**Heat Exchangers for Thermoelectric Devices**

David Astrain and Álvaro Martínez *Public University of Navarre* 

*Spain* 

#### **1. Introduction**

288 Heat Exchangers – Basics Design Applications

[22] T.T. Dang, J.T. Teng and J.C. Chu (2010): Effect of flow arrangement on the heat

[24] T.T. Dang and J.T. Teng (2010): Numerical and experimental studies of the impact of

[27] T.T. Dang, J.T Teng, and J.C. Chu, Influence of Gravity on the Performance Index of

[28] T.T. Dang and J.T. Teng (2010): Effect of the substrate thickness of counter-flow

[29] T. Dang and J.T. Teng (2011): The effects of configurations on the performance of

[30] T.T. Dang and J.T. Teng (2011): Comparison on the heat transfer and pressure drop of

[31] T.T. Dang and J.T. Teng (2010): Numerical simulation of a microchannel heat

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Heat exchangers play an important role in the performance of thermal machines, namely, electric power generators, engines and refrigerators. Regarding thermoelectrics, this influence is even higher, owing to the difficulty of transferring heat from the small surface area of a typical thermoelectric module to a bigger one. Particularly, in the hot face of an average 40 mm x 40 mm Peltier module, the heat flux readily yields 40600 W/m2. The thermoelectric effects, namely, Joule, Seebeck, Peltier and Thomson, describe the interaction between thermal and electric fields, and are well known since the XIX century (Rowe, 2006).

German physicist Thomas J. Seebeck discovered in 1821 that an electric circuit composed of two dissimilar conductors *A* and *B* connected electrically in series and exposed to a thermal gradient induces an electric current -or an electromotive force (*EAB*) if the circuit is openedwhich depends on the materials and the temperature difference between junctions (*∆T*). This phenomenon is called **Seebeck effect**, characterized by the *Seebeck coefficient* .

$$
\alpha\_{AB} = \frac{\Delta E\_{AB}}{\Delta T} = \alpha\_A - \alpha\_B \tag{1}
$$

Likewise, in 1834, French physicist Jean Peltier discovered that if an electrical current (*I*) is applied across the electric circuit composed of two dissimilar conductors, the inverse effect takes place, that is, heating occurs at one junction whereas cooling occurs at the other. This phenomenon is called **Peltier effect**, described by the *Peltier coefficient π*.

$$
\dot{Q}\_P = \pm I \pi\_{AB} = \pm I \, T \left( \alpha\_B - \alpha\_A \right) \tag{2}
$$

In 1851, William Thomson stated the *Thomson effect*, which indicates that a homogeneous material exposed to thermal and electrical gradients absorbs or generates heat. Moreover, he described the relation between Seebeck and Peltier effects, given by *Thomson coefficient τ*.

$$
\tau\_A - \tau\_B = -T\frac{\partial \alpha\_A}{\partial T} + T\frac{\partial \alpha\_B}{\partial T} = T\frac{\partial}{\partial T}(\alpha\_B - \alpha\_A) \tag{3}
$$

The possibility of using thermoelectric devices to produce electric power was raised by John W. Strutt in 1885. Subsequently, between 1909 and 1911, Edmund Altenkirch proved that thermoelectric materials must feature high Seebeck coefficient (), high electrical conductivity () and low thermal conductivity (), in order for the material to retain heat in

Heat Exchangers for Thermoelectric Devices 291

presented different types of heat exchangers specifically designed for dissipating high heat fluxes from the cold and the hot side of thermoelectric devices. After that, the chapter studies the improvement in the efficiency of thermoelectric devices achieved with these heat exchangers. Finally, the concept of thermoelectric self-refrigeration is introduced; this application uses thermoelectric technology for the refrigeration and temperature control of a

A thermoelectric pair can be used to generate electric power, since Seebeck effect indicates that if the junctions of two thermoelectric legs type "p" and "n" are exposed to different temperatures, an electric current is induced. On the other hand, if an external electric source supplies power to the thermoelectric pair, Peltier effect states that one junction absorbs heat whereas the other one generates heat, so that the thermoelectric pair performs like a thermal machine that receives electric work, removes heat from a cold reservoir and emits heat to a hot reservoir. There are in the market different types of Peltier modules, composed of several thermoelectric pairs connected electrically in series and thermally in parallel. Figure 1 shows an average thermoelectric module working as refrigerator. In order to improve the heat transfer both in the hot and the cold side, a heat exchanger must be installed at either

device, without electricity consumption.

**2. Influence of heat exchangers on thermoelectric devices** 

side of the Peltier module to increase the heat transfer area.

Fig. 1. Sketch of a Peltier module working as refrigerator.

The Peltier module is a small device that emits –or absorbs- large amounts of heat, so that the heat density or heat flux is significantly high. The face of a Peltier module is so small that increasing the heat transfer surface area of the heat exchanger (finned dissipator and cold plate in Figure 2) is virtually useless, since the effectiveness of the heat exchanger

the junctions and minimize losses due to Joule effect. These three parameters were combined to form the Figure of merit (*Z = 2/*), key parameter in the characterization of thermoelectric materials. By then, further developments had been rejected because of the low efficiencies attained, and it was not until the application of semiconductor materials to thermoelectric devices by Abram F. Ioffe in 1957, that thermoelectric technology contemplated its major breakthrough. Since that moment, scientific efforts focused on increasing the Figure of merit via new thermoelectric materials.

Although the thermoelectric effects were discovered almost two centuries ago, the application of thermoelectric technology to either heating or cooling (Peltier effect), and electric power generation (Seebeck effect) was not relevant until the fifties of the last century, when this technology was successfully used for military and aerospace purposes. The application to other fields was then rejected because of the high price of thermoelectric materials, but now has become a reality. In this regard, some in-depth reviews on the state of the art of thermoelectric technology can be found in the literature (Goldsmid, 1964, 1986, 1995; Riffat & Xiaoli, 2003). Nowadays, the successful development of thermoelectrics for civil purposes depends mainly on two aspects: thermoelectric materials development and heat exchangers thermal design. Whereas the first one intends to increase the Figure of merit and efficiency of the devices via new thermoelectric materials, the second one focuses on enhancing the heat transfer via improving the heat exchangers.

Thermoelectric technology presents significant advantages with respect to common devices used for refrigeration or electric power generation, since thermoelectric devices have no moving parts (no compressor, turbine, etc. must be installed), which makes them virtually noiseless and increases their lifespan to a great extent. Furthermore, thermoelectric devices are easily and accurately controlled. All these advantages, along with the fact that the prices of Peltier modules are constantly decreasing, boosted the development of highly interesting thermoelectric applications, competing nowadays in the civil market with good prospects for the future (Bell, 2008; Chang et al., 2009; Chein & Huang, 2004; Gordon et al., 2002; Hongxia & Lingai, 2007; Khattab & El Shenawy, 2006; Martínez et al., 2010; Min & Rowe, 1999, 2006; Omer et al., 2001; Riffat et al., 2006; Vian et al., 2002; Vian & Astrain, 2009a, 2009b; Yang & Stabler, 2009; Yodovard et al., 2001). Regarding the last comment, it is common knowledge that efficiency of thermoelectric devices represents the key point to bear in mind, in order for these prospects to become reality.

A proper analysis of thermoelectric applications requires detailed studies on heat transfer between the thermoelectric modules, the heat source and the heat sink. In this sense, wrong selection of either the dissipation method (natural or forced convection, thermosyphons, etc.) or the refrigerant (air, water, eutectic fluids, etc.) leads to poor heat transfer and finally to low efficiencies. Although published improvements on heat transfer processes for other fields of knowledge are very common in scientific literature, thermoelectric developers have not been able to use all this information and apply it to the thermoelectric field, though this fact is being corrected nowadays. Thus, several studies have come out recently which address the application of different dissipation techniques to thermoelectric modules (Astrain et al., 2003, 2005, 2010; Knight et al., 1991; Omer et al., 2001; Ritzer & Lau, 1994, 2000; Rowe et al., 1995, Stockholm & Stockholm, 1992; Vian & Astrain, 2008, 2009a).

This chapter shows in the first place the influence of heat exchangers on the performance of both thermoelectric generation and thermoelectric refrigeration devices. Then, there are

the junctions and minimize losses due to Joule effect. These three parameters were

thermoelectric materials. By then, further developments had been rejected because of the low efficiencies attained, and it was not until the application of semiconductor materials to thermoelectric devices by Abram F. Ioffe in 1957, that thermoelectric technology contemplated its major breakthrough. Since that moment, scientific efforts focused on

Although the thermoelectric effects were discovered almost two centuries ago, the application of thermoelectric technology to either heating or cooling (Peltier effect), and electric power generation (Seebeck effect) was not relevant until the fifties of the last century, when this technology was successfully used for military and aerospace purposes. The application to other fields was then rejected because of the high price of thermoelectric materials, but now has become a reality. In this regard, some in-depth reviews on the state of the art of thermoelectric technology can be found in the literature (Goldsmid, 1964, 1986, 1995; Riffat & Xiaoli, 2003). Nowadays, the successful development of thermoelectrics for civil purposes depends mainly on two aspects: thermoelectric materials development and heat exchangers thermal design. Whereas the first one intends to increase the Figure of merit and efficiency of the devices via new thermoelectric materials, the second one focuses on

Thermoelectric technology presents significant advantages with respect to common devices used for refrigeration or electric power generation, since thermoelectric devices have no moving parts (no compressor, turbine, etc. must be installed), which makes them virtually noiseless and increases their lifespan to a great extent. Furthermore, thermoelectric devices are easily and accurately controlled. All these advantages, along with the fact that the prices of Peltier modules are constantly decreasing, boosted the development of highly interesting thermoelectric applications, competing nowadays in the civil market with good prospects for the future (Bell, 2008; Chang et al., 2009; Chein & Huang, 2004; Gordon et al., 2002; Hongxia & Lingai, 2007; Khattab & El Shenawy, 2006; Martínez et al., 2010; Min & Rowe, 1999, 2006; Omer et al., 2001; Riffat et al., 2006; Vian et al., 2002; Vian & Astrain, 2009a, 2009b; Yang & Stabler, 2009; Yodovard et al., 2001). Regarding the last comment, it is common knowledge that efficiency of thermoelectric devices represents the key point to

A proper analysis of thermoelectric applications requires detailed studies on heat transfer between the thermoelectric modules, the heat source and the heat sink. In this sense, wrong selection of either the dissipation method (natural or forced convection, thermosyphons, etc.) or the refrigerant (air, water, eutectic fluids, etc.) leads to poor heat transfer and finally to low efficiencies. Although published improvements on heat transfer processes for other fields of knowledge are very common in scientific literature, thermoelectric developers have not been able to use all this information and apply it to the thermoelectric field, though this fact is being corrected nowadays. Thus, several studies have come out recently which address the application of different dissipation techniques to thermoelectric modules (Astrain et al., 2003, 2005, 2010; Knight et al., 1991; Omer et al., 2001; Ritzer & Lau, 1994,

2000; Rowe et al., 1995, Stockholm & Stockholm, 1992; Vian & Astrain, 2008, 2009a).

This chapter shows in the first place the influence of heat exchangers on the performance of both thermoelectric generation and thermoelectric refrigeration devices. Then, there are

), key parameter in the characterization of

*2/*

combined to form the Figure of merit (*Z =* 

increasing the Figure of merit via new thermoelectric materials.

enhancing the heat transfer via improving the heat exchangers.

bear in mind, in order for these prospects to become reality.

presented different types of heat exchangers specifically designed for dissipating high heat fluxes from the cold and the hot side of thermoelectric devices. After that, the chapter studies the improvement in the efficiency of thermoelectric devices achieved with these heat exchangers. Finally, the concept of thermoelectric self-refrigeration is introduced; this application uses thermoelectric technology for the refrigeration and temperature control of a device, without electricity consumption.
