Design of the SiemensWestinghouse SOFC

Tgenerate commercial current, numerous cells must be stacked together in layers (Figure 3) inta generator module. The stack is cooled using process air; during normal operation, requires nexternal water.An individual cell is closed at one end and is typically 2.2 cm (0.867 inches) in diameter by 150 cm (59 inches) in length. Air is delivered teach cell by a delivery tube, and fuel is delivered tthe cell exterior (Figure 4). The open circuit potential of a single cell is in the range of 900 mV t1V.Power produced is proportional tthe active surface area of the cells. At atmospheric pressure and uniform temperature of 1000° C, a single tubular SOFC will generate potential of about 900 mV t1V and up t210 W DC. Fuel Cell Energy, Inc. reports emissions from their SOFC systems are: NOx < 0.1 ppmv, SOx<0.01 ppmv, CO <10 ppmv, and VOC <10 ppmv. Overall these concentrations are nearly 50 times less than today's average natural gas turbine according tthe DOE Office of Fossil Energy. Emissions are slow that that the South Coast Air Quality Management District in southern California and regulatory authorities in Massachusetts and Connecticut have exempted fuel cells from air quality permitting requirements. In addition thigh efficiency and low emissions SOFCs are extremely quiet; typically 1 t250 kW units produce about 60 dBA of noise at 1 meter (normal speech is around 70 dBA). The combination of low emissions and quiet operation mean that the units can be located almost anywhere. With moving parts limited to circulating blowers, they have high reliability and very low maintenance.

The first production SOFC units available will be as small as 250 kW commercial products will range from 0.25 - 5 MW range.Because of their low emissions and quiet operations, SOFCs are ideally suited for distributed generation at large commercial buildings such as offices and malls, large complexes such as hospitals and government facilities, and industrial complexes particularly those that make use of the co-gen potential of the heat. Other important applications include applications with DC power requirements for computer and telecommunication centers, mini-grids at universities, port and military facilities, as well as new and existing housing developments. Diesel generation costs $800 to $1,500 per kW, and natural gas turbine generators can cost less. At this time SOFC fuel cells cost around $4000 per kW, but the target cost of SOFC units by the end of the decade is $400 per kW.

Additional types of Fuel Cells, from Society of Automotive Engineers (SAE) website

  • Alkaline fuel cells - AFC
    • First used in the Gemini-Apollspace program tproduce drinking water and electrical energy.
    • Operate on compressed hydrogen.
    • Generally use a solution of potassium hydroxide (chemically, KOH) in water as their electrolyte.
    • Output of alkaline fuel cell ranges from 300 watts (W) t5 kilowatts (kW).
  • Direct methanol fuel cells - DMFC
    • Use methanol instead of hydrogen.
    • Operating temperatures are in the same range as PEM (see below) fuel cells – 50 t100°C (122 t212°F).
    • Direct methanol fuel cells are being considered for use in the transportation industry.
  • Molten carbonate fuel cells - MCFC
    • Use a liquid solution of lithium, sodium, and/or potassium carbonates soaked in a matrix.
    • Units with output up t2 megawatts (MW) have been constructed, and designs exist for units up t100 MW.
    • The nickel electrode-catalysts are inexpensive compared tthose used in other cells, but the high temperature alslimits the materials and safe uses of MCFCs.
  • Phosphoric acid fuel cells - PAFC
    • Use phosphoric acid as the electrolyte tmake electricity.
    • Efficiency ranges from 40 t80 percent and operating temperature is 150 t200° C (about 300 t400° F).
    • Have outputs up t200 kW, and 11 MW units have been tested.
  • Proton exchange membrane fuel cells - PEM
    • The most common type of fuel cell being developed for transportation use.
    • Operate at the one kW per liter of volumetric powered level at a temperature under 100°C (212 °F)
    • React quickly tchanges in electrical demand and will not leak or corrode.
    • Use inexpensive manufacturing materials (plastic membrane).
  • Regenerative fuel cells - RFC
    • Separate water inthydrogen and oxygen by a solar-powered electrolyser.
    • Hydrogen and oxygen are fed intregenerative fuel cells, generating electricity, heat and water.
    • Water is then recirculated back tthe electrolyser of the regenerative fuel cell and the process repeats.
  • Solid oxide fuel cells - SOFC
    • Use a hard, ceramic compound of metal (like calcium or zirconium) oxides (chemically, O2) as electrolyte.
    • Output is up t100 kW.
    • Reformer is not required textract hydrogen from the fuel due thigh temperature.

Visit the SAE website for more information about fuel cells.