Exergy Analysis of an Absorption Refrigeration System Using an Iconic Liquid as a Working Fluid in the Chemical Compressor

The first and second law of thermodynamics are applied to an ionic-liquid (IL) based absorption refrigeration system for high power electronics cooling. The IL is a salt in a liquid state usually with an organic cation and inorganic anion. It provides an alternative to the normally toxic working fluids used in the chemical compression loop, such as ammonia in conventional absorption systems. The use of ILs also eliminates crystallization and metal-compatibility problems of the water/LiBr system. In this study, mixtures of refrigerants and imidazolium-based ILs are theoretically explored as the working fluid pairs in a miniature absorption refrigeration system, so as to utilize waste-heat to power a refrigeration/heat pump system for electronics cooling. A mathematical model based on exergy analysis is employed to characterize the performance for specific refrigerant/IL pairs. Both the coefficient of performance (COP) and the exergetic coefficient of performance (ECOP) of the absorption system and components are evaluated. The thermodynamic properties of ILs are evaluated using the correlations based on group contribution methods. A non-random two-liquid (NRTL) model is built and used to predict the solubility of the mixtures. The properties of the refrigerants are determined using REFPROP 6.0 software. Saturation temperatures at the evaporator and condenser are set at 25 o C and 50 o C, respectively. The power dissipation at the evaporator is fixed at 100 W with the operating temperature set at 85 o C, which are the benchmark conditions for high performance microprocessor chip cooling. The desorber and absorber outlet temperatures are adjusted to evaluate the system performance variation with respect to the operating condition change. The effect of the refrigerant/IL compatibility, alkyl chain length of the IL cation, and thermodynamic properties of the refrigerants, such as latent heat of evaporation, on the ECOP is investigated. Also the exergy destruction of each component of the cycle is evaluated and discussed as a means to identify the critical component(s) of the system that would require optimization.