Thermodynamic analysis of CO₂ transcritical two-stage compression refrigeration cycle systems with expanders

ABSTRACT

The key to improving the efficiency of carbon dioxide (CO₂) transcritical refrigeration systems is to select an appropriate two-stage compression system. In this study, the thermodynamic performance of six different CO₂ transcritical two-stage refrigeration systems with expanders is compared. The design parameters are analysed to determine their respective impacts on the system coefficient of performance (COP) in order to ascertain which system configuration produces the maximum system COP. It is concluded that the system with a two-stage expander and incomplete intercooling has great advantages in terms of performance and equipment selection for the high-pressure stage in certain circumstances. Under the given conditions, the design parameters including the evaporating temperature, the condensing temperature, the compressor efficiency of the high-pressure stage, and the compressor efficiency of the low-pressure stage impact on the system performance. It should be noted that the gas cooler outlet temperature and the compressor efficiency of the low-pressure stage are the major factors which affect the efficiency of the system; thus, the system design and equipment selection should minimise the gas cooler outlet temperature and select a high-efficiency compressor for the low-pressure stage to ensure efficient and safe operation.

KEYWORDS:

• CO₂
• transcritical cycle
• two-stage compression
• two-stage expander
• COP
• single-stage expander

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [grant number 51006073]; and the Tianjin Research Program of Application Foundation and Advanced Technology [grant number 15JCTPJC62600].

Notes on contributors

Shengchun Liu

Dr Shengchun Liu is an Associate Professor in the School of Mechanical Engineering at the Tianjin University of Commerce (TJCU), People’s Republic of China. Dr Liu obtained his doctoral degree in Thermal Physical Engineering at the TJCU in 2006 and attended the West Virginia University in the United States, the Laboratory of Mechanics and Technology (LMT) Laboratory/ENSC in Paris and the University of Tokyo as a Visiting Scholar. He is currently a member of the Heat and Mass Transfer Young Society of China. His research works focus on heat and mass transfer, phase change mechanisms, refrigeration system optimisation and energy saving, heat transfer and heat exchange studies on refrigeration and air conditioning systems, along with performance studies on natural refrigerants.

Zhili Sun

Dr Zhili Sun is a Lecturer in the Department of Refrigeration and Air-conditioning at the TJCU. He graduated from the Tianjin University and the TJCU, respectively. His research interests include the energy saving and optimisation of refrigeration systems, the scientific problems of the equal mass flow distribution of two-phase fluid, and heat transfer enhancement technology.

Hailong Li

Dr Hailong Li is an Associate Professor in the School of Sustainable Development of Society and Technology at the Mälardalen University, Sweden. His research interests include energy saving and distributed energy resources.

Baomin Dai

Dr Baomin Dai is a Lecturer in the Department of Refrigeration and Air-conditioning at the TJCU. He graduated from the Tianjin University and the TJCU, respectively. His research interests include energy saving and optimisation of refrigeration systems and the scientific problems of heat and mass transfer.

Yong Chen

Mr Yong Chen is an Engineer at the VECK (Tianjin) Co Ltd. His research interests include the energy saving and optimisation of refrigeration systems.