Absorption heat pump cycles with NH₃ – ionic liquid working pairs

Abstract

Ionic liquids (ILs), as novel absorbents, draw considerable attention for their potential roles in replacing water or LiBr aqueous solutions in conventional NH₃/H₂O or H₂O/LiBr absorption refrigeration or heat pump cycles. In this paper, performances of 9 currently investigated NH₃/ILs pairs are calculated and compared in terms of their applications in the single-effect absorption heat pumps (AHPs) for the floor heating of buildings. Among them, 4 pairs were reported for the first time in absorption cycles (including one which cannot operate for this specific heat pump application). The highest coefficient of performance (COP) was found for the working pair using [mmim][DMP] (1.79), and pairs with [emim][Tf₂N] (1.74), [emim][SCN] (1.73) and [bmim][BF₄] (1.70) also had better performances than that of the NH₃/H₂O pair (1.61). Furthermore, an optimization was conducted to investigate the performance of an ideal NH₃/IL pair. The COP of the optimized mixture could reach 1.84. Discussions on the contributions of the generator heat and optimization results revealed some factors that could affect the performance. It could be concluded that the ideal IL candidates should show high absorption capabilities, large solubility difference between inlet and outlet of the generator, low molecular weights and low heat capacities. In addition, an economic analysis of the AHP using NH₃/[emim][SCN] working pair with plate heat exchangers was carried out based on heat transfer calculations. The results indicated that the NH₃/IL AHP is economically feasible. The efforts of heat transfer optimization in the solution heat exchanger and a low expense of ILs can help the IL-based AHP systems to become more promising.