Experimental investigation on the influencing factors of a transcritical CO2 heat pump

As one of the earliest natural refrigerants, CO2 has a lot of unique advantages, such as zero ODP, negligible GWP, excellent heat transfer coefficient, high volumetric capacity, non-toxicity, non-flammability, and cost effectiveness. Since it was proposed to applying in the mobile air-conditioning, the transcritical CO2 cycle has been attracting great attentions to replace CFC and HCFC refrigerants in many areas, including refrigeration, heat pump, and airconditioning, etc.Unlike the conventional refrigerants, the heat rejection pressure and temperature of the gas-cooler in the transcritical cycle are usually decoupled in the transcritical cycle. Besides, there exists an optimal heat rejection pressure under which the maximum cycle efficiency can be achieved. The concept of “the optimal heat rejection pressure” has attracted wide attentions in the scientific community. Therefore, the interaction effect between heat rejection pressure and system performance has been studied by many researchers.The heat rejection pressure of the gas-cooler has a great impact on the COP of the transcritical CO2 system, but the investigation on the influence factors of the heat rejection pressure is quite rare in the open literature. In this paper, the effects of the water inlet temperatures and the water flow rates on the heat rejection pressure of a water-to-water transcritical CO2 refrigeration heat pump with single-stage expansion system have been investigated. Furthermore, the operation parameters and the performance of the system are also evaluated.