Performance comparisons of NH₃/ionic liquid absorption–compression heat pump for increasing the utilization of geothermal energy

Geothermal energy is becoming increasingly attractive to mitigate global energy and environment crisis. Various hybrid heat pump systems were introduced for increasing the utilization of geothermal energy, and then the absorption–compression heat pump (ACHP) using novel NH₃/IL (ionic liquid) mixtures was analyzed. The property model of NH₃/IL mixtures and the thermodynamic model of the ACHP cycle were established. Parameter studies showed that there was an optimal pressure ratio to achieve the highest coefficient of performance (COP). The optimal pressure ratio increased and the highest COP decreased as the allowed absorption pressure decreased. In addition, a higher pressure ratio was needed to produce a larger temperature lift. A higher absorber temperature (heat sink) led to a lower COP, while a higher generation temperature (heat source) was beneficial to a higher absorber temperature. Among the investigated ILs, [OMIM][BF₄] (1-octyl-3-methylimidazolium tetrafluoroborate) yielded the highest heating COP with the lowest pressure ratio.