Ionic liquids for microchannel membrane-based absorption heat pumps: Performance comparison and geometry optimization

Abstract

Microchannel membrane-based absorption heat pump (MMAHP) shows great potential in efficiency enhancement and size reduction towards renewable/waste energy utilization. Ionic liquid (IL) mixtures with H2O as refrigerant present excellent merits in avoiding crystallization and obtaining a high coefficient of performance (COP). This study investigates the feasibility of MMAHP using five H2O/ILs (i.e., [BMIM][BF4], [DMIM][DMP], [EMIM][DMP], [EMIM][OAc], and [EMIM][OMs]) under various working conditions, with H2O/LiBr as the baseline. However, MMAHP and IL mixtures are constrained by the high solution pressure drop due to narrow flow channels and high IL viscosities; therefore, the geometries are optimized to maximize efficiency and compactness. The results show that under a desired cooling capacity of 0.5 kW, [EMIM][OAc] and [EMIM][OMs] can yield higher COPs than other ILs and LiBr. The smallest system volume of each working pair increases with the targeted COP to satisfy the requirement of membrane liquid entry pressure (LEP). Nevertheless, [DMIM][DMP], [EMIM][DMP], and LiBr cannot achieve COPs above 0.70, 0.75, and 0.80, respectively, within the involved geometries range due to the limitation of LEP (80 kPa). Comparing the Pareto frontiers that show the trade-off between the COP and compactness, LiBr is the best choice with targeted COPs below 0.778, while [EMIM][OAc], [EMIM][OMs], and [BMIM][BF4] perform better with targeted COPs above 0.778. Meanwhile, although considering the constraint of LEP would make the Pareto frontier moving towards lower COP and smaller compactness, the MMAHP still outperforms the conventional falling-film systems in efficiency and compactness when the targeted COP is below 0.80.