HFC refrigerants are a great alternative to R-22, which depletes the ozone layer, but HFCs contribute to global warming. Is CO2 the answer?
Global warming — or more precisely, impending federal legislation aimed at reducing global warming — has large supermarket retailers looking at their carbon footprint.
Take Safeway. As reported in SN's Sept. 21, 2009, issue, the Pleasanton, Calif.-based national food retailer is actively assessing its creation of carbon gases, as well as advising other supermarket chains to do the same before the enactment of cap-and-trade legislation it expects next year. Or consider an even larger retailer, Wal-Mart Stores. The Bentonville behemoth has not only analyzed its contribution to global warming, but reported it for all to see.
And in Wal-Mart's carbon analysis is a reminder of what is probably the food industry's biggest Achilles' heel when it comes to global warming: refrigeration emissions. In 2006, Wal-Mart contributed the equivalent of 2.95 million tons of carbon dioxide (CO2) in just refrigerant to the atmosphere, or 51% of the 5.8 million tons of direct CO2 emissions that the company generated, and 14.5% of its total CO2 emissions (direct and indirect).
In keeping food cold, supermarkets rely on complex mechanical systems that, despite all manner of technological remedies, tend to leak refrigerant. These gaseous emissions rise into the upper reaches of the atmosphere. In the case of R-22, still the industry's primary refrigerant, the effect is to deplete the atmospheric ozone layer that protects humans from most of the sun's ultraviolet light, and also to contribute to global warming.
The ozone effect was enough to cause the U.S. Environmental Protection Agency, in concert with other industrialized nations, to phase out use of R-22 (the Montreal Protocol), a process that will accelerate next year.
But the phaseout of R-22 has only led to a new dilemma for the food industry: R-22's replacement, various HFC refrigerants such as R-507 and R-407A, don't deplete the ozone in the atmosphere but they are greenhouse gases that add substantially — even more than R-22 — to global warming. Their global warming potential (GWP) is thousands of times more potent than that of the biggest global warming contributor, carbon dioxide (CO2).
And here's the final twist in the story: CO2 itself can also be used as a refrigerant — hundreds of European supermarkets have adopted it — and may be the industry's ultimate answer to the refrigeration dilemma.
But isn't CO2 the biggest greenhouse gas of them all? Yes, but with several caveats. First, unlike R-22 and HFCs, CO2 is a naturally occurring gas that can be taken out of the atmosphere for industrial uses. So even if it leaks out of supermarkets, that CO2 is merely going back to where it came from, in contrast to the CO2 generated by cars and power plants, which is new CO2.
But it's the bloated GWP of synthetic refrigerants that makes the case for replacing them with a natural refrigerant like CO2 so compelling. For example, R-404A has a GWP of 3,900, compared with CO2's GWP of 1, which means that 100 pounds of R-404A is equivalent to 378,400 pounds of CO2. Moreover, HFCs are among the gases regulated under the Waxman-Markey cap-and-trade bill, making them another potential hot potato for supermarkets like R-22.
In the meantime, food retailers are at least seeking to use an HFC with a relatively low GWP, such as R-407A (2,100). Last year, for example, Food Lion decided to make R-407A its preferred HFC as it replaces R-22 in existing stores and ramps up remodeled and new stores, said Wayne Rosa, energy and maintenance manager for the Salisbury, N.C.-based chain.
But at the same time, Food Lion is testing the merits of CO2 as a refrigerant that would not harm the environment and thereby obviate any regulatory concerns. “CO2 really seems to be the [industry's] direction,” Rosa said in a session he led at the Food Marketing Institute's Energy & Technical Services Conference last month in Indian Wells, Calif. “Europe is going in that direction and the U.S. is looking at going in that direction. It fulfills our need from a carbon footprint standpoint.”
Food Lion's motivation in testing CO2 was reinforced recently when it determined that refrigerant emissions accounted for 24% of its carbon footprint. “When you look at the bills in front of Congress, this becomes very important,” he said.
Even a chain like Stop & Shop, Quincy, Mass., which is not currently testing CO2 systems, is taking them seriously. “That's where the industry is heading for low-temperature applications,” said Ken Welter, Stop & Shop's manager of refrigeration engineering. “There are so many drivers — global warming, energy efficiency and capital costs.”
Last month, the EPA gave CO2 a boost by endorsing it as a replacement for HCFC refrigerants like R-22 that are being phased out to protect the ozone layer; the announcement came under the EPA's Significant New Alternatives Policy (SNAP) program, which evaluates alternatives to ozone-depleting substances. It was triggered by a request from refrigeration manufacturer Hill Phoenix, Conyers, Ga., but will apply to all refrigeration manufacturers. The EPA approval also allows CO2 to be used in lieu of HFC refrigerants.
“Now that EPA's SNAP Program has found carbon dioxide acceptable for use as a primary refrigerant in commercial refrigeration, I expect to see more and more of these systems, which will be beneficial for our environment,” said Keilly Witman, manager of EPA's GreenChill Advanced Refrigeration Partnership.
Currently, Hill Phoenix has supplied CO2 systems to four stores in Canada and 21 in the U.S., said Scott Martin, the company's director of sustainable technologies.
Over the past two years, Food Lion has tested two secondary loop refrigeration systems that use CO2 as a secondary refrigerant that travels throughout the store to cool frozen-food display cases. The chain is now about to embark on two more test stores, but these will employ what are called cascade (or subcritical) refrigeration systems.
At the FMI Energy Conference, Rosa reported on the progress in the two secondary loop CO2 stores as well as the prospects for the two new cascade CO2 stores.
As a charter member of the EPA's GreenChill Advanced Refrigeration Partnership, Food Lion has committed itself to addressing the leaks in its existing refrigeration systems as well as exploring advanced systems such as secondary loop that minimize leaks by reducing the amount (charge) used and confining it largely to the machine room.
After starting with a medium-temperature secondary system that employs propylene glycol as the secondary coolant in a store in Dinwiddie, Va., Food Lion launched its first low-temperature CO2 secondary system at a store in Montpelier, Va., in late 2006. In 2008, the chain opened a store in Portsmouth, Va., that employs both medium- and low-temperature secondary systems, the latter with CO2.
What has Food Lion accomplished so far with its secondary loop CO2 installations? They unquestionably reduce the amount of HFC used (just 300 pounds of R-507, compared with 3,000 pounds in a conventional system), helping to reduce its carbon footprint. The HFC that is used circulates only between an enclosed refrigeration house, which abuts the back of the two stores, and rooftop condensers.
Almost all of the equipment using the HFC is manufactured in a factory, reducing the chances of leaks and making them easier to identify and fix. As a result, the leak rates in the Montpelier and Portsmouth stores are comparatively low — 6.7% and 2.2%, respectively — and represent a major savings in refrigerant and maintenance costs.
In addition, the copper lines carrying CO2 out to the cases are narrower than what would carry conventional refrigerant, cutting the installation cost of the system. High-grade CO2 works well with the piping, costs under $1 per pound (compared with $5 to $8 per pound for conventional refrigerants) and offers “great thermodynamic properties,” making it “super efficient,” said Rosa.
Indeed, perhaps the most significant finding is that both low-temperature secondary loop CO2 systems employed by Food Lion have been able to consistently draw up to 3% less energy than conventional DX (direct-expansion) systems for the past two to three years, said Rosa. In the Portsmouth store, Food Lion sub-metered the compressor rack and condenser to determine their energy usage.
“There is an energy savings for this technology,” he said, adding that with new training the chain may be able to reduce its energy consumption by 8% to 10% compared with DX systems.
On the other hand, Food Lion has observed an “energy penalty” with the medium-temperature secondary loop systems in the Portsmouth and Dinwiddie stores.
Energy, maintenance and installation savings are helping Food Lion to justify the investment in the secondary loop system. “Since this is a new technology, everybody's worrying about how you can cost-justify it,” noted Rosa. For example, the CO2 stores saved a considerable amount in installation costs, requiring less than half the weight of copper used in standard stores (and copper prices have since increased) and 70% less HFC refrigerant charge.
On the other hand, at the Montpelier store, Food Lion used “industrial grade insulation” on piping, increasing its costs. In its second CO2 store in Portsmouth, the chain went back to cheaper standard insulation with no loss of energy.
As a result, the overall cost of the Montpelier CO2 system was roughly equivalent to that of a standard DX system, while the cost of the Portsmouth system was 10% less. However, that doesn't take into account Food Lion's investment in back-up generators needed in the event of a power outage to support the higher pressures CO2 require. The chain didn't include the cost of the generators because it was able to offset the cost with lower utility rates for those stores based on the use of the generators.
Rosa acknowledged that there was a learning curve in implementing a CO2 system, as with any new refrigerant, although “going to CO2 is not as big a learning curve as everybody thinks it is.”
For example, CO2 systems require an electric process to defrost the coils in cases; Food Lion had to adjust since it normally uses hot gas in that process. Another factor: CO2 is available but not necessarily stocked in a lot of supply houses. “You need a good relationship with a supply house,” said Rosa. In addition, CO2 leaks are more challenging to track down.
But the biggest considerations with CO2 secondary systems, said Rosa, are the need for a start-up guide as well as a comprehensive training program for technicians. Both Virginia stores used a contractor familiar with starting up and maintaining CO2 secondary systems, but Food Lion has since invested in training. “If something goes down on a Sunday afternoon in the middle of the summer, you can't always find your contractor to help you, so we want as many resources trained on this as possible,” he said.
Last week, Food Lion opened its first store with a cascade CO2 system in College Park, Ga., provided by Kysor Warren, Columbus, Ga. A second store, with a similar system from Hill Phoenix, is slated to open in Columbia, S.C., in December.
In contrast with the low-temperature secondary loop CO2 system used in the Virginia stores, the cascade system will service not only low-temperature but medium-temperature cases. The cascade system utilizes a relatively small amount of HFC refrigerant as a primary refrigerant; the College Park store will use about 450 pounds of R-507 (compared with 1,200 pounds of CO2); the Columbia store will have R-407A.
The cascade system used by Food Lion is effectively a blend of a conventional DX system (for low-temperature) and a secondary system (for medium-temperature), with CO2 as the cooling agent circulating through display cases for both. It uses two sets of compressors, one for HFCs on the medium-temperature (upper-cascade) side and one for CO2 on the low-temperature (lower-cascade) side.
The pressure in the cascade system gets up to 450 psig (pounds per square inch gauge). To release the pressure in case of a power failure, the cascade system includes expansion valves in the display cases. The cascade system for both stores received approval from Underwriters Laboratories (UL) last month. Rosa said compressor and component selection for cascade systems is “limited but improving.”
Like the Virginia stores, the cascade refrigeration stores will have back-up generators, in this case to preserve food after a power failure — an additional cost of about $200,000, said Rosa. (Food Lion will not get utility breaks in these stores.) “It's not required, but we don't feel comfortable about the risk of product loss,” said Rosa. The cascade systems also have a rack premium of about $100,000 compared with a conventional DX system, he added. Compressors for cascade systems must be imported, resulting in a cost premium, noted Hill Phoenix's Martin.
However, Rosa expects the cascade system to offset its cost by yielding a significant energy savings compared with conventional DX and even secondary systems. “We're hoping it uses less energy than the low-temperature secondary loop systems,” which consume up to 3% less than a conventional system.
In addition, as with the low-temperature secondary system, the cascade system uses smaller copper line sizes than DX and low-cost CO2.
Food Lion's cascade refrigeration implementation is not the first in the U.S. — that distinction belongs to a Price Chopper store in Saratoga, N.Y., which launched its system in March 2008. Also provided by Hill Phoenix, the system is different from Food Lion's in that it uses propylene glycol rather than CO2 as the secondary coolant for medium-temperature cases. Price Chopper did not respond to a request for comment on the status of the system.
While rare so far in the U.S., cascade CO2 systems are booming in Europe, especially in Scandinavia and Switzerland, where taxes and regulations on HFCs have spurred implementation. Starting about a decade ago, there are now more than 1,000 installations of subcritical (mostly cascade) systems in Europe, according to Raphael Gerber, project manager for Frigo Consulting, a refrigeration engineering firm based in Bern, Switzerland, who spoke at the FMI Energy Conference. There are also about 300 installations (generally in colder climates) of transcritical CO2 systems, which use only DX technology with much higher pressures than cascade systems.
“CO2 refrigeration is an effective and safe alternative, especially in subcritical cascade applications,” said Gerber. “There are hardly any technical challenges left.”