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Article Excerpt
INTRODUCTION
Because of environmental concerns such as global warming, ozone depletion, and a lack of energy efficiency, it is necessary to investigate alternative cooling technologies to the refrigeration that uses refrigerants (1). Thermoelectric cooling and heat pumping are alternatives that have recently attracted attention. Thermoelectric devices are solid-state devices in which electrons or holes--equivalent to refrigerants in traditional vapor-compression systems--carry electricity and thermal energy under an electric field (2-8). Therefore, they have many inherent, attractive features such as a long life and no moving parts, and they don't emit toxic gases, are lightweight, are low-maintenance, and are very reliable. In the past decade, there has been rapid development when it comes to the fundamental theory, materials, and devices related to thermoelectrics. This paper provides a critical review of thermoelectric technology and assesses its potential applications in air conditioning and refrigeration. It should be noted that the information in this paper is influenced by the research focus of the present authors and reflects their assessment of the field.
The remainder of this paper is structured as follows: the next section provides the fundamentals of thermoelectric technology, and then the potential application of thermoelectric technology to air conditioning and refrigeration will be discussed.
FUNDAMENTALS OF THERMOELECTRIC TECHNOLOGY
In this section, thermoelectric effects will be discussed first, then the overview of thermoelectric materials, modules, and systems will be presented.
Thermoelectric Effects
In thermoelectric materials, electrons and holes operate as both charge and energy carriers. Thermoelectric cooling is the direct conversion of electric voltage to temperature difference. The related effects include the Seebeck, Peltier, and Thomson effects. The Peltier and Seebeck effects are reversals of one another.
Seebeck Effect. The Seebeck effect was discovered by Thomas Seebeck in 1821. It is associated with the generation of a voltage along a conductor subject to a temperature gradient (9). As shown in Figure 1, if a temperature gradient, [DELTA]T = ([T.sub.2]-[T.sub.1]) or [DELTA]T = ([T.sub.cold]-[T.sub.hot]), applies to a conductor, an electromotive force (EMF), [DELTA]V = ([V.sub.2]-[V.sub.1]), will occur between the hot and cold ends due to charge carrier diffusion and phonon drag. The whole system is in semi-equilibrium; chemical potential due to the concentration is balanced by the built-in electrostatic potential, namely the Seebeck voltage. The Seebeck coefficient of the conductor is defined as
[FIGURE 1 OMITTED]
[alpha] = -[[DELTA]V]/[[DELTA]T],
with a positive value when the electrical carriers are holes. Thermoelectric power generators are based on this phenomenon. Note that the Seebeck coefficient is sometimes called the thermal EMF coefficient or thermoelectric power.
Peltier Effect. The thermoelectric cooling phenomenon is physically based on the Peltier effect, which was discovered by Jean Peltier in 1834-13 years after the Seebeck effect was unveiled (10). The Peltier coefficient is a measure of the amount of heat carried by electrons or holes. This amount of heat is proportional to the electrical current flowing in the circuit. The proportionality constant is defined as the Peltier coefficient
[PI] = [Q/I],
where Q is the heat current and I is the electrical current.
When two different materials are joined together to form a loop, as shown in Figure 2, there will be an abrupt change in heat flow at the junctions because the two materials have different Peltier coefficients. The excess energy released to the lattice causes heating; the deficiency in energy supplied by the lattice causes cooling. An interesting consequence of this effect is that the direction of heat transfer is controlled by the polarity of the electric current. Reversing the electric polarity will change the direction of transfer and, thus, the sign of the heat absorbed/evolved.
[FIGURE 2 OMITTED]
The Peltier effect is the principle at work behind thermoelectric modules (also called Peltier coolers) or refrigerators that are used for transferring heat from one side of the device to the other.
Thomson Effect. The Thomson effect describes the heating or cooling of a current-carrying material subject to a temperature gradient and was discovered by William Thomson (Lord Kelvin) in 1851 (11). Any current-carrying conductor with a temperature difference between two points will either absorb or emit heat, depending on the material. The Thomson coefficient [tau] is defined as
[[dQ]/[dx]] = [tau]I[[dT]/[dx]],
where [[dQ]/[dx]] is the rate of the heating per unit length, I is the electrical current, and [[dT]/[dx]] is the temperature gradient.
Kelvin Relations. It is of great importance in thermoelectric theory that there exist thermodynamic relationships between these thermoelectric coefficients, called the Kelvin Relations or Thomson Relations (12):
[PI] = [alpha]T
[tau] = Td[alpha]/[dT]
Thermoelectric Element (4), (6), (13-16)
An element of a thermoelectric module consists of p and n branches, as shown in Figure 3. When a current I flows through this thermoelectric element, the total heat flow, Q, within each branch (p or n) is expressed as:
[FIGURE 3 OMITTED]
[Q.sub.p] = [[alpha].sub.p]TI-[[lambda].sub.p][A.sub.p][[dT]/[dx]]
[Q.sub.n] = [[alpha].sub.n]TI-[[lambda].sub.n][A.sub.n][[dT]/[dx]]
where A is the section area of each branch, dT/dx is the temperature gradient, and [lambda] is the thermal conductivity. The coefficient of performance (COP) can be expressed as the quotient of the total cooling power, [Q.sub.C], by the electric power W:
[phi] = [[Q.sub.C]/W] = [[([[alpha].sub.p]-[[alpha].sub.n])[IT.sub.C]-K[DELTA]T-0.5[I.sup.2]R]/[I[([[alpha].sub.P]-[[alpha].sub.n])[DELTA]T + IR]]]
where K and R are the total thermal conductance and the total electrical resistance in the circuit, respectively.
By setting [dQ.sub.C]/[dI]] = 0, the maximum cooling power can be obtained:
[Q.sub.max] = [[[([[alpha].sub.p]-[[alpha].sub.n]).sup.2][T.sub.C.sup.2]]/[2R]]-K[DELTA]T
By setting [Q.sub.C] = 0, the maximum temperature difference [DELTA][T.sub.max] can be obtained:
[DELTA][T.sub.max] = [[[([[alpha].sub.p]-[[alpha].sub.n]).sup.2][T.sub.C.sup.2]]/[2KR]]
where [T.sub.C] is the temperature at the cold side of thermoelectric element.
By setting [d[phi]/dI] = 0, the maximum COP can be obtained:
[[phi].sub.max] = [[[T.sub.C][[[(1 + Z[T.sub.M])].sup.[1/2]]-([T.sub.H]/[T.sub.C])]]/[([T.sub.H]-[T.sub.C])[[[(1 + Z[T.sub.M])].sup.[1/2]] + 1]]]
where [T.sub.M] is the average temperature of the hot side, [T.sub.H], and the cold side, [T.sub.C].
The figure-of-merit of the thermoelectric element is defined as
Z = [[([[alpha].sub.p]-[[alpha].sub.n]).sup.2]/[[[([[rho].sub.p][[lambda].sub.p]).sup.[1/2]] + [([[rho].sub.n][[lambda].sub.n]).sup.[1/2]]].sup.2]],
where [rho] is the electrical resistivity of the thermoelectric materials.
For a single n- or p-type thermoelectric material, the figure-of-merit reduces to
Z = [[[alpha].sup.2]/[[rho][lambda]]].
The figure-of-merit Z has a unit of inverse Kelvin...
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