Evaluation of water + imidazolium ionic liquids as working pairs in absorption refrigeration cycles

Abstract

Ionic Liquids (ILs) have been proposed as alternative absorption media for water in absorption refrigeration cycles to replace the currently used lithium bromide, which suffers from problems like crystallization and corrosiveness. This work investigates the use of imidazolium based ionic liquids as water absorbents in absorption refrigeration cycles. A predictive procedure is followed for calculating the parameters that mostly affect the performance of the cycle, that is the Vapor-Liquid Equilibrium (VLE) of the working pair and its heat capacity: UNIFAC is employed for the VLE calculations while a new predictive method has been developed for the heat capacity of the ILs. The effect of the alkyl chain length of the cation and the type of the anion that compose the IL is investigated, and it is shown that ILs consisting of imidazolium cation with short alkyl chain and the anions [Cl] and [DMP] exhibit the best performance in terms of Coefficient of Performance (COP) and circulation ratio (f). Comparison with the conventional LiBr absorbent indicates that the best ILs exhibit higher COP values but higher f ratios as well. Also, double cycle configurations are studied revealing the major effect of the supplied heat temperature on COP, even for the higher temperatures, compared to the single effect one. Finally, it is shown that there is a clear correlation between the COP of the refrigeration cycle and the product of the circulation ratio and the specific heat capacity of the strong solution, which is proposed in this work as a new reliable index for screening different ILs as absorbents, without the necessity of solving the mass and energy balances of the cycle.