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Advanced Refrigeration

Most of the 35,000+ supermarkets in the United States use centralized direct expansion (DX) 

systems to chill their products. Typically, these refrigeration systems are charged with 3000 – 4000 

pounds of refrigerant and can leak in excess of twenty percent of their charge each year. Commonly 

used refrigerants include ozone-depleting hydrochlorofluorocarbon (HCFC) refrigerants, often 

HCFC-22, and blends consisting entirely or primarily of hydrofluorocarbons (HFCs), both of which 

are potent greenhouse gases. Fortunately, in recent years there have been several advancements in 

refrigeration technology that can help food retailers reduce both refrigerant charges and refrigerant 

emissions.

 Below is a general description of a centralized DX system as well as an overview of the different 

advanced refrigeration options.

On this page:

• Centralized DX System

• Distributed System

• Secondary Loop System

• Cascade System

• Low GWP Refrigerants

Centralized Direct Expansion System

At least 70 percent of stores in the United States use centralized DX systems to cool their display 

cases and walk-ins. In a DX system, the compressors are mounted together and share suction and 

discharge refrigeration lines that run throughout the store, feeding refrigerant to the cases and 

coolers. The compressors are located in a separate machine room, either in the back of the store or 

on its roof, to reduce noise and prevent customer access, while the condensers are usually aircooled and hence are placed outside to reject heat.

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The multiple compressor racks operate at various suction pressures to support display cases 

operating at different temperatures. The hot gas from the compressors is piped to the condenser and 

converted to liquid. The liquid refrigerant is then piped to the receiver and distributed to the cases 

and coolers by the liquid manifold. After cycling through the cases, the refrigerant returns to the 

suction manifold and the compressors.

Supermarkets tend to have one DX system for low-temperature refrigeration (e.g., ice cream, 

frozen foods, etc.) and one or two DX systems for medium-temperature refrigeration (e.g., meat, 

prepared foods, dairy, refrigerated drinks, etc.).

Schematic of a Centralized DX System

Source: International Energy Agency (IEA). 2003. "IEA Annex 26: Advanced Supermarket 

Refrigeration/Heat Recovery Systems, Final Report Volume 1-Executive Summary."

Distributed System

Unlike centralized direct expansion refrigeration systems, distributed systems use multiple smaller 

units that are located close to the display cases that they serve. For instance, compressors in a 

distributed system may be located on the roof above the cases, behind a nearby wall, or even on top 

of or next to the case in the sales area. The close proximity of the compressors to the cases and 

coolers allows the system to use considerably less piping and additionally use a smaller refrigerant 

charge than traditional DX systems. This reduction in charge often results in a decrease in total 

refrigerant emissions.

Schematic of a Distributed System

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Source: International Energy Agency (IEA). 2003. "IEA Annex 26: Advanced Supermarket 

Refrigeration/Heat Recovery Systems, Final Report Volume 1-Executive Summary."

Secondary Loop System

Secondary loop systems use a much smaller refrigerant charge than traditional direct expansion 

refrigeration systems, and hence have significantly decreased total refrigerant emissions. In 

secondary loop systems, two liquids are used; a primary refrigerant and a secondary fluid. The 

secondary fluid is cooled by the primary refrigerant in the machine room and then pumped 

throughout the store to remove heat from the display equipment. Secondary loop systems typically 

ate with two to four chiller systems depending on the temperatures needed for the display cases.

Schematic of a Secondary Loop System

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Source: Southern California Edison and Foster-Miller, Inc. 2004. "Investigation of Secondary Loop 

Supermarket Refrigeration Systems." Report prepared for California Energy Commission, Public 

Interest Energy Research Program.

Cascade System

Cascade systems consist of two independent refrigeration systems that share a common cascade 

heat exchanger. Each system of a cascade system uses a different refrigerant that is most suitable 

for the given temperature range. High temperature system uses high boiling point refrigerants such 

as R-404A, R-507A, R-134A, propane, butane, and ammonia, whereas low temperature systems 

use low boiling refrigerants such as R-744 (CO2) and R-508B. The advantages of a cascade system 

include a reduction in the refrigerant charge and a reduced carbon footprint.

Low-GWP Refrigerants

Similar to advanced refrigeration technologies, low-GWP refrigerants can also reduce the climate 

impacts of supermarket refrigeration systems. Refrigerants such as carbon dioxide (CO2), ammonia, 

hydrocarbons, and hydrofluoro-olefins (HFOs) have potential to be used in commercial 

refrigeration systems in the U.S. market. Currently only CO2 is being used as a primary refrigerant 

in commercial refrigeration system applications in the United States. Refrigeration systems that use 

CO2 as a primary refrigerant are commonly referred to as transcritical CO2 systems. Transcritical 

CO2

refrigeration is a type of refrigeration cycle in which CO2is the sole refrigerant, evaporating in 

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the subcritical region and rejecting heat at temperatures above the critical point in a gas cooler 

instead of a condenser. For more information on acceptable low-GWP refrigerants, see Acceptable 

Substitutes in Retail Food Refrigeration.

Last updated on March 16, 2014

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