Semnan University Press Journal of Heat and Mass Transfer Research 2345-508X 13 4 2026 12 01 Advances in Subcooling Techniques for Transcritical CO₂ Refrigeration Systems: A Comprehensive Review 387 412 10563 10.22075/jhmtr.2026.39481.1865 EN Saif Nawaz Ahmad Research and Development Cell, Cool-Max Innovation, Bhiwadi, Rajasthan, India 0000-0001-7936-6096 Nitin Kumar Department of Mechanical Engineering, RKGIT Ghaziabad, Uttar Pradesh, India Rajnish Kumar Department of Aeronautical Engineering, NCE Chandi, Bihar, India Adil Nawaz Ahmad Department of Civil Engineering, Aliah University Kolkata, India Journal Article 2025 10 23 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. https://jhmtr.semnan.ac.ir/article_10563_1f2b8183cd1e469ad3a7ad1eadce49d1.pdf