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About every 10 years, refrigeration equipment manufacturers are forced to redevelop their products for newer, safer, more climate-friendly, alternative refrigerants. This sounds positive, but it comes at a substantial cost, which is primarily covered by the end users. These mandates drive up costs in the form of engineering, development and testing to ensure that the systems are reliable and perform under real-world conditions. In most cases, the newer refrigerant has higher operating pressures than the previous one and, instead of increasing cooling capacity, the changes result in decreased cooling or condensing capacity.

As this article is being written, the dawn of another transition to a new refrigerant is at hand. Companies are again being mandated to convert equipment such as process chiller products from R404a, which has a global-warming potential rating of 3900, to the newest HFC solution, refrigerant R448a (GWP 1390). This change will bring us another step closer to the goal of reaching a global-warming potential of 1.

Some groups believe that now might be the time to consider a more future-proof solution to end another decade-long cycle of diminishing returns. They believe it starts with a refrigerant produced naturally, that will never be phased out, that is not chemical based and that takes the global-warming potential of current refrigeration equipment from a factor of almost 4000 all the way down to 1.

And so, we make the case that now is the time to consider a refrigeration system using carbon dioxide (CO2) as a refrigerant. There are three steps involved in doing so:

  • Step 1: Show how a CO2 system stands up in a head-to-head assessment operating under the same conditions. CO2 does not require a sacrifice in cooling or condenser capacity.
  • Step 2: Prove the impact of CO2 systems with improved heat recovery performance.
  • Step 3:Demonstrate how a CO2-based system can help customers reach their net-zero and sustainability initiatives.

Head-to-Head Assessment

Step 1 is important because it demonstrates that a CO2-based refrigeration system can operate at the same (or better) operating efficiency as current R404a systems. Because of the added cost for CO2 equipment, to be a viable alternative to HFC refrigerants, it must be able to compete from a head-to-head perspective.

Bakersfield, Calif., was chosen as the location of the company’s first CO2 field installation. The opportunity allowed testing that could show whether a CO2 system could operate reliably in one of the hottest regions in the United States.

Toward that end, the engineers did an in-depth ambient study of the site. The study confirmed that in the hot days of summer, a conventional R404A system can deliver a higher operating efficiency. When the operating conditions across the entire year are factored in, however, even with air-cooled gas coolers, the benefits shifted to the CO2-based system. The study showed a CO2 system could deliver lower operating costs with a higher cooling capacity (figure 1).

PC 0322 Pro Refrigration 02 Figure 1

FIGURE 1. A CO2-based system provided lower operating costs in a recent real-world comparison. Shown here are the cooling capacity and energy consumption data comparing a conventional R404A system and the CO2-based system. Image provided by Pro Refrigeration Inc. (Click on the image to enlarge.)

With each new refrigerant there comes an added cost. CO2 system’s base costs are currently 50 to 70 percent higher than comparable systems utilizing HFC refrigerants; however, price differences could close as supply-chain channels begin to fill and U.S. manufacturers and contractors adapt to new materials, standards and processes.

Heat Recovery

One way to cover — or offset — the added price of a CO2 system is to factor in the increased waste heat recovery. The principle of any refrigeration system is to remove heat where it is not wanted (making a medium cold) and reject this heat into a secondary medium. Typically, waste heat is rejected into the surrounding ambient. Instead, this waste heat can be utilized as a valuable heat source. In fact, when combined with CO2 refrigeration system, it could even eliminate the end user’s need for mechanical or fossil fuel heating.

Some processors have been using heat recovery to offset water heating needs for decades. Dairy producers, wineries, bakeries and breweries all utilize a tremendous amount of hot water for sanitation and clean-in-place (CIP) systems.

Using waste heat from a CO2 refrigeration system enables such facilities to reduce the load on their water heaters (or boilers) while often improving the operation and capacity of the refrigeration system. CO2 enables customers to recover more waste heat — 100 percent recovery versus 30 percent with HFC — at higher water temperatures to 185°F (85°C).

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The impact of CO2 heat recovery from a CO2 system can be seen on a system deployed in August 2021 on a 3,000-cow dairy farm in Pixley, Calif. Previously, the farm consumed 21 to 25 gal of propane daily to generate hot water to sanitize their milk tanks, milking parlors and pipelines. Image provided by Pro Refrigeration Inc. (Click on the image to enlarge.)

An example of the impact of CO2 heat recovery comes from a system deployed in August 2021 at a 3,000-cow dairy farm in Pixley, Calif. Previously, the farm consumed 21 to 25 gal of propane daily to generate hot water to sanitize their milk tanks, milking parlors and pipelines. Following the installation of the CO2 chiller system, and by utilizing higher discharge refrigerant temperatures, the dairy farm now generates 3,000 gal of 150°F (65°C) water daily. Annually, this eliminates an additional 40 to 57 tons of carbon and the use of over 9,000 gal of propane.

Sustainable Initiatives

The final step is to show sustainable value and explain how systems using refrigerant-grade CO2 — also known as R744 — make good business sense. Such systems provide a real and measurable way for companies to reduce carbon emissions (table 1).

PC 0322 Pro Refrigration 04 Table 1

TABLE 1. The sustainable value of CO2 systems is demonstrated in the reduction in carbon emissions. Image provided by Pro Refrigeration Inc. (Click on the image to enlarge.)

The topic of sustainability and terms like “net zero”— previously the focus of think tanks and academia — have shifted. Today, the timelines and expectations are being driven by corporations with aggressive and measurable climate pledges that go beyond the targets set by international groups committed to climate change.

These pledges are driving companies to develop climate-friendly solutions. People are now understanding the real weight of their carbon emissions. These changes and others are defining why CO2 should be considered a replacement for HFC-based refrigerants. How exciting to utilize a refrigerant recovered, or “borrowed,” from the atmosphere, especially knowing that in the future, it will likely return to the atmosphere.

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CO2-based systems often stand up in a head-to-head assessment for process cooling applications. Image provided by Pro Refrigeration Inc. (Click on the image to enlarge.)

CO2 has the potential to be as common of a refrigerant as R404A is today, at least for industrial process refrigeration systems. EPA mandates will continue to tighten the restrictions on HFC refrigerants, and they will continue to frustrate equipment manufacturers, suppliers and contractors as we work to adopt and adapt, while always striving to keep our customers cool. We think it’s time to consider CO2, and natural refrigerants deserve to be a bigger part of the climate-saving solution.

Benefits and Differences Between CO2 and HFC Heat Recovery

CO2/R744-based systems utilize additional control valves that are specifically equipped with a high pressure valve and a flash-gas valve. Together, these valves control the discharge pressure of the CO2 chiller and the receiver pressure of the CO2 chiller. This allows added control of the discharge pressure of the CO2 chiller system, enabling the system to operate at higher discharge pressures and temperatures when needed.

HFC (404A)-based systems are dependent on and limited by ambient conditions to determine the discharge pressure and temperature of the system.

CO2/R744 operates at discharge temperatures that can exceed 200°F (93°C). Producers can eliminate the need for fossil fuel heaters by using only waste heat to deliver water temperatures to 185°F (85°C).

HFC (R404A) operates at discharge temperatures reaching 150°F (65°C). This enables preheating of water to 125 to 130°F (51 to 54°C) using conventional heat recovery methods.

CO2/R744 systems can recover up to 100 percent of the rejected heat as well as operate as a heat pump. They also can provide water heating more efficiently than is possible heating with propane when the producer does not have a cooling demand.

HFC (R404A) can recover up to 30 percent of the waste heat.

CO2/R744 systems can operate transcritical; that is, without a pressure-to-temperature correlation. This means the refrigerant does not condense or undergo a phase change. This allows the high pressure valve to determine the amount of heat that is rejected. CO2 systems can be forced to run in a transcritical manner even under low ambient conditions, allowing the maximum heat rejection at the desired temperature.

HFC (404A) heat recovery systems are limited to the point of condensing.

Jim VanderGiessen Jr. cofounded Pro Refrigeration Inc., Auburn, Wash. Engineers at PROGreen Solutions, a team at Pro Refrigeration Inc. focused on natural refrigerants, also contributed to this article. For more information from Pro Refrigeration Inc., call 800-845-7781 or visit

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