A number of technologies have been developed to help increase the efficiency of transcritical CO2 refrigeration systems, especially in high ambient conditions.

When the ambient temperature is higher than the critical point of CO2 (88°F/31°C), CO2 enters the supercritical phase and can’t be fully condensed. It can be cooled, and the cooled gas-liquid mixture is further treated to create liquid CO2 for the evaporator.

Adiabatic gas coolers, ejectors, parallel compression and subcooling are some of the key technologies that are used to reduce the workload in a transcritical CO2 system in high ambients, thereby improving its efficiency.

A new research paper proposes another method: the use of an absorption chiller for subcooling. The paper, “Enhancing the performance of a CO2 refrigeration system with the use of an absorption chiller,” was published in the December issue of the International Journal of Refrigeration and can be accessed here.

In this scenario, an absorption chiller, which does not use a compressor, is fed by waste heat generated by the transcritical compressors. The authors of the study – Evangelos Bellos and Christos Tzivanidis of the Thermal Department, School of Mechanical Engineering, National Technical University of Athens – found that in all cases, the examined system is more efficient than the reference, with a mean COP (coefficient of performance) enhancement of about 23% and a maximum improvement of 75%. The enhancements are greater in the cases with a higher compressor pressure ratio.

No external energy

According to the paper’s abstract, the objective of the study was to “examine a transcritical CO2 refrigeration system coupled to a single-effect absorption chiller.” The role of the absorption machine is to create subcooling after the gas-cooler in order to increase the COP. The analysis is conducted with a validated numerical model, which is developed in Engineering Equation Solver.

Because the absorption system is fed by waste heat from the CO2 compressors, “there is not any need for any external energy source,” the abstract noted.

The system was studied and optimized for different heat rejection temperatures from 35°C (95°F) up to 50°C (122°F), and for different refrigeration temperatures from -35°C (-31°F) up to 5°C (41°F). The study found that “there is COP enhancement in all the operating scenarios and especially in the cases with higher heat-rejection temperature and lower refrigeration temperature,” said the abstract. “The mean COP enhancement is about 23.4%, which is an important enhancement for designing more efficient systems.” 

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