Development of a metal hydride compressor

The National Chemical Laboratory for Industry (NCL) model 4 chemical engine has been completed. This is a type of hydrogen compressor using LaNi₅ as the energy conversion medium. The container, consisting of 19 copper tube elements with aluminium inner fins and covered with a stainless steel heat-insulated water jacket, is filled with 18.9 mol of LaNi₅. The compressor is composed of a pair of these containers, two electrical boilers (9 kW), a circulating pump, valves and other components. The container is able to absorb 1.3 m³ (at normal temperature and pressure (NTP)) of hydrogen and the time needed for preheating (90 °C) and cooling (27 °C) is less than 3 min. In the present investigation, using this container, the basic operating characteristics such as the hydrogen-absorbing and hydrogen-desorbing ability, the thermal efficiency, the heat transfer coefficient and the reaction velocity have been examined. Because of the circulating hot water (90 °C; 30 1 min^-1), the NCL model 4 compressor is able to desorb 360 1 min^-1 (NTP) of high pressure (18-12 atm) hydrogen gas continuously in 3 min. However, this is its maximum ability under stable conditions. The overall heat transfer coefficient estimated is about 300-1200 kcal m^-2 h^-1 °C^-1. Using a plunger pump, the NCL model 4 compressor has been combined with a reverse osmosis type of desalination apparatus and a practical test is now under way.