Optimal Design of Power Gradient Limited Solid Oxide Fuel Cell Systems with Hybrid Storage Support for Ship Applications

Solid oxide fuel cells are a much discussed power technology for ship applications as they exhibit high energy efficiency and fuel versatility. However, due to their strict power gradient limitations, their application in ship power systems with fluctuating load profiles is not given without support. An adequately designed energy storage consisting of batteries and potentially supercapacitors could increase the dynamic behavior of a power system to a sufficient level. To prove that only moderate storage support is required, a model based system design optimization is conducted for two real-life case studies. In doing so, the influence of the models' levels of detail on the optimal system design and the cost estimations is demonstrated. For the first study, a yacht load profile with high storage capacity demand and a maximum load of 487 kW was investigated. A cost optimal battery capacity of 129 kWh fulfills the required power supply aspects for a 251 kW fuel cell system without the need for a supercapacitor. In the second study, a cargo ship resembles an example for a high storage power demand and a peak load of 560 kW. Here, a hybrid storage composed of a 49.4 kWh battery and a 71 Wh supercapacitor sufficiently supports a 195 kW fuel cell system. The storage model assessment shows, that life estimations and the nonlinear behavior of supercapacitors need to be covered with particular care when designing a power system. By contrast, the lithium-ion battery's physical behavior can be simplified more easily. Based on both the straightforward and the revised analysis, the usability of solid oxide fuel cells on ships with dynamic load profiles can be assumed given, when combined with an energy storage unit.