Case Study Contributed by shecco
Whole Foods - Propane/CO2 Cascade System
Technology: Centralised cascade refrigeration system
Refrigerant: Propane and CO2 – Global Warming Potential (GWP): 3 and 1 respectively
Performance/Energy Efficiency: Not calculated yet
Costs: Upfront cost of about 50% more than that of a conventional HFC Direct Expansion (DX) system
In August 2016, Whole Foods opened a 49,000-square-foot store in Santa Clara, California that is the first in the U.S. to employ a propane/CO2 cascade refrigeration system.
The system contains 265 lbs. of propane, spread across seven separate chiller units on the roof, including three for low temperature and four for medium-temperature applications. Whole Foods applied for and received permission from the EPA to test market the system in this store.
The propane at the Santa Clara store, which never leaves the roof, is used strictly to condense its refrigeration partner, CO2 (about 1,700 lbs. in total), which is piped up from a rack directly below, inside the building; the liquid CO2 is then piped down to DX evaporators in low-temperature cases, and pumped to liquid-overfeed coils in medium-temperature cases. “It’s very clean, sharp design,” said Coffin.
The system’s structure is not unlike some other CO2 cascade systems Whole Foods is testing except that it uses propane on the “high side” rather than NH3 or HFCs, and the propane is spread across multiple chilling units. The capacity for the medium temperature CO2 equipment is 592,000 Btuh, and 158,300 British thermal units (Btuh) for the low temperature CO2 equipment. The system also includes four propane units with a medium temperature capacity of 700,717 Btuh total and the three propane units with a low temperature capacity of 230,524 Btuh total.
“There is precedent for this type of system, but this is the first installation of the technology in the U.S.”
— Tristam Coffin, Director of Sustainability and Facilities for Whole Foods’ Northern California division
The propane charge in each medium-temperature module is between 40 and 45 lbs, while the propane charge in each low-temperature module is about 35 pounds (lbs). The CO2 charge is of 1,730 lbs. Each propane module is equipped with a reciprocating compressor, while the CO2 rack has 3 low-temperature reciprocating compressors. Each module includes a SWEP heat exchanger, as well as condenser fans.
The system also includes Micro Thermo controls, and a heat reclaim system for domestic water and space heating. There is also a gas defrost system for low temperature.
Like its other natural refrigerant ventures, the propane/CO2 cascade project represents another chapter in Whole Foods’ relentless quest for real-world understanding of natural refrigerant technology. “We know there’s no silver bullet for natural refrigerant systems,” said Coffin. “We need different choices for climate zones and building types so that we can make an educated decision rather than rely on outside lab data that may or may not be applicable to the actual operating environment in our stores. That’s my goal for the company and my personal goal.”
Though propane is safely used in a host of everyday applications in larger quantities than those allowed for refrigeration, the flammability of the gas remains a concern.Thus, Whole Foods has taken special pains to ensure the safety of its cascade system, especially given its relatively large charge.
For example, the chain has set up a “very comprehensive leak detection system” for the seven rooftop propane modules that has several leak sensors on each unit as well as on other “points of possible ignition,” including a generator and two heating, ventilation and air-conditioning (HVAC) units, explained Coffin.
The propane system and other equipment on the roof have also been designed to shut down in the unlikely event that a propane leak comes within 25% of the LEL, removing any source of ignition. “We are very confident that there would never be any potential for a [flammability incident] because of the R290,” he said.
What may help propane be accepted as a safe refrigerant is that the gas and other hydrocarbons have been routinely used for other applications for many generations. For example, on the roof of the Santa Clara store sits a generator tied to 200 lbs of propane, and the building has numerous natural gas lines for heating and cooking.
“So it’s a matter of helping folks understand that, yes, this is flammable A3 refrigerant, but we have flammable gases all around the building,” said Coffin. “We just have to describe the refrigeration application, how it works and what measures we have implemented to ensure occupants of the building are going to be safe.”
As part of its analysis of the propane/CO2 cascade system, Whole Foods will be comparing it to other CO2 cascade systems it uses that employ an HFC or ammonia on the high side. “Our expectation is that it is going to be more efficient than those systems,” Coffin said. He also expects the propane system to be “in line with, if not more efficient than, a transcritical system without any [energy-improving] bells and whistles.”
Whole Foods is hoping that the efficiency of the propane cascade system will be similar to that of a standard Direct Expansion (DX) hydrofluorocarbon (HFC) system. “But even if the energy impact is a little more, we’ll settle for that because the leak impact is going to be that much less,” Coffin said.
The chain considered Total Equivalent Warming Impact (TEWI), which combines direct greenhouse gas emissions from leaks with indirect emissions from electricity usage, “really the most important thing”. For all evaluations, he is seeking at least one year of data to account for ambient temperature swings before coming to any conclusions.
Another energy-saving factor is the heat reclaim achieved with the propane system; waste heat captured by glycol is used to pre-heat domestic water and to supplement space heating, per Title 24 of the California Code of Regulations.
Total Cost of Ownership
The first cost of the propane/CO2 (as well as Whole Food’s NH3/CO2 system) was about 50% more than that of a conventional HFC DX system, and 20% more than the cost of some of the chain’s early transcritical systems, Coffin said. The marked uptick in adoption of transcritical technology, along with the greater number of suppliers, has driven down the cost of those systems significantly, but the relative newness of propane and ammonia cascade systems means their costs are still high.
“We went into this knowing that we would pay a premium for a propane system, but also knowing that if the system works – and of course, we get SNAP approval – this could be a viable option for the future, and you would expect prices to drop as they have with transcritical,” said Coffin.
One of Coffin’s earlier concerns about natural refrigerant systems was the cost of installations, which were driven higher by contractors who weren’t familiar with the systems and wanted to budget in extra time and resources. More recently, installations costs have begun to fall as contractors have become “more comfortable” with the technology, realizing “it’s just another refrigerant and most of the equipment used is almost identical” with traditional models, he said