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Calcium hydride has shown great potential as a hydrogen storage material and as a thermochemical energy storage material. To date, its high operating temperature (above 800 °C) has not only hindered its opportunity for technological application but also prevented detailed determination of its thermodynamics of hydrogen sorption. In addition, calcium metal suffers from high volatility, high corrosivity from Ca (and CaH 2), slow kinetics of hydrogen sorption, and the solubility of Ca in CaH 2. In this work, a literature review of the wide-ranging thermodynamic properties of CaH 2 is provided along with a detailed experimental investigation into the thermodynamic properties of molten and solid CaH 2. The thermodynamic values of hydrogen release from both molten and solid CaH 2 were determined as Δ H des (molten CaH 2) = 216 ± 10 kJ mol−1.H 2 , Δ S des (molten CaH 2) = 177 ± 9 J K−1 mol−1.H 2, which equates to a 1 bar hydrogen equilibrium temperature for molten CaH 2 of 947 ± 65 °C. Similarly, in the solid-state: Δ H des (solid CaH 2) = 172 ± 12 kJ mol−1.H 2 , Δ S des (solid CaH 2) = 144 ± 10 J K−1 mol−1.H 2. Moreover, the activation energy of hydrogen release from CaH 2 was also calculated using DSC analysis as E a = 203 ± 12 kJ mol−1. This study provides the first thermodynamics for the Ca–H system in over 60 years, providing more accurate data on this emerging energy storage material. • The thermodynamics for hydrogen desorption of calcium hydride were determined. • This is the first study in over 60 years to determine the thermal properties. • The kinetics of hydrogen release were measured using the Kissinger method. • The melting point of CaH 2 was measured under a hydrogen atmosphere. • A literature review of previous thermodynamic studies on CaH 2 is included. [ABSTRACT FROM AUTHOR]

Metal hydrides have been demonstrated as energy storage materials for thermal battery applications. This is due to the high energy density associated with the reversible thermochemical reaction between metals and hydrogen. Magnesium iron hydride (Mg 2 FeH 6) is one such material that has been identified as a thermal energy storage material due to its reversible hydrogenation reaction at temperatures between 400 and 600 °C. This study demonstates an automated thermal battery prototype containing 900 g of Mg 2 FeH 6 as the thermal energy storage material with pressurised water acting as the heat transfer fluid to charge and discharge the battery. The operating conditions of the system were optimised by assessing the ideal operating temperature, flow rate of the heat transfer fluid, and hydrogen pressures. Overall, excellent cyclic energy storage reversibility was demonstrated between 410 and 450 °C with a maximum energy capacity of 1650 kJ which is 87% of the theoretical value (1890 kJ). [Display omitted] • A prototype thermal battery was developed using Mg 2 FeH 6 as the thermal energy storage material. • The automated battery consisted of a heat transfer fluid system to charge and discharge the material. • Complete optimisation of the operating conditions of the system was established. • Excellent cycle reversibility was demonstrated between 410 and 450 °C. [ABSTRACT FROM AUTHOR]

Hydrogen and ammonia are both potential energy carriers being considered for large scale energy transport. Sodium borohydride (NaBH 4) has been widely applied as a potential solid-state hydrogen storage material. It can produce hydrogen gas and sodium metaborate (NaBO 2) after hydrolysis with water at room temperature. The regeneration of NaBH 4 from NaBO 2 could significantly reduce the cost of NaBH 4 and enable its wide-spread industrial use. In this work, we demonstrate a simple method for regenerating NaBH 4 from NaBO 2 ·4H 2 O using Mg 2 N 3 , which combines NaBH 4 production with NH 3 gas production in a single step at room temperature. NaBH 4 is successfully synthesized from NaBO 2 ·4H 2 O using the Mg 3 N 2 reducing agent via ball milling under a hydrogen atmosphere. NaBH 4 is formed at a 76.6% yield by planetary ball milling at 600 rpm under 40 bar hydrogen for 12 h. NH 3 gas is also formed, which can be easily separated from the solid products. Therefore, this one-step process could produce two different types of carbon-free hydrogen carriers suitable for energy export from renewable sources. • One step process to store hydrogen in NaBH 4 and NH 3. • Lower cost for NaBH 4 regeneration. • Mg 3 N 2 can be considered as a promising reductant for simultaneous synthesis of NaBH 4 and NH 3. • The hydrogen pressure have a effect on the ball milling. [ABSTRACT FROM AUTHOR]

Thermochemical energy storage offers a cost-effective and efficient approach for storing thermal energy at high temperature (∼1100 °C) for concentrated solar power and large-scale long duration energy storage. SrCO 3 is a potential candidate as a thermal energy storage material due to its high energy density of 205 kJ/mol of CO 2 during reversible CO 2 release and absorption. However, it loses cyclic capacity rapidly due to sintering. This study determined that the cyclic capacity of SrCO 3 was enhanced by the addition of either reactive SrTiO 3 or inert SrZrO 3 , where the molar ratios of SrCO 3 to SrZrO 3 were varied from 1:0.125 to 1:1. Thermogravimetric analysis over 15 CO 2 sorption cycles demonstrated that both materials retained ∼80 % of their maximum cyclic capacity on the milligram scale. Repeated measurements using gram scale samples revealed a decrease in maximum capacity to 11 % using a sample of SrCO 3 – 0.5 SrZrO 3 over 53 cycles, while the use of SrTiO 3 additives allowed for the retention of 80 % maximum capacity over 55 cycles. These findings highlight the potential of reactive additives in enhancing the performance of thermochemical energy storage systems, while providing valuable insights for the development of cost-effective materials. [Display omitted] • SrCO 3 is a cost-effective thermochemical energy storage material. • TGA shows both SrZrO 3 and SrTiO 3 additives increase cyclic capacity up to ∼80 %. • SrTiO 3 reduces sintering whereas SrZrO 3 does not prevent sintering. [ABSTRACT FROM AUTHOR]

• Carbonation reaction kinetics of limestone-based CaCO 3 /Al 2 O 3 mixture is estimated. • Numerical modelling of CaCO 3 based reactor for thermal storage application is performed. • The influence of operating parameters on the reactor's performance is examined. • The effect of incorporating graphite fin into the CaCO 3 reactor is studied. This study conducts kinetic analyses of the carbonation reaction of CaCO 3 (doped with Al 2 O 3) as well as parametric analyses of the performance of a thermochemical reactor, which can act as a thermal battery. Kinetic measurements of CO 2 release and absorption were carried out using thermogravimetric analysis (TGA) at 815, 830 and 845 °C on a CaCO 3 /Al 2 O 3 sample that had been previously cycled over 500 times. The rapid reaction kinetics revealed that the Avrami nucleation growth model with exponent 3 fits well to explain the carbonation reaction. The numerical study considered a cylindrical reactor with a height and diameter of 100 mm. According to numerical analysis, at an applied CO 2 pressure of 1 bar, increasing the thermal conductivity of the reactor bed from 1.33 to 5 W/m.K increases the rate of carbonation reaction by 74%. When the applied CO 2 pressure is increased from 1 to 2 bar, the performance of the reactor bed with thermal conductivity of 1.33 W/m.K improves by 42%; however, when the applied CO 2 pressure is increased from 2 to 3 bar, the performance improves by only 18%. Additionally, when the boundary temperature of the reactor was lowered by 30 °C, performance was enhanced by 43% at an applied CO 2 pressure of 1 bar. This study also examined the effect of using a graphite fin as a heat extraction system. The graphite fin allowed for more rapid heat extraction and increased the carbonation reaction by 44% in the reactor bed with poor thermal conductivity (1.33 W/m.K) but had no effect in the reactor with modest thermal conductivity of (5 W/m.K) due to its ability to already transfer heat effectively to the reactor shell. [ABSTRACT FROM AUTHOR]

The inherent nature concerning the intermittency of concentrating solar power (CSP) plants can be overcome by the integration of efficient thermal energy storage (TES) systems. Current CSP plants employ molten salts as TES materials although metal hydrides (MH) have proven to be more efficient due to their increased operating temperatures. Nonetheless, the heat exchange between the MH bed and the heat transfer medium used to operate a heat engine is a critical factor in the overall efficiency of the TES system. In this work, a computational study is carried out to investigate the performance of a magnesium hydride TES packed bed using a multiphase (boiling) medium instead of single-phase heat absorption methods. The boiling heat transfer behaviour is simulated by using the Eulerian two-fluid framework. The simulations are conducted at a transient state using SST- k -ω Reynolds-Averaged Navier-Stokes equations. It is observed that, unlike the single-phase heat collection method, the multiphase heat absorption method maintains a constant temperature in the heat transfer fluid throughout the reactor. Consequently, a higher temperature gradient is realised between the MH bed and heat transfer fluid (HTF), leading to improvements in the overall reaction rate of the hydrogenation process. • Performance enhancement of a magnesium hydride reactor due to multiphase heat absorption was studied. • Boiling heat transfer behaviour was simulated using Eulerian two-fluid framework. • The percentage of improvement in reactor performance was variable at different reacted fractions. • Dry out condition was critical in the reactor efficiency. [ABSTRACT FROM AUTHOR]