Power Plant Engineering

78 POWER PLANT ENGINEERING

Input power – Polarization losses = Output power

and, Output power/Input power = Efficiency(η)

Efficiency of Cells. In terms of the final energy output these options are only 40% to 60%
efficient — the exception being electrolysis which can be up to 80% efficient. The argument of fuel cell
advocates is that this cycle still represents an improvement for cars. The overall efficiency is something
like 40% if running fuel rather than 20% if running an internal combustion engine.

The simple method of calculating the efficiency (η) of a fuel cell is

η =

Cell voltage on load

No load voltage of cell

=

c

o

V

V

η =

No load voltage – Polarization voltage
No load voltage
The efficiency of a fuel cell varies with the current density at electrode surface due to the
Polarization Effect. Fig. 2.24 gives a typical characteristic. The power loss is converted to waste heat
and released to atmosphere.

PowerEfficiency

Pc

B

A

Efficiency

Power

Current Density

Fig. 2.24. Power and Efficiency Curve.
After reaching saturation level, the and power per cell starts decreasing. The losses increase and
are converted to waste heat.

2.18 Thermionic Systems and Thennionic Emission

A thermionic converter transforms heat directly into electrical energy by utilizing thermionic
emission. All metals and some oxides have free electrons which are released on heating. These elec-
trons can travel through a space and collected on a cooled metal. These electrons can return to hot
metal through an external load thereby producing electrical power.

A thermionic converter has two elec-trodes enclosed in a tube. The cathode is called an emitter
and is heated enough to release electrons from its surface. The electrons cross a small gap and accumu-
late on a cooled metal anode called the collector. The space between the electrodes is maintained at
high vacuum or filled with a highly conducting plasma like ionised cesium vapour to minimal energy
losses. The external load R is connected through anode to cathode. The electrons return to cathode
through the external load and electrical power is produced.