Switching from synthetic refrigerants to CO2 (R744)-based free cooling can slash cooling-related energy use in data centers by up to 70%, according to M&M Carnot, a U.S.–Canadian manufacturer specializing in natural refrigerant-based HVAC&R technologies owned by Johnson Controls.
Speaking at the ATMO America 2025 Summit in Atlanta, John Miranda, M&M Carnot’s Director of CO2 Strategy for Industrial Refrigeration, explained how the company’s patented free cooling approach uses cool ambient air to circulate CO2 refrigerant passively, dramatically reducing compressor use and, therefore, energy consumption.
“Depending on the data center’s location, we’re only running a compressor 20–30% of the time,” he said.
ATMO America 2025 took place June 11–12 and was organized by ATMOsphere, publisher of NaturalRefrigerants.com. M&M Carnot was one of the event’s Gold sponsors.
CO2 free cooling
A CO2 system can operate in free-cooling mode in ambient temperatures of −10 to 20°C (14 to 68°F) before switching to subcritical and transcritical operation as temperatures rise. In warmer conditions, an adiabatic gas cooler can help improve efficiency further by reducing time spent in energy-intensive transcritical operation.
“CO2 is such a great refrigerant in free-cooling mode, and we’re able to switch to it seamlessly, giving an advantage over synthetic [alternatives],” said Miranda. “The only synthetic free cooling today uses a pump, which involves a lot of work.”
R744’s pressure sensitivity also enables thermosiphon cooling, a form of free cooling, with an indoor–outdoor temperature difference of just 3 to 4°C (5 to 7°F) enough to drive the system.
This feature becomes increasingly valuable as data center temperatures rise. Return water temperatures are already hitting 32°C (90°F), and outlet air is reaching 27°C (80°F), up from 21°C (70°F) just a few years ago.
“Eventually you’ll reach temperatures where you don’t need a compressor anymore, just heat rejection equipment,” he explained.
“CO2 is such a great refrigerant in free-cooling mode, and we’re able to switch to it seamlessly, giving an advantage over synthetic [alternatives].”
John Miranda, Director of CO2 Strategy for Industrial Refrigeration, M&M Carnot
The role of CO2
Cooling strategies vary widely depending on data center size and application. Hyperscale data centers may not adopt CO2 across entire systems, but there is still potential for air-side cooling, which can still account for 20% of thermal loads even in facilities dominated by liquid cooling, Miranda noted.
Edge computing, which processes data closer to the source, is another major opportunity. These smaller, distributed facilities are a strong fit for CO2 systems.
“If we do our job, I see edge computing being all transcritical CO2,” he added.
While CapEx for CO2 free cooling is on par with synthetic systems, the company reports that it offers lower installation and operating costs.
“Transcritical CO2 doesn’t have a chance in the data center sector if it doesn’t use free cooling,” said Miranda.
In 2022, M&M Carnot published a white paper – “Refrigerant Restrictions: Is Your Data Center Ready for the Transition?” – to help owners of data centers replace high-GWP refrigerants with CO2 solutions.
A decade of innovation
M&M Carnot has been developing CO2-based cooling technologies for over a decade. The company installed its first R744 data center system at a telecoms facility in 2014 and patented its free-cooling design for transcritical CO2 in 2015. Since then, it has continued to scale its technology across North America.
During his ATMO America 2025 presentation, Miranda highlighted several recent data center deployments:
- In 2019, M&M Carnot installed two 15TR (52.8kW) CO2-based CRAC units at a data center in Montreal, Canada. The system included a two-fan gas cooler and used orbital-welded stainless steel tubing.
- In 2024, the company added three 80TR (281.3kW) CO2 chillers to an existing California data center. Though free cooling was not applied in this case, the move helped the client transition away from synthetic refrigerants.
- Another 2024 project in Montreal saw the installation of a 26.7TR (94kW) CO2-based CRAH unit, where free cooling met 90% of the load, with compressors used only for conditions in which free cooling was not feasible.
- A third Montreal facility added a 39.8TR (140kW) in-row CO2 cooling system featuring thermal siphon technology, a form of free cooling. The unit delivers 40% more capacity than chilled water alternatives, according to Miranda.
The company is currently scaling its air- and liquid-based systems to handle installations that are three to four times larger than current deployments.
“We’re doing it as fast as we can right now,” Miranda concluded.
