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applications (Figure 7), displacement type machines are more appropriate to the small-scale ORC units, because they are characterized by lower flow rates, higher pressure ratios and much lower rotational speeds than turbo-machines (Persson, 1990). Figure 7: Approximate range of chiller cooling capacity range by compressor type (ASHRAE, 2008) While technically mature turbomachines are available on the market for large ORC units, it seems that almost all positive displacement expanders that have been used up to now are prototypes, often derived from existing compressors (Zanelli and Favrat, 1994; Yanagisawa et al., 2001; Aoun and Clodic, 2008; Lemort et al., 2008). Most of the Rankine cycle systems used in automotive applications employ positive displacement machines. One exception is the Rankine cycle system proposed by Cummins (Nelson, 2008), which is associated to a truck engine and uses a turbo machine. Toyota (Oomori and Ogino, 1993) used a scroll expander. BMW (Freymann et al., 2008) initially used 2 vane expanders (one for both cycle) but decided to replace them by axial piston machines showing efficiencies of 55% (which could probably be improved, given that the prototype is non-optimized). Honda (Endo et al., 2007) designed a compact swash plate axial piston type expander, comprising an oil gear pump and a generator motor mounted coaxially with the expander. Steam Rankine cycle recovering heat from high temperature gas, should operate at high evaporating pressure to improve the cycle performance. Most of the existing expander technologies can withstand pressures up to 35 bar (El Chammas and Clodic, 2005). One exception is the swash-plate piston expander developed by Endo et al. (2007) that operates with pressures up to 100 bars. Another difficulty associated with the use of a positive displacement machine is its lubrication. An oil separator could be installed at the expander exhaust. Unlike with compressors, an oil pump is necessary to drive the separated oil back to the expander suction. Kane (2002) proposed to let the oil traveling with the refrigerant through the system and to install an oil separator at the evaporator exhaust. Separated oil is injected into the scroll expander bearings, while the lubrication of the two spirals relies on the slight inefficiency of the separator. Alternatively, oil-free machines could be used, but generally show lower volumetric performance due to larger tolerances between moving parts (Yanagisawa et al., 2001; Peterson et al., 2008). In some operating conditions (wet fluids with limited superheat at the expander supply), liquid may appear at the end of the expansion. This could be a threat of damage for piston (reciprocating) expanders but not for scroll and screw expanders, since the latter do not use valves (the timing of suction and discharge is determined by the machine geometry). Some types of expanders (scroll, screw, vanes) are characterized by a fixed built-in volume ratio. To optimize the performance of the expander, this built-in volume ratio should match the operating conditions (in order to limit under-expansion and over-expansion losses). Volume expansion ratios achieved in Rankine cycle systems are typically larger than those achieved in vapor compression refrigeration systems, which justifies developing adapted design of such expanders rather than retrofitting existing compressors. Generally speaking, piston expanders are more appropriate for the applications with large expansion volume ratio. Finally, previous studies carried out by the authors also indicated that the tightness of the machine (in case of open-drive expanders) is an important issue (Quoilin, 2007).PDF Image | Technological and Economical Survey of Organic Rankine Cycle Systems
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