Solid Oxide Fuel Cells for Transportation

The well-known compatibility of the SOFC with hydrocarbon fuels makes the solid oxide fuel cell a strong contender for transportation applications. Gasoline and diesel will continue to be the dominant fuels for decades to come. The 500 million automobiles, the existing fuel distribution system and the economic power of oil and automobile companies combined form a strong inertia in favour of liquid fuels and internal combustion engines. The chances are slim that hydrogen will soon appear at gasoline stations to power "PEMobiles", i.e. vehicles equipped with polymer electrolyte fuel cells. Therefore, because of their compatibility with vaporised liquid hydrocarbon fuels, solid oxide fuel cells may play an important role in transportation. Basically, a typical "SOFCar" would be a hybrid fuel cell vehicle equipped with a small buffer battery and a relatively small onboard SOFC power source which is more or less continuously converting conventional gasoline or diesel fuel into DC power. The temperature of the fuel cell will be adjusted to the actual power demand. The paper presents arguments for the use of light weight, compact and efficient solid oxide fuel cells in transportation. Furthermore, the SOFC is compared to the PEFC which today is highly favoured for transportation applications. Suggestions are provided for design and operation of SOFCars 1. PEFC: today's transportation favourite! It seems to be a forgone conclusion that only the polymer electrolyte fuel cell PEFC (or PEM) has a realistic chance to replace the internal combustion engine in transportation applications. In fact, this notion is reconfirmed by convincing presentations of experimental results. Impressive investments and global business partnerships have been formed. It has certainly been established that fuel cells having solid polymer electrolytes can power vehicles. Again and again, the players in the field assure each other by reports of success. But there are still unsolved critical issues, many of which would not exist if solid oxide fuel cells were considered for transportation applications. Therefore, some of the open questions shall be recalled. Solid polymer fuel cells need clean hydrogen. But hydrogen is expensive, not generally available and difficult to be handled safely by ordinary drivers. It must be carried along in liquid or adsorbed form at cryogenic temperatures or as compressed gas in voluminous tanks. For the onboard conversion of gas station fuels to hydrogen vehicles must carry bulky, complex and costly chemical converters. As a consequence, the driving range of hydrogenfuelled cars, with fuel cells and internal combustion engines alike, is limited to about 200 km. Certainly, the hydrogen fuel issue is reduced for "PEMobiles" serving routine tours and returning to the same location every night. But most automobiles are operated at random, but not in well-organised fleets. The low operating temperature of solid polymer fuel cells, often praised as key advantage, may turn out to be the Achilles heel of the technology. In hot climates the temperature of the ambient air may be too high for removing the waste heat from a stack. As a consequence, the stack temperature may rise, thus drastically increasing the need for membrane humidification. But under said conditions the required water can no longer be recovered from the exhaust air. As a result, the car must be operated at reduced power to protect the fuel cel Similarly, solid