A review of carbon dioxide as a refrigerant in refrigeration technology

Review Article Page 1 of 10 AUTHORS: Paul Maina1,2 Carbon dioxide as a refrigerant A review of carbon dioxide as a refrigerant in refrigeration technology Zhongjie Huan1 EMAIL: Tough environmental laws and stringent government policies have revolutionised the refrigeration sector, especially concerning the cycle fluid known as the refrigerant. It has been observed that only natural refrigerants are environmentally benign. When other refrigerant qualities are considered, especially those relating to toxicity and flammability, carbon dioxide emerges as the best among the natural refrigerants. However, carbon dioxide based refrigerants are not without drawbacks. Even though the use of R744 – a carbon dioxide based refrigerant gas – has solved the direct effect of emissions on the environment, studies to investigate the indirect effects of these systems are needed. Improvement in existing technical solutions and the formulation of additional solutions to existing R744 refrigeration problems is paramount if this technology is to be accepted by all, especially in areas with warm climates. National policies geared to green technologies are important to clear the way and provide support for these technologies. It is clear that carbon dioxide is one of the best refrigerants and as environmental regulations become more intense, it will be the ultimate refrigerant of the future. POSTAL ADDRESS: Introduction AFFILIATIONS: Department of Mechanical Engineering, Tshwane University of Technology, Pretoria, South Africa 1 Department of Mechanical Engineering, Moi University, Eldoret, Rift Valley, Kenya 2 CORRESPONDENCE TO: Paul Maina Department of Mechanical Engineering, Tshwane University of Technology, Private Bag X680, Pretoria 0001, South Africa DATES: Received: 31 July 2014 Revised: 13 Nov. 2014 Accepted: 19 Jan. 2015 KEYWORDS: environment; safety; heat pump; energy efficiency; transcritical HOW TO CITE: Maina P, Huan Z. A review of carbon dioxide as a refrigerant in refrigeration technology. S Afr J Sci. 2015;111(9/10), Art. #2014-0258, 10 pages. http://dx.doi.org/10.17159/ sajs.2015/20140258 Most refrigerators use a liquefiable vapour to transfer heat. This fluid is known as the refrigerant. Refrigerant selection is a key design decision that influences the mechanical design of the refrigeration equipment. Factors that must be considered in refrigerant selection include performance, safety, reliability, environmental acceptability and cost. However, the primary requirements are safety, reliability and, nowadays, environmental friendliness (in terms of ozone depletion and global warming potential). Table 1 summarises the properties of some refrigerants and indicates that no progress has been made in terms of global warming potential (GWP) when switching from hydrochlorofluorocarbons (HCFCs) to the hydrofluorocarbon (HFC) family. When securely contained in a properly operating system, refrigerants do not impact climate change; however, system leaks and improper recovery of refrigerants during repairs or at end of life result in these harmful gases entering the atmosphere. Furthermore, during production of refrigerants, toxic and harmful wastes are released into the environment, which cause air, water and land pollution in addition to releasing greenhouse gases. An alternative to HFCs is to apply naturally occurring and ecologically safe substances, the so-called natural working fluids. The most important substances in this category are hydrocarbons, ammonia and carbon dioxide, although when safety concerns are raised (toxicity and flammability), R744, a carbon dioxide based refrigerant gas, becomes the best substitute. Carbon dioxide (CO2) is a clear gas (at atmospheric conditions) without a particular smell when the concentration is below suffocation level. When the concentration reaches toxic levels, it has a slightly pungent smell and somewhat acidic taste. It has a higher density than air, which has its own advantages with respect to refrigeration and disadvantages with respect to safety. CO2 is made both naturally and artificially – artificially through the burning of fuel and other industrial processes.1,3 Approximately 0.04% of atmospheric air is CO2, thus CO2 is at a concentration of approximately 380 parts per million (ppm) in air. Exhaled air from the body has a CO2 concentration of about 4%. History of R744 as a refrigerant Since the invention of the vapour-compression cycle by Evans and Perkins in 1834, R744 has been a candidate for a refrigerant. Documented studies state that Alexander Twining was the first to propose R744 refrigeration using a steam compression system in his British patent of 1850. However, Thaddeus Lowe was the first to actually build a refrigerator running on R744 for ice production in 1866 after discovering its potential while using it in military balloons. Carl Linde followed suit and built a better refrigerator running on R744 in 1881, just after Windhausen had built the first R744 compressor in 1880. In 1884, W Raydt built a R744 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. The British company J and E Hall built the first R744 marine refrigerator in 1890 using Windhausen’s compressor designs, while in the USA, continuous production of these refrigerators was started in 1897, mainly by Kroeschell Bros. Ice Making Company. Owing to its safety aspects when compared to other refrigerants during this period, R744 refrigerators grew in number, especially in the marine sector. At the same time, its technology was improving. For example, in 1889, J and E Hall created a two-stage R744 compressor which was more efficient, and in 1905, Voorhees created a flash chamber which was very similar to a liquid-vapour separator.4-7 © 2015. The Author(s). Published under a Creative Commons Attribution Licence. South African Journal of Science http://www.sajs.co.za Calcium chloride solution was used in most refrigerators as a secondary fluid. The salt solution was cooled to around -10 ºC (evaporation temperature of -15 ºC). Originally, the evaporator and condensers used galvanised steel pipes, 32 mm in diameter for small refrigerators and 51 mm in diameter for large cold rooms. Tank and coil heat exchangers were the first to be used, before tube in tube (double pipe) technology was introduced in 1902. The shell and tube type were invented in the early 1930s and fin technology in the 1920s. Copper replaced steel pipes during this decade too, with pipe diameters being reduced to 13 mm because of the increased heat transfer offered by the fins and copper. Air circulating fans were introduced around this time for improved cooling, especially in cold rooms. R744 used to cost around 9 cents per kg but the price increased to 12 cents per kg in the late 1920s. 1 Volume 111 | Number 9/10 Sep (...truncated)