Case Study contributed by shecco
Piggly Wiggly - Ammonia/CO2 Cascade System
Technology: Cascade refrigeration system
Refrigerant: Ammonia (low-charge) and CO2 – Global Warming Potential (GWP): 0 (both refrigerants)
Performance/Energy Efficiency: 28.5% energy savings over a seven-month period compared with an HFC-407 system
Costs: Higher upfront cost compared to a traditional HFC-based system.
Piggly Wiggly opened a 36,000-square-foot store in Columbus, Georgia in September 2015 using an NH3/CO2 refrigeration system, the fourth in the U.S. to use such technology at that time.
The Piggly Wiggly store uses an ultra-low charge (53 pounds) –– less than any of the other NH3/CO2 stores – which is confined to the roof in the ammonia rack. The CO2 charge is of 1,400 lbs.
The ammonia condenses the CO2, which circulates throughout the store; cooling low-temperature cases via direct expansion and medium-temperature through pumped liquid overfeed. For energy comparison purposes, an HFC (R407A) rack alternates every few weeks with the ammonia rack in condensing the CO2.
One of the unique features of the NH3/CO2 system is its automatic oil recovery system.
Because it is immiscible in ammonia, oil does not suction back to the compressor from the case evaporators as it does in a conventional HFC system. Normally, that means a technician must periodically recover the oil from the evaporators, but the oil recovery system eliminates that task. In fact, the oil never reaches the evaporators, as it is captured in the receiver and channelled back to the compressors.
The NH3/CO2 system has a refrigeration capacity for medium temperature cases of 462.1 thousand of international British thermal units (IT) per hour (MHB) (38.5 Tons of Refrigeration (TR)), and 221.5 MHB (18.5 TR) for low temperature cases. The total capacity of the ammonia cycle is of 70 TR.
The NH3 rack consists of 3 open-drive reciprocating compressors, and the CO2 rack consists of two reciprocating compressors.
The system also includes an automatic oil recovery system, a water diffusion tank and stratus display cases with electronic expansion valves. It also has case and rack controls, a BAC evaporative fluid cooler and variable frequency drives.
A heat reclaim system for hot water has also been added. Defrost at low temperature is electric, while defrost at medium temperature is off-cycle.
The retailer has recorded energy savings averaging 28.5% over a seven-month period in its Georgia store, thanks mainly to its NH3/CO2 refrigeration system, according to Keith Milligan, CIO for the JTM Corp., Phoenix City, Alabama, a U.S. family-owned retailer that operates 19 Piggly Wiggly stores along the border separating central Alabama and Georgia.
When Keith Milligan received the first utility bill for his Columbus store, he was flabbergasted at how low it was. “I called the lady at the power company [Georgia Power] and said, ‘I just want to make sure you got this right,’” said Milligan. “I didn’t want her to come back in two years and say, ‘I billed you wrong and you owe us $200,000.’ But she checked it, and it was correct,” Milligan added.
The retailer’s data comes from a comparison between the Columbus store’s power consumption and an R407A Piggly Wiggly store in La Grange, Georgia. For the period ranging from October 2015 to April 2015, the new Piggly Wiggly store consumed 23% to 33% less energy than the conventional outlet.
The new store was equipped with other energy-saving elements, including LED lights, skylights, occupancy and daylight controls, doors on display cases, and heat reclaim for hot water. But the NH3/CO2 system, accounting for 60% of the store’s electricity consumption, was by far the most impactful on efficiency.
Milligan said he considered installing a transcritical CO2 system instead of the NH3/CO2 system, but decided not to because of central Georgia’s warm climate. “The [transcritical] technology is improving [for warm climates] but at the time it would have increased my power bill.” (Nb: Recent technology developments allow CO2 technology to run efficiently even in climate up to 113 °F, but come with a higher investment cost).
By contrast, ammonia systems are unaffected by high ambient temperatures. “The ammonia system has worked very well,” Milligan said. “Ammonia has been around forever as a refrigerant, though not in such tiny quantities.”
Ali added that in warmer climates, the NH3/CO2 system offers “the biggest bang for the buck”.
Ammonia poses its own challenges, notably its toxicity in certain concentrations and its pungent odor. “I asked a lot of questions about that,” said Milligan. “But because there is such a small amount [53 lbs.], I didn’t see much danger. I’m very comfortable with it.” The Piggly Wiggly distribution centre in Bessemer, Ala., he noted, uses thousands of pounds of ammonia.
In the first year of operation, there have been no ammonia leaks. If there was a leak, “ammonia is lighter than air, and it’s above the store,” said Milligan. “So it stays outside.” In the event of a cold day with temperature inversion, leaked ammonia could move toward the ground, but “normally it would rise,” he noted. And with its self-alarming odor, “ammonia lets you know you have a leak and you have to leave the area. So I don’t think there’s any danger at all for our techs.”
The ammonia system, which is designed in conformance with all major ammonia safety standards (IIAR-2, IBC, IMC, IFC, EPA and ASHRAE-15), easily passed muster with the local fire marshal. Because of the small ammonia charge (under 100 lbs.), the store does not fall under the Emergency Planning and Community Right to Know Act’s reporting requirements, nor does it require an on-site engineer to monitor it.
The system was also designed to mitigate against any ammonia exposure. The rack is enclosed by removable panels; in the piping, valves use seal caps, and as a safety precaution in abnormal high pressure situations the pressure release valves would discharge ammonia into a water diffusion tank rather than the atmosphere, so nobody can smell it.
Beyond its impact on energy consumption, the NH3/CO2 system offers significant reduction in direct greenhouse gas emissions stemming from refrigerant leaks. That’s due to the virtually non-existent combined GWP of ammonia (zero) and CO2 (one).
For that reason, the Piggly Wiggly store received the highest certification level – Platinum – from the US Environmental Protection Agency’s GreenChill Partnership.
The environmental benefits of his Columbus store are aligned with Milligan’s values as well: “We wish all of our stores were like that,” he said. “I have grandkids and I want to leave a good place for them. As a nation and a world, we are making progress, but we still have a long way to go.”
Miligan would like to see more supermarkets follow his lead. “Every store you change makes a big difference.” To that end, he invites other retailers, including competitors, to tour his Columbus store.
Milligan intends to use an NH3/CO2 system in future stores. As for existing outlets, he is hoping manufacturers will come up with a retrofit solution that encompasses low-GWP refrigerants.