Carbon dioxide has high density and low viscosity, its flow loss is small, heat transfer effect is good, and through the enhancement of heat transfer, it can make up for the shortcomings of its low cycle. At the same time, carbon dioxide environmental performance is good, low cost, easy to obtain, good stability, which is conducive to reduce the unit volume. *Importantly, it is safe, non-toxic and non combustible, which has obvious advantages over R290.

Of course, using carbon dioxide as refrigerant has its disadvantages. The high critical pressure and low critical temperature of carbon dioxide also bring many difficulties to it. No matter subcritical cycle or transcritical cycle, the operating pressure of carbon dioxide refrigeration system will be higher than that of traditional refrigeration and air conditioning system, which will inevitably bring many new requirements to the design of system and components. At the same time, the efficiency of carbon dioxide refrigeration system is relatively low.

At present, the research and application of carbon dioxide mainly focus on three aspects

On the one hand, in the field of automobile air conditioning, the refrigerant with no harm to the environment must be used as soon as possible due to the large amount of refrigerant emission;

The second is the heat pump water heater. There is a considerable temperature slip in the heat release of carbon dioxide under supercritical conditions, which is conducive to heating the hot water to a higher temperature;

The third aspect is to use carbon dioxide as low-temperature refrigerant in cascade refrigeration cycle considering its good low-temperature flow performance and heat transfer characteristics.

In cascade refrigeration system, carbon dioxide cycle operates under subcritical condition. At this time, carbon dioxide is used as low-pressure refrigerant, and NH3 is used as refrigerant for high-pressure stage. Compared with other low-pressure refrigerants, even at low temperatures, the viscosity of carbon dioxide is very small, and its heat transfer performance is good, because the use of latent heat, its refrigeration capacity is quite large.

At present, several cascade refrigeration systems using carbon dioxide as low-temperature refrigerant have been established in supermarkets in Europe. The operation shows that the system is technically feasible, and this system is also suitable for low-temperature freeze-drying process.

At present, there are mainly two technical schemes for the replacement of R22 refrigerant in the world

One is represented by Nordic countries and South Korea, which advocates using natural working fluids as substitutes, such as pure working fluids R290, R1270, R744, R600a, R600, R717, etc., as well as mixtures of HCs;

The other is to use HFCS as substitutes, such as the non azeotropic mixture R410A of United signal company, R407C of DuPont company and i.c.i company, and R32 and R152a. The ODP of these refrigerants is 0, which can protect the ozone layer, but it will produce greenhouse effect.

At present, the prospect of carbon dioxide in the domestic market is a bit like "seeing flowers in the fog", as Wang Liqun said, they all know its good, but it is rarely used. The development of carbon dioxide has not been seen in the domestic air-conditioning industry for the time being, and it has just taken steps in the domestic refrigeration and refrigeration market. However, in the field of heat pump water heater, the standard for core accessories of carbon dioxide heat pump water heater - GB / t26181-2010 will be issued soon. Le Hongsheng, director of the heat pump propulsion Office of Shanghai Hitachi Electric Co., Ltd., who participated in the standard formulation, believes that although the heat pump water heater using carbon dioxide refrigerant in China has not yet come out, and the core component compressor is also in the research and development stage, "but the formulation of this advanced standard will play a good role in the market promotion of the products."

（1） Main advantages of CO2 refrigerant

1. CO2 is a natural substance, ODP = 0, GWP = 1. The use of CO2 as a refrigerant has no destructive effect on the atmospheric ozone layer, can reduce the global greenhouse effect, has a wide range of sources, does not need to be recycled, can greatly reduce the cost of refrigerant replacement, save energy, fundamentally solve the environmental pollution problem of compounds, with good economy.

2. CO2 is safe, non-toxic, non combustible, and has good thermal stability. It will not decompose harmful gases even at high temperature. In case of leakage, there will be no damage to human body, food and ecology.

3. CO2 has thermophysical properties suitable for refrigeration cycle and equipment. With small molecular weight and large refrigeration capacity, the unit refrigerating capacity at 0 ℃ is 5-8 times higher than that of conventional refrigerants. Therefore, for the refrigeration system with the same cooling load, the size and weight of the compressor can be significantly reduced, and the whole system is very compact; the lubrication conditions are easy to meet, and the common materials in the refrigeration system are not corroded, which can improve the sealing performance of the open compressor and reduce leakage.

4. The viscosity of CO2 is small. At 0 ℃, the kinematic viscosity of CO2 saturated liquid is only 5.2% of NH3 and 23.8% of R12. The flow resistance of the fluid is small, and the heat transfer performance is better than CFC refrigerants, which can improve the heat dissipation of totally enclosed refrigeration compressor.

（2） Main shortcomings and analysis of CO2 refrigerant

1. The critical pressure of CO2 is high (7.38mpa), so the working pressure of CO2 transcritical refrigeration cycle is much higher than that of traditional subcritical two-phase refrigeration cycle, which is about 6-8 times of that of CFC or HCFC system.

Therefore, the refrigerant pipeline system in the refrigeration system must be subject to safety analysis. However, since the refrigeration capacity of CO2 per unit volume is about 5-8 times of that of conventional refrigerant, the volume flow of CO2 required by the system is very small, and the explosion energy of gas in the equipment is a function of the product of pressure and volume, so although the working medium pressure of the system is high, the volume is small, the product of pressure and volume is not different from that of conventional refrigerant, and the explosion energy of gas in the equipment is not increased much.

Based on the reliability theory, the reliability of CO2 transcritical refrigeration system under different pipe conditions is studied and analyzed according to the different influencing factors and changing rules of pipeline reliability. The conclusion is that when the outer diameter of pipeline system is given, as long as the pipe material and pipe wall thickness are reasonably selected, the reliability of the system under the given pressure can be guaranteed Safety: the higher operation pressure of CO2 transcritical cycle can be reasonably solved; the existing steel pipe can be directly applied, while the existing copper pipe should be selected after safety analysis according to the pipe diameter and wall thickness. Therefore, the safety problems caused by high operating pressure of CO2 will not affect the promotion and application of CO2 as an environmental protection refrigerant.

2. The cop of CO2 single-stage compression transcritical cycle is lower than that of R12, R22, R134a and other conventional refrigerants at the same temperature. In view of the low coefficient of performance of CO2 refrigeration cycle, scholars have found that it can be solved by improving the system cycle mode and optimizing the system equipment, such as adopting two-stage compression and adopting expander to recover part of expansion work to improve the refrigeration cycle efficiency. Theoretical analysis shows that the efficiency of CO2 transcritical cycle with expander is higher than that of throttling expansion cycle with conventional refrigerant.

Application prospect of CO2 refrigerant

Since g. Lorentzen, the former president of the international refrigeration society, proposed the use of CO2 as an environmentally friendly refrigerant and the theory of transcritical cycle [7], the advantages of CO2 in environmental protection and performance have attracted more and more attention of scholars all over the world. The development of its systems and components has also been greatly developed. The existing research results show that when the CO2 system is operating at high ambient temperature (above 45 ℃) The cooling performance of the system is lower than that of the traditional system, which is similar to that of the traditional system at 35-45 ℃, and the performance is better when the temperature is below 35 ℃. At present, the application prospect of CO2 transcritical cycle is very good in automobile air conditioning, heat pump, commercial refrigeration equipment, food refrigeration and so on. The performance of CO2 transcritical cycle is equivalent to or even better than that of the original R22 or R12 or R134a refrigeration device. It is especially suitable for the occasion where large temperature change is required, and the performance is better at lower evaporation temperature.

As a natural refrigerant, CO2 has incomparable advantages over other non natural refrigerants in terms of its physical properties. With the deepening of the research on CO2 refrigeration system, CO2 as a new generation of refrigerant will be further promoted. It is believed that in the near future, CO2 will be widely used in automobile air conditioning system, commercial refrigeration system, residential air conditioning system and heat pump drying system of various production enterprises.

As a refrigerant, people want it to be environmentally friendly, efficient and economical, but in fact, there is no perfect refrigerant. Compared with other refrigerants, CO2 has obvious advantages in environmental protection, safety, economy and refrigeration capacity per unit volume, but also has deficiencies in operating pressure and efficiency. The current research shows that the safety and reliability of CO2 system under transcritical pressure can be ensured as long as the thickness of pipe and pipe wall is reasonably selected, and the reasonable improvement of CO2 transcritical cycle mode can effectively reduce throttling loss and improve the cycle efficiency of the system. As one of the environmental protection refrigerants, CO2 has a good application prospect Development will be widely used. Xuemei has always been at the forefront of technology. Our company actively participates in the project implementation, R & D and promotion of energy-saving products, implements the commitment of energy-saving and emission reduction, and develops new energy-saving and efficient carbon dioxide compressor with natural working medium and reliable environmental protection.