“Low GWP” HFO refrigerants R1234yf and R1234ze(E)
The ban on the use of R134a in mobile air-conditioning systems within the EU has triggered a series of research projects. Apart from the CO2 technology (page 35), new refrigerants with very low GWP values and similar thermodynamic properties as R134a have been developed.
In early 2006, two refrigerant mixtures were introduced under the names “Blend H” (Honeywell) and “DP-1” (DuPont). INEOS Fluor followed with another version under the trade name AC-1. In the broadest sense, all of these refrigerants were blends of various fluorinated molecules.
During the development and test phase it became obvious that not all acceptance criteria could be met, and thus further examinations with these blends were discontinued. Consequently, DuPont (meanwhile Chemours) and Honeywell bundled their research and development activities in a joint venture which focused on 2,3,3,3-tetrafluoropropene (CF3CF=CH2). This refrigerant, designated R1234yf, belongs to the group of hydro fluoro olefins (HFO). These refrigerants are unsaturated HFCs with a chemical double bond.
The global warming potential is extremely low (GWP100 = 4). When released to the atmosphere, the molecule rapidly disintegrates within a few days, resulting in a very low GWP. This raises certain concerns regarding the long-term stability in refrigeration circuits under real conditions. However, extensive testing has demonstrated the required stability for mobile air-conditioning systems.
R1234yf is mildly flammable as measured by ASTM 681, but requires significantly more ignition energy than R152a, for instance. Due to its low burning velocity and the high ignition force, it received a classification of the new safety group “A2L” according to ISO 817.
In extensive test series, it has been shown that a potentially increased risk of the refrigerant flammability in MAC systems can be avoided by implementing suitable constructive measures. However, some investigations (e.g. by Daimler-Benz) also show an increased risk. This is why various manufacturers have intensified again the development of alternative technologies.
Toxicity investigations have shown very positive results, as well as compatibility tests of the plastic and elastomer materials used in the refrigeration circuit. Some lubricants show increased chemical reactivity which, however, can be sup-pressed by a suitable formulation and/or addition of “stabilizers”.
Operating experiences gained from laboratory and field trials to date allow a positive assessment, particularly with regard to performance and efficiency behaviour. For the usual range of mobile air-conditioning operation, refrigerating capacity and coefficient of performance (COP) are within a range of 5% compared with that of R134a. Therefore, it is expected that simple system modifications will provide the same performance and efficiency as with R134a.
The critical temperature and pressure levels are also similar, while the vapour densities and mass flows are approximately 20% higher. The discharge gas temperature with this application is up to 10 K lower.
With a view to the relatively simple conversion of mobile air-conditioning systems, this technology prevailed up to now over the competing CO2 systems.
However, as already explained before, due to the flammability of R1234yf, investigations focus on other technical solutions. This includes active fire-extinguishing devices (e.g. with argon) but also enhancements of CO2 systems.
Further applications for HFO refrigerants
The use of R1234yf in other mobile air-conditioning applications is also being considered, as well as in stationary A/C and heat pump systems. However, this must take into account the charge limitations for the A2L refrigerants (e.g. EN378), which will restrict their use accordingly. Additional concerns are those regarding the long-term stability in refrigeration circuits, given the usually very long life cycles of such systems.
For applications requiring the use of refrigerants of safety group A1 (neither flammable nor toxic), R134a alternatives of lower GWP based on HFO/HFC blends have already been developed. They have been applied for some time in real systems.
R1234yf, as well as R1234ze(E), described below, are also used as base components in HFO/HFC blends. In view of legal regulations for the reduction of F-Gas emissions (e.g. EU F-Gas Regulation), these blends have been developed as “Low GWP” alternatives to R134a, R404A/R507A, R22/R407C and R410A. Some of these refrigerants have already been tested with regard to refrigerating capacity and efficiency as parts of the “Alternative Refrigerants Evaluation Program” (AREP) initiated by AHRI and have also been used in real systems. For further information on HFO/HFC blends, see page 24.
From the group of hydro fluoro olefins, another substance under the name R1234ze(E) is available, which until now has been used predominantly as blowing agent for polyurethane foam and propellant. R1234ze(E) differs from R1234yf by having a different molecular structure. Its thermodynamic properties also provide favourable conditions for the use as refrigerant. Its global warming potential is also very low (GWP100 = 7).
Often there is a degree of uncertainty concerning flammability. In safety data sheets, R1234ze(E) is declared as non-flammable. However, this only applies to transport and storage. When used as a refrigerant, a higher reference temperature of 60°C for flammability tests is valid. At this temperature, R1234ze(E) is flammable and therefore classified in the same safety group A2L as R1234yf.
R1234ze(E) is sometimes called an R134a substitute, but its volumetric refrigerating capacity is more than 20% below that of R134a or R1234yf. Moreover, the boiling point (-19°C) considerably limits its use for lower evaporation temperatures. Therefore, it is preferably used in liquid chillers and for high temperature applications. For further information, see page 36, “Special applications”.