The optimization of mixing ratio of expanded graphite mixed chemical heat storage material for magnesium oxide/water chemical heat pump

• The expanded graphite (EG) mixture was developed for MgO/H2O chemical heat pump.
• Optimization of mixing molar ratio between Mg(OH)2 and CaCl2 was conducted.
• The hydration reactivity of mixture was decreased as the mixing molar ratio increased.
• Heat output rate and capacity of optimized EG mixture was estimated.
• Optimized EG mixture shows better heat output performance than pure Mg(OH)2.

A chemical heat storage composite material (EMC), a mixture of expanded graphite (EG), magnesium hydroxide (Mg(OH)2), and calcium chloride (CaCl2), was developed as a magnesium oxide/water chemical heat pump reactant. The potential of the EMC was confirmed and optimized mixing weight ratio between EG and Mg(OH)2 was suggested in previous study. In this study, the optimization of mixing molar ratio between Mg(OH)2 and CaCl2 for practical application was conducted; total six kinds of EMC mixtures, which have different mixing molar ratio from 0, to 0.01 to 0.20 with optimized mixing weight ratio, were prepared then dehydration and hydration experiments were carried out. From experimental results, it was confirmed that hydration reacted conversion was increased as increasing amount of CaCl2 in an EMC and the optimized mixing molar ratio was suggested as mixing molar ratio, α, is 0.1 at mixing weight ratio, n, is 0.8 by considering chemical rate constant and reacted conversion. Hydration under various vapor pressures and temperatures of optimized EMC was also conducted and optimized EMC showed better performance than pure Mg(OH)2. Finally, the heat output performance of optimized EMC was estimated numerically. In conclusion, optimized EMC performed better on dehydration and hydration than pure Mg(OH)2 by adding EG, which has high thermal conductivity and large specific surface, and CaCl2, which has hydrophilic property.