Advanced NH3 based absorption refrigeration cycles

Abstract

Absorption refrigeration cycles are energy saving as they can be driven by waste heat instead of electricity. By doing so, they reduce the use of fossil fuels, resulting in a reduction of emissions. However, many challenges exist for the traditional working fluids of the cycle, such as the crystallization problem of the water-lithium bromide pair and the difficulty in separating ammonia from the ammonia-water working pair. Ionic liquids, as novel absorbents in absorption cycles, have attracted considerable attention because of their unique properties like negligible vapor pressure, negligible flammability, thermal stability, low melting temperatures, liquid state over a wide temperature range and good solubility.
In this thesis, a thermodynamic model of the ammonia based double-effect absorption cycle in series configuration was used to predict the performance and suitability of nine ionic liquids as potential absorbents. The performance of the ammonia-ionic liquid systems was compared with that of ammonia-water systems and the influence of the operating conditions and the design parameters on the performance was investigated.
Furthermore, a method to experimentally determine the excess enthalpy of ammonia and ionic liquid mixtures is proposed and the suitability of using absorption refrigeration on a fishing ship, with ionic liquids as absorbents, is studied.
The results show that the ionic liquids [Bmim][BF4], [Mmim][DMP] and [Emim][SCN] used as absorbents in the investigated absorption cycle reached the best performance and could be a promising replacement of water in the ammonia-water working pair. The best performing ionic liquid, [Bmim][BF4], was found to be a better performing absorbent than water in the double-effect cycle.