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Direct Energy, 2018a

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8 THERMOELECTRICS 197<br />

8.8. A thermoelectric device has a gure of merit of Z =0.7 K −1 . A<br />

second device is made out of the same semiconducting materials, but<br />

it has been doped so that the electrical conductivity is 20% higher.<br />

Find the gure of merit of the second device.<br />

8.9. A thermoelectric device is made from a material with resistivity 5 ·<br />

10 −8 Ω·m and Seebeck coecient 8.5 · 10 −5 K V . A cube, 1 cm on each<br />

side, was used to determine the thermal conductivity. One side of the<br />

cube was heated. At a steady state, the rate of energy transfer by<br />

conduction through the cube is 1.8 W. The temperature distribution<br />

through the material is linear, and a temperature dierence across is<br />

measured to be 20 K across the cube. Find the thermal conductivity<br />

κ, and nd the gure of merit Z for the material.<br />

8.10. As shown in Fig. 8.3, a heater supplies heat to one side of an iron rod.<br />

The rod is cylindrical with length 30 cm and radius 2 cm. The heater<br />

supplies 2 W of power to the edge of the rod. Iron has a thermal<br />

conductivity of κ =80<br />

m·K W . Two thermocouples are are spaced 15<br />

cm apart as shown in the gure. What is the dierence in temperature<br />

(in degrees Celsius) measured between the two thermocouples?<br />

8.11. A thermoelectric device is used to build a small refrigerator that can<br />

hold two pop cans. When the device is operating, the cold side of<br />

the device is at T =10 ◦ C while the hot side of the device, outside<br />

the refrigerator, is at T =42 ◦ C. What is the maximum possible<br />

eciency of this device?<br />

8.12. The cold side of a thermoelectric device, used to generate electricity, is<br />

at a temperature of 100 ◦ C. What is the minimum temperature of the<br />

hot side of the device needed to achieve an eciency of η eff = 15%?<br />

8.13. The Carnot eciency describes the limit of the eciency for some<br />

devices. Does it apply to the following types of devices? (Answer yes<br />

or no.)<br />

• Hall eect device<br />

• Semiconductor laser<br />

• Photovoltaic device<br />

• Piezoelectric device<br />

• Pyroelectric device<br />

• Thermoelectric device used as a temperature sensor

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