Thermal Energy Storage by Chemical Heat Pump System Using NiCl₂⋅6NH₃ Application of Liquid Ammoniate NH₄NO₃⋅nNH₃ to Low Temperature Material

Abstract

Chemical heat pump systems are useful for thermal energy storage. The most important factor for the actual operation of chemical heat pump is reaction rate. In the present work, liquid ammoniate NH₄NO₃⋅nNH₃ was selected as a low temperature material, because the dissociation rate of liquid ammoniate is relatively fast even at low temperatures. NiCl₂⋅6NH₃ has been adopted as a high temperature material. The present chemical heat pump system consists of the following reactions;
High temperature reaction:
Low temperature reaction:
On the basis of preliminary experiments, a bench scale system was constructed. NH₄NO₃(677 g), charged in a low-temperature vessel made of Pyrex glass, was ammoniated at 0°C until NH₄NO₃⋅2.4NH₃, was formed. The low temperature vessel was connected to high temperature vessels (280 mm in length and 50 mm diameter, made of SUS 304) containing NiCl₂⋅6NH₃ (480 g in total weight in 5 vessels). At the heat storage stage, NH₃ gas generated from the high-temperature vessels by heating at 195°C in a silicone oil bath was successfully absorbed by the liquid ammoniate within 8 h. At the heat-releasing stage, the reaction of NiCl₂⋅2NH₃ with NH₃ proceeded in the high-temperatture, vessels, which were immersed in a water bath, whereas the NH₃ gas was generated from NH₄NO₃⋅nNH₃ at 0°C. The flow rate of NH₃ gas was well suited for NH₃ absorption with Ni salt, and the reaction was completed in 2 h. The amount of released heat was calculated from the temperature increase of the water bath. The recovered thermal energy was 388 kJ amounting to 75% of the ideal value 520 kJ. Similar results were obtained even after 20 cycles of operation.