Hydrogen Storage Alloys And Their Performance In Metal Hydride Heat Pump Refrigeration System

Hydrogen energy is an important energy source in the 21st century. Due to its featureslike clean, efficient, safe and sustainable development hydrogen energy shows the greatestpotential for development of clean energy in this century. Metal hydride heat pump is animportant application field of hydrogen energy. Metal hydride heat pump utilizes less heat andgenerates refrigerating effect by hydrogen reaction enthalpy of hydrogen storage alloys.Compared with the traditional freon air-conditioners, metal hydride heat pump has suchbenefits as no pollution to environment and energy saving, and has become a for developmentdirection with great potential in the 21st century.Hydrogen storage alloys and performance in metal hydride heat pump refrigerationsystem are investigated in this dissertation. Crystal structure and hydrogen storage propertiesof the hot end alloys including LaNi_4.61Mn_0.26Al_0.13 and LaNi_4.4Mn_0.26Al_0.34 and the cold endalloys including La_0.6Y_0.4Ni_4.8Mn_0.2 and La_0.5Y_0.5Ni_4.8Mn_0.2 are studied. Dependence ofcoefficients of performance (COP) on various parameters for metal hydride heat pumprefrigeration system is investigated theoretically. Based on the existing system, the factorsinfluencing both refrigeration temperature and COP are discussed.The experimental results show that compared with LaNi_4.61Mn_0.26Al_0.13 alloy, theequilibrium hydrogen pressure and hydrogen absorbing capacity of LaNi_4.4Mn_0.26Al_0.34 alloyget decreased. The equilibrium hydrogen pressure is high, which can increase the differenceof hydrogen pressure between paired alloys and is beneficial to the system. But the flatplateau slope and hysteresis degree of LaNi_4.4Mn_0.26Al_0.34 alloy are smaller than that ofLaNi_4.61Mn_0.26Al_0.13 alloy. Compared with La_0.6Y_0.4Ni_4.8Mn_0.2 alloy, the equilibrium hydrogen pressure and hydrogen absorbing capacity of La_0.5Y_0.5Ni_4.8Mn_0.2 alloy get increased, but theflat plateau slope and hysteresis degree of La_0.5Y_0.5Ni_4.8Mn_0.2 alloy become smaller, which arebeneficial to achieving the higher COP. The proper mass ratio of paired alloys, initialhydrogen-charging content, cycling time and working temperature are also beneficial toreaching high COP. When the initial hydrogen-charging pressure increases, the refrigerationtemperature decreases, but COP increases. However, when the initial hydrogen-chargingpressure is over 15MPa, the refrigeration temperature and COP almost remain the constant.The refrigeration temperature decreases with increasing the cycling time, but turns to becomestable gradually. COP continually increases with increasing the cycling time. When the highworking temperature increases, the COP increases sharply and then decreases marginally.When the low working temperature increases, the COP increases. The operating parametersfor the system are selected as the initial hydrogen-charging content as pressure of 15MPa, thecycling time as 20-25 minutes, the high working temperature of 170℃～180℃and the lowworking temperature of 25℃～40℃. It is noted that the difference in the calculated andexperimental refrigeration temperature and COP are quite large. It means that heattransferring property between metal hydride and reaction bed need to be improved.