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Sustainability 2021, 13, 1213 6 of 34 (v) DMFC. Compared to PEMFC, DMFC is slightly higher cost and lower efficiency, but it has the benefit that liquid methanol is appreciably easier to handle than hydrogen. Using methanol as fuel leads to CO2 emissions but no NOx emissions. The major challenge with DMFC is that methanol crosses over the membrane to the cathode and reacts directly with oxygen, leading to low efficiency [37]. Thus, the prerequisite of DMFC application is to enhance membrane performance, which would facilitate increased efficiency and power capacity of the resulting fuel cell stack. (vi) MCFC. MCFC is relatively high cost and operates at temperatures in the range 600–700 ◦C. Due to the higher temperature, LNG, methanol and hydrocarbons other than pure hydrogen can be used as fuels. There are no CO2 emissions if hydrogen is used as fuel, CO2 only circulates in fuel cells to regenerate carbonate in the electrolyte. In addition, high operating temperatures dictate that a WHR system is also suitable for MCFC. Using hydrocarbons as fuel leads to CO2 emissions, but no NOx emissions exist as no air is present when the reforming takes place at the anode. However, potential NOx emissions might exist from the subsequent WHR systems. MCFC is a highly efficient fuel cell, with low cost catalyst and electrolytes, and high flexibility towards fuels and contaminants [38]. While the high operating temperature makes it suitable for energy recovery systems, it also makes MCFC systems vulnerable to negative cycling effects like corrosion and cracking of components. MCFC has a slow start-up, and is less flexible towards changing power demands than low temperature fuel cells [11]. Combining MCFCs with batteries/supercapacitors or an electrolyser to allow for more steady operation of the fuel cell could significantly reduce the strain from thermal cycling. This could also allow for more flexible operation with a faster start-up and the ability to cater to changing power demands. MCFCs are commercially available, but still struggle with high cost, limited lifetime and low power density [34,39]. (vii) SOFC. Unlike a MCFC, no CO2 is required to be circulated to the cathode of a SOFC. SOFCs are relatively high cost and work at temperatures ranging between 500–1000 ◦C. Due to the higher temperature, direct internal reforming of hydrocarbon fuels, such as LNG and methanol, and direct thermal cracking of ammonia (NH3) in a SOFC stack are possible. Meanwhile, a WHR system is also suitable for SOFC, and like MCFC, the high temperature makes SOFC vulnerable to negative cycling effects. In spite of the stability of tubular SOFC in terms of thermal cycling effects, the planar SOFC is more favorable due to higher energy density and easier manufacture [11]. Combining a SOFC with a battery will reduce thermal strain and ensure a more flexible operation as well. Regarding NH3-fed SOFCs, both oxygen ion-conducting and proton-conducting elec- trolytes have been reported [40]. Where an oxygen ion-conducting electrolyte (namely SOFC-O) is employed, the main reactions are same as reactions for either hydrogen or hydrocarbon fuels. But if a proton-conducting electrolyte (namely SOFC-H) is employed, the main reactions are as follows: Anode reaction: Cathode reaction: 2H2 →4H+ +4e 4H+ +O2 +4e− →2H2O In the case of SOFC-O, water vapor is produced at the anode and the exhaust gases at the anode include N2, H2O, and the remaining NH3 and H2. As for SOFC-H, water vapor is produced at the cathode and the exhaust gases at the anode include N2 and the remaining NH3 and H2. Thus, one of the major advantages of SOFC-H is that hydrogen is not diluted by the water vapor generated in the electrochemical reaction [41]. Key characteristics of different types of fuel cells are summarized in Table 2. Among the seven types of fuel cell, differences in performance in terms of technical, environmental and economic issues, as well as the applicability of the technology onboard ships, are usually considered. However, it is difficult to choose the promising pathways based on an individual indicator. Therefore, a multi-criterion decision-making approach, orPDF Image | Fuel Cell Power Systems for Maritime Applications
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