CO2 System Architectures: Opportunities in Food Retail

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Food retailers seeking to make the move to CO₂ refrigeration have two primary system types from which to choose: CO₂ transcritical booster and cascade systems. A closer look at each option may help you decide which is a better fit for your operations.

CO₂ transcritical booster

A complete R-744 system, CO₂ transcritical booster utilizes direct expansion (DX) for low- and medium-temperature suction groups. This system is called transcritical because it is designed to operate at pressures above CO₂’s critical pressure (1,500 psig). Heat produced from low-temperature case compressors is rejected into the medium-temperature suction group’s compressors. The medium-temperature compressors are the workhorses of the system and must be sized to handle:

• • The total heat of rejection of low-temperature loads
  • 100 percent of the medium-temperature load
  • The flash tank bypass load

Only one condenser or gas cooler is needed for all low- and medium-temperature cases. CO₂ pumped technology — where CO₂ is used as a secondary fluid — is also available on both low- and medium-temperature stages.

CO₂ cascade

CO₂ cascade systems utilize two distinct refrigeration circuits: a CO₂ circuit for the low-temperature suction group, and an HFC-based circuit (such as HFC-134a) for the medium-temperature needs. It’s called cascade because the heat produced from the low-temperature circuit is discharged into the suction stage of the medium-temperature circuit via an intermediate heat exchanger. Medium-temperature compressors send gas to an air-cooled condenser on the roof. Like a standard refrigerant, CO₂ is maintained below its critical point (or subcritical mode) of 88 °F.

For an all-natural cascade alternative, some retailers have even experimented with using NH₃ (ammonia) as the medium-temperature refrigerant. These low-charge ammonia circuits are typically housed on the facility roof, far removed from the store’s occupied spaces.

Evolution of electronics

Unlike traditional HFC systems, CO₂ system architectures introduce the requirement for additional electronic components, including: case controllers, pressure transducers, temperature sensors and electronic expansion valves. While these components may contribute to increased system costs, they’re necessary for optimizing the refrigerant quality and pressures to the cases. From an end user and servicing perspective, these case controllers provide quick access to precise temperature controls and ongoing, optimized energy efficiencies.

The benefits of going green

For those U.S. retailers who have experimented with CO₂ refrigeration, the benefits are obvious. New Seasons is a Northwestern grocer whose first CO₂ system earned a GreenChill Platinum Certification award for green refrigeration. Their CO₂ transcritical booster system delivered the following improvements:

• • Up to 30 percent lower total equivalent warming impacts (TEWI)
  • 95 percent fewer refrigerant emissions
  • Smaller refrigeration footprint

The retailer is currently planning additional CO₂ installations.

1. 1. http://www.atmo.org/presentations/files/5907fb91732731493695377MIjnG.pdf, pg. 3

Read the full Accelerate America article on CO₂ system architectures [pg.16].