Engineering Tomorrow enables natural refrigerant solution for Brooklyn supermarket

"The goal for the Brooklyn store was to be completely free of synthetic refrigerants," says Michael Marotta, president of Supermarket Technical Services, Inc., Whole Foods’ Northeast refrigeration and controls design firm. "Whole Foods Market is a real leader in using green technologies to reduce their carbon footprint. In fact, the store on the Upper West Side was the first LEED Gold-certified supermarket in New York City. For the Brooklyn store, they wanted to meet or exceed that LEED-certification level. Plus, they wanted to earn Platinum GreenChill Certification from the U.S. Environmental Protection Agency, which recognizes food retailers who reduce refrigerant emissions and their impact on climate change.

"With these goals in mind,” Marotta continues, “Whole Foods tasked us to design a system to handle every fixture in the store – including self-contained equipment like juice bubblers – using CO₂. This was a first for us, and, as far as we know, a first in the nation."

How Whole Foods Market used CO₂ refrigerant for the whole store

Although used in the 1920s and 1930s, CO₂ refrigerant, or R-744, mostly disappeared after World War II when synthetic refrigerants became popular. But over the last two decades, CO₂ has been making a comeback, especially in supermarkets. It is a non-flammable and non-toxic refrigerant used in proven technology, and it provides a naturally available, environmentally friendly choice to meet sustainability goals. It has an ozone depletion potential of zero, and the lowest possible global warming potential of 1.0. Currently, hundreds of U.S. grocery stores are using CO₂ – but only for a portion of the refrigeration system, either as a secondary refrigerant or in a cascade system that also uses hydrofluorocarbon (HFC) refrigerants.

"Entirely eliminating HFC-based refrigerants from a grocery store is an interesting challenge," Marotta says. "That's because when you're cooling mid-temp and low-temp fixtures with CO₂, you've got to use a transcritical CO₂ booster system."

"A unique aspect of a transcritical CO₂ system is that the pressure is higher on the high side," explains Derek Gosselin, systems product manager for Hillphoenix. "With any kind of refrigerant, a compressor is used to pressurize the refrigerant gas that is then condensed into a liquid. When the pressure is released in the evaporator, the liquid expands back into a gas to absorb heat. With typical refrigerants, the entire compression-expansion cycle occurs below the pressure point where the gas can be easily liquefied, the so-called 'critical point.' But with CO₂ at high ambient temperatures, the cycle crosses above the critical point into a 'transcritical' condition. Under those conditions, CO₂ will not condense into a liquid; it stays in a gas phase. So instead of a condenser to turn the CO₂ gas into a liquid, a gas cooler is used to reject heat from the gas so it can absorb heat to keep the refrigeration cycle going. This cycle occurs at very high supercritical pressures – which are over 70 times higher than normal atmospheric pressure at sea level."

Gosselin notes that "in Europe and Canada, transcritical CO₂ systems have been used for years. Supercritical operation is less of an issue because they have the benefit of lower ambient temperatures to stay at subcritical conditions – where CO₂ has outstanding efficiency. But the U.S. has higher ambient temperatures. Under those conditions, CO₂ gas will go transcritical with high gas pressures that affect the design of racks, loops, controllers, valves, and system controls."

Assembling the team for the first store-wide transcritical CO₂ system

To bring all the pieces together for a transcritical CO₂ booster system in their Brooklyn store, Whole Foods hand-selected their team. Tristam Coffin was the grocer’s green-mission specialist for the Northeast Region in 2012, when the partners where chosen. Along with Danfoss, Hillphoenix and Supermarket Technical Services, his team also included BL Companies as the architectural, engineering and HVAC system design firm, Engineering & Refrigeration, Inc. (E&R) as the installing contractor, and e2s Energy Efficiency Services, which is Whole Foods’ national commissioning and energy consulting firm.

"A lot of the technologies used in a transcritical system have been used before," says Coffin. "But we needed partners who could combine them into a game changer for our industry in regards to sustainability."

The store site itself was a game changer as well. As a brownfield site located at 214 Third Street in Brooklyn's Gowanus neighborhood, the land for the 56,000-square-foot supermarket required significant clean-up. When designing the facility, Whole Foods worked with the New York State Energy Research and Development Authority and captured incentives used to make the building 60 percent more energy efficient than required by code. Advanced technologies included a solar energy system from NY-Sun, the state’s solar initiative, and a combined heat and power (CHP) system utilizing a gas fired generator for a portion of the electrical demand with the waste heat combining with two absorption chillers to handle the baseline space heating and cooling.

"We wanted to create a dichotomy on this site between before and after," says Coffin. "I brought partners to the table who could collaborate with our team to make a difference. As you can imagine, we went through several design charrettes for the project. Initially we considered a cascade system with ammonia on the roof and subcritical CO₂ loops in the store.

"But I recognized that Hillphoenix was committed to bringing broad, global expertise in transcritical technologies to the United States. And because we worked with Danfoss on other projects, it was clear they were the global CO₂ leader. So bringing them on board was a no-brainer. They showed our designers how to control the system."