Technology & Engineering
Thermoelectric
Thermoelectric technology involves the conversion of temperature differences into electric voltage or vice versa, known as the Seebeck and Peltier effects. This technology has applications in power generation, cooling, and heating systems. It is based on the thermoelectric properties of certain materials that can generate electricity when there is a temperature gradient.
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eBook - PDFThermal Design
Heat Sinks, Thermoelectrics, Heat Pipes, Compact Heat Exchangers, and Solar Cells
- HoSung Lee(Author)
- 2022(Publication Date)
- Wiley(Publisher)
5 Thermoelectric Design 5.1 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 temperature. Basically, there are two devices: Thermoelectric generators (TEGs) and Thermoelectric coolers (TECs). These devices have no moving parts and require no maintenance. TEGs s 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 in deep space and mountaintop telecommunication sites. TECs provide refrigeration and temperature control in electronic packages and medical instruments. Thermoelectrics have become increasingly important with numerous applications. Since Thermoelectricity was discovered in the early nineteenth century, there has not been much improvement in efficiency or materials until the recent development of nanotechnology, which has led to a remarkable improve- ment 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 poten- tial difference could be produced by a circuit made from two dissimilar wires when one of the junctions was heated. This is called 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.
- Saim Memon(Author)
- 2019(Publication Date)
- IntechOpen(Publisher)
When the sides of TE materials are exposed to different temperatures, then a voltage is created across the two sides of the material. Conversely, when a voltage is applied, a temperature difference can be created, known as the “ Peltier effect ” . At the atomic scale, when a temperature gradient is applied at the two end sides of a thermocouple, the electrons and holes move faster, and they have a lower density at the hot side, resulting in diffusion of electrons/holes towards the cold side as schematically demonstrated in Figure 2 . This movement of carriers (electrons for n-type and holes for p-type materials) is translated into the generation of an electric field across the thermocouple. This is called as the “ Seebeck effect ” , and the voltage created for a temperature difference, Δ T , under thermodynamic equilibrium is S Â Δ T , where S is the Seebeck coeffi-cient. TE materials are therefore one potential candidate for harvesting waste ther-mal energy, due to their ability to convert it into electricity, even under very-low-temperature gradients relative to the environmental temperature. This technology exhibits distinct advantages over other energy-harvesting technologies: (i) TE con-version is reliable and operates in silence as it works without mechanical move-ment, (ii) it is an environmentally friendly green technology, since no heat and no gaseous or chemical wastes are produced during operation, and (iii) it can be widely Figure 1. Schematic representation of different sectors contributing to large amounts of wasted thermal energy. 8 Advanced Thermoelectric Materials for Energy Harvesting Applications used in places where other energy conversion technologies are unavailable, such as in the remote outer space, etc. [6, 7]. The “ Thermoelectric effect ” encompasses three distinct effects: (a) the Seebeck effect, (b) the Peltier effect and (c) the Thomson effect.
eBook - PDF- Lidong Chen, Ruiheng Liu, Xui Shi(Authors)
- 2020(Publication Date)
- Elsevier(Publisher)
C H A P T E R 1 General principles of Thermoelectric technology 1.1 Introduction The first Thermoelectric effect, namely the Seebeck effect, was discov-ered in 1821, which describes the electromotive force generated by the temperature difference. In the following thirty years or more, Peltier effect and Thomson effect were successively discovered. These effects are the three main physical effects in Thermoelectric technology that describe the direct conversion between thermal and electrical energies [1 3] . Although the discoveries of both Seebeck and Peltier effects were made using a circuit composed of two different conductors and the effects were only observed at the junctions between dissimilar conduc-tors, they are actually the bulk properties of the materials involved, not the interfacial phenomena. Solid state physics developed in the follow-ing century reveals that all the three Thermoelectric effects originate from the energy difference of carriers in different materials and/or in the different parts of materials under different temperatures. Thomson built the relationship among the three effects, and devel-oped the basic thermodynamic theories for Thermoelectric effects [3] . Thomson’s work showed that a circuit composed of two conductors with positive and negative Seebeck coefficients (usually called the ther-mocouple) is a type of heat engine. Such heat engine can generate elec-trical power by virtue of the temperature difference, or pump heat to realize refrigeration. However, since the reversible Thermoelectric effects are always accompanied by the irreversible Joule heat and heat conduc-tion, its energy conversion efficiency is principally low. Thermoelectric effects have been widely used for temperature calibrations as thermo-couples, but they had no practical application as heat engine, and there 1 Thermoelectric Materials and Devices DOI: https://doi.org/10.1016/B978-0-12-818413-4.00001-6 Copyright © 2021 China Science Publishing & Media Ltd.
eBook - PDF- Charles R. Russell(Author)
- 2013(Publication Date)
- Pergamon(Publisher)
Electrochemical pre-parations of some elements were accomplished with the minute currents produced in this way. The only use of Thermoelectricity over the next century was for temperature measurements. Even in this application the important advantages of Seebeck's mineral semiconductors were overlooked. The next step in the history of Thermoelectricity came in 1834 when Peltier, a French watchmaker, observed that a thermal effect is produced by an electric current flowing through a junc-tion of dissimilar conductors. However, Peltier also failed to comprehend the fundamental nature of his observation. He concluded only that Ohm's law might not apply for very weak currents. A revival of interest in Thermoelectric power came with the development after 1939 of synthetic semiconductor materials hav-ing Thermoelectric effects orders of magnitude greater than those ThermoelectricITY 275 in metals. The application of these new materials to Thermoelectric systems was recognized by Maria Telka who made a generator of some 5 percent efficiency using zinc antimonide and lead sulfide. Thermoelectric PHENOMENA The flow of heat and the flow of electrons through a material are related. The flow of electrons transports some thermal energy and the flow of heat transports some electricity. In addition are the irreversible conversion of electrical energy into heat by electri-cal resistance, designated as Joule or PR heating, and the irrever-sible flow of heat by thermal conduction. A Thermoelectric generator is illustrated in Fig. 7-2. Two dissim-ilar materials, n and p, are connected by metal conductors mak-ing good thermal and electrical contact at the hot and cold junctions. The electrical energy generated from thermal energy does work in an external circuit; the remainder of the thermal energy is rejected from the cold junction at 0 . The potential depends upon both the temperature difference and the properties of the materials.
- Peter Spies, Markus Pollak, Loreto Mateu, Peter Spies, Markus Pollak, Loreto Mateu(Authors)
- 2015(Publication Date)
- Jenny Stanford Publishing(Publisher)
Chapter 6 Thermoelectric Generators Robert Hahn a and Jan D. K¨ onig b a Fraunhofer Institute Reliability and Microintegration, Gustav-Meyer-Allee 25, 13355 Berlin, Germany b Fraunhofer IPM, Heidenhofstrasse 8, 79110 Freiburg, Germany [email protected], [email protected] 6.1 Physical Principles The discovery of Thermoelectricity was done a long ago. In 1821, Thomas J. Seebeck observed that the needle of a compass was deflected in the vicinity of two metallic conductors connected to one another when different temperatures prevailed at the joints. The degree of deflection here was proportional to the temperature difference. The reason for the movement of the compass needle was an electrical field that had apparently been created by the difference in temperature between the conductors. The effect observed by Seebeck also functions in the opposite direction and was first described by Jean C. A. Peltier in 1834: If electricity is applied to the two connected conductors, a temperature gradient occurs at the contact points. Heat energy is transported from one connection point to the other, leading to a cooling effect. Handbook of Energy Harvesting Power Supplies and Applications Edited by Peter Spies, Loreto Mateu, and Markus Pollak Copyright c 2015 Pan Stanford Publishing Pte. Ltd. ISBN 978-981-4241-86-1 (Hardcover), 978-981-4303-06-4 (eBook) www.panstanford.com 218 Thermoelectric Generators 6.1.1 The Seebeck Effect The Seebeck effect is the phenomenon underlying the conversion of heat energy into electrical power. Its physical significance can be appreciated by considering the effect of imposing a temperature gradient along a finite conductor. Without temperature gradient, the carriers in the conductor have a distribution according to the thermal equilibrium Fermi–Dirac distribution.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Academic Studio(Publisher)
This effect can be used to generate electricity, to measure temperature, to cool objects, or to heat them or cook them. Because the direction of heating and cooling is determined by the polarity of the applied voltage, Thermoelectric devices make very convenient temperature controllers. Traditionally, the term Thermoelectric effect or Thermoelectricity encompasses three separately identified effects, the Seebeck effect , the Peltier effect , and the Thomson effect . In many textbooks, Thermoelectric effect may also be called the Peltier–Seebeck effect . This separation derives from the independent discoveries of French physicist Jean Charles Athanase Peltier and Estonian-German physicist Thomas Johann Seebeck. Joule heating, the heat that is generated whenever a voltage is applied across a resistive material, is somewhat related, though it is not generally termed a Thermoelectric effect (and it is usually regarded as being a loss mechanism due to non-ideality in Thermoelectric devices). The Peltier–Seebeck and Thomson effects can in principle be thermodynamically reversible, whereas Joule heating is not. Seebeck effect The Seebeck effect is the conversion of temperature differences directly into electricity. Seebeck discovered that a compass needle would be deflected when a closed loop was formed of two metals joined in two places with a temperature difference between the junctions. This is because the metals respond differently to the temperature difference, which creates a current loop, which produces a magnetic field. Seebeck, however, at this time did not recognize there was an electric current involved, so he called the phenomenon the thermomagnetic effect, thinking that the two metals became magnetically polarized by the temperature gradient. The Danish physicist Hans Christian Ørsted played a vital role in explaining and conceiving the term Thermoelectricity.
eBook - PDFLow-Grade Heat Harvesting
Materials, Devices, and Technologies
- Xiaogang Zhang(Author)
- 2023(Publication Date)
- Wiley-VCH(Publisher)
21 2 Conventional Thermoelectric Devices for Low-Grade Heat Harvesting 2.1 Basic Structure and Working Principle of Electronic Thermoelectric Device The traditional Thermoelectric devices are designed in series and in parallel with the “π” structure of P-type materials and N-type materials to ensure the required output power. The main carrier of P-type Thermoelectric materials is a hole, and the car- rier movement direction is the same as the current direction. The main carrier of N-type Thermoelectric materials is electrons, and the carrier movement direction is opposite to the current direction. Therefore, in Thermoelectric devices with a fixed hot end and cold end, the series connection of P-type Thermoelectric material and N-type Thermoelectric material can realize the sequence of current direction and provide a more powerful electrical output. Benefiting from rational Thermoelectric designs, significant progress has been made on high-performance and cost-effective Thermoelectric materials. Figure 2.1 presents the reported state-of-the-art thermo- electric materials with maximum ZT values (ZT max ), cost, and cost-effectiveness (ZT max /cost). 2.1.1 Working Principle of Electronic Thermoelectric Devices The reason why the traditional Thermoelectric material energy can be converted to each other is based on three effects. These three effects are not independent but are linked by Kelvin’s relationship. At the same time, they are also the basis for ther- moelectric devices to realize the mutual conversion of thermal energy and electric energy. 2.1.1.1 Seebeck Effect The Seebeck effect is a phenomenon that can convert heat energy into electric energy. The basic principle can be shown in Figure 2.2a. Two different conductor materials A and B are joined to form a circuit. If the temperature of the two joint ends is maintained at T 1 and T 2 , respectively.
eBook - PDF- Nathan Snyder(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
The introduction to Thermoelectric generator design given in the next section forms the basis for the selection and evaluation of materials. In the last section of the paper, some indication of the pos-sible performance of Thermoelectric devices using presently known materials will be given. II. DESIGN CONSIDERATIONS A schematic diagram for a Thermoelectric generator is shown schematically in Fig. 1. The maximum efficiency of this device, defined as power dissipated in the load R ^ ~ rate of heat removal from hot reservoir is given by (see, for instance, Ref. 1 or 2) T h (1 + z T) 1 ' 2 + (T c /T h ) where T = — (T + T, ). If the two arms of the device have the 2 N c h' same resistivity p, thermal conductivity κ, and the same magnitude but opposite sign for their Seebeck coefficients a, the figure of merit is given by z = α 2 /ρκ (2) Note that the only way that the properties of the material affect the efficiency is through this figure of merit. A number of assumptions (Ref. 2) have been made in deriving Eq· 1; those which are most important for the considerations of this paper are (a) the resistance of the contacts between the Thermoelectric materials and the heat reservoirs has been taken as negligibly small; (b) the ratio of the area of the n arm to the area of the p arm and the load resistance have been adjusted to the value which maximizes the efficiency; and (c) there is perfect thermal insulation, i.e. there is no heat loss from the hot reservoir except through the Thermoelectric arms, n and p. 28 ENERGY CONVERSION FOR SPACE POWER If it is desired to have maximum power output per unit weight rather than maximum efficiency, the requirements for the Thermoelectric materials are somewhat different. A consideration of the design of the Thermoelectric unit only shows that the material parameter which should be as large as possible (Ref. 3) is α /ρ instead of z.
eBook - PDF- Jianyong Ouyang(Author)
- 2022(Publication Date)
- Wiley(Publisher)
An external electricity source is connected to the two legs, and electrical current flows from the n -type leg to the p -type leg. The curved arrows indicate the transportation of the charge carriers from the cold side to the hot side in the Thermoelectric legs. 1 Fundamental Knowledge on Thermoelectric Materials 36 where R is the electrical resistance of the Thermoelectric leg, K the thermal conductance of the Thermoelectric material, and Δ T = T H − T C . The input power by the electricity includes the heat pumping and the Joule heat, P S TI I R in = + Δ 2 . (1.74) The ratio of the heat absorbed at the cold side to the input electrical power is the energy efficiency of cooling. It is called as the coefficient of perfor-mance (COP), C OP heat absorbed electrical power input = = --ST I I R K T S C t h 1 2 2 Δ Δ TI I R + 2 . (1.75) As shown in Eq. (1.73), the rate of heat pumping is a quadratic equation of the current I . There is a maximum rate of heat pumping. The correspond-ing current at the maximum heat pumping rate is given by I ST R c max . = (1.76) At this maximum cooling rate, the corresponding COP is given by C OP H C max . = -1 2 2 ZT T ZT T C Δ (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 I S T R ZT max ( . ′ = + -Δ 1 1 (1.78) The maximum COP is shown by the Eq. (1.79), C OP C H C H C max . ′ = + - -( ) + + ( ) T ZT T T T T ZT 1 1 1 (1.79) The maximum temperature difference between the hot and cold sides is given by 1.5 Summary 37 T T ZT C H C -( ) = 1 2 2 (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.
eBook - PDF- Qiang Xu, Tetsuhiko Kobayashi, Qiang Xu, Tetsuhiko Kobayashi(Authors)
- 2015(Publication Date)
- CRC Press(Publisher)
131 Development of Thermoelectric Technology from Materials to Generators three. example. cases,. the. TE. material. dimensions. and. number. of. p–n. pairs. are. optimized.here . .The.thermal.conditions.were.set.to. Q h =.400.kW/m 2 ,. T 0 .=.1200.K,. T 1 .=.800.K,. T 2 .=.500.K,.and. T 3 .=.300.K . .The.heat.from.the.hot.surface.of.the.module. simulates.the.case.of.waste.heat.recycling.in.the.burner-type.furnace,.where.the. top.surface.of.the.module.is.heated.at.a.very.high.temperature.as.the.TE.module . . The.temperature.conditions.are.temporarily.fixed.to.apply.the.targeted.materials . . In.addition,. n 2 .and. n 3 .were.varied.although. n 1 .is.fixed.as.100.for.all.calculations . . These.boundary.conditions.are.not.so.far.from.the.realistic.conditions.when.we. have.to.consider.1D.analysis . Figure.4 .40a .through.c.shows.the.calculated.ratio,. l i ,.of.the.elements.at.all.the. stages.for.Cases.1,.2,.and.3,.respectively . .As.shown.in.Figure.4 .40a, .the.ratio,. l 1 ,. seems.flat.without.any.strong.dependency.of. n 2 .and. n 3 . .In.order.to.satisfy.the.con-dition.that.the.terminal.temperatures,. T 0 ,. T 1 ,. T 2 ,.and. T 3 .are.set.constant,.the.ther-mal.resistance.in.a.stage.should.be.constant.even.if.the.number.of.TE.elements. n . increases. .The.fact.that.the.ratio. l .should.decrease.as.shown.in.Figure.4 .40b .and. c.as. n .increases.indicates.that.the.TE.elements.should.be.longer.and.thinner.as. n . Number of p–n pairs of third stage, n 3 Ratio of A to d of second stage, l 1 p (mm) Number of p–n pairs of second stage, n 2 400 400 200 0 200 0 0.0 0.5 1.0 (a) Number of p–n pairs of third stage, n 3 Ratio of A to d of second stage, l 2 p (mm) Number of p–n pairs of second stage, n 2 400 400 200 0 200 0 0.0 0.5 1.0 (b) Number of p – n pairs of third stage, n 3 Ratio of A to d of third stage, l 3 p (mm) Number of p – n pairs of second stage, n 2 400 400 200 0 200 0 0.0 0.5 1.0 (c) FIGURE 4.40 Calculated.ratio.
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