Performance investigation on NH3/ionic liquid absorption cycles for geothermal energy utilization.

Geothermal heat is a promising renewable energy resource and is becoming increasingly attractive to mitigate the global energy and environment crisis. In this study, we introduced various geothermal heat pump systems and focused on the absorption-compression heat pump (ACHP) using novel NH3/IL working pairs for cascade utilization of geothermal energy. The thermodynamic model of the ACHP cycle and the property model of four NH3/IL working pairs 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 high-side pressure decreased. In addition, a higher pressure ratio was needed to produce a larger temperature lift. a higher absorber temperature led to a lower COP; a higher generation temperature was beneficial to a higher absorber temperature. [OMIM][BF4] (1-octyl-3-methylimidazolium tetrafluoroborate) was the best IL among the four studied ILs, yielding the highest heating COP with the lowest pressure ratio.