Development of a two-stage metal hydride system as topping cycle in cascading sorption systems for cold generation

Abstract

Thermally driven sorption heat pumps compete to be an alternative to mechanically driven vapor compression heat pumps. They do not use CFC refrigerants and therefore have no ozone depletion potential and only a negligible global warming potential. However, their performance lacks behind, even if in the case of compression devices the efficiency of electricity generation is taken into account. The currently available sorption devices have a coefficient of performance (COP) for cooling of about 0.75 for single-effect systems and of 1.2 for double-effect systems. Since the temperatures and pressures under which sorption systems are operated differ widely, it has been suggested to combine sorption systems operating with different working pairs to form a cascading system, in which a topping cycle is producing cold and heat at a sufficiently high temperature level to be able to drive a bottoming cycle which also produces cold, thus increasing the COP. In this study, a two-stage metal hydride sorption device is investigated, which is used as a topping cycle in a cascading system. The system comprises the three metal hydrides LmNi 4.91Sn 0.15, LaNi 4.1Al 0.52Mn 0.38 and Ti 0.99Zr 0.01V 0.43Fe 0.09Cr 0.05Mn 1.5 in two reactors each. It is operated with a driving temperature of 310 °C, releasing heat for driving a bottoming cycle at a temperature of 125 °C and producing cold at a temperature of 2 °C. With a half-cycle time of 15 min and using the reaction enthalpies and the exchanged amount of hydrogen, the heat and cold output of the system can be determined. The total cold production is 1.8 kW and the heat generation is around 1.5 kW. The COP is in the range of 0.9 and the coefficient of heat amplification around 0.75. If a double-effect lithium bromide–water system with the above-mentioned COP is used as the bottoming cycle of the cascading system an overall COP of 1.8–2 is expected.