In order to solve the problems of a heat pump air conditioner for an electric bus under extreme conditions, such as the large compression ratio, the high discharge temperature, the reduced system performance, the frequent shutdown of the compressor due to the overheat protection, this paper proposes a vapor injection technology with an economizer, and carries out a theoretical analysis of the process to reveal the effect of the vapor injection technology on the performance of the air conditioner. The results show that at ultra-low temperature heating operating conditions, when the compressor speed increases from 2000 rpm to 5000 rpm, the heating capacity of the vapor injection system increases from 16.2% to 22.7%, and the heating performance coefficient increases from 2.8% to 14.2%.

In response to the call for carbon neutrality and control of CO2 emission, this paper proposes a high temperature cascade heat pump system with double medium and low temperature heat sources. Compared with those with a single heat source, the new system has obvious advantages in energy saving and carbon emission. An experiment is designed where the temperature of medium temperature heat source keeps unchanged, revealing that, compared with a single low temperature heat source, the heating capacity of low temperature heat source reduces by 2.9%, the heating capacity of medium temperature heat source increases by 31.1%, the total power consumption of the heat pump increases by 3%, and the heating capacity of the heat pump increases by 4.3%.

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In order to improve the thermal performance of a high temperature heat pump system, to realize the efficient heat recovery of industrial waste heat, and to make the combined utilization of low and medium temperature heat sources, a high temperature heat pump system with a double heat source cascade is proposed, which can absorb heat from industrial waste heat and the system is capable of heating water up to the required high temperature. The performance of the system is tested and analyzed. It is found that the inlet temperature of the medium temperature heat source has a significant impact on the performance of the heat pump system under working mode of the medium and low temperature heat sources. When the inlet temperature of the intermediate temperature heat source rises from 42?C to 53?C, the heating capacity of the cascade heat pump increases by 18.6%, and its performance coefficient increases by 8.9%. This paper provides a reference for the high efficiency of the cascade high temperature heat pump technology.