A modeling study to investigate performance of SOFC-ICE hybrid systems for marine applications

Abstract

Emissions restrictions imposed by the International Maritime Organization (IMO) is forcing ship owners and builders to look into alternative fuels and prime movers. The high efficiency of fuel cells could help to decrease emissions in marine power generation. Solid oxide fuel cells (SOFCs) are the most fuel flexible among fuel cells, the high operating temperature and the possibilities for direct internal reforming (DIR)makes this technology of great interest for natural gas fueled systems. SOFCs operating in hybrid system configuration could even achieve higher efficiencies, due to effective utilization of left-over fuel in anode off-gas and adequate system heat integration. In literature extensive research is found about hybridization of SOFCs and gas turbines. For such hybrid configurations high efficiencies are projected, however poor part load performance and high system complexity are tempering the interest for marine applications. SOFC integration with an internal combustion engine (ICE) also has high projected efficiencies and is expected to enable system integration in marine applications with limited complexity, higher robustness, and lower costs compared to SOFC-gas turbine integration. However, due to the novelty of SOFC-ICE hybrid systems, not much research has been published as of yet and the research that is found shows a variety in system configurations and performance results. This observation justifies additional SOFC-ICE hybrid system research, particularly if the system has to operate on marine applications. In this work an integration of an SOFC and ICE is proposed. Both SOFC and ICE share the load on the system: the SOFC can operate on a base load, while the ICE can handle majority of the transient load. A pre-reformer is proposed, which supplies partially reformed methane to the SOFC. The ICE is supplied with natural gas mixed with excess fuel from the SOFC-anode. This additional natural gas supply to the engine makes it a combined cycle, instead of a bottoming cycle, and allows better dynamic load control and increases reliability. Also system heat integration is an essential requirement, as the steam required for methane pre-reforming is produced with heat from engine exhaust system. System component models are developed and individually analysed. Thereafter both component models are combined to an SOFC-ICE hybrid system model and a study is conducted to investigate the sensitivity of the following operating parameters and system configurations on system performance and efficiency: SOFC current density, SOFC fuel utilization, anode off-gas recycling, methane pre-reforming ratio, pre-reformer integration, and power split ratio. The SOFC component model provides insights of performance behaviour when varying operating parameters. The ICE model clearly indicates the advantages when hydrogen is added to natural gas, both improved engine efficiency and improved combustion stability are demonstrated. Finally, it is found that operating the hybrid system model consisting of a 375 kWe (AC) SOFC and a 375 kWe ICE leads to an electric efficiency of 45.7 % (LHV). This is a 5 to 10 percent point improvement compared to conventional diesel engines operating in this power range [1]. In this hybrid system the SOFC current density is set to 5000 Am¡2, anode off-gas recycling is not applied, and 30 % of the SOFC fuel is pre-reformed. The SOFC fuel utilization is set to 86 % in order to avoid too large hydrogen-natural gas blending ratios at the ICE intake, which are currently not substantiated with engine experiments. Future modeling of the ICE should make it possible to extend the hydrogen-natural gas blending ratio, such that even higher hybrid system efficiencies can be demonstrated. This work demonstrates that the SOFC-ICE hybrid system operating at a 50 % SOFC and 50 % ICE power split provides a firmefficiency improvement compared to conventional diesel driven power plants in the range upto 1MW. An higher efficiency means a lower fuel consumption and thus a CO2-emissions reduction. Also NOx-formation is reduced, due to the absence of expansive high temperature combustion in the fuel cell part of the system. Considering volumetric power density, the SOFC-ICE hybrid system installation volume is more than twice as large as conventional marine power plants. Taking the energy conversion efficiency into account, the LNG storage space for proposed hybrid system is two times larger than that of a diesel fueled generator set. A 50 % SOFC and 50 % ICE power split leads to these numbers, depending on the operating profile a different power split can lead to other efficiencies and volume and weight constraints. The impact of power density and energy density depend on the practical application (e.g. ship design, required power, endurance, operating profile, and costs) and must be considered case-by-case.