64 Efficiency

The efficiency of a fuel cell is closely related to the voltage level that can be practically produced. The total cell voltage includes contributions from the anode and cathode as well as the ohmic polarization. Each of the electrodes can be affected by activation and concentration overpotentials as follows:

Vcathode Ecathode Vact ,cathode n conc,cathode Vanode Eanode Vact ,anode nconc,anode

The total cell voltage is then:

Vcell Vcathode Vanode iRcell

Thus, overall cell performance is reduced by five primary overpotentials as presented in the equations above. Typically, cathodic losses far exceed losses at the anode, primarily due to the stability of the oxygen molecule compared to the primary reactants participating in anodic electrochemistry (e.g., hydrogen). In addition, activation overpotentials are typically greater than ohmic losses, which are greater than concentration overpotentials for typical fuel cells at typical operating conditions. In general, the losses lead to practical cell voltages in the range of 0.6 to 1.0 volts.

Figure 6.4 presents the voltage levels achieved by each of the fuel cell types versus temperature as well as the reversible cell potential. Note that some of the highest voltage levels are achieved for the higher temperature fuel cell types even though the reversible potential decreases with increasing temperature. This is primarily due to reductions in activation overpotentials at higher temperatures.

The voltage levels presented in Figure 6.4 roughly correlate with system efficiency such that practical fuel cell efficiencies are greatest for alkaline fuel cells, followed by the high-temperature molten carbonate and solid oxide fuel cells. The lower-temperature proton exchange membrane fuel cells can achieve practical efficiencies that exceed those observed for phosphoric acid fuel cells but are less than either alkaline or higher-temperature fuel cell systems. When operated on natural gas, system fuel-to-electricity conversion efficiencies have been observed as presented in Table 6.4.

TABLE 6.4

Typical Efficiency Ranges for Fuel Cell Systems Operating on Natural Gas

TABLE 6.4

Typical Efficiency Ranges for Fuel Cell Systems Operating on Natural Gas

Range of Fuel-to-Electricity Efficiency

Fuel Cell Type

(Natural Gas Operation, LHV Basis)

Molten Carbonate (MCFC)

50-60%

Phosphoric Acid (PAFC)

38-45%

Proton Exchange Membrane (PEMFC)

33-45%

Solid Oxide (SOFC)

40-55%

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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