Dynamic Modelling of a Solid Oxide Fuel Cell System

Abstract

The constant pressure on the maritime sector to reduce Greenhouse Gas (GHG) emissions has led the shipping industry to search for alternatives, such as zero-emissions propulsion systems, which would allow meeting the 2050 target imposed by International Maritime Organization (IMO) regulation. Fuel cells have demonstrated to substantially contribute to the greening of energy conversion technologies. Specifically, Solid Oxide Fuel Cells have proven to be a reliable technology to produce energy from Liquid Natural Gas (LNG). However, the limited understanding of the effect of the components around the stack (also known as Balance of Plant, BoP) in SOFC power generation system represents one of the reasons of the slow development of this technology. The main objective of this research is to gain insight in the performance of the BoP components and their influence on the SOFC power generation system for maritime applications. A dynamic model describing a complete SOFC power generation system is developed in this work. The chosen system configuration consists of three blowers, two heat exchangers, a mixer, an external pre-reformer, an SOFC stack and an afterburner. Specifically, each BoP component is modelled dynamically using a 0-D approach and verified individually by using the software Cycle-Tempo. Then, the BoP models are integrated with an existing 1-D SOFC stack model. A dedicated control system is implemented and the load following capabilities of the complete system are studied. The model developed is able to simulate the time variation of all the BoP component characteristics and provides insights in the system efficiency when varying operating parameters such as stack current, anode recirculating ratio and fuel utilization. In particular, it is proven that working at low current enables higher cell voltage and, thus, higher system and stack efficiency. System fuel utilization significantly contributes to the system efficiency, which reaches the highest value for the highest fuel utilization. Additionally, the effect of the fuel utilization rate on the stack and system is the highest at lower currents. System and stack efficiency of respectively 58 % and 66 % are possible with the chosen system configuration. Anode recirculating contributes more to the system efficiency than the stack efficiency. The highest system efficiency is obtained for low current values and high recirculating ratio. Moreover, significant CO2 emission reduction is obtained for high recirculating ratio. The chosen control strategy succeeds on ensuring thermal safe operation, but does not guarantee fast response to load changes. In particular, a system response within 2 hours is achieved with the controller developed when the stack current is changed from 27 A to 23 A . Moreover, a load ramp of the stack current is a better choice in terms of thermal safe operation than a stepped change. The developed model represents a solid base for future development and research in the modelling of SOFC power generation systems for maritime applications. Nevertheless, future investigations on model validation, control system, start-up operations and system optimization are recommended.