How to improve CO2 efficiency in commercial refrigeration

Improving CO₂ system efficiency attracted a lot of attention at the 11th IIR Gustav Lorentzen Conference (GL 2014) on Natural Refrigerants. R744.com looks at the four main options that can be implemented to enhance efficiency - heat recovery, integrated expansion-compression-unit (ECU), adsorption chiller for sub cooling, and parallel compression – analysed in two of the GL 2014 research papers.

Due to its increased popularity, transcritical CO₂ technology has obtained an important market share and has become one of the most successful new refrigerants to be used in commercial refrigeration. This trend has generated scientific and technological research focusing on evaluating existing CO₂ technologies and exploring innovative solutions to make transcritical CO₂ technology even more efficient for such applications.

The Swedish Royal Institute of Technology (KTH) and Swiss company Frigo-Consulting Ltd. presented two scientific papers on this topic at the 2014 Gustav Lorentzen Conference.

Potential to improve heat recovery at high heating COP

The paper “Heat recovery investigation of a supermarket refrigeration system using carbon dioxide as refrigerant”, written by A. Abdi, S. Sawalha, M. Karampour, analysed the heat reclaim of a transcritical CO₂-booster refrigeration unit operating in a Swedish supermarket. The analysis focused on a comparison between the store’s existing heat recovery control strategy and an optimum control strategy, which was based on theoretical analysis.

The field measurement analysis revealed that:

Only between 30-60 % of the available heat recovered is utilised
The rest of the heat is released to the outdoors

The theoretical analysis revealed that:

The amount of heat that can be recovered from the refrigeration system is about 1.3 times (130 %) the cooling demand in the system

Comparing the field measurements with the theoretical analysis the paper concludes that:

Heat can be recovered at heating COP of 3-4.5
It is possible to improve heat recovery from the refrigeration system at a relatively high heating COP compared to what is achievable with a heat pump

How do methods of enhancing transcritical CO₂ efficiency perform?

E. Wiedenmann, J. Schoenenberger, M. Baertsch prepared a paper titled “Efficiency analysis and comparison of innovative R744-refrigerating systems in commercial applications”,,discussing an ongoing project (expected to be concluded by the end of 2014) which aims to enable the selection of the most appropriate methods for increasing transcritical CO₂ efficiency, as well as to facilitate their dissemination.

The project entails an analysis and comparison of new alternative methods of increasing the efficiency of transcritical CO₂ refrigerating systems based on a pros and cons list, which is complemented by information on savings and market potential. At a second stage, the project compares two selected methods with the following three already implemented methods of increasing the efficiency of transcritical CO₂:

Integrated-expansion-compression-unit (ECU)
Absorption chiller for sub cooling
Parallel compression

The paper evaluates the above methods by running real tests on systems operating in Swiss supermarkets. The main findings of the empirical analysis based on measured data are:

The ECU technology necessitates further development in order to be adopted on a wide scale
The use of absorption technology with CO₂ refrigeration results in high additional costs, which limits its market potential
Parallel compression shows high market potential, due to lower associated additional costs

In addition, efficiency can also been improved by investing in adiabatic gas cooling of gas coolers and better control systems.