During the 11th IIR Gustav Loretzen 2014 Conference on Natural Refrigerants, a unique event dedicated to knowledge sharing on CO2, ammonia, hydrocarbons, and water as refrigerants, the Oak Ridge National Laboratory, Frigo-Consulting, Enex, Sintef and the Norwegian University of Science and Technology (NTNU), presented results of studies that examined the efficiencies of different CO2 refrigeration technologies in warm climates. Whilst different design alternatives exist to enhance the COP of R744 systems for high ambient temperature regions, such as implementing external subcooling units, the papers presented at GL 2014 focused on parallel compression technology and ejectors.
Standard CO2 transcritical system remains most efficient in Northern climates
In the GL 2014 paper by Oak Ridge National Laboratory, “CO2 supermarket refrigeration systems for southeast Asia and the USA” by V. Sharma, B. Fricke, P. Bansal, a comparative analysis of CO2 refrigeration system installations in supermarket refrigeration applications in 88 Chinese cities from all climate zones was presented.
Confirming results seen in other studies, the performance comparison of R404A multiplex Direct Expansion (DX), CO2/ R404A cascade, and CO2 transcritical systems with bypass compressors, demonstrated that in the northern regions of Southeast Asia (China and Japan) CO2 transcritical systems perform better or have an equivalent performance to the R404A Multiplex DX. In contrast, R404A multiplex DX and CO2/ R404A cascade systems perform better in the southern regions of Southeast Asia (India, Bangladesh and Burma). Similarly, in the US CO2 transcritical systems were shown to be most efficient in the northern half of the country.
CO2 transcritical systems achieve:
- The highest COP (Coefficient of Performance) at temperature less than 10°C
- COP of 12% higher than DX at a temperature of 5°C
For an ambient temperature range of between 22 and 33°C, the R404A multiplex DX system and the CO2 transcritical booster system exhibit an equivalent system COP.
Ejectors could improve CO2 transcritical system efficiency by up to 20% in southern climates
To illustrate the fact that the efficiency challenge related to ordinary R744 booster systems operated at high ambient temperatures can be overcome, the paper titled “Experience with ejectors implemented in a R744 booster system operating in a supermarket,” by J. Schönenberger, A. Hafner, K. Banasiak, and S. Girotto analysed the performance of a CO2 commercial refrigeration system with parallel compressor technology and three ejectors, installed in a 5,000m2 supermarket in Fribourg, Switzerland in 2013.
Thanks to the ejectors, the annual energy efficiency improvement potential compared to traditional CO2 systems with parallel compression is between 10 to 20%, depending on the climatic region and the integration of heat recovery systems. In addition, further efficiency increase can be achieved by increasing the evaporation temperature of the low temperature refrigeration.
The efficiency gains are attributed to the fact that the three ejectors partly recover the expansion work, which is applied to return liquid and vapour from the low pressure receiver downstream of the MT evaporators back to the medium pressure receiver, located upstream of all evaporator refrigerant supply valves. What is more, frost formation on the evaporators and the number of defrost cycles is reduced.
Optimising ejector geometry to improve CO2 system COP
The paper titled “CFD Case Study of R744 Ejectors”, by K. Banasiak, A. Hafner T. Eikevik, O. Haddal, focused on understanding the fluid dynamic phenomena that affect ejector performance, results that could contribute to optimising ejector performance and in turn improving CO2 refrigeration system COP.
The study presented numerically analysed the geometry of the CO2 vapor ejector based on a three-dimensional CFD code developed in Fluent ANSYS environment. The energy performance, represented by the ejector efficiency, was simulated for three distinctive sets of boundary conditions, representing different operational conditions.