Metal hydride based heating and cooling systems: A review

Dry (solid) sorption systems are attractive competitors to wet (liquid) sorption systems in providing useful cold and/or useful heat. Among the dry sorption systems, those based on the absorption/desorption of hydrogen in/from metal alloys reveal advantageous features, and this has stirred up the interest of researchers already since the 1970s. In recent years, many attempts have been made to develop metal hydride based heating and cooling systems. Of special interest was and is the possibility to utilize low temperature heat (waste heat, solar heat) to drive those systems. Major applications are seen in air-conditioning and heat supply for buildings and in air-conditioning of automobiles. In this paper, the research and development work on metal hydride based heating and cooling systems is reviewed which has been published in the last three decades. Emphasis is given primarily to cooling/air-conditioning. The objectives are to provide the fundamental understanding of metal hydride based heating and cooling systems and to give useful guidelines regarding various design parameters. The operation principles of various types of systems are explained and the importance of the metal hydride reaction bed heat and mass transfer characteristics is stressed. Possible ways for improving the coefficient of performance and specific cooling capacity are discussed. Besides a brief characterization of many experimental and theoretical investigations, the worldwide status of the development of metal hydride based heating and cooling systems is summarized in a tabular form.