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means to obtaining both objectives seems to be to allow relatively high pressures and design the ORC process with a recuperator. The IMO SOLAS regulations state that the flash point of a fluid in a machinery space may not be lower than 60◦C. This represents a significant reduction in the number of feasible fluid candidates. Hence all the hydrocarbons can be excluded as solutions. RC318, R245fa and R236ea are all non-flammable and can as such be used. However, they have a relatively high GWP, especially RC318 with a value of 10,900 on a 100-years time horizon (CO2-equivalent). R245fa and R236ea have GWP values of 1,020 and 1,350 respectively [26]. Domingues et al. [27] recently investi- gated R245fa as ORC working fluid applied to recover heat from a combustion engine. It was found that the proper- ties of R245fa lead to high heat exchanger effectiveness and that the fluid was suitable for the application. Other non-flammable fluids among the tested are: de- cafluorobutane with a GWP100 of 7,000, sulfur fluoride with a GWP100 of 23,900 (among the highest for all sub- stances) and nitrous oxide with a relatively low GWP100 of 310 [23]. CO2 is another non-flammable alternative with a low GWP. This fluid requires very high pressures to be effi- cient though (optimum of 18.1% efficiency at 210 bar). No other non-flammable fluids suitable for ORC were found. Further analysis of the simulations suggested that if a fluid fire hazard level of 3 could be accepted, a simplified process without superheater could achieve efficiencies as high as the highest found in this study. Within this group the siloxane fluid MM is likely a good candidate with high efficiency at a low maximum pressure and low GWP. A drawback is the relatively low condensing pressure (0.06 bar at 25◦C). Bombarda et al. [28] state that MM has been proposed in the literature and is in use currently as working fluid for Rankine cycles recovering heat from combustion engines. One of the leading ORC companies uses siloxanes in the same type of application [29]. This indicates that this fluid has also proven its durability and usefulness in this context. Another fluid worth emphasizing is ethanol, which was superior within a large temperature range. Possibly mixed with water to increase the flash point (55◦C) ethanol might be a good candidate as working fluid in a low pressure Rankine process with no recuperator. The maximum effi- ciency is nearly as good as the highest in this investigation, and the environmental profile also is good with low GWP and ODP, as well as low ecotoxicity. 4.3. Thermal stability Toluene is already in use in the industry by a Dutch company in high temperature applications. It was selected due to its high chemical stability at elevated temperatures [30]. The stability is a key point, while information on these characteristics is only available for a few of the fluids considered in this work. Andersen et al. [31] tested the decomposition rate of normal-pentane, iso-pentane, neo- pentane, toulene and benzene under conditions relevant to high temperature ORC processes, i.e. up to 315◦C and 41 bar. Benzene was found to be the most stable fluid, but decomposition was found after only a few days, though in small amounts. A 50% loss of the fluids was predicted to be in a time frame within the order of years for all of the fluids. The Andersen study highlights the need for further studies on fluid stability, as the long term consequences of using many of the ORC fluids are not described adequately. As in the present study, benzene was also found to be the best among candidates in a recent study by Vaja et al. [32] investigating a combustion engine and high temperature ORC combined cycle. 5. Conclusions A generally applicable methodology, based on the prin- ciples of natural selection, was presented and used to deter- mine the optimum working fluid, boiler pressure and Rank- ine process layout for scenarios related to marine engine heat recovery. Different solutions were obtained according to the heat source inlet temperature. The dry type organic fluids (toluene, pentanes, hexanes and heptanes) showed to be leading to the highest efficiencies in recuperated pro- cesses. In non-recuperated cycles, wet and isentropic flu- ids presented higher efficiencies, especially ethanol showed promising properties within the temperature range 240- 360◦C. Imposing a pressure limit of 20 bar on the ORC process resulted in slightly lower cycle efficiency. Super- critical pressures did result in higher efficiencies, but only with heat sources of about 300◦C and hotter. At the engine design point condition with a heat trans- fer fluid temperature of 255◦C, the effects of pressure, pro- cess constraints and fluid hazard level were studied. Re- sults suggested that no single fluid can be used in an ORC process satisfying the requirements of process simplicity, low pressure, high efficiency, low hazard and low environ- mental impact. The requirements were shown to cause accumulated reductions in the maximum achievable cycle efficiency. The high fire hazard and low flash point of the organic dry fluid type may not be accepted within the ma- rine regulations, and only a few options remain among the studied fluids. However, R245fa and R236ea seem feasible with low hazard and near optimal efficiency at reasonable pressures, but the high GWP represents a drawback envi- ronmentally. Acknowledgements The authors wish to thank the Lighthouse Maritime Competence Centre (http://www.lighthouse.nu) for the fi- nancial support making this study possible. Susan Canali is acknowledged for her proof reading assistance. References [1] Wang Z, Zhou N, Guo J, Wang X. Fluid selection and parametric optimization of organic Rankine cycle using low 9PDF Image | organic Rankine cycles for waste heat recovery in marine settings
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