Refrigerants are the working fluids used in a refrigeration system that extracts heat from a body or space and thus, producing some cooling effect. The system performance is dependent on the thermo-physical properties of refrigerants. The choice of refrigerant for a particular application depends on safety, reliability, cost and performance of the overall system. Presently, more emphasis is given on the environmental aspect because of ozone layer depletion and global warming occurring due to the uses of different refrigerants. The evolution of various refrigerants falls under different generations which are dictated by different levels of priority. Different generations of refrigerants categorised under different levels of priority have been presented in Figure 1. At the beginning of mechanical refrigeration, in the early 19th century natural refrigerants were mainly used. Refrigerators that were built in the late 1800s to 1929 used the first-generation refrigerants such as methyl chloride, ammonia and sulphur dioxide. The common refrigerants for the first hundred years included whatever worked and whatever was available. Nearly all the first-generation refrigerants were flammable, toxic or both and some were also highly reactive. The second-generation refrigerants were distinguished by a shift to chloro-fluoro chemicals for safety and durability. The third-generation refrigerants were mostly low ozone depletion potential refrigerants. Besides having low ODP and GWP values, they can also be used in existing refrigeration system designs. Ultimately, the fourthgeneration refrigeration considered all the above-mentioned factors along with high performances.
Evolvement of CO2 as Refrigerant
At the end of the nineteenth century different refrigerants began to evolve. Carbon dioxide was one among them. Carbon dioxide was first solidified in 1835 by a French physicist, Thilorier and from then it is being used as a cooling agent, popularly known as dry ice to solidify mercury. Lowe was first to suggest that carbon dioxide could be used as a refrigerant in 1867. In 1884, Raydt built a R744 (CO2) refrigeration system for making ice using a vapour compression mechanism while, at the same time, J Harrison was the first person to build a device for manufacturing R744 purely for refrigeration use. Franz Windhausen patented a refrigerating compressor machine that used carbon dioxide in 1886. The British Company J&E Hall built a carbon dioxide compressor taking permission from Windhausen. Later the same company built the first two-stage CO2 compressor also. That was the beginning for extended use of carbon dioxide as refrigerant in refrigeration system. Later its application was enhanced in various systems as such onboard of refrigerated ships and also in other sectors of refrigeration. The use of equipment working with CO2 as a refrigerant mainly involved when beer brewing and meat were started to transport from Australia and Latin America to Great Britain in 1890.
Abandonment and Revival of CO2 as Refrigerant
After the evolution of the second-generation refrigerants like chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), the use of CO2 was phased out eventually in most of the industries. The cause for doing so is it has got a very low critical temperature of 31.1°C which is not ideal for sub-critical operation. However, with increase in ambient temperature, the system gradually changes from sub-critical operation to trans critical operation accompanied by a high operating pressure. When the system is in trans critical operation, the expansion valve has a very large pressure drop causing a great energy loss resulting in a drop in COP. But the environmental issues associated with other thermodynamically efficient refrigerants paved the way for CO2 as refrigerant. Figure 2 shows about the timeline of different refrigerants and refrigeration systems evolved along with CO2.
Although, the use of CFCs and HCFCs were in vogue for a longer period of time, but at the end of twentieth century, it is established that these halogenated refrigerants were the root cause of stratospheric ozone layer depletion. These are eventually phased out in many countries as per recommendations of the Montreal and Kyoto protocols in 1987 and 1997 respectively. Under these circumstances, CO2 has once again found its importance as an eco-friendly, safe and reliable refrigerant.
Professor Gustav Lorentzen was the pioneer of the revival of R744 refrigeration in the early 1990s. By the end of the early 1990s, CO2 has earned its significance in most of the refrigeration and industrial sectors due to its excellent thermodynamic as well as environmental properties. CO2 as a natural refrigerant is mostly implemented in low temperature refrigeration systems such as in food processing industry and in supermarket refrigeration systems. This is so because at low temperatures, CO2 has got many unique thermo-physical properties. Some of them are: high latent heat of vaporisation, high vapour density and low surface tension which enable the sub-critical refrigeration systems to achieve high COP. Properties of different refrigerants used in the past, being used at present and promising future refrigerants are listed in table 1 for comparison.
CO2 is abundantly found in the atmosphere mainly as derivatives of industrial waste products. It has been considered one of the ideal refrigerants in mechanical vapour compression refrigeration (VCR) systems. Presently, it has found its place in marine refrigeration and also as an alternate refrigerant to ammonia and methyl chloride due to its non-toxic nature.
Transcritical Vapour Compression Refrigeration using CO2
Transcritical cycle is the thermodynamic cycle where the working fluid goes through both subcritical and supercritical states. As CO2 has a low critical temperature (31.1°C), therefore, during operating at high ambient temperature, vapor compression system for refrigeration will work nearly or just above the critical pressure (7.38 MPa). Heat rejection will take place mostly at supercritical and low side condition in subcritical pressure making it a transcritical system (single stage). The schematic and the P-h diagrams of a transcritical CO2 vapour compression system have been shown in Figure 3(a) and Figure 3(b) respectively. It may be noted that a gas cooler is being used instead of the condenser in the transcritical system.
Cascade Refrigeration System using CO2
The cascade refrigeration system is a refrigerating system that generally works with two kinds of refrigerants having different boiling points, which run through their own individual freezing cycle and are connected by a heat exchanger. Many industrial applications like food storage, liquefying of petroleum vapour and natural gases, precipitation hardening of some special alloys, dry ice production, storing of blood etc., required low temperature refrigeration in the temperature range from -30°C to -100°C. For low temperature refrigeration of that range, single stage systems become inefficient. A cascade system can be considered a suitable option for this low temperature refrigeration system if proper refrigerants are used which in turn depends upon the desired temperature required for various applications. Apart from that heat transfer characteristics, safety, environmental issues and lubricants compatibility of refrigerant pair should be considered. In the design phase for CO2 cascade refrigeration system, CO2 is used in low temperature circuit (LTC) and NH3 (R-717) in the high temperature circuit (HTC). Two circuits are thermally joined to each other through a cascade-condenser, which acts as an evaporator for the HTC and a condenser for the LTC. The arrangement of different components of a CO2/NH3 cascade refrigeration system has been shown in Figure 4(a). The corresponding T-s diagram has also been presented in Figure 4(b).
Applications of CO2 as Refrigerant
As mentioned earlier, CO2 was first used as a cooling agent popularly known as dry ice that sublimes at -78.3°C. However, in refrigeration, there are enormous applications for R744, some of which have been commercialised. A few of the important application areas where CO2 is used as refrigerant are discussed below:
Water Heat Pumps
Production of hot water is the best application for R744 heat pumps as the temperature slide in the transcritical cycle suits the thermodynamic properties of water well. Very efficient heat transfer and very high-water temperatures are achieved with water heating applications using a counter-flow gas cooler. The percentage energy saving of a heat pump (ΔE) when compared to another heating system with an efficiency of h is given by:
(1)
For electric heaters, h is nearly equal to 1 and it varies from 0.5-0.95 for fuel-fired heaters.
Mobile Air-Conditioning
Mobile air-conditioning system consumes 31 per cent of the world’s refrigerant, which adds up to more than 150,000 ton/year. Compact R744 systems coupled with good heat transfer characteristics between air and R744 encourage the use of these systems in a sector in which equipment space and weight are limited while energy efficiency is predominant. With the phasing out of HFC in progress, R744 is one of the best alternatives for mobile air conditioning. R744 air conditioning will work especially well with fuel-efficient hybrid or electrical cars with little waste heat available. With electrical cars specifically, if the air-conditioning system is efficient enough, more energy will be used to drive the car and thus more travelling distance will be covered before the electricity runs out. However, the only limitation of R744 in mobile air-conditioning applications is its high heat rejection temperature.
Commercial Refrigeration
Commercial refrigeration consumes about 28 per cent of worldwide refrigerants. This makes it one of the largest emitters of refrigerants into the environment and accounts for approximately 37 per cent of worldwide emissions. This equipment includes refrigerated display counters in supermarkets, refrigerated vending machines, water coolers or heaters and ice generating machines. Till the year 2000, R744 applications in commercial refrigeration were not considered viable. The perception has now changed with its use either as a heat transfer fluid, in a cascade system, or on its own in either a transcritical cycle or a subcritical cycle, depending on the environmental temperature. External factors like safety requirements, extra tax on HFC systems and limitations on the maximum amount of HFC charge that can be used on a single are the main reasons for R744 acceptability in commercial refrigeration.
Residential Air-Conditioning
The use of CO2 as refrigerant in air conditioning systems has been the focus of investigations by both research institutions and industry because of the high demand for such equipment and the requirement for HFC alternatives. The annual demand for residential air-conditioning units is more than 40 million units and further market growth is expected. Air conditioning is the second largest consumer of energy after water heating in most residential areas. Environmental concerns in this application are more focused on the indirect impacts of emissions due to energy use, than on the direct impacts of refrigerant leaking. Therefore, energy efficiency is paramount.
Environment Control Units
Military operations usually require space conditioning for their temporary shelters, command modules and vehicles, which should be able to withstand the unique operational environment. The compactness of R744 equipment in addition to its availability globally has led to an increased interest in R744 space-conditioning systems for the military.
Future Applications for R744
The potential of R744 refrigeration is far wider than the applications discussed above. Experts assert that R744 as a refrigerant show promise of capturing more markets, even outside the refrigeration industry, although currently, it is the natural refrigerant with the widest range of use. Some future potential uses of R744 as a refrigerant are information technology (IT) equipment cooling, industrial heat pumps and industrial waste heat recovery.
Conclusion
CO2 commonly referred to as R744 has emerged as an excellent natural refrigerant for low temperature refrigeration applications. The use of a sub-critical refrigeration cycle for CO2 at low temperatures can greatly reduce the condensation pressure within the pressure range of standard refrigeration applications. Besides, it has also got zero ODP and very low GWP value which made its place very much stabilised in the field of refrigeration. However, due to its low critical temperature, it had been abandoned for some time and it was replaced by the second and third generation refrigerants such as the CFCs and the HCFCs.
But they were also being phased out due to their high ODP and GWP values. So, due to prior necessity CO2 has been again introduced in refrigeration systems. Presently, this refrigerant is widely implemented in food processing industries and supermarket applications. Apart from these industries, CO2 is also used in cascade refrigeration systems due to its non-toxic and non-flammable and odourless nature. Moreover, as compared with ammonia two-stage refrigeration system, the CO2/NH3 cascade refrigeration system has a significantly lower charge amount of ammonia, and the COP of the cascade system exceeds that of a two-stage system at low temperatures.
