Ionic liquid screening and performance optimization of transcritical carbon dioxide absorption heat pump enhanced by expander

Absorption technologies show great potential in utilizing renewable energy and waste heat. In this study, novel CO₂/ionic liquid (IL) mixtures are investigated to overcome the shortcomings of conventional working fluids. To screen the optimum ILs for a transcritical CO₂/IL absorption heat pump (TsCO₂-AHP), the absorption cycle models using 34 IL species are established to estimate the cycle performance. Within the selected candidates, [OMIM][Tf₂N] yields the best cycle performance. For imidazolium-based ILs with different anions, the coefficient of performance (COP) is in the order of [Tf₂N] > [C(CN)₃] > [OTF] > [DCA] ≈ [PF₆]. [EMIM][C(CN)₃] yields the highest heat transfer coefficient of 0.520 kW/(m²·K) due to its better transport properties compared to other candidates. The high-side pressure optimization is conducted to maximize the COP. At the generator temperature of 138 °C, the COP of CO₂/[OMIM][Tf₂N] reaches the highest 0.2614 with the optimum P_high of 10.289 MPa. By equipping an expander, the COP of the TsCO₂-AHP is improved by 11.9% because of a larger cooling capacity and less electricity consumption. The main objectives are contributing novel potential working fluids, optimizing the cycle parameters, as well as selecting the best-performance CO₂/IL mixture for improved development of absorption technologies.