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3414 E.H. Wang et al. / Energy 36 (2011) 3406e3418 Dh 1⁄4 hmax hmin (24) The thermal efficiencies of the contour lines are configured according to the following vector: 1⁄2hmax 0:02Dh hmax 0:2Dh hmax 0:5Dh hmax 0:8Dh (25) This approach is also applied in all the following contour maps. The operating region, where the degradation of the thermal effi- ciency is within 20% of the maximum value hmax, is defined as the optimal working region. The relative optimal working regions of various working fluids are shown as the “belt” shape in Fig. 8. When the ORC system is assembled on the vehicle, the opera- tional conditions during the transient process should be controlled within the optimal working region to maintain a high thermal efficiency. Because the ambient temperature varies with the seasons during the year, the condensing temperature of the ORC can be controlled by a fan to regulate it as close as possible to the current ambient temperature. Accordingly, the evaporating pres- sure should be adjusted to operate inside the optimal working region. When the vehicle is running in a transient process, the waste heat generated by the engine also varies. This causes evap- orating pressure variation in the evaporator. When this occurs, the mass flow rate of the working fluid needs to be controlled to keep the evaporating pressure in the optimal region with regard to the relevant condensing temperature. An optimal working region that can cover a wide range of condensing temperatures as well as evaporating pressures is advantageous. From the results in Fig. 8, the optimal working regions of R245fa, R245ca, R141b, R11, R123, and R113 are shown to satisfy this requirement. The ORC system exergy destruction rate comparison is delin- eated in Fig. 9. The corresponding contour maps are shown in Fig. 10. On the same surface, the exergy destruction rate is relatively small when the ORC is working in the high thermal efficiency region. But it increases drastically as the ORC moves to its low thermal efficiency region. The working fluid with a higher thermal efficiency manifests lower exergy loss. The reason is, for a specific 10 kW net power output, the heat addition will increase rapidly if thermal efficiency becomes smaller. Comparisons of expanding pressure ratios are shown in Figs. 11 and 12. Those working points with a higher pressure ratio result in larger thermal efficiency in the feasible working region. The maximum pressure ratio is limited to eight based on the constraints of the design of the single screw expander, such as leakage. Thus the maximum thermal efficiency could not be improved further. Fig. 16. Heat absorption rate comparison for A type. When the pressure ratio rises in the feasible region, the deviation between the evaporating temperature and condensing tempera- ture increases. This causes an incremental change to the thermal efficiency, which can be evaluated using equation (21). When the condensing temperatures of the selected working fluids are given a specific constant value, the corresponding vapor pressures can be calculated based on the Riedel method [34]: lnPvpr 1⁄4AB=TrþClnTrþDTr6 (26) where the reduced vapor pressure Pvpr and the reduce temperature Tr are defined as Pvpr 1⁄4 Pvp =Pcr (27) Tr 1⁄4 T=Tcr (28) Because various working fluids have distinct coefficient values, the corresponding condensing pressures are also different. When the pressure ratios are set to the maximum value, the pertinent maximal evaporating pressures are also distinct. This is why the lower boundary lines of the feasible working regions deviate among the selected working fluids. Fig. 15. Mass flow rate comparison. Fig. 17. Pump power comparison.PDF Image | Study of working fluid selection of organic Rankine cycle (ORC) for engine waste heat recovery
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