Power Plant Engineering

82 POWER PLANT ENGINEERING

Carnot efficiency

ηc = 1 – a

c

T

T

= (1 – θ)

If β = 18,

ηmax = 0.9(1 – θ)

ηmax occurs when βa = βc

or c
c

V

T =

a

a

V

T

2.19 Thermoelectric Systems

A loop of two dissimilar metals develops an e.m.f. when the two junctions of the loop are kept
at different temperatures. This is called Seebeck effect. This effect is used in a thermocouple to meas-
ure temperature.

Thermoelectric generator is a device which directly converts heat energy into electrical energy
using the Seebeck thermoelectric effect. The device is very simple but thermal efficiency is very low of
the order of 3%. Efficiency of thermoelectric generator depends upon the temperature of hot and cold
junctions and the material properties. The semiconductor materials have more favourable properties
which can withstand high temperatures and can give reasonable efficiency. The probability of develop-
ing peak load power stations of the order of 100 mW working at 20 percent thermal efficiency is high.
Where cheap fuels are available thermoelectric generators can be developed for base load and standby
power generation also. Another important application is the use of radioactive decay heat to generate
power in space and other remote locations. The use of solar energy to supply heat for generating elec-
tricity can be an attractive application of thermoelectric devices if high efficiency materials can be
developed.

2.19.1 Principle of Working

The operation of a thermoelectric generator is shown in
Fig. 2.27. The net useful power output is given by

W = I^2 R [W]

where I = current [A]

R = External load resistance [Ω]

The current in the circuit is given by

I =

()i

T

RR

α∆

+

[A]

where α = Seeback coefficient (V/K]

∆T = Temperature difference between hot and cold junctions [K]
Ri = Internal resistance of thermoelectric generator [Ω]
The magnitude of potential difference depends on the pair of conductor materials and on the
temperature difference between the junctions.

R

–+Qout

Qin

I

Fig. 2.27. Thermoelectric Generator.