The term “hydrofluoroolefins” (HFO), which are also often referred to as the “fourth generation refrigerants”, entered the cooling technology industry in 2013, when common use of the refrigerant R1234yf was started in automobile air conditioning. It was only a matter of time before the fourth generation refrigerants appeared – the previously common refrigerants from the HFC (hydrofluorocarbons) group are mostly greenhouse gases with a high GWP (Global Warming Potential). Pursuant to the Kigali Amendment to the Montreal Protocol and in accordance with the EU Regulation 517/2014 (so called F-gas Regulation), such substances are gradually withdrawn from production and use. That is why it was necessary to find an alternative.
As a brief chemistry revision, hydrofluoroolefins are derivates of the unsaturated hydrocarbon – propylene (propene) – and are designated as R1234. They have one double carbon bond and two hydrogen atoms, three carbon atoms and four fluorine atoms, which are variously distributed within the molecule – that is why the compound occurs in several isomers with appropriate letter designations and significant differences in terms of physical properties. R1234yf and R1234ze are the most important in cooling technology. Their common characteristic include a very low GWP (4 and 7 respectively) that results from the double chemical bond between the carbon atoms. However, obtaining such beneficial values is not without a cost – this very double carbon bond is also responsible for the refrigerants’ slight flammability. In accordance with the EN 378 standard, they are classified as A2L – slightly flammable (an A2L class refrigerant is characterized by flame propagation under test conditions in air at 60°C and 101.3 kPa, at max combustion speed ≤10 cm/s under the test conditions of 23°C and 101.3 kPa).
The R1234yf refrigerant was created as a result of the cooperation between the chemical giants, DuPont and Honeywell, who normally compete with each other. Their intention was to create a product that would meet the requirements of Directive 2006/40/EC (MAC Directive) concerning air conditioning systems in motor vehicles. Air conditioning systems of cars that received their type approval after 2011 were supposed to be filled with a refrigerant that would ensure a GWP value of <150. This was equivalent to eliminating the R134a refrigerant (GWP = 1430) in the long term. In practice, this provision started to apply from 1 January 2013 and from 1 January 2017 it applies to all new vehicles. Introducing the new refrigerant was not without controversies – its spontaneous ignition temperature is 405ºC, which, in the opinion of some car manufacturers (e.g. German Daimler), may cause a higher risk of fire originating from the air conditioning system in the case of a collision. Additionally, a by-product of HFO refrigerant combustion, namely hydrogen fluoride, constitutes a serious risk for e.g. rescue teams working at a burning car. However, studies conducted by the German government agency Kraftfahrt-Bundesamt, as well as those originating from the International Society of Automotive Engineers, show that in this case the risk of fire caused by spontaneous refrigerant combustion is negligible. Ultimately, even Daimler uses the R1234yf refrigerant in their vehicles, but with additional fire safeguards.
Except for automobile air conditioning, there are virtually no applications where the R1234yf refrigerant would be used as a stand-alone refrigerant. It does however constitute an important component of refrigerants in the form of mixtures. A key example is the R513A refrigerant – a nonflammable azeotropic mixture (without temperature glide) composed of R1234yf (56%) and R134a, with GWP = 573, used as a substitute for the R134a refrigerant in both the existing and new systems.
R1234ze is available in two forms: R1234ze(E) and R1234ze(Z). The isomer R1234ze(Z) is not usable in cooling applications because of its high normal boiling point of 9.8°C and low unitary volumetric cooling capacity. This refrigerant was originally created as a foaming agent and its production cost (and consequently the cost of the final product) is lower than those of R1234yf. R1234ze(E), which offers a GWP of 7, may be successfully used in newly designed systems instead of R134a (GWP = 1430). In comparison these have similar pressure and COP, but the former offers lower volumetric efficiency and cooling capacity (approx. 75% of the cooling capacity of R134a).
Following a period of the manufacturers’ initial distrust, R1234ze(E) has found a wide range of applications in equipment operating under moderate and high temperatures, especially in large systems for commercial and industrial cooling. It can be found in air- and water-cooled liquid cooling packages, air conditioning systems, air dryers, refrigerators, vending machines and dispensers, as well as heat pumps. Manufacturers have recently been shifting their focus to using R1234ze(E) in high-temperature heat pumps for commercial and industrial applications. A screw compressor with R1234ze(E) has an extended operating envelope, with the condensing point at the outlet reaching up to 80°C and evaporation point above 30°C.
Due to its slight flammability, R1234ze cannot be used for retrofitting and there are filling limitations for systems operating with this medium. Maximum filling levels are indicated in the EN 378 standard and depend on device locations, presence of people in the cooled area and system type. For direct evaporation systems (DX) and for general occupancy rooms (class A), the maximum filling level is a function of the low flammable limit (LFL) which is equal to 0.303 kg/m3 for R1234ze(E). Based on this, the maximum filling level may be 11.5 kg (38ᐧLFL) and if the entire system is located in the machine room or outdoors – 40 kg (132ᐧLFL). For supervised occupancy (class B) the filling level may be from 10 to 25 kg, depending on whether the devices are located in the zone occupied by people or the compressor and the receiver are in a separate machine room or outdoors. If the entire system is located in a machine room or outdoors, there are no filling level limitations.
Just like R1234yf, the R1234ze(E) refrigerant constitutes a component of new refrigerants in the form of mixtures, for example the nonflammable R450A mixture with a GWP of 547 consists of 58% of R1234ze(E) (R134a being the other component).