CO2 Refrigeration Fundamentals: Energy Efficiency Strategies

Mar 08, 22 | Cold Chain

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Welcome to the final installment of our CO2 Refrigeration Fundamentals blog series. So far, we’ve examined R-744’s unique properties, servicing tips, system operation and design strategies— all of which correspond with topics in our CO2 Chats educational videos. For our final blog, we’ll explore strategies for achieving CO2 transcritical booster system energy efficiencies in warm climates.

CO2 Refrigeration Fundamentals

How do you achieve energy efficiency in a CO2 system?

The energy consumption of a refrigeration system is a key factor when evaluating its total cost of ownership (TCO). For a CO2 transcritical booster system, energy efficiency is dependent on many factors, including:

    • The ambient temperature range
    • The humidity of the region
    • The availability of and cost of water
    • The cost of peak demand charges

Among these factors, the goal of improving system energy efficiencies in warmer climates has become a major focus area for equipment manufacturers. Leading high ambient strategies include:

    • Adiabatic gas cooler
    • Parallel compression
    • Mechanical sub-cooling
    • Zero superheat control of medium-temperature (MT) evaporators
    • Ejector controls

Retailers must consider all of these variables and strategies when they work with their equipment suppliers and design engineers to specify a CO2 transcritical booster system — which also includes ensuring that qualified service technicians are available in their region to support CO2 refrigeration.

What is an adiabatic gas cooler used in CO2 systems?

An adiabatic gas cooler is very similar to a dry gas cooler, except that it uses adiabatic pre-cooling pads outside the condenser coils. When the ambient temperature reaches about 72 °F on a CO2 transcritical booster system, a water solenoid valve is energized, causing water to be sprayed along the top of the adiabatic pads. As the water trickles down the pads, a condenser pulls air through these wetted pads, causing moist, cooler air to hit the coils. In turn, the condenser reacts to this cooler air and drops the temperature and pressure, making the system significantly more energy-efficient by operating at lower pressures.

What is parallel compression in a CO2 system?

In a CO2 transcritical booster system, parallel compression refers to the practice of adding a separate suction group to the system. This could be confusing to service technicians, who may think we’re simply referring to a parallel rack used on a traditional hydrofluorocarbon (HFC)-based system.

The concept is relatively simple. A “parallel” compressor is added to the MT suction group, which essentially provides a separate suction group to the system. Thus, instead of the bypass gas circulating from the flash tank to the MT suction as in a standard transcritical booster system, the parallel compressor suction group compresses excess flash gas and circulates it to the gas cooler.

This allows the parallel compressor to operate at a suction pressure of about 550 psi (the same as the flash tank), instead of the MT suction of 425 psi. The net effect of leveraging the higher suction pressure is achieving higher compressor capacity for less effort, which translates into a lower heat of compression and reduces energy consumption. As a result, parallel compression is considered a leading high ambient strategy. When annualized in a typical environment, it can potentially save up to 10% in energy costs. Further, parallel compression can be used in combination with an adiabatic gas cooler to achieve additional energy efficiencies.

Thank you for following our CO2 Refrigeration Fundamentals blog series. To learn more about any topic discussed herein, please view the companion installments in our CO2 Chats video series. For more information about Emerson’s comprehensive CO2 products and capabilities, please visit