Wisconsin-based retailer Kwik Trip has become one of the first convenience store (c-store) chains in the U.S. to use transcritical CO2 (R744) refrigeration technology after installing a CO2 rack at its news 836m2 (9,000ft2) store in Dayton, Minnesota.

Having now proven the concept, the chain has committed to transcritical CO2 in all of its new and remodeled stores from July 2024. The chain will conduct its first retrofit with CO2 this winter.

Details of Kwik Trip’s plans were shared by its Senior HVAC/R Engineer, Carl Klemp, during yesterday’s End User Panel at the ATMOsphere (ATMO) America Summit 2023 on natural refrigerants. The conference is currently taking place in Washington D.C. and is organized by ATMOsphere, publisher of R744.com.

Klemp presented Kwik Trip’s Dayton case study alongside project partners Bruce Hierlmeier, Director of Regulatory Compliance and Refrigeration Technology at Zero Zone, and Brent Cheshire, Manager of End User Services at Copeland (formerly Emerson Climate Technology).

Transcritical CO2 racks for c-stores

The Midwest brand has over 800 stores across Illinois, Iowa, Michigan, Minnesota, South Dakota and Wisconsin.

Kwik Trip’s stores vary in size, ranging from 372–1,394m2 (4,000–15,000ft2), and have traditionally used HFC-based refrigeration systems.

According to Klemp, 554 of the chain’s 800 sites have parallel racks, only two of which are now transcritical CO2. The remaining stores use condenser refrigeration units, which is more common for this type of application.

Kwik Trip first introduced refrigeration racks to its brand back in 2009. It then started discussing CO2-based systems a few years later in 2013 due to the changing regulation of refrigerants and a growing interest in sustainability.

“At the time, we were told that [CO2] wasn’t the right fit for a small format, and we also got priced out,” said Klemp.

However, as the cost for transcritical CO2 came down and Kwik Trip introduced a larger store concept that placed greater emphasis on refrigeration, it became a viable option for the retailer.

“We now meet the threshold of a smaller transcritical CO2 booster,” he added.

“At the time, we were told that [CO2] wasn’t the right fit for a small format and we also got priced out. [However,] we now meet the threshold of a smaller transcritical CO2 booster.”

Carl Klemp, Kwik Trip

Kwik Trip partnered with local OEM Zero Zone and local contractor St. Cloud Refrigeration (SCR) for the project, and in October 2022, the CO2 rack was commissioned at its new store in Dayton.

Familiarity of components

With transcritical CO2 not being commonly used in c-stores, Klemp said Kwik Trip wanted the new system to be as close to a traditional HFC-based system as possible. It needed to be simple to use and service to ensure an easy transition to the technology.

The company opted for Copeland compressors and controls due to familiarity and performance, he explained.

The system includes three reciprocating semi-hermetic transcritical CO2 compressors for medium temperature (MT), one of which is variable-speed. Low-temperature demand is served by two subcritical CO2 scroll compressors.

In addition to the variable-speed MT compressor, the system includes a split condenser and digital scroll on the booster side to optimize load management during winter months when load can vary significantly.

The system also uses heat reclaim for domestic hot water production and space heating.

To help technicians easily oversee and manage the entire system, Copeland’s E3 supervisory control was included.

Proof of concept

While Kwik Trip has encountered some challenges with its first transcritical CO2 installation, such as higher upfront costs and refrigerant supply issues, Klemp said that it has largely been a positive and beneficial experience.

In addition to being future-proof to regulatory changes, the CO2 rack provides hotter water – up to 38°C (100°F) in the winter and 60°C (140°F) in the summer – from the heat reclaim system compared to an HFC alternative. According to a case study on the installation, this has helped to reduce the store’s energy consumption and costs and contribute to the brand’s sustainability targets.

With the new E3 control system, the store is able to achieve more precise temperature control than with a traditional HFC rack.

To further assess the benefits of transcritical CO2 in its c-stores, Kwik Trip is partnering with the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) to compare the energy efficiency of its new system with an existing HFC system in a nearby store. The chain will also be evaluating the need for adiabatic gas coolers for its more southern locations.

According to Klemp, Kwik Trip will be rolling out a CO2 technician training program with Zero Zone.