Advances in Subcooling Techniques for Transcritical CO₂ Refrigeration Systems: A Comprehensive Review

Document Type : Review Article

Authors

1 Research and Development Cell, Cool-Max Innovation, Bhiwadi, Rajasthan, India

2 Department of Mechanical Engineering, RKGIT Ghaziabad, Uttar Pradesh, India

3 Department of Aeronautical Engineering, NCE Chandi, Bihar, India

4 Department of Civil Engineering, Aliah University Kolkata, India

Abstract

Transcritical CO₂ refrigeration systems are increasingly adopted as low-GWP alternatives; however, their performance deteriorates due to high gas cooler pressures and large throttling losses. Subcooling the refrigerant before expansion is a key thermodynamic solution to mitigate these losses, yet a comprehensive and updated synthesis that directly compares the performance and applicability of diverse subcooling techniques within a unified framework is lacking. This review addresses that gap by providing a structured, critical analysis of both internal and external subcooling methods, including internal heat exchangers (IHX), ejector- and expander-assisted systems, and mechanical and thermoelectric subcoolers. The consolidated findings indicate that for high-ambient conditions, mechanical subcooling consistently emerges as the most effective approach, offering substantial COP improvements (20-40%) and in the case of DMS, significant discharge pressure reduction. This review further examines integration challenges, advanced control strategies, and emerging hybrid and renewable-assisted systems, offering valuable insights for researchers and designers aiming to enhance the sustainability and performance of next-generation CO₂ refrigeration technologies.

Keywords

Main Subjects

References

[1]   Tamura, T., Yakumaru, Y., Nishiwaki, F., 2005. Experimental study on automotive cooling and heating air conditioning system using CO₂ as a refrigerant. International Journal of Refrigeration, 28, pp. 1302–1307. doi: 10.1016/j.ijrefrig.2005.09.010.
[2]   Sarkar, J., 2014. A review on thermodynamic optimization of irreversible refrigerator and verification with transcritical CO₂ system. International Journal of Thermodynamics, 17, pp. 71–79. doi: 10.5541/ijot.519.
[3]   Wang, D., Chen, Z., Gu, Z., Liu, Y., Kou, Z., Tao, L., 2020. Performance analysis and comprehensive comparison between CO₂ and CO₂/ethane azeotropy mixture as a refrigerant used in single-stage and two-stage vapor compression transcritical cycles. International Journal of Refrigeration, 115, pp. 39–47. doi: 10.1016/j.ijrefrig.2020.03.003.
[4]   Song, X., Lu, D., Lei, Q., Cai, Y., Wang, D., Shi, J., Chen, J., 2021. Experimental study on heating performance of a CO₂ heat pump system for an electric bus. Applied Thermal Engineering, 190, p. 116789. doi: 10.1016/j.applthermaleng.2021.116789.
[5]   Qayyum, U., Tassou, S.A., Torrens, D., Tavares, J., 2025. Thermoeconomic performance of a CO₂ heat pump for space and water heating of a 4-bedroom house in the South of England. Building Services Engineering Research and Technology, 46, pp. 545–559. doi: 10.1177/01436244251340364.
[6]   Bellos, E., Tzivanidis, C., 2019. A comparative study of CO₂ refrigeration systems. Energy Conversion and Management: X, 1, p. 100002. doi: 10.1016/j.ecmx.2018.100002.
[7]   Llopis, R., Nebot-Andrés, L., Sánchez, D., Catalán-Gil, J., Cabello, R., 2018. Subcooling methods for CO₂ refrigeration cycles: A review. International Journal of Refrigeration, 93, pp. 85–107. doi: 10.1016/j.ijrefrig.2018.06.010.
[8]   Chen, Y., Gu, J., 2005. The optimum high pressure for CO₂ transcritical refrigeration systems with internal heat exchangers. International Journal of Refrigeration, 28, pp. 1238–1249. doi: 10.1016/j.ijrefrig.2005.08.009.
[9]   Sánchez, D., Patiño, J., Llopis, R., Cabello, R., Torrella, E., Fuentes, F.V., 2014. New positions for an internal heat exchanger in a CO₂ supercritical refrigeration plant: Experimental analysis and energetic evaluation. Applied Thermal Engineering, 63, pp. 129–139. doi: 10.1016/j.applthermaleng.2013.10.061.
[10] Torrella, E., Sánchez, D., Llopis, R., Cabello, R., 2011. Energetic evaluation of an internal heat exchanger in a CO₂ transcritical refrigeration plant using experimental data. International Journal of Refrigeration, 34, pp. 40–49. doi: 10.1016/j.ijrefrig.2010.07.006.
[11] Liu, S., Wang, J., Dai, B., Yang, X., Nian, V., Li, H., Yuan, J., 2021. Alternative positions of internal heat exchanger for CO₂ booster refrigeration system: Thermodynamic analysis and annual thermal performance evaluation. International Journal of Refrigeration, 131, pp. 1016–1028. doi: 10.1016/j.ijrefrig.2021.05.003.
[12] Cao, F., Wang, Y., Ye, Z., 2019. Theoretical analysis of internal heat exchanger in transcritical CO₂ heat pump systems and its experimental verification. International Journal of Refrigeration, 106, pp. 506–516. doi: 10.1016/j.ijrefrig.2019.05.022.
[13] Sánchez, D., Larrondo, R., Vidan-Falomir, F., Cabello, R., 2024. Experimental evaluation of the CO₂-based mixtures CO₂/R32, CO₂/R1234yf and CO₂/R1270 in a transcritical refrigerating plant considering the effect of the internal heat exchanger (IHX). Applied Thermal Engineering, 236, p. 121473. doi: 10.1016/j.applthermaleng.2023.121473.
[14] Ituna-Yudonago, J.F., Belman-Flores, J.M., Elizalde-Blancas, J., Serrano-Arellano, F., Pérez-García, V., Gutiérrez-León, D.G., 2017. Transient behavior of CO₂ in the internal heat exchanger during the start-up of the transcritical refrigeration system. Science and Technology for the Built Environment, 23, pp. 594–607. doi: 10.1080/23744731.2016.1234876
[15] Fang, J., Yin, X., Wang, A., Sun, X., Liu, Y., Cao, F., 2021. Cooling performance enhancement for the automobile transcritical CO₂ air conditioning system with various internal heat exchanger effectiveness. Applied Thermal Engineering, 196, p. 117274. doi: 10.1016/j.applthermaleng.2021.117274.
[16] Zendehboudi, A., 2025. Thermo-economic evaluation of low-GWP CO₂-based zeotropic mixtures in space heating heat pumps with and without internal heat exchanger. International Journal of Refrigeration, 173, pp. 1–17. doi: 10.1016/j.ijrefrig.2025.01.036.
[17] Elbel, S., Hrnjak, P., 2004. Effect of internal heat exchanger on performance of transcritical CO₂ systems with ejector. In: International Refrigeration and Air Conditioning Conference, School of Mechanical Engineering,  Purdue University, West Lafayette, Indiana, in the United States, pp. 1–8.
[18] Xu, X.X., Chen, G.M., Tang, L.M., Zhu, Z.J., Liu, S., 2011. Experimental evaluation of the effect of an internal heat exchanger on a transcritical CO₂ ejector system. Journal of Zhejiang University Science A, 12, pp. 146–153. doi: 10.1631/jzus.A1000212.
[19] Zhang, Z.Y., Ma, Y.T., Wang, H.L., Li, M.X., 2013. Theoretical evaluation on effect of internal heat exchanger in ejector expansion transcritical CO₂ refrigeration cycle. Applied Thermal Engineering, 50, pp. 932–938. doi: 10.1016/j.applthermaleng.2012.08.022.
[20] Nakagawa, M., Marasigan, A.R., Matsukawa, T., 2011. Experimental analysis on the effect of internal heat exchanger in transcritical CO₂ refrigeration cycle with two-phase ejector. International Journal of Refrigeration, 34, pp. 1577–1586. doi: 10.1016/j.ijrefrig.2010.03.007.
[21] Zeng, M.Q., Zhang, X.L., Mo, F.Y., Zhang, X.R., 2022. Thermodynamic analysis of the effect of internal heat exchanger on the dual-ejector transcritical CO₂ cycle for low-temperature refrigeration. International Journal of Energy Research, 46, pp. 12702–12721. doi: 10.1002/er.8038.
[22] Robinson, D.M., Groll, E.A., 1998. Efficiencies of transcritical CO₂ cycles with and without an expansion turbine. International Journal of Refrigeration, 21, pp. 577–589. doi: 10.1016/S0140-7007(98)00024-3.
[23] Zhang, Z., Tian, L., Chen, Y., Tong, L., 2014. Effect of an internal heat exchanger on performance of the transcritical carbon dioxide refrigeration cycle with an expander. Entropy, 16, pp. 5919–5934. doi: 10.3390/e16115919.
[24] Joneydi Shariatzadeh, O., Abolhassani, S.S., Rahmani, M., Ziaee Nejad, M., 2016. Comparison of transcritical CO₂ refrigeration cycle with expander and throttling valve including/excluding internal heat exchanger: Exergy and energy points of view. Applied Thermal Engineering, 93, pp. 779–787. doi: 10.1016/j.applthermaleng.2015.09.017.
[25] Li, J., Wang, Q., Su, D., Ren, X., Meng, L., 2020. Thermodynamic evaluation of the effect of internal heat exchanger and expander on the CO₂ trans-critical cycle cascade refrigeration system. In: Energy Proceedings, pp. 1–6. doi: 10.46855/energy-proceedings-7306.
[26] Shi, B., Chen, M., Chi, W., Yang, Q., Liu, G., Zhao, Y., Li, L., 2023. Effects of internal heat exchanger on two-stage compression trans-critical CO₂ refrigeration cycle combined with expander and intercooling. Energies, 16, article number: 115 pp. 1–15. doi:10.3390/en16010115
[27] Nebot-Andrés, L., Calleja-Anta, D., Sánchez, D., Cabello, R., Llopis, R., 2020. Thermodynamic analysis of a CO₂ refrigeration cycle with integrated mechanical subcooling. Energies, 13, pp. 1–17. doi: 10.3390/en13010012.
[28] Cecchinato, L., Chiarello, M., Corradi, M., Fornasieri, E., Minetto, S., Stringari, P., Zilio, C., 2009. Thermodynamic analysis of different two-stage transcritical carbon dioxide cycles. International Journal of Refrigeration, 32, pp. 1058–1067. doi: 10.1016/j.ijrefrig.2008.10.001.
[29] Nebot-Andrés, L., Sánchez, D., Cabello, R., Calleja-Anta, D., Fossi, C., Llopis, R., 2021. Current limits of CO₂ compressors working in integrated mechanical subcooling cycles. IOP Conference Series: Materials Science and Engineering, 1180, p. 012058. doi: 10.1088/1757-899X/1180/1/012058.
[30] Nebot-Andrés, L., Calleja-Anta, D., Fossi, C., Sánchez, D., Cabello, R., Llopis, R., 2022. Experimental assessment of different extraction points for the integrated mechanical subcooling system of a CO₂ transcritical plant. International Journal of Refrigeration, 136, pp. 8–16. doi: 10.1016/j.ijrefrig.2022.01.006.
[31] Cui, C., Ren, J., Rampazzo, M., Song, Y., Yin, X., Wang, W., Cao, F., 2023. Real-time optimization of integrated subcooling transcritical CO₂ heat pump water heater via phasor extreme seeking control (ESC). Applied Thermal Engineering, 232, p. 121045. doi: 10.1016/j.applthermaleng.2023.121045.
[32] Martínez-Ángeles, M., Nebot-Andrés, L., Calleja-Anta, D., Llopis, R., 2024. Experimental assessment of CO₂/R-152a mixtures in a refrigeration plant with integrated mechanical subcooling. International Journal of Refrigeration, 158, pp. 288–302. doi: 10.1016/j.ijrefrig.2023.12.005.
[33] Boewe, D.E., Bullard, C.W., Yin, J.M., Hrnjak, P.S., 2001. Contribution of internal heat exchanger to transcritical R-744 cycle performance. HVAC&R Research, 7, pp. 155–168. doi: 10.1080/10789669.2001.10391268.
[34] Kim, S.G., Kim, Y.J., Lee, G., Kim, M.S., 2005. The performance of a transcritical CO₂ cycle with an internal heat exchanger for hot water heating. International Journal of Refrigeration, 28, pp. 1064–1072. doi: 10.1016/j.ijrefrig.2005.03.004.
[35] Cho, H., Ryu, C., Kim, Y., 2007. Cooling performance of a variable speed CO₂ cycle with an electronic expansion valve and internal heat exchanger. International Journal of Refrigeration, 30, pp. 664–671. doi: 10.1016/j.ijrefrig.2006.10.004.
[36] Aprea, C., Maiorino, A., 2008. An experimental evaluation of the transcritical CO₂ refrigerator performances using an internal heat exchanger. International Journal of Refrigeration, 31, pp. 1006–1011. doi: 10.1016/j.ijrefrig.2007.12.016.
[37] Rigola, J., Ablanque, N., Pérez-Segarra, C.D., Oliva, A., 2010. Numerical simulation and experimental validation of internal heat exchanger influence on CO₂ trans-critical cycle performance. International Journal of Refrigeration, 33, pp. 664–674. doi: 10.1016/j.ijrefrig.2009.12.030.
[38] Tao, Y.B., He, Y.L., Tao, W.Q., Wu, Z.G., 2010. Experimental study on the performance of CO₂ residential air-conditioning system with an internal heat exchanger. Energy Conversion and Management, 51, pp. 64–70. doi: 10.1016/j.enconman.2009.08.024.
[39] Wang, Z., Gong, Y., Wu, X.H., Zhang, W.H., Lu, Y.L., 2013. Thermodynamic analysis and experimental research of transcritical CO₂ cycle with internal heat exchanger and dual expansion. International Journal of Air-Conditioning and Refrigeration, 21, p. 1350005. doi: 10.1142/S2010132513500053.
[40] Karampour, M., Sawalha, S., 2014. Investigation of using internal heat exchangers in CO₂ trans-critical booster system. In: 11th IIR Gustav Lorentzen Conference on Natural Refrigerants (GL2014), Hangzhou, China, August 31-September 2, 2014, pp. 453–460.
[41] Kim, M.S., Shin, C.S., Kim, M.S., 2014. A study on the real time optimal control method for heat rejection pressure of a CO₂ refrigeration system with an internal heat exchanger. International Journal of Refrigeration, 48, pp. 87–99. doi: 10.1016/j.ijrefrig.2014.09.014.
[42] Wang, Z., Han, F., Sundén, B., 2018. Parametric evaluation and performance comparison of a modified CO₂ transcritical refrigeration cycle in air-conditioning applications. Chemical Engineering Research and Design, 131, pp. 617–625. doi: 10.1016/j.cherd.2017.08.003.
[43] Ye, Z., Wang, Y., Song, Y., Yin, X., Cao, F., 2020. Optimal discharge pressure in transcritical CO₂ heat pump water heater with internal heat exchanger based on pinch point analysis. International Journal of Refrigeration, 118, pp. 12–20. doi: 10.1016/j.ijrefrig.2020.06.003.
[44] Hou, Y., Wu, W., Li, Z., Yu, X., Zeng, T., 2023. Effect of drying air supply temperature and internal heat exchanger on performance of a novel closed-loop transcritical CO₂ air source heat pump drying system. Renewable Energy, 219, p. 119516. doi: 10.1016/j.renene.2023.119516.
[45] He, Y.J., Tai, Y.D., Fakrouche, N., Zhang, C.L., 2023. Applicability evaluation of internal heat exchanger in CO₂ transcritical cycle considering compressor operation boundaries. Applied Energy, 349, p. 121579. doi: 10.1016/j.apenergy.2023.121579.
[46] Gullo, P., Elmegaard, B., Cortella, G., 2016. Energy and environmental performance assessment of R744 booster supermarket refrigeration systems operating in warm climates. International Journal of Refrigeration, 64, pp. 61–79. doi: 10.1016/j.ijrefrig.2015.12.016.
[47] Bellos, E., Tzivanidis, C., 2019. CO₂ transcritical refrigeration cycle with dedicated subcooling: Mechanical compression vs. absorption chiller. Applied Sciences, 9, pp. 1–27. doi: 10.3390/app9081605.
[48] Llopis, R., Cabello, R., Sánchez, D., Torrella, E., 2015. Energy improvements of CO₂ transcritical refrigeration cycles using dedicated mechanical subcooling. International Journal of Refrigeration, 55, pp. 129–141. doi: 10.1016/j.ijrefrig.2015.03.016.
[49] Bush, J., Beshr, M., Aute, V., Radermacher, R., 2017. Experimental evaluation of transcritical CO₂ refrigeration with mechanical subcooling. Science and Technology for the Built Environment, 23, pp. 1013–1025. doi: 10.1080/23744731.2017.1289056.
[50] Dai, B., Liu, S., Li, H., Sun, Z., Song, M., Yang, Q., Ma, Y., 2018. Energetic performance of transcritical CO₂ refrigeration cycles with mechanical subcooling using zeotropic mixture as refrigerant. Energy, 150, pp. 205–221. doi: 10.1016/j.energy.2018.02.111.
[51] Catalán-Gil, J., Llopis, R., Sánchez, D., Nebot-Andrés, L., Cabello, R., 2019. Energy analysis of dedicated and integrated mechanical subcooled CO₂ boosters for supermarket applications. International Journal of Refrigeration, 101, pp. 11–23. doi: 10.1016/j.ijrefrig.2019.01.034.
[52] Wang, Z., Zhao, H., Wang, X., Han, J., Lai, Y., 2020. Thermodynamic performance evaluation of the CO₂ parallel compression supermarket refrigeration system with a subcooler. International Journal of Energy Research, 44, pp. 6709–6724. doi: 10.1002/er.5408.
[53] Dai, B., Cao, Y., Liu, S., Ji, Y., Sun, Z., Xu, T., Zhang, P., Nian, V., 2022. Annual energetic evaluation of multi-stage dedicated mechanical subcooling carbon dioxide supermarket refrigeration system in different climate regions of China using genetic algorithm. Journal of Cleaner Production, 333, p. 130119. doi: 10.1016/j.jclepro.2021.130119.
[54] Cortella, G., Coppola, M.A., D’Agaro, P., 2021. Sizing and control rules of dedicated mechanical subcooler in transcritical CO₂ booster systems for commercial refrigeration. Applied Thermal Engineering, 193, p. 116953. doi: 10.1016/j.applthermaleng.2021.116953.
[55] Dai, B., Wang, Q., Liu, S., Zhang, J., Wang, Y., Kong, Z., Chen, Y., Wang, D., 2024. Multi-objective optimization analysis of combined heating and cooling transcritical CO₂ system integrated with mechanical subcooling utilizing hydrocarbon mixture based on machine learning. Energy Conversion and Management, 301, p. 118057. doi: 10.1016/j.enconman.2023.118057.
[56] Sánchez, D., Catalán-Gil, J., Cabello, R., Calleja-Anta, D., Llopis, R., Nebot-Andrés, L., 2016. Improvements in a CO₂ transcritical plant working with two different subcooling systems. In: 12th IIR Gustav Lorentzen Conference on Natural Refrigerants, pp. 1014–1022. doi: 10.18462/IIR.GL.2016.1170.
[57] Zheng, Z., Yang, Q., Zhang, W., Zhao, Y., Liu, G., Li, L., 2025. Thermodynamic analysis and comparison of mechanical subcooling transcritical CO₂ refrigeration system with expander and throttling valve. Case Studies in Thermal Engineering, 65, p. 105625. doi: 10.1016/j.csite.2024.105625.
[58] Liu, X., Fu, R., Wang, Z., Lin, L., Sun, Z., Li, X., 2019. Thermodynamic analysis of transcritical CO₂ refrigeration cycle integrated with thermoelectric subcooler and ejector. Energy Conversion and Management, 188, pp. 354–365. doi: 10.1016/j.enconman.2019.02.088.
[59] Sarkar, J., 2013. Performance optimization of transcritical CO₂ refrigeration cycle with thermoelectric subcooler. International Journal of Energy Research, 37, pp. 121–128. doi: 10.1002/er.1879.
[60] Aranguren, P., Sánchez, D., Casi, A., Cabello, R., Astrain, D., 2021. Experimental assessment of a thermoelectric subcooler included in a transcritical CO₂ refrigeration plant. Applied Thermal Engineering, 190, p. 116826. doi: 10.1016/j.applthermaleng.2021.116826.
[61] Schoenfield, J., Hwang, Y., Radermacher, R., 2012. CO₂ transcritical vapor compression cycle with thermoelectric subcooler. HVAC&R Research, 18, pp. 297–311. doi: 10.1080/10789669.2012.625348.
[62] Astrain, D., Merino, A., Catalán, L., Aranguren, P., Araiz, M., Sánchez, D., Cabello, R., Llopis, R., 2019. Improvements in the cooling capacity and the COP of a transcritical CO₂ refrigeration plant operating with a thermoelectric subcooling system. Applied Thermal Engineering, 155, pp. 110–122. doi: 10.1016/j.applthermaleng.2019.03.123.
[63] Sánchez, D., Aranguren, P., Casi, A., Llopis, R., Cabello, R., Astrain, D., 2020. Experimental enhancement of a CO₂ transcritical refrigerating plant including thermoelectric subcooling. International Journal of Refrigeration, 120, pp. 178–187. doi: 10.1016/j.ijrefrig.2020.08.031.
[64] Dai, B., Liu, S., Zhu, K., Sun, Z., Ma, Y., 2017. Thermodynamic performance evaluation of transcritical carbon dioxide refrigeration cycle integrated with thermoelectric subcooler and expander. Energy, 122, pp. 787–800. doi: 10.1016/j.energy.2017.01.029.
[65] Kwan, T.H., Shen, Y., Wu, Z., Yao, Q., 2020. Performance analysis of the thermoelectric device as the internal heat exchanger of the trans-critical carbon dioxide cycle. Energy Conversion and Management, 208, p. 112585. doi: 10.1016/j.enconman.2020.112585.
[66] Casi, Á., Aranguren, P., Araiz, M., Sanchez, D., Cabello, R., Astrain, D., 2022. Experimental evaluation of a transcritical CO₂ refrigeration facility working with an internal heat exchanger and a thermoelectric subcooler: Performance assessment and comparative analysis. International Journal of Refrigeration, 141, pp. 66–75. doi: 10.1016/j.ijrefrig.2022.05.024.
[67] Nebot-Andrés, L., Llopis, R., Sánchez, D., Catalán-Gil, J., Cabello, R., 2017. CO₂ with mechanical subcooling vs. CO₂ cascade cycles for medium temperature commercial refrigeration applications: Thermodynamic analysis. Applied Sciences, 7, pp. 1–22. doi: 10.3390/app7090955.
[68] Fu, R., Wang, J., Zheng, M., Yu, K., Liu, X., Li, X., 2019. Thermodynamic analysis of transcritical CO₂ ejector expansion refrigeration cycle with dedicated mechanical subcooling. Entropy, 21, pp. 1–24. doi: 10.3390/e21090874.
[69] Sun, Z., Wang, Q., Su, D., Liu, S., Dai, B., 2019. Comparative analysis of thermodynamic performance of CO₂ cascade refrigeration system assisted with expander and mechanical subcooling. International Journal of Energy Research, 43, pp. 7891–7907. doi: 10.1002/er.4719.
[70] Sánchez, D., Catalán-Gil, J., Cabello, R., Calleja-Anta, D., Llopis, R., Nebot-Andrés, L., 2020. Experimental analysis and optimization of an R744 transcritical cycle working with a mechanical subcooling system. Energies, 13, pp. 1–27. doi: 10.3390/en13123204.
[71] Hazarika, M.M., Ramgopal, M., Bhattacharyya, S., 2021. Studies on performance improvement of an R744 transcritical refrigeration system using dedicated mechanical subcooling. Lecture Notes in Mechanical Engineering, pp. 33–44. doi: 10.1007/978-981-15-6360-7_4.
[72] Na, S.I., Kim, M., Kim, M.S., 2022. Performance simulation of CO₂ transcritical cooling system with mechanical subcooling cycle for automobile air conditioning. Journal of Mechanical Science and Technology, 36, pp. 4797–4807. doi: 10.1007/S12206-022-0838-7.
[73] Illán-Gómez, F., García-Cascales, J.R., Velasco, F.J.S., Otón-Martínez, R.A., 2023. Numerical performance of a water source transcritical CO₂ heat pump with mechanical subcooling. Applied Thermal Engineering, 219, p. 119639. doi: 10.1016/j.applthermaleng.2022.119639.
[74] Caliskan, O., Ersoy, H.K., 2024. Energy and environmental analysis of transcritical CO₂ supermarket refrigeration cycles with dedicated mechanical subcooler. Kahramanmaras Sutcu Imam University Journal of Engineering Sciences, 27, pp. 1582–1601. doi: 10.17780/ksujes.1477931
[75] Yazawa, K., Dharkar, S., Kurtulus, O., Groll, E.A., 2015. Optimum design for thermoelectric in a sub-cooled trans-critical CO₂ heat pump for data center cooling. Annual IEEE Semiconductor Thermal Measurement and Management Symposium, 2015-April, pp. 19–24. doi: 10.1109/SEMI-THERM.2015.7100133.
[76] Jamali, S., Yari, M., Mohammadkhani, F., 2017. Performance improvement of a transcritical CO₂ refrigeration cycle using two-stage thermoelectric modules in sub-cooler and gas cooler. International Journal of Refrigeration, 74, pp. 105–115. doi: 10.1016/j.ijrefrig.2016.10.007.
[77] Hu, Y., Shen, B., Krishnamoorthy, S., Shirey, D., 2025. Heating performance of an air source heat pump with a portable thermoelectric subcooler. Case Studies in Thermal Engineering, 73, p. 106542. doi: 10.1016/j.csite.2025.106542.
[78] Choudhary, K.D., Dasgupta, M.S., Sunder, S., 2017. Investigation of effect of suction nozzle pressure drop and degree of sub-cooling on CO₂ transcritical ejector system. In: Proceedings of the 24th National and 2nd International ISHMT-ASTFE Heat and Mass Transfer Conference, pp. 1201–1210. doi: 10.1615/IHMTC-2017.1670.
[79] Liu, X., Yu, K., Wan, X., Li, X., 2021. Performance evaluation of CO₂ supermarket refrigeration system with multi-ejector and dedicated mechanical subcooling. Energy Reports, 7, pp. 5214–5227. doi: 10.1016/j.egyr.2021.08.110.
[80] Somasundaram, V., Singha, P., Dasgupta, M.S., Hafner, A., Bhattacharyya, S., 2024. Performance analysis of an ejector based CO₂ transcritical refrigeration system with integrated mechanical subcooling for dairy application. AIP Conference Proceedings, 2960, p. 040006. doi: 10.1063/5.0182904.
[81] Liu, X., Yu, K., Wan, X., Zheng, M., Li, X., 2021. Conventional and advanced exergy analyses of transcritical CO₂ ejector refrigeration system equipped with thermoelectric subcooler. Energy Reports, 7, pp. 1765–1779. doi: 10.1016/j.egyr.2021.03.023.
[82] Casi, Á., Aranguren, P., Araiz, M., Alegría, P., Astrain, D., 2022. Impact of a thermoelectric subcooler heat exchanger on a carbon dioxide transcritical refrigeration facility. Applied Thermal Engineering, 215, p. 118866. doi: 10.1016/j.applthermaleng.2022.118866.
[83] Casi, Á., Aranguren, P., Araiz, M., Sanchez, D., Cabello, R., Astrain, D., 2022. Performance assessment of an experimental CO₂ transcritical refrigeration plant working with a thermoelectric subcooler in combination with an internal heat exchanger. Energy Conversion and Management, 268, p. 115963. doi: 10.1016/j.enconman.2022.115963.
[84] Chi, W., Yang, Q., Chen, X., Liu, G., Zhao, Y., Li, L., 2022. Performance evaluation of NH₃/CO₂ cascade refrigeration system with ejector subcooling for low-temperature cycle. International Journal of Refrigeration, 136, pp. 162–171. doi: 10.1016/j.ijrefrig.2022.01.005.
[85] Nebot-Andrés, L., Del Duca, M.G., Aprea, C., Žerovnik, A., Tušek, J., Llopis, R., Maiorino, A., 2022. Improving efficiency of transcritical CO₂ cycles through a magnetic refrigeration subcooling system. Energy Conversion and Management, 265, p. 115766. doi: 10.1016/j.enconman.2022.115766.
[86] Li, H., Huang, Q., Yu, J., 2024. Thermodynamic analysis of a modified two-stage transcritical CO₂ refrigeration cycle with an ejector and a subcooler. International Journal of Refrigeration, 168, pp. 492–505. doi: 10.1016/j.ijrefrig.2024.09.019.
[87] Zou, L., Liu, Y., Yu, J., 2025. Thermodynamic and environment analysis of a modified transcritical CO₂ refrigeration cycle integrated with ejector and subcooler. International Journal of Refrigeration, 173, pp. 123–138. doi: 10.1016/j.ijrefrig.2025.02.012.
[88] Lv, D., Yang, Q., Jiang, J., Lv, J., Liu, G., Zhao, Y., Li, L., 2025. Comparative energy and exergy analysis of an ejector-enhanced vapor injection transcritical CO₂ refrigeration system integrating with subcooler and flash tank. Thermal Science and Engineering Progress, 64, p. 103795. doi: 10.1016/j.tsep.2025.103795.
[89] Lv, J., Lv, D., Yang, Q., Zhao, Y., Liu, G., Li, L., 2025. Comparative performance analysis of a self-heat recuperation driven ejector subcooling transcritical CO₂ refrigeration systems using different throttling devices. International Journal of Refrigeration, 176, pp. 106–119. doi: 10.1016/j.ijrefrig.2025.04.031.
[90] Syngounas, E., Tsimpoukis, D., Koukou, M.K., Vrachopoulos, M.G., 2022. Energetic, exergetic and financial evaluation of a solar-powered absorption chiller integration into a CO₂ commercial refrigeration system. Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy, 236, pp. 1192–1209. doi: 10.1177/09576509221077540.
[91] Ran, D., Yang, Q., Yin, T., Li, L., Zhao, Y., Liu, G., 2025. Performance evaluation and discharge pressure discussion of supermarket CO₂ refrigeration system integrated with ejector subcooling using solar energy. International Journal of Refrigeration, 172, pp. 27–40. doi: 10.1016/j.ijrefrig.2025.01.022.
[92] Yin, T., Yang, Q., Jiang, J., Zhao, Y., Liu, G., Li, L., 2025. Optimization of optimal discharge pressure for a novel solar-assisted ejector subcooling CO₂ transcritical refrigeration system based on NSGA-II. Applied Thermal Engineering, 265, p. 125574. doi: 10.1016/j.applthermaleng.2025.125574.
[93] Tarawneh, M., Melhem, E.B., 2025. Thermoelectric and solar photovoltaic synergy for optimized transcritical CO₂ refrigeration in hot climates. Heat Transfer, 54, pp. 2865–2880. doi: 10.1002/htj.23329.
[94] Singha, P., Das, C., Dasgupta, M.S., Bhattacharyya, S., Hafner, A., 2025. New approach to improve COP and heat recovery in transcritical CO₂ refrigeration system for milk processing application. Scientific Reports, 15, pp. 1–14. doi: 10.1038/s41598-025-90067-3.
[95] Syngounas, E., Tsimpoukis, D., Bellos, E., Koukou, M.K., Tzivanidis, C., Vrachopoulos, M.G., 2025. Energy and exergy performance investigation of a transcritical CO₂ vapor ejector-based refrigeration system for marine provision plants. Applied Thermal Engineering, 269, p. 126036. doi: 10.1016/j.applthermaleng.2025.126036.
Journal of Heat and Mass Transfer Research
Volume 13, Issue 4 - Serial Number 28
In Progress
December 2026
Pages 387-412

Files

History

  • Receive Date: 23 October 2025
  • Revise Date: 05 February 2026
  • Accept Date: 16 April 2026

Share

How to cite

Statistics