recycled materials into metal hydride synthesis would not only be beneficial environmentally, but also economically. As an example, Guerrero-Ortiz et al. [272] have developed a viable synthetic route from Al-cans into NaAlH₄ metal hydride using NaH and TiF₃ as a catalyst. This approach could create a much needed synergy between the recycling and energy industries, potentially leading to reduced costs with further development [272]. Passing et al. [273] studied the use of scrap Mg/Al metal waste for hydrogen storage. Al served as an enhancement additive to MgH₂, increasing its thermal conductivity. The incorporation of recycling processes in the production of metal hydride hydrogen storage systems is still in the early stages of development but holds significant promise in driving the overall economic viability of hydrogen storage technologies. Finally, it is worth noting that coupled MH–TES hydrogen storage systems are made-up of numerous components which are usually expensive. Unless the production costs of MH materials can be reduced, MH–TES systems will present themselves as financially unattractive solutions. Therefore, maintaining a balance between material and system costs remains a critical factor.

6. Conclusions

Despite the growing interest in the development of MH–TES systems, different aspects impact their economic and technical viability. Regarding MH materials themselves, much effort has been dedicated to not only develop new materials, but also to improve the kinetic behavior and propose efficient ways to improve the effective heat conductivity of powders and compacts. Other important developments in this field are related to the production methods of these materials, the utilization of low-cost and earth-abundant raw materials, and the use of recycled materials.

Developments related to the systems themselves have focused on the optimization of geometry, with numerous works numerically investigating the behavior of these systems. Special emphasis has been given to the heat management of these systems. The positioning of heat exchange surfaces and the use of fins and baffles are some of the most prominent examples of investigated aspects in the field.

The diversity of TES materials is overwhelming and the properties of each class of materials affect their use. Further developments of TES materials being currently used in renewable energy sites might also produce new opportunities for MH–TES systems.

While no systematic bibliographical review was carried out, some conclusions with the sampling presented here can be pointed out. The surveyed works suggest that sensible heat storage materials have been studied the least in relation to the other two TES types, which might suggest that further consideration and a careful analysis of the merits and shortcomings of this pairing should be made to avoid overlooking good potential candidates.

A significant amount of the studies surveyed sought to couple MH systems with latent TES materials. This trend suggests that researchers are looking for materials with higher thermal capacity. For the thermochemical TES materials coupled with MH systems, few studies were found. This might be related either to the complexity of these systems or the amount of knowledge, models and data required to enable the development of such systems. Still, due to their outstanding heat storage capacities, the further development of new pairings might be one of the alternatives to obtain systems with promising characteristics for hydrogen and energy storage. It is possible that one of the bottlenecks for the development of thermochemical TES materials with MH systems is the inherent difficulty and complexity of finding candidates that match well in operation temperature as well as thermodynamic and kinetic behavior. The development of a framework for the selection of material pairing in this regard would be highly desirable.

Most of the studies available in the literature for MH–TES systems use different numerical methods to evaluate the system. However, the vast majority of the experimental studies also involved numerical investigations or were based on previous work, in which the designed system was first investigated numerically. This shows how fundamental these numerical studies have become for the development of laboratory-scale prototypes and proof-of-concept designs.

In spite of the current achievements, an accelerating demand for zero-emission energy technologies is putting more pressure on the race to develop economically competitive solutions. Most of the possible MH and TES combinations are yet to be explored, and the leap from theory to practice is only just beginning. Therefore, this work emerges as a starting point for future research efforts on the matter, be it in the form of optimizing already studied MH–TES systems or conceiving new ones.