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In this paper, a transcritical heat pump using CO 2  as the refrigerant was developed, and the heating performance test experiment was carried out in the standard bus air conditioning performance enthalpy difference laboratory to explore the influence of outdoor temperature, compressor speed and electronic expansion valve (EEV) opening on the performance of the heat pump system. Operating at −25 °C/20 °C (outdoor temperature is −25 °C, indoor temperature is 20 °C), the heating capacity (Q) of the CO 2  heat pump is 10.5 kW, the coefficient of performance (COP) is 1.24, indicating that the system can also effectively save energy under ultra-low temperature conditions. As the compressor speed rises, the COP of system drops sharply, so the compressor operating frequency should be reduced as much as possible under the premise of satisfying the Q and outlet air temperature (T a.out ). Increasing the opening of the EEV can effectively reduce the compressor exhaust temperature (T com.out ) and pressure(P com.out ) to ensure the safe and stable operation of the system.

Experimental study on the heating performance of transcritical CO 2 heat pump for electric buses