Exhaustion of fossil energy sources and problems of global warming and air pollution highlight the necessity of developing environment-friendly energy sources and equipment/system for their use. On such demand, in case of automobiles and ground transportation vehicles, their energy source has shifted from diesel and gasoline to hydrogen, and their power generating equipment from internal combustion engines to motors.
The International Maritime Organization(IMO) recently established a control limit of air pollutant emission through the air pollution control agreement(MARPOL Annex VI), and it has enforced gradational sailing ban against larger-than-400 GT ships that discharge more pollutant than the limit. In addition, some regions of the world have applied much stricter control limit according to their own regulations, and the IMO control limit is also expected inevitably to increase gradually.
In this way, even for ships, maritime transportation vehicles, the environment-friendliness has become a realistic problem which has to be solved right now. Though they are currently taking actions for the problem through treatment of fuel oil and efficiency improvement of internal combustion engines, because those are not fundamental solutions for a pollution-free environment the introduction of such a new power generation device as a fuel cell system that uses hydrogen as its fuel has been considered prudently.
The fuel cell system characterized by high efficiency and environment-friendly feature has been applied to automobiles and is about to be industrialized, but its application to ships remains at an experimental stage. In case of ships, because characteristics required for its power generating device differ according to its size, navigation area and use, the kind and system composition of suitable fuel cells cannot be decided indiscriminately. Especially, because ships sail in unique isolated locations where they cannot receive outside support, the equipment and system must be designed according to much stricter safety standards than those for ground transportation vehicles.
The objective of this thesis is to evaluate by simulation the performance of various systems adopted as fuel cell systems for ships. First of all, we built a methane-fueled, external-reforming SOFC single system, a fuel cell system that reflects as a power generation device the uniqueness and safety of a large LNG carrier which uses natural gas as a hydrogen energy source, and then we examined, through simulation and modeling, the effects of cell operating temperature(COT) and cell current density(CD), reformer steam/carbon ratio(S/C), recuperator temperature efficiency, and hydrogen-fuel utilization rate on the performance and safety of the system.
In addition, the waste heat from the fuel cells was actively utilized in order to convert as much as possible the chemical energy possessed by hydrogen to electricity for the purpose of high efficiency of the fuel cell system. For such purposes, in order to use the waste heat we have built a hybrid system that combines a gas turbine and a steam turbine.
We have built both a cooled turbine system and a non-cooled turbine system depending on the turbine inlet temperature(TIT) for the SOFC/GT system, both a supplementary fuel supply system for steam temperature and pressure of LNG ships and a waste heat-dependent system without supplementary fuel supply for the SOFC/ST system, and both a cooled turbine system and a non-cooled turbine system for the SOFC/GT/ST system. We evaluated the performance of a variety of systems on the basis of various conditions and variables.
Through the evaluation, we found that all the combined systems were more efficient than the SOFC single system. The change in efficiency of the SOFC/GT system was more gradual than that of the SOFC/ST system. And it was found that although the SOFC/GT/ST system was the most efficient the simple SOFC/GT system is suitable because building such a complex system is inefficient for the case of low-powered ships.
In addition, we have built an internal reforming system, though risky, in order to reduce the amount of cooling air that accounts for most of air supplied into the stack. Thus, we found that the efficiency of the internal reforming system was improved due to use of heat source from the stack over the external reforming system.
Prior to practical application of the fuel cell system to ships, further studies on characteristics of the fuel cell system are required to answer the questions on whether the fuel cell system is suitable to the loading characteristic of ships, which system can be built, what performance characteristics the system has, and whether the system is safe.