Republished with permission of Bitzer
Stratospheric ozone depletion as well as atmospheric greenhouse effect due to refrigerant emissions have led to drastic changes in the refrigeration and air conditioning technology since the beginning of the 1990s.
This is especially true for the area of commercial refrigeration and air conditioning systems with their wide range of applications. Please see part 1 and 2 in prior issues of RACA Journal for other references.
In addition to the flammable HFO refrigerants R1234yf and R1234ze(E) already described, non-flammable mixtures are now also available as R134a alternatives. As previously mentioned, the initial situation is most favourable for these. They achieve GWP values of approx. 600 − less than half of R134a (GWP = 1430). In addition, this type of blend can have azeotropic properties, so that they can be used like pure refrigerants.
For quite some time a blend has been applied on a larger scale in real systems – this was developed by Chemours and is called OpteonTM XP-10. Results available today are promising. This is also true for an R134a alternative designated Solstice® N-13 and offered by Honeywell which, however, differs regarding the blend composition.
The refrigerants are listed in the ASHRAE nomenclature under R513A (Chemours) and R450A (Honeywell). The same category also includes the refrigerant blends ARM-42 (ARKEMA) as well as R456A (Mexichem AC5X). All options show refrigerating capacity, power input, and pressure levels similar to R134a.
Thus, components and system technology can be taken over, only minor changes like superheat adjustment of the expansion valves are necessary. Polyolester oils are suitable lubricants which must meet special requirements, e.g. for the utilisation of additives. Prospects are especially favorable for supermarket applications in the medium temperature range in a cascade with CO₂ for low temperature, just as in liquid chillers with higher refrigerant charges where the use of flammable or toxic refrigerants would require comprehensive safety measures.
A special case is the refrigerant R515B: an azeotropic mixture of R1234ze(E) and small amounts of R227ea. This combination, declared by the manufacturer Honeywell as an R134a alternative, is nonflammable (A1) despite the very low GWP of approx. 300. However, as with the previously described R1234ze(E), this can only be considered an alternative under certain restrictions. The volumetric refrigerating capacity is also more than 20% lower than that of R134a or R1234yf.
Substitutes for R404A/R507A and R410A
Since the available HFO molecules (R1234yf und R1234ze) show a considerably smaller volumetric refrigerating capacity than the above mentioned HFC refrigerants, relatively large HFC proportions with high volumetric refrigerating capacity must be added for the particular alternatives. The potential list of candidates is rather limited, one option is R32 with its relatively low GWP of 675.
However, one disadvantage is its flammability (A2L), resulting also in a flammable blend upon adding fairly large proportions in order to increase the volumetric refrigerating capacity while maintaining a favorable GWP. For a non-flammable blend, on the other hand, a fairly large proportion of refrigerants with high fluor content (for example R125) must be added. A drawback here is the high GWP of more than approx. 900 for non-flammable R22/R407C alternatives and more than approx. 1300 with options for R404A/ R507A.
Compared to R404A/ R507A, however, this means a reduction down to a third. The future drastic ‘phase-down’ of F-Gases, e.g. as part of the EU F-Gas Regulation, already leads to a demand for R404A/ R507A substitutes with GWP values clearly below 500. Although this is possible with an adequate composition of the blend (high proportions of HFO, R152a, possibly also hydrocarbons), the disadvantage will be its flammability (safety groups A2L or A2).
In this case, the application will have higher safety requirements and will need an adequately adjusted system technology. R410A currently has no non-flammable alternatives for commercial applications. Either R32 as pure substance or blends of R32 and HFO can be used. Due to its high volumetric refrigerating capacity, this requires a very high proportion of R32, which is why only GWP values from approx. 400 to 500 can be achieved. With a higher HFO proportion, the GWP can be reduced even further, but at the cost of a clearly reduced refrigerating capacity.
All blend options described with R1234yf and R1234ze(E) show a more or less distinct temperature glide due to boiling point differences of the individual components. The same criteria apply as described in context with R407C. Beyond that, the discharge gas temperature of most R404A/R507A alternatives is considerably higher than for these HFC blends. In single stage low temperature systems this may lead to restrictions in the compressor application range or require special measures for additional cooling.
In transport applications or in low temperature systems with smaller condensing units, the compressors used can often not meet the required operating ranges, due to the high discharge gas temperatures. This is why refrigerant blends based on R32 and HFO with a higher proportion of R125 have also been developed. The GWP is slightly above 2 000, but below the limit of 2 500 set in the EU F-Gas Regulation from 2020.
The main advantage of such blends is their moderate discharge gas temperature, which allows the operation within the typical application limits of R404A. Tab 4 shows the potential blend components for the alternatives described above. With some refrigerants the mixture components for R22/R407C and R404A/R507A substitutes are identical, but their distribution in percent is different. In the meantime, Chemours, Honeywell, Arkema, Mexichem and Daikin Chemical have offered corresponding chemical variants for laboratory and field tests, and in some cases already for commercial use.
A number of refrigerants are still declared as being under development and are only made available for testing purposes under special agreements. Until now, trade names are often used although a larger number of HFO/HFC blends are already listed in the ASHRAE nomenclature.
Table 5 lists a range of currently available refrigerants or refrigerants declared as development products.
Due to the large number of different versions and the potential changes in development products, Bitzer has so far tested only some of the new refrigerants. This is why for the time being, only refrigerant properties of nonflammable alternatives for R134a and R404A/R507A (GWP < 1500) are listed which have already received an ASHRAE number and are commercially available.
For testing the ‘Low GWP’ refrigerants, AHRI (USA) has initiated the ‘Alternative Refrigerants Evaluation Program (AREP)’. It was established to investigate and evaluate a series of the products including halogen-free refrigerants. Some of these are also listed in Table 5.
Bitzer Refrigerant report_A-501-20-27_table4 and 5