CO2 (R744) refrigeration systems are often subjected to criticism about their energy efficiency in warm climates and other operational aspects.
But these criticisms are really “myths,” said Scott Martin, Senior Director of Industry Relations and Compliance at U.S. OEM Hillphoenix, who set out to debunk them at the Food Industry Association (FMI) Energy and Store Development Conference, held in Baltimore, Maryland, October 2‒4.
Conyers, Georgia-based Hillphoenix designs and manufactures both HFC and CO2 systems and has close to 1,500 Booster CO2 refrigeration systems operating in North America, with locations as far south as Mexico City. According to Martin, the manufacturer’s sister company, Denmark-based Advansor ‒ with over 15,000 installed CO2 systems ‒ is currently “building many” systems for Mexico and Central and South America.”
“There are energy savings possibilities with CO2, which are compounded by elements like heat reclaim and electronic expansion valves (EEVs),” Martin said. “About 15 years ago, I was a non-believer in CO2, but I have changed because it is a future-proof solution.”
The first three myths Martin challenged revolved around the energy consumption of CO2 systems compared to those using traditional refrigerants. The last three deal with practical aspects of CO2 systems, including complexity, high pressures and leak rates.
No CO2 equator
Coined around 2010, the “infamous CO2 equator” was designed for energy evaluation, indicating that CO2 systems should only operate as cascade systems and, as such, not be considered in states south of the Mason–Dixon line in the U.S.
“By 2017, [this equator] was gone, and transcritical CO2 systems were applicable below that line,” Martin said, adding that the “so-called” CO2 equator “never actually existed.” because it was an “energy only” equator.
Not an energy hog
Martin indicated that CO2 systems are incorrectly compared to traditional ones with the same design parameters; however, CO2 operates at high pressures, allowing a higher saturated suction temperature (SST) to get the same evaporation temperature. “You do not design and size a CO2 system the same way as an HFC.”
In a comparison, three design changes were added for an efficient CO2 system using the same air-cooled condenser as an HFC one.
- The CO2 system has a 1°F (0.55°C) higher SST for both low (LT) and medium-temperature (MT) loads.
- The CO2 gas cooler is sized using a dynamic approach based on ambient temperature variations instead of the fixed one used for air-cooled condensers in HFC systems.
- The CO2 system has a 50°F (10°C) minimum condensing temperature instead of the 70°F (21.1°C) used for HFC systems. “We are not giving an advantage to CO2 here,” Martin said. “We have pressure drop available for CO2 and can afford to run a lower condensing temperature.”
Another issue Martin outlined is comparing the systems at 100% load. “In our analysis, we tried to be fair and vary the load,” Martin said, showing an analytical energy consumption comparison of a CO2 system to an HFC system using R449A, with results for seven U.S. cities as far north as Minneapolis, Minnesota, and as far south as Miami, Florida.
In a comparison using a standard gas cooler, the CO2 systems are less energy efficient (-3.5% lower in Minneapolis and -19.4% in Miami) than the R449A one. However, adding an adiabatic gas cooler and parallel compression makes CO2 more efficient (6.4% in Minneapolis and 3.0% in Miami).
Technological advances make additional savings possible for CO2 systems. In an actual store that the owner and contractor agreed was a “finely tuned” system, Hillphoenix switched out the mechanical thermostat expansion valve (TEV) for an electronic one (EEV). “We did not change anything out but the expansion valve and saw an 8.7% energy savings in that store,” Martin said.
The analytical study did not include the energy savings of using an EEV. Neither did it account for additional savings available from heat reclaim.
Martin shared data generated by Flō Energy Solutions, comparing CO2 system heat reclaim savings over an R449A one. The compressor discharge temperature of CO2 is, on average, 30°F (16.7°C) hotter than that of R449A, which provides 116.7% more energy savings in heat reclaim in Minneapolis and 51.2% in Miami.
“There are energy savings possibilities with CO2.”Scott Martin, Senior Director of Industry Relations and Compliance at Hillphoenix
No supercritical energy spike
When a CO2 system moves to supercritical operation with ambient temperature rising to 87°F (30.5°C), the energy requirement increases on roughly the same linear path. “There is no huge spike,” Martin said.
Not more complexity
A CO2 system running in supercritical mode on a hot summer day only requires a high-pressure valve and a gas bypass valve. “These components are like the valves we have always used ‒ just better, more reliable,” Martin noted. “With a well-programmed controller, everything responds properly, and you have an optimal system.”
Less pressure than paintball guns
In today’s CO2 refrigeration systems, “the only high pressure is in the machine room or on the roof that varies from 650‒1,660psi [44.8‒114.5bar] on a super hot summer day,” Martin said. “We send approximately 50 psig [3.4bar] out to the sales floor with 200psig [13.8bar] in the LT, -20°F [-8.9°C] suction line and 407psig [28.1bar] in the MT, 20°F [-6.7°C] line.
He admitted that 15 years ago in the U.K., an improperly designed and not thoroughly tested fitting caused an issue with CO2 pressure on the sales floor. “But that is not the design anymore; we don’t send high-pressure CO2 to the sales floor.”
Martin compared the safety of CO2 pressures in a store to those found in paintball guns. In the past, a paintball gun CO2 cylinder was pressurized to 894psig (61.6bar) at 75°F (23.9°C). “We didn’t think anything about playing paintball with a CO2 cartilage under our arm or giving one to our kids,” Martin said.
“The pressures in the CO2 system inside the store are lower than a paintball gun,” he noted, with next-generation guns using nitrogen pressurized at 3,000 to 4,500psi (206.8 to 310.3bar).
10% leak rate
Martin said some people contend that CO2 leak rates are over 100%. However, leaks are different from emissions. “Emissions come from three primary sources: actual leaks, venting for service and repair, and relief valve opening during a power failure,” he said.
In surveying its CO2 installations, Hillphoenix finds stores operating without power failures generally add 40lbs (18.1kg) of CO2 yearly to a 400lbs (181.4kg) system. “These are real stores, so the leak rate is about 10%,” Martin contended.
Having to add CO2 due to power failures is a concern. According to Martin, the North American Sustainable Refrigeration Council (NASRC) is organizing a committee and surveying OEMs about the use of 60 to 90psig (870.2 to 1,305.3bar) evaporators in the display cases to provide longer standstill pressure in the event of a power failure compared to the current 45bar (653.7psig) rating.
“The majority of evaporators in European display cases have 80bar [1,160.3psig] evaporators for longer reliability [that] cuts venting of CO2 during power failures,” Martin said, inviting the industry to join the NASRC discussion.
“CO2 is a great environmentally benign refrigerant,” Martin said in conclusion. “If we partner and stop arguing about the best refrigerant, we can make tremendous gains, as an industry, on a CO2 solution.”
During the question-and-answer session, a representative of Weis Markets volunteered that the findings his company has seen from its four CO2 stores “speak exactly” to what Martin showed, even though they, too, had concerns five years ago about these myths. He added that CO2 will be its prototypical refrigeration system moving forward.
“We have invested significant research and development resources towards building [a] comprehensive, environmentally friendly product portfolio,” said Eduardo Navarro, Vice President of Product Segments for Dover Food Retail, of which Hillphoenix, Anthony and Advansor are brands, in a company video. The company offers large to small CO2 system solutions and propane (R290) cases.
“CO2 is a great environmentally benign refrigerant.”Scott Martin, Senior Director of Industry Relations and Compliance at Hillphoenix