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Sustainability 2021, 13, 1213 17 of 34 The operating temperature of the SOFC stack affects the layout of a SOFC module, in terms of fuel reforming, fuel preheating, air preheating and WHR system. SOFC modules can also have a number of distinct layouts, e.g., external reforming or internal reforming, atmospheric system or pressurized system, standalone system or hybrid system, indirect hybrid system or direct hybrid system [75–77]. Internal reforming is more attractive than external reforming due to higher efficiency and lower capital cost. However, the endothermic reaction of the fuel reforming process may lead to significant temperature gradients and inhomogeneous current distributions inside the cells [40,74,76,77], so indirect internal reforming is preferable due to reduced thermal stress. In addition, depending on the source of steam for fuel reforming, there may be recirculation of water from the anode or external water supply. Anode recirculation provides the steam for the fuel reforming process through recirculating part of the exiting gas from the anode. Anode recirculation is normally performed by a blower or an ejector. However, if there is no anode recirculation arrangement, an external water supply is heated by a heat recovery steam generator to provide the steam for the fuel reforming process [75]. Anode recirculation seems to be more attractive for marine applications since it is typically less expensive and more efficient. In addition, the limited fresh water storage available onboard a ship is another consideration. However, it is difficult to control the steam-to-carbon ratio inside the stack accurately [75–77]. 3.3.2. Standalone SOFC System A standalone SOFC system consists of a SOFC stack, a fuel supply unit, an air supply unit, a reforming unit, a catalytic burner and a WHR system [75], as shown in Figure 5. Anode recirculation and indirect internal reforming are supposed to be employed for the SOFC modules discussed in Section 3.3. Therefore, water supply and steam generation from the WHR system are not needed. The reformer is thermally coupled with the SOFC stack, i.e., the heat used for the fuel reforming process is from the electrochemical reactions and is transferred to the reforming unit either by heat radiation or by direct physical contact between the reforming unit and the SOFC stack. Part of the exiting gas from the anode is recirculated for fuel reforming. Another part of the anode exiting gas reacts with the residual air from the cathode in an after catalytic combustor. The exhaust gas from the combustor flows through the air preheater, the fuel preheater and the economizer before discharging to the atmosphere. Other options of the layout include: the air being preheated by the residual air from the cathode; the fuel being preheated by the exiting gas from the anode; the catalytic combustor providing heat for an external fuel reforming unit; and the economizer generating steam for the fuel reforming. [74,78,79] Depending upon the operating temperature, varying grades of waste heat can be recovered for distinct applications, which can significantly impact the system economics and environmental issues. 3.3.3. Indirect Hybrid SOFC System Combined SOFC-ST System An indirect hybrid SOFC-ST system consists of a SOFC stack, a fuel supply unit, an air supply unit, a reforming unit, an after catalytic combustor and a WHR system [74,80]. The high temperature exhaust gas from the SOFC stack can be utilized for pre-heating the fuel, the air and for the reforming unit. When the operating temperature of the SOFC stack is lower, Rankine cycles could be considered to generate steam. Thus, additional electrical energy could be generated by a ST and the overall efficiency of the system could be increased to more than 80% [74,80]. The common working fluid of Rankine cycle is water. However, when the temperature of the heat source is lower, organic fluids are typically utilized to substitute water due to their low critical temperature. The combination of a SOFC system and an organic Rankine cycle (ORC) has been investigated and verified by many researchers in recent years [81–84].PDF Image | Fuel Cell Power Systems for Maritime Applications
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