Part Ⅰ : The Development of High Performance Ti-Mn Based Metal Hydrides
$TiMn_2-based$ alloys (AB2-type) are promising hydrogen storage materials for their easy activation, good hydriding-dehydriding kinetics, high hydrogen storage capacity (~1.9 wt% H, LaNi5-based alloys = ~1.5 wt% H) and low cost. However, they have two critical problems for various applications, especially for a Compressor-driven metal hydride heat pump (CDMHHP). One is a high plateau pressure of alloys at room temperature at over 20 atm. The other is a poor hysteresis or sloping characteristics. It is important to solve both problems simultaneously. In this work, $TiMn_2-based$ multi-component alloys have been extensively examined by measuring P-C-isotherms in order to develop a hydrogen storage alloy which has a small hysteresis and sloping, a high hydrogen storage capacity and a suitable plateau pressure for CDMHHP applications. In this work, two aspects is considered. First is element substitution and second is hypo-stoichiometry. The plateau pressure was decreased by the partial substitutions of Zr for Ti, while the slope was increased. The plateau pressure was decreased by the change of stoichiometry. In this case, the slope was decreased. It was found that the sloping characteristics of the Ti-Mn based Laves phase alloys is affected by the strain energy which is induced by size mismatch between Zr and Ti. The substitution of Cu, which has smaller atomic radius than Cr, were very effective in improving the sloping properties. But, the storage capacity decreases. So, V is substituted to increase the storage capacity. After a careful balancing of the substitution effects of the alloying elements, it was found that $(Ti_{0.8}Zr_{0.2})_{1.05}Mn_{0.8}Cr_{1.05}V_{0.05}Cu_{0.1}$ alloy showed a good plateau characteristics with considerably small hysteresis and sloping.
Part Ⅱ : The Development of Compressor-Driven MHHP System
In recent years, various thermally driven solid sorption systems using different working hydride pairs (Metal Hydride Heat Pump, MHHP) have been investigated regarding their heating, cooling and heat transformation applications. It is for the use of the freon type refrigerant is restricted in the aspect of the destruction of ozone layer and the green house effect. The goal of this investigation is to make the MHHP system competitive with the conventional vapor compression systems and also with commercially available liquid sorption systems. For this, the cooling power must be increased. However, the increase of cooling power at MHHP system is restricted for its discontinuous cooling operation. So, to solve this problem Compressor-Driven MHHP (CDMHHP) was suggested. This system showed high cooling power compared with any other metal hydride cooling systems for its continuous cooling operation. However, the cooling power of reported CDMHHP is still lower than the commercialized cooling system for the hydrogen transfer rate was still lower. In this work, we improved the CDMHHP system by adapting hydrogen by-path method to increase the hydrogen transfer rate and investigated the operating characteristics of the system. The dependence of cooling power on operating parameters such as the hydrogen charged amount, cycle time and air flow rate were investigated. The optimum operating conditions was evaluated as follows; (1) the amount of hydrogen charged = 450 liter, (2) cycle time = 3 min, (3) air flow rate = 7℃/min. The maximum cooling power and efficiency (COP) of this system is 353 kcal/kg-alloyh and 1.8 respectively. And the minimum cooling temperature is 6℃ at 7 N㎥/min airflow rate.
