High temperature transcritical CO2 heat pump with optimized tube-in-tube heat exchanger

Abstract

CO2 heat pump operating on tanscritical cycle is widely adopted to provide hot water whose temperature is as high as about 90 °C. Recently, it is recognized that high temperature heat pump has promising application in various industries such as papermaking, chemical, automotive, and metallurgical. Then, in this paper a model of a tube-in-tube heat exchanger was established, and its parameters including tube length, fin height, fin number, fin thickness and helix angle are optimized through simulation to enhance its heat transfer performance, Accordingly, a transcritical CO2 heat pump system which can provide about 100 °C pressurized hot water is proposed and investigated. Subsequently, an experimental setup is constructed and operated under ambient temperatures, inlet and outlet temperatures of cooling water to demonstrate the feasibility of obtaining hot water at 100 °C. When the ambient temperature and inlet water temperatures are 40 °C and 9 °C respectively, the highest COP of 3.64 is obtained in experiments and the corresponding simulated value is 3.87. When the ambient temperature is 40 °C, inlet and outlet water temperatures are 9 °C and 85 °C, respectively, the study recorded the highest COP of 4.47. It is worth noting that both the heating capacity and COP increase with rising ambient temperature.