Thermoelectrics (eBook)

Design and Materials

Hosung Lee (Autor)

eBook Download: EPUB

2025 | 2. Auflage
965 Seiten
Wiley (Verlag)
978-1-394-31736-3 (ISBN)

Infos

Complete introduction to the field of thermoelectrics, covering materials, applications, recent developments, and more, with end-of-chapter problems included throughout

Thermoelectrics provides an introduction to the fundamental theories in the fast developing and interdisciplinary field of thermoelectrics. The topics covered are in sync with contemporary technology advancement happenings within the TEC/TEG electronics cooling community and include discussion of challenges and concerns surrounding practical applications.

The first section covers thermoelectric generators and coolers (refrigerators) before examining optimal design with dimensional analysis. A number of applications are considered, including solar thermoelectric generators, thermoelectric air conditioners and refrigerators, thermoelectric coolers for electronic devices, thermoelectric compact heat exchangers, and biomedical thermoelectric energy harvesting systems. The second section focuses on materials and covers the physics of electrons and phonons, theoretical modeling of thermoelectric transport properties, thermoelectric materials, and nanostructures.

In this Second Edition, many new examples and end-of-chapter problems have been added. New results from the theories have been added in certain chapters, along with new design charts and many examples showing how to use the charts. A companion website hosts solution manuals and appendices.

Sample topics covered in Thermoelectrics include:

• Thermoelectric effects, including the Seebeck, Peltier, and Thomson effects as well as Thomson/Kelvin relationships
• Performance, maximum, abnormal parameters for thermoelectric modules as well as effective material properties
• Thermal and electrical contact resistances for micro and macro devices, with information on modeling and validation
• Thermoelectric transport properties, covering Seebeck coefficient, electrical conductivity, lattice and electronic thermal conductivities
• Low-dimensional nanostructures, covering quantum wells, wires, and dots and supporting proof-of-principle studies

Thermoelectrics is an ideal resource on the fundamentals of the subject for professionals in the electronics cooling industry, solid state physicists, and materials scientists and engineers. It is also a valuable reference for early career scientists and undergraduate and graduate students in related programs of study.

HOSUNG LEE is Professor Emeritus of Mechanical and Aerospace Engineering at Western Michigan University.

Textauszug

1
Introduction

1.1 Introduction

Thermoelectrics is literally associated with thermal and electrical phenomena. Thermoelectric processes can directly convert thermal energy into electrical energy or vice versa. A thermocouple uses the electrical potential (electromotive force) generated between two dissimilar wires to measure the temperature. Basically, there are two devices: thermoelectric generators and thermoelectric coolers. These devices have no moving parts and require no maintenance. Thermoelectric generators have great potential for waste heat recovery from power plants and automotive vehicles. Such devices can also provide reliable power in remote areas such as deep space and mountaintop telecommunication sites. Thermoelectric coolers provide refrigeration and temperature control in electronic packages and medical instruments. Thermoelectrics has become increasingly important with numerous applications. Since thermoelectricity was discovered in the early nineteenth century, there has not been much improvement in the efficiency of materials until the recent development of nanotechnology, which has led to a remarkable improvement in performance. It is thus very important to understand the fundamentals of thermoelectrics for development and thermal design. We start with a brief history of thermoelectricity.

In 1821, Thomas J. Seebeck discovered that an electromotive force or a potential difference could be produced by a circuit made from two dissimilar wires when one of the junctions was heated. This is called the Seebeck effect.

Thirteen years later in 1834, Jean Peltier discovered the reverse process that the passage of an electric current through a thermocouple produces heating or cooling depending on its direction. This is called the Peltier effect. Although the above two effects were demonstrated to exist, it was very difficult to measure each effect as a property of the material because the Seebeck effect is always associated with two dissimilar wires, and the Peltier effect is always followed by the additional Joule heating, that is, heat generation due to the electrical resistance to the passage of a current. Joule heating was discovered in 1841 by James P. Joule.

In 1854, William Thomson (later Lord Kelvin) discovered that if a temperature difference exists between any two points of a current-carrying conductor, heat is either liberated or absorbed depending on the direction of the current and material, which is in addition to the Peltier heating. This is called the Thomson effect. He also studied the relationships between the above three effects thermodynamically, showing that the electrical Seebeck effect results from a combination of thermal Peltier and Thomson effects. Although the Thomson effect itself is small compared to the other two, it leads to a very important and useful relationship called the Kelvin relationship.

The mechanisms of thermoelectricity were not understood well until the discovery of electrons at the end of the nineteenth century. Now it is known that solar energy, an electric field, or thermal energy can liberate some electrons from their atomic binding, even at room temperature (from the valence band to the conduction band of a conductor), where the electrons become free to move. This is the reason why we have electrostatics everywhere. However, when a temperature difference across a conductor is applied as shown in Figure 1.1, the hot region of the conductor produces more free electrons and diffusion of these electrons (charge carriers including holes) naturally occurs from the hot region to the cold region. On the other hand, the electron distribution provokes an electric field, which also causes the electrons to move from the hot region to the cold region by the Coulomb forces. Hence, an electromotive force (emf) is generated in a way that an electric current flows against the temperature gradient. As mentioned before, the reverse is also true. If a current is applied to the conductor, electrons move and interestingly carry thermal energy. Therefore, a heat flow occurs in the opposite direction of the current, which is shown in Figure 1.1.

Figure 1.1 Electron concentrations in a thermoelectric material.

Source: Lee (2017) [1]/John Wiley & Sons

In many applications, a number of thermocouples, each of which consists of p-type and n-type semiconductor elements, are connected electrically in series and thermally in parallel by sandwiching them between two high-thermal conductivity but low-electrical conductivity ceramic plates to form a module, which is shown in Figure 1.2.

Figure 1.2 Cutaway of a typical thermoelectric module.

Source: Lee (2010) [2]/John Wiley & Sons

Figure 1.3 A thermocouple

Consider two wires made from different metals joined at both ends, as shown in Figure 1.3, forming a closed circuit. Ordinarily, nothing will happen. However, when one of the junctions is heated, something interesting happens. Current flows continuously in the circuit. This is the Seebeck effect, in honor of Thomas Seebeck, who made this discovery in 1821. A circuit that incorporates both thermal and electrical effects is called a thermoelectric circuit. A thermocouple uses the Seebeck effect to measure temperature, and the effect forms the basis of a thermoelectric generator.

In 1834, Jean Peltier discovered the reverse of the Seebeck effect by demonstrating that cooling can take place by applying a current across the junction. Heat pumping is possible without a refrigerator or compressor. Thermal energy can possibly be converted into electrical energy without a turbine or an engine.

There are some advantages of thermoelectric devices in spite of their low thermal efficiency. There are no moving parts in the device; therefore, there is less potential for failure in operation. The controllability of heating and cooling is very attractive in many applications such as lasers, optical detectors, medical instruments, and microelectronics.

1.2 Thermoelectric Effect

The thermoelectric effect consists of three effects: the Seebeck effect, the Peltier effect, and the Thomson effect.

1.2.1 Seebeck Effect

The Seebeck effect is the conversion of a temperature difference into an electric current. As shown in Figure 1.3, wires A and B are joined at both ends, and a voltmeter is inserted in wire B. Suppose that a temperature difference is imposed between two junctions, then a potential difference or voltage V will appear on the voltmeter. The potential difference is proportional to the temperature difference. The potential difference V is

(1.1)

where ΔT = Th − Tc and αAB = αA − αB; αAB is called the Seebeck coefficient (also called the thermopower), which is usually measured in μV/K. The sign of α is positive if the electromotive force, emf, tends to drive an electric current through wire A from the hot junction to the cold junction as shown in Figure 1.3. In practice one rarely measures the absolute Seebeck coefficient because the voltage meter always reads the relative Seebeck coefficient between wires A and B. The absolute Seebeck coefficient can be calculated from the Thomson coefficient.

1.2.2 Peltier Effect

When the current flows across a junction between two different wires, it is found that heat must be continuously added or subtracted at the junction in order to keep its temperature constant, which is illustrated in Figure 1.4. The heat is proportional to the current flow and changes the sign when the current is reversed. Thus, the Peltier heat absorbed or liberated is

(1.2)

where πAB is the Peltier coefficient and the sign of πAB is positive if the junction at which the current enters wire A is heated and the junction at which the current leaves wire A is cooled. The Peltier heating or cooling is reversible between heat and electricity. This means that heating (or cooling) will produce electricity and electricity will produce heating (or cooling) without a loss of energy.

1.2.3 Thomson Effect

When the current flows as shown in Figure 1.4, heat is absorbed in wire A due to the negative temperature gradient and liberated in wire B due to the positive temperature gradient, which is experimental observation [3], depending on the material. The Thomson heat is proportional to both the electric current and the temperature gradient, which is schematically shown in Figure 1.4. Thus, the Thomson heat absorbed or liberated across a wire is

(1.3)

where τ is the Thomson coefficient. The Thomson coefficient is unique among the three thermoelectric coefficients because it is the only thermoelectric coefficient directly measurable for individual materials. There is another form of heat, called Joule heating (I2R), which is irreversible and is always generated as the current flows in a wire. The Thomson heat is reversible between heat and electricity.

1.2.4 Thomson (or Kelvin)...

Produkt-Details

• Erscheint lt. Verlag: 11.4.2025
• Sprache: englisch
• Themenwelt: Technik ► Maschinenbau
• Schlagworte: Seebeck coefficient • thermoelectric cooler • Thermoelectric design • thermoelectric figure merit • Thermoelectric Generator • thermoelectric heat pump • Thermoelectric refrigerator • thermoelectric system design • thermoelectric transport
• ISBN-10: 1-394-31736-0 / 1394317360
• ISBN-13: 978-1-394-31736-3 / 9781394317363