Carbonated Cooling

MICHAEL GARRY

The shadow cast by global warming takes in a critical part of supermarket operations — refrigeration.

That's because the refrigerants used by store systems have a propensity for escaping into the atmosphere. Once there, they act as “greenhouse gases” with a high global warming potential (GWP), the measure of global warming that starts with carbon dioxide's GWP of one.

For example, R-22, the common HCFC refrigerant still widely used by supermarkets (but being phased out because of its ozone-depleting effect), has a GWP that is 1,700 times that of carbon dioxide. R-404A, one of the HFC refrigerants that stores are using in place of R-22, has an even higher GWP — 3,500 times that of carbon dioxide.

The regulations imposed on supermarkets by the U.S. Environmental Protection Agency so far only apply to R-22 and its ilk. But should federal global warming legislation come to pass — as many observers expect with a change in the White House next year — supermarkets may eventually be regulated in their use of HFC refrigerants. “Who knows where we'll be in 10 years?” said Benny Smith, director of facilities for Price Chopper Supermarkets, Schenectady, N.Y.

What this means is that food retailers are beginning to look seriously at refrigeration systems that reduce the amount of high-GWP refrigerants required. To date, these systems have included secondary loop refrigeration (being tested by Food Lion among others), distributed units (used by Albertsons among many others), and units based on redesigns of conventional direct expansion (DX) systems (employed by Hannaford Bros., Farm Fresh and Raley's).

Another store refrigeration design that has caught on in Northern European countries features a conventional DX design but uses carbon dioxide — the lowest GWP gas and a naturally occurring substance — as the primary refrigerant rather than an HFC or HCFC. The more popular version of this is called a “Cascade” or subcritical refrigeration system.

While carbon dioxide is understood to be the predominant greenhouse gas, the excess amounts of it in the atmosphere are caused primarily by the burning of fossil fuels for energy production. The contribution of carbon dioxide to the atmosphere from refrigeration leaks would be miniscule by comparison.

Carbon dioxide is also used in a secondary loop system marketed by Hill Phoenix, Conyers, Ga., and being tested in two stores by Food Lion. But in this case the carbon dioxide is secondary coolant while the primary coolant is an HFC refrigerant.

In late March, Price Chopper, which operates 116 stores in seven states, became the first North American food retailer to install a Cascade carbon dioxide system in a store in Saratoga, N.Y., that is being remodeled. “We were looking for a way to be more environmentally friendly by eliminating R-404A refrigerant altogether on the low-temperature side,” said Mark Hankle, mechanical engineer for Price Chopper.

Price Chopper's decision to explore an environmentally beneficial advanced refrigeration system like the Cascade system ties into its membership in the EPA's GreenChill Advanced Refrigeration partnership, which the chain joined in January. The GreenChill program calls for food retailers to track their refrigerant usage, transition to HFC refrigerants and look into advanced refrigeration designs.

In testing this system, Price Chopper, Smith said, “is at the leading edge of refrigeration technology.”

The Cascade system, from Hill Phoenix, uses only carbon dioxide as a refrigerant for low-temperature frozen food and ice cream cases. But under the Cascade design, the low-temperature system is connected to a secondary loop, medium-temperature system. The latter system uses R-404A to cool the carbon dioxide condensers, keeping pressures under control; it also cools propylene glycol that in turn chills the medium-temperature dairy, beverage and floral cases.

Overall, the Cascade system handles about 90% of the store's low-temperature product cases and about half of the medium-temperature cases.

HFC refrigerant reduction is one of the benefits of the Cascade system. It uses approximately 600 pounds of R-404A, compared to 1,400 pounds used in a conventional DX system, said Hankle.

The Cascade system also offers some cost advantages. Notably, carbon dioxide costs much less than standard refrigerant — 50 cents per pound compared with about $8 per pound — and has favorable heat transfer properties. The system also uses smaller-diameter copper tubing and smaller compressors. Price Chopper is estimating that the system will be 3% more energy efficient than a conventional system, though that remains to be seen.

The overall cost of the system is “in the process of being analyzed,” said Hankle. “We're paying a premium for being the first installation and still figuring everything out.”

Price Chopper's priority now is to evaluate the system's “functionality, efficiency and reliability” over the next year, particularly the summer months, said Hankle. “Then we'll evaluate the pricing if we go forward [with other installations].”

Since the installation in late March, the Cascade system has been “functioning pretty well — no worse than any conventional system we've ever put in,” said Hankle.

But Smith acknowledged that testing a system like this, which has not been used before in North America, is a little risky. “Sometimes you need to take a risk to be a leader,” he said.

Hill Phoenix is not yet prepared to release the Cascade system as a commercial product, said Scott Martin, director, advanced engineering, Hill Phoenix. When it does, the cost of the system will be “comparable” to traditional systems “especially over time,” he added. “The Cascade system has a total-cost-of-ownership story to tell.”

According to Knud Jacobsen, OEM sales manager for refrigeration component maker Danfoss' U.S. division in Baltimore, the initial cost of a Cascade system in Europe is 20% higher than a conventional system, but that the cost comes down after 10 to 20 installations.

HIGHER PRESSURES

The higher pressures used in the Cascade system represent one of the key points of difference with other systems — and one of the main areas of concern from a safety/operational perspective. According to Hankle, pressures in the system run up to 420 pounds per square inch gauge (psig), compared to about 200 psig in a system that uses only R-404A. “It was a little bit of a concern, but we've been dealing with R-410A (315 psig) in our air conditioning systems for a few years so our techs are used to it,” he said.

The higher pressures associated with the Cascade system have prevented system components like the rack compressors from receiving Underwriters Laboratories approval. Martin said that Hill Phoenix and other manufacturers are pushing for UL approval of compressor racks used in Cascade carbon dioxide systems. “If we go forward with further installations [of the Cascade system], we will look for UL-listing of the rack compressors,” said Hankle. For the test store in Saratoga, noted Smith, Price Chopper received approval from state and local authorities to use the Cascade system.

To compensate for the higher pressures, numerous safeguards are included in the Cascade system. For example, pressure valves, designed to meet the refrigeration safety code, allow the carbon dioxide to vent outside the store. In addition, the condensing unit for the carbon dioxide is connected to the store's emergency generator, so even during a power outage the pressure remains under control. Leak detectors are used inside the store, machine room and main freezer.

Smith noted that the chain's technicians became acquainted with the system's higher pressures, Cascade design and other oddities — such as plate-to-plate heat exchangers on the carbon dioxide compressor racks — by working from the beginning with the system's installation contractor, ABC Refrigeration, Syracuse, N.Y., and by taking a two-day training course provided by Hill Phoenix. He credited ABC Refrigeration with being hugely helpful with the system's implementation.

“Anything new is a little more acceptable [to the technicians] if they are involved in the process at the front end,” Smith said, adding that the technicians who went through training “are up to speed.”

“Everybody was nervous up front, but once they worked with it they got comfortable,” Hankle added.

Another adjustment required by the Cascade system is that it required product cases with thicker evaporation coils for carbon dioxide. “We had to coordinate with our purchasing/procurement department to make sure they got the right cases,” said Smith. At the time, Hill Phoenix was the only provider who could supply these cases.

Every refrigeration system is subject to leaks, and the Cascade system is no exception. Indeed, its higher pressures may lead to more leaks than other systems. On the other hand, compared to refrigerants like R-404A, carbon dioxide causes negligible harm to the environment when it leaks out of the system, and it is not currently subject to EPA regulations.

TREND IN EUROPE

Cascade systems using carbon dioxide are far more common in Northern Europe, where “at least 300 stores” have installed them over the past three to four years, said Christin Callesen, sales director for food retail, Danfoss, based in Nordborg, Denmark. The Danish Supermarket Group is one food retailer employing them, he said. British food retailer Tesco is also testing a Cascade system.

Carbon dioxide systems are favored in countries like Denmark that heavily tax HFC refrigerants. Denmark also passed a law in 2007 restricting the amount of HFC that can be used in a refrigeration system to no more than 22 pounds.

Another carbon dioxide refrigeration design is the transcritical system, which unlike the Cascade is not linked to any other system. It is being tested in about 100 Northern European stores, including Lido and Coop supermarkets in Denmark, said Callesen. Transcritical systems, best suited to cooler climates, operate at pressures as high as 1,200 psig, which require steel piping. Those are refrigeration pressures that U.S. food retailers are not “used to dealing with,” said Martin of Hill Phoenix.

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The global warming potential of R-404A refrigerant.