From a regulatory standpoint, this sector has already shifted to eco-friendly refrigerants.

Ammonia (R717) has been a well-known refrigerant in large scale industrial applications for more than 120 years [5]. It has excellent thermodynamic and transport properties, and consequently high energy efficiency in refrigeration systems. Today, more than 90% of large industrial refrigeration facilities in Europe use R717 as a refrigerant [1]. Moreover, it is used in food processing and cold storage industries, ice rinks, wineries, breweries, and large scale air conditioning for universities, office buildings and airports. In numerous applications it exceeds R22, which is considered to be the most effective refrigerant from the HCFC group. Since the developing countries are still using R22, ammonia could potentially serve as a viable replacement refrigerant in the future. What limits its application in other sectors are safety concerns, incompatibility with copper materials, higher gas discharge temperatures and the need for open type compressors [22]. For instance, since there are no compressors small enough, it is not suitable for domestic heat pump systems [3]. Table 2 shows the ratio between the thermophysical properties of R717 and R22. For a certain cooling effect, systems using R717 have smaller piping dimensions, less refrigerant charge, and the pressure drops are considerably smaller, which is especially useful in long, branched systems.

Following the restrictions of F-gas regulation, which ban the use of refrigerants in the domestic sector with a GWP of 150 or more from 2015, R600a was introduced as a replacement for R134a. R134a is one of the most commonly used HFC refrigerants which was developed for the purpose of replacing R12. Since then, it has had a wide application across many different refrigeration sectors, but now it is being replaced due to proposed HFC phase down steps. On the other hand, HC’s are excellent refrigerants in many ways – energy efficiency, critical point, solubility, transport and heat transfer properties [5]. Furthermore, they are fully chemically compatible with almost all of the lubricants used in the refrigeration systems, but special care must be taken due to the greater solubility which in turn could cause oil dilution. The most important concern regarding the application of HC’s is their flammability, safety classification A3 (Table 1). Nevertheless, more than 700 million HC-based domestic refrigerated appliances worldwide are proving that HC’s can be safely used [1]. The study on the risk associated with using R600a in domestic refrigerators shows that the risk is negligible [8]. Moreover, R1270 and R290 can be used in a mixture with R134a to produce an efficient alternative refrigerant to substitute R22 in older systems, more specifically in developing countries [9]. R134a is added in the mixture in order to reduce the flammability of HC’s.

Commercial refrigeration is an example of a sector where the F-gas Regulation has had a very prominent impact [1]. This is due to an upcoming ban on the use of HFC’s with a GWP higher than 150 as of January 2022 in multipack centralised refrigeration systems for commercial use with a rated capacity of 40 kW or more, except in the primary refrigerant circuit of cascade systems where fluorinated greenhouse gases with a GWP of less than 1,500 may be used [2]. The greatest impact of the Regulation in the near future will be on users of R404a (R143a/R125/R134a – 52/44/4), which is the global industry standard HFC refrigerant for commercial refrigeration in both small and large systems [10]. It is subject to phase-out in all sectors since it has a GWP value of 3,922. One of the possible non-flammable replacements are R407F (R125/R32/R134a – 30/30/40) and R407A (R125/R32/R134a – 40/20/40), but it is important to note that these refrigerants are only temporary alternatives due to medium - high values of GWP - 1,824 and 2,107, respectively [11]. Another non-flammable option is the recently developed R448A which offers greater GWP reduction. Despite having higher Coefficient of Performance (COP) values, it has a lower cooling capacity which implies higher run time in supermarket systems, so it is more recommended to use this refrigerant in redesigned systems [12]. In addition, a recent study [13] states that R449A is a viable retrofitting option for R404A supermarket refrigeration systems due to comparable COP’s and much better TEWI numbers. Also, there is the possible use of R290 as a substitute for R404a in the sector of light commercial refrigeration, which could potentially reduce the energy consumption by 34% [14]. Even though R290 is quite different from a thermodynamic point of view, its major advantage is the value of latent heat which is around two times greater, which in the turn reduces the mass flow rate in the system.

The primary reliable natural refrigerant option in this sector is CO₂ (R744), whose usage has become increasingly popular over the last few years. After the interest in CO₂ virtually disappeared in the 1950’s, the idea of using it again was brought up by Lorentzen in the late 1980’s [24]. It represents an excellent alternative among the natural refrigerants, especially in applications such as these, where the toxicity and flammability of R717 and HC’s may be a problem [25]. CO₂ can also be found in small to medium sized industrial refrigeration plants, either as a sole refrigerant or in combination with R717 in cascade systems. The biggest challenge in CO₂ refrigeration system technology is dealing with a unique critical point of about 31 °C with a corresponding absolute pressure of 74 bar [24]. Therefore, pressures in refrigeration systems with CO2 are usually up to 10 times higher than those systems using R717 or R404A. Moreover, because of such distinct properties, most modern CO₂ systems operate in transcritical mode. In fact, the CO₂ transcritical refrigeration systems have become the mainstream-cutting edge HFC-free technology for the food retail sector [1]. The number of European stores using CO₂ transcritical systems have tripled between the years 2013–2016 [1], with the effect most pronounced in Western and Northern Europe.

The increase of new HFC-free products in these sectors is not very pronounced, partly due to the fact that there are no bans for HFC’s with GWP > 150 (except moveable room AC by 2020). The only major ban by 2025 will be on single split AC systems with a refrigerant charge of less than 3 kg, containing fluorinated greenhouse gases with GWP > 750.

R410A, which was introduced as a replacement for R22, is currently widely used in air-conditioning applications ranging from residential unitary air-conditioning, heat pump systems to large commercial chillers [19]. With a GWP of 1,924, it has thus become a target of the Regulation. In that context, several low-GWP R410A replacements are under evaluation by the industry. The blends in question are R447B (R32/R125/R1234ze – 68/8/24) and R452B (R32/R125/R1234yf – 67/7/26). Both of these refrigerants have A2L ASHRAE classification and GWP’s of 714 and 675, respectively. Theoretical system simulations in a R410A reversible heat pump indicate that both of the low GWP alternatives can match the performance of R410A without significant system modifications for ambient temperatures below 35 °C and can show 3% to 4% higher efficiency at elevated ambient temperatures [15]. In addition, R32 as a pure substance can also be used as a reliable R410A replacement. Its GWP of 675 and A2L classification make it a favorable candidate for new equipment in many applications [16]. Through experimentation, it was found that R32 produces better results in terms of higher COP and higher capacity [17]. However, different system components need to be optimized for a better R32 application. The main concern is the high compressor discharge temperature when using R32 at extreme cooling and heating conditions. High compressor discharge temperature also reduces the reliability of system operation due to the possibility of lubricating oil performance degradation [17]. A recent study on natural refrigerant options for residential heat pump water heaters states that R290a is a viable drop-in replacement for R134a, while R600a requires a larger compressor size to meet a similar heating capacity [18]. Significant reductions in refrigerant charge, along with lower condenser discharge temperatures are also additional benefits.

Until recently, R134a was the primary refrigerant for the mobile air conditioning systems in Europe, but in regard to the MAC Directive, which prohibits the use of refrigerants with a GWP of 150 or more in all new vehicles from January 2017, it is being replaced with R1234yf. This refrigerant is part of the HFO refrigerant group, which is widely recognized as the next generation of refrigerants because of their environmental friendliness (GWP < 10), cost-effectiveness, and great energy efficiencies [19]. One of the reasons why R1234yf is such a good alternative to R134a is the similarity of their thermophysical properties. A study on R134a substitutes also proposes R1234yf as a viable alternative, yet states that another HFO refrigerant – R1234ze(E) may perform even better under specific conditions [20]. However, it is worth noting that the particular study did not consider pressure drops in their analyses. Nevertheless, R1234ze(E) has found its application in various refrigerant mixtures, which have a lower potential of flammability risk because of their content in the mixture. A study on alternative low-GWP refrigerants in MAC systems [21] shows that R444A (R1234ze(E)/R152a/R32 – 83/5/12) and R445A (R1234ze(E)/R134a/R744 – 85/9/6) could be considered as alternatives to R1234yf for the air conditioning systems of large vehicles for which flammability is a major problem. Finally, Daimler AG has recently developed CO2 air conditioning systems which are, as of 2017 offered for the first time in their luxury class vehicles as a replacement for R134a [26].

The need to control emissions of greenhouse gases under the provisions of the Kyoto Protocol requires careful consideration and evaluation of energy consuming activities. TEWI is a measure of the relative impact on global warming (relative to CO₂) of a specific refrigeration system during its operation. It is based on the total greenhouse gas emissions during the operation of the equipment and the subsequent disposal of the refrigerant at the end of life. TEWI takes into account both the direct and indirect emissions produced through the energy consumed in operating the equipment. Direct emissions refer to refrigerant released during the lifetime of the equipment, including unrecovered losses on final disposal, while indirect emissions are related to CO₂ emissions from fossil fuels used to generate energy to operate the equipment throughout its lifetime [27]. Refrigerant leakage is imminent during maintenance and servicing due to a high pressure in the system compared to the ambient [28]. TEWI calculations depend on a number of assumptions regarding the performance of the equipment, properties of refrigerants and electricity generation efficiencies. The CO₂ equivalent emissions from electricity generation depend on the region and the source of the electricity [29]. Therefore, the final values are subject to various deviations. In addition, for a TEWI comparison to be of real value, it must relate to systems of equal duty and function. Energy demand equivalent for manufacturing and transporting refrigerant and refrigeration equipment is excluded from TEWI calculations. Although it is possible to calculate the impact of these in TEWI terms, the values have been shown to be insignificant [27].

The method of calculating TEWI is provided below:

where GWP is the global warming potential of refrigerant, relative to CO₂ (GWP CO₂ = 1), L_annual is the leakage rate per year [kg], n is system operating life [years], m is refrigerant charge [kg], α_rec is recovery/recycling factor from 0 to 1, E_annual is energy consumption per year [kWh] and β is indirect emission factor [kg CO₂/kWh].

The first component of the equation represents the impact of leakage losses, the second – the impact of recovery losses, and the final one is the impact of energy consumption [30].

The calculation of a TEWI number is performed for an indirect refrigeration system with a known cooling load and energy efficiency ratio. Based on the given European Seasonal Energy Efficiency Ratio (ESEER) and cooling capacity from the manufacturer, the annual energy consumption has been calculated for an assumed number of full load hours of 1,200 per year for a total equipment lifetime of 15 years. Refrigerant emissions due to leakage and recovery factor have been based upon [27] where the leakage rate is estimated at 7%, while the recovery factor is approximated to 0.9 [27]. The β factor for CO2 equivalent emissions due to electricity used is the figure published by the Croatian Ministry of Construction and Physical Planning [31] at 0.23481 kg/kWh. Refrigerant charge has been determined according to the manufactures of the chillers with a similar cooling capacity (~650 kW):

• “Manufacturer Z” – R134a, 116 kg;

• “Manufacturer Z” – R410A, 154 kg;

• “Manufacturer Y” – R290, 51 kg;

• “Manufacturer X” – R717, 29 kg.

The TEWI results calculated for refrigerants R134a, R410A, R290 and R717 are shown in Table 3.

The impact of the relative emissions associated with the total energy consumption, leakage and recovery losses are shown in Figure 1.

Figure 1.

Detailed TEWI calculations for different types of refrigerant

It is apparent that the emissions are the highest when the system contains fluorinated refrigerants. They are the highest for R410A due to the biggest refrigerant charge in combination with the largest GWP value. On the other hand, natural refrigerants have the most environmentally favourable results because of a variety of factors. Smaller system charges, coupled with the fact that the GWP values are near zero, makes their direct impact on the global warming practically non-existent. However, results also indicate that most of the emissions refer to the indirect impact, and due to the negligible differences between annual energy consumptions, those are mutually comparable for the refrigerants considered. A similar study on alternative refrigerants presented similar results [32], favouring refrigerants from the HC group over the HFC group in terms of total TEWI values.