Optimization of trans-critical CO₂ high-temperature heat pump cycle and study of maximum heating temperature.

High-temperature heat pump (HTHP) applications can be expanded from building heating to broader industrial heating fields, which is an important direction for developing heat pumps. A detailed study of five trans-critical CO2 HTHP cycles was conducted: basic cycle, basic cycle with an internal heat exchanger (IHX), ejector cycle, ejector cycle with an IHX, and dual-temperature evaporation ejector cycle. A thermodynamic model and a multiobjective optimization model of each cycle were established, considering the heat transfer pinch temperature difference (PTD) of the heat exchanger and the non-equilibrium phase change phenomenon of the ejector. A cycle optimization method was proposed for the ejector heat pump to solve the phase equilibrium problem. The maximum heating temperature of the trans-critical CO2 HTHP cycle is limited by the compressor discharge pressure and discharge temperature; the maximum heating temperature is up to 124.0 ◦C at a 20 ◦C ambient temperature. When the heating temperature is 85 ◦C, the COP of each cycle is equal at approximately 4. However, when the heating temperature exceeds 85 ◦C, the ejector cycle with an IHX demonstrates enhanced performance, attributable to the capability of the IHX to elevate the compressor inlet temperature and reduce the optimal discharge pressure. Conversely, when the heating temperature is <85 ◦C, the dual-temperature evaporation ejector cycle exhibits superior performance due to higher average evaporation temperature, which increases the COP of the cycle.