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Everything R744

Frequently Asked Questions

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The natural refrigerant R744 (CO2)

Characteristics of R744

  • What is R744?
  • R744 is the chemical reference for carbon dioxide (CO2) used as refrigerant. It is a naturally occurring substance that can be applied as a working fluid in different heating and cooling applications, due to its excellent heat transfer properties and its high volumetric cooling capacity.
  • Are R744 and CO2 the same thing?
  • Yes. According to industry standards, each refrigerant receives a nomenclature starting with the letter R (for Refrigerant) and a number, which follows an order depending on the "family" the substance belongs to. R744 / R-744 is hence the standard nomenclature for CO2 used as a refrigerant: "7" indicates the natural fluid while "44" refers to the fluid's weight. The term R744 is applied when referred to the refrigerant itself, and CO2, and/ or CO2 Technology is used when referring to an application or use.
  • What are the main characteristics of R744?
  • R744 is:
    • Non-toxic
    • Non-flammable
    • Non-ozone-depleting
    • Environmentally friendly, with a Global Warming Potential = 1
    A more detailed explanation of each of these features follows below.
  • Why is R744 environmentally friendly?
  • Applying R744 in vapour compression applications reduces significantly the amount of greenhouse gas (GHG) emissions released into the atmosphere, which is at the origin of climate change. Besides, R744 systems, in many applications, have the potential to be more efficient than current systems in most climate conditions, and therefore indirect emissions of GHG resulting from energy consumption are also reduced.

    For example: Given that, on average, Mobile Air Conditioning (MAC) is activated about 60% of the time a car is in use over its lifetime, the choice of the low global warming refrigerant R744 is a key element for improving the environmental performance of a car. For other applications, such as heat pump water heating or commercial refrigeration, the potential for emission reduction by using R744 is also significant.

    For more details see the section Environment under Mobile Air Conditioning.
  • Does R744 have any impact on the Ozone layer?
  • No, R744 does not contribute in any way to the depletion of the ozone layer.
  • Is R744 flammable?
  • No, R744 is not flammable and presents no risks in case of interaction with other substances or chemical blends. CO2 is applied in many fire extinguishers.
  • Is R744 toxic?
  • R744 is not toxic at moderate concentrations. Given the small system charge used for heating and cooling applications, R744 thus presents minimal health risks. In the US, for example, this is reflected by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) with the common standard ASHRAE 34, which lists R744 as Safety Class A1 - a low toxic, non-flammable refrigerant.

    When inhaled at high concentrations, however, carbon dioxide can lead to a risk of asphyxiation, since it displaces oxygen. In such case it would produce a sour taste in the mouth and a stinging sensation in the nose and throat combined with dizziness.
  • Where does the CO2 used in refrigeration come from?
  • The refrigerant used in MAC or heat pumps is an industrial waste product. Some industrial companies have developed systems to extract CO2 from their plants to process it and sell it as a refrigerant.
  • Is R744 a totally new refrigerant?
  • Not exactly. CO2 was already used as a refrigerant in the mid-nineteenth century and its use steadily increased, reaching a peak in the 1920s. After that, the natural refrigerant was displaced by chlorofluorocarbons (CFCs) that operated at much lower pressures. Due to the impact of CFCs, and later HFCs, on the environment, CO2 was re-evaluated as an alternative refrigerant in the late 1980s. A new way to contain the high pressure was discovered by Professor Gustav Lorentzen and his group of researchers in Norway, giving way to the application of transcritical R744 to different heating and cooling systems.


  • For which applications is R744 suitable?
  • R744 can be applied in most heating and cooling systems such as Mobile Air Conditioning (MAC) in vehicles and buses, vending machines, coolers, commercial cabinets for supermarkets, containers and climate control systems for residences. CO2 Technology (R744) has also shown to be extremely efficient in heating water. This explains the success of the Japanese "EcoCute" Heat Pump Water Heaters, which can also be combined efficiently with floor heating. As an illustration, in Japan more than 300,000 CO2 based EcoCute units of water heaters were sold in 2006.
  • What are the main benefits of R744 in Hot Water Heat Pumps?
  • Today, directly-fired hot water heaters are common, mostly fired either with gas or oil. A R744-based heat pump is more environmentally friendly, features high energy efficiency and provides safer operation. In addition, it is more cost-efficient as it provides storage for hot water needed during the day that can be heated at night when electricity is available at a lower price (night tariff).

    Compared to the water heat pumps operated with HFCs, R744 heat pumps will provide higher water temperatures and thereby eliminate risks of bacteria. R744 also has the clear advantage of a lower global warming impact.
  • What is the state of development in this field?
  • The first "EcoCute" units were commercialised in Japan in 2001. The Japanese government had originally targeted to reach, by the year 2010, over 5 million units in operation in the Japanese market. By the end of 2007, there were over 1 million units installed. Other markets (i.e. Europe, rest of Asia) are foreseen to follow soon.
  • What are the main benefits of R744 in commercial refrigeration?
  • R744-based commercial refrigeration and freezing features:

    • Reduced emissions of greenhouse gases
    • Space savings for piping arrangements (self-contained system)
    • Reduced costs of piping arrangements and insulation
    • Energy efficiency
  • What is the state of development in this field?
  • There are several ongoing projects with R744 at advanced degrees of completion. Some refrigeration cabinets working with R744 are already part of supermarkets in Europe (i.e. Scandinavia, UK, Switzerland, Belgium), and large-scale production of components could follow soon if the market conditions evolve favourably.

Mobile Air Conditioning


  • How does a R744 MAC system work?
  • The R744 Mobile Air Conditioning (MAC) uses the same process as ordinary automotive air conditioners to cool a vehicle. The liquid refrigerant, in this case R744, is evaporated inside the passenger compartment heat exchanger where it absorbs heat from the incoming ambient air, thereby cooling the vehicle's cabin. By compressing the evaporated refrigerant to a higher pressure level, its temperature increases. The heat of this high-pressure refrigerant is then rejected to the ambient air through the gascooler. After the refrigerant's expansion to liquid at a lower pressure level, the refrigerant is again able to absorb heat from the incoming ambient air. In this way a closed refrigerant cycle will keep the passenger compartment cool also at high summer temperatures.
  • What pressure can a R744 system reach in operation?
  • R744 systems can reach a pressure of up to 133 bars. With current systems based on R134a, the pressure in operation does not usually exceed 30 bars. Components already available ensure that a R744 system operates safely and efficiently also under these conditions.
  • How do R744 systems differ from those using HFC-134a?
  • The main difference between the R744 and HFC-134a systems is the pressure level (up to 133 bar and 30 bar respectively). This high pressure in the R744 system corresponds to a heat rejection mainly at pressure levels above the critical point, i.e. in the transcritical area. In a transcritical cycle no condensation takes place. As a result, in a R744 system the traditional R134a condenser is replaced by a gascooler optimised to reject heat from the R744 gas to the ambient air.

    At low ambient temperature the R744 cycle might operate below the critical point, i.e. in a subcritical process. In this case, R744 will condensate during the heat exchange in the gascooler like R134a in the condenser.
  • What is the benefit of high-pressure operation?
  • An operation above the critical point, as it is the case in transcritical R744 systems, is highly favourable as gas cooling with a temperature glide is more efficient than a heat exchange by condensation in current systems. In addition, the higher energy density of high-pressure R744 systems allows a reduced size of lines and components.

    Due to the combination of high energy efficiency, high pressures and reduced internal system volume by the use of R744 as refrigerant, the relative pressure losses in such systems will be negligible compared to the pressure losses in traditional HFC-134a systems.
  • Which components are different in R744 systems compared to current ones?
  • A new generation of components capable of handling high pressure and temperature are needed for R744 systems. Most automotive component and system manufacturers have already succeeded in developing them, and today there are compact, lightweight R744 compressors and heat exchangers (including gascoolers, evaporators and internal heat exchangers) offering optimal performance. Besides, there are only small variations compared to a traditional MAC device: the internal heat exchanger, and the duty of the expansion device, which mainly controls the high side pressure instead of controlling the superheat of the evaporator as in current HFC systems. Also, the gascooler takes the place of the condenser in current systems.
  • Are there any additional benefits resulting from the design of R744 systems?
  • Yes. Since the energy density of R744 is higher compared to HFC systems, R744 MAC can be more compact due to a reduced size of lines and components. This has several advantages:

    • The system is smaller and about 2 kg lighter which means reduced material costs, reduction of extra weight in the car caused by MAC, easier packaging.
    • The reduced inner diameters provide safer handling of the high pressure.
    • The relative pressure losses in a R744 MAC are negligible compared to HFC-134a systems, due to the combination of higher energy efficiency and reduced internal system volume.
  • What is the performance of a R744 MAC system compared to a current one?
  • It has been long agreed that R744 outperforms R134a at medium temperature conditions (up to 30º Celsius). Latest tests show, however, that R744 also performs better at high ambient conditions (35°C), in all sorts of vehicles, including small cars. Even at those conditions, R744 systems need 10% less fuel to function than a state-of-the art HFC-134a system. In some cases, as latest tests shows, the efficiency gains at 35°C can be as high as 20% over HFC-134a systems. Overall, compared to current HFC-134a systems, the energy demand for providing the same amount of cooling into the passenger compartment can be reduced by up to 30% for the R744 unit, depending on the ambient conditions. At very high ambient temperatures (above 40°C), equal or slightly higher fuel consumptions may occur. However, these operation conditions are of minor importance (~5% of the car operation) to the energy consumption in an annual cycle.

    For any ambient temperatures, the R744 performs much better in reducing the time to cool down and heat the car, thus allowing a safer operation of the vehicle.
  • Are there special requirements for using a R744 MAC system?
  • To ensure a safe system operation it is not allowed to place line fittings or component joints inside the passenger compartment; the only part of the system allowed in this part of the car is the evaporator. This will minimize the risk of refrigerant vent to the passenger compartment in case of an accident.

    Components used in R744 systems also have to undergo leakage and pressure tests to withstand high pressure and extreme operating conditions which may occur during its lifetime in a car. The Society of Automotive Engineers (SAE) provides established test conditions and sets down restrictions for R744 in motor vehicle air conditioning systems in the standard J639.

    In the United States, the Environmental Protection Agency (EPA) requires that systems are designed to avoid occupant exposure to concentrations above the CO2 short term exposure limit (STEL) of 3% averaged over 15 minutes in the passenger cabin. Given the small charge of the system (about 300 grams), there are minimal risks to the passengers even in the case of leakage.

    For extra safety a CO2 sensor may also be integrated to measure the CO2 concentration in the passenger cabin.
  • Can I also use the R744 MAC for heating my car?
  • Yes, a R744 MAC can also be used for heating a vehicle without the need for an additional device. In fact, the properties of CO2 make it especially effective to heat fast and efficiently the passenger cabin. In this case, the system would operate in a reversed cycle, i.e. in heat pump mode, where the gascooler functions as an evaporator providing heated ambient air to the passenger compartment.
  • What are the benefits for the driver?
  • For the driver, R744 present additional benefits:

    • Faster cooling
    • Faster heating
    • Increase of driving distance between refuelling
    • No need for additional / auxiliary device for heating
    • Lighter, smaller, noiseless components
    • Faster defrost, increased visibility: allows for safer driving
  • How fast can a R744 system cool a car?
  • Tests have shown that R744 systems can cool down the temperature of a cabin car from 75º Celsius to less than 25º in 10 minutes. With current systems, based on HFC-134a, this transition would take at least 16 minutes. When used in reversed mode, R744 is also faster to heat the car cabin, and defrost the windows to allow a safer and more comfortable drive during the cold season.
  • How much CO2 is needed during one life cycle?
  • The initial charge for R744 systems ranges from 200 to 400 grams. Over its entire lifetime, estimations point to a total use of less than 1.0 kg of CO2. As a comparison: In the same time a car using HFC-134a systems emits around 2,000 kg of CO2 through the tailpipe!
  • Does a R744 MAC system require servicing?
  • Yes, R744 systems need servicing over its lifetime like every other MAC system. When the charge of CO2 in the system is too small, due to accidents or leakage, the system will need to be re-filled by staff trained to handle high-pressure systems. However, the complicated infrastructure to avoid CFC/HFC-refrigerant leakage into the atmosphere will no longer be needed since leakage of small amounts of R744 does not carry an important burden to the atmosphere as it is the case now with high global warming refrigerants.


  • Is CO2 not supposed to contribute to global warming?
  • CO2 is an important component of the earth's atmosphere, being present in the air at a concentration of 0.03%-0.06%. Together with other natural greenhouse gases (GHG), it absorbs infrared radiation to warm the planet's surface and enable human life. However, its steadily rising concentration in the atmosphere due to the anthropogenic burning of fossil fuels has made it the world's predominant GHG.

    The controversial reputation of CO2 is partly due to the fact that the Global Warming Potential (GWP), the contribution of a substance to global warming over a timescale of 100 years, is measured in CO2 equivalents. That does not mean that CO2 per se in small concentrations poses a threat to the earth's climate. As an example, 1kg of the current refrigerant used in vehicles, R134a, has a GWP of 1,410. The same amount of R744 (CO2) has a GWP of 1!

    In addition, CO2 used in cooling systems is an industrial waste product. In Germany alone, 1/3 of CO2 from natural reserves remains industrial waste gas. By cleaning this CO2 and reusing it as the refrigerant R744, CO2 becomes environmentally neutral!
  • What are direct emissions?
  • Direct emissions from Mobile Air Conditioning (MAC) occur when refrigerant is released from the system and emitted to the surrounding air, thereby contributing to global warming. The total amount of direct emissions is a sum of "regular" gradual and continuous leakage from the MAC during its lifetime, of "irregular" accidental releases while the system is serviced, and losses at the end of life when the refrigerant is removed from the car.

    Normally, a MAC system is guaranteed to be service-free for about 5 years. It might happen that the initial charge is totally gone after these 5 years, meaning that the refrigerant has been leaking with a rate of up to 20% from line fittings, joints and the compressor over the years. The amount of refrigerant escaping from the system, and thereby causing direct emissions, will mainly depend on system size, charge and component tightness.
  • How does a R744 MAC system reduce direct emissions?
  • Using a R744 (CO2) MAC system would reduce direct emissions substantially. Given that R744 has by far the lowest global warming potential of all current and proposed refrigerants, the unintentional leakage of small amounts would hardly contribute to climate change. In addition, highly resistant, leak-tight and efficient compressors, connections and control elements in R744 systems are designed to prevent even the smallest of leaks.

    In total, R744 systems will save an average of 7% of direct emissions from MAC.
  • What are indirect emissions?
  • Indirect climate change emissions, also known as tailpipe emissions, result largely from the car engine consuming fuel to transfer mechanical and electrical power to the cooling system. Regardless of whether the air conditioning is working or not, its additional weight further raises a car's exhaust CO2 emissions.

    Depending on the usage profile, region and climate conditions, estimations point to additional fuel consumption through MAC at 2.5 to 7.5% per year, corresponding to GHG emissions of around 55 to 221 kg CO2 per vehicle.
  • How does a R744 MAC system reduce indirect emissions?
  • R744 MAC can reduce a car's indirect emissions by up to 5% through:

    • smaller and lighter components: their low total weight reduces the additional load that is placed on the engine and thereby the overall exhaust emissions
    • higher energy density: a better performance in a given component volume than for the HFCs used in current low-pressure systems leads to less fuel and energy needed to reach the same level of cooling
    • operation as a heating system: R744 MAC makes auxiliary heaters that would consume additional fuel unnecessary
    Faster cooling and heating while saving fuel... although it may sound like a paradox, by using CO2 as refrigerant a car can substantially reduce its indirect CO2 emissions!
  • How can I help the environment by using R744?
  • One fifth of all HFC-134a produced worldwide is emitted to the atmosphere every year. Already now, leaking HFC-134a MAC systems are responsible for raising a car's direct greenhouse gas emissions by up to 15% every year. With an expected global car sale of 81.5 million vehicles in 2017, these direct emissions alone may amount to 390 million tons of CO2 equivalents only in 2017.

    The early global use of CO2 as a refrigerant (R744) would mean avoiding most of these direct emissions. Given that HFC-134a has a 1,410 times higher Global Warming Potential (GWP) than CO2, the leakage of even small amounts will pose a threat to the environment. Visualize this figure: 1 kg of released HFC-134a is heating up the atmosphere like 1.4 tons of CO2!

    Regarding indirect tailpipe emissions due to higher fuel consumption, R744 MAC features a higher cooling performance than HFC-134a in more than 90% of operating conditions.
  • Concretely, how much greenhouse gases can R744 save in the atmosphere?
  • Overall, R744 can save up to 12% of total greenhouse gas emissions from a car. Hence, in Europe alone, we could save 30 x 106 metric tons of greenhouse gases until 2011 if 3 million R744 MAC units were introduced every year, starting from 2008!

Economy & Market

  • Are R744 MAC systems already available?
  • Yes. Leading automotive suppliers have announced that their product lines for R744 Mobile Air Conditioning (MAC) are ready for serial production. Within the last decade, CO2 technology has been tested in a wide range of vehicles by most major car manufacturers. Today there are more than 100 vehicles operating with R744 MAC. It is estimated that in total, suppliers and carmakers have invested about 500 million euros in the development of R744 MAC so far.
  • For what kind of cars is R744 suitable?
  • Its proven efficiency, reliability and compliance with highest safety ratings makes R744 MAC suitable for all types of passenger cars, fuel cell hybrid vehicles and large buses.

    Even though R744 MAC was believed only to be suitable for medium-sized and luxury cars, recent tests have proved a higher cooling performance of R744 even in small cars. In addition, the standard R744 MAC system will fit into compact cars due to variable packaging as well as reduced component size and weight.
  • How much fuel does a MAC system normally consume?
  • Depending on model type, ambient temperature and velocity, the working air conditioning can increase a car’s total fuel consumption by around 10% while driving on a highway and as much as 40% in the city. Recent test have shown that only cooling down a car from 25°C to 20°C in the city traffic will consume at least 2 additional litres per 100 km.

    Calculated over the whole year, this corresponds to an average increase of 5% for a vehicle equipped with Mobile Air Conditioning (MAC).
  • How much fuel can be saved by using a R744 MAC system?
  • At ambient temperatures of 25°C and 35°C, where 95% of all MAC operation takes place in Europe, the use of R744 in small cars will lead to an absolute fuel reduction of 0.3 and 0.5 litres per 100 km respectively. Other tests from leading suppliers indicate that an optimized R744 climate system can reduce fuel consumption by up to 25% compared to current HFC-134a systems. As an example, in southern Europe a R744 MAC can save up to 17 litres of petrol per year now used only for cooling a car, whereas in Asian conditions (New Delhi) it could even save up to 58 litres annually.
  • Is a car equipped with R744 MAC more expensive than a present one ... when producing and buying a car?
  • Estimations of the total or additional costs are difficult as R744 (CO2) MAC has not yet entered mass production. It is estimated that additional initial costs for the first system generation may be around 20 euros/unit. However, leading suppliers have designed components that will be even more cost-efficient than current systems due to their simplicity. In addition, several components will have the same shape as current HFC-134a parts to enable manufacturers the use of existing assembly tools. Modular concepts using common parts for all components within a product family - e.g. valves, compressors - will further reduce development and assembly costs.

    Depending on the actual expenditure for R&D, design and production of R744 MAC systems, the final consumer price can suffer a further increase. However, this first investment is paid off by annual fuel savings due to faster and more efficient cooling and heating.
  • ... when driving and servicing a car?
  • There are no additional costs when driving a car equipped with R744 MAC. On the contrary: Its high cooling and heating capacity will lead to substantial fuel savings in more than 90% of the operation time. Depending on the region where the car is operated, fuel savings could be as high as 50 euros per year.

    Regarding the servicing of R744 MAC, initial costs may arise from training staff to handle high-pressure systems and the investment in new charging systems. However, economic benefits are expected to outweigh primary investments as the current complicated infrastructure to avoid refrigerant leakage will become unnecessary. Once in place, largely automated charging systems with easy operation will make the maintenance of R744 MAC a low cost procedure (25-100 euros).
  • ... when disposing of a car?
  • Due to its low total charge ( approx. 300 g) and negligible Global Warming Potential, R744 MAC does not require refrigerant recovery or recycling at the vehicle's end of life, only the capture of oil. Using R744 would not only save costs for sophisticated recovery equipment, but may also reduce recycling fees currently paid by car owners in some countries.
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