Study on the performance of hybrid adsorption–compression type II heat pumps based on ammonia salt adsorption

Abstract

Sorption heat pumps based on monovariant reactions, such as ammonia-salt systems, can operate at low driving temperatures and achieve high power densities in comparison with multi-variant sorption systems. The disadvantage of monovariant systems, however, is the inflexibility towards required temperature levels. Where multivariant systems scale over a large range of temperatures, for the monovariant system, the temperature range is limited by the discrete transition from (fully) adsorbed to desorbed state. To increase flexibility towards changes in operating temperatures of the monovariant sorption systems, the extension of such systems with a compressor has been studied. Focus of this research is on the use of ammonia salts for type II heat pump for upgrading low temperature industrial waste heat to low-medium pressure steam. At ECN, a system based on LiCl-MgCl2 ammonia reactions has proved to achieve sufficient temperature lift (.508C) and cyclic stability (.100 cycles) but requires a minimum temperature of 1208C for proper operation. To add flexibility to this system, i.e. to be able to use waste heat below 1208C, the performance of a hybrid variant containing both thermally driven sorption reactors and a compressor has been evaluated. This evaluation focuses on extension in temperature range, and exergy efficiency and economic consequences of such a hybrid system. In addition, the possibility to use other ammonia-salt combinations has been investigated. The conclusions are that hybrid systems can reduce primary energy consumption and be economically feasible. It also shows that salt combinations other than LiCl-MgCl2 could be more suitable for a hybrid thermo-chemical adsorption-compression system.