CO2/Ammonia Cascade System Proves Effective in Retail Settings
Supermarkets can future-proof their refrigeration systems using natural alternatives
Ongoing and evolving U.S. Environmental Protection Agency (EPA) and Department of Energy (DOE) regulations have created a complex landscape for supermarket owners. The pressure to reduce emissions and find safe, sustainable, low-global warming potential (GWP) refrigerants has led to the incorporation of natural refrigerants, such as CO2, in supermarket refrigeration systems around the world.
So, when Keith Milligan, financial controller of JTM Corp., decided to build a new Piggly Wiggly grocery store in Columbus, Georgia, he knew he wanted to install a refrigeration system that was both energy-efficient and environmentally friendly. JTM Corp. operates 19 Piggly Wiggly supermarkets in Georgia and Alabama.
Meanwhile, at its headquarters just a few miles down the road from where Milligan was planning to build the new store, Kysor/Warren, a division of Heatcraft Worldwide Refrigeration, was looking to test a new method of refrigeration designed specifically for warmer climates. The new Piggly Wiggly store was the ideal opportunity for Kysor/Warren to collaborate with a retailer to trial the use of a natural CO2/ammonia refrigeration system.
THE SEARCH FOR ALTERNATIVES
“It is important for refrigeration systems to be both energy-efficient and environmentally friendly,” said Michael Lehtinen, marketing director at Kysor/Warren. “In warmer climates, where temperatures can exceed 88°F during the summer months, a CO2 system is forced to operate above its critical point for several hours each day, which can lead to efficiency losses.”
Looking for ways to take advantage of the benefits of CO2 while also addressing the concerns of energy efficiency, engineers at Kysor/Warren started thinking about incorporating another natural refrigerant — ammonia — in a cascade system. Ammonia is an effective and efficient refrigerant and has been widely used in industrial operations, such as refrigerated warehouses. However, it generally has not been considered for use in a retail environment because it is toxic, corrosive, and can be flammable when exposed to heat in enclosed areas. Engineers at Kysor/Warren researched options for using this natural refrigerant to create a superior application for the retail environment and took their idea to Milligan.
“For this Piggly Wiggly store, we proposed two systems: a standard hydrofluorocarbon (HFC) system using R-407A, and a CO2/ammonia cascade system,” said Lehtinen. “We knew the only way we would be able to prove the effectiveness of the ammonia system would be to install two completely redundant systems and alternate between the two on a regular basis, collecting the energy consumption data from both.”
Lehtinen explained that both systems were set up to run CO2 through the retail environment as the main heat transfer component. The difference between the two systems was the second component used to chill the CO2. The system that utilized HFC was a typical installation with both the HFC rack and the CO2 rack located within the same room in the store. The installation for the CO2/ammonia system was similar to the installation for any typical cascade system, but instead of placing both racks in the store, the CO2 rack was placed in the store, and the ammonia rack was placed on the roof.
AMMONIA AS A VIABLE SOLUTION
Where the ammonia would be housed and the amount of ammonia used were both considered when building the new system. Plans called for the relatively small 53-pound charge of ammonia to be contained in just one open area on the roof of the building — far away from the store’s personnel and customers. To prove the system’s safety and that the impact of an ammonia leak would be minimal, Kysor/Warren engineers conducted a plume study. This was performed by creating models to simulate the effects of the concentrated ammonia being dispersed into the air.
“The results showed that even in the event the entire 53-pound charge of ammonia was released into the air, it would dissipate quickly — before reaching the parking lot — causing no harmful effects,” said Lehtinen.
PUTTING THE TECHNOLOGIES TOGETHER
To help create the complex system they planned to pilot at the new Piggly Wiggly store, Kysor/Warren relied on the experience Danfoss had with developing innovative solutions for refrigeration units utilizing HFCs, CO2, and ammonia to provide the components necessary to make the system operational.
“From a technical point of view, Danfoss had the engineering expertise to develop the cascade evaporative condenser and separate valve, one of the key features of our system design,” said Masood Ali, global leader, Heatcraft’s Center of Excellence for Alternative Systems. “To my knowledge, Danfoss is the only manufacturer that could supply this critical component. Plus, we already had a good working relationship with Danfoss, so this was another opportunity to collaborate.”
“We have spent the last 10 years researching and developing innovative CO2 technologies for both transcritical and subcritical refrigeration systems,” said Jim Knudsen, food retail segment manager at Danfoss. “Our experience with more than 7,000 industrial installations around the world helped prepare us for the new challenge presented by Kysor/Warren.”
The engineers at Danfoss started by providing an AK-SM 850 system manager or rack controller. The system manager contains the functionality and tools needed to provide full coverage for the refrigeration control and monitoring application. Serving as the central box or main hub, it is a single viewpoint to access the system, make set point changes, view sensor readings, and see alarms within the store.
“Once the rack controller was in place, we started incorporating specialized controls made with this application in mind,” said Chris Brown, OEM applications engineer at Danfoss. “This project utilized two different models of case controllers — the AK-CC 210 and the AK-CC 550A. In medium-temperature cases, where pumped CO2 was being used, the AK-CC 210 controlled the liquid solenoid, fans, lights, and other secondary features within the case. The AK-CC 550A was used in low-temperature cases to control the same secondary features as well as the DX electronic expansion valve.”
On the rack side, where the compressors and condenser fans are located, Danfoss used an AK-PC 781 rack controller for each of the three refrigerants — CO2, ammonia, and R-407A. These devices were used to stage the compressor, monitor the suction pressures and temperatures, and monitor the discharge pressures and temperatures in that part of the system.
Danfoss incorporated multiple heat exchangers for temperature correlation when going between CO2 and the other refrigerants. Superheat controllers were also included to measure temperatures within the system. An EKD 316 superheat controller was used for the HFC heat exchangers, and an EKC 313 superheat controller was used for the ammonia heat exchangers.
In addition, Danfoss provided controls for monitoring power consumption at the racks and monitoring the power on the fluid cooler and multiple variable-frequency drives (VFDs) throughout the system.
“This new system was more complex than a normal system, so we had to focus on identifying the right product in our broad portfolio of solutions that would be ideal for use with low-GWP refrigerants,” added Brown. “As the project went along, we added features and made adjustments to meet the store’s needs.”
On the ammonia side, Danfoss incorporated an ICF valve station, which includes a stop valve, strainer, solenoid valve, ICF pressure regulation valve, and an ICM motorized valve. In addition, there were a number of SVA-type isolation valves used throughout the system. These valves made it possible to cut the flow of refrigerant into a line, making it easy to service valves or other components.
As construction progressed on the new store, preparations to install the two refrigeration systems began. Although the contractor selected for the task had never worked with CO2 or ammonia — and despite the dual system complexity — both Kysor/Warren and Milligan were pleased with the ability to get the systems installed and ready with relative ease.
“We spent a lot of time with the contractor during the installation process,” said Dale Sizemore, director of technical services at Kysor/Warren. “Although the installation process is comparable to that of a traditional refrigeration system, we did have to follow specific procedures to ensure the multiple systems where piping was contained were equally distributed among the three heat exchangers. We worked with the contractor through the entire process to train the team and help them become comfortable with the components of the new system.”
“Our installer was excited for the opportunity to gain new skills working with this new technology,” added Milligan. “The increase in the number of controls compared to a standard refrigeration system made the installation slightly more complicated, but as questions arose, the installer was able to call Kysor/Warren for additional guidance.”
PROVING THE SYSTEM’S VALUE
Once the store was up and running, Kysor/Warren wasted no time comparing the energy between the ammonia system and the standard HFC system.
“Our energy consumption data is a true account of the energy we’re pulling from the grid,” said Lehtinen. “We’ve been able to put the data from each system side by side to clearly differentiate between the times of day, different days of week, and different months.”
In the first year of operation, the results were clear: The ammonia system consistently outperformed the standard system, using an average of 18 percent less energy than the HFC system.
“One of the measures we value is the life cycle climate performance, or LCCP,” said Robert DelVentura, director of research and development for Heatcraft. “This globally recognized metric is an overall measure of the impact a system has on the indirect component (electrical use) and the direct component (refrigerant leaks). The LCCP of this store is probably the lowest you will see in any store.
“From an environmental perspective, it’s also important to mention the GWP of the ammonia/CO2 cascade system,” DelVentura added. “HFC refrigerants typically have a GWP value of anywhere from 1,300-3,800 per pound, whereas ammonia has zero, and CO2 has 1 per pound. With the ammonia/CO2 cascade system, we’ve reduced the direct GWP by more than 99 percent while, at the same time, improving the energy efficiency of the overall system.”
“With the new store, we saw more than a 28 percent decrease in energy usage when compared to an existing store,” said Milligan. “The energy savings were based on the use of both refrigeration systems as well as the use of energy-efficient LED lighting, skylights for daylight harvesting, occupancy sensors for lighting control, extra insulation, hot water heat reclaim, and a reflective roof. With all of these features, the energy savings we’ve seen have exceeded the projections originally set forth by Kysor/Warren.”
In this side-by-side comparison, ammonia proved to be a viable, environmentally friendly refrigerant option for use in a retail setting alongside CO2.
“Our partnership with Danfoss to discover new ways to expand the use of natural refrigerants and provide a better alternative to the systems available today will have a lasting impact, not only in our industry but also on the environment,” said Lehtinen. “This system has proven to be a sustainable option for the future of retail refrigeration.”
Milligan was even more enthusiastic.
“The use of ammonia in a CO2 cascade system solves one of the biggest problems we faced with our refrigeration systems — operating systems that allow us to future-proof our store against changing regulations and also save energy and, therefore, costs,” he said. “By incorporating two natural refrigerants, the need to alter our system to meet new environmental regulations will no longer be an issue. This is the end game. There is no better solution than this one.”
Information courtesy of Jim Knudsen, food retail segment manager, Danfoss; Chris Brown, OEM applications engineer, Danfoss; and Adam Anderson, applications engineer, Danfoss. They can be contacted via email at firstname.lastname@example.org.