Assessment of a methanol-fueled integrated hybrid power system of solid oxide fuel cell and low-speed two-stroke engine for maritime application

Abstract

An integrated solid oxide fuel cell (SOFC)-internal combustion engine (ICE) system was proposed in this paper. Methanol was used as the fuel in the system, meaning that the SOFC exhaust gas could not enter the engine with methanol, due to the high-injection pressure of fuel in engine and difficulty of pressurizing SOFC exhaust gas. Therefore, this paper proposed SOFC exhaust gas was delivered with engine intake air to achieve waste gas reutilization, which could also reduce oxygen concentration and increase specific heat capacity of in-cylinder gas for low NOx emissions. Two modes were proposed, including delivering SOFC anode-off gas into engine for Tier II requirements and delivering SOFC cathode and anode off gas into engine for Tier III requirements. The effects of power ratio of SOFC and engine on proposed system were analyzed based on the electrochemical model of SOFC and filling and emptying model of engine. The results indicated that increasing the power ratio in both modes led to an increase in brake power but a decrease in indicated efficiency, mainly due to the increase in energy input and the decrease in exergy-to-power conversion efficiency. In addition, at Tier III mode, the Tier III requirements (weighted average NOx was 3.35 g/kWh) were met at power ratio of around 1:2, at which the energy and exergy efficiency of system reached 55.6% and 57.9%. Furthermore, an improvement for a tanker was proposed, substituting proposed system for combination of marine engine and diesel genset. Specifically, SOFC and ICE were used for auxiliary power unit and propulsion, respectively. The results showed that it could achieve better carbon-emission and techno-economic performances. The payment period of such improvement was 3.08 years taking carbon trading and government support into account.