Power Plant Engineering

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.

2.17.4 Efficiency of Cells

The performance evaluation of fuel cells is represented in terms of current density at electrode
surface (range 100 to 400 mA/cm^2 ) at specified temperature and reactant partial pressures and voltage.

Let, Vo = No load voltage of cell, Volts, DC
Vc = Cell voltage on load
Ic = Cell current on load, Ampere
Pc, = Cell power, Watts
Vp = Polarization voltage = Voltage drop in the cell?
= No load voltage Vo – On load voltage V,
A = Surface area of on face of an electrode, m
Id = Current density of cell, = Ic/A.... A/m....
η = Efficiency
During no current (no load or open circuit), the cell voltage is maximum and is called no load
voltage (Vo).

The performance is illustrated by actual Vc vs. Id curve. Increase in operating temperature and

partial pressure, improves the fuel cell performance (increase in Vc and Pc). There is a trade-off be-

tween the higher performance and higher cost (for high temperature, pressure design).

VOLTAGE VC-CURRENT DENSITY Id CHARACTERISTIC (POLARIZATION CURVE)

The performance of a fuel cell is evaluated by the cell voltage Vc vs. electrode current density Id

curve (Fig. 2.22). Cell voltage Vc drops with increase in current density due to polarization within the

cell. Hence, the curve is also called the polarization curve of the fuel cell.