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M.E. Mondejar et al. Renewable and Sustainable Energy Reviews 91 (2018) 126–151 Fig. 6. Average at-sea main engine load factor in 2007 and 2012 as a function of the size category, which is given in Twenty Foot Equivalent Unit (TEU) for container ships (a), and in Deadweight tonnage (DWT) for bulk carriers (b). each of the available heat sources, and therefore, the economic viability of the WHRS [57]. The container ship was assumed to sail with a load profile typical of slow steaming practice. Higher average power capa- cities are common for bulk carriers (moderate slow steaming) and oil tankers (average speed close to the design point). Ship owners endeavor slow steaming activities in periods of increasing fuel prices, declining freight rates and high overcapacity [35]. These factors occurred si- multaneously during 2007 and 2008, and led to operation at reduced speeds and to the design of new-buildings with smaller engines [35]. Fig. 6 shows the average load factor of the main engine during voyage for the container and the bulk carrier fleet in 2007 and 2012, as a function of the vessel size. As customary, the vessel size is expressed in twenty foot equivalent unit (TEU) for container ships, and in dead- weight tonnage (DWT) for bulk carriers. The data are retrieved from the third IMO greenhouse gases study on ship emissions [3]. The container ships sailed in both years at lower mean power capacities compared to bulk carriers and oil tankers (not shown in the plots). This tendency is due to the comparatively high overcapacity of the container shipping segment. All ship types experienced a decrease of average speed. Such a trend is more pronounced for container ships whose mean load factor decreased by 40% in five years. Bulk carriers and oil tankers experi- enced a more moderate reduction of mean power capacity, i.e., 18.8% and 26.0%, respectively. 3.3.2. Container ship The container ship was assumed to operate at low speed corre- sponding to typical slow steaming operation with 30% main engine power being the most frequent load of the engine (≈ 1000 h yr−1). Fig. 7 shows the energy recovered with the ORC and SRC technologies and the net power production. The values are given as a function of the engine load accounting for the yearly operational profile of the ship. The WHRSs exploiting the low-sulfur exhaust gas heat yields the highest yearly electricity production (ORC: 2.70 GW h and SRC: 2.13 GW h), while the electricity production from the high-sulfur ex- haust gases is second highest (ORC: 1.08 GW h and SRC: 1.04 GW h). The utilization of the high-sulfur exhaust gas is hindered due to the heating of service steam and the minimum required boiler feed tem- perature. The WHR units recovering low-sulfur exhaust gas heat pro- duce significantly more electrical power since these constraints are not applicable. The electricity produced by the ORC units from the sca- venge air and jacket water are 0.20 GW h and 0.47 GW h, respectively. Although a large amount of energy is available from the scavenge air, this case represents the lowest potential for WHR. The heat available in the air cooler decreases rapidly with the engine load compared to that of the jacket water, where the temperature and mass flow rate are kept constant during operation. This implies lower off-design efficiencies when using the scavenge air heat. The electricity produced by the ORC unit is supplied to the grid on board the ship, where it is distributed to various electricity consumers by the power management system. In some situations, the electrical power of the WHR units utilizing exhaust gases can be higher than the electricity demand on the ship. If this is the case, it can be beneficial to install a shaft motor such that the remaining electricity can be used for propulsion. When covering the on board electricity demands, the power from the WHR unit replaces that of the four-stroke auxiliary diesel engines which operate at lower efficiencies than the main engine. The fuel-saving potential of installing WHR units to recover exhaust heat was estimated by considering the following two extreme cases: (1) all the produced electricity replaces the electricity production from four- stroke auxiliary engines with an average fuel consumption of 210 g kWh−1, and (2) all the produced electricity is used for propulsion via a shaft motor. The fuel savings were calculated as the fraction of fuel energy saved by the WHR units compared to the fuel energy used in the main engine. The fractions were calculated by considering only the operation between 25% and 100% main engine load, since the engine data was only available in this load range. In the first case the saved fuel was 7.8% and in the second case it was 5.9%, when considering the ORC unit utilizing low-sulfur exhaust gas. The design of this ORC unit is characterized by a volume flow rate ratio of 23 kg s−1 and an enthalpy difference of 119 kJ kg−1 across the turbine. The low enthalpy differ- ence enables turbine designs with moderate peripheral speeds and centrifugal stresses, while the low volume flow rate ratio enables tur- bine stages with low Mach numbers and small rotor blade height var- iation [58]. Compared to steam turbines, these features enable eco- nomically attractive and efficient turbine designs employing few stages [56,58]. This indicates that it is economically realistic to reach high turbine efficiencies for the cyclopentane ORC unit, and that the turbine efficiency of 72% is a conservative value. In case the turbine efficiency of the cyclopentane turbine was 90%, the fuel savings reached for the ORC unit would be 10.0% when the produced electricity replaces production from the four-stroke auxiliary engines. 3.3.3. Bulk carrier Fig. 8 shows the results of the energy analysis. Compared to the 131PDF Image | organic Rankine cycle power systems for maritime applications
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