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M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 123 as devised by Voorhees in 1905 [10], is one example of the improvements that were made. When supercritical high-side operation was needed, this was obtained by charging more refrigerant into the system. As the CFC fluids were introduced in the 1930s and 1940s, these ‘safety refrigerants’ eventually replaced the old working fluids in most applications. Although the major argument in their favor was improved safety compared to fluids like ammonia and sulfur dioxide, CO2 was also displaced by this transition to CFC. There is no single reason why the use of CO2 declined, but a number of factors probably contributed. These factors included high-pressure containment problems, capacity and efficiency loss at high temperature (aggravated by the need to use air cooling instead of water), aggressive marketing of CFC products, low-cost tube assembly in competing systems, and a failure of CO2 system manufacturers to improve and modernize the design of systems and machinery. With the CFC problem becoming a pressing issue in the late 1980s, the whole industry was searching for viable refrigerant alternatives. In Norway, Professor Gustav Lorentzen believed that the old refrigerant CO2 could have a renaissance. In a 1989 international patent appli- cation [11], he devised a ‘transcritical’ CO2 cycle system, where the high-side pressure was controlled by the throttling valve. One of the intended applications for this system was automobile air-conditioning, a sector that dominated the global CFC refrigerant emissions, and also an application where a non-toxic and non-flammable refrigerant was needed. The potential for more compact components due to high pressure was also an interesting feature. In 1992, Lorentzen and Pettersen [3] published the first experimental results on a prototype CO2 system for automobile air conditioning. A comparison was made between a state-of-the-art R-12 system and a laboratory prototype CO2 system with equal heat exchanger dimen- sions and design-point capacity. Although simple cycle calculations indicated that the CO2 system efficiency would be inferior, a number of practical factors made the actual efficiencies of the two systems equal. Based on these and other results, the interest in CO2 as a refrigerant increased considerably throughout the nineties, in spite of resistance from the fluorocarbon industry [12] and conservative parts of the automotive industry [13]. A number of development and co-operation projects were initiated by the industry and the research sector, including the European industry consortium project ‘RACE’ on car air conditioning, the European ‘COHEPS’ project on CO2 heat pumps, and the CO2 activities within the international IEA (International Energy Agency) Annexes on Natural Work- ing Fluids and Selected Issues in CO2 systems. 1.3. Structure of paper This article provides a critical review of transcritical CO2 cycle technology in various refrigeration, air-conditioning and heat pump applications. Recent research results in the world are introduced suggesting the possible applications for the particular purpose and the barriers that should be overcome before commercialization. The history and reinvention of CO2 have been introduced in Section 1 since it is not a new refrigerant. The thermodynamic and transport properties of CO2 are quite different from all the conventional refrigerants and are important for the system design, especially for cycle simulation, heat transfer and pressure drop calculations. Section 2 presents the properties of CO2 and its comparison with other refrigerants. Section 3 discusses some peculiarities of transcritical cycles and systems. A large number of cycle modifications are possible, including staging of compression and expansion, splitting of flows, use of internal heat exchange, and work-generating expansion instead of throttling. Some of these options are discussed in Section 4. Section 5 presents the heat transfer and pressure drop issues in CO2 systems, which focus on supercritical flow and flow vaporization. Section 6 deals with issues and design characteristics related to high operating pressure. Operating pressures in CO2 systems are typically 5–10 times higher than with conventional refrigerants, and this gives several effects that influence the design of components and their performance. In addition, high pressure may create perceived safety problems unless the underlying issues are addressed properly. Section 7 presents component design issues for CO2 system and those barriers that should be overcome before commercialization. Section 8 introduces some possible applications for the particular purpose such as mobile and residential air conditioning and heat pump applications, environmental control unit, heat pump water heaters which are available in the market, dehumidifier, commercial refrigeration, and heat recovery system. Future research challenges and concluding remarks are summarized in Section 9. 2. Properties of CO2 The refrigerant properties are important for the design of the heat pump system and its components. The properties of CO2 are well known and they are quite different from all the conventional refrigerants. Table 1 compares the characteristics and properties of CO2 with other refrigerants [14,15]. CO2 is a non-flammable natural refrigerant with no Ozone Depletion Potential and a negligible GWP. Its vapor pressure is much higher and its volumetric refrigeration capacity (22,545 kJ/m3 at 0 8C) is 3 – 10 times larger than CFC, HCFC, HFC and HC refrigerants. The critical pressure and temperature of CO2 are 7.38 MPa (73.8 bar) and 31.1 8C, respectively, and it is not possible to transfer heat to the ambient above this critical temperature by condensation as in the conventional vapor compression cycle. This heat transfer process (gas cooling) above thePDF Image | CO2 Vapor Compression Systems
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