第四代制冷劑是HFO（氫氟烯烴）和天然制冷劑（如氨、二氧化碳、碳氫化合物和水）。 這些制冷劑不會消耗臭氧層，并且全球變暖潛勢 (GWP) 較低或為零12。
HFO 是一種含有氫、氟和碳原子的合成化合物，但碳原子之間至少有一個雙鍵。 它們的通式為C<sub>n</sub>H<sub>x</sub>F<sub>y</sub>，其中n、x和y是整數3。
最常用作制冷劑的 HFO 是 R1234yf（2,3,3,3-四氟丙烯）、R1234ze（反式 1,3,3,3-四氟丙烯）、R1233zd（反式 1-氯-3,3,3-） 三氟丙烯）和R1233zde（順式1-氯-3,3,3-三氟丙烯）4。
HFO 是在 2000 年代和 2010 年代開發的，作為第三代制冷劑 HFC（氫氟碳化物）的替代品，后者被發現具有高 GWP 并會導致氣候變化。 HFO 的 ODP 為零或接近于零，GWP 非常低，范圍為 1 至 105。
HFO作為制冷劑具有許多優點，如穩定性高、毒性低、不易燃、沸點低、汽化潛熱高以及與潤滑油和金屬相容等。 它們還廣泛用于其他應用，例如發泡劑、溶劑、滅火器和氣溶膠推進劑。
由于越來越多的科學證據和公眾對其環境效益的認識，預計未來幾年全球重油的生產和消費將迅速增長。 2016年，197個國家簽署了《蒙特利爾議定書》基加利修正案，同意在具體期限內逐步減少氫氟碳化合物和其他強效溫室氣體的生產和使用。
基加利修正案被認為是全球應對氣候變化的重要一步，因為它有可能在本世紀末避免全球變暖0.4°C。 根據聯合國最新評估，逐步減少氫氟碳化合物還可以提高冷卻系統的能源效率，減少電力需求和碳排放。
天然制冷劑是自然存在于環境中的物質，其 ODP 為零，GWP 極低或為零。 它們包括氨 (NH<sub>3</sub>)、二氧化碳 (CO<sub>2</sub>)、碳氫化合物（如丙烷、丁烷和異丁烷）和水 (H<sub>2</sub>) >O）。
天然制冷劑自制冷早期以來一直被用作制冷劑，但隨著合成制冷劑（如 CFC、HCFC 和 HFC）的發展，其使用量逐漸減少。 然而，近年來，天然制冷劑作為合成制冷劑的環保替代品重新受到歡迎。
天然制冷劑作為制冷劑具有許多優點，例如高性能、低成本、廣泛可用性和對環境影響小。 它們還廣泛用于各種應用，例如家用、商業和工業制冷、熱泵、空調和運輸制冷。
然而，天然制冷劑也有一些缺點，如易燃、有毒、高壓或效率低等。 因此，天然制冷劑的選擇和使用需要認真考慮安全性、技術性和經濟性等方面。
因此，制冷劑的選擇和開發是一個復雜的動態過程，涉及熱力學、物理、化學、安全、經濟和環境等多個因素。 理想的制冷劑應該具有高性能、低環境影響、低成本和廣泛的可用性。
The fourth generation of refrigerants are HFOs (hydrofluoroolefins) and natural refrigerants (such as ammonia, carbon dioxide, hydrocarbons and water). These are refrigerants that do not deplete the ozone layer and have low or zero global warming potential (GWP)12.
HFOs are synthetic compounds that contain hydrogen, fluorine and carbon atoms, but have at least one double bond between the carbon atoms. They have the general formula of C<sub>n</sub>H<sub>x</sub>F<sub>y</sub>, where n, x and y are integers3.
The most common HFOs used as refrigerants are R1234yf (2,3,3,3-tetrafluoropropene), R1234ze (trans-1,3,3,3-tetrafluoropropene), R1233zd (trans-1-chloro-3,3,3-trifluoropropene) and R1233zde (cis-1-chloro-3,3,3-trifluoropropene)4.
HFOs were developed in the 2000s and 2010s as alternatives to the third generation of refrigerants, HFCs (hydrofluorocarbons), which were found to have high GWP and contribute to climate change. HFOs have zero or near-zero ODP and very low GWP, ranging from 1 to 105.
HFOs have many advantages as refrigerants, such as high stability, low toxicity, non-flammability, low boiling point, high latent heat of vaporization and compatibility with lubricants and metals. They are also widely used in other applications, such as foam blowing agents, solvents, fire extinguishers and aerosol propellants.
The global production and consumption of HFOs are expected to increase rapidly in the coming years, due to the growing scientific evidence and public awareness of their environmental benefits. In 2016, the Kigali Amendment to the Montreal Protocol was signed by 197 countries, which agreed to phase down the production and use of HFCs and other potent greenhouse gases by specific deadlines.
The Kigali Amendment is considered a major step forward in the global fight against climate change, as it has the potential to avoid up to 0.4°C of global warming by the end of the century. According to the latest assessment by the United Nations, the phase down of HFCs could also improve the energy efficiency of cooling systems and reduce the electricity demand and carbon emissions.
Natural refrigerants are substances that occur naturally in the environment and have zero ODP and very low or zero GWP. They include ammonia (NH<sub>3</sub>), carbon dioxide (CO<sub>2</sub>), hydrocarbons (such as propane, butane and isobutane) and water (H<sub>2</sub>O).
Natural refrigerants have been used as refrigerants since the early days of refrigeration, but their use declined with the development of synthetic refrigerants, such as CFCs, HCFCs and HFCs. However, in recent years, natural refrigerants have regained popularity as environmentally friendly alternatives to synthetic refrigerants.
Natural refrigerants have many advantages as refrigerants, such as high performance, low cost, wide availability and low environmental impact. They are also widely used in various applications, such as domestic, commercial and industrial refrigeration, heat pumps, air conditioning and transport refrigeration.
However, natural refrigerants also have some drawbacks, such as flammability, toxicity, high pressure or low efficiency. Therefore, the selection and use of natural refrigerants require careful consideration of the safety, technical and economic aspects.
Therefore, the selection and development of refrigerants is a complex and dynamic process that involves multiple factors, such as thermodynamic, physical, chemical, safety, economic and environmental aspects. The ideal refrigerant should have high performance, low environmental impact, low cost and wide availability.
