Cascade Systems

A cascade system design typically uses two refrigerants in separate circuits that cascade the load from one lower-temperature circuit to another higher-temperature circuit through a heat exchanger. For example, CO₂ may be used inside a building to handle refrigeration loads, while another refrigerant, such as ammonia, HFC, or HFO, is used outside the building to reject heat to the environment.

Here are some of the pros and cons of these systems:

Pros

• Highly efficient for low-temperature (LT) applications like freezers;
• Keeps CO₂ confined to the low side, avoiding extreme pressures above the critical point, which is very desirable;
• Limits CO₂ to sales floor areas, while the high-temperature (HT) refrigerant stays in the machine room; and
• Reduces the amount of synthetic refrigerant needed if HFC/HFO is used.

Cons

• Requires two refrigerant types, making the system more complex due to multiple layers of controls and the components that must be managed; and
• Higher upfront installation and ongoing maintenance costs.

Booster Systems

A booster system is 100% CO₂ and handles both LT and medium-temperature (MT) loads. Since CO2 LT discharge gas is only compressed to an intermediate pressure, LT compressors discharge into the MT suction line, so the MT compressors can further compress the gas to “boost” it to a usable pressure. The now-boosted-pressure gas can reject heat at the gas cooler. This is a transcritical system that will operate as subcritical or supercritical, depending on the ambient temperature of the gas cooler location.

Here are some of the pros and cons of these systems:

Pros

• Uses only natural refrigerant (CO₂), eliminating HFCs/HFOs, which is better for the environment;
• Simplifies refrigerant management with a single fluid;
• Proven efficiency in cold to moderate climates; and
• Already widely adopted in supermarkets and retail settings.

Cons

• Efficiency drops in hot climates without advanced features (parallel compression, ejectors, subcooling, etc.);
• High operating pressures require specialized components; and
• Control strategies can be more complex than cascade systems.

Secondary Loop Systems

Secondary loop systems use CO₂ as a pumped liquid. A primary refrigeration system (HFC, HFO, ammonia or even CO₂) chills the CO₂ to a subcritical liquid state, which is then pumped through insulated piping to cases or coolers. The CO₂ absorbs heat and returns to a central heat exchanger.

Here are some of the pros and cons of these systems:

Pros

• Very low charge of HFC/HFO or ammonia in the primary loop;
• CO₂ has high cooling capacity, allowing smaller pipes and pumps compared to glycol/brine, offsetting some upfront cost while increasing efficiency; and
• Improved safety on sales floors (low pressure and contained charge).

Cons

• A primary chiller/refrigeration plant is still required;
• Typically, less efficient than direct expansion systems; and
• Risk of dry ice forming if pressures get too low.

Comparison

Each CO₂ refrigeration architecture presents specific advantages and limitations. Cascade systems deliver reliable performance and maintain CO₂ in subcritical operation but require two refrigerants and higher system complexity. Booster systems use only CO₂, offering efficiency and sustainability in cold and moderate climates, though advanced strategies such as parallel compression, ejectors, and subcooling may be needed for efficiency in warm conditions. Secondary loop systems improve safety and minimize primary refrigerant charges but introduce additional pumping energy and lower overall efficiency.

System selection depends on climate conditions, facility type, sustainability objectives, and capital investment. With ongoing advances in CO₂ technology, booster systems are gaining wider adoption, while cascade and secondary loop designs continue to serve niche and application-specific roles. Together, these three CO₂ strategies form a critical foundation for understanding CO₂ architecture — an increasingly vital component as the industry accelerates its shift toward sustainable refrigeration solutions.