ASHRAE Journal - August 2013 - 71

Technical FeaT
FeaTure

the same footprint and in the same climatic zone. The
results show an annual energy savings of 10% in the population with transcritical booster systems.
Regarding subcritical (cascade) systems, there are also
encouraging signs that CO2 is competitive with traditional systems. Hinde4 reports on preliminary findings
of a field investigation. Here a DX R-404A system is
compared to a R-404A/CO2 hybrid system where CO2 is
used on the low temperature cases as a volatile brine.
Although difficult to compare, there was an approximate
2% to 3% efficiency gain. On another occasion, Danish
discount supermarket chain Netto5 tested different system configurations including secondary coolants as well
as CO2 cascades. The outcome of the analysis indicated
an advantage of cascade systems over R-404A benchmark on the level of 10%.

Conclusions
The transcritical CO2 booster system provides the best
efficiency in cold and moderate climates. Compared to
alternatives, the simplicity of this system has made it
the most popular alternative in Europe and Canada. The
newest trend is to add heat reclaim, where the properties
of transcritical CO2 allow for the refrigeration system to
provide both space and sanitary water heating.
Current research is focusing on ways to increase
efficiency during the hottest hours of the year, either
by adiabatic cooling, liquid subcooling, load shifting
(i.e., ice bank systems) or other technology. Ammonia
or propane/CO2 cascade systems provide the best efficiency in hot climates. The choice between ammonia
and propane is more a question of the refrigerants’
acceptance, as well as investments, rather than that of
efficiency. To deal with toxicity, low charge modular
chiller systems as well as technician training may ease
the way for ammonia. Regarding the configuration
with propane, it has already been successfully used
outside of North America (Lidl discount stores), also
in low charge systems, but due to flammability will
require changes to current rules and standards before
it could gain widespread acceptance. If ammonia
or propane is not acceptable, a cascade system with
R-134a also provides a reasonable efficiency and good
environmental performance, especially if best practices are applied to make the chiller as leak-tight as
possible.

HFC/CO2 hybrid systems offer similar efficiencies to
the benchmark system but have a higher TEWI than
other alternatives due to the high HFC charge. Solutions
with pumped glycol were found to have significantly
higher energy consumption both due to the required
pumping power and due to the increased temperature
lift required of the brine.
There are clear benefits provided in terms of operational efficiency and a reduced TEWI (carbon footprint)
where system architectures are chosen and designed
to match the climatic conditions. Though not treated
in this article, investment costs are also a major factor when deciding between system types. In speaking
to both OEMs and end users it is clear that experiences
with investment costs vary significantly and that they are
dependent on the prevalence of any given solution in the
region. As with any new technology, costs will decrease
as scale is achieved.

References
1. Mikhailov, A., H.O. Matthiesen, S. Suindykov. 2011. “Comparison
of energy efficiency of systems with natural refrigerants.” ASHRAE
Transactions 117(2):116.
2. EPEE. 2010. Excerpt of “Eco-efficiency Study on Supermarket
Refrigeration.” http://tinyurl.com/mgbcbfp.
3. Wozny, M. 2010. “Datenauswertung der Gaskühler von transkritischen CO2 –Anlagen.” Chillventa 2010 Proceedings.
4. Hinde, D., S. Zha, L. Lan. 2009. “Carbon dioxide in North American
supermarkets.” ASHRAE Journal (2).
5. SCM FRIGO. 2010. “Test results: SCM FRIGO and Grundfos Field
Test the Performance of Pumped CO2 system at Netto Supermarket in
Denmark.” http://tinyurl.com/m49jqwx.

Bibliography
ATMOsphere International Workshop On Natural Refrigerants. 2010.
“How To Bring Natural Refrigerants Faster To Market Summary
Report.”
Handschuh, Roland. 2008. “Design Criteria for CO2 Evaporators.”
Published in Natural Refrigerants–sustainable ozone– and climate-friendly
alternatives to HCFCs.
IPCC/TEAP. 2005. Special Report: Safeguarding the Ozone Layer and the
Climate System.
Stenhede C. 2007. “Heat Exchangers in Carbon Dioxide Cascade
Systems.” Scandinavian Refrigeration (6):36.
Wilson, I. and D. Maier. 2006. “Carbon dioxide for use as a refrigerant.” International Institute of Refrigeration Conference. Innovative
Equipment and Systems for Comfort and Food Preservation. http://tinyurl.
com/pzxvro3.
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