Thermoelectric technology assessment: application to air conditioning and refrigeration.
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.
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 ([K.sup.-1]) and, therefore, often appears as a dimensionless product with an absolute temperature, ZT. The figure-of-merit Z or ZT is the key parameter that determines the efficiency of thermoelectric devices. A plot of the COP versus cold-side temperature, [T.sub.C], for a different figure-of-merit, ZT, is shown in Figure 4. The Carnot cycle efficiency is also presented for comparison. Thermoelectric devices approach Carnot cycle efficiency for infinite ZT.
[FIGURE 4 OMITTED]
It can be seen from Figure 4 that thermoelectric modules, based on commercially available materials that have a ZT of about 1, cannot compete in efficiency with traditional vapor-compression systems when operating at a relatively large temperature lift ([T.sub.H] - [T.sub.C]), e.g., 30[degrees]C. However, the efficiency of thermoelectric modules increases rapidly with decreasing temperature lift. This is different from traditional vapor-compression systems where the pressure drop of refrigerant flow always has a finite value. It is expected that the efficiency of thermoelectric modules could eventually surpass that of traditional vapor-compression and possibly all other competing concepts when the temperature lifts are small, such as 5[degrees]C. This feature of thermoelectric technology can be used to significantly enhance the efficiency and cooling capacity of traditional vapor-compression systems, which will be discussed later in this paper.
A broad search has been under way to identify new materials with high figure-of-merit ZT. The best ZT materials are found in heavily-doped semiconductors, as shown in Figure 5. Insulators have poor electrical conductivity that leads to a low thermoelectric effect. Metals are also poor thermoelectric materials because of their low Seebeck coefficient and high thermal conductivity. Thermoelectric materials that are most relevant to air conditioning and refrigeration are discussed in the following sections.
[FIGURE 5 OMITTED]
[Bi.sub.2][Te.sub.3]-Based Thermoelectric Materials. The commercially available, best, and simple compound thermoelectric material for refrigeration around room temperature is bismuth telluride ([Bi.sub.2][Te.sub.3]), which was developed in the late 1950s (2), (4), (17-23). One of the most interesting features of [Bi.sub.2][Te.sub.3] is that its Seebeck coefficient depends on its composition, as shown in Figure 6. The undoped [Bi.sub.2][Te.sub.3] (corresponding to 60 atomic percent Te) is p-type with a Seebeck coefficient of about 230[mu]V/K. As the concentration of Te increases, the Seebeck coefficient gradually falls to zero and then changes its sign and becomes negative (n-type). The maximum ZTs of p-type and n-type [Bi.sub.2][Te.sub.3] crystalline materials (not alloys) at room temperature are about 0.75 and 0.86, respectively.
[FIGURE 6 OMITTED]
The ZT of crystalline [Bi.sub.2][Te.sub.3] can be improved through alloying (3), (19-21), (26-29). There is complete solid solubility among the three compounds [Bi.sub.2][Te.sub.3], [Sb.sub.2][Te.sub.3], and [Bi.sub.2][Se.sub.3]. The addition of [Sb.sub.2][Te.sub.3] or [Bi.sub.2][Se.sub.3] to [Bi.sub.2][Te.sub.3] improves the ZT mainly by reducing its lattice thermal conductivity without causing too much degradation of electronic properties. It turns out that the optimal compositions for thermoelectric refrigeration are normally [Bi.sub.0.5][Sb.sub.1.5][Te.sub.3] and [Bi.sub.2][Te.sub.2.7][Se.sub.0.3] for the p-type and n-type, respectively. The commonly used [Bi.sub.2][Te.sub.3]-based alloys are summarized in Figure 7, among which the highest ZT is around 1.1.
[FIGURE 7 OMITTED]
Lead Telluride-Based Thermoelectric Materials. Lead telluride (PbTe) is typically used for thermoelectric refrigeration or cooling at higher temperatures, around 500-800 K, than [Bi.sub.2][Te.sub.3] (28), (29), (33-36). Both n-type and p-type PbTe samples can be produced by making PbTe departures from its stoichiometry and also by addition of impurities, such as the chalogenides ([PbCl.sub.2], [PbBr.sub.2], [PbI.sub.2], etc.) and chalcogenides (SnTe, [Bi.sub.2][Te.sub.3], [TaTe.sub.2], [MnTe.sub.2], etc.) as donors and alkali metals ([Na.sub.2]Te, [K.sub.2]Te, etc.) as acceptors.
The Z value of PbTe is just over 1 x [10.sup.-3] [K.sup.-1] at room temperature but eventually surpasses that of [Bi.sub.2][Te.sub.3] at higher temperatures. PbTe alloys, rather than the simple compound, are usually used, which can be formed by substituting Sn for Pb to form [Pb.sub.1-x][Sn.sub.x]Te or Se for Te to form Pb[Te.sub.x][Se.sub.1-x]. It is possible to obtain a value of ZT slightly above unity for n-type PbTe alloys at a temperature of about 600 K, but the ZT for the p-type PbTe alloys is only about 0.7, as shown in Figure 8.
[FIGURE 8 OMITTED]
Nanostructured Thermoelectric Materials. The use of nanostructures for thermoelectric applications was triggered by conceptual studies in the early 1990s that identified the potential benefits of quantum confinement of electrons and phonons and phonon interface scattering (37-41). Since then, much attention has been paid to the development of nanostructures for enhancing ZT (16), (42-61). Figure 9 shows some samples of nanostructures currently under study.
[FIGURE 9 OMITTED]
The prediction of ZT enhancement in low-dimensional materials has been experimentally demonstrated in [Bi.sub.2][Te.sub.3]/[Sb.sub.2][Te.sub.3] superlattices (52) and [PbSeTe/PbTe] quantum-dot superlattices (54). [Bi.sub.2][Te.sub.3]/[Sb.sub.2][Te.sub.3] superlattices were reported to have a ZT of ~2.5 around room temperature, the highest ZT to date. [PbSeTe/PbTe] quantum-dot superlattices exhibit a ZT of ~2.0 at elevated temperature (about 500 K). Recently, a significant ZT increase has been reported in bulk materials made from nanocrystalline powders of p-type BiSbTe, a peak ZT of 1.4 at 100[degrees]C (212[degrees]F) (61). This nanocomposite fabrication method is cost effective and can be scaled for mass production. Figure 10 is a snapshot of state-of-the-art thermoelectric materials, including both bulk and nanostructured materials.
[FIGURE 10 OMITTED]
Thermoelectric Cooling System
Construction of Thermoelectric Modules. A single-stage thermoelectric module or device is typically composed of thermoelectric elements (n- and p-types) that are connected electrically in series and thermally in parallel, as shown in Figure 11. The ceramic plates form the cold and hot surfaces of the module, providing mechanical integrity and both electrical insulation and thermal conduction to the heat sink and the object to be cooled. The plates are commonly made of alumina, but when large lateral heat transfer is required, higher thermal conductivity materials such as beryllia and aluminum nitride are desired. There are also few thermoelectric modules with no ceramic plates to support the thermoelectric elements. This arrangement could eliminate the thermal resistance associated with the ceramic plates, but it may reduce the mechanical rigidity of the system.
[FIGURE 11 OMITTED]
A multistage module arrangement is needed when a large temperature difference is required. The multistage module arrangement is essentially two or more single-stage modules stacked on top of each other. The lower stage requires greater cooling power to pump the heat dissipated by the upper stage, so the multistage module is pyramid shape, as shown in Figure 12.
[FIGURE 12 OMITTED]
Commercial Thermoelectric Modules. Commercial thermoelectric modules are available in a wide variety of sizes, shapes, operating voltages and currents, and cooling capacities. Most thermoelectric modules are not larger than 50 mm (2 in.) in length due to mechanical constraints. The module tends to become concave, similar to a deformed thermostatic bimetallic element, when its two sides are subject to different temperatures. The applied thermal stress could cause a crack or break in the materials due its thermal expansion. For this reason, when the heat exchanger surface is very large or a large cooling capacity is required, several modules are assembled onto heat exchangers rather than mounting one large module, e.g., in the application of air conditioning and refrigeration.
The commercial thermoelectric modules are typically characterized by four parameters: [I.sub.max], [DELTA][T.sub.max], [Q.sub.c-max], and [V.sub.max]:
[I.sub.max] = the DC current that yields the maximum junction temperature difference [DELTA][T.sub.max] when the heat load is zero, A
[DELTA][T.sub.max] = the maximum junction temperature difference across the module at [I.sub.max] without heat load. [DELTA][T.sub.max] of commercial single-stage modules is about 70[degrees]C (158[degrees]F) with the hot-junction temperature at room temperature, [degrees]C ([degrees]F)
[Q.sub.c-max] = the cooling power when the module operates at [DELTA]T = 0 and I = [I.sub.max], W
[V.sub.max] = the terminal voltage for [I.sub.max] without heat load, V
The performance and geometry of typical thermoelectric modules are summarized in Table 1 (62-66). Some micromodules, e.g., the microbulk module, HWD0500-4040, possess a cooling power density of more than 30 [W/[cm.sup.2]].
Table 1. Specifications of Commercial Thermoelectric Modules Performance Model [DELTA]T, [Q.sub.max], [I.sub.max], [degrees]C W A DT12-2.5 74 23 2.5 RC12-4 74 36 3.7 HWD0500-4040 58 513 60 CP 2-31-06L 67 29.3 14.0 CP 2-127-10L 68 77.1 9.0 CP 2-127-06L 67 120 14.0 930-7 66 1.8 3.7 960-127 66 26.0 3.0 TE-7-1.0-2.5 72 1 1.9 TE-32-2.8-1.5 70 60 24.4 TE-83-1.0-1.3 69 22.5 3.6 KSM-06127A 73 67.2 6 KSM09071C 73 51.2 9 MI2011T 87 0.47 0.7 2 CP 085 100-31-20 77 9.74 5.9 TE-2-(ll-4)-1.5 93 0.4 1 MI3021T 109 0.33 1.3 3 CP 040 065-127-71-31 96 6.48 1.8 TE-3-(31-ll-4)-1.5 109 0.4 0.9 4 CP 055 065-127-71-31-17 107 6.84 3.1 MI4012T 129 0.76 1 TE-4-(83-18-4-l)-1.3 131 0.5 2.8 5 CP 055 065-127-71-31-17-7 118 3.37 3.0 K5MB002 161 1.8 5.2 MI6030 150 0.58 3.6 6 CP 055 065-127-71-31-17-7-2 131 1.22 3.0 Performance Dimension, mm Model [V.sub.max], Base Base Top Top V Width Lengt Width Length DT12-2.5 14.7 30.0 34.0 30.0 30.0 RC12-4 14.7 30.0 34.0 30.0 30.0 HWD0500-4040 15.2 44 40 40 40 CP 2-31-06L 3.8 30 30 30 30 CP 2-127-10L 15.4 62 62 62 62 CP 2-127-06L 15.4 62 62 62 62 930-7 0.8 9.6 9.6 9.6 9.6 960-127 15.4 30.0 30.0 30.0 30.0 TE-7-1.0-2.5 0.9 8 8 8 8 TE-32-2.8-1.5 4 40 40 40 40 TE-83-1.0-1.3 10.3 22 19 22 19 KSM-06127A 16.5 40 40 40 40 KSM09071C 9.2 44 44 44 44 MI2011T 1.9 6.60 6.60 3.96 3.96 2 CP 085 100-31-20 3.8 23 26 30 30 TE-2-(ll-4)-1.5 1.3 6 4 2 4 MI3021T 1.9 6.60 6.60 2.54 2.54 3 CP 040 065-127-71-31 15.4 15 15 30 30 TE-3-(31-ll-4)-1.5 3.5 10 8 2 4 4 CP 055 065-127-71-31-17 14.6 15 15 40 40 MI4012T 6.7 13.21 17.17 4.06 7.98 TE-4-(83-18-4-l)-l.3 8.9 24 20.6 4.5 2.4 5 CP 055 065-127-71-31-17-7 14.5 10 10 40 40 K5MB002 18 5.3 5.3 19.25 20.75 MI6030 6.3 0.85 21.72 28.27 5.21 6 CP 055 065-127-71-31-17-7-2 14.5 5 5 40 40 Model Height # Company of Stages DT12-2.5 4.0 1 Marlow RC12-4 3.4 1 Marlow HWD0500-4040 1.2 1 Marlow CP 2-31-06L 4.6 1 Melcor CP 2-127-10L 5.6 1 Melcor CP 2-127-06L 4.6 1 Melcor 930-7 4.7 1 TECA 960-127 3.6 1 TECA TE-7-1.0-2.5 4.8 1 TEtech TE-32-2.8-1.5 4 1 TEtech TE-83-1.0-1.3 3.6 1 TEtech KSM-06127A 3.8 1 Komatsu KSM09071C 5.5 1 Komatsu MI2011T 4.29 2 Marlow 2 CP 085 100-31-20 10.7 2 Melcor TE-2-(ll-4)-1.5 < 6.7 2 TEtech MI3021T 5.38 3 Marlow 3 CP 040 065-127-71-31 9.5 3 Melcor TE-3-(31-ll-4)-1.5 < 9.3 3 TEtech 4 CP 055 065-127-71-31-17 13.8 4 Melcor MI4012T 8.71 4 Marlow TE-4-(83-18-4-l)-l.3 <13.6 4 TEtech 5 CP 055 065-127-71-31-17-7 16.9 5 Melcor K5MB002 10.3 5 Komatsu MI6030 5.21 6 Marlow 6 CP 055 065-127-71-31-17-7-2 20.1 6 Melcor
Thermoelectric System Accessories. The accessories of a thermoelectric system include power supplies, temperature controllers, thermal interface materials, and heat exchangers, as discussed in the following paragraphs.
The direct current (DC) power sources can be battery/battery piles or power converters. The operating power of thermoelectric modules ranges from hundreds of milliwatts to hundreds of watts. The quality of DC current could affect the performance of thermoelectric modules. To achieve the optimum performance of the modules, the DC current should be smooth with little ripple or noise. Unfiltered, fully rectified alternating current (AC) voltage has a ripple factor of approximately 48% that may decrease the modules' performance by as much as 23%. A ripple factor of less than 10% will result in less than 1% degradation in the maximum achievable temperature difference (63), (67).
Temperature controllers are needed for those applications requiring a high degree of temperature stability. Linear proportional, proportional-integral (PI), or proportional-integral-derivative (PID) controllers are typically used and integrated with thermoelectric modules (68), (69). PI control is better suited for the thermal load and ambient temperature with large variation. PID control is more complex than PI and is typically used when large thermal loads must be removed/controlled immediately. With these temperature controllers, it is possible to acquire temperature accuracy within 0.01[degrees]C (0.0018[degrees]F) or even down to 0.001[degrees]C (0.00018[degrees]F) (70).
Thermal interface material (TIM) is as important as any other component of a thermoelectric system. Its function is to minimize contact resistance between thermoelectric modules and heat source/heat sink. With heat transfer, thermal interface material is a bottleneck for conducting heat from and to the thermoelectric modules. Therefore, it directly affects system performance as well as heat exchanger sizing.
Adequate heat sinking is required to dissipate the heat load and power of the module to avoid an excessive hot-side temperature rise. A typical design consideration might be to limit the temperature difference between the heat sink and the ambience in the range of 10[degrees]C to 20[degrees]C (18[degrees]F to 36[degrees]F) (71), (72). An inefficient heat sink may significantly decrease the COP of the whole refrigeration system. Moreover, mechanical failures could occur where the solder joints on the hot side of the pellets are melted due to inadequate heat transfer to the heat sinks.
POTENTIAL APPLICATIONS OF THERMOELECTRIC TECHNOLOGY TO AIR CONDITIONING AND REFRIGERATION
The concept of solid-state refrigeration dates back almost to the very beginning of the field of thermoelectrics. Since the discovery of the Peltier effect in the early 19th century, it has been possible to construct a solid-state cooling mechanism. Given currently available materials, thermoelectric technology is suited for applications where its compact size, high reliability, absence of moving parts, and silent operation outweigh its relatively low efficiency. Examples of applications it can be used for include cooling diode lasers (73-75), cooling electronics (76-78), portable refrigerators (79), (80), cooling/heating car seats (81), (82), and even air conditioning in a railroad passenger car (83-85).
However, thermoelectric technology has been very seldom used in large-scale air conditioning and refrigeration, due mainly to its relatively low efficiency when compared with traditional vapor-compression systems (86-90). Thermoelectric systems might become more economically competitive if the heat load is varied frequently, such as in a train carriage. An assessment of thermoelectric air conditioning has been carried out by Stockholm and Pujol-Soulet (86), who installed a thermoelectric system in a train carriage in France (86). Other successful large-scale applications include a 27 kW industrial water cooler (87).
Instead of utilizing a full-fledged thermoelectric cooling system, it is possible to use thermoelectric technology to improve the performance of traditional vapor-compression systems, so called "hybrid systems" or "integrated systems" (91-94). These hybrid systems take advantage of the fact that thermoelectric systems exhibit excellent efficiency at small temperature lifts, as shown in Figure 4. For example, the COP could reach a value of 10 for a temperature lift of 5 K. This behavior is different from vapor-compression systems and other systems that involve fluid flow where the pressure drop will always have a finite value. As suggested by several groups, the hybrid systems are expected to be the most productive neat-term applications of thermoelectric technology in large-scale air conditioning and refrigeration. The following sections offer more detailed discussions of hybrid systems.
Thermoelectrically Enhanced Liquid Subcooling
Figure 13 shows a schematic of a hybrid system in which a vapor-compression cycle is integrated with a thermoelectric subcooling element (indicated after the condenser). In this hybrid system, the thermoelectric module is used to cool the liquid refrigerant down to a temperature below that of the heat sink. Most importantly, the thermoelectric subcooler can operate at a higher COP than the original vapor-compression system because of its small temperature lifts. The thermoelectric subcooler could be even more efficient if a staged thermoelectric device, rather than a single module, is utilized, as indicated in Figure 13. The first few thermoelectric elements after the condenser outlet provide a small amount of subcooling with a very high COP, thus increasing the total performance of the subcooling device.
[FIGURE 13 OMITTED]
The performance simulation of a hybrid system with a staged thermoelectric subcooler was carried out, and the results for COP and cooling capacity are shown in Figures 14a and 14b, respectively (91), (92), (94). It can be seen in Figure 14a that up to a subcooling level of about 15 K, the COP of the hybrid system improves at a decreasing rate and reaches a maximum of about 11%. While the COP peaks at 15 K of subcooling, about a 20% capacity increase could be achieved according to Figure 14b. These simulation results suggest that the introduction of thermoelectric subcooling can significantly enhance efficiency and capacity of the conventional vapor-compression system without adding any moving parts. The thermoelectric subcooler could also be seen as a simple add-on for an existing system, just for the purpose of increasing capacity.
[FIGURE 14 OMITTED]
Thermoelectrically Enhanced Heat Exchangers
Thermoelectric modules can also be used to enhance the efficiency of heat exchangers (94), (95). An example is shown in Figure 15, where the thermoelectric modules are inserted between the tube and the fins. This can be done by simply using a flat tube (also called a microchannel) heat exchanger. During operation, the thermoelectric module pumps heat from the tube to the fin, at very high efficiency when the temperature lift is small, and consequently increases the fin temperature. For a condenser, for example, a few degrees increase in the temperature of the fins would lead to a considerable increase in efficiency of the respective vapor-compression system or a considerable reduction in heat exchanger size.
[FIGURE 15 OMITTED]
Some observations on thermoelectric technology, especially those relevant to large scale air conditioning and refrigeration.
* Thermoelectric modules are solid-state electronic devices that directly convert electricity to temperature difference. Thermoelectric devices have no moving parts and therefore are inherently reliable and require little maintenance. Furthermore, the lack of refrigerants used in the systems provides many benefits to the environment as well as for packaging and safety.
* The use of thermoelectric devices and systems has been limited by their relatively low energy conversion efficiency. Present commercially available thermoelectric devices operate at about 10% of Carnot efficiency if used as home refrigerators, whereas compressor-based refrigerators usually operate at about 30% of Carnot efficiency.
* A broad search for thermoelectric materials with high efficiency has been conducted. Currently, there is no known theoretical impediment to significant increases in thermoelectric energy conversion efficiency. A breakthrough in thermoelectric materials could spark many applications that use thermoelectric technology as a safe, efficient, and reliable alternative.
* Thermoelectric technology is suitable for applications where its compact size, reliability, absence of moving parts, and silent operation outweigh its relatively low efficiency. Thermoelectric devices have been used in situations where the heat load is small (e.g., <25 W), the required temperature lift is small (e.g., <10[degrees]C [18[degrees]F]), or the variation of the heat load is large (e.g., train passenger cabin). It is important to note that the COP of thermoelectric modules increases significantly with decreasing temperature lift, as shown in Figure 4.
* Instead of utilizing a fully designed thermoelectric cooling system, it is also possible to use a small thermoelectric system as a subcooler to improve the performance of a traditional system. This is a "hybrid system" since it combines a solid-state cooling device together with a conventional vapor-compression-type air conditioning and refrigeration.
A = cross section area, [m.sup.2] ([in.sup.2])
I = electrical current, A
K = thermal conductance, [W/K]
Q = heat current or heat flow rate, W
R = electrical resistance, [V/A]
T = temperature, K or [degrees]C ([degrees]F)
V = electric voltage, V
W = electric power, W
x = position, m (in.)
ZT = thermoelectric figure-of-merit
[alpha] = Seebeck coefficient, V/K
[lambda] = thermal conductivity, [W/m].K (W/in..[degrees]F)
[PI] = Peltier coefficient, V
[rho] = electric resistivity, ohm[??]m (ohm.in.)
[sigma]= electric conductivity, [1/ohm.m] (1/ohm.in.)
[tau] = Thomson coefficient, [V/K]
[phi] = coefficient of performance (COP)
c = cold side
n = n-type thermoelectric materials
p = p-type thermoelectric materials
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Received December 2, 2007; accepted May 12, 2008
Bao Yang, PhD
Thanh N. Tran, PhD
Bao Yang is an assistant professor and Herwin Ahuja is a graduate student at the Center for Environmental Energy Engineering, Department of Mechanical Engineering, University of Maryland, College Park, MD. Thanh N. Tran is a scientist at the Naval Surface Warfare Center, Carderock Division, West Bethesda, MD.
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|Author:||Yang, Bao; Ahuja, Herwin; Tran, Thanh N.|
|Publication:||HVAC & R Research|
|Date:||Sep 1, 2008|
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