Power Plant Engineering

Fig. 2.19
Ideal Fuel Cell Potentials. Overpotentials always reduce fuel cell potentials, so that less volt-
age is delivered per electron transferred.
Conversely, over potentials always increase
electrochemical cell potentials so that more voltage is
required per electron transferred.
Ideal Cell Potentials
E = E
c
 – E
a
 = (E
eq, c
 – e
c
) = (E
eq, a
 + e
a
)
Ideal fuel cell Energy Conversion
1. Reactant/product transport
3. Ion transport through e-lyte
4. Electron transport
2. Reaction at electrocatalyst.
2.17.3 OTHER TYPES OF FUEL CELLS
Fuel cells are a means of converting a fuel
to electrical energy using an electrochemical
membrane. The most popular to date has been the
proton exchange hydrogen fuel cell. It takes two mol-
ecules of hydrogen and one molecule of oxygen and
produces two molecules of water leaving behind four
spare electrons to generate an electric current. In
terms of the energy value of the hydrogen, the con-
version process is around 75% to 80% efficient.
Some fuel cells use other chemical fuels as
a source of hydrogen such as methanol, which is
processed into hydrogen for the use by the fuel cell.
H O
2
(CO )
2
Air
Fuel
(–)
(+)
Anode
Electrolyte
Cathode
Fig. 2.20
Product Water
O + 4H + 4e
2
+
–
2H O
2
2H
2
4H + 4e
+
–
Hydrogen
Oxygen
4e
–
(+)
(–)
Oxygen
electrode
Hydrogen
electrode
Proton Exchange
Membrane
Fig. 2.21

---

76
POWER PLANT ENGINEERING
Although this means that the system doesn’t have to store large quantities of highly explosive hydro-
gen, it does reduce the efficiency of the electricity generation process to 30% or 40%. This is still more
efficient than burning the methanol directly combustion engines are only around 20% efficient in terms
of the energy of the fuel that is actually transferred into motion on the ground.
The problem with hydrogen fuel cells is generating the hydrogen to fuel them. To get those 4
electrons out by combining two hydrogen molecules with an oxygen molecule, you have to put them in
at the point you manufacture of the hydrogen. There are four common processes:
Reformation of hydrocarbons. Hydrogen can be produced from any fossil fuel, such as oil or
coal, by heating and then ‘reforming’ the hydrogen with steam.
Steam reformation of natural gas. Like the above, but without the need to initially turn the solid
hydrocarbons into hydrocarbon gases.
Biomass pyrolysis organic matter can be gasified/pyrolysed to produce hydrogen rich gases than
can then be reformed with steam to hydrogen.
Electrolysis. Producing hydrogen from water directly using electricity.
In general, the case for fuel cells in mobile uses is marginal to the use of other fuels. They
potentially have an application in balancing out the variations from certain forms of renewable energy
such as wind or tidal power.