R744

R744 refrigerant

R744 is the designation used when carbon dioxide (CO₂) is employed as a refrigerant, and it is increasingly being adopted as an alternative to conventional fluorocarbon-based refrigerants, particularly hydrofluorocarbons (HFCs). R744 has an ozone depletion potential (ODP) of zero, and its global warming potential (GWP) is defined as 1, as CO2 is used as a reference. For these reasons, R744 is characterized by its low environmental impact.
Carbon dioxide (CO₂) was widely used as a primary refrigerant in the 19th century, but its use declined significantly with the widespread adoption of fluorocarbon-based refrigerants.
However, from the 1980s onward, growing concerns over ozone layer depletion and global warming caused by fluorocarbon refrigerants led to renewed attention to CO₂, which is known for its excellent environmental properties. As a result, it was designated as R744 within the international refrigerant numbering system (ASHRAE designation).
Today, R744 is utilized as a refrigerant in applications such as water heaters as well as refrigeration and freezing equipment.

Comparison of R744 and HFC Refrigerants (R134a, R410A)

Advantages of R744 refrigerant

low environmental impact

The global warming potential (GWP) of hydrofluorocarbon (HFC) refrigerants is 1,430 for R134a and 2,090 for R410A, while R32, which is considered to have a relatively low GWP, still has a value of 675.
Thus, a common challenge with HFC refrigerants is their generally high GWP.
In contrast, R744 has a GWP of 1 and an ozone depletion potential (ODP) of zero.
These characteristics contribute to its recognition as an alternative to HFC refrigerants.

Nonflammable、Low Toxicity

Natural refrigerants, which occur naturally in the environment, include R744 as well as R290 (propane) and R717 (ammonia). However, these refrigerants present handling challenges due to their flammability and toxicity, requiring special precautions.
In contrast, R744 is classified as an A1 refrigerant and is non-flammable with low toxicity.

Suitable for use as a high-temperature refrigerant

Conventional refrigerants such as HFCs release heat to the surrounding environment when they transition from a gaseous state to a liquid state. However, because R744 has a low critical temperature, defined as the temperature above which it cannot be liquefied, it is difficult to utilize the gas-to-liquid phase-change process.
The critical temperature of R744 is 31 ℃. In other words, during summer, the ambient temperature may exceed this value, making condensation-based liquefaction difficult to achieve.
For this reason, R744 uses a method known as transcritical operation to reject heat.
Transcritical operation is an operating method in which heat is rejected in a supercritical state, where the refrigerant is neither a gas nor a liquid.
For HFC refrigerants that utilize the condensation process from gas to liquid, the practical temperature range for high-efficiency operation is approximately 40-60 ℃, and efficiency declines at higher temperatures.
In contrast, R744 enables the supply of hot water at temperatures of around 90 ℃ through transcritical operation.
Owing to this characteristic, R744 is well suited as a refrigerant for equipment that requires high temperatures, such as water heaters.

Easily available and cost-effective

R744 is carbon dioxide (CO₂) and can be recovered from the atmosphere for use.
For this reason, its production cost can be kept lower than that of fluorocarbon-based refrigerants.

Precautions for Use of R744

Requires high-pressure operation

R744 must be operated at very high pressures, typically in the range of 8-12 MPa.
For this reason, pressure-resistant components, such as piping, are required.
As a result, the initial installation cost of the system tends to be higher.

Limited efficiency for air-conditioning (cooling) applications

Because R744 has a critical temperature of 31 ℃, it becomes difficult to reject heat under conditions such as high ambient temperatures in midsummer, resulting in reduced cooling efficiency. For this reason, R744 is not very suitable for air-conditioning (cooling) applications.

Requires leak detection

If R744 accumulates at high concentrations, there is a risk of oxygen deficiency.
Therefore, leaks must be detected using sensors.

Applications of R744

R744 is widely used as a refrigerant for heat pump water heaters.
It is also used in commercial refrigeration and freezing equipment.
However, it is not well suited for cooling applications such as air conditioning.

Outlook for R744

R744 is a refrigerant for which development will continue as a key option supporting the reduction of fluorocarbons and the mitigation of global warming.
In particular, improving pressure-resistant design to address its high-pressure characteristics is an important challenge.
In the future, in addition to applications such as water heating and refrigeration, expansion into applications that require both cooling and heating, such as air-conditioning systems, is also expected.