Refrigerants for a Sustainable Future

Definition

Worldwide use of high global warming potential (GWP) hydrofluorocarbon (HFC) refrigerants for space conditioning and food storage results in significant equivalent greenhouse gas (GHG) emissions. This is further exacerbated in developed countries by the current transition from hydrochlorofluorocarbon (HCFC) refrigerants to HFC refrigerants. Under the Kigali amendment to the Montreal Protocol, the proposed phase-out of currently used HFC and HCFC refrigerants has initiated a re-evaluation of some pre-existing refrigerants as well as the development and evaluation of new refrigerants. Making the ideal refrigerant selections for heating, ventilation, air-conditioning, and refrigeration (HVAC&R) applications is thereby difficult in an already overabundant refrigerants market. In this paper, a study of key parameters required of a good refrigerant is conducted, followed by the analysis of refrigerants desired and refrigerants used in two major sectors of the HVAC&R industry, namely commercial refrigeration and residential air-conditioning and heat pumps. Finally, keeping in consideration the global environmental regulations and safety standards, a recommendation of the most suitable refrigerants in both sectors has been made.

1. Introduction

In 2015, the 21st session of the United Nations Framework Convention on Climate Change (COP21) convened in Paris, aiming to limit global temperature increase to 2 °C (Paris Agreement: 2016). Considering that the reduction in the use of high global warming potential (GWP) refrigerants is crucial to achieving this goal, the Kigali amendment to the Montreal Protocol was subsequently adopted during the 28th Meeting of the Parties held in October 2016 [1]. Under the Kigali amendment, a legally binding mandate for phasing out the production and use of hydrofluorocarbon (HFC) refrigerants was agreed upon to avoid 70 billion metric tons of carbon dioxide equivalent emissions cumulatively through 2050. According to the agreement, the phase down of HFCs begins on 1 January 2019 for developed countries (non-Article 5 countries), and for developing countries (Article 5 countries), the phase down begins on 1 January 2024. A summary of the phase-down schedule of HFC refrigerants dictated by the Kigali amendment is given in

Table 1. HFC refrigerant phase-down per the Kigali amendment to the Montreal Protocol.

	Non-Article 5, Group 1 a	Non-Article 5, Group 2 b	Article 5, Group 1 c	Article 5, Group 2 d
Baseline years	2011, 2012, 2013	2011, 2012, 2013	2020, 2021, 2022	2024, 2025, 2026
Baseline Calculation	Average production/consumption of HFCs in 2011, 2012, 2013, plus 15% of HCFC baseline production/consumption	Average production/consumption of HFCs in 2011, 2012, 2013, plus 25% of HCFC baseline production/consumption	Average production/consumption of HFCs in 2020, 2021, 2022, plus 65% of HCFC baseline production/consumption	Average production/consumption of HFCs in 2024, 2025, 2026, plus 65% of HCFC baseline production/consumption
Reduction Steps
Step 1	2019, 10%	2020, 5%	2029, 10%	2032, 10%
Step 2	2024, 40%	2025, 35%	2035, 30%	2037, 20%
Step 3	2029, 70%	2029, 70%	2040, 50%	2042, 30%
Step 4	2034, 80%	2034, 80%	2045, 80%	2047, 85%
Step 5	2036, 85%	2036, 85%

^a Non-Article 5, Group 1: Non-Article 5 countries, excluding those in Group 2 (45 countries). ^b Non-Article 5, Group 2: Belarus, the Russian Federation, Kazakhstan, Tajikistan, and Uzbekistan. ^c Article 5, Group 1: Article 5 countries, excluding those in Group 2 (137 countries). ^d Article 5, Group 2: Bahrain, India, Iran, Iraq, Kuwait, Oman, Pakistan, Qatar, Saudi Arabia, and the United Arab Emirates.

.

Ever since the beginning of the heating, ventilation, air-conditioning, and refrigeration (HVAC&R) industry, the pursuit for the ideal refrigerant has been an ongoing challenge. The primary refrigerant selection criteria used during the initial years of the industry was whatever works; however, with advancements in technology and increasing environmental awareness, the refrigerant selection criteria have been modified to include not only energy efficiency but also zero ozone depletion potential (ODP) and recently low global warming potential. As a result, several alternative low-GWP refrigerants have been proposed for various HVAC&R applications, including natural refrigerants such as carbon dioxide (CO₂), ammonia (NH₃), and propane, as well as synthetic refrigerants such as R-1234yf and R-1234ze(E). However, these refrigerants have drawbacks, such as high pressure, flammability, and toxicity. In this paper, the pros and cons of potential alternative refrigerants will be studied, and these alternatives will be rated with respect to the current refrigerants used in several HVAC&R applications. The critical parameters used for the selection of the best refrigerant for an application will be discussed.

Since energy efficiency is also key to achieving reduced global warming, the thermodynamic properties and performance of refrigerants are important parameters to consider. Furthermore, thermophysical properties of a refrigerant also dictate refrigeration system component sizing and required piping strength. The selection of refrigerants is also dependent upon the application. For example, in centralized refrigeration systems, a low operating temperature (−40 °C) and high refrigerant charge require the use of refrigerants with low normal boiling points and no flammability. However, flammable refrigerants may be permissible for use in small air-conditioning and refrigeration systems, such as window air conditioners and self-contained commercial display cabinets, due to their low refrigerant charge requirement. Toxicity is another major parameter to consider for indoor applications, where refrigerant may come into contact with occupants, but not necessarily for remote and industrial applications. In the following sections, the history of refrigerants and the characteristics of an ideal refrigerant are discussed, followed by an analysis of refrigerant options for the commercial refrigeration and air-conditioning sectors.

2. History of Refrigerants

Mechanical vapor compression refrigeration cycles employ a working fluid (refrigerant) that undergoes phase changes to transfer heat. The refrigerant absorbs heat from a low-temperature source, evaporating in the process. It then releases heat to a higher-temperature sink, condensing back to a liquid state. This cycle can be utilized for both refrigeration and heating applications.

The early vapor compression refrigeration systems, developed in the late 19th and early 20th centuries, employed readily available refrigerants such as sulfur dioxide (R-764), ethyl chloride (R-160), methyl chloride (R-40), and ammonia (R-717). These early refrigerants, while effective, often posed significant environmental and safety risks. However, these early refrigerants proved to be hazardous due to their toxicity and flammability. In response to these concerns, chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) were developed in the 1930s as safer, non-toxic, and non-flammable alternatives [2,3]. The widespread adoption of CFCs and HCFCs in various applications, including aerosol propellants, cleaning agents, and foam insulation, led to a significant surge in their production during the 1950s and 1960s.

In the 1970s, scientists discovered that CFCs and HCFCs were depleting the Earth’s ozone layer [4,5]. CFC and HCFC molecules are highly stable, allowing them to persist in the atmosphere for extended periods. Once they reach the stratosphere, these molecules break down, releasing chlorine atoms. These chlorine atoms then react with ozone, depleting it and forming diatomic oxygen.

The Montreal Protocol, an international treaty ratified in 1987, mandated the phase-out of ozone-depleting substances like CFCs and HCFCs. The U.S. fully banned CFC production and import in 1996 and is phasing out HCFCs with a complete ban scheduled for 2030 [6,7].

After the Montreal Protocol phased out ozone-depleting refrigerants, hydrofluorocarbons (HFCs) emerged as safer alternatives. However, HFCs have a high global warming potential (GWP), contributing significantly to climate change. To mitigate these effects, there’s a strong push to minimize the use and emissions of HFCs. Hydrocarbons like propane (R-290) and isobutane (R-600a), ammonia (R-717), carbon dioxide (R-744), and hydrofluoroolefins (HFOs) such as R-1234yf and R-1234ze(E) are emerging as low-GWP refrigerant options [8,9,10].

3. Refrigerant Selection

The evolution of refrigerants has been shaped by various factors, including safety and environmental concerns, as discussed above. While safety and environmental impact are crucial, other characteristics also influence refrigerant selection for specific applications, as discussed below.

Transport properties: Thermal conductivity, viscosity, and specific heat are important transport properties to consider for the selection of a good refrigerant [11]. All these properties contribute to the refrigerant-side heat transfer coefficient. A higher refrigerant-side heat transfer coefficient is desirable to improve the heat transfer through the evaporator and condenser. Thus, refrigerants with high thermal conductivity, low viscosity, and high specific heat are desired.

Evaporator pressure: To prevent air and moisture from entering the refrigeration system through leaks, the system’s lowest evaporating pressure—or minimum operating pressure—must always remain above atmospheric pressure. If air or moisture infiltrates through a leak, system performance will degrade [12]. Non-condensable gases, such as air, reduce cooling capacity and system efficiency. Moisture poses additional risks by reacting with refrigerants and oils to form acids, leading to component corrosion and lubricant gelling, which can significantly shorten compressor lifespan. Furthermore, moisture can freeze at expansion devices, obstructing refrigerant flow.

Condensing pressure: Compressor energy consumption depends on the ratio of condensing to evaporating pressures. Lowering the condensing pressure helps reduce energy use [13]. Additionally, components on the high-pressure side must endure high operating pressures without failing. Reducing condensing pressure decreases the material requirements for these components.

Freezing temperature: The evaporator is where the refrigerant in a refrigeration system reaches its lowest operating temperature. To prevent the refrigerant from solidifying at this stage, its freezing point must be significantly lower than the required evaporator temperature [14].

Latent heat of vaporization: A refrigerant’s latent heat of vaporization determines how much heat it can absorb per unit mass. Therefore, a refrigerant with a high latent heat of vaporization is preferred, as it allows for a lower mass flow rate or refrigerant charge [15]. However, care should be taken to ensure that the refrigerant flow rate is not too low; otherwise, temperature control issues may arise due to instability in the flow as a result of non-homogeneous evaporation and inefficient heat transfer.

Critical point: Refrigeration systems achieve optimal efficiency when the refrigerant temperature stays significantly below its critical temperature [16]. As illustrated in Figure 1, refrigerants with low critical temperatures experience a sharp decline in cooling capacity and heat rejection as the condensing temperature nears their critical point. Additionally, power consumption rises significantly under these conditions, making refrigerants with higher critical temperatures more desirable.

Compressor discharge temperature: An appropriate compressor discharge temperature is necessary to ensure the proper operation and longevity of the compressor. At very high discharge temperatures, compressor oil may break down, resulting in reduced compressor life [17]. When using refrigerants that exhibit high discharge temperatures, it may be necessary to cool the compressor oil with an oil cooler or use a liquid or vapor injector to reduce the discharge temperature. Conversely, if the compressor discharge temperature is very low, there might be a risk of compressing two-phase refrigerant, resulting in mechanical damage to the compressor. In such cases, it may be necessary to superheat the suction gas prior to compression.

Temperature glide: A zeotropic refrigerant blend that undergoes a constant pressure phase change from liquid to vapor or from vapor to liquid will do so over a range of temperatures, rather than at a single temperature as would be the case with a pure refrigerant or azeotropic blend [18]. The temperature change that occurs during the phase change is referred to as the temperature glide. This behavior is visualized on a T-s diagram as a sloped line rather than a horizontal line. In properly designed heat exchangers, temperature glide can offer an energy benefit by reducing the finite temperature difference between the refrigerant blend and the fluid being cooled or heated [19]. However, temperature glide results in the fractionation of the refrigerant, where the vapor contains an excess of the refrigerants with the lower boiling points and the liquid with an excess of the refrigerants with the higher boiling points. Thus, if leaks develop in the system, the composition of the refrigerant may change over time.

High ambient refrigeration capacity: In general, as ambient temperature increases, refrigeration capacity and system performance decrease [20]. Since the molecular structure and intermolecular bonds are different for various refrigerants, the rate of decrease in capacity and system performance with increasing ambient temperature varies among the different refrigerants. Thus, for high ambient temperature conditions, the behavior of the refrigerant at these temperatures should be considered.

Inertness and stability: The refrigerant must remain chemically stable and non-reactive with all system materials, including metals, lubricants, plastics, elastomers, and components like valves and fittings. Also, the chemical composition of the refrigerant should remain stable at the extreme operating temperatures found in the evaporator and compressor discharge [21].

Dielectric strength: For refrigeration applications involving the use of hermetic compressors, the incoming refrigerant vapor at the suction port of the compressor is used to cool the compressor motor windings [22]. Since the refrigerant is in direct contact with the motor windings, the refrigerant must maintain its dielectric strength or electrical insulating properties to ensure safe electrical operation of the compressor.

Oil solubility: The lubricating oil for the compressor must be highly miscible and soluble with the refrigerant to ensure that it returns to the compressor rather than accumulating in different areas of the system [23]. Improper oil solubility in HVAC&R systems can lead to reduced lubrication, oil logging, increased pressure drops, and freezing issues, all of which compromise efficiency and reliability, especially in low-temperature applications. For large refrigeration systems, it is recommended to install an oil separator after the compressor discharge to collect the lubricating oil and return it to the compressors [24]. In addition, refrigerant piping should be sized appropriately to ensure adequate refrigerant velocity to entrain the oil so that any oil that has migrated to other parts of the system may be returned to the compressors.

Global warming potential: The global warming potential is a comparative parameter that represents the contribution to global warming of a given mass of a chemical over a given time period compared to the same mass of carbon dioxide. In recent years, international efforts have been established, culminating in the development of the Kigali amendments to the Montreal Protocol, to curb the ever-increasing global warming effect by controlling the emissions of key greenhouse gases, including high GWP refrigerants. Thus, low GWP is a major selection criterion for selecting an appropriate refrigerant for a given application [25].

Flammability and toxicity: Flammable and toxic refrigerants pose a significant risk in regards to liability to manufacturers, contractors, and building owners and safety concerns for the customers and the entire HVAC&R supply chain. Ideally, a refrigerant should be non-toxic and non-flammable, with an ASHRAE safety classification of A1 (where “A” denotes non-toxic, while “1” denotes non-flammable). However, as pressure mounts to phase out HFCs due to their high GWPs, many next-generation low GWP refrigerants under consideration, including HFOs and HFO-HFC blends, exhibit mild flammability and are classified as A2L for safety [26]. In addition, several hydrocarbon refrigerants are being considered that have higher flammability, with a safety classification of A3 [27]. Figure 2 summarizes the general trend of increasing flammability with decreasing GWP for the refrigerants listed in Table 2. Research efforts are currently underway to reduce the risks associated with the use of flammable refrigerants [28].

Cost: Depending upon the size of the refrigeration system, the amount of refrigerant charge may vary from tens of grams in a domestic refrigerator to over one thousand kilograms in a commercial or industrial refrigeration system [29]. Also, the annual refrigerant leakage rate from some systems, in particular, commercial refrigeration systems, can be as high as 25% [30]. Thus, the selected refrigerant should be readily available at a low cost.

Table 2. Properties of selected refrigerants.

Refrigerant	Critical Temperature (°C) a	Critical Pressure (MPa) a	Boiling Point at 101 kPa (°C) a,b	Freezing Point (°C) a	Glide (°C) c	GWP d	Safety Classification e
R-22	96.1	4.99	−40.8	−157.4	–	1760	A1
R-32	78.1	5.78	−51.7	−136.8	–	677	A2L
R-123	183.7	3.66	27.8	−107.2	–	79	B1
R-125	66.0	3.62	−48.1	−100.6	–	3170	A1
R-134a	101.1	4.06	−26.1	−103.3	–	1300	A1
R-143a	72.7	3.76	−47.2	−111.8	–	4800	A2L
R-152a	113.3	4.52	−24.0	−118.6	–	138	A2
R-1234yf	94.7	3.38	−29.5	−53.2	–	1	A2L
R-1234ze(E)	109.4	3.63	−19.0	−104.5	–	1	A2L
R-290	96.7	4.25	−42.1	−187.6	–	3	A3
R-600	152.0	3.80	−0.5	−138.3	–	4	A3
R-600a	134.7	3.63	−11.7	−159.4	–	3	A3
R-1270	91.1	4.56	−47.6	−185.2	–	2	A3
R-717	132.3	11.33	−33.3	−77.7	–	0	B2L
R-744	31.0	7.38	−78.5	−56.6	–	1	A1
R-404A	72.0	3.73	−46.2	–	0.5	3943	A1
R-407A	82.3	4.52	−45.0	–	5.6	1923	A1
R-407C	86.0	4.63	−43.6	–	6.2	1624	A1
R-407F	82.7	4.75	−46.1	–	5.6	1674	A1
R-410A	71.3	4.90	−51.4	–	0.1	1924	A1
R-441A	117.3	4.40	−41.5	–	19.2	3	A3
R-444B	92.1	5.21	−45.4	–	8.7	295	A2L
R-446A	84.2	5.63	−49.7	–	4.3	461	A2L
R-447A	82.6	5.54	−49.7	–	3.7	572	A2L
R-447B	81.3	5.50	−50.0	–	3.1	714	A2L
R-448A	83.7	4.50	−44.8	–	5.8	1273	A1
R-449A	83.9	4.39	−44.0	–	5.7	1282	A1
R-449B	83.9	4.43	−44.2	–	5.7	1296	A1
R-449C	86.1	4.26	−42.1	–	5.6	1147	A1
R-450A	105.6	4.08	−24.9	–	0.8	547	A1
R-451A	95.4	3.45	−29.1	–	0.0	133	A2L
R-451B	95.5	3.46	−29.1	–	0.0	146	A2L
R-452A	75.6	3.91	−45.8	–	3.9	1945	A1
R-452B	79.7	5.06	−49.3	–	4.1	676	A2L
R-452C	74.8	3.97	−46.4	–	3.7	2019	A1
R-454A	85.7	4.21	−42.7	–	5.3	238	A2L
R-454B	80.9	5.04	−48.7	–	4.5	467	A2L
R-454C	88.5	3.88	−38.8	–	4.2	146	A2L
R-455A	87.5	4.20	−49.5	–	11.8	146	A2L
R-457A	93.0	4.02	−36.9	–	4.2	139	A2L
R-459A	81.5	5.12	−48.8	–	4.4	461	A2L
R-463A	76.0	5.07	−58.1	–	11.3	1377	A1
R-507A	70.6	3.70	−46.7	–	0.0	3985	A1
R-511A	96.9	4.29	−42.0	–	0.0	3	A3
R-513A	97.7	3.68	−28.0	–	0.0	573	A1
R-515A	108.7	3.60	−19.0	–	0.0	403	A1

^a Data from [31]. ^b Bubble point used for blends. ^c Glide reported at a midpoint temperature of 0 °C (32 °F). ^d GWP values for pure refrigerants from [32]; GWP values for blends based on mass-weighted values from [32]. ^e As specified by ASHRAE [33].

Table 2. Properties of selected refrigerants.

Refrigerant	Critical Temperature (°C) a	Critical Pressure (MPa) a	Boiling Point at 101 kPa (°C) a,b	Freezing Point (°C) a	Glide (°C) c	GWP d	Safety Classification e
R-22	96.1	4.99	−40.8	−157.4	–	1760	A1
R-32	78.1	5.78	−51.7	−136.8	–	677	A2L
R-123	183.7	3.66	27.8	−107.2	–	79	B1
R-125	66.0	3.62	−48.1	−100.6	–	3170	A1
R-134a	101.1	4.06	−26.1	−103.3	–	1300	A1
R-143a	72.7	3.76	−47.2	−111.8	–	4800	A2L
R-152a	113.3	4.52	−24.0	−118.6	–	138	A2
R-1234yf	94.7	3.38	−29.5	−53.2	–	1	A2L
R-1234ze(E)	109.4	3.63	−19.0	−104.5	–	1	A2L
R-290	96.7	4.25	−42.1	−187.6	–	3	A3
R-600	152.0	3.80	−0.5	−138.3	–	4	A3
R-600a	134.7	3.63	−11.7	−159.4	–	3	A3
R-1270	91.1	4.56	−47.6	−185.2	–	2	A3
R-717	132.3	11.33	−33.3	−77.7	–	0	B2L
R-744	31.0	7.38	−78.5	−56.6	–	1	A1
R-404A	72.0	3.73	−46.2	–	0.5	3943	A1
R-407A	82.3	4.52	−45.0	–	5.6	1923	A1
R-407C	86.0	4.63	−43.6	–	6.2	1624	A1
R-407F	82.7	4.75	−46.1	–	5.6	1674	A1
R-410A	71.3	4.90	−51.4	–	0.1	1924	A1
R-441A	117.3	4.40	−41.5	–	19.2	3	A3
R-444B	92.1	5.21	−45.4	–	8.7	295	A2L
R-446A	84.2	5.63	−49.7	–	4.3	461	A2L
R-447A	82.6	5.54	−49.7	–	3.7	572	A2L
R-447B	81.3	5.50	−50.0	–	3.1	714	A2L
R-448A	83.7	4.50	−44.8	–	5.8	1273	A1
R-449A	83.9	4.39	−44.0	–	5.7	1282	A1
R-449B	83.9	4.43	−44.2	–	5.7	1296	A1
R-449C	86.1	4.26	−42.1	–	5.6	1147	A1
R-450A	105.6	4.08	−24.9	–	0.8	547	A1
R-451A	95.4	3.45	−29.1	–	0.0	133	A2L
R-451B	95.5	3.46	−29.1	–	0.0	146	A2L
R-452A	75.6	3.91	−45.8	–	3.9	1945	A1
R-452B	79.7	5.06	−49.3	–	4.1	676	A2L
R-452C	74.8	3.97	−46.4	–	3.7	2019	A1
R-454A	85.7	4.21	−42.7	–	5.3	238	A2L
R-454B	80.9	5.04	−48.7	–	4.5	467	A2L
R-454C	88.5	3.88	−38.8	–	4.2	146	A2L
R-455A	87.5	4.20	−49.5	–	11.8	146	A2L
R-457A	93.0	4.02	−36.9	–	4.2	139	A2L
R-459A	81.5	5.12	−48.8	–	4.4	461	A2L
R-463A	76.0	5.07	−58.1	–	11.3	1377	A1
R-507A	70.6	3.70	−46.7	–	0.0	3985	A1
R-511A	96.9	4.29	−42.0	–	0.0	3	A3
R-513A	97.7	3.68	−28.0	–	0.0	573	A1
R-515A	108.7	3.60	−19.0	–	0.0	403	A1

^a Data from [31]. ^b Bubble point used for blends. ^c Glide reported at a midpoint temperature of 0 °C (32 °F). ^d GWP values for pure refrigerants from [32]; GWP values for blends based on mass-weighted values from [32]. ^e As specified by ASHRAE [33].

4. Application Specific Refrigerant Selection

Both increasing globalization and the advancement of refrigeration technology have led to an increase in the use of air-conditioning and refrigeration equipment in many parts of the world. Selection of appropriate refrigerants for the two major applications, including commercial refrigeration and air-conditioning, is discussed below.

For commercial refrigeration and air-conditioning applications, the relative performance of currently available refrigerants and lower GWP alternative refrigerants was determined using seven different refrigerant characteristics, including the following:

• Global warming potential;
• Refrigerating capacity (latent heat of vaporization, critical point);
• High ambient temperature (HAT) capacity;
• Temperature glide;
• Transport properties;
• Flammability and toxicity (safety);
• System efficiency (evaporator pressure, condensing pressure).

A simple thermodynamic analysis of a vapor compression refrigeration cycle was used to determine the refrigerating capacity, high ambient temperature refrigerating capacity, and system efficiency temperature of the various refrigerants, with the required refrigerant properties determined using REFPROP (Version 10) [31,34]. For commercial refrigeration applications, the saturated evaporating temperature was assumed to be −34.4 °C (−30 °F), and the saturated condensing temperature was assumed to be 40.6 °C (105 °F), while for air-conditioning applications, the saturated evaporating and condensing temperatures were assumed to be 7.2 °C (45 °F) and 40.6 °C (105 °F), respectively. For high ambient temperature conditions, the saturated condensing temperature for both commercial refrigeration and air-conditioning applications was raised to 51.7 °C (125 °F) while the saturated evaporating temperatures remained unchanged. Furthermore, in the cycle analysis, it was assumed that the isentropic efficiency of the compressor was 65%.

The transport properties of refrigerants were determined by evaluating the heat transfer coefficient of the refrigerant flowing through a pipe. The simple Nusselt–Reynolds–Prandtl heat transfer correlation developed by Dittus and Boelter [35] for fully developed turbulent flow inside a pipe was used to determine the heat transfer coefficient:

$$Nu=0.023R{e}^{0.8}P{r}^{0.4}$$

(1)

where Nu is the Nusselt number, Re is the Reynolds number, and Pr is the Prandtl number. In evaluating Equation (1), it was assumed that the Reynolds number of the flow was 300,000. The density, viscosity, and Prandtl number for the refrigerants were determined at the saturated vapor evaporating temperature using REFPROP [31].

Finally, environmental and safety characteristics of refrigerants, such as global warming potential, flammability, and toxicity, were obtained from UNEP [32] and ASHRAE [33].

For each of the seven characteristics, the relative performance of the various refrigerants was then rated from 0 (worst) to 1 (best), where the outermost web represents a value of 1, whereas the center point represents a value of zero, as discussed below.

4.1. Commercial Refrigeration

The commercial refrigeration sector includes refrigeration systems utilized in supermarkets, grocery stores, convenience stores, restaurants, cafeterias, and other food service establishments. In the United States, there are more than 38,000 supermarkets, 14,000 grocery stores, and 154,000 convenience stores [36,37]. In the U.S. commercial building sector, refrigeration systems used in food retail outlets account for roughly 14% of the total electrical energy consumption [38]. Globally, there are approximately 90 million commercial refrigeration units in operation, which include condensing units, stand-alone equipment, and centralized systems [39,40].

Commercial refrigeration systems are employed to store products such as frozen foods, beverages, deli and dairy items, and fresh produce, maintaining temperatures that ensure food safety and quality [41]. These products are usually stored within two temperature ranges: the low-temperature (LT) range of −40 °C to −18 °C (−40 °F to 0 °F) and the medium-temperature (MT) range of −18 °C to 5 °C (0 °F to 41 °F) [42].

Commercial refrigeration includes a diverse array of system configurations and sizes, from compact, self-contained refrigerated cabinets used in food service to large, field-assembled centralized refrigeration systems designed for supermarkets [43].

Refrigerant options: several alternative lower GWP refrigerant options are available for commercial refrigeration applications [44], and these refrigerant options are summarized in

Table 3. Alternative refrigerants for commercial refrigeration applications.

Application	Refrigerants
Currently Used Refrigerants
Self-Contained Refrigeration	R-22, R-134a, R-404A, R-407A, R-407F
Centralized Refrigeration	R-22, R-404A,R-407A, R-407F
Alternative Refrigerants
Self-Contained Refrigeration	R-32, R-1234yf, R-1234ze(E)
	R-446A, R-447A, R-450A
	R-451A, R-451B, R-454A, R-454C, R-455A, R-513A
	R-290, R-600a, R-744
Centralized Refrigeration	R-32, R-1234yf, R-1234ze(E)
	R-448A, R-449A, R-450A
	R-451A, R-451B, R-454A, R-454C, R-455A, R-513A
	R-717, R-744

.

4.1.1. Analysis of Refrigerant Options for Commercial Refrigeration

A total of eighteen potential refrigerants for commercial refrigeration applications were analyzed and compared, including two commonly used refrigerants, R-22 and R-404A. For each of the seven refrigerant characteristics, the relative performance of the alternative refrigerants for commercial refrigeration applications was rated from 0 (worst) to 1 (best), as summarized in Figure 3, Figure 4 and Figure 5 and discussed below. Figure 3 and Figure 4 represent the influence of different refrigerant characteristics associated with both pure and blended compositions, respectively. Figure 5 presents a detailed overview of each characteristic and how individual refrigerants compare with regards to some of the older refrigerants, such as R-404A, R-407F, and R-22.

Global warming potential: The candidate refrigerants for commercial refrigeration applications were rated in terms of their global warming potentials on a scale of 0 to 1, where a rating of 1 indicates a refrigerant with no global warming potential while a rating of 0 indicates a refrigerant with a very high global warming potential (in this case, the GWP value of R-404A, which is 3943). The refrigerants with low GWP, such as R-744, R-290, R-600a, R-1234yf, R-1234ze(E), R-450A, R-451A, and R-513A, rate high on this scale, with scores nearly equal to one [45]. The lowest-rated refrigerant in the group in terms of GWP is R-404A, with a normalized GWP score of 0, as shown in Figure 4. Several alternative refrigerant options for R-404A, including R-134a, R-448A, R-449A, R-449B, R-407A, R-407C, and R-407F, have intermediate GWP values.

Refrigerating capacity: The capacities of the alternative refrigerants for commercial refrigeration applications were compared on a normalized basis to the capacity of R-22, where a refrigeration capacity (the difference in enthalpy between the evaporator outlet and inlet) equal to or greater than that of R-22 was set to 1. The refrigerant capacity of each refrigerant was determined at a saturated evaporating temperature of −34.4 °C (−30 °F) and a saturated condensing temperature of 40.6 °C (105 °F).

The normalized capacities of the hydrocarbon refrigerants (R-290 and R-600a) are high, being equivalent to or better than that of R-22. The relative capacity of R-404A, which is 0.61, is significantly lower than that of R-22. Several of the direct replacements for R-404A, including R-407A, R-407C, R-407F, R-448A, R-449A, R-449B, R-450A, R-451A, R-1234ze(E), and R-513A, exhibit higher capacities than R-404A. Also, R-134a and R-744 show higher capacity than R-404A.

High ambient temperature (HAT) capacity: The refrigerating capacity of each of the candidate refrigerants at high ambient temperature [20] was determined for a saturated condensing temperature of 51.7 °C (125 °F) and normalized with respect to R-22. Many refrigerants performed well in terms of HAT capacity, including R-290, R-600a, R-134a, R-407A, R-407C, R-407F, R-448A, R-449A, R-449B, R-450A, R-451A, R-513A, R-1234yf, and R-1234ze(E), with normalized HAT capacities of greater than 0.9 as compared to R-22. The HAT capacity of R-717 is higher than that of R-22, and R-404A was found to have approximately 15% lower capacity than that of R-22.

Temperature glide: The temperature glide of each refrigerant was determined at a saturated evaporating temperature of −34.4 °C (−30 °F). It was assumed that a rating of 1 (best) indicates a refrigerant with no temperature glide, while a rating of 0 (worst) indicates a refrigerant with a very high temperature glide (in this case, a temperature glide of 10 °C or more). From Figure 3 and Figure 5d, it can be seen that the pure refrigerants, including R-744, R-290, R-600a, R-22, R-134a, R-1234yf, and R-1234ze(E), do not exhibit temperature glide (normalized temperature glide of 1). The refrigerant blends R-404A, R-450A, R-451A, and R-513A exhibit negligible glide (normalized glide of approximately 0.9), while R-407F, R-407C, R-448A, R-407A, R-449B, and R-449A exhibit significant temperature glide (normalized glide ranging between 0.30 and 0.42).

Transport properties: As noted earlier, the transport properties of the refrigerants were evaluated by estimating the heat transfer coefficient of fully developed turbulent flow of the refrigerants inside a pipe. The heat transfer coefficient depends upon the thermal conductivity, specific heat, and viscosity of the fluid.

The normalized heat transfer coefficient for the selected refrigerant, h_norm,ref, is calculated using the following equation:

$$h_{\mathrm{n}\mathrm{o}\mathrm{r}\mathrm{m},\mathrm{r}\mathrm{e}\mathrm{f}}=1-{\displaystyle \frac{h_{R-22}-h_{\mathrm{r}\mathrm{e}\mathrm{f}}}{h_{R-22}-h_{\mathrm{m}\mathrm{i}\mathrm{n}}}}$$

(2)

where h_ref is the heat transfer coefficient of the selected refrigerant, h_min is the minimum value of the heat transfer coefficient among all the selected refrigerants, and h_R-22 is the heat transfer coefficient of R-22.

On comparing the heat transfer coefficient of all refrigerants with respect to that of R-22, it was seen that R-744, R-290, R-600a, R-407a, R-407C, R-407F, and R-448A. R-449A and R-449B perform better than R-22. Good alternatives with normalized heat transfer between 0.6 and 0.9 include R-134a and R-450A. In this comparison, the refrigerants with normalized heat transfer below 0.5 included R-404A, R-1234ze(E), R-513A, R-451A, and R-1234yf.

Flammability and Toxicity The flammability and toxicity of the alternative refrigerants were normalized according to their ASHRAE safety classification [46] as follows:

• Class A1: normalized as 1
• Class A2 and A2L: normalized as 0.3
• Class A3: normalized as 0.05
• Class B1: normalized as 0.3
• Class B2 and B2L: normalized as 0.05
• Class B3: normalized as 0

From Figure 3, Figure 4 and Figure 5, it can be seen that R-744, R-134a, R-407A, R-407C, R-407F, R-448A, R-449A, R-449B, R-450A, R-451A, and R-513A are the most ideal alternative refrigerants in terms of low flammability and low toxicity, while ammonia and the hydrocarbons (R-290 and R-600a) are the least favorable alternatives due to their flammability and/or toxicity.

System Efficiency System efficiency was characterized by comparing the coefficient of performance (COP) of a direct expansion refrigeration system using the different refrigerants. For this comparison, the saturated evaporating temperature was assumed to be −34.4 °C (−30 °F), and the saturated condensing temperature was assumed to be 40.6 °C (105 °F). The system efficiency of each refrigerant is normalized with respect to that of R-22.

As shown in Figure 3, Figure 4, and Figure 5g, the system efficiency of an ammonia-based system is higher than R-22, while the COP of many of the refrigerants, including R-600a, R-134a, R-290, R-450A, R-1234ze(E), R-513A, R-407C, R-407F, R-451A, R-1234yf, R-449B, R-449A, R-448A, R-407A, and R-404A, ranges between 0.97 to 0.8 times that of R-22. Due to its low critical temperature, R-744 is the worst performer among all the refrigerants studied in terms of system efficiency, with an efficiency of almost 45% less than that of R-22. However, when used in booster and cascade refrigeration systems, the COP of R-744 at various ambient conditions can be equivalent to or better than an R-404A direct expansion refrigeration system.

4.1.2. Summary: Commercial Refrigeration

Under the Montreal Protocol and the Kigali amendment [1], the phase-out of R-22 and phase-down of R-404A, the two most commonly used refrigerants in the U.S. commercial refrigeration industry, are scheduled as per Table 1.

Thus, in search of an ideal alternative refrigerant for commercial refrigeration applications, eighteen current and proposed refrigerants were analyzed and are summarized in Figure 3, Figure 4 and Figure 5. In general, the suitable refrigerants for commercial refrigeration applications include those that lie mainly near the outside ring of the spider plot, such as R-744, R-717, and R-290; however, the specific refrigeration application will dictate the appropriate options.

For systems with low refrigerant charge, such as self-contained systems and beverage vending machines, both flammable and non-flammable low GWP refrigerants would be appropriate options. In terms of flammable refrigerant alternatives, R-290 is an ideal alternative to the traditional refrigerant options of R-404A and R-134a. Among the non-flammable refrigerant alternatives, R-448A and the R-407 series provide good performance. However, due to their higher GWP (∼1300), these refrigerants may not be long-term solutions. R-744 is another non-flammable refrigerant option for self-contained display cabinets; however, since R-744 is a high-pressure gas, it cannot be used as a drop-in replacement and would require a specifically designed system with high-pressure-bearing components and tubing. Under the current toxicity regulations, use of R-717 in self-contained systems is not permissible.

In centralized refrigeration systems, R-744 is a good alternative to R-404A since R-744 has a refrigeration capacity approximately 36% higher than that of R-404A, as well as three times a higher heat transfer coefficient, no glide, no flammability, and a low GWP of 1. However, due to its low critical temperature, the performance of R-744 is 18% lower than that of R-404A in high ambient temperature conditions. This reduced performance limits the use of R-744 refrigeration systems to cooler climates. Technology enhancements such as parallel compression, subcooling, adiabatic gas cooling, ejectors, and expanders could enhance the performance of R-744 systems, thereby extending their reach to warmer climates. R-744 as a refrigerant option in warmer climates could also be made feasible by coupling a subcritical R-744 system in a cascade system, where refrigerants that perform well in warmer climates, such as R-407C, R-407F, R-448A, R-717, and R-600a, are used in the upper cycle of the cascade. However, being a high-pressure gas, R-744 cannot be used as a retrofit refrigerant in typical R-404A direct expansion systems. R-744 systems must be specifically designed and constructed with piping and components rated for use at high pressure. For retrofitting of systems in warmer ambient conditions, R-448A, R-407A, R-407C, and R-407F could be used as drop-in refrigerants in typical R-404A direct expansion systems.

4.2. Air-Conditioning and Heat Pumps

The residential and small commercial air-conditioning (AC) and heat pump (HP) sectors feature a range of systems, including small packaged and split AC/HP units for residential and commercial use, compact window air conditioners for homes, and larger packaged AC/HP units designed for commercial applications [47].

Air-conditioning and fans constitute nearly 20% of building site energy consumption across the globe [48]. Globally, more than 2 billion air conditioners and heat pumps are in operation across the residential, commercial, and industrial sectors [39,49]. Due to rising global temperatures [50] and growing purchasing power worldwide, the annual demand for air-conditioning and heating is increasing at an average rate of 14%, resulting in the installation of 5800 million AC and heat pump units per year across the globe [51,52].

Refrigerant Options Several alternative lower GWP refrigerant options are available for the range of air-conditioning applications, and these refrigerant options are summarized in

Table 4. Alternative refrigerants for air-conditioning and heat pump applications.

Application	Refrigerants
Currently used refrigerants
Window and small packaged units	R-22, R-410A
Split systems	R-22, R-410A
Large packaged units	R-22, R-410A
Alternative refrigerants
Window and small packaged units	R-32, R-1234yf, R-1234ze(E)
	R-444B, R-446A, R-447A
	R-447B, R-452B
	R-454B, R-459A
	R-511A, R-290, R-600a
	R-744
Split systems	R-32, R-444B, R-446A
	R-447A, R-447B, R-452B
	R-454B
	R-459A, R-511A
Large packaged units	R-32, R-444B, R-446A
	R-447A, R-447B, R-452B
	R-454A, R-454B, R-455A
	R-459A, R-511A

.

4.2.1. Analysis of Refrigerant Options for Air-Conditioning and Heat Pumps

A total of twenty-three potential refrigerants for air-conditioning applications was analyzed and compared, including two commonly used refrigerants, R-22 and R-410A. Similar to commercial refrigeration, the relative performance of the alternative refrigerants for air-conditioning applications was rated from 0 (worst) to 1 (best) for each of the seven refrigerant characteristics, as summarized in Figure 6, Figure 7 and Figure 8 and discussed below.

Global warming potential: With respect to GWP, the refrigerants investigated for the air-conditioning sector were normalized on a scale of 0 to 1, where 0 represents the GWP value of 2400 (slightly higher than that of R-410A) and 1 represents the GWP value of zero. From Figure 6, Figure 7, and Figure 8a, it can be seen that R-410A, R-22, R-134a, and R-32 are the least favorable in terms of GWP (with normalized GWP values ranging from 0.2 to 0.47), while R-744, R-290, R-600a, R-1234yf, R-1234ze(E), R-444A, R-454A, R-455A, R-459A, and R-511A have very low global warming potential values (with normalized GWP values close to 1). Other alternative refrigerants with moderately low GWP values (normalized GWP values greater than 0.7) include, in descending order of normalized GWP, R-444A, R-451A, R-455A, R-459A, R-454A, R-454B, R-446A, R-450A, R-452B, R-447A, and R-444B.

Refrigerating capacity: The refrigeration capacities of all the alternative refrigerants were normalized to the capacity of R-22 for a fixed operating condition consisting of an evaporator temperature of 7.2 °C (45 °F) and a condensing temperature of 40.6 °C (105 °F). Figure 6, Figure 7 and Figure 8b show that refrigerants with higher refrigerating capacity than R-22 (normalized capacity of approximately 1) include R-511A, R-600a, R-290, R-32, R-446A, R-447A, R-447B, R-459A, R-454B, R-452B, and R-444B. Refrigerants with capacities close to that of R-22 (normalized capacity of approximately 0.9) include R-410A, R-444A, R-134a, R-454A, R-457A, and R-450A. Refrigerants such as R-1234ze(E), R-455A, R-451A, R-1234yf, and R-744 have capacities lower than R-22 (normalized capacity ranging between 0.8 to 0.57).

High ambient temperature (HAT) capacity: The cooling capacity of a refrigerant decreases with an increase in ambient temperature. However, the magnitude of the drop in capacity varies depending upon the thermodynamic properties of the refrigerant. The refrigerating capacity of each of the candidate refrigerants at high ambient temperature was determined for a saturated condensing temperature of 51.7 °C (125 °F) and normalized with respect to R-22. The HAT refrigeration capacities of all alternative refrigerants are comparable to that of R-22 (having normalized HAT capacity values greater than 0.9). R-744, with a normalized capacity of 0.88, has the lowest normalized refrigeration capacity at high ambient conditions.

Temperature glide: As noted earlier, pure refrigerants exhibit no glide while the amount of glide varies dramatically among the refrigerant mixtures. The temperature glide of each refrigerant was determined at a saturated evaporating temperature of 12.8 °C (55 °F). Figure 6, Figure 7 and Figure 8d show that pure refrigerants such as R-22, R-32, R-1234yf, R-1234ze(E), R-290, R-744, R-600a, and R-134a have no glide (a normalized value of 1). However, for refrigerant mixtures, the temperature glide varies from 0.1 K (0.2 R) for R-410A (a normalized value of 0.98) to approximately 8 K (14 R) for R-455A (a normalized value of 0).

Transport properties: The heat transfer coefficients of the selected refrigerants were determined for an evaporator temperature of 12.8 °C (55 °F) and were then subsequently normalized according to Equation (2). The normalized heat transfer coefficient values thus range from a lower limit of zero to an upper limit of 1 (for R-22). Many candidate refrigerants were found to have equal or greater heat transfer performance compared to R-22, including R-744, R-32, R-290, R-447A, R-459A, R-452B, R-410A, R-444B, R-600a, R-407F, R-407C, R-444A, R-407A, R-457A, R-134a, R-450A, and R-1234ze(E). The refrigerants with heat transfer performance less than R-22 include R-451A and R-1234yf.

Flammability and toxicity:Figure 6 and Figure 7 show the comparison of the candidate refrigerants for air-conditioning applications based upon the ASHRAE safety classification [33], where flammability and toxicity were normalized as discussed in Section 4.1.1. As per the current regional safety standards, A1 refrigerants, such as R-410A, R-22, R-134a, R-450A, and R-744A, are ideal for air-conditioning, followed by A2L refrigerants, which include R-1234yf, R-1234ze(E), R-32, R-444A, R-444B, R-446A, R-447A, R-447B, R-451A, R-454A, R-454B, R-455A, R-457A, and R-459A. The A3 category refrigerants, R-290, R-600a, and R-511A, are permissible for use in small air-conditioning and heat pump systems as per regional safety standards.

System efficiency: System efficiency was characterized by comparing the coefficient of performance (COP) of a direct expansion air-conditioning system using the different refrigerants. For this comparison, the saturated evaporator temperature was assumed to be 12.8 °C (55 °F), and the saturated condensing temperature was assumed to be 35 °C (95 °F). The system efficiency of each refrigerant is normalized with respect to that of R-22.

The coefficients of performance of R-600a, R-134a, R-450A, and R-1234ze(E)-based air-conditioning systems are higher than that of the baseline R-22-based system. Many of the alternative refrigerants exhibited a coefficient of performance slightly lower than R-22, including R-451A, R-32, R-1234yf, R-444A, R-454B, R-452B, R-459A, R-511A, R-447B, R-290, R-447A, R-410A, and R-446A. For these refrigerants, the normalized coefficient of performance ranged from 0.98 to 0.90. Other refrigerants with lower system efficiency, in descending order, are R-444B, R-457A, R-454A, R-455A, and R-744, respectively.

4.2.2. Summary: Air-Conditioning

As discussed in the previous section, air-conditioning and heat pump applications consist of low refrigerant charge systems, including window, packaged, and small split air conditioners, as well as high refrigerant charge systems such as large split air conditioners and rooftop units. Due to the low charge requirements of window and small split air conditioners, the use of refrigerant from any flammability category (A1, A2, A2L, and A3) is permitted under current safety regulations. However, in warmer climate regions, because of higher cooling loads, the refrigerant charge required by these small systems may exceed the maximum charge limit for A2, A2L, and A3 refrigerants. In the case of larger split units with their higher refrigerant charge requirements, the replacement of current high GWP refrigerants is restricted to non-flammable (A1) refrigerants only.

In pursuit of viable future refrigerants for these applications, twenty-three potential alternative refrigerants were studied. As shown in Figure 6, Figure 7 and Figure 8, R-511A and R-290 are ideal refrigerants for window and small packaged air-conditioning systems due to their low GWP, higher cooling capacity (76%), and high heat transfer coefficient (20%) compared to R-410A. Isobutane (GWP = 3), with 76% greater cooling capacity and an equivalent heat transfer coefficient compared to R-410A, is another good alternative refrigerant for small air-conditioning applications; however, R-511A, R-290, and R-600a are flammable (A3) refrigerants. Among the lesser flammable refrigerants (A2L), R-446A (GWP = 461) has 30% to 35% greater cooling capacity than R-410A at standard and high ambient conditions. Thus, it is a potential refrigerant for window and small packaged units in moderate and warmer climate regions. Among the non-flammable refrigerant options, R-450A (GWP = 547) is the closest possible replacement for R-410A, with 9% lower cooling capacity at standard ambient conditions and 7% lower capacity at higher ambient conditions, as compared to R-410A.

5. Conclusions

The global heating, ventilation, air-conditioning, and refrigeration (HVAC&R) industry, a significant contributor of greenhouse gas emissions, has taken measures to reduce its emissions in order to reduce the rate of increase in the earth’s temperature caused by these greenhouse gas emissions. A major step in the right direction would be replacing the currently used high-GWP refrigerants in equipment for applications such as food storage and human comfort with efficient and stable alternative refrigerants with low GWP.

In this paper, twenty-seven current and alternative refrigerants from all classifications of flammability and toxicity (A1, A2, A2L, A3, B1, B2, B2L, and B3) were studied for application in two major sectors of the HVAC&R industry, namely, refrigeration and air-conditioning. A general trend seen among these refrigerants is that the refrigerants that have low GWP and higher system performance are flammable and/or toxic and are thus allowed only in restrictive use under current regulations. On the other hand, the use of non-flammable alternative refrigerants with moderately lower GWPs with respect to current refrigerants leads to a compromise in system efficiency.

For the refrigeration industry, the ideal refrigerant is dependent upon system type and geographical location of the refrigeration system. Considering charge limitations on the use of flammable refrigerants, hydrocarbons are most suited for low-charge, self-contained systems. For large, centralized refrigeration systems, transcritical carbon dioxide booster systems are recommended for new installations in cold and moderate temperature regions. For warmer temperature regions, carbon dioxide could be used in the low-temperature cycle of a cascade system with ammonia, R-290, R-600a, R-407A, R-407C, R-407F, R-448A, and R-449A used in the high-temperature cycle. For retrofit systems, R-407A, R-407C, R-407F, R-448A, and R-449A could be used in both A5 and non-A5 group countries.

Based upon the parametric analysis conducted in Section 4.2, there is not a single clear ideal refrigerant to satisfy the global air-conditioning industry. Depending upon the regional safety and environmental regulations along with ambient conditions, the ideal refrigerant would vary geographically. For example, hydrocarbons such as R-290 and R-600a, along with azeotropic refrigerants such as R-511A, have low GWP, highly desirable thermodynamic properties, and high cooling capacity. However, due to the high flammability of hydrocarbons, under current regulations in some countries, these refrigerants either cannot be used in air-conditioning systems or can be used with refrigerant charge limits. Thus, this suggests the use of the next best available refrigerant, R-446A, a lower flammability HFO refrigerant blend with a GWP of 461. R-466A would be a suitable solution for split air-conditioning systems, but not for small, portable air-conditioning units [53].

The trade-off between GWP and safety (i.e., flammability and toxicity) of refrigerants is critical. Considering the irreversible environmental impacts caused by high-GWP refrigerants, the long-term solution should be the utilization of refrigerants with high cycle performance and negligible GWP. To make this happen, the research priority of the HVAC&R industry should shift from developing low GWP refrigerants with no flammability to focusing on the safety aspects related to the use of flammable refrigerants, such as safe equipment design (e.g., spark-proof electrical components), minimization of refrigerant charge, flame-free joining techniques, and safe handling and maintenance procedures. In addition, it is noted that the temperature glide of many alternative low GWP refrigerant blends is large. Additionally, a comprehensive sustainability assessment of refrigerants must consider their entire lifecycle, including production, transportation, usage, and end-of-life disposal. While alternative refrigerants may have lower global warming potential, they often require significant energy input for production and may pose challenges in terms of recycling and disposal. Finally, the transition to these refrigerants can incur economic costs for equipment upgrades and may introduce safety concerns related to their toxicity or flammability. To address these complexities, future research should employ a holistic approach, integrating metrics like energy performance, lifecycle emissions, resource efficiency, and socio-economic impacts. Tools such as Life Cycle Assessment (LCA) and multi-criteria decision analysis can aid in evaluating the trade-offs associated with different refrigerant options. The effects of refrigerant blend fractionation throughout the cycle and changing composition due to leaks should be considered during the system and component design in order to optimize performance.