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M.E. Mondejar et al. Renewable and Sustainable Energy Reviews 91 (2018) 126–151 whether their flash temperature is below or above 60°C. Though nothing is specified with regards to the flammable character of re- frigerants, it can be inferred that refrigerants with flash temperatures below 60 °C would be considered as potentially more flammable. Ex- amples of working fluids with flash temperatures below that limit are hydrocarbons (e.g. isobutene, cyclopentane), but also some HFC (e.g. R245fa). With regards to refrigeration systems, it is mentioned that refrigerating machinery using toxic and/or flammable refrigerants should be located outside the main machinery space in a separate gastight compartment. This requirement could affect the integration of ORC power systems on board when using flammable working fluids. Moreover, independently of the flammability of the working fluid, it is specified that leak detectors should be installed, and when the amount of working fluid is above 300 kg, monthly tests should be carried out [97]. 4.2.3. Working fluids of the future New working fluids for ORC power systems to be integrated on ships should comply with the aforementioned regulations, provide good performance, and desirably have low or moderate flammability and toxicity risks. These criteria are demanding given that the development of new organic molecules which simultaneously meet thermodynamic, safety, and environmental requirements is limited [98]. In addition, other factors, such as the availability, cost, and influence on the com- ponent design must be considered. The use of hydrofluoroolefins (HFOs) as environmentally-friendly working fluids has recently been suggested by different authors [98,99]. These compounds contain at least one double carbon bond, which is susceptible to degradation in the troposphere and thus reduces the atmospheric lifetime of the molecule. The addition of fluorine provides stability and reduces the flammability of the molecule. The most recent research regarding the use of HFOs focusses on fluor- opropenes. Among them, R1233zd, R1234ze(Z), and R1243ye(E) pre- sent very low ODP and low GWP. Their critical parameters are close to those of R245ca and R245fa, which were proposed as possible candi- dates for WHR on board ships. For the same application, Kontomaris [100] recently presented two new working fluids, i.e., DR-2 and DR- 40A, developed by DuPont. DR-2 is a hydrofluoroolefin, and DR-40A is a near-azeotropic mixture. Both fluids could likely be used as replace- ments for R245fa. The potential of halogenated propenes is limited for medium-tem- perature energy sources, such as the exhaust gas heat, because of the low critical temperatures of those that are commercially available. However, butene-based or pentane-based HFOs could arise as alter- native fluids, as their saturation properties may be closer to those of the hydrocarbons, such as benzene or toluene. Siloxanes, which have no ODP and very low GWP, are well-known options for medium-tem- perature heat sources. They could be used in the regenerative ORC unit recovering the exhaust gas energy. Suitable siloxanes for this applica- tion could be hexamethyldisiloxane (MM) or octamethyltrisiloxane (MDM). However, they present relatively low vapor pressures com- pared to those of hydrocarbons, thus increasing the risk of air infiltra- tion in the condenser. Additionally, their strong dry behavior entails higher recuperator heat transfer areas. Thereby, further research is needed to evaluate the prospects of using siloxanes on board ships. In conclusion, the available working fluids that meet all the en- vironmental requirements can be classified into three groups: HFOs (with moderate flammability and high price), hydrocarbons (with high flammability and low price), and siloxanes (with moderate flamm- ability and low price). ORC units used for WHR of exhaust gases and other high temperature sources may use hydrocarbons or siloxanes as working fluids. The choice of the former or the latter would be sig- nificantly influenced by the required operational pressures. ORC units recovering the heat from the jacket cooling water may use HFOs, which avoid the risks associated with the high flammability of hydrocarbons, but could increase notably the cost of the installation. Nevertheless, it should be considered that the use of flammable working fluids may, as well, increase the costs due to extra safety equipment. 4.3. Component design and selection This section provides guidelines on the most suitable equipment (heat exchangers, pumps and expanders) for ORC power systems on board ships. The analysis focuses on the components and technologies for the ORC units fed by exhaust gas and jacket water heat; see Figs. 10a and 10b. 4.3.1. Heat transfer equipment The selection of the HEXs is of great importance for the viability of ORC power systems as this equipment represents a significant part of the total capital cost [101] (e.g., according to Lecompte et al. [102] HEXs could represent up to 35% of the total cost of an ORC unit). Furthermore, it usually has the largest influence on the total volume of the installation, which is important as space is a valuable commodity on board ships [103]. In the ORC units shown in Figs. 10a and 11a, the boiler is arguably the most critical component, as it needs to withstand the highest tem- perature and pressure of the working fluid. In large ships, the exhaust gases are typically available at temperatures above 230 °C and volume flow rates higher than 50000 m3 h−1; see Table 1. Due to the sulfur content of the fuel, corrosion may occur if acids are formed on the exhaust gas side [104]. This should be considered, especially if no in- termediate loop is present between the exhaust gases and the ORC evaporator, as the low wall temperature could promote the condensa- tion of the exhaust gases on the tubes. An additional factor to consider is the potential complications derived from the soot deposits on the boiler. This phenomenon has been on the increase in the last decade as a consequence of the lower quality of the heavy fuels used, and the lower exhaust gas temperatures (from about 375 ◦C to about 245 ◦C) and velocity of the exhaust gases due to more optimized designs of the engines [105]. Besides a reduction of the efficiency in the heat transfer process, soot deposits can lead to more severe events such as soot fire or iron fire (i.e., the combustion of the boiler itself). These events have been observed more frequently when the exhaust gases flow inside tubes, especially in case of gilled or pinned tubes. In order to avoid this, soot-blowing systems or manual cleaning can be used. Soot deposits can be avoided if the boiler is designed for high gas flow velocities. This, however, entails high pressure drops. In order to ensure efficient op- eration of the turbochargers the pressure drop across the gas side of the boiler should be below 0.015 bar as recommended by MAN [105]. The requirements of high gas flow velocities and low pressure drops limit the heat transfer surface area of the boiler and thereby the minimum pinch point temperature. The evaporator of an ORC unit for WHR of the exhaust gases can be a once-through boiler, with the working fluid in the tubes [59], leading to a higher pressure on the working fluid side than in the exhaust gas side. Alternatively, a drum-type boiler can be used. Unlike steam, or- ganic substances have a relatively small difference between the specific volumes in the liquid and vapor phases, which makes it possible to achieve the evaporation of all the working fluid by using once-through boilers [8], therefore avoiding the use of drums [106]. The elimination of the drum allows faster start-ups of the unit, which are commonly constrained by the saturation temperature rise imposed by the thickness of the walls of the drum. In principle, the elimination of the drum minimizes the working fluid inventory [10] and also implies a reduc- tion of the total volume of the installation, but this should be carefully considered as it may also increase the size needed for the boiler, which would need to accommodate all the working fluid volume flow rate. An exhaust gas heated drum boiler of the finned tube type is com- monly employed for steam generation aboard ships [105]. Here, the steam flows inside the tubes and the exhaust gases flow outside, in contact with the fins. A finned tube boiler is also a reasonable option for 138PDF Image | organic Rankine cycle power systems for maritime applications
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