Study on working condition adaptability of transcritical CO₂ heat pump based on intermediate temperature heating

In waste heat source transcritical CO₂ heat pump, due to high-temperature waste heat leads to a smaller system compression ratio, lower discharge temperature. It is difficult to directly heat low-temperature water to 80-100 ℃. If this is to be achieved by conventional procedures, the discharge pressure needs to be increased to more than 14.5 MPa. But this pressure far exceeds the compressor pressure limit, there is a risk of equipment damage. In order to solve the problems, this study proposes an intermediate temperature regenerative waste heat source transcritical CO₂ heat pump system. By dividing the gas cooler into a first gas cooler and a second gas cooler, which are respectively set at the two side of the internal heat exchanger. The refrigerant at the compressor inlet is sufficiently superheated by the refrigerant at the outlet of the first gas cooler, effectively solve the problem of the discharge temperature being too low. This study also analytically compares the single-parameter optimization of the conventional system with the coupled optimization of the new system at three inlet water temperatures (5 ℃, 15 ℃, 30 ℃) and a fixed outlet water temperature of 90 ℃ for the operating conditions. The main research results show that under the same condition, the new system can achieve the discharge temperature of 15.0 MPa in the conventional system under the discharge pressure of 12.3 MPa. This configuration significantly reduces the required discharge pressure while enabling the expansion valve inlet temperature and the inlet water temperature closer to each other, enhancing the system's heat exchange efficiency. Compared to conventional systems, this innovation achieves a COP improvement of approximately 16.34 %. This demonstrates the new system not only slove the challenge of direct single-stage heating of low-temperature water to 80–100 ℃ under high waste heat and low-pressure ratio conditions, but also further improves energy efficiency. Providing critical technical support for the application of high-temperature industrial heat pumps.