European distribution centers are increasingly opting for transcritical CO2 (R744) refrigeration systems for reliability, efficiency, ease of installment and safety, said Anders Mønsted, Business Development Manager for Advansor’s industry segment.

According to Mønsted, Danish OEM Advansor has produced 14,000 transcritical CO2 systems for commercial applications since 2006. The company has also built approximately 100 CO2 racks for 30 European distribution centers in the past three years. 

“It’s a small number, but these are much bigger systems based on the same [retail] design,” he noted.

Mønsted discussed refrigeration trends in European distribution centers in a presentation at the ATMOsphere (ATMO) Europe Summit 2023. Roy Arends, a Refrigeration Design Engineer at Dutch contractor Veld Koeltechniek, also participated in the presentation and provided information about an industrial installation. The event, held in Brussels last September, was organized by ATMOsphere, publisher of R744.com.

According to Mønsted, the “robust” nature of transcritical CO2 compressors in retail use shows their reliability. “They can run 100,000 hours without maintenance,” he said.

When it comes to the efficiency of a distribution center, CO2 decreases overall facility energy use with heat recovery, providing free heating, Mønsted continued. A CO2 system provides a facility with complete climate solutions.

“We can do cooling, freezing, air-conditioning and heating with one system,” said Mønsted. “There may be more initial work in the design to get the most out of a system, but supplying full solutions is a big plus for CO2.”

Technological advances that improve the efficiency of transcritical CO2 refrigeration mentioned by Mønsted included parallel compression, permanent magnet and ultra-low superheat, with or without ejectors. 

Mønsted shared data showing that adding just parallel compression to a standard CO2 booster system reduced energy consumption by 11% in a distribution center in Holland. However, he indicated that the biggest energy savings for distribution centers come from running part load. “You can have many compressors in a CO2 rack standing idle, making the part-load efficiency as good as the full-load efficiency,” he said. 

Choosing CO2 over ammonia

Working with Advansor, Veld Koeltechniek helped build and install a transcritical CO2 system for a European distribution center owned by an international supermarket chain.  

The chain required a natural refrigerant solution using CO2 or ammonia (R717) for a total area of 5,000m2 (53,820ft2) with 48,000m3 (1,695,104ft3) of cold storage. Refrigeration capacity requirements included 335kW (95.3TR) of 2°C (35.6°F) cold room storage, 40kW (11.4TR) of 0‒2°C (32‒35.6°F) for meat storage and two freezing rooms, each with 20kW (5.7TR) at −20°C (−4°F) for gel packs.

According to Arends, a CO2 system was chosen over ammonia for the following reasons:

  • CO2 has “far fewer” safety requirements.
  • CO2 maintains temperature better over large pipe distances.
  • Ammonia requires a larger capital investment.

Ammonia systems require APTEX (atmospheric explosive) gas detection, ventilation, machinery room fire protection, a separate room for control cabinets due to copper components and approval from authorities.

Large pipe distances in distribution centers create large pressure drops. In CO2 systems, one bar of pressure drop equates to losing roughly 1K (1°C/1.8°F) in temperature.

“For ammonia, the drop in Kelvin is much higher,” said Arends.

Due to the different system requirements, Arends said that an ammonia system would cost €100,000‒€250,000 ($108,305‒$270,763) more than a transcritical CO2 one.

Veld met the chain’s redundancy requirements with two separate Advansor CO2 direct expansion racks, each carrying 75% of the calculated cooling load, with multiple compressors on each rack and at least two fans per evaporator. “The cold room and the freezing room evaporation interconnect, with both racks supplied with parallel compression,” said Arends.

The system was built and installed in 2022.

Each rack supplies the system with 250kW (71.1TR) at −8°C (17.6°F) for cooling and 30kW (8.5TR) at −30°C (−22°F) for freezing. The system also uses a gas cooler rated at 84bar (1,218psi) with a 470kW (133.6TR) capacity and a 35‒45°C (95‒113°F) outlet temperature. Arends indicated that roughly 400kW (113.7TR) of recovered heat is available for offices and floor heating in the freezing rooms.

While reflecting on his career, Mønsted said, “After 30 years of advances, we see 140bar (2,031psi) components for industrial installations. It’s a great achievement and a milestone for our industry that we can do full transcritical for industrial systems.”

At the ATMOsphere (ATMO) LATAM Summit 2023 in Mexico City in November, Advansor reported installing its 48th industrial transcritical CO2 system in Latin America.

“It’s a great achievement and a milestone for our industry that we can do full transcritical [CO2] for industrial systems.”

Anders Mønsted, Business Development Manager for Advansor’s industry segment