Reaction control of CaSO4 during hydration/dehydration repetition for chemical heat pump system

Chemical heat pump (CHP) systems have been proposed by the authors as significant technologies for effective energy utilization. For lower temperature heat source, we have proposed CHP materials using a reversible calcium sulfate/water (CaSO4/CaSO4·1/2H2O) reaction for effective waste energy utilization. As one of the CHP systems, we proposed a novel refrigeration truck system which stores the engine waste heat by means of the CHP and recycles/releases it as cold heat for cooling/refrigerating the refrigeration room during an idling stop i.e. engine stop. However, the CHP materials sometimes showed the degradation of hydration/dehydration reaction during about 500 times repetition in the CHP refrigeration mode experiments etc. We need to explain the major cause and the resolution of the degradation. The effective repetition conditions and the increasing repetition cycles are important in order to improve the efficiency of the CHP systems. s study, the samples recovered after 3–550 times repeated reactions of hydration/dehydration in the CHP system were analyzed by several methods. Differential thermogravimetry (DTG) was used to analyze the conversion of III-CaSO4/β-CaSO4·1/2H2O repeated reaction. The crystal phases of samples were examined by powder X-ray Diffraction (XRD). The microstructures and specific surface area of the samples were observed by Scanning Electron Microscopy (SEM) and N2 adsorption. esult, the major cause of the reaction degradation within the CHP system was considered to be the irreversible transformation of III-CaSO4 into II-CaSO4 of the CHP materials occurred by the repeated reaction in high pressure depending on the temperature and pressurized time. The transformation occur in the dehydration step of CaSO4·1/2H2O particles. The appropriate temperature together with water vapor pressure in the repeated dehydration more than 288 times were found to be 423 K at 30 kPa, 413 K at 151 kPa, and 403 K at 453 kPa. From these results, the CHP system is able to increase the repeated reaction times by the appropriate pressure control for increasing the durability of the CHP materials.