U.S. component manufacturer Emerson and South African consultant Future Green Now have partnered on a new study designed to help supermarkets in North America identify which add-on technologies they should use to boost the efficiency of transcritical CO2 (R744) refrigeration systems in warm climates.

The study – “New to the World Climate Study for the Americas: Climate Zone Impact of CO2 System Selection” – looks at the amount of time CO2 systems spend operating in less-efficient “transcritical mode” in 166 cities across 13 climate zones in the Americas. It then examines how this time could be reduced by adding an adiabatic gas cooler, parallel compression, ejector, or some combination of these technologies to the system.

Findings from the study were presented by Andre Patenaude, Director Solutions Strategy – Cold Chain at Emerson Commercial and Residential Solutions, and Samantha Bothma, Design Engineering Consultant at Future Green Now, during a refrigeration case study session at the ATMOsphere (ATMO) America Summit 2022 on natural refrigerants. The conference took place June 7–8 in Alexandria, Virginia, and was organized by ATMOsphere, publisher of R744.com.

“This study was commissioned to provide industry stakeholders with an unbiased third-party engineering evaluation of energy comparison of the most common high ambient strategies to support the uptake with CO2 transcritical booster systems for the supermarket industry,” explained Patenaude.

The research shows that adding an adiabatic gas cooler alone to a transcritical CO2 booster system can reduce the amount of time the system spends in transcritical mode, though the amount of energy saved varies across climate zones.

The study found that adiabatic gas coolers have the greatest impact in hot, dry climates. For example, in Palm Springs, California, which the study categorized as having a “hot dry 3B” climate, a transcritical CO2 booster system spends 54% of its time in transcritical mode with a dry gas cooler. This falls to just 6.5% with an adiabatic gas cooler.

A similar system in Arcata, California, which has a “marine 4C” climate, would operate in transcritical mode 0.1% of the time with a dry gas cooler and 0% of the time with an adiabatic gas cooler. As pointed out by Bothma during her presentation, adding an adiabatic gas cooler to a transcritical CO2 booster system in this climate “makes less sense.”

Parallel compression and ejectors

The study also assessed how combining other technologies, such as parallel compression and high-pressure gas ejectors, with dry and adiabatic gas coolers could further improve R744 system efficiency.

In the study’s four dry climate zones, adiabatic gas coolers produced greater energy savings than the combination of dry gas coolers and parallel compression. The opposite was seen in the study’s nine humid climates, with dry gas coolers plus parallel compression performing better.

Bigger energy savings were seen across all 13 climate zones when adiabatic gas coolers were combined with parallel compression. For example, in climates categorized as “hot dry 2B/3B,” like Palm Springs, the energy consumption of a transcritical CO2 booster system was reduced by 8% with a dry gas cooler plus parallel compression, 14% with an adiabatic gas cooler and 19% with an adiabatic cooler plus parallel compression.

Additional energy savings were achieved by adding a high-pressure gas ejector to a transcritical CO2 booster system with an adiabatic gas cooler and parallel compression. This was particularly true in locations with the highest ambient temperatures. For example, in “hot humid 1A/2A” climates, the average energy savings with all three technologies was 16.5%. In Miami, Florida – the city with the highest ambient temperature in this climate zone – energy consumption was reduced by 19.5%. In this case, the addition of a high-pressure gas ejector resulted in a further 1.7% of energy savings.

Regardless of which add-on technologies are ultimately selected, “the industry has proven that there’s no reason not to use transcritical CO2 in warmer climates,” said Patenaude.

“The industry has proven that there’s no reason not to use transcritical CO2 in warmer climates.”

Andre Patenaude, Emerson Commercial and Residential Solutions,

Other factors

As pointed out by Patenaude and Bothma during their presentation, optimizing transcritical CO2 systems is not just about energy savings; end users must also consider other factors during the decision-making process.

“Adiabatic can be a simple way of improving your energy efficiencies,” said Bothma. “But whether it’s that valuable is assessed on a case-by-case basis.”

Capital expenses are often the most important factor, according to Patenaude, but electrical rates and peak demand charges may justify the extra spend to reduce costs and save on energy bills.

Similarly, the availability and cost of water also play a role, particularly when considering adiabatic gas coolers, which use small amounts of water to help cool the system’s CO2. However, as explained by Bothma, by “considering things like condensate reclaim, the majority of your water that you use for an adiabatic gas cooler can be free.”

Other considerations include service and maintenance, the need for heat reclamation and carbon intensity.

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