GLOBAL WARMING DUE TO REFRIGERANTS
By: Madhura Kunachi
Tags: Global Warming, Refrigerants

( Rohan Kulkarni(11), Vineet Kumbhar(12), Kshitija Kumbharkar(13), Madhura Kunachi(14), Ganesh lohiya(15))

INTRODUCTION TO REFRIGERANTS

A refrigerant is a substance or mixture, usually a fluid, used in a heat pump and refrigeration cycle. In most cycles it undergoes phase transitions from a liquid to a gas and back again. Many working fluids have been used for such purposes. Fluorocarbons, especially chlorofluorocarbons, became commonplace in the 20th century, but they are being phased out because of their ozone depletion effects. Other common refrigerants used in various applications are ammonia, sulphur dioxide, and non-halogenated hydrocarbons such as propane.

PROPERTIES A REFRIGERANT SHOULD POSSES:

The ideal working fluid or often called refrigerant would have favourable thermodynamic properties, be noncorrosive to mechanical components, and be safe, including freedom from toxicity and flammability. It would not cause ozone depletion or climate change. Since different fluids have the desired traits in different degree, choice is a matter of trade-offs. The desired thermodynamic properties are a boiling point somewhat below the target temperature, a high heat of vaporization, a moderate density in liquid form, a relatively high density in gaseous form, and a high critical temperature. Since boiling point and gas density are affected by pressure, refrigerants may be made more suitable for an application by appropriate choice of operating pressures.

REFRIGERANT CRITERIA:

Summarizing the points mentioned in the previous article in two main points; a working fluid in a compression refrigeration system must satisfy a number of requirements that can be divided into two groups:

1. The refrigerant should not cause any risk of injuries, fire or property damage in case of leakage.

2. The chemical, physical and thermodynamic properties of the refrigerant must suit the system and the working conditions at a reasonable cost.

The criteria can be specified more precisely as follows:

It is not possible to fulfil all the requirements above at the same time. The most important criterion is chemical stability within the refrigeration system. All the other criteria are meaningless if the refrigerant decomposes or reacts with the materials used in the system. However, the chemical criterion can also be a problem. When a refrigerant is emitted to the atmosphere, it should not be so stable that it can exist indefinitely. The ideal refrigerant would be totally stable in use within the system but would decompose easily in the atmosphere without the formation of any harmful substances.

The thermodynamic and transport properties determine the performance of the refrigeration system. The pressure should be neither too high nor too low. A pressure below atmospheric is undesirable, because air can then be sucked into the system, resulting in problems such as inert gases in the condenser or ice plugs in the expansion valve. The latent heat multiplied by the vapor density gives an estimate of the capacity of a refrigeration system. A refrigerant with a high pressure gives a higher capacity, but a refrigerant with a low critical pressure compared with the pressure in the system leads to a low coefficient of performance (COP). This trade-off between high capacity and high efficiency must be considered when choosing the refrigerant. Another desired criterion is that the specific heat should be small compared to the latent heat of vaporization, in order to achieve small losses in the expansion device.

Oil is normally present in a refrigeration system, and the interaction between the oil and the refrigerant must be considered. High oil solubility is used in hermetic compressors, but immiscible oils are normally used when the working fluid is ammonia.

DESIGNATION OF REFRIGERANTS

According to an international agreement, refrigerants are represented by the letter R (as in Refrigerants) followed by a two- or three-digit number and, in some cases, one or two letters. The designation ‘Rxyz’ is determined by the chemical composition of the molecule, as described below.

The methane, ethane and propane series:

Here, (x) gives the number of carbon atoms in the chemical formula, minus one. ​

(x) = 0 is the methane series, but the 0 is ignored for these compounds. Examples are R12 and R22.

(x) = 1 is the members of the ethane series, such as R114, R124 and R134a.

(x) = 2 is the propane series, e.g. R290 (propane).

For these groups, (y) describes the number of hydrogen atoms plus one and (z) describes the number of fluorine atoms.

ZEOTROPIC AND AZEOTROPIC MIXTURES:

(x) = 4 refers to zeotropic mixtures. The components in the mixture have different boiling points, and thus the refrigerant mixture has a temperature glide. R407A and R407C are examples of such refrigerants.

(x) = 5 refers to azeotropic mixtures. These act like homogeneous substances with one specific boiling point, and therefore they have no glide. R502 and R507 are examples of azeotropic mixtures.

Here, (y) and (z) are ordinal numbers.

HIGH ORGANIC COMPOUNDS:

(x) = 6 means that the composition is organic, e.g. butane, R600, and isobutene, R600a. This group has several subgroups, for example hydrocarbons, oxygen compounds, sulfuric compounds and nitrogen compounds.

The subgroups have been assigned different number series within the main group, so (y) and (z) describe the subgroup and order within the subgroup.

INORGANIC COMPOUNDS:

(x) = 7 refers to inorganic compounds, such as ammonia, R717, water, R718, and carbon dioxide, R744.

In this group, (y) and (z) are the molar mass.

UNSATURATED ORGANIC COMPOUNDS:

(x) = 11 stands for unsaturated ethane compounds, such as R1150 (ethylene).

(x) = 12 stands for unsaturated propane compounds, such as R1270(propylene). The (y) and (z) are the same as for the ethane and propane series.

LETTERS AT THE END:

The last letter, if any, in the designation number means different things:

Lower-case letters describe the structure of the molecule. For example, R600 is butane and R600a is isobutane. These two compounds have the same chemical formula, but different spatial arrangements, and they therefore have slightly different properties.

Capital letters describe specific mixing proportions of different components. For example, R407 A-E are mixtures of the refrigerants R32, R125 and R134a. R407A has the following mixing proportions: 20% R32, 40% R125 and 40% R134a, while R407C consists of 23% R32, 25% R125 and 52% R134a.

ENVIRONMENTAL EFFECTS DUE TO REFRIGERANTS

In addition to being toxic or explosive and thereby dangerous to people’s health, there are other problems associated with refrigerants. Environmental aspects are increasingly being taken into consideration. Refrigerants can thus also be ranked according to their impact on the stratospheric ozone layer (the Ozone Depletion Potential, ODP) or as greenhouse gases (the Global Warming Potential, GWP).

OZONE DEPLETION POTENTIAL, ODP:

Refrigerants containing chlorine or bromine contribute to the breakdown of the ozone layer. The reaction is as follows:

CL +O3 àO2 +ClO

However, the CIO molecule is unstable. It breaks down and reacts with ozone molecules (in accordance with the equation above) repeatedly until a more stable compound is created.

The ODP is the ratio of the impact on ozone of a chemical compared with the impact of a similar mass of CFC-11 (R11). Thus, the ODP of CFC-11 is 1.0 by definition. Other CFCs and HCFCs have ODPs ranging from 0.01 to 1.0. The halons have ODPs ranging up to 10. Carbon tetrachloride has an ODP of 1.2, and methyl chloroform’s ODP is 0.11. HFCs have zero ODP because they do not contain chlorine.

The inert nature of many haloalkanes, chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFC), particularly CFC-11 and CFC-12, made them preferred choices among refrigerants for many years because of their non-flammability and non-toxicity. However, their long lifetimes in the atmosphere along with their corresponding global warming potential and ozone depletion potential raised concerns about their usage. In order from the highest to the lowest potential of ozone depletion are: Bromo chlorofluorocarbon, CFC then HCFC.

The discovery of stratospheric ozone holes over polar regions in the early 1980’s led to a transition to hydrofluorocarbons (HFCs) which are not-ozone depleting. These include R-134a, R-143a, and R-410a (a 50/50 blend of R-125/R-32), These refrigerants still have global warming potentials thousands of times greater than CO2 and atmospheric lifetimes that can extend for decades.

New refrigerants were developed in the early 21st century that are safer for the environment, but their introduction has been held up due to concerns over toxicity and flammability. HFCs are now being replaced in markets where leaks are likely by using a fourth generation of refrigerants, most prominently HFO-1234yf trademarked by Chemours as Opteon-YF, that have global warming potentials much closer to that of CO2.

As of year, 2019, CFCs, HCFCs, and HFCs are responsible for about one-tenth of the direct radiative forcing from all long-lived anthropogenic greenhouse gases. A 2018 study by the non-profit organization “Drawdown” put proper refrigerant management and disposal at the very top of the list of climate impact solutions, with an impact equivalent to eliminating over 17 years of US carbon dioxide emissions.

Compared to halogenated refrigerants, hydrocarbons like isobutane (R-600a) and propane (R-290) offer several advantages: low cost and widely available, zero ozone depletion potential and very low global warming potential. They also have good energy efficiency but are flammable and can form an explosive mixture with air if a leak occurs. Despite the flammability, they are increasingly used in domestic refrigerators. EU and US regulations set maximum charges of 57 or 150 grams of refrigerant, keeping the concentration in a standard kitchen below 20% of the lower explosive limit. The LEL can be exceeded inside the appliance, so no potential ignition sources can be present. Switches must be placed outside the refrigerated compartment or replaced by sealed versions, and only spark-free fans can be used. In 2010, about one-third of all household refrigerators and freezers manufactured globally used isobutane or an isobutane/propane blend, and this was expected to increase to 75% by 2020.

Observed stabilization of HCFC concentrations (left graphs) and the growth of HFCs (right graphs) in earth’s atmosphere.

GLOBAL WARMING POTENTIAL, GWP

Due to their stability in the atmosphere, CFCs as well as HCFCs and HFCs are often very effective greenhouse gases. The GWP factor is used to reflect their impact on global warming.

The GWP is the ratio of the warming caused by a substance to the warming caused by a similar mass of carbon dioxide. Thus, the GWP of CO2 is 1.0. CFC-12 has a GWP of 8500, while CFC-11 has a GWP of 5000. Various HCFCs and HFCs have GWPs ranging from 93 to 12100. Water, a substitute in numerous end-uses, has a GWP of 0.

When using GWP values from different sources, it is important to consider that the values may differ due to different integration times or calculation models.

Another measurement of the impact on global warming is the TEWI value (Total Environmental Warming Impact). This includes not only the direct impact of any release of the refrigerant (GWP), but also the impact during the generation and use of primary energy in the system.

Calculation of global warming potential (GWP) values of refrigerants is a complex issue. For the great majority of cases there are several different values of GWP for each specific refrigerant. This is due to several reasons, including the following:

1. There is variety of sources from which the GWP values could be obtained

2. GWP values are periodically updated, based on the most recent research and as scientific understanding improves

3. GWP values are calculated over different time horizons. Typically, GWP values are quoted over a 100-year time horizon, although 20 year and to a lesser extent 500 year integrated values are also commonly provided.

The higher the GWP value, the more that gas warms the Earth compared to carbon dioxide. GWP values for ozone depleting substances can range, for example, from 2 up to about 14,000. The GWPs of commonly used HFCs can range from <1 to about 12,500.

Table 1 provides some sample GWP values. Examples of CFC-12, HCFC-22 and HFC134a have been selected to illustrate the progression in knowledge over time leading to the updating of the GWP values. The examples also indicate some of the different sources of GWP values.

The GWP values in Table 2 are for single component refrigerants. In the case of refrigerants which are mixtures (or blends) of more than one refrigerant, the GWP is calculated as a mass weighted average of the individual components.

CONCLUSION

Refrigeration equipment must be adapted to the global warming challenge. It needs long-term policy. The current phase-out of HCFCs, even if it requires a huge effort on the part of the refrigeration sector, provides an opportunity to design new equipment using renewables as energy sources and using technologies with good energy efficiency. Existing equipment can and must be improved. However, the progressive renewal, within about 20 years (lifespan of equipment) on the average, of refrigeration and air–conditioning equipment, using environmentally friendly technologies, is a key factor and could represent a major part of worldwide efforts for mitigating global warming until 2030.

There’s always room for improvement. In order to replace harmful refrigerants like CFCs and HFCs, Distributors DuPont and Honeywell have turned to HFO (hydrofluoro olefin) as an alternative.

In the meantime, you can do your part by maintaining your appliances, replacing them when they’re outdated, and properly recycling them. Turn off your AC when you’re not home so you aren’t wasting energy, too.