rate of heat pumping. The corresponding current at the maximum heat pumping rate is given by
(1.76)
At this maximum cooling rate, the corresponding COP is given by
(1.77)
The electrical current at the maximum cooling rate is different from that corresponds to the maximum COP. The electrical current corresponding to the maximum COP is given by
(1.78)
The maximum COP is shown by the Eq. (1.79),
The maximum temperature difference between the hot and cold sides is given by
(1.80)
This indicates that thermoelectric materials with a high Z value are needed for efficient thermoelectric cooling. When the Z value is higher, lower temperature at the cold side can be achieved.
1.4 Thermoelectric Sensors
The Seebeck effect can be used to measure the temperature difference between two objects. One example is the thermocouples used for high temperature measurement. One end of a thermocouple is put to an object of constant temperature. The temperature of the other end can be read in terms of the Seebeck voltage.
1.5 Summary
Thermoelectric materials can be used for the direct energy conversion between heat and electricity. Apart from inorganic semiconductors and metals, intrinsically conducting polymers and organic molecules and composites of nanomaterials have also gained great attention. The ZT value is the parameter to characterize thermoelectric materials. A high ZT value requires high Seebeck coefficient, high electrical conductivity, but low thermal conductivity. Inorganic thermoelectric materials usually have a high Seebeck coefficient. But their thermal conductivity is very high. Organic or polymeric thermoelectric materials usually have a low thermal conductivity. But their Seebeck coefficient is lower than their inorganic counterparts by one to two orders of magnitude. The energy band structure and doping mechanism of conducting polymers is fundamentally different from metals or inorganic semiconductors. Thus, the approaches to improve the thermoelectric properties of conducting polymers can be quite different from that of inorganic thermoelectric materials as well. The thermoelectric properties of composites depend on the structure and properties of the fillers and the matrix and the microstructure of the composites. In‐depth understanding of the thermoelectric fundamental knowledge is the key for the development of high‐performance thermoelectric materials.
The applications of thermoelectric materials include thermoelectric generators, Peltier coolers, and thermoelectric sensors. There are heat transfer, drift electrical current, and thermoelectric effects during the operation of these devices. Their performance is thus strongly dependent of the physical properties of the thermoelectric materials, particularly the ZT value. Apart from the intrinsic properties of the thermoelectric materials, other factors such as the electrodes, the contact between the electrodes and thermoelectric materials and dimensions of the thermoelectric legs should be taken into consideration in designing a thermoelectric device.
Acknowledgment
This work was financially supported by a grant from the Ministry of Education, Singapore (R‐284‐000‐228‐112).
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