Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

Chapter 11 | 635

this manner form a thermocouple,which is the most versatile and most
widely used temperature measurement device. A common T-type thermocou-
ple, for example, consists of copper and constantan wires, and it produces
about 40 mV per °C difference.
The Seebeck effect also forms the basis for thermoelectric power genera-
tion. The schematic diagram of a thermoelectric generatoris shown in Fig.
11–25. Heat is transferred from a high-temperature source to the hot junction
in the amount of QH, and it is rejected to a low-temperature sink from the
cold junction in the amount of QL. The difference between these two quanti-
ties is the net electrical work produced, that is,WeQHQL. It is evident
from Fig. 11–25 that the thermoelectric power cycle closely resembles an
ordinary heat engine cycle, with electrons serving as the working fluid.
Therefore, the thermal efficiency of a thermoelectric generator operating
between the temperature limits of THand TLis limited by the efficiency of a
Carnot cycle operating between the same temperature limits. Thus, in the
absence of any irreversibilities (such as I^2 Rheating, where Ris the total
electrical resistance of the wires), the thermoelectric generator will have the
Carnot efficiency.
The major drawback of thermoelectric generators is their low efficiency.
The future success of these devices depends on finding materials with more
desirable characteristics. For example, the voltage output of thermoelectric
devices has been increased several times by switching from metal pairs to
semiconductors. A practical thermoelectric generator using n-type (heavily
doped to create excess electrons) and p-type (heavily doped to create a defi-
ciency of electrons) materials connected in series is shown in Fig. 11–26.
Despite their low efficiencies, thermoelectric generators have definite weight
and reliability advantages and are presently used in rural areas and in space
applications. For example, silicon–germanium-based thermoelectric genera-
tors have been powering Voyagerspacecraft since 1980 and are expected to
continue generating power for many more years.
If Seebeck had been fluent in thermodynamics, he would probably have
tried reversing the direction of flow of electrons in the thermoelectric circuit
(by externally applying a potential difference in the reverse direction) to cre-
ate a refrigeration effect. But this honor belongs to Jean Charles Athanase
Peltier, who discovered this phenomenon in 1834. He noticed during his
experiments that when a small current was passed through the junction of two
dissimilar wires, the junction was cooled, as shown in Fig. 11–27. This is
called the Peltier effect,and it forms the basis for thermoelectric refrigera-
tion.A practical thermoelectric refrigeration circuit using semiconductor
materials is shown in Fig. 11–28. Heat is absorbed from the refrigerated space
in the amount of QLand rejected to the warmer environment in the amount of
QH. The difference between these two quantities is the net electrical work that
needs to be supplied; that is,WeQHQL. Thermoelectric refrigerators
presently cannot compete with vapor-compression refrigeration systems
because of their low coefficient of performance. They are available in the
market, however, and are preferred in some applications because of their
small size, simplicity, quietness, and reliability.

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High-temperature source

TH

Low-temperature sink

TL

QH

QL

I

I

Hot junction

Cold junction

FIGURE 11–25

Schematic of a simple thermoelectric

power generator.

SOURCE

SINK

Hot plate

Cold plate

QH

QL

+ –

I

pn pn pn

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FIGURE 11–26

A thermoelectric power generator.