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Sustainability 2021, 13, 1213 27 of 34 6.4. Durability Durability primarily means the lifetime of a fuel cell stack. The lifetimes of a PEMFC for stationary and transportation applications are expected to be 40,000 and 5000 h, re- spectively, by the U.S. Department of Energy [44]. Some MCFC and SOFC plants have achieved a lifetime of more than 30,000 h [17,33,106]. The lifetime of a fuel cell stack is mainly dependent on the degradation of electrolyte, electrode and bipolar plate [107,108]. For instance, the degradation mechanisms of PEMFC include loss of catalyst, reduced conductivity of electrolyte, corrosion, poisoning and flooding. [109,110]; the degradation mechanisms of SOFC include loss of catalyst and electrolyte, cracking or corrosion. [111] Based on the degradation mechanisms of a fuel cell stack, novel materials and technologies have been employed to improve performance and durability. However, for large-scale commercialization, the focus also includes maintaining the performance throughout the operational life or mitigating the degradation rates to an acceptable level [108]. The realistic operating conditions include load cycling, thermal cycling and impurities in the fuel and air, which will influence the chemical and mechanical stability of the fuel cell systems and components. Maintaining steady-state operation is important to obtain prolonged durability, and can only be achieved through appropriate system design and an optimized control strategy encompassing all elements of the power system. In addition, marine fuel cell power systems operate in a sea water environment. Accordingly, some precautions need to be taken to prevent ingression of salt mist in the cathode air since the performance of the fuel cells is proved to be degraded by the sea mist [19]. 6.5. Operability Operability could be reflected by start-up time and transient dynamic response. Con- sidering the fuel cell stack, the start-up time ranges from a few seconds for a PEMFC to tens of minutes for a SOFC since high temperature fuel cells need more time for stack and reformer preheating [106]. However, for maritime applications, a long start-up time is not a significant flaw and could be accepted to some extent. After all, several hours are normally required for engine standby of large maritime ships powered by diesel engines at present. Dynamic response characteristics reflect the response of fuel cell power systems to external load changes. The transient response time ranges from less than 10 s for PEMFC to 15 min for SOFC [106]. Meanwhile, the transient response time of reforming systems is typically a few minutes. Therefore, for standalone fuel cell systems, batteries or supercapacitors are required to offset the sudden changes of external loads since the transient response time of batteries or supercapacitors is normally less than 10 s [112]. When the working conditions of the fuel cell stack adapt to the external loads, the batteries or supercapacitors no longer contribute power to the grid. 6.6. Economic Costs The application of any new technology onboard a ship has a cost associated with it, which can of course act to inhibit the transition to novel power and propulsion systems. However, estimating the premium of innovation of a ship by applying the net present value formula is an effective tool which could assist in the evaluation of the financial performance on acquisition and operational decisions [113]. The development of fuel cells for maritime applications commenced half a century ago, but high costs are commonly regarded as the primary factor restricting their widespread use. High costs emanate from the following aspects: (i) High stack costs. The unit prices of fuel cell stacks are more than 50 $/kW for PEMFC and about 4000–9000 $/kW for MCFC and SOFC at present [34], which are signifi- cantly more expensive than conventional diesel engine power plants. Achieving reduced stack costs depends on the use of cheaper electrode materials rather than precious metal catalysts. In addition, with increasing annual production volume, the unit prices would decrease gradually due to economies of scale. For example, the SOFC stack cost is ex- pected to reduce to a competitive price, 170 $/kW, through mass production and cellPDF Image | Fuel Cell Power Systems for Maritime Applications
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