Direct Energy, 2018a

8 THERMOELECTRICS 193
Figure 8.4: Labeled pull-apart viewshowing the major components of the
NASA Multi-Mission Radioisotope Thermoelectric Generator. This gure
is used with permission [120].
6% [119]. This technology is not new. The Apollo 12 mission in 1969 used
a similar type of power supply, but that supply produced only 70 W and
had a lifetime of 5-8 years. Thermoelectric devices have also been used
in nuclear power plants as a secondary system to recover some electricity
from heat produced [5].
While thermoelectric eects are often fundamental to the operation of
sensors and power supplies, the eects are sometimes unwanted [23, p. 457].
Electrical circuits contain junctions of wires made out of dierent metals.
Such a junction occurs, for example, when an aluminum trace on a printed
circuit board meets the tin wire of a resistor or when a tin lead solder joint
meets a copper wire. The Seebeck eect occurs at all of these junctions.
The Seebeck coecient at a junction of copper and tin lead solder, for
example, is 2 μV K
[23, p. 457]. These unwanted voltages that develop can
introduce noise or distortions into sensitive circuits.
Electrical engineers often think of heat as wasted energy. Almost
every electrical circuit contains resistors which heat up when current ows
through them. In some applications, this heating is the desirable outcome.