CO₂ as a Refrigerant – Introduction to Retail Cascade Systems

In post 11 of our CO₂ as a Refrigerant blog series, I provide an introduction to CO₂ cascade systems used in commercial refrigeration systems.

A CO₂ cascade system is comprised of two distinct stages and systems:

1. A low stage provides the cooling using R-744, always operating in subcritical mode of operation.
2. A high stage absorbs heat from the condensing R-744 at the cascade heat exchanger, leveraging a lower-global warming potential (GWP) A1 or A2L refrigerant in a more traditional system architecture.

A CO₂ cascade system offers an alternate architecture for retailers who want to deploy a low-GWP option but may not want to deal with the complexities of CO₂ supercritical operation. In a CO₂ cascade system, the high, medium-temperature (MT) and low-temperature (LT) stages are completely independent of each other, except for a heat exchanger that connects them.

Typically, the high stage of a supermarket system would use a lower-GWP, medium-pressure HFC refrigerant such as R-513A which referred to as a “hybrid” system. In the future, this may also include lower-flammability A2L alternatives. Not only does the high stage serve the MT loads, but it is also used to condense the CO₂ in the low stage. In the low stage, CO₂ is generally discharged into a plate heat exchanger acting as a condenser; R-744 is condensed and then recirculated for LT loads. The simple high-stage architecture typically employs a close-coupled system controlled by the pressure in the low-stage receiver.

R-744 is a very effective refrigerant for LT loads; and with a GWP of 1, it contributes to a cascade architecture that could meet many retailers’ sustainability objectives. Within the cascade heat exchanger, the evaporating high-stage refrigerant absorbs heat rejected by the condensing R-744. The condensing temperature is always maintained below the critical point.

In the final post of this CO₂ as a Refrigerant blog series, I’ll explore the advantages and disadvantages of the systems examined thus far and review system selection best practices.