HFO/HFKW-blends as alternatives to HFKWs

HFO / HFKW-blends as alternatives to HFKWs

The decision on the use of " low GWP" refrigerant 1234yf in vehicle air conditioning systems and the development of alternatives for other mobile applications and stationary systems has meanwhile initiated.

Primary objectives are the formulation of mixtures with significantly reduced GWP with similar thermodynamic properties as the HFKWs used today.

The basic components for this are the refrigerants R1234yf and R1234ze (E), which belong to the group of hydro-fluoro-olefins (HFO) with chemical double bond. They are characterized as preferred candidates in the sum of their characteristics. However, both refrigerants are flammable ( safety group A2L ). In addition, the volumetric cooling capacity is relatively low; it is approximately at R134a level for R1234yf and even more than 20% lower for R1234ze (E).

The list of other potential refrigerants from the HFO group is relatively long. However, there are only a few substances that meet the high requirements regarding thermodynamic properties, flammability, toxicity, chemical stability, compatibility with materials and lubricants. These include, for example, low-pressure refrigerants such as R1336mzz (Z) and R1233zd (E), which are primarily an option for liquid chillers with large turbocompressors or can be used with positive displacement compressors in high-temperature applications. R1233zd (E) has a (very) low ozone depletion potential (ODP). Upon release into the atmosphere, however, a rapid disintegration of the molecule occurs.

On the other hand, no suitable candidates with similar volumetric cooling capacity as R22 / R407C, R404A / R507A and R410A are currently in prospect for commercial application. Direct alternatives for these refrigerants with significantly lower GWPs must therefore be "formulated" as a mixture of R1234yf and / or R1234ze (E) with HFKW refrigerants, possibly even small amounts of hydrocarbons or CO2.

However, due to the properties of the HFKW refrigerants suitable as mixture components, flammability and GWP are diametrically related. In other words, mixtures as alternatives to R22 / R407C with a GWP

For R134a alternatives , the situation is more favorable. Due to the already relatively low GWP of R134a, a mixture with R1234yf and / or R1234ze (E) makes it possible to formulate non-flammable refrigerants with GWP approx. 600.

So currently two development directions are pursued:

• Non-flammable HFKW alternatives (mixtures) with GWP values ​​according to the previously mentioned limits - Safety group A1. These refrigerants can then be used in terms of safety requirements in the same way as currently used HFKWs.
• Flammable HFKW alternatives (mixtures) with GWP values ​​below any possible limits mentioned above - according to safety group A2L (for flame retardant refrigerants).

This group of refrigerants will then be subject to, among other things, fill quantity restrictions according to the future valid specifications for A2L refrigerants.

Non-flammable R134a alternatives

As previously mentioned, the starting point for developing non-combustible blends is most favorable for R134a alternatives. GWP values ​​of approx. 600 can be reached here. This is less than half compared to R134a (GWP100 = 1430). In addition, such mixture variants may have azeotropic properties, they can therefore be applied as single-component refrigerant.

For some time now, a blend developed by Chemours called Opteon ® XP-10 has been used on a larger scale in real plants. The results available so far are promising.

This also applies to a Honeywell-offered R134a alternative called Solstice N-13, which, however, differs in the mixture composition. Meanwhile, the refrigerants are listed in the ASHRAE nomenclature under R513A (Chemours) and R450A (Honeywell). The same category also includes the blends ARM-42 (ARKEMA) and R456A (Mexichem AC5X).

With these options, cooling capacity, power requirements and pressure levels are similar to R134a. This also allows components and system technology to be adopted. Only minor changes, such as overheating adjustment of the expansion valves, are required. Suitable lubricants are polyol ester oils, which, however, have to fulfill special requirements, for example when using additives.

Particularly favorable perspectives arise in supermarket applications in the standard refrigeration area in cascade with CO 2 for deep freezing. Likewise in liquid chillers with larger refrigerant charges, where operation with flammable or toxic refrigerants would require extensive safety measures.

Alternatives for R22 / R407C, R404A / R507A and R410A

Since the available HFO molecules (R1234yf and R1234ze) have a significantly lower volumetric cooling capacity than the above-mentioned HFKW refrigerants, relatively large proportions of HFKWs with high volumetric cooling capacity have to be mixed for the respective alternatives. The potential candidate list is quite limited, including R32 with the benefit of a relatively low GWP of 675.

With kind permission of Bitzer Kühlmaschinenbau GmbH 

Source: Bitzer Refrigerant Report 19