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"Low GWP" HFOs and HFO/HFC blends as alternatives to HFCs The decision to use the "low GWP" refriger- ant R1234yf in mobile air conditioning sys- tems for passenger cars (see pages 11/12) also led to the development of alternatives for further mobile applications as well as stationary refrigeration, air conditioning and heat pump systems. Primary objectives are the use of single- component refrigerants and of mixtures with significantly reduced GWP and similar thermodynamic properties as the HFCs currently used predominantly. An essential basic component for this is R1234yf (CF3CF=CH2). This refrigerant belongs to the group of hydro-fluoro-olefins (HFO), i.e. unsaturated HFCs with molecu- lar double bonds. This group of HFOs also includes another substance called R1234ze(E), which has been mainly used as a propellant for PU foam and aerosol. R1234ze(E) (CF3CH=CFH) differs from R1234yf in its molecular structure. Both substances are the preferred choice in terms of their properties and are also used as basic components in HFO/HFC blends. The Global Warming Potential is very low − R1234yf with GWP 4 and R1234ze(E) with GWP 7. However, these refrigerants are flammable (safety class A2L), meaning the refrigerant quantity in the system must be considered in light of the installation location. In addition, there remain open questions concerning the long-term stability in stationary systems where long life cycles are common. Fur- thermore the volumetric refrigerating capacity is relatively low; for R1234yf it is close to the level of R134a, and more than 20% lower for R1234ze(E). There is also some uncertainty concerning flammability. In safety data sheets, R1234ze(E) is declared as non-flammable. However, this only applies to its transport and storage. When used as a refrigerant, a higher reference temperature for flammabil- ity tests of 60°C applies. At this tempera- ture, R1234ze(E) is flammable and there- fore classified in safety class A2L, like R1234yf. HFOs and HFO/HFC blends R1234ze(E) is sometimes referred to as a R134a substitute, but its volumetric refriger- ating capacity is more than 20% lower than that of R134a or R1234yf. The boiling point (-19°C) also greatly restricts the appli- cation at lower evaporation temperatures. Its preferred use is therefore in liquid chillers and high temperature applications. For further information see chapter Refriger- ants for special applications, page 37. The list of further potential HFO refriger- ants is relatively long. However, there are only few fluids that meet the requirements in terms of thermodynamic properties, flammability, toxicity, chemical stability, compatibility with materials and lubricants. These include e.g. the non-flammable (safe- ty group A1) low-pressure refrigerants R1224yd(Z), R1233zd(E), R1336mzz(E), R1336mzz(Z) and R514A (blend of R1336mzz(Z)/R1130(E)). These are primari- ly an option for liquid chillers with large turbo-compressors, and they can be used with positive displacement compressors in high-temperature applications as well as ORC systems. Further information: see chapter Refrigerants for special applica- tions, page 37. R1224yd(Z) and R1233zd(E) belong to the group of HCFO (hydro-chloro-fluoro-olefins); they have a (very) low ozone depletion potential (ODP). Upon release into the atmosphere, however, the molecule rapidly disintegrates. On the other hand, there are currently no candidates from the HFO family with similar volumetric refrigerating capacity such as R22/R407C, R404A/R507A and R410A available for commercial use. Direct alter- natives for these refrigerants with signifi- cantly lower GWPs must therefore be “con- structed” as a mixture of R1234yf and/or R1234ze(E) with HFC refrigerants, possibly also small proportions of hydrocarbons, CO2 or other suitable molecules. However, due to the properties of the HFC refrigerants suitable as blend components, flammability and GWP are related diametri- cally to one another. In other words: Blends as alternatives to R22/R407C of GWP < approx. 900 are flammable. This is also true with alternatives for R404A/R507A in blends of GWP < approx. 1300 and for R410A in blends of GWP < approx. 2000. The reason for this is the high GWP of each of the required non-flammable com- ponents. There are a few exceptions, which are discussed in chapter Further develop- ment projects with “Low GWP” refrigerants, page 26. For R134a alternatives, the situation is more favorable. Due to the already quite low GWP of R134a, a blend with R1234yf and/or R1234ze(E) allows a formulation of non-flammable refrigerants with a GWP of approx. 600. Thus, primarily two directions for develop- ment are pursued: o Non-flammable HFC alternatives (blends) with GWP values according to the above mentioned limits – safety group A1. Regarding safety require- ments, these refrigerants can then be utilized similar to currently used HFCs. o Flammable HFC alternatives (blends) with GWP values below the above men- tioned possible limits – according to safety group A2L (for refrigerants of lower flammability). This group of refrigerants is then subject to charge limitations according to future re- quirements for A2L refrigerants. Meanwhile, there are development projects using refrigerant components with a much higher volumetric refrigerating capacity and pressure than R1234yf and R1234ze(E). These can then be used to "formulate" mix- tures with R32 as an alternative to R410A, which are optimised for certain properties. See additional information in chapter Fur- ther development projects with “Low GWP” refrigerants, page 26. R134a alternatives In addition to the flammable HFO refriger- ants R1234yf and R1234ze(E) already described, non-flammable mixtures are now also available as R134a alternatives. As previously mentioned, the initial situa- tion is most favorable for these. They achieve GWP values of approx. 600 − less than half of R134a (GWP = 1430). In addition, this type of blends can have azeotropic properties, so that they can be used like pure refrigerants. 24PDF Image | REFRIGERANT REPORT 21
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