Measurements of Operating Parameters of a Metal Hydride Compressor with a Heat Pump

Abstract

The hydrogen compression that occurs in a metal hydride compressor is based on the chemical and thermal processes of hydrogen absorption and desorption into an intermetallic structure of metals. The concept presented in this article is centred on the use of an optimal metal alloy that is capable of absorbing hydrogen into its structure at a low temperature and pressure. After such an alloy is heated up, its pressure will significantly increase, and this facilitates the compression of hydrogen without it being in direct contact with the movable parts of the compressor. As a result, the safety of the compression process is increased. The hydrogen compressor contains a pair of tandem metal hydride (MH) tanks with an integrated heat exchanger. The tanks are alternately heated and cooled, so that while hydrogen is absorbed in one of the tanks it is concurrently desorbed in the other tank. The unique nature of the prototype single-stage metal hydride compressor consists in using a heat pump that facilitated a significant reduction of the consumed electric energy in the heat transport between the tandem-arranged tanks. The purpose of this investigation was to examine the possibility of integrating a heat pump as a source of heat and cold for the process of absorption- and desorption-based hydrogen compression. With the prototype MH compressor presented in this article, a compression ratio of ε = 2.0 was achieved while the tanks were operated at temperatures ranging from 12 to 55 °C. In a single cycle, the compressor worked with 166 litres of hydrogen, which was absorbed into 3.125 kg of the La0.85Ce0.15Ni5 alloy. The average heating power that was achieved on the side of the condenser that heated the MH1 tank was 362 W, and the cooling power that was achieved on the evaporator side was 300 W. The achieved value of the heat pump COP was 2.92.