第四代制冷剂是HFO（氢氟烯烃）和天然制冷剂（如氨、二氧化碳、碳氢化合物和水）。 这些制冷剂不会消耗臭氧层，并且全球变暖潜势 (GWP) 较低或为零12。
HFO 是一种含有氢、氟和碳原子的合成化合物，但碳原子之间至少有一个双键。 它们的通式为C<sub>n</sub>H<sub>x</sub>F<sub>y</sub>，其中n、x和y是整数3。
最常用作制冷剂的 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（顺式1-氯-3,3,3-三氟丙烯）4。
HFO 是在 2000 年代和 2010 年代开发的，作为第三代制冷剂 HFC（氢氟碳化物）的替代品，后者被发现具有高 GWP 并会导致气候变化。 HFO 的 ODP 为零或接近于零，GWP 非常低，范围为 1 至 105。
HFO作为制冷剂具有许多优点，如稳定性高、毒性低、不易燃、沸点低、汽化潜热高以及与润滑油和金属相容等。 它们还广泛用于其他应用，例如发泡剂、溶剂、灭火器和气溶胶推进剂。
由于越来越多的科学证据和公众对其环境效益的认识，预计未来几年全球重油的生产和消费将迅速增长。 2016年，197个国家签署了《蒙特利尔议定书》基加利修正案，同意在具体期限内逐步减少氢氟碳化合物和其他强效温室气体的生产和使用。
基加利修正案被认为是全球应对气候变化的重要一步，因为它有可能在本世纪末避免全球变暖0.4°C。 根据联合国最新评估，逐步减少氢氟碳化合物还可以提高冷却系统的能源效率，减少电力需求和碳排放。
天然制冷剂是自然存在于环境中的物质，其 ODP 为零，GWP 极低或为零。 它们包括氨 (NH<sub>3</sub>)、二氧化碳 (CO<sub>2</sub>)、碳氢化合物（如丙烷、丁烷和异丁烷）和水 (H<sub>2</sub>) >O）。
天然制冷剂自制冷早期以来一直被用作制冷剂，但随着合成制冷剂（如 CFC、HCFC 和 HFC）的发展，其使用量逐渐减少。 然而，近年来，天然制冷剂作为合成制冷剂的环保替代品重新受到欢迎。
天然制冷剂作为制冷剂具有许多优点，例如高性能、低成本、广泛可用性和对环境影响小。 它们还广泛用于各种应用，例如家用、商业和工业制冷、热泵、空调和运输制冷。
然而，天然制冷剂也有一些缺点，如易燃、有毒、高压或效率低等。 因此，天然制冷剂的选择和使用需要认真考虑安全性、技术性和经济性等方面。
因此，制冷剂的选择和开发是一个复杂的动态过程，涉及热力学、物理、化学、安全、经济和环境等多个因素。 理想的制冷剂应该具有高性能、低环境影响、低成本和广泛的可用性。
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.
