Carbon dioxide (CO₂)

Forward Naturally

As we transition toward a more environmentally friendly future, your refrigerant choice becomes an important factor for your business and the planet. CO₂ is a natural cooling agent that delivers sustainable and energy-efficient refrigeration in everything from warehouses to ice machines. Allowing businesses to move forward naturally.

General properties

CO₂ has several unique thermo-physical properties making it an ideal refrigerant:

Excellent heat transfer coefficient
High energy content
Relatively insensitive to pressure losses
Very low viscosity of the liquid phase

In practical applications, CO₂ systems deliver a very high-performance output. The main reasons are:

Superior heat exchange.
Smaller piping sizes
Low pumping power as secondary fluid.
Superior heat recovery

For all refrigerants, there is a decline in system efficiency with increasing condensing temperatures. High condensing temperatures are often seen as limiting factor for usage of CO₂. However, the inherent thermo-physical properties of CO₂ and the use of the expansion energy with ejectors can help to compensate for and offset this factor.

In addition, CO₂ refrigeration systems possess heat recovery options that can help further future-proof your business. CO2 and its high energy content, which continues to scale at greater temperatures, can be reclaimed, increasing overall system efficiency.

CO₂ cooling and commercial refrigeration

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Applications of CO₂ in food retail

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Benefits of CO₂ in industrial refrigeration

Environmental impact

From an environmental perspective, CO₂ is a very attractive refrigerant with zero ODP and a GWP of 1. It is a naturally occurring substance and abundant in the atmosphere.

Pressure and temperature

CO₂ is a high-pressure refrigerant where high operating pressures are required for efficient operation. During standstill, the ambient temperature can reach and exceed the critical temperature and the pressure can exceed the critical pressure. Hence systems are typically designed to withstand pressures up to 90 bar, or sometimes even equipped with a small standstill condensing unit to keep pressures low.

At the same time, CO₂ has a low compression pressure ratio (20 to 50% less than HFCs and ammonia), which improves volumetric efficiency. With evaporation temperatures in the range of -55 ºC to 0 ºC, the volumetric performance of CO₂ is for example four to twelve times better than that of ammonia, which allows compressors with smaller swept volumes to be used.

The triple point and critical point of CO₂ are very close to the working range. The critical point may be reached during normal system operation. During system service, the triple point may be reached, as indicated by the formation of dry ice when liquid containing parts of the systems are exposed to atmospheric pressure. Special procedures are necessary to prevent the formation of dry ice during service venting.

Material interaction

CO₂ does not react with common metals or with Teflon®, PEEK, or neoprene components. However, it diffuses into elastomers and can cause swelling with butyl rubber (IIR), nitrile rubber (NBR), and ethylene-propylene materials (EPDM).

The density of liquid CO₂ is about 1.5 times that of ammonia, resulting in higher mass charge in evaporators, such as large plate chillers in large industrial systems. Higher density means higher oil circulation as well, which in turn requires effective oil separators for industrial systems.

Cost efficiency

CO₂ is a by-product in a number of industries, so the price of CO₂ is low. However, CO₂ systems tend to be more expensive than traditional systems due to higher pressures (in transcritical systems) or increased complexity (in both transcritical and subcritical systems). Complexity of systems seems to be decreasing with the entrance of Booster systems and as the number of CO₂ installations has increased, history has shown that the cost approaches the cost of the reference systems using HFCs.

Secondary, large CO₂ systems, especially in industrial refrigeration, may be less expensive to build than their glycol counterparts and thus offer lower initial and life-cycle costs.

Applications

Unlike most other refrigerants, CO₂ is used in practice in three different refrigeration cycles:

Subcritical (cascade systems)
Transcritical (CO₂-only systems)
Secondary fluid (CO₂ used as a volatile brine)

The technology used depends on the application and the intended location of the system. There are a several applications where CO₂ is attractive and already widely used today:

Industrial refrigeration. CO₂ is generally used in combination with ammonia, either in cascade systems or as a volatile brine
Food/retail refrigeration
Heat pumps
Transport refrigeration

Danfoss believes that CO₂ will be the main refrigerant in multipack commercial refrigeration systems. The F-gas regulation is a clear push in this direction.

CO₂ systems can also be extended to cover heat recovery. In many cases the extra investment for gaining the waste heat is negligible and several cases have demonstrated that.

CO₂ Applications in Food Retail and Industrial Refrigeration

Read all about CO₂ applications in our dedicated site for Food Retail and Industrial Refrigeration

CO₂ applications in Food Retail

CO₂ applications in Industrial Refrigeration

Save energy and protect the environment

Read how Danfoss can help you save energy and protect the environment with our CO₂ solutions

Save energy and protect the environment with our CO₂ solutions

Danfoss Learning

Mobile training unit for CO₂ applications

Danfoss presents the mobile training unit equipped with CO₂ solutions and interactive learning modules for transport across the globe to support our worldwide community to address growing CO₂ concerns.

Mobile CO2 training unit