2.5. FUEL CELL TYPES
2.5.6. Solid Oxide Fuel Cell (SOFC)
The SOFC is viable for generating electricity from hydrocarbon fuels and being efficiently inte-grated with reformers. The higher operating temperature, from 700 C to 1000 C, allows internal reforming and promotes rapid kinetics with non-precious materials. The high tempera-ture of the SOFC, however, places stringent requirements on its materials. The development of suitable low-cost materials and low-cost
fabrication of ceramic structures has been a focus of research. The SOFC integrates well with large coal gasification plants that are able to take advan-tage of synergies to produce high efficiency, 99%
CO2capture, and low water consumption[32].
Two different design configurations are being developed for SOFCs motivated by (1) consider-ations of how to seal the anode and cathode compartments, (2) ease of manufacturing, and (3) minimizing losses due to electric resistance.
The two principal types are tubular and planar as shown inFig. 2.4. Operating between 900C and 1000 C, the tubes have relatively high electrical resistance but are simple to seal.
Some tubular designs eliminate the need for seals and allow for thermal expansion. Several tubular units are presently operating in the field, with tens of thousands of hours of demon-strated operation. In the tubular configuration, bundles of tubes that are closed at one end are arranged in parallel. Air is introduced to the inside of each tube while fuel flows over the outside of the cells. Oxygen ions migrate through the electrolyte to react with fuel; the flow of ions produces electricity.
The planar SOFC is composed of flat, thin ceramic plates. The planar SOFC operates at 800 C or even less. Ultra-thin electrode/
electrolyte sheets have low electrical resistance in order to achieve high power density and effi-ciency. Operation at temperatures lower than the tubular SOFC enables less exotic materials
FIGURE 2.4 Tubular and planar cell configurations[1].
FUEL CELL TYPES 25
of construction, thus saving on cost. In the planar configuration, the anode, electrolyte, and cathode form thin, flat layers that are sintered together, and then separated by bipolar plates similar to the design of other types of fuel cells.
The plates can be rectangular, square, circular, or segmented in series and can be manifolded externally or internally. Many of the planar SOFCs use metallic bipolar plates and are oper-ated at a lower temperature than the all-ceramic tubular design. With currently hundreds of stack tests having been run by Versa and Delphi and record-breaking electrochemical performance, the planar SOFC has emerged as the low-cost potential, high-performance SOFC. Since 2000, DOE’s Solid-State Energy Conversion Alliance has made an order of magnitude improvement in SOFCs increasing power density and lowering the projected costs[32]. The typical embodiment of direct methane oxidation fuel cell is a variation of the SOFC.
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REFERENCES 27
C H A P T E R
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Fuels for Fuel Cells
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