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150 M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 material requirements by 50% while eliminating the need for long suction and liquid lines, and increasing effectiveness by 10%. 7.3. Other components 7.3.1. Lubricants A variety of lubricants can be used for CO2 applications In certain systems synthetic hydrocarbons such as poly alpha olefins (PAOs) and alkyl benzenes (ABs) can be still used even though they have poor solubility The poor solubility of the synthetic hydrocarbons is compensated for by their excellent low temperature flow properties and can be improved still further by blending with more miscible lubricants (e.g. polyalkylene glycols (PAGs), esters, etc.). A range of individual and blends of synthetic lubricants are therefore being evaluated to find the more cost effective solution for a particular application. Quite often lubricant selection will be based on logistic factors, i.e. a lubricant that can work with a variety of refrigerants [121]. Kawaguchi et al. [122] reported that polyalkylene glycol (PAG) was the primary lubricant candidate since it was partially miscible with CO2. It had excellent lubricity in boundary condition and supercritical condition, and showed good stability under supercritical condition. Other lubricants tested were polyolester (POE), polycarbonate (PC) and polyvinyl ether (PVE). Li and Rajewski [123] conducted a screening study on various lubricant candidates for CO2 systems, including mineral oil, polyalphaolefin (PAO), polyolester (POE), polyalkylene glycol (PAG) and alkyl naphthalene (AN). The experimental results raised concern about the compat- ibility between POE and CO2 after significant degradation was found for this lubricant. The authors speculated that carbonic acids formed by dissolution of CO2 into the moisture in the polyolester (POE) lubricant accelerated the degradation. Dissolved aluminum was found in the polyalkylene glycol (PAG) lubricant, and this was probably caused by a phosphate anti-wear additive which had been converted into aluminium-aggressive alkyl phosphates. Seeton et al. [124] studied solubility, viscosity, boundary lubrication and miscibility of CO2 and synthetic lubricants. They concluded that the polyalkylene glycol (PAG) lubricant seemed to give the best lubricity for transcritical applications. Heide and Fahl [125] studied the miscibility in CO2 of several candidate lubricants for transcritical systems, including alkyl benzene (AB), alkyl naphthalene (AN), polyalkylene glycol (PAG), polyolester (POE), and polymer ester (PME). The POE lubricants showed good miscibility, while the other candidates had large regions of immisci- bility. The results indicated a strong influence of chemical structure of lubricant on phase behavior with CO2. Investigations on thermal stability of lubricants by Fahl et al. [126] showed that PME and aromatic ester derivates have a good potential for applications with high temperatures. 7.3.2. Elastomers Some issues are being studied with respect to elastomer materials for seals and hose connections in CO2 systems Permeation rates are quite high, thus giving potential problems regarding desired leakage rates in automobile air conditioning systems. Explosive decompression may occur when CO2 systems or components are rapidly depressur- ized, leading to fractured and ruptured sealing elements [127]. A fluorite elastomer, FKM was regarded as promising due to its wide temperature range of application and the negligible impact of explosive decompression. 7.3.3. Valves and controls Jain et al. [128] has developed components for CO2 air- conditioning systems, including electronic expansion valves, accumulators, hoses, o-rings and fittings. The expansion valve is a linear proportional solenoid valve. Several manufacturers are working on expansion valves and controls for CO2 systems. Saginomiya and Fujikoki in Japan are working on valves (especially expansion valves) and controls for transcritical CO2 systems, especially heat pump water heaters on the market. In Europe, Danfoss AS is working on various concepts, including mechanical (automatic, thermostatic) and elec- tronic (modulating coil) valves. The German company Otto Egelhof GmbH and Co. is working on a step-motor controlled valve, mainly intended for the mobile air- conditioning market. In addition, Obrist Engineering in Austria is offering valves, controls and other components for prototyping purposes. 8. Application areas Recent research on transcritical CO2 systems has investigated a variety of possible applications, mostly with funding from the affected industries. While many govern- ment-sponsored R&D programs can afford the luxury of starting with basic research, industry-sponsored efforts generally proceed in two stages. The first task is to prove that the technology will work for a specified application, and then in stage two the state of the art is advanced to enable the technology to compete in the market. From the standpoint of experimental design, first stage is the horseless carriage stage. Any new technology that challenges an existing one must demonstrate that it is workable, usually on the old technology’s terms. The first CO2 prototype systems were therefore built to mimic the conventional technology in size, weight, air flow rates, etc. In the long run, however, that is an unrealistic and non-sensical demand: like requiring Henry Ford’s new invention to compete with a horse-drawn carriage on muddyPDF Image | CO2 Vapor Compression Systems
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