Aspects on the development of HFO and HFO/HFC refrigerants

The decision to use the "low GWP" refrigerant R1234yf in mobile air conditioning systems for passenger cars (R1234yf) 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 that are 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 molecular 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, open questions remain concerning the long-term stability in stationary systems where long life cycles are common. Furthermore 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 flammability tests of 60°C applies. At this temperature, R1234ze(E) is flammable and therefore classified in safety class A2L, like R1234yf.

R1234ze(E) is sometimes referred to as a R134a substitute, but its volumetric refrigerating capacity is more than 20% lower than that of R134a or R1234yf. The boiling point (-19°C) also greatly restricts the application at lower evaporation temperatures. Its preferred use is therefore in liquid chillers and high temperature applications. For further information: Refrigerants for special applications.

The list of further potential HFO refrigerants 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 (safety group A1) low-pressure refrigerants R1224yd(Z), R1233zd(E), R1336mzz(E), R1336mzz(Z) and R514A (blend of R1336mzz(Z)/R1130(E)). These are primarily 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: Refrigerants for special applications

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 alternatives for these refrigerants with significantly lower GWPs must therefore be “constructed” 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 diametrically to one another. In other words: Blends as alternatives to R22/R407C of GWP < approx. 900 are flammable. This is also true for 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 components. There are a few exceptions: Further development projects.
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 development are pursued:

• Non-flammable HFC alternatives (blends) with GWP values according to the above mentioned limits – safety group A1. Regarding safety requirements, these refrigerants can then be utilized similar to currently used HFCs.
• Flammable HFC alternatives (blends) with GWP values below the above mentioned possible limits – according to safety group A2L (for refrigerants of lower flammability). This group of refrigerants will be subject e.g. to charge limits according to the current standards 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" mixtures with R32 as an alternative to R410A, which are optimised for certain properties. Additional information: Further development projects.