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Metal Hydride Heat Pumps for Thermal Management of Devices
Chemical and Materials Engineering
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Metal hydrides have very high volumetric energy densities which makes metal hydride heat pumps (MHHPs) particularly efficient devices. Further, a metal hydride heat pump (MHHP) can be tuned to cover a wide range of temperatures. Most phase change materials (PCMs) have a fixed phase change temperature, but metal hydrides are more versatile since they undergo phase change at all temperatures by adjusting the hydrogen equilibrium pressure. Currently, there is knowledge of heat pumps of large sizes, but we do not have data for small/miniature heat pump devices for either heating or cooling. The motivation for this work was to develop MHHPs for small scale applications. In the present work, experimental studies have been carried out on heat pumps working with different hydride pairs, such as MmNi<sub>4.15<sub>Fe<sub>0.85<sub>/LaNi<sub>4.6<sub>Sn<sub>0.4<sub>, MmNi<sub>4.15<sub>Fe<sub>0.85<sub>/LaNi<sub>4.78<sub>Sn<sub>0.22<sub>, and LaNi<sub>4.78<sub>Sn<sub>0.22<sub>/LaNi<sub>4.6<sub>Sn<sub>0.4<sub>, where Mm represents Mischmetal. The issues addressed in this work are miniaturization of a MHHP, electronic control of heat pump hydrogen gas transfer, heat pump performance and its dependence on gas pressure ratio in a MHHP, and mitigation of parasitic losses by reducing the amount of excess H<sub>2<sub> in the system. The electronic control of MHHP was accomplished by using a proton exchange membrane (PEM) with catalyst, thus replacing the mechanical valve which separates the two hydrides in a conventional heat pump. Such a heat pump can be recharged just with the reversal of polarity of the applied voltage to the PEM thereby obviating the need to heat the low pressure side of a metal hydride heat pump for system recharge. In a typical MHHP, the equilibrium plateau pressures of the hydrides (in two different cylinders) are used to control the non-equilibrium thermodynamics. Minimum temperatures in the range of +16<super>o<super>C to -12<super>o<super>C were obtained by varying the equilibrium pressure ratios (P<sub>1<sub>/P<sub>2<sub>) of the two hydrides from 0.006 to 150 respectively. It was found that the minimum temperature or the degree of cooling is logarithmically dependent on the pressure ratio. The minimum pressure ratio for optimum functioning of the heat pump was found to be P<sub>1<sub>/P<sub>2<sub> = 0.2. Also, the presence of excess H2 is detrimental to heat pump performance, for example, reducing the excess amount of H<sub>2<sub> gas in the system from ~2.25 moles to ~0 moles led to the reduction of minimum temperature from 20.5<super>o<super>C to 7<super>o<super>C. A minimum temperature of 7<super>o<super>C was obtained with the heat pump (LaNi<sub>4.78<sub>Sn<sub>0.22<sub>/LaNi<sub>4.6<sub>Sn<sub>0.4<sub>) in the absence of an external heat load (and under thermal insulation). Experiments conducted on the same system with an external heat load (and without thermal insulation) showed a drop in temperature of the heat load from ~79<super>o<super>C to ~46<super>o<super>C by the metal hydride, a reduction by 33<super>o<super>C. The advantage of this system is that it operates at sub-atmospheric pressures, which has implications for safety.The results of the current investigations are indicative of the utilization of MHHPs with enhanced features such as electronic control of hydrogen flow for small scale heating and cooling applications.