Pushing the CO2 equator further south

With a clear shift towards CO₂ refrigeration by European retailers, a hot topic of discussion for several retailers at the 2013 ATMOsphere conference was the need for cost effective natural solutions for hot climates and pushing the “CO₂ equator” further south, bringing viable CO₂ solutions to all.

Presentations by enEx, Carel, Frigo-Consulting and Green & Cool all indicated promising results through the use of parallel compression, ejectors and mechanical subcoolers, helping to push the “CO₂ equator” further south towards the Mediterranean.

Cost effective CO₂ solutions for warmer climates

One of the retailers that expressed the need for environmentally friendly, efficient and cost effective solutions at the ATMOsphere conference was Delhaize. Megan Hellstedt from Delhaize Group highlighted: “We know there are companies testing solutions for warmer climates, and they are testing effective systems, and they will share their learnings quickly, and we will be able to adopt it. So we believe there is a momentum on that issue and that we have some solutions that are emerging. The question is how to quickly make them cost effective for the industry.”

Measures to improve efficiency of CO₂ transcritical systems by up to 25% in warm climates

Sergio Girotto from enEx focused his presentation on the efficiency improvements achieved for CO₂ use in warm climates. According to Girotto 85% of the world’s population lives in warm ambient climates, therefore for CO₂ heating and cooling solutions to become mainstream, the technology must be adapted to a wide range of ambient conditions, including those with short-lived high peak temperatures and those with high temperatures for long periods of time.

Three key solutions have been identified by enEx to improve the energy efficiency of CO₂ in warm ambient temperatures:

System with auxiliary compressors for flash vapour compression
System with evaporator overfeed
Energy recovery from throttling process

What energy savings can be expected using these solutions in a climate such as that found in Southern Italy? According to the results presented by Girotto, using evaporator overfeeding a 12 to 15% improvement in energy efficiency can be achieved. Adding auxiliary compressors to this improves energy efficiency by 15 to 20%, and also adding expansion energy recovery improves energy efficiency by 20 to 25%.

Parallel compression and flash gas valve synchronisation provide comparable efficiencies with CO₂ hybrid systems in warm climates

Diego Malimpensa, Application Management of “Carel retail sistema”, in his presentation also looked at using parallel compression and flash gas valve synchronisation to improve CO₂ efficiencies in warm climates. The Carel system has a parallel compressor controlled by the receiver pressure.

Looking at the performances of systems only with the flash gas valve or with a parallel compressor Malimpensa showed that the efficiency of the systems improves at higher ambient temperatures. In fact, not only does the efficiency of a system with a parallel compressor increase with higher ambient temperature, the parallel compressor can only run when the outside air temperature is 15°C or higher.

To evaluate the performance of these adaptations, Carel compared a booster with flash valve and booster with parallel compression, in three different cities: Munich, which has an average temperature of 8°C, Venice, which has an average temperature of 13°C and Palermo, which has an average temperature of 18°C. Using these systems a 5% energy saving was achieved in Munich, a 7% energy saving in Venice, and a 10% energy saving in Palermo. Comparing these results to those of a cascade CO₂/R134a system, it was found that in Palermo the energy saving results of a booster with parallel compression is comparable with those of a cascade CO₂/R134a system, which a few years ago was unthinkable.

“I doubt that there is something like a CO₂ equator”

Jonas Shönenberger, working with Swiss Consultancy Frigo-Consulting, presented the case of an installed CO₂ transcritical installation equipped with ejectors. There are two key benefits of the ejector system when compared to the standard booster system: the evaporation temperature can be increased, and a part of the vapour is shifted from the medium temperature compressors to the parallel compressors, so you reduce the work on the other compressors. Thanks to these factors the measured efficiency increase of using the ejectors to recover throttling losses was 12% compared to a CO₂ booster system with a parallel compression unit. Travelling further south, the recovery of these throttling losses increases, and so does the potential to increase efficiency.

Improved control systems achieve higher COP even in warm climates

Stig-Göran Lind Marketing Director for Green & Cool, also mentioned in his presentation the R&D efforts undertaken to make their “ECO” products suitable for warm climates. These developments include flash gas removal and parallel compression. With their improved control systems Green & Cool can achieve a high COP even under high ambient temperatures.