As a natural working medium, CO2 is a key research orientation of chlorofluoro carbons (CFCs). In this article, we will cover the basics about CO2 refrigeration compressors, the differences between a CO2 compressor and other types of compressors, and the advantages of CO2 as a refrigerant.
Due to the negative impacts of CFCs on our atmosphere, environment protection and research of refrigerants to replace them have become a global concern. To address this, international communities have been making laborious efforts for decades, as outlined below:
- 1985: Vienna Convention for the Protection of the Ozone Layer signed.
- 1987: Montreal Protocol established.
- 1990: London Conference about the ozone layer held.
- 1992: The Copenhagen Amendment made to Montreal Protocol.
- 1994: United Nations Framework Convention on Climate Change (UNFCCC)
- 1997: Kyoto Protocol; 37 industrialized countries agreed on the first commitment period and their targets of greenhouse gas emissions.
- 2007: Bali Action Plan launched.
- 2011: Durban Platform for Enhanced Action
- 2015: International Climate Change Conferences in Rio, Copenhagen, and Paris.
- 2016: Paris Agreement entered into force.
- 2021: Glasgow Climate Change Conference scheduled.
Since 1994, after the UNFCCC was adopted, there has been a Conference of Parties (COP) every year in different cities across the world. As we can clearly see from the timeline, the process of CFCs replacement is accelerating all over the world. Phasing out Ozone Depleting Substances (ODS) has become an international responsibility.
The alternative working medium should meet three requirements: Safety, Environmental Acceptability, and Device Applicability. With years of unremitting efforts, scientists have developed many transitional or long-term substitutes for CFCs and hydrochlorofluorocarbons (HCFCs) such as R134a, R407C, R410A, and R290. Relevant technologies and equipment are also under study. Some of them have been widely used in refrigeration and the Heating, Ventilating and Air Conditioning (HVAC) industry already.
The Montreal Protocol requires countries to phase out CFCs, HCFCs, and other substances that deplete the ozone layer within a time limit, and stipulates the use period of these substances for developed and developing countries, respectively. The HCFC refrigerants widely used in industry right now are listed in the greenhouse gases by Kyoto Protocol, because of their obvious greenhouse effect.
Some European countries have banned the use of HCFCs in areas of refrigeration and HVAC. Meanwhile, they further propose to phase out HCFCs from other areas. The legislation of some countries strictly restrict or require the phasing out of the R134a refrigerant in the 2020s, which in fact makes a true dilemma for the refrigeration and HVAC industry—the need to adapt to the elimination of CFCs and HCFCs refrigerants, and to find alternatives.
In the research of environmental protection and refrigerant substitutes, natural refrigerants such as water, ammonia, hydrocarbons, and CO2 are the focus. Gustav Lorentzen of Norway, Former Chairman of the International Refrigeration Society, believed that natural refrigerants are the ultimate solution to environmental problems.
Why Use CO2 as a Refrigerant?
As a natural refrigerant, CO2 has many advantages over other substances, such as:
- Low kinematic viscosity
- Low dynamic viscosity
- Small pressure ratio (about 2.5 ~ 3.0)
- Small volume
- Large refrigerating capacity per unit volume
- High thermal conductivity
- High heat capacity at constant pressure
- Low surface tension
- Environment protective
- Easy to buy
- Physical stability and safety
Due to its preferable performance in heating and cooling, CO2 has become a hot research topic worldwide. Lorentzen, in his research, had the opinion that CO2 was, “an irreplaceable refrigerant.” Indeed, it is one of the most promising natural refrigerants and is expected to become one of the ideal refrigerants of the 21st century.
At present, CO2 refrigeration technology is primarily applied in three areas: Automobile air conditioning, heat pumps, and multi-stage refrigeration systems. Since CO2 is an environmentally friendly refrigerant, CO2 compressors have been designed and manufactured for different uses. There are six types of CO2 compressors, as follows:
- Piston Compressors
- Rolling Piston Compressors
- Swing Piston Compressors
- Scroll Compressors
- Vane Compressors
- Screw Compressors
Compared with compressors of other refrigerants, CO2 compressors are characterized by their high working pressure; small structure size and pressure ratio; large pressure differences of suction and exhaust; and high cooling and heating efficiency.
Manufacturer Innovations in CO2 Compressors
More and more enterprise manufacturers have joined in the trend to research and develop CO2 refrigeration compressors. Big manufacturers around the world such as Danfoss, Bitzer, Dorin, and Emerson not only have launched carbon dioxide compressors, but are also providing carbon dioxide solutions for different commercial sectors.
Danfoss
Danfoss’ carbon dioxide gas cooler solutions have the advantage of easy installation, simple debugging, and maximum system operation efficiency. As a positive responder to reduce carbon emission, Danfoss’ CO2 Adaptive Liquid Management Solution won the 2020 AHR Expo Innovation Award.
Bitzer
As the first manufacturer to produce carbon dioxide refrigeration compressors, Bitzer has been keeping close contact with major customers in the world for many years. Together with its clients, the company has been jointly researching and developing carbon dioxide refrigeration technology. At present, the earliest carbon dioxide unit developed by Bitzer and its customers has been operating safely for nearly 20 years.
Dorin
Dorin is the manufacturer of the world's first semi-hermetic reciprocating compressor (piston compressor) of CO2 refrigerant, which can be used in HPWH and light, medium-sized commercial refrigeration equipment. Compressor motors have power ranging from 0.75KW to 15kw and can operate in medium and high-speed modes (1450 rpm and 2900 rpm). The diameter of the compressor is between 18mm and 34mm.
Emerson
As the world's leading provider of HVAC and refrigeration solutions, Emerson's subsidiary brand Copeland’s scroll compressor can be used in low-temperature multi-stage refrigeration systems, which is the only compressor in the US that meets the UL requirements for multi-stage systems of high-pressure subcritical carbon dioxide.
Features of CO2 Scroll Compressors
- High Efficiency: The process of suction, compression and discharge of scroll compressors are continuous and unidirectional, so the harmful overheating of suction gas is small and the pressure difference between the two adjacent chambers is low, helping to avoid gas leakage. As there’s no clearance volume in a scroll compressor, it leaves no expansion process that causes the decline of gas transmission coefficient. As a result, the volumetric efficiency is high, and it usually can be higher than 95%.
- Minimum Vibration and Noise: During the suction procedure of a scroll compressor, the compression and exhaust is continuous at the same time. The pressure rises slowly, so the torque change is relatively small; naturally there is little vibration and noise. The torque of scroll type compressors is only 1/10 that of rotary compressors and reciprocating compressors.
- Simple Structure and Small Volume: The ratio of number of parts of CO2 scroll compressors to that of rotor compressors and reciprocating compressors is 1:3:7, making the size of scroll compressors 40% smaller compared with reciprocating compressors, and the weight 15% lighter. Since it has no suction and exhaust valves, less vulnerable parts, and is a flexible mechanism with adjustable axial and radial clearance, a scroll compressor can avoid various damages caused by possible liquid strike. In terms of structure, scroll refrigeration compressors can maintain high efficiency and high reliability even under high-speed operation, and its maximum performance can be 13,000 r/min.
- Easy Discharge of Pressure: Due to screw compressors adopting a gas support mechanism, it allows liquid compression. Once the pressure in the compression chamber is too high, the end faces of the moving plate and the static plate can be separated, and the pressure can be released immediately.
- High Gas Transmission Coefficient: The inner cavity of the compressor case is an exhaust chamber, which can reduce the preheating of suction and improve the gas transmission coefficient of the compressor.
- Tight Sealing: The manufacturing accuracy of scroll compressor body line is very strict, with special precision machining equipment required. The sealing requirement for a scroll compressor is high as well, and the sealing structure is complex.
- Wide Application Scenarios: The cooling capacity of scroll compressor is between 0.75 and 15kw (excluding special models), and most of them are between 3 ~ 5kW. Most models can be used for small household air conditioning and commercial air conditioning system. Nevertheless, scroll compressors are not suitable for the refrigeration condition of - 5 ℃.
Comparison of CO2 Scroll Compressor with Other Types of Compressors
Table 1 (Click to view in high-res): Performance comparison of different types of compressors. Note: Due to limited information, the symbol “-” indicates that we couldn’t find the data.
CO2 scroll refrigeration compressors are playing an irreplaceable role among other types of compressors. In view of its features of low vibration, low noise, longevity, reliability, and high efficiency, CO2 scroll compressors are promised to occupy a large share in the market of small and medium cooling capacity compressors. Thus, we believe since scroll compressors are more conducive to improve efficiency of the refrigeration system, its development prospect is brighter than other types.
For a complete refrigeration system, one cannot work with only a compressor and other mechanical components. Refrigerants, or cooling mediums, play a vital role too. Refrigerant is the working medium in the refrigeration equipment. It circulates in the refrigeration system, and by the change of its own thermodynamic state, refrigerant can complete the work of energy conversion and transfer with the outside environment to achieve the purpose of refrigeration.
Pros and Cons of CO2 Refrigerants
CO2 refrigerant (R744) is a kind of natural working medium, colorless and tasteless at normal atmospheric temperatures. The critical temperature and pressure are 31.1 ℃ and 7.37 MPa, respectively.
The physical properties of CO2 are as follows:
- The evaporation latent heat of CO2 is large and the refrigerating capacity per unit volume is high (22.6 MJ/m3 at 0 ℃), about 5 to 8 times that of traditional refrigerants.
- The kinematic viscosity of CO2 is very low and can remain at low temperatures.
- The thermal conductivity of CO2 is high, and the ratio of liquid density to vapor density is low. After throttling, the refrigerant can be distributed evenly in the refrigeration pipes. These excellent flow and heat transfer performances of CO2 can significantly reduce the size of compressor and refrigeration equipment, making the whole system very compact.
As a refrigerant, CO2 has the following three advantages over other refrigerants:
- First, regarding environment protection, the Ozone Depletion Potential (ODP) of C02 is 0 and the Global Warming Potential (GWP) is 1, which is far less than that of the refrigerant’s CFCs and HCFCs. Since most of the CO2 used for refrigeration is chemical and an industrial by-product emitted from factories, using CO2 as a refrigerant is like recycling or delaying the emission of this exhaust gas, which is quite beneficial to the environment.
- Secondly, from the thermophysical properties of the working medium, CO2 is perfectly suitable for refrigeration cycles and equipment.
- Thirdly, the chemical properties of CO2 are stable. It’s non-toxic, harmless, noncombustible, and will not decompose into toxic gases at high temperatures. Moreover, CO2 is cheap, easy to obtain, and economical.
The disadvantages of CO2 as a refrigerant are its low critical temperature (31.1 ℃) and high critical pressure (7.37 MPa). Especially for the latter, if a trans-critical cycle is adopted in a refrigeration system, the maximum working pressure of CO2 will be 10 MPa, which requires higher standards for the design of refrigeration systems and components. See Table 2. 
Table 2 (Click to view in high-res): Comparison of CO2 with other types of refrigerants. Note: Due to limited materials, symbol “-” in the table indicates that we couldn’t find the data. The relative price is relative to R12. We assume the price of R12 to be 1.
Summary
Energy-saving, environment protection and low carbon life are the development direction of modern technologies. As a natural working medium, CO2 has been widely used because of its advantages of environment friendliness, non-toxicity, and easy access.
According to Table 2, we can conclude that, compared with common refrigerants, the refrigerating capacity per unit volume of CO2 is 5.2 times that of NH3; 7.9 times that of R134a at 0-10 ℃; 4 times that of R410A; 8.4 times that of R600a; 8.2 times that of R152a, and 5.8 times that of R290.
From the perspective of thermodynamics, CO2 is in the state of supercritical fluid, with high specific heat, high thermal conductivity, and low kinematic viscosity, which makes it have better transmission performance, heat transfer characteristics, and higher heat transfer efficiency than liquid and other gases.
Compared with other refrigerants, the adiabatic index of CO2 is higher, the compression ratio of trans-critical refrigeration cycle is smaller (about 2-4), and the compression process is closer to an isentropic process. All these factors help to improve the indication efficiency of refrigeration.
From the perspective of environment protection, except for CO2 and NH3, other refrigerants have higher GWP. The GWP index of R410A is 2100 times that of CO2, and R134a is 1300 times that of CO2. Both can be put in the category of greenhouse gas that needs to be reduced. As a result, refrigerants like R410A and R134a can only be used as an alternative working medium and should not be used for a long time. As a natural gas in the earth's biosphere, CO2 has the lowest ODP and GWP, which are respectively 0 and 1.
Finally, considering usage safety, CO2 is non-toxic and nonflammable as a natural working fluid, making it a very safe substance. On the contrary, NH3, R600a, R152a, and R290 are all combustible, thus much more dangerous than CO2. By now, the industry hasn’t solved the safety problems caused by these four refrigerants yet.
CO2 refrigerants have better properties than other refrigerants and is gradually used in refrigeration compressors. Though strictly speaking, CO2 is not perfect either. With its low critical temperature (31.1 ℃) and high critical pressure (7.37 MPa), the requirements for compressors and pipelines are particularly high, which will surely increase the costs of product manufacturing. Nevertheless, the world’s top compressor manufacturers are studying improvement methods, such as the use of trans-critical circulation systems. We believe that the development prospect of CO2 refrigerant is bright, and it will play a leading role in the refrigeration compressor market in the future. As a part of natural refrigerants, there’s a promising future and market for CO2 in the immediate future when the voices of environmental protection are rising.
