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I. Historical Development Since American inventor Poulson invented the first steam-compression refrigeration machine in 1834, refrigerants have accompanied refrigeration machines throughout a history of 172 years. Ether was the first refrigerant to be used, and later various natural substances such as air, CO2, ammonia, and SO2 were employed as refrigerants. Among them, CO2 and SO2, due to their increased effectiveness as refrigerants, were used for a long time; SO2 was in use for as long as 60 years before it was phased out, while CO2 was utilized in marine refrigeration systems for 50 years, until it was replaced by Freon refrigerants in 1955. Ammonia, as a refrigerant with excellent thermodynamic properties, has been used in large-scale refrigeration systems and is still in use to this day. The emergence of the Freon refrigerant in 1929 led to rapid development of compression refrigerators, which surpassed ammonia refrigerators in terms of applications. This advancement spurred swift growth in the refrigeration industry and became one of the milestones in its development. Azeotropic mixed refrigerants were put into use in the 1950s, while non-azeotropic mixed refrigerants were introduced in the 1960s ； Thereafter, the development of various refrigerants based on halohydrocarbons reached a state of considerable maturity. By the 1980s, the formal recognition of the need to phase out ozone-depleting substances such as CFCs, the signing of the Montreal Protocol, and subsequently the signing of the Kyoto Protocol aimed at limiting greenhouse gas emissions in developed countries to curb global warming, all contributed to the development of refrigerants that are based on HFCs and are designed to be environmentally friendly and energy-efficient. Overall, the development of refrigerants has progressed in line with increasing demands for safety, cost-effectiveness, and environmental protection. It has evolved from initial natural refrigerants that were flammable, explosive, and toxic, to refrigerants that are safer for human health and more cost-effective, and has now entered an era focused on environmentally friendly and energy-efficient refrigerants. II. Requirements imposed on refrigerants by environmental protection issues In 1974, Professors Molina and Rowland of the University of California in the United States proposed that the chlorine atoms in halocarbons could destroy the atmospheric ozone layer. Halogenated hydrocarbon refrigerants, including CFCs and HCFCs, all cause damage to the ozone layer, with CFCs having the greatest destructive effect. It has been proven that ozone layer depletion causes the following effects: (1) damage to the immune system and an increased incidence of skin cancer ； (2) Increased incidence of cataracts ； (3) Marine food webs will be severely disrupted ； (4) Disrupting terrestrial ecosystems ； (5) Exacerbate air pollution ； (6) Accelerates the aging of outdoor plastic materials. For this reason. In 1987, an United Nations environmental organization held a conference in Montreal, Canada, and adopted the Montreal Protocol on Substances that Deplete the Ozone Layer, which officially established a schedule for gradually reducing the production and consumption of CFCs. The Seventh Meeting of the Parties to the Montreal Protocol, held in Vienna in December 1995, established a schedule for restricting the production and consumption of CFCs and HCFCs in both developed and developing countries ahead of schedule. An appropriate greenhouse effect is essential for the Earth. Without the greenhouse effect, the average surface temperature of Earth would be only –18°C, whereas the actual average surface temperature is 15°C. However, an excessive accumulation of greenhouse gases in the atmosphere causes global warming, which leads to some adverse or unpredictable effects: (1) rising average sea levels ； (2) Climate change is difficult to estimate accurately ； (3) Crop yields are unpredictable ； (4) Ecosystem changes in local areas are highly sensitive. In December 1997, the Third Conference of the Parties to the United Nations Framework Convention on Climate Change, held in Kyoto, Japan, adopted the Kyoto Protocol, which aimed to limit greenhouse gas emissions from developed countries in order to curb global warming. It stipulated that by 2010, emissions of six greenhouse gases, including carbon dioxide, from all developed countries should be reduced by 5.2% compared to 1990 levels. On February 16, 2005, the Kyoto Protocol officially entered into force. The United States signed the Kyoto Protocol in 1998. However, in March 2001, Bush announced his refusal to ratify the Kyoto Protocol, using as excuses that \"reducing greenhouse gas emissions would affect the economic development of the United States\" and that \"developing countries should also bear the responsibility for reducing and limiting greenhouse gas emissions.\" Chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs) refrigerants are all considered greenhouse gases. As a result, refrigerants that have been widely used in the past and are still in use today, such as CFC11, CFC12, HCFC22, HCFC123, etc., all have an ODP value of not zero; they are all refrigerants that are harmful to the atmospheric environment, and their use should be restricted or prohibited ； HFC refrigerants such as R134a, and ones like R407C and R410A, have GWP values that are high or not zero, thereby contributing to the greenhouse effect. Both the issue of ozone layer depletion and the greenhouse effect impose restrictions and requirements on refrigerants: the ODP value of the refrigerants used in refrigerators should be 0, and their GWP value should be 0 or as low as possible. III. Current Status of Refrigerant Substitution 1. Substitution for R12 Internationally, there are mainly the following approaches to replacing R12: (1) The use of R134a, as adopted in the United States and Japan ； (2) Adopting HC600a as in Germany and other European countries ； (3) Some manufacturers in our country use HFC152a/HCFC22. Each of these three options has its own advantages and disadvantages. R134a was proposed as a replacement refrigerant for R12. R134a has an ODP value of 0 and a GWP of 875, and many of its thermodynamic properties are very similar to those of R12. Its thermal conductivity for liquids and gases is significantly higher than that of R12. However, R134a has certain weaknesses in terms of physical properties; factors such as a high pressure ratio, low latent heat, strong polarity leading to susceptibility to hydrolysis, and insolubility in mineral oils make the replacement process complex. This requires the development of specialized compressors, polar oils, and heat exchangers, as well as corresponding adjustments to the refrigeration system and production lines. Its performance is not entirely satisfactory; it has high energy consumption and low cooling capacity at low temperatures. Due to its high GWP value, R134a has been included in the greenhouse gas inventory under the Kyoto Protocol, and the international community recognizes it as a mere transitional refrigerant. Due to the characteristics of its policies and regulations, manufacturers in the United States place a high emphasis on safety issues, and prefer to use refrigerants that are not particularly efficient but are safe and reliable. R600a is a hydrocarbon with an ODP value and a GWP value of 0, making it an environmentally friendly refrigerant. However, it is flammable, with a safety category of A3. R600a has a higher critical temperature and critical specific volume than R12, resulting in no significant efficiency loss when operating at higher condensation temperatures ； The pressure ratio is higher than that of R12, but the exhaust temperature is lower, which is more favorable for the compressor’s operation. In addition, it has the advantages of high cooling efficiency, low cost, and good miscibility with mineral oil. Many people advocate using R600a as a permanent replacement for R12 in applications with low cooling temperatures, such as refrigerators. Many manufacturers use it widely in household refrigerators; half of the refrigerator products on the domestic market use R600a. Due to the slightly lower cooling capacity, the refrigeration system needs to be redesigned and the production line requires modification ； Due to its flammability, high standards of fire prevention measures are required during production and maintenance. HFC152a/HCFC22 is a refrigerant proposed by Xi’an Jiaotong University. Since it contains HCFC22, which is restricted and prohibited under the Montreal Protocol, its use in China is limited to 20–30 years, and it can only serve as a transitional alternative product. 2. Replacement of R22: Compared to the replacement of R12, replacing R22 is more complex. Practice has shown that no single-component vapor pressure curve is similar to that of R22 ； Simulation results of the theoretical air-conditioning cycle show that the coefficient of performance (COP) and volumetric cooling capacity of any HFCs are inferior to those of R22. Therefore, the alternatives to R22 are now more diverse. Currently, there are mainly two alternative options: one is the use of HFCs, such as R407C and R410A, represented by the United States and Japan ； Another approach is adopted by countries such as Germany and some Nordic nations, which use natural refrigerants such as HC290, HC1270, CO2, NH3, etc. R407C is a ternary azeotropic refrigerant that was proposed as a substitute for R22. It is immiscible with mineral oil, and its volumetric cooling capacity and coefficient of performance under air-conditioning conditions are slightly lower than those of R22. When replacing the air conditioning system, only the lubricant and refrigerant need to be replaced. However, under low-temperature conditions, the volumetric cooling capacity is much lower than that of R22, and the difference between the bubble point and dew point is large (up to 7°C); therefore, it is best to design the heat exchanger as a counterflow type. R407C has an ODP value of 0, but it still has a high GWP value. R410A is a binary near-azeotropic refrigerant with a bubble-dew point difference of only 0.2°C, and its properties are similar to those of a pure substance. Under air-conditioning conditions, the volumetric cooling capacity and coefficient of performance are similar to those of R22; whereas under low-temperature conditions, the volumetric cooling capacity is about 60% higher than that of R22, and the coefficient of performance is also about 5% higher. Compared to R407C, R410A refrigeration systems have a smaller volume and higher energy efficiency under low-temperature conditions. However, the pressure of R410A is much higher than that of R22, so it cannot be used as a direct replacement in existing systems; instead, the compressor and the system itself need to be redesigned. R410A has an AODP value of 0, but still has a high GWP value. Compared to R407C, R410A has a larger cooling capacity per unit volume and a smaller system size, which reduces the material costs associated with it; as such, it holds promise for use in new household air conditioning and heat pump systems. R290, also known as propane, has standard boiling points, critical temperature, and pressure that are very similar to those of R22; its saturated vapor pressure curve is also quite similar to that of R22. Its other thermal properties are similar to or even better than those of R22 in many aspects. At 7℃, the latent heat of vaporization is 84.4% higher than that of R22 ； At the same temperature, R290 has a lower exhaust temperature than R22, a dynamic viscosity lower than that of R22, and a thermal conductivity higher than that of R22. It is also highly miscible with mineral oils, has an ODP value of 0, and a GWP value of 3. All these indicate that R290 has great potential as a substitute for R22. However, it is a flammable substance and poses a risk of combustion and explosion within certain concentration ranges. The production of high-purity R290 requires a very strict manufacturing process, which results in a relatively high price for R290. When safety measures such as fire protection are properly in place, R290 will be an excellent alternative to R22. CO2 has been used as an important refrigerant for a long time in history, and in recent years it has been put to use again due to the issue of ozone layer depletion. CO2 is a natural substance, inexpensive, and easily available ； Non-toxic, non-flammable ； With an ODP of 0 and a GWP value of 1, there is no issue related to recycling, making it an environmentally friendly refrigerant ； It has stable physicochemical properties, good miscibility with lubricants, and low viscosity ； High adiabatic index, resulting in high compressor exhaust temperature ； High latent heat of vaporization ； High working pressure results in a very small specific volume of intake air, leading to a higher cooling capacity per unit volume. This allows the compressor size to be reduced, thereby making the system structure more compact ； The critical temperature is 31.3°C, and the critical pressure is 7.372 MPa. As early as 1995, CRIEPI in Japan, Tokyo Electric Power Company, and DENSO Corporation began collaborating on research into CO2 heat pump systems using transcritical cycles. They continued to improve these heat pump devices, and commercial versions of heat pump water heaters were developed and brought to market in 2002. Subsequently, companies such as Sanyo and Daikin also successively developed their own heat pump water heater products. The United States, Japan, and some countries in Europe **have already developed CO2 car air conditioning systems, which have been installed in vehicles for trial use. The main challenges facing current CO2 transcritical cycle air conditioning systems are improving efficiency and reducing costs. This depends on breakthroughs and improvements in certain key technologies, such as the design and selection of compressors, issues related to the system’s pressure resistance and high-pressure protection, the design of efficient heat exchangers, and the recovery of expansion losses. THR03 is a ternary mixed refrigerant developed by Tsinghua University. Its ODP value is 0, and its GWP value is 830, which is lower than that of R407C and R410A ； The latent heat of vaporization is high, the specific heat of the gas phase is high, the exhaust temperature is low, the thermal conductivity in the two-phase region is high, the viscosity in the two-phase region is low, and the flow resistance is low ； It has good compatibility with existing compressor materials; aside from replacing mineral oil (MO) with ester oil (POE), the structural design of the compressors in R22 systems and the motor insulation materials generally require no changes ； The THR03 refrigerant is non-flammable. According to relevant literature, THR03 has a higher cycle performance coefficient than both R407C and R410A. Its volumetric cooling capacity is very close to that of R22, its exhaust temperature is lower than that of R22, its evaporation pressure is similar to that of R22, and its condensation pressure is lower than that of R407C. However, it shares the same problem as R407C and R410A: it has a relatively high GWP value, meaning it still exerts a significant greenhouse effect. Liquefied petroleum gas, LNG, is mentioned in foreign literature as a potential alternative refrigerant to R22; that is, LNG, which is produced locally and sold in large quantities, can be used as a substitute for R22 as a refrigerant. Some studies have conducted comparative experiments on R22, R290, and LNG. The composition of the natural gas used in these experiments was 98.95% propane (R290), 1.007% ethane (R170), 0.0397% isobutane (R600a), with very small amounts of other components. The results show that when R290 or LNG is used as a substitute for R22 in heat pump cycles or cooling applications, system performance can be achieved or even exceeded that of systems using R22, although their cooling and heating capacities are reduced. As a refrigerant, LNG performs better than pure R290, and it is an excellent alternative to R22 as a refrigerant. IV. Conclusion An ideal refrigerant should meet the requirements in four aspects: ozone depletion potential (ODP), global warming potential (GWP), flammability, and toxicity. Through extensive screening of possible alternative substances, the academic community has reached a consensus that no pure fluid can fully meet the requirements in these 4 aspects; they all have at least one deficiency. Looking at the replacement of the two most widely used refrigerants, R12 and R22, there are currently two main trends in the development of alternative substances: one is HFCs and their mixtures, and the other is natural substances such as ammonia, R600a, propane, CO2, etc., as well as their mixtures. The choice of refrigerant as a substitute should be determined by taking into account specific factors such as the environment, policies, and application conditions. For environmentally friendly refrigerants with poor performance, further research should be conducted to overcome the key challenges and improve their operational efficiency ； For environmentally friendly refrigerants with good performance but certain drawbacks such as flammability or toxicity, their use should be retained as much as possible; meanwhile, progress in related fields such as materials science, automatic control, and safety should be accelerated through research and application.