During the 11^th 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) CO₂transcritical systems perform better or have an equivalent performance to the R404A MultiplexDX. In contrast, R404A multiplex DX and CO₂/ R404A cascade systems perform better in the southern regions of Southeast Asia (India, Bangladeshand 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) a ttemperature less than 10°C

·COP of 12% higher than DX at a temperature of 5°C

For an ambient temperature range of between 22and 33°C,the R404A multiplex DX system and the CO₂transcritical boostersystem 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 ejector simplemented in a R744 booster system operating in a supermarket,” by J.Schonenberger, A. Hafner, K. Banasiak, and S. Girotto analysed the performance ofa CO₂ commercia lrefrigeration 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.

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