Thermodynamics

Chapter 11 | 635this manner form a thermocouple, which is the most versatile and mostwidely used temperature measurement device. A common T-type thermocouple,for example, consists of copper and constantan wires, and it producesabout 40 mV per °C difference.The Seebeck effect also forms the basis for thermoelectric power generation.The schematic diagram of a thermoelectric generator is shown in Fig.11–25. Heat is transferred from a high-temperature source to the hot junctionin the amount of Q H , and it is rejected to a low-temperature sink from thecold junction in the amount of Q L . The difference between these two quantitiesis the net electrical work produced, that is, W e Q H Q L . It is evidentfrom Fig. 11–25 that the thermoelectric power cycle closely resembles anordinary heat engine cycle, with electrons serving as the working fluid.Therefore, the thermal efficiency of a thermoelectric generator operatingbetween the temperature limits of T H and T L is limited by the efficiency of aCarnot cycle operating between the same temperature limits. Thus, in theabsence of any irreversibilities (such as I 2 R heating, where R is the totalelectrical resistance of the wires), the thermoelectric generator will have theCarnot efficiency.The major drawback of thermoelectric generators is their low efficiency.The future success of these devices depends on finding materials with moredesirable characteristics. For example, the voltage output of thermoelectricdevices has been increased several times by switching from metal pairs tosemiconductors. A practical thermoelectric generator using n-type (heavilydoped to create excess electrons) and p-type (heavily doped to create a deficiencyof electrons) materials connected in series is shown in Fig. 11–26.Despite their low efficiencies, thermoelectric generators have definite weightand reliability advantages and are presently used in rural areas and in spaceapplications. For example, silicon–germanium-based thermoelectric generatorshave been powering Voyager spacecraft since 1980 and are expected tocontinue generating power for many more years.If Seebeck had been fluent in thermodynamics, he would probably havetried reversing the direction of flow of electrons in the thermoelectric circuit(by externally applying a potential difference in the reverse direction) to createa refrigeration effect. But this honor belongs to Jean Charles AthanasePeltier, who discovered this phenomenon in 1834. He noticed during hisexperiments that when a small current was passed through the junction of twodissimilar wires, the junction was cooled, as shown in Fig. 11–27. This iscalled the Peltier effect, and it forms the basis for thermoelectric refrigeration.A practical thermoelectric refrigeration circuit using semiconductormaterials is shown in Fig. 11–28. Heat is absorbed from the refrigerated spacein the amount of Q L and rejected to the warmer environment in the amount ofQ H . The difference between these two quantities is the net electrical work thatneeds to be supplied; that is, W e Q H Q L . Thermoelectric refrigeratorspresently cannot compete with vapor-compression refrigeration systemsbecause of their low coefficient of performance. They are available in themarket, however, and are preferred in some applications because of theirsmall size, simplicity, quietness, and reliability.High-temperature sourceT HQ HHot junctionIW netICold junctionQ LLow-temperature sinkT LFIGURE 11–25Schematic of a simple thermoelectricpower generator.Hot plateCold plateSOURCESINKQ HQ Lp n p n p n+ –IW netFIGURE 11–26A thermoelectric power generator.