Transcritical CO2 Refrigeration Systems and Heat Pump Technologies

Transcritical CO₂ refrigeration systems and CO₂-based heat pumps have emerged as sustainable alternatives to conventional HFC-based solutions, leveraging the favourable thermophysical properties, negligible ozone depletion potential and low global warming potential of carbon dioxide (R744). In a transcritical cycle, CO₂ operates above its critical point in the gas cooler, enabling efficient heat rejection and compact heat exchanger design. Booster system architectures employ multi-stage compression to serve both low- and medium-temperature refrigeration demands simultaneously, while vapour injection and mechanical subcooling strategies enhance capacity and efficiency, particularly in warm ambient conditions. CO₂ heat pump technologies extend these principles to water heating and space conditioning, offering competitive performance through optimised compressor configurations, advanced cycle control and tailored gas cooler geometries. Recent developments have focused on integrating ejector devices, refining heat exchanger layouts and hybridising with auxiliary circuits to overcome efficiency challenges near the critical region. The high volumetric capacity of CO₂ contributes to reduced refrigerant charge and smaller component footprint, supporting the global transition towards carbon-neutral refrigeration and heat pump applications across residential, commercial and transport sectors.

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Transcritical CO2 Refrigeration Systems and Heat Pump Technologies publication trend

The graph below shows the total number of articles in transcritical co2 refrigeration systems and heat pump technologies across all publications each year (not limited to Nature Index journals).

Technical terms

Transcritical cycle: A refrigeration or heat pump cycle in which the refrigerant passes above its critical pressure and temperature during heat rejection.

R744 (CO₂): Carbon dioxide used as a refrigerant with zero ozone depletion potential and very low global warming potential.

Booster system: A multi-stage compression configuration that simultaneously satisfies low- and medium-temperature cooling loads.

Vapour injection: The introduction of refrigerant vapour between compression stages to increase capacity and improve efficiency.

Gas cooler: A heat exchanger in a transcritical cycle where refrigerant rejects heat above its critical state.

Ejector device: A passive flow component that recovers expansion work and enhances cycle performance by reducing throttling losses.

Coefficient of performance (COP): The ratio of cooling or heating output to electrical input, used to evaluate system efficiency.

References

1. Global selection appraisal study for heat pump system of electric vehicle based on energetic, economic, and environmental analysis. Carbon Neutrality (2024).
2. Energy analysis of alternative CO2 refrigeration system configurations for retail food applications in moderate and warm climates. Energy Conversion and Management (2017).
3. CO2 with Mechanical Subcooling vs. CO2 Cascade Cycles for Medium Temperature Commercial Refrigeration Applications Thermodynamic Analysis. Applied Sciences (2017).
4. Multi-Ejector Concept: A Comprehensive Review on its Latest Technological Developments. Energies (2019).
5. Recent Advances in Transcritical CO2 (R744) Heat Pump System: A Review. Energies (2019).