Reduction of the Greenhouse Gasses Emissions in Refrigeration

From the mid-nineteenth century the first refrigerating fluids to be used were natural fluids such as: carbon dioxide, ammonia and sulfur dioxide. Because of their dangerous use and toxicity, natural fluids were replaced by synthetic fluids during the 1930s.

The initial synthetic fluids to be available as vapor compression refrigerants were the CFCs and subsequently the HCFCs. Chlorine and fluorine contained in these chemical compounds ensured good thermodynamic properties in order to guarantee safety to people during their use. In 1974 two American scientists, Rowland and Molina, discovered that the chlorine contained in these compounds was causing the thinning of the ozone layer in the stratosphere. This discovery, corroborated also by other scientific studies, led many states to stipulate agreements and protocols aimed at to take countermeasures toward the phenomenon of ozone depletion, through the progressive replacement of CFC and HCFC fluids with other less harmful fluids. The first important international agreement was the Montreal Protocol, signed on September 16, 1987. To date, 192 nations joined this agreement. Following the prescription of the Montreal Protocol, pure or mixed HFC fluids not dangerous for the ozone were introduced. Indeed, with the introduction of HFC fluids, chlorine has been completely eliminated and replaced by hydrogen, solving the problem of stratospheric ozone depletion. However, a new environmental problem has arisen: the increase temperature of our planet caused by human activities and, in part, by this new generation of refrigerant fluids. The Kyoto Protocol, drafted in 1997 and with more than 180 countries adhering, classifies HFC refrigerants as substances responsible for the direct greenhouse effect [5].

The aforementioned restrictions are forcing the transition toward a fourth generation of refrigerants with ODP and GWP satisfying the regulations. In this new generation of refrigerants there are HydroFluoroOlefins (HFO) which derive from olefins rather than from alkanes. Two known and promising HFOs are: HFO1234yf and HFO1234ze [6]. These refrigerants have zero ODP and very small GWP which guarantee a very short lifetime once released in the atmosphere [7, 8]. The current state of the art reveals that the HFOs have been tested as drop-in replacement of domestic refrigerators previously operating with HFC134a [9-13].

Furthermore, in some fields, such as domestic refrigeration, isobutane (R600a) has already been proposed to replace HFC134a [14, 15]. However, although this solution has already been accepted by the European market, it involves some technical difficulties linked to the high flammability of the R600a. While the small amount of charge present in a domestic refrigerator for the European market is not of particular concern, domestic refrigerator manufacturers have had to redesign some areas of their production plant (e.g. refrigerant storage and charging stations), with consequent huge investments by the companies involved. No less important is the adoption of specific components (i.e. compressors) able to work with HydroCarbons (HC). Furthermore, as evidenced by Bansal et al. [16] the use of R600a may have high barriers to the American market (US market) due to stricter fire prevention standards (UL 250 standard), as well as other markets may be reluctant to such a solution [17].

Figure 1 shows a block diagram that describes the historical evolution of refrigerant fluids.

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Figure 1. Historical evolution of refrigerant fluids

The refrigerants currently on the market are divided into two main categories: natural fluids and synthetic fluids.

4.1 Natural fluids

The term natural refrigerants mean all those substances or compounds that can be found directly in nature or whose composition does not occur by chemical synthesis. They are substances that have a low environmental impact by not containing chlorine or fluorine. Ammonia, carbon dioxide, hydrocarbons and water are natural refrigerants.

4.1.1 Ammonia

Ammonia was one of the first fluids to be used as a refrigerant. In 1854 F. Carrè built a water-ammonia absorption refrigerator while in 1876 Boyle developed an ammonia compressor.

Ammonia, whose chemical formula is NH3 while the abbreviation ASHRAE is R717, is a colorless gas with a pungent smell. The thermodynamic properties of ammonia are:

• high energy efficiency, both in high and low temperature applications;
• operating pressures comparable to those of other common refrigerants;
• the possibility of using a lower refrigerant charge than the average, thanks to the high latent heat of evaporation that it possesses.

Ammonia is also inexpensive, widely available on the market and, in terms of global overheating and stratospheric ozone damage, has no environmental impact. The disadvantages that have limited its use are toxicity and flammability; in fact, ammonia belongs to the B2 safety group. Furthermore, it is corrosive towards certain alloys and some metals. It has only spread in the industrial sector as a refrigerant fluid for cooling, freezing and deep-freezing plants, whereas in the commercial sector it has always been replaced by HFCs. Some important technical advances have allowed ammonia to spread to other fields besides the industrial one. These advances include:

• the development of some hermetic components of the refrigeration system such as the compressor;
• the use of alloys compatible with the fluid;
• the use of new soluble lubricants in the fluid.

These technical interventions have led to higher plant production costs that are not always sustainable for smaller capacity systems developing.

4.1.2 Carbon dioxide

The first uses of CO₂ as a refrigerant date back to almost 150 years ago. In 1866 Thaddeus Lowe was able to adapt a compressor used for ice production to CO₂. Carbon dioxide is a colorless and odorless gas. The thermophysical properties of CO₂ are:

• very small kinematic viscosity;
• large density;
• high working pressures that require specific plant interventions;
• inertia at ordinary temperatures and compatible with all the most common materials, although in the wet state it is corrosive towards almost all metals except stainless steels;
• low critical temperature (30°C) such as to make the thermal gradient between the cold source and the hot source too limited, with a consequent small quantity of heat exchanged.

It is easily available in the environment and is often the waste product of technological processes, so its cost is very low. Carbon dioxide is not very polluting and is used as a reference gas to quantify the impact of other refrigerant gases on the environment. It is non-flammable, chemically stable and not dangerous in water. However, although it is not toxic, if present in a considerable quantity in an enclosed space it can lead to asphyxia. Compared to other refrigerants, the CO₂ system efficiency is more influenced by the type of application and the climate. In fact, with the increase in temperature the efficiency of a system that uses carbon dioxide decreases. This is an important aspect because it constitutes a strong limitation to the use of CO₂ in countries characterized by warm climate.

4.1.3 Hydrocarbons

Hydrocarbons are organic compounds, mainly of fossil origin, composed only of carbon and hydrogen atoms. They have good thermodynamic properties, including a high heat transport that allows a lower refrigerant charge. In terms of efficiency, they are able to compete with HFCs and HCFCs, reaching high COP values in most applications. Among the advantages of hydrocarbons there is compatibility with many lubricants used in the equipment with the exception of those containing silicone and silicates. They are also used in mixtures (together, for example, with HFCs), allowing an easier retrofit operation of plants loaded with CFCs. They have a low environmental impact, are not toxic but are highly flammable belonging to the A3 safety class. For this reason, they have limitations on use in terms of the quantities allowed in the equipment. Hydrocarbons such as propane and isobutane find application in the refrigeration sector, while other hydrocarbons are used as alternative foaming agents in the production of foams.

4.1.4 Water

The water is odorless, colorless, has no environmental impact and is the most economical natural refrigerant. It has high values of latent heat of evaporation and of critical temperature so it can be used even in very hot climates. It is not toxic, flammable or explosive, therefore it represents, from this point of view, a valid solution. It has the following characteristics that limit its use as a primary refrigerant:

• low solidification temperature at atmospheric pressure;
• over time it causes corrosion and oxidation in metals;
• high specific volume value that makes the compression process at low pressures in vapor compression refrigeration cycles challenging.

From a plant engineering point of view, interventions such as the use of large compressors and the use of titanium alloys are necessary. It is a substance still under study. Table 1 below summarizes the main aspects of natural fluids.

Table 1. Properties of natural fluids

4.2 Synthetic fluids

The synthetic refrigerating fluids which have been used most often belong to the family of alkyl halides. Based on the presence or absence of chlorine in the compound, these fluids can be divided into three large groups: CFC, HCFC, HCF.

4.2.1 CFC

Chlorine-fluoro-carbides are synthesis products deriving from the first hydrocarbons of the saturated paraffin series, methane (CH₄) and ethane (C₂H₆), by replacing all the hydrogen atoms in the molecule with chlorine (Cl) and fluorine atoms (F).

This compound was first synthesized by Thomas Midgley, who announced its invention in April 1930 and from 1931 was commercially available. At first they were used only as refrigerant fluids, but over the years they became widespread in various sectors becoming central in the production, for example, of detergents in the electronics industry, of foaming agents in plastic foams, of solvents in the chemical industry, of tracer fluids and chemical products to extinguish fires. The replacement of a hydrogen atom with a fluorine one causes an increase in density while the substitution of a hydrogen atom with one of chlorine generally causes an increase in the latent heat of evaporation and a reduction in the boiling temperature. CFCs are low boiling: they boil at atmospheric pressure and at temperatures lower than room temperature range. They are colorless, odorless or with a faint odor of ether. All CFCs have the following important properties:

• not flammable;
• not toxic for humans;
• chemically stable;
• guarantee good performances of the refrigeration systems in relation to their charge;
• production costs are relatively low.

Although CFCs possess good properties, the characteristics that have determined their success in various sectors are non-toxicity and non-flammability. The great chemical stability, which represents an advantage during use, has proved over time to be the most important disadvantage from the environmental point of view given their high potential for destruction of the stratospheric ozone (ODP).

Some important CFCs are shown in Table 2.

Table 2. CFC refrigerants

4.2.2 HCFC

HCFCs characterize the second generation of synthetic fluids and also belong to the family of alkyl halides. From the point of view of the chemical composition they are distinguished from the CFCs for having replaced a part of chlorine in the compound with hydrogen. This substitution has two direct consequences:

• greater instability once dispersed in the atmosphere, where it tends to decompose more rapidly than CFCs;
• moderate flammability and toxicity increases compared to CFCs.

Furthermore, this replacement does not affect the chemical and physical stability of the refrigerant, preventing the formation of substances or new compounds when it is inside a refrigeration circuit. They are generally also colorless and odorless. Since the fields of application of HCFCs have been conceived as CFC substitutes, they are the same.

Some important HCFCs are reported in Table 3. The R22, present in this list, is the most used refrigerant fluid in the field of commercial and industrial air conditioning [18, 19]. It has a refrigerating effect equal to that of ammonia with the difference that it has more reduced compression ratios

Table 3. HCFC refrigerants

4.2.3 HFC

Introduced as a substitute for CFCs and then HCFCs from the early 1990s, HFCs are the third generation of refrigerants. They are compounds formed by hydrogen, fluorine and carbon atoms. They are average non-flammable, with low toxicity and good energy efficiency [20]. They are colorless, odorless and non-aggressive with respect to materials such as copper and plastic and non-reactive. The success of this type of refrigerants has not been exciting, especially with regards to its use as pure fluids. The cause must be attributed to the fact that HFC fluids do not in most cases offer comparable performance with CFC and HCFC refrigerants, so that the retrofitting of old systems is not always simple and possible. Currently HFCs are used more as components for mixtures. With these, it is possible to calibrate the thermodynamic characteristics of the refrigerant that must go to replace the CFCs and HCFCs in existing plants. The absence of chlorine in the compound guarantees HFCs a zero impact on the environment from the point of view of stratospheric ozone destruction, unlike CFCs and HCFCs; however, HFCs are harmful from the point of view of global warming potential and therefore of the greenhouse effect. Some HFCs are shown in Table 4.

Table 4. HFC refrigerants

R134a is perhaps the most widespread among HFCs. It has been used, for example, as a substitute for R12 (CFC) at medium and high temperatures in refrigeration and as an alternative to sulfur hexafluoride in magnesium metallurgy. In the conversion of plants from R12 to R134a, however, with the same compressor power, R134a provides a slightly lower cooling capacity than that obtained using R12. Furthermore, R134a is harmful if inhaled, it is not flammable at room temperature and atmospheric pressure but it can become combustible if mixed with pressurized air.

Table 5 summarizes the main aspects of synthetic fluids.

Table 5. Aspects of synthetic fluids