Green Technology & Innovation 2979-1456 Caravel Press 10.36922/GTI025320011 2025.016 Experimental investigation on the heating characteristics of a transcritical CO2 heat pump https://orcid.org/0009-0002-8145-5375 YangLingxiao ylx_zzu@163.com ChenZhenqian zqchen@seu.edu.cn Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing, Jiangsu, China Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing, Jiangsu, China; Jiangsu Provincial Key Laboratory of Solar Energy Science and Technology, School of Energy and Environment, Southeast University, Nanjing, Jiangsu, China 24112025 1 1 9 24 November 2025 © 2025 The Author(s). Published by Caravel Press. 2025 This is an open access article under the CC BY 4.0 licence.

Inadequately configured operating parameters can have a detrimental effect on the performance of a transcritical CO2 heat pump due to the unique thermophysical properties of CO2 . This study investigates the heat transfer characteristics of the gas cooler (GC) and the effects of three operating parameters on the heating process of the system. The results showed that: The temperature distribution of CO2 and water in the GC exhibited typical “two-region” and “three-zone” patterns; and the increase in discharge pressure enhanced the thermal quality of the GC CO2 side, thereby improving the heat output temperature. Nevertheless, higher thermal quality on the GC CO2 side can exacerbate the heat capacity mismatch between the two fluids, thereby reducing the system coefficient of performance (COP). Furthermore, increasing compressor speed (ncom) mitigated the mismatch in heat capacity caused by elevated thermal quality. Taken together, ncom resulted in a 14.7°C increase in heating temperature within the experimental range, accompanied by an 18.4% decrease in COP, showing the best performance among the three parameters studied. This study offers a critical reference for both the theoretical analysis and the control strategy of transcritical CO2 heat pumps, directly supporting the pursuit of efficient heating.

 Transcritical CO2 heat pump Gas cooler Compressor speed Heating performance Dai B, Qi H, Dou W, et al. Life cycle energy, emissions and cost evaluation of CO2 air source heat pump system to replace traditional heating methods for residential heating in China: System configurations. Ener Conv Manag. 2020;218:112954. doi: 10.1016/j.enconman.2020.112954 10.1016/j.enconman.2020.112954 Ge TS, Weng ZC, Huang R, Hu B, Eikevik TM, Dai YJ. High temperature transcritical CO2 heat pump with optimized tube-in-tube heat exchanger. Energy. 2023;283:129223. doi: 10.1016/j.energy.2023.129223 10.1016/j.energy.2023.129223 Liao SM, Zhao TS, Jakobsen A. A correlation of optimal heat rejection pressures in transcritical carbon dioxide cycles. Appl Ther Eng. 2000;20(9):831-841. doi: 10.1016/S1359-4311(99)00070-8 10.1016/S1359-4311(99)00070-8 Lorentzen G. Revival of carbon dioxide as a refrigerant. Int J Refriger. 1994;17(5):292-301. doi: 10.1016/0140-7007(94)90059-0 10.1016/0140-7007(94)90059-0 Song Y, Cui C, Yin X, Cao F. Advanced development and application of transcritical CO2 refrigeration and heat pump technology-a review. Ener Rep. 2022;8:7840-7869. doi: 10.1016/j.egyr.2022.05.233 10.1016/j.egyr.2022.05.233 Liu R, Gao F, Liang K, et al. Thermodynamic evaluation of transcritical CO2 heat pump considering temperature matching under the constraint of heat transfer pinch point. J Therm Sci. 2021;30(3):869-879. doi: 10.1007/s11630-021-1373-z 10.1007/s11630-021-1373-z Wang X, Yang L, Xu B, Chen Z. Analysis of coupled heat transfer and flow behaviors of supercritical CO2 in horizontal circular tube. Int J Heat Fluid Flow. 2026;117:110064. doi: 10.1016/j.ijheatfluidflow.2025.110064 10.1016/j.ijheatfluidflow.2025.110064 Zhao W, Zhang Y, Sun C, Li L, Li B, Xu J. Thermodynamic analysis of a transcritical CO2 heat pump for heating applications. Energy. 2025;318:134896. doi: 10.1016/j.energy.2025.134896 10.1016/j.energy.2025.134896 Zou H, Li X, Tang M, et al. Temperature stage matching and experimental investigation of high-temperature cascade heat pump with vapor injection. Energy. 2020;212:118734. doi: 10.1016/j.energy.2020.118734 10.1016/j.energy.2020.118734 Chen YG. Pinch point analysis and design considerations of CO2 gas cooler for heat pump water heaters. Int J Refriger. 2016;69:136-146. doi: 10.1016/j.ijrefrig.2016.05.003 10.1016/j.ijrefrig.2016.05.003 Chen YG. Optimal heat rejection pressure of CO2 heat pump water heaters based on pinch point analysis. Int J Refriger. 2019;106:592-603. doi: 10.1016/j.ijrefrig.2019.04.003 10.1016/j.ijrefrig.2019.04.003 Ye Z, Wang Y, Song Y, Yin X, Cao F. Optimal discharge pressure in transcritical CO2 heat pump water heater with internal heat exchanger based on pinch point analysis. Int J Refriger. 2020;118:12-20. doi: 10.1016/j.ijrefrig.2020.06.003 10.1016/j.ijrefrig.2020.06.003 Cui Q, Wei D, Gao E, Zhang X. Compressor control strategy for CO2 heat pump toward efficient and stable domestic hot water production: An experimental investigation. Therm Sci Eng Prog. 2023;38:101681. doi: 10.1016/j.tsep.2023.101681 10.1016/j.tsep.2023.101681 Cui Q, Wang C, Gao E, Zhang X. Pinch point characteristics and performance evaluation of CO2 heat pump water heater under variable working conditions. Appl Therm Eng. 2022;207:118208. doi: 10.1016/j.applthermaleng.2022.118208 10.1016/j.applthermaleng.2022.118208 Kline S, McClintock F. Describing uncertainties in single- sample experiments. J Mech Eng. 1998;1:3-17. doi: 10.1016/0894-1777(88)90043-X 10.1016/0894-1777(88)90043-X Yang L, Qin X, Zhao L, Ye S, Wei X, Zhang D. Analysis and comparison of influence factors of hot water temperature in transcritical CO2 heat pump water heater: An experimental study. Energy Conv Manag. 2019;198:111836. doi: 10.1016/j.enconman.2019.111836 10.1016/j.enconman.2019.111836 Yang LX, Wei XL, Zhao LH, Qin X, Zhang DW. Experimental study on the effect of compressor frequency on the performance of transcritical CO2 heat pump system with regenerator. Appl Therm Eng. 2019;150:1216-1223. doi: 10.1016/j.applthermaleng.2019.01.091 10.1016/j.applthermaleng.2019.01.091 Badache M, Ouzzane M, Eslami-Nejad P, Aidoun Z. Experimental study of a carbon dioxide direct-expansion ground source heat pump (CO2-DX-GSHP). Appl Therm Eng. 2018;130:1480-1488. doi: 10.1016/j.applthermaleng.2017.10.159 10.1016/j.applthermaleng.2017.10.159 Rabelo SN, Paulino TF, Duarte WM, Maia AAT, Machado L. Experimental analysis of the influence of the expansion valve opening on the performance of the small size CO2 solar assisted heat pump. Solar Ener. 2019;190:255-263. doi: 10.1016/j.solener.2019.08.013 10.1016/j.solener.2019.08.013 Liu X, Wang D, Peng X, Wang G, Yang Y. Experimental study on performance and compressor characteristics of transcritical CO2 heat pump system. Appl Therm Eng. 2024;250:123524. doi: 10.1016/j.applthermaleng.2024.123524 10.1016/j.applthermaleng.2024.123524 Baek C, Heo J, Jung J, Cho H, Kim Y. Optimal control of the gas-cooler pressure of a CO2 heat pump using EEV opening and outdoor fan speed in the cooling mode. Int J Refriger. 2013;36(4):1276-1284. doi: 10.1016/j.ijrefrig.2013.02.009 10.1016/j.ijrefrig.2013.02.009