The hydrogen absorption into and desorption from hydride materials are accompanied by a high heat release and consumption, respectively. This has made hydride materials desirable for various heat-related applications, such as heat pumps, heat transformation, and, more recently, heat storage for solar energy applications [
1,
2]. Among these hydride materials, Mg-based hydrides have stimulated worldwide interest in their utilization in hydrogen/heat storage and high-temperature fuel cell technologies. This is particularly true due to their relatively high hydrogen storage capacity (3.6–7.6 wt% and 110–150 kg-H
2/m
3) and the high heat of the reaction (60–80 kJ/mol-H
2) [
3,
4]. As a result, their use in concentrated solar power (CSP) plants can improve performance in terms of energy storage density compared to the state-of-the-art two-tanks-based molten salts [
5,
6], one tank-thermocline [
7,
8], and latent heat phase change materials [
9,
10], although the economic analysis shows that metal hydrides are more expensive [
11,
12].
Few studies on the design and optimization of Mg hydride material-based thermal energy storage for CSP plants have been reported, not only experimentally but also theoretically [
2,
3,
4,
11,
12,
13,
14,
15,
16,
17,
18,
19,
20,
21,
22,
23,
24]. Nyallang et al. [
2] proposed a procedure for choosing metal hydrides pairs for thermal energy storage systems on the basis of energy storage density and efficiency criteria. The findings showed that the coupling of Mg-based hydrides with the most commercially-available low-temperature hydrides, such as LaNi
5, could lead to an energy storage efficiency of 0.7–0.8, with an energy storage density reaching up to 1 GJ·m
−3. In addition, during the heat discharging process, the average output temperature of the heat transfer fluid (HTF) could be enhanced by 25 °C, which is essential for the power plant’s exergetic efficiency located downstream of the thermal energy storage system. Reiser et al. [
3] experimentally prepared and characterized Mg-based hydride materials for heat storage applications. The results showed that these materials are stable within the temperature range 250–550 °C, with thermal energy densities reaching up to 2.257 MJ·kg
−1. Bogdanovic et al. [
4] experimentally reported the performance of a pilot process steam generator equipped with a Mg-based hydride heat storage device. The heat storage system provided 9.08 kWh of heating output at 370 °C, with an energy efficiency of up to 0.796. Corgnale et al. [
12] reported a screening procedure of suitable high-temperature metal hydrides for thermal energy applications. The screening procedure included a techno-economic analysis that analyzed the cost, the energy storage density, and the exergetic efficiency of these materials. The results showed that TiH
2, CaH
2, and NaMgH
3 showed a desirability for CSP plants, since their operating temperature is above 600 °C and their volumetric energy density exceeds 25 kWh/m
3. Sekhar et al. [
13] reported experimental tests on a heat storage reactor filled with Mg–30% MmNi
4. They investigated the effects of operating conditions on key performance indicators such as hydrogen capacity and energy storage efficiency. The results showed that at a fixed absorption temperature (150 °C), the increase in hydrogen pressure from 10 to 30 bars augments the thermal energy storage efficiency, which increases from 0.5 to 0.74, respectively. Given its high energy density of ~2.9 MJ/kg and high operating temperature (580–600 °C), the perovskite-type hydride NaMgH
3 showed great potential as a solar energy material [
14]. In addition, the hydride showed minimum kinetic degradation during a cycling stability study. Fang et al. [
15] proposed and tested a proof-of-concept thermal battery based on a metal hydrides pair. The pair consisted of 50 g of a Mg-based hydride composite (MgH
2 + TiMn
1.5 + 5 wt%ENG) with 150 g of TiMn
1.5V
0.62. The results showed that the total cooling and heating energies were 13.6 kWh and 35.4 kWh, respectively. This corresponds to a coefficient of performance (COP) of 0.384 for cooling, which is close to the theoretical one. Paskevicius et al. [
16] proposed a prototype comprising of a metal hydride bed (19 g of MgH
2) and a 2250 cm
3 H
2 bottle for TES in concentrated solar power plants. The results showed that the hydride was thermally cycled at 420 °C, with a limited H
2 capacity loss. Moreover, for very small-scale applications, there are many issues, such as powder agglomeration and heat loss to the environment, thereby decreasing the energy utilization of the storage system. Poupin et al. [
17] experimentally investigated a high-temperature (HT) thermal battery pair with a low-temperature (LT) metal hydride for solar-related power plants. The battery consisted of 40.7 g of 2 Mg–Fe for HT heat charging/discharging and 85.2 g of a TiMn
1.5 alloy 5800 for the hydrogen storage medium. The results showed that a maximum energy storage density of 1.488 MJ/kg was achieved at a temperature of 520 °C. In addition, the effect of the thermal cycling and volumes of the LT and HT reactors on the thermal energy storage performance was emphasized and discussed. Bogdanovic et al. [
18] proposed a TES based on a MgH
2/AB2 (code 5800) metal hydride pair. The system could exchange a maximum of 69 g of H
2 between the hydrides beds. The system consisted of 1.054 kg of MgH
2 paired with 5.9 kg of an AB2-type hydride (Ti
0.98Zr
0.02Fe
0.09Cr
0.05Mn
1.2), which could simultaneously produce ice and high-temperature heat. The results showed that the magnesium container produced 0.64 kWh of heat after absorbing 59 g of H
2 after 3 h. In the meantime, in the low-temperature metal hydride, the formation of 1.9 kg of ice was observed, which accounts for 0.18 kWh of cooling effect (28% of the heating effect). Ward et al. [
19] compared two metal hydrides pairs, such as NaMgH
2F/Na
3AlH
6 and NaMgH
3/NaAlH
4, for heat storage systems. It was shown that the installed cost of a heat storage system integrating the former pair was 11% lower than that integrating the latter pair, which is <30
$/kWh. Moreover, the energetic efficiency was increased by 5%. Bao and Yuan [
20] reported the performance of metal hydride-based heat storage systems adopting multi-step operating conditions. The findings showed that the adoption of this operating condition allows for a constant outlet temperature of the heat transfer fluid during the heat discharging process. Bao [
21] investigated the effect of heat-transfer enhancement measures on the performance of a magnesium hydride-based high-temperature thermochemical heat storage. These measures included extended surfaces (fins) and thermal conductivity augmentation based on compacted metal hydride-graphite. The results showed that the latter provides uniform temperature distribution inside the reactor; thus, it should be recommended for metal hydride-based heat storage applications. d’Entremont et al. [
22] numerically investigated the performance of the metal hydrides pair NaMgH
2F/TiCr
1.6Mn
0.2 for heat storage. The results showed that this thermal energy storage system could achieve an output energy density of 226 kWh/m
3, nine times the DOE SunShot target, with moderate temperature and pressure swings. Besides, simulations indicate that passive heat-transfer enhancement strategies can significantly improve performance. Malleswararao et al. [
23] carried out the performance prediction of a metal hydride pair for TES using a 3D model simulation into COMSOL. The pair was made of Mg
2Ni/LaNi
5 hydrides, which was selected using a thermodynamic compatibility check. The findings showed that the energy storage density of 156 kWh/m
3 at an energy storage efficiency of 89.4% was achieved. Mellouli et al. [
24] conducted a numerical analysis of a heat storage system based on metal hydrides pair made of Mg
2FeH
6/Na
3AlH
6. The results demonstrated that under the given operating conditions, the heat storage could deliver an energy density of 90 kWh/m
3, with a 96% energy storage efficiency.
From the studies mentioned above, it is clear that Mg-based hydrides in heat storage applications suffer from two undeniable challenges, namely, slow intrinsic kinetics and a thermal management based on a tank design and optimization viewpoints. While the use of catalysts and the pre-processing of hydride powder by mechano-synthesis using high-energy reactive ball milling (HRBM) can considerably alleviate the former, the latter remains a problem. Regardless of the intensive efforts given to the design of a metal hydride tank, the rate at which these hydride materials store/restore hydrogen depends strongly on the system’s configuration, the operating conditions in terms of pressure and temperature, and, more importantly, upon the heat and mass transfer abilities [
25]. Numerous studies have discussed the effects of heat-transfer enhancement methods on the performance behavior of metal hydride reactors. According to this review [
26] and references therein, the heat-transfer-enhancement measures consist of the following two techniques: passive and active. Passive heat-transfer-enhancement techniques comprise the insertion of inert materials that have a high intrinsic thermal conductivity (e.g., extended surfaces (fins), meshes, or expended natural graphite (ENG) compacts). As can be seen, the passive heat transfer measures consist mainly of thermal conductivity augmentation and represent 43% of the heat transfer mechanisms studied in the literature. Yang et al. [
27] numerically investigated the effect of heat-transfer-enhancement measures in a metal hydride reactor for heat pumps. They discussed the performance of three reactors adopting a passive heat-transfer-enhancement using aluminum foam or highly compacted metal hydrides. The simulation results showed that for metal hydride heat pumps utilizing the heat-transfer enhancement mentioned above, the performance’s coefficient slightly reduces, while the specific heating power remarkably increases. Several experimental studies [
28,
29,
30,
31,
32,
33] have shown that effective thermal conductivity greatly enhanced the hydrogen flow transfer between coupled beds. This thermal conductivity enhancement was shown to be the most decisive aspect in the overall heat transfer coefficient [
34].