Transcritical CO₂ refrigeration cycle modifications: thermodynamic performance, practical challenges and future directions

Abstract

Carbon dioxide (CO₂, R744) has emerged as a leading natural refrigerant alternative owing to its negligible ozone depletion potential, unity global warming potential, chemical stability, and favourable heat transfer characteristics. However, the low critical temperature (31.1 °C) and high critical pressure (7.38 MPa) of CO₂ impose significant thermodynamic penalties under transcritical operation, including elevated throttling losses, high optimal discharge pressure, and substantial exergy destruction in the gas cooler and expansion device. These limitations become particularly pronounced under moderate-to-high ambient temperature conditions, where coefficient of performance (COP) reductions of up to 38% have been experimentally reported. This paper presents a comprehensive and critical review of efficiency-enhancing modifications for transcritical CO₂ vapor compression refrigeration systems, synthesizing experimental, field-based, and simulation studies from recent literature. The reviewed modifications are systematically classified into four categories: heat transfer enhancement, expansion work recovery, cycle-architecture and refrigerant-management modifications and working-fluid and hybrid-cycle modifications. For each category, the underlying thermodynamic mechanisms, reported performance improvements, practical implementation considerations, and inherent trade-offs are critically evaluated. The review demonstrates that COP improvements vary widely subject to the modification strategy and operating conditions adopted, with performance gains generally below 50%, although substantially higher gains of up to 207% have been recorded under ultra-low temperature operating conditions. Expansion work recovery devices, including ejectors, mechanical expanders and rotary pressure exchangers, consistently reduce throttling irreversibility, while parallel and two-stage compression configurations improve performance under high ambient temperatures and low evaporating temperature conditions. Subcooling techniques and internal heat exchangers offer moderate but reliable gains across diverse operating conditions. CO₂-based refrigerant blends and compression–absorption cycles present promising pathways for pressure reduction and COP improvement, though practical deployment remains constrained by system complexity and limited long-term experimental data. The findings highlight that no single modification universally optimizes performance across all operating conditions, and that hybrid configurations combining complementary strategies consistently deliver superior results. Future research priorities are identified, including transient and part-load performance characterization, comprehensive exergy-based system optimization, and long-term reliability validation of advanced components under real operating conditions.