A CO₂ refrigeration system is a type of cooling system that uses carbon dioxide (CO₂), also known as R-744, as the refrigerant. It serves the same function as conventional refrigeration systems — transferring heat from one place to another — but does so using CO₂ instead of synthetic refrigerants like HFCs or HCFCs.
CO₂ has a Global Warming Potential (GWP) of 1, making it one of the least environmentally damaging options currently available. It is classified as a safety group A1 refrigerant, meaning it is non-toxic and non-flammable under normal operating conditions. These characteristics allow CO₂ systems to be used safely in public and commercial spaces, including supermarkets and food processing areas.
Compared to traditional systems, CO₂ refrigeration operates at significantly higher pressures, which influences both the system design and component selection. The technology has evolved over the years to accommodate these requirements, resulting in systems that are reliable, efficient, and increasingly adopted in both commercial and industrial applications.
CO₂ systems can be used in both subcritical and transcritical modes, depending on the ambient temperature and system configuration. In subcritical mode, CO₂ behaves like a conventional refrigerant. In transcritical mode, the system operates above the critical point of CO₂, and specific design adaptations are required. These modes are discussed further in the section on system types.
How does a CO₂ refrigeration system work
A CO₂ refrigeration system follows the same basic thermodynamic cycle as other vapour-compression systems but operates under much higher pressures. The process involves compressing, cooling, expanding, and evaporating the refrigerant to move heat from one place to another. Below is a simplified explanation of how a transcritical CO₂ booster system works, which is one of the most common designs in commercial settings.
Compression
The cycle begins at the compressor. Low-pressure CO₂ vapour enters the compressor, where it is compressed to a much higher pressure and temperature. In booster systems, there are typically two groups of compressors:
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Medium-temperature (MT) compressors handle loads around -10 °C (approx. 28 bar).
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Low-temperature (LT) compressors handle loads around -32 °C (approx. 13 bar).
These compressors work together to manage both cooling and freezing demands.
Gas cooling / condensation
After compression, the high-pressure CO₂ enters the gas cooler. This component releases the heat absorbed during evaporation. If the ambient temperature is below the critical point of CO₂ (31 °C), the system operates in subcritical mode and the gas condenses. If the ambient temperature is higher, the system enters transcritical mode, and the gas cooler acts to reduce the temperature without condensing the refrigerant.
Pressure regulation and expansion
Next, the refrigerant passes through several valves that control pressure and prepare the fluid for evaporation:
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The high-pressure valve reduces pressure from the gas cooler to the receiver.
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The flash gas bypass valve helps manage pressure at the receiver.
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The MT and LT expansion valves control the pressure drop before the refrigerant enters the respective evaporators.
This step produces a mixture of cold liquid and vapour, which is essential for effective heat absorption in the next stage.
Evaporation
The refrigerant enters the evaporator coils, where it absorbs heat from the surrounding area. As it evaporates, it turns back into low-pressure vapour. MT evaporators handle chilled applications, while LT evaporators are used for freezing. The vapour then returns to the compressors, and the cycle repeats.
This entire process happens in a closed loop and allows for precise temperature control across multiple applications. Proper system design is essential to handle the high operating pressures and ensure stable, efficient operation.
Key characteristics of CO₂ as a refrigerant
CO₂ (R-744) has several physical and chemical properties that set it apart from conventional refrigerants. Understanding these characteristics is essential when deciding whether this type of system suits your application.
Low global warming potential
CO₂ has a GWP of 1, which is the reference value for all other refrigerants. Unlike HFCs and HCFCs, it does not contribute significantly to climate change when released into the atmosphere. This makes it a long-term, regulation-proof option.
High operating pressure
CO₂ systems typically operate at much higher pressures than traditional systems. For example, discharge pressures can exceed 90 bar in transcritical mode. This requires specialised components, reinforced pipework, and proper safety measures. System designers must account for this in both component selection and installation practices.
Non-toxic and non-flammable
R-744 is categorised as an A1 safety class refrigerant. It poses no significant health or fire risk under normal conditions, which allows it to be used safely in public spaces and food environments.
Efficient heat transfer
CO₂ has excellent thermodynamic properties. It offers high heat transfer rates in heat exchangers and evaporators, which can result in more compact system designs and reduced material usage.
High volumetric cooling capacity
Because CO₂ is a dense refrigerant, it allows for smaller pipe diameters and lower compressor swept volumes compared to HFC systems. This can lead to space savings and reductions in refrigerant charge volumes.
Stable chemical properties
CO₂ is chemically stable and does not degrade easily within the system. It is compatible with most materials used in refrigeration, including copper, brass, and standard lubricants, provided water contamination is avoided.
No planned phase-out
Unlike many synthetic refrigerants, CO₂ is not subject to phase-down under the F-Gas Regulation. This makes it a reliable choice for companies looking for long-term security in their refrigeration investment.
Types of CO₂ refrigeration systems
There are several configurations of CO₂ refrigeration systems. The right choice depends on the required cooling temperatures, the climate where the system will operate, and the complexity you are prepared to manage. Below are the main types currently in use.
Transcritical CO₂ systems
These systems operate above the critical point of CO₂ (31 °C and 73.8 bar). In transcritical mode, there is no condensation—heat is rejected in a gas cooler instead. This type is commonly used in moderate to warm climates, especially in supermarkets and commercial installations. Transcritical systems are fully CO₂-based, without the need for additional refrigerants.
Transcritical booster systems are a popular sub-type. They use a two-stage compression cycle to handle both medium and low temperature loads, improving efficiency and reducing equipment count.
Subcritical CO₂ systems
Subcritical systems operate entirely below CO₂’s critical point. These systems are usually part of a cascade design, where CO₂ is used on the low-temperature side, and another refrigerant—such as ammonia or an HFC—is used to condense the CO₂.
This setup is suitable for colder climates or where high efficiency is required at low evaporating temperatures. It allows for better control of pressure and temperature but adds complexity and cost due to the involvement of two refrigerants.
Cascade CO₂ systems
A cascade system combines two refrigerants: CO₂ on the low side and a secondary refrigerant on the high side. The CO₂ circuit handles the evaporators, while the second refrigerant condenses the CO₂. These systems are used in applications requiring very low temperatures, such as cold storage or industrial freezing.
Hybrid and parallel compression systems
Some systems use parallel compression or ejectors to improve transcritical efficiency, especially in warmer climates. These are advanced designs aimed at reducing energy consumption and improving performance during high ambient conditions.
Each type of system has its own advantages and considerations. The decision depends on your cooling needs, energy goals, and site conditions.
Disadvantages and limitations of CO₂ systems
While CO₂ refrigeration systems offer clear environmental and performance benefits, they also present several challenges that must be addressed during system design, installation, and operation.
Higher operating pressure
One of the main limitations is the significantly higher pressure compared to conventional systems. Pressures in transcritical mode can exceed 90 bar. This requires reinforced components, specialised valves, and pipework rated for high pressure, all of which increase material and equipment costs.
Increased component and installation costs
CO₂ systems often involve more complex components—such as high-pressure valves, gas bypass circuits, and pressure safety systems. This adds to the upfront cost. Installation also requires trained technicians familiar with CO₂ systems and their safety requirements, which can increase labour time and cost.
Dry ice formation during charging
CO₂ can solidify directly into dry ice if the pressure drops below 4.2 bar (its triple point). This can happen during system charging or in poorly designed relief lines. Care must be taken to avoid this condition, as solid CO₂ can block lines or damage equipment.
Water contamination risks
CO₂ is highly sensitive to moisture. If water enters the system, it can form corrosive acids or unusual compounds, particularly in systems with water heat exchangers. To prevent this, all components must be properly dried, and system charging must be done under controlled conditions.
Leak detection requirements
CO₂ has a lower “practical limit” compared to many HFCs, which means that leak detection equipment is more frequently required — especially in enclosed areas. In addition, due to the high pressures involved, the chance of small leaks increases, making detection systems an important part of compliance and safety.
Efficiency loss in hot climates
In very warm ambient conditions, transcritical CO₂ systems can become less efficient unless they incorporate advanced designs like parallel compression or ejectors. Without these enhancements, energy consumption can rise compared to other refrigerants under the same conditions. But that is not the case for the UK region.
Although these limitations require careful planning and design, most of them can be mitigated with proper system configuration and engineering expertise.
Business applications of CO₂ refrigeration systems
CO₂ refrigeration systems are used in a range of commercial and industrial applications where reliable cooling and regulatory compliance are essential. The system’s characteristics make it particularly suitable for businesses aiming to reduce environmental impact while maintaining operational efficiency.
Supermarkets and food retail
CO₂ systems are widely adopted in supermarkets across Europe and the UK. They can serve both chilled and frozen displays using a single refrigerant in a booster system. Since these systems are safe for public areas and compatible with heat reclaim setups, they help reduce overall energy usage. Their performance in both medium and low-temperature ranges also simplifies the refrigeration plant design.
Food processing and cold storage
Industrial food processing facilities benefit from CO₂ systems due to their high cooling capacity and efficiency at low temperatures. In cold storage, especially in applications requiring temperatures below -30 °C, CO₂ cascade or subcritical systems provide consistent performance. Their ability to meet strict hygiene and safety standards is also important in this sector.
Logistics and distribution centres
Temperature-controlled logistics, such as refrigerated warehouses and transport hubs, rely on CO₂ refrigeration for its ability to manage large loads and operate reliably in varying ambient conditions. Heat reclaim options can also reduce heating costs in such facilities.
Industrial and manufacturing facilities
In broader industrial applications, CO₂ systems are used for process cooling and environmental control. These include beverage production, dairy plants, and pharmaceutical storage, where precision and reliability are required. The non-toxic nature of CO₂ makes it suitable for use in environments with strict safety regulations.
Future-proofing and regulatory compliance
Businesses choosing CO₂ now avoid the uncertainty of future bans and restrictions on synthetic refrigerants. Because CO₂ is not subject to the F-Gas phase-down and has no known regulatory phase-out, systems using R-744 are less likely to face legal or market pressure in the coming decades.
As you can see, CO₂ refrigeration systems are already proven across multiple sectors. They are particularly beneficial for companies looking to modernise their cooling infrastructure without risking non-compliance or future obsolescence.
| Feature | CO₂ Refrigeration System (R-744) | Traditional Refrigeration (e.g. HFC, HCFC) |
|---|---|---|
| Global Warming Potential (G
WP) |
1 (negligible) | 1300–4000 depending on refrigerant |
| Safety classification | A1 – Non-toxic, non-flammable | Varies: some toxic, some mildly flammable |
| Operating pressure | Very high (up to 120 bar in transcritical systems) | Moderate (usually below 30 bar) |
| System cost (initial) | Higher due to reinforced components and controls | Lower, components widely available |
| Component availability | Increasing, but still more specialised | Widely available globally |
| Energy efficiency | High in moderate/cool climates, needs enhancements in hot areas | Depends on refrigerant and system design |
| Leak detection requirement | More likely due to higher pressure and lower charge limit | Less frequent, larger allowable charge volumes |
| Regulatory outlook | Not subject to F-Gas phase-down | Many refrigerants being phased out or heavily taxed |
| Heat recovery potential | Very good, especially in transcritical systems | Possible, but less efficient |
| Service & maintenance | Requires trained personnel familiar with CO₂ systems | Standard technicians available |
| Environmental compliance | Fully compliant with long-term regulations | May require future system replacement or retrofit |
| Applications | Supermarkets, cold storage, food processing, industrial cooling | Widely used in all sectors, including legacy systems |
Final word
Choosing the right refrigeration partner is key to ensuring efficiency, reliability, and long-term performance. At MGI Ltd, we design and install CO₂ refrigeration systems that meet the latest environmental standards while reducing your operational costs. Our team has extensive experience in delivering tailored solutions for supermarkets, food processors, and industrial facilities across Oxfordshire and beyond.
Contact us today to arrange a survey or discuss your next refrigeration project.