Fuel Cell Handbook, Fourth Edition, November 1998

J.H. Hirschenhofer, D.B. Stauffer, R.R. Engleman, and M.G. Klett
Department of Energy/Fossil Energy Technology Center

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Characteristics

Figure 1-5 Relative Emissions.gif (5699 bytes)

    Figure 1-5     Relative Emissions of PAFC Fuel Cell Power Plants
Compared to Stringent Los Angeles Basin Requirements

Fuel cells have many characteristics that make them favorable as energy conversion devices. Two that have been instrumental in driving the interest for terrestrial application of the technology are the combination of relatively high efficiency and very low environmental intrusion (virtually no gaseous or solid emissions). Efficiencies of present fuel cell plants are in the range of 40 to 55% based on the lower heating value (LHV) of the fuel. Hybrid fuel cell/reheat gas turbine cycles that offer efficiencies up to 70%, LHV, using demonstrated cell performance, have been proposed. Figure 1-5 illustrates demonstrated low emissions of installed PAFC units compared to the Los Angeles Basin (South Coast Air Quality Management District) requirements, the strictest requirements in the US. Measured emissions from the PAFC unit are < 1 ppm of NOx, 4 ppm of CO, and <1 ppm of reactive organic gases (non-methane) (5). In addition, fuel cells operate at a constant temperature, and the heat from the electrochemical reaction is available for cogeneration applications. Because fuel cells operate at nearly constant efficiency, independent of size, small fuel cell plants operate nearly as efficiently as large ones. [The fuel processor efficiency is size dependent; therefore, small fuel cell power plants using externally reformed hydrocarbon fuels would have a lower overall system efficiency.]

Table 1-2 Summary of Major Fuel Constituents.gif (6641 bytes)
a - In reality, CO, with H2O, shifts to H2 and CO2, and CH4, with H2O,
      reforms to H2 and CO faster than reacting as a fuel at the electrode.
b - A fuel in the internal reforming MCFC.                                            

Table 1-2     Summary of Major Fuel Constituents Impact on PAFC, MCFC, SOFC, and PEFC

Thus, fuel cell power plants can be configured in a wide range of electrical output, ranging from watts to megawatts. Fuel cells are quiet and even though fuel flexible, they are sensitive to certain fuel contaminants that must be minimized in the fuel gas. Table 1-2 summarizes the impact of the major constituents within fuel gases on the various fuel cells. The reader is referred to Sections 3 through 6 for detail on trace contaminants. The two major impediments to the widespread use of fuel cells are 1) high initial cost and 2) high-temperature cell endurance operation. These two aspects are the major focus of manufacturers’ technological efforts.

Other characteristics that fuel cells and fuel cell plants offer are

  • Direct energy conversion (no combustion).
  • No moving parts in the energy converter.
  • Quiet.
  • Demonstrated high availability of lower temperature units.
  • Siting ability.
  • Fuel flexibility.
  • Demonstrated endurance/reliability of lower temperature units.
  • Good performance at off-design load operation.
  • Modular installations to match load and increase reliability.
  • Remote/unattended operation.
  • Size flexibility.
  • Rapid load following capability.

General negative features of fuel cells include

  • Market entry cost high; Nth cost goals not demonstrated.
  • Endurance/reliability of higher temperature units not demonstrated.
  • Unfamiliar technology to the power industry.
  • No infrastructure.

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