Theoretical analysis of internal heat exchanger in transcritical CO2 heat pump systems and its experimental verification

Abstract

Many studies have investigated the effect of internal heat exchanger (IHX) on the transcritical CO2 heat pump (TCHP) system. However, little information is available regarding the length of IHX under various water temperature conditions. In this paper, the theoretical model was established to compare the heating performance with and without IHX. A prototype TCHP test-rig was conducted to validate the model, and the experimental results showed a good agreement. The results show that the effect of IHX on the coefficient of performance (COP) is related with inlet water temperature. The IHX is beneficial for the COP with higher temperature 40 °C. Meanwhile, the optimal discharge pressure is reduced by more than 1 MPa with the IHX length increasing from 0 m to 4 m. Although the longer length is profitable, the discharge temperature will tend to rise 25 °C and easily exceed 145 °C when the ambient temperature depresses to 2 °C. Therefore, the length in this paper is recommended to be approximately 2.5 m considering the conflict between the discharge pressure and discharge temperature.