Anhydrous SrCl₂ powder (purity >98.0%) was obtained from Kanto Kagaku, Japan. The median diameter (dp50) of the SrCl₂ particles was 100 µm. Prior to use, the powder was dried at 200˚C for 12h in an electric furnace.

Figure 2 shows a schematic representation of the experimental apparatus employed herein, which consisted of a reactor, an ammonia condenser, an ammonia cylinder, a vacuum pump, and a liquid level indicator. The various components of the setup were connected by a range of tubes for the transport of ammonia. In addition, flows of water connected to thermostat baths were connected to the reactor and the ammonia condenser.

Photographic images of the top view of the reactor and the stainless steel mesh and plate can be seen in Figure 3. The SrCl₂ particles were deposited on a heat exchange

plate (100 mm × 100 mm) with a thickness of 1 mm, onto which were placed a stainless steel mesh and a stainless steel plate. The purposes of the stainless steel mesh and plate were to prevent the outflow of SrCl₂ particles and to limit particle scattering through curtailing the momentum of the liquid ammonia flow. The heating medium (i.e., water) flowed under the heat exchange plate, and the inlet and outlet water temperatures were measured during using a platinum resistance temperature detector. The heat generated by the reactant bed was transferred to the water through the heat exchange plate, resulting in an increase in water temperature during discharge.

The discharge characteristics of the liquid-solid reaction and the gas-solid reaction were evaluated. The two exothermic reactions of interest are shown in Equations (4) and (5) below. The coordination change of 0 to 8 ammonia molecules, which isn’t strictly based on the practical conditions, was selected to create uniform initial packed bed conditions.

In this process, the condensation of ammonia was carried out prior to beginning the discharge operation of the liquid-gas reaction. Ammonia was supplied from the NH₃ cylinder at the pressure beyond equilibrium pressure and was condensed in the isotherm condenser and the liquid level indicator. The successful condensation of ammonia was confirmed visually. Subsequently, the valve above the liquid level indicator was closed. The liquid NH₃ level in the indicator was 13 mL, which is equivalent to the quantity of SrCl₂ employed to fill the reactor. Following condensation, the liquid NH₃ was supplied to the reactor by opening the valve below the liquid level indicator, which triggered the start of the liquid-solid reaction. For the gas-solid reaction, gaseous NH3 was supplied directly from the NH₃ cylinder to the reactor. The various experimental conditions employed are outlined in Table 1.

The average power, Q_average, as defined in Equation (6), was based on the measured water temperature (see nomenclature list for definitions).

*Mass of SrCl₂: 9.6 g; Thickness of packed bed: 1 mm; Flow rate of heat transfer water: 0.8 L/min.

Furthermore, the characteristic heat transfer coefficient, U, was determined to evaluate the heat transfer resistance in the reactor, and was calculated from the experimental results as outlined in Equation (7).

where Q is the momentary power, based on the measured water temperature. T_fl is the temperature of the heat transfer water, which was calculated as a logarithmic mean of the inlet and outlet temperatures of the heat transfer water. Moreover, A is the area of the heat transfer surface (i.e., 100 mm²), and T_r is the characteristic temperature of the reactant packed bed, which was determined by measuring the temperature using a thermocouple at 0.5 mm above the heat exchange plate, as indicated in Figure 2. The characteristic heat transfer coefficient is the parameter that reflects the heat resistances 1) through the packed bed, 2) between the packed bed and the heat transfer plate surface, 3) the heat transfer plate, and 4) the thermal boundary layer at the bottom of the heat transfer plate. The heat resistances at 3) and 4) are same under all the experimental conditions, therefore, the change in this parameter implies heat resistance changes at 1) and 2).

We then aimed to evaluate the short-term cycling capabilities of the discharge operation of the liquid-solid reaction, as outlined in Equation (3).

Following the condensation of NH3, the discharge and charge operations were repeated 10 times as per the reaction conditions outlined in Table 2. Each reaction cycle was operated as follows. During discharge, liquid NH3 was supplied to the reactor from the liquid level indicator, allowing the liquid-solid reaction to commence. Following the exothermic reaction, the reactor was heated to 80˚C using the thermostat bath, allowing the endothermic reaction to take place. The desorbed NH₃ then transferred to the condenser and liquid level indicator and was condensed. This charge operation took place

over 6 h, and was completed by closing the valve between the reactor and the condenser, and by subsequently cooling the reactor to 5˚C. After precooling, the heat release operation was repeated once more. The condenser temperature was maintained at 5˚C for the duration of this experiment. It should be noted that as the initial reactant was SrCl₂, the exothermic reaction shown in Equation (3) takes place upon the supply of liquid NH3. This initial reaction was defined as cycle 0, while the subsequent exothermic reaction shown in Equation (1) was defined as cycle 1.

The average power, Q_average, defined by Equation (6), and the conversion ratio, X, defined by Equation (8), were calculated

The variation in average power of the liquid-solid and gas-solid reactions with time over a range of pressures is outlined in Figure 4. As shown, for the gas-solid reactions at both 5˚C and 15˚C, an increase in pressure resulted in an increase in the average power, due to the difference in equilibrium temperature of the reaction. Gaseous ammonia should be supplied at higher pressure for higher performance. The average power of the liquid-solid reaction was generally lower than that of the gas-solid reactions at 0.4 MPa or 0.6 MPa. However, the average power of the liquid-solid reaction was comparable with that of the 0.2 MPa gas-solid reaction it was slightly higher at 15˚C, while at 5˚C, it became higher after 200 s. Thus, the liquid-solid reaction can potentially become effective at low temperatures.

We then examined variations in the characteristic heat transfer coefficient, U, with time (Figure 5). Comparable results were obtained for all gas-solid reactions, and the average value obtained over 400 s was U = 165 W/m²/K at 15˚C and U = 161 W/m²/K at 5˚C. However, a larger heat transfer coefficient was obtained for the liquid-solid reaction at both 15 and 5˚C, giving an average value of U = 302W/m²/K and U = 304 W/m²/K over 400 s respectively. This indicates the decreased thermal resistance imparted by the liquid ammonia.

Indeed, this can be explained by the pictorial representation shown in Figure 6. In the gas-solid reaction, the heat of reaction transfers through the reactant particle prior

to reaching the heat transfer plate (Figure 6(a)). While in the liquid-solid reaction, as the ammonia is expected to remain in its pure form prior to the reaction with SrCl₂, and due to the insolubility of this salt in ammonia [17] , the reactant particles come into contact with pure liquid ammonia during the reaction (Figure 6(b)). Thus, the presence of liquid ammonia on the heat exchange surface or in the interparticle voids results in additional heat migration pathways and decreased thermal resistance. Decrease in the heat transfer coefficient over time in the liquid-solid reaction likely indicates the reduction of liquid ammonia with progress of the reaction. This enhancement of thermal conductivity in the reactor might be influenced by the distribution of the liquid present, therefore, the discharge performance of the liquid-solid reaction can be improved by devising a liquid supply method.

The thermal conductivity in the reactor was evaluated roughly using the limited measuring point of the packed bed in this experiment. Therefore, detailed research on the mechanism of the liquid-solid reaction and the reactor design will be conducted in the future.

We then examined the cycling performance of the liquid-solid discharge operation, where the reactions were cycled 10 times. Figure 7 shows the variation in the average power and conversion ratio, where a clear trend was observed for both measurements, despite small differences being detected over the 10 cycles. More specifically, the peak value of the average power fluctuated between 14 and 16 W, while the conversion ratio fluctuated between 0.6 and 0.7 after 400 s. Interestingly, no apparent degradation was observed in the short term for this system. Figure 8 shows photographic images of the SrCl₂ packed bed particles after evaluation of the 10th cycle performance, where scattering of the reactant particles from the heat exchange surface and agglomeration between the particles were observed. Scattering was likely caused by the uncontrolled momentum of the supplied liquid NH₃, while agglomeration was assumed to be caused

by increased adhesion between the particles due to the presence of liquid NH₃. We therefore expect that the varying physical contact between the heat exchange surface, the packed bed, and the liquid ammonia led to the fluctuation in results. As this could potentially prevent reliable cycling over the long term, alternative routes to liquid ammonia supply or particle fixing should be examined.

However, in the context of the short-term durability of the proposed system, no apparent degradation was observed, with comparable performances being obtained in each discharge operation.