Reactivity Improvement of Magnesium Chloride by Ammonia Pre-coordination for Thermochemical Energy Storage at Approximately 100°C

The concern over climate change has led to a rapid transformation of the global energy system, such as the expansion of renewable energy on a global scale.1) However, renewable energy (including solar, wind, wave, etc.) is a natural energy source, and its output itself is unstable depending on time and place. Therefore, even if large power generation capacity from renewable energies is installed, it cannot directly reduce other installed capacities of conventional power stations since backup power source is required for stable power supply. To further advance a sustainable energy system, energy storage technologies is useful.

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Rapid progress in development among energy storage technologies for thermal energy storage has been seen in recent years owing to its low cost and the capability of large-scale storage over long periods of time.2,3,4) However, most of the materials used in demonstration tests were limited to sensible heat storage materials such as concrete and rocks, which have low energy storage performance; development using higher performance heat storage materials is desired.

Thermochemical energy storage (TcES) materials are a promising candidate as advanced energy storage materials. It has many advantages such as high energy density (over 1 MJ kgmaterial−1), thermal output at a constant temperature based on chemical equilibrium, chemical heat pump (CHP) drive, and no loss of stored chemical energy during long-term storage.5,6) However, further development of each material and its reaction are required for target temperatures to control thermal output and storage performances to be determined. Among the various thermal energy storage temperature ranges, the temperature range around 100°C is particularly important not only for energy-leveling purposes but also for heat waste utilization. It is well known that there is a great deal of heat waste globally, especially in the industrial sector.7,8) Forman et al.9) showed that 72% of the global primary energy consumption became rejected energy (341 EJ/year in 2012) and that 63% of this energy arose at a temperature below 100°C. Therefore, the development of TcES materials that can be used in this temperature range is extremely important. The reaction of magnesium chloride and water (MgCl2/H2O) is one of the major TcES materials and reactions in this temperature range.10)

MgCl2 has been reported as an excellent chemical heat storage material because of its abundance, low cost, high reactivity, and high storage density.6) However, problems have also been reported regarding its repeated reaction characteristics due to liquefaction caused by the low melting points of the produced hydrates and high deliquescence.11,12) MgCl2/ammonia (NH3), whereby the reaction gas is changed from H2O to NH3, has also been reported to be used as a TcES material in this temperature range.13,14,15,16) By using NH3 instead of H2O, NH3 can be supplied without a heat source even if the operating environment is below freezing, and there is no concern regarding material liquefaction which can cause reaction degradation in this temperature range.

The reactions between MgCl2 and NH3 are shown in the following reaction equations.17)

MgCl2 reacts with NH3 and produces mono-, di-, and hexa-ammine complexes (Mg(NH3)Cl2, Mg(NH3)2Cl2, and Mg(NH3)6Cl2). The equilibrium lines reported for each reaction are shown in Fig. 1.17) The reaction temperature ranges are different for each of the reactions depending on NH3 pressure. The reaction in Eq. (3), which has a high NH3 reaction capacity (4 molNH3 molMgCl2−1) and a high energy storage density of 1722 kJ kg-MgCl2−1, is particularly desirable when it is used as a TcES material at approximately 100°C. Hereafter, for concise expression of reactions in Eqs. (1), (2), (3), an abbreviation is used to refer to the coordination numbers before and after reactions such as MgCl2-0/1, MgCl2-1/2, and MgCl2-2/6 corresponding to Eqs. (1), (2), (3), respectively.

Some TcES studies using the reaction (MgCl2-2/6) have been reported. Touzain et al.14,16) demonstrated TcES operation of MgCl2-2/6 in a laboratory pilot in 1994. They reported absorption experiments at 50°C and under various NH3 pressures (3.2, 5, and 8 atm) using a mixture of graphite and MgCl2 samples to promote heat transfer. Although heat transfer enhancement of the graphite was confirmed, the reaction took more than 100 min under 3.2 atm of NH3 pressure, 60 min under 5 atm, and 30 min under 8 atm to complete NH3 absorption, despite the relatively high pressure and low temperature of the reaction. Also, Udell et al.15) and Modifi et al.13) used several kilograms of a sample prepared by mixing Mg(NH3)6Cl2 and expandable graphite, introduced it into a tube-type closed system reactor (length: 43 cm, ID: 4.1 cm), and conducted reaction experiments using ten of these tube-type closed system reactors. They demonstrated thermal output/storage operation of MgCl2-2/6 as a TcES material, and achieved a temperature increase from room temperature to 160°C and 100-150°C in the middle of hot bed cells under approximately 860 kPa as maximum feed ammonia pressure.

Also, some evaluation at the powder level of MgCl2-2/6 has been reported for not only TcES18,19) but also NH3 storage material.20,21,22,23) Bevers et al.18) reported thermodynamic properties of MgCl2-2/6 as a system for CHP, the output to approximately 230°C using high-pressure differential scanning calorimetry, and thermogravimetry with 3.65 and 15.27 mg of the measured sample. The heating-cooling test was performed using the temperature swing method between 300 K and 873 K under 1.20-6.00 bar, and a thermal output/storage energy density of 1783 and 1786 kJ kgMgCl2−1 was reported. However, due to the effects of high temperature, significant performance degradation was confirmed. Aoki et al.20) reported the effect of structural change due to pretreatment of materials on the reaction with MgCl2 and NH3. Their experimental results indicated that the high kinetic barrier for the formation of low coordinated ammine complexes can be decreased by the structural change during NH3 absorption and ball-milling treatments. Hummelshøj et al.21) reported creation of mesopores of approximately 20 nm during desorption of Mg(NH3)6Cl2, and they confirmed that these mesopores help NH3 diffusivity and enhance desorption kinetics.

For the purpose of developing NH3 storage materials for metal chlorides, Kubota et al.22) conducted NH3 absorption experiments under 303 K and a NH3 pressure of 84 kPa for MgCl2. However, MgCl2 showed its inferior absorption performance (0.47 kg kgMgCl2−1 for 10 min). On the other hand, Iwata et al.19) reported NH3 pressure dependency in the MgCl2-2/6 reaction at the temperature range of 130-180°C under 120-400 kPa of NH3. This achieved a relatively high absorption conversion rate of MgCl2-2/6 of approximately 0.002 s−1 at 130°C under 120 kPa of NH3 pressure. However, characteristic information of the sample was not provided in detail, and the reason for the performance differences between different research groups is unclear.

While TcES demonstrations of MgCl2-2/6 have already been conducted on a scale of several kilograms, the evaluation at the powder level has not yet been sufficiently developed. Specifically, the structural conditions of the experimental sample should be characterized for performance evaluation. Our research group conducted a detailed exploration of the MgCl2/NH3 reaction including the selection of initial materials, optimal pretreatment, and material characterization, and found an appropriate activation process of MgCl2 to achieve superior performance of MgCl2/NH3 as TcES at the target temperature below the atmospheric pressure of NH3. Therefore, in this study, we report the details of the optimum activation process of MgCl2 for TcES, and the effects of the activation and the performance of the activated sample are also shown in detail, including cyclic durability.

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