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M.E. Mondejar et al. Renewable and Sustainable Energy Reviews 91 (2018) 126–151 EEDI values are lowered every five years. The regulatory scheme en- tailed a 10% reduction of the minimum EEDI value in 2015, and a 30% decrease by 2030, with respect to the reference EEDI. In order to regulate SOx emissions, the IMO committee also estab- lished the so-called emission control areas (ECAs). Inside the ECAs, the maximum allowable sulfur content of the fuel was reduced from 1.5% to 0.1% in 2015. Globally, the limit decreased from 4.5% to 3.5% in 2012. A global limit of 0.5% will be established after 2020 [46]. Scrubber technologies and the use of fuels with low-sulfur content, e.g., low-sulfur marine diesel oil (MDO) or marine gas oil (MGO), biofuels, dimethyl ether (DME), methanol, and liquefied natural gas (LNG) (see Section 4.7), are arguably the most viable solutions to respect these limits. As for the NOx emissions, the higher the combustion pressure and temperature in the cylinders, the higher the engine performance and NOx emissions. In this regard, the IMO has gradually established rules to prevent air pollution due to the shipping activity. Before the IMO regulations were put into force, the main propulsion engines were often tuned so as to minimize the fuel consumption, thus leading to high NOx emissions. Currently there are three IMO emission standards that set increasingly restrictive NOx emission limits as a function of the max- imum operating speed of the engine, and the year of construction of the ship [47]: Tier I (2000) and Tier II (2011), which are global, and Tier III (2016), which is only applicable in NOx ECAs. For ships constructed on or after 1st January 2016, the main engine must emit only 20% of the Tier I limit (17 g kWh−1) within the ECAs. For ships constructed on or after 1st January 2011, the global limit is currently 14.4g kWh−1, according to Tier II. Within this regulatory scheme, the installation of WHRSs is a viable measure to increase the overall energy conversion efficiency and reduce emissions, since less fuel will be required to produce the same amount of power (lower EEDIs). It is important to emphasize that the regula- tions on NOx emissions relate only to the main engine (propulsion) and not to the auxiliary engines (power generation). Because of this, the main engine may be tuned to minimize NOx emissions, and the WHRSs installed on the propulsion side will need to be optimized as integral parts of the propulsion system [48]. Although the implementation of WHRSs does not have a direct impact on the fuel sulfur content, using alternative fuels alters the temperature and mass flow rates of the available heat sources. Moreover, the operating costs related to low- sulfur fuels can be lowered, thus facilitating the transition to these fuels. 3. Waste heat recovery on ships In this section an analysis of the WHR potential is presented. The vessel types responsible for the highest CO2 emissions are first identi- fied. Secondly, the general features of a typical marine propulsion system are outlined. An energy analysis of three selected operating ships is then presented to quantify the waste heat and the energy re- coverable using the ORC technology. Finally, important remarks and conclusions from the energy analysis are reported. 3.1. Mapping of the world fleet The total world fleet comprised 107.749 operative units in 2012 according to data tracked by the automatic identification system (AIS) and reported in the third IMO greenhouse gases study [3]. Fig. 2 shows the number of units, installed propulsion power, and CO2 emissions by ship type in 2012. The values of installed power refer to the main en- gine only. The CO2 emissions were estimated considering the con- tribution of the auxiliary engines and boilers for heat supply on board. The data were taken from the third IMO greenhouse gases study on ship emissions [3]. Note that the tanker category includes vessels trans- porting oil, gas, chemicals, and liquids. Bulk carriers, tankers, general cargo ships, and service vessels constitute around 60% of the total world fleet. About 30% of the Fig. 2. Number of units, propulsion power and bottom-up CO2 emissions by ship type in 2012. The data are taken from the third IMO greenhouse gases study [3]. cumulative power is installed on container ships, although the number of operative units is relatively low (8.3%). On average, these vessels are characterized by high design speeds (21kn) and deadweights (42.231 t). The average propulsion power installed on board is 27 MW. Bulk carriers and tankers have relatively lower design speeds (< 15 kn) and high average deadweights (> 70.000 t), thus ranking second (21.5%) and third (20.1%), respectively, in terms of installed power. Fig. 2 shows that container ships, tankers and bulk carriers are re- sponsible for more than 65% of the total yearly CO2 emissions. These ships also have the highest utilization factor (> 200 d per year on sail [49]) with mean speeds ranging from 12 to 15 kn. This, in turn, entails high yearly emissions. The share for general cargo, fishing vessels and Ro-Ro ranges between 5% and 7%. These ships are on an average more than 160 d per year at sea and account for around 26.6% of the total world fleet. All other ship types have CO2 productions lower than 5%. The breakdown of the CO2 emissions stresses the need for improving the energy conversion efficiency of tankers, bulk carriers, and container vessels, as they share the highest contribution to the total world fleet emissions and have the largest propulsion units. For these ship types, more than 70% of the total emissions relate to the main propulsion engine. Auxiliary engines account for 21.9% and boilers 6.1% of the yearly CO2 production [50]. Fig. 3 depicts the number of units and propulsion power supplied by four-stroke and two-stroke diesel engines for bulk carriers, container ships, and tankers. The plot shows that two-stroke low-speed engines dominate the market. In addition to a higher efficiency the advantages compared to the four-stroke counterpart are: (i) a higher power density in kW m−3, (ii) direct coupling to the propeller, thus avoiding the losses associated to the use of a gear box, and (iii) the possibility of designing propellers with large diameters, improving the mechanical efficiency [51]. Although four-stroke engines could be initially more interesting from the stand point of WHRS as they have higher exhaust gas tem- peratures due to their lower efficiency, their smaller sizes make WHRS more feasible on two-stroke engines. 3.2. Machinery system aboard Fig. 4 depicts a sketch of a state-of-the-art machinery system for large ships. The main engine is equipped with receivers for exhaust and scavenge air to accommodate the constant pressure operation of the turbochargers. At loads below 40% or 50%, auxiliary blowers provide the required scavenge air flow. Conversely, at high loads, a fraction of the exhaust gas flow can be bypassed and converted into power in a separate turbine, i.e., the power turbine (PT). A SRC plant can be added 128PDF Image | organic Rankine cycle power systems for maritime applications
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