Experimental study and optimization analysis of start control strategy for the transcritical carbon dioxide heat pump

Abstract

Transcritical carbon dioxide heat pump (TCHP) water heaters hold great promise for widespread adoption in the field of sustainable energy. Due to the high discharge temperature and pressure, inappropriate control strategies can easily lead to the shutdown of the system during the start-up phase. However, the current research has focused on control strategies for the stable operation of TCHP system, without experimental research on the control strategies for the start-up stage. In this study, a TCHP system was constructed to explore the optimal initial opening degree of the electronic expansion valve (EEV) using experimental methods, and the EEV control rules and compressor frequency rising rules in the start-up phase were optimized. Furthermore, the changing characteristics of key parameters under different strategies were analyzed. The results demonstrate that the stability of the system during the start-up phase is maximized when the initial EEV opening degree is set to 300. Furthermore, by employing a compressor frequency rising rule of running at 50 r/s for 60 s, 60 r/s for 60 s, and 90 r/s for 60 s, the total power fluctuation is minimized, resulting in a total power in the three stages of 1073, 1686, and 2152 W, respectively. Additionally, when targeting a discharge pressure range of 8.5–––9.0 MPa controlled by the EEV, the maximum discharge pressure at a water temperature of 60 ℃ is 9.8 MPa, with the EEV opening degree falling within the range of 200–––450. The results are of significant reference value for determining the initial EEV opening degree and the target value of the discharge pressure during the start-up phase of TCHP systems. The optimized start control strategy effectively improves the TCHP system stability during the start-up phase.