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M.E. Mondejar et al. Renewable and Sustainable Energy Reviews 91 (2018) 126–151 modification allows the ORC unit recuperating the jacket water heat to attain higher energy conversion efficiencies compared to the values achievable with current engines. Integration of ORC units on ships for WHR can be done by either direct heat exchange with the heat source or using an intermediate loop, for example an oil loop, as mentioned in Section 4.1. On ships, the existing service steam system can be used as an alternative option for integrating ORC units by placing the ORC unit in parallel with the service steam demands. In this scenario, the steam generator should be oversized in order to provide enough steam for both the steam services and the ORC unit. Heat for steam evaporation can be recovered from exhaust gases after the turbochargers and/or from the EGR stream in the case of EGR engines, while scavenge air, jacket water and lube oil heat can be used for feed water preheating. In order to achieve op- timum waste heat utilization, the steam pressure should be optimized in order to reach the optimum trade-off between steam temperature and mass flow rate. At high steam pressures the saturation temperature is high and thereby the ORC unit efficiency is high; however, high steam pressures also result in less heat extraction from the waste heat sources and thereby less steam flow to the ORC unit. The integration of ORC units on the service steam system represents a practical solution to harvesting heat from multiple heat sources at different temperatures. However, this integration option generally results in lower WHR unit efficiencies due to increased heat transfer irreversibilities compared to the option of integrating the ORC unit directly with the waste heat sources. 4.7. Alternative fuels Upcoming technologies for marine machinery systems can offer new possibilities for the ORC technology aboard ships. As mentioned in Section 2, alternative low-sulfur fuels, e.g., low-sulfur MDO or MGO, biofuels, dimethylether (DME), alcohols (e.g., methanol, ethanol), hy- drogen, and LNG, may replace the HFO as fuel for the main engine. The use of these alternative fuels may require modifications of the engine and/or engine tuning, which in turn may influence the availability of waste heat sources on board. This section discusses how the use of al- ternative fuels for propulsion may modify the results given in Section 3 with respect to the expected net power output of ORC units on board, and how their use and integration with the ORC technology can be a step forward towards sustainability in the shipping industry. However, it needs to be stressed that it is not possible to draw quantitative con- clusions on how the temperature and mass flow rates of waste heat sources on board are affected by the use of alternative fluids. This is because it is common practice to tune the engines specifically for each fuel in order to minimize the energy losses from the engines and deviate as little as possible from the operating conditions of an engine using a conventional fuel, while complying with legislation regarding NOx emissions. Therefore, it would be necessary to know how the engine manufacturing industry would adapt the operation of their engines to the potential upcoming renewable fuels, in order to be able to draw more detailed conclusions about the impact of these fuels on the WHRS onboard. Since such information is not available, this section is limited to the discussion of the effects of alternative fuels on the prospects for WHR primarily in qualitative terms. Initially, the analysis presented in Section 3 considered the case of a HFO and a low-sulfur fuel, irrespective of their nature. The use of a low- sulfur fuel allows exploiting more waste heat from the exhaust gases, as sulfuric acid condensation is not expected within the operating tem- peratures, and there is no demand of service steam to preheat the fuel and the fuel tanks. As for the above-mentioned fuels, only MDO and MGO contain sulfur, although in a proportion of less than 0.1% in mass. This sulfur content may generate sulfuric acid condensation only at very low temperatures, and therefore, both MDO and MGO comply with the assumptions made in Section 3 for the minimum boiler feed tem- perature. As a consequence, the main differentiating factor of these fuels with regards to WHR on board comes from the distribution and temperature of the available heat sources. For instance, low-sulfur MDO is preheated in the exhaust boilers before its injection in the burner, using about 25% of the energy available in the exhaust gases [87]. Considering the average mass flow rate of exhaust gases in Ref. [87] this could imply a reduction of the exhaust gas temperature of around 80 K, which could reduce the maximum efficiency of the ORC unit. The use of biofuels consisting of vegetable oils (i.e., biodiesel) has been pointed out as a possible alternative to, or in combination with, HFOs, in order to reduce the particle and sulfur emissions without the need of engine modifications [147,148]. A possible increase of NOx emissions derived from their use may happen and may be compensated for by re- tuning the engine, therefore varying the engine operation conditions. However, as of today their use as shipping fuel is marginal due to their high cost and limited availability. Regarding the use of alcohols and DME as alternative fuels, both the emissions of NOx and particles are reduced [148]. In the case of alco- hols, the combustion improves due to the presence of oxygen in their molecule, leading to a decrease of the engine heat losses and of the exhaust gas temperatures [149], which could affect the maximum ef- ficiency of the ORC unit. Although both methanol and ethanol have been suggested as potential alternative fuels for ships, so far there is no usage of ethanol on commercial vessels [149]. Methanol is widely available, can be produced both from natural gas and renewable re- sources, and as it is liquid at ambient temperature its storage and dis- tribution are similar to conventional fuels. However, the use of me- thanol requires adaptation of the engines, larger storage tanks, and its toxicity and corrosivity requires stricter safety measures [148]. Me- thanol can be used in internal combustion engines blended with an- other fuel or in a dual-fuel engine. According to the MAN Diesel CEAS engine calculation tool [54] engines working with methanol present relatively lower flow rates of exhaust gases, which would reduce the potential for power conversion with an ORC unit. Currently, a number of companies (i.e., Stena Line, Wärtsila and MAN Diesel & Turbo) are working on adapting their diesel engines for operation with methanol as a fuel [150]. Dimethyl ether (DME), which is a liquefied gas with similar char- acteristics as liquefied petroleoum gas, can be derived from both nat- ural gas and biomass resources, is non-toxic, and conversely to me- thanol, can be directly used in diesel engines using liquefied petroleum gases. As a disadvantage, both DME and methanol, present heating values lower than MDO (29 MJ/kg and 22.7 MJ/kg, respectively, compared to approximately 45 MJ/kg), thus requiring the consumption of higher amounts of fuel, although NOx emissions are reported to be lower [152]. An additional consideration for DME and methanol, and other fuels that do not produce soot during combustion, is that the lack of soot allows for extensive use of EGR for reduction of NOx emissions without compromising the engine reliability due to soot deposition in the EGR cooler and the scavenging chamber. The use of high rates of EGR tends to increase the exhaust gas temperature without any penalty in fuel consumption of the engine. Owing to the increased exhaust gas temperatures, the potential for WHR of the exhaust gases is increased when using fuels that do produce any soot. Hydrogen is a gaseous fuel, well-known as a renewable energy carrier, with a high heating value. However, its high self-ignition temperature makes it unfeasible for direct use in existing engines, but it is an ideal fuel if it is combined with others [151]. The main dis- advantage that it presents is that the storage tanks of compressed hy- drogen require 6–7 times more space those of standard HFOs, making it more challenging to integrate WHR systems on board. Liquefied natural gas (LNG) is claimed to have the greatest potential as an alternative fuel due to its zero emissions of SOx, lower emissions of NOx, large availability worldwide, and a more competitive price compared to distillates [148]. Fig. 18 illustrates the increasing orders of LNG fueled ships registered by DNV-GL [146]. As for disadvantages, LNG requires more expensive tanks and piping, and increased port 144PDF Image | organic Rankine cycle power systems for maritime applications
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