Transcritical CO2 refrigeration systems enhanced by ejector technology: state-of-the-art review

• Comprehensive state-of-the-art review of ejector-integrated transcritical CO 2 systems. • Ejector integration improves COP by 10–25%, reaching up to 40–60% in hybrid cycles. • CFD–ML surrogate models reduce entrainment-ratio and pressure-lift errors to < 3 %. • Multi-ejector and VGE systems sustain COP under large ambient-temperature variations. • Key research gaps identified: standardization, long-term reliability, and smart control. The demand for sustainable and environmentally benign refrigeration technologies has accelerated the adoption of carbon dioxide (CO 2 ) as a natural refrigerant. Despite its thermodynamic benefits and negligible global warming potential, the use of CO 2 in transcritical refrigeration cycles is constrained by significant inefficiencies, particularly related to throttling losses and high discharge pressures. Ejector technology has emerged as a potential addition mechanism that could enhance the overall cycle performance by recuperating the expansion work and distributing the pressure to optimal points. This review paper gives an in-depth and critical description of ejector-integrated transcritical CO 2 refrigeration systems. It explores the basics of ejectors, including ejector-based system configurations, their performance enhancement, control strategies, and industrial applications. Quantitative analyses from recent studies indicate that ejector integration can improve the system Coefficient of Performance (COP) by 10 to 25 % compared with conventional throttling cycles, while hybrid designs employing internal heat exchangers or parallel compression achieve gains up to 40 %. In addition, recent developments such as Computational Fluid Dynamics (CFD) and machine learning, are also discussed. The integration of CFD and ML frameworks has reduced prediction errors in the entrainment ratio and pressure lift to below 3%. Critical gaps are found in standardization, long-term reliability, and smart system integration. The review outlines preliminary directions including the establishment of unified testing protocols, the development of long-duration reliability studies, and the design of adaptive, sensor-integrated ejector systems for intelligent control. This review is cross-disciplinary and systematic in its scope to the critical role ejector technology has played in enhancing the development of high-efficiency and low-emission refrigeration technology.