**3. Heat exchangers analysis**

Once we have demonstrated the enormous importance that thermal resistances of heat exchangers have on the performance of thermoelectric devices, it is of high interest to show the most significant designs of heat exchangers applied to thermoelectrics.

#### **3.1 Finned dissipator**

This type of heat exchanger represents the most used heat dissipation system in thermoelectric refrigeration, essentially because of its low manufacturing cost. However, this is not the best option whatsoever. Major problems of this design relate to constriction thermal resistances (Lee et al., 1995), which are inherent to the small surface areas of Peltier modules. This fact entails that a significant surface area of the dissipator is useless, as can be seen at the top of Figure 5.

Some works (Astrain & Vian, 2005) have already addressed the optimization of a finned dissipator for the hot end of a Peltier module. The optimization parameters were: position of the module, position and type of fan, thickness of the dissipator base, and height of the fins. The most outstanding conclusions were:


Fig. 4. Electric power generated (Pmax) versus thermal resistance of the hot side heat

Once we have demonstrated the enormous importance that thermal resistances of heat exchangers have on the performance of thermoelectric devices, it is of high interest to show

This type of heat exchanger represents the most used heat dissipation system in thermoelectric refrigeration, essentially because of its low manufacturing cost. However, this is not the best option whatsoever. Major problems of this design relate to constriction thermal resistances (Lee et al., 1995), which are inherent to the small surface areas of Peltier modules. This fact entails that a significant surface area of the dissipator is useless, as can be

Some works (Astrain & Vian, 2005) have already addressed the optimization of a finned dissipator for the hot end of a Peltier module. The optimization parameters were: position of the module, position and type of fan, thickness of the dissipator base, and height of the fins.

 When the fan is placed at one end of the dissipator so that the air crosses the dissipator from one side to the other, the optimal position of the module is not the exact centre but a bit closer to the fan. Moreover, this fact gains significance as the air flow increases. If an axial fan is installed over the dissipator, the thermal resistance decreases by 5.5 %

 Increasing the base thickness of the dissipator leads to a decrease in the thermal resistance without affecting the pressure losses. Specifically, if this parameter increases from 8 mm to 16 mm, the thermal resistance decreases by 13.2 %. However, this fact

exchanger (Rdc) and thermal resistance of the cold side heat exchanger (Rdf).

the most significant designs of heat exchangers applied to thermoelectrics.

**3. Heat exchangers analysis** 

**3.1 Finned dissipator** 

seen at the top of Figure 5.

The most outstanding conclusions were:

with respect to the previous case.

also leads to heavier and more expensive dissipators.

 Increasing the height of the fins is also beneficial from both a thermal and hydrodynamic point of view. Specifically, if this parameter rises from 40 mm to 60 mm in a dissipator with an axial fan over it, the thermal resistance reduces by 10.4 %.

Fig. 5. Temperature distribution in a finned dissipator, with and without thermosyphon.

Finally, this work presents a prototype thermoelectric refrigerator that served to experimentally prove that the COP improves by 10 % if the thermal resistance of the heat exchanger installed at the hot side of the Peltier modules decreases by 13 %.

In conclusion, this work makes evident the important role that holds the thermal resistance of the heat exchangers in the efficiency of a thermoelectric refrigeration device. Likewise, it also indicates that the thermal resistance of a finned dissipator is too high despite the optimization process. This fact indicates that it is absolutely necessary to design new types of heat exchangers in order to reduce even more the thermal resistance and increase the efficiency of thermoelectric devices. In this line of work, there have been developed the phase-change thermosyphons, described in the following section.

#### **3.2 Thermosyphon for the hot end of a Peltier module**

A thermosyphon is a hermetically sealed container in the shape of a straight prism, enclosing a fluid. The Peltier module is attached to the bottom of the rear surface, so that the heat flux produced by the module is transmitted to the fluid, which begins to boil. Vapour produced in the process rises up to the top of the thermosyphon by natural convection. Likewise, the cold reservoir (usually the ambient) is connected to the front surface of the thermosyphon, where several fins are installed. Thus, when the vapour touches this cold

Heat Exchangers for Thermoelectric Devices 299

290 295 300 305 310 315 **Ambient temperature (K)**

Fig. 7. Thermal resistance of a real thermosyphon and a similar finned dissipator versus

et al., 2006a). Figure 10 shows a sketch of a thermoelectric device that incorporates a cylindrical TSV for the hot side of the Peltier module. Its basic concept is similar to that behind the TSF, so that a deposit for the liquid and a condensation zone must be included in the design. The latter represents the major difference with respect to the TSF, since it must be cylindrical now, thus increasing the heat transfer surface area, which makes TSV work properly with natural convection. Experimental values of TSV's thermal resistances are showed in Figure 8, where they are compared with those obtained with a TSF for both

At the cold side of a Peltier module, the problem remains similar to that at the hot side, though in this case the heat flux is not emitted but absorbed by the module, and the objective is to improve the heat transfer between the thermoelectric module and the

Like in the previous case, the most used heat exchanger is a finned dissipator due to its low cost. However, new designs combining thermosyphon and capillarity lift technologies have been proposed, such as the Bosch-Siemens patented thermosyphon TMP (Astrain et al., 2006b), which improves significantly the thermal resistance of this heat exchanger. The TMP is installed in the refrigerator so that one face is attached to the cold end of the module, and the opposite face is inside the refrigeration chamber. This thermosyphon increases the heat transfer surface area from the small surface of the Peltier module to the significantly bigger surface area of a finned dissipator, taking advantage of the high heat transfer inherent to

Rtot tsf Rtot disip

**3.3 Thermosyphon and capillarity lift for the cold end of a Peltier module** 

refrigeration chamber of a thermoelectric refrigerator.

0.04

ambient temperature.

natural and forced convection.

0.06

0.08

0.1

**R tot. (K/W)**

0.12

0.14

0.16

0.18

surface, it cools down, then condensates and finally gravity makes it go down to the bottom of the thermosyphon. As a result, the fluid forms a cycle completely closed and selfsufficient. Figure 6 describes the process.

Fig. 6. Phase-change thermosyphon for the hot end of a Peltier module.

The heat flux emitted by the module (*Qc*) is uniformly distributed along the base area of the finned dissipator, as can be seen at the bottom of Figure 5, thus increasing significantly the efficiency of the system. Likewise, the heat flux produced by the condensation process (*Qh*) is transferred to the ambient. A fan enhances the heat transfer.

Figure 7 presents experimental values of thermal resistances of a prototype thermosyphon (called TSF) attached to a commercial 40 mm x 40 mm Peltier module, for different ambient temperatures, along with the thermal resistance of a similar-in-weight commercial dissipator (Astrain et al., 2003). It can be seen that the thermal resistance of this TSF decreases as the ambient temperature increases, owing to the fact that the boiling and condensation coefficients improve with temperature. This thermosyphon attains a thermal resistance ranging from 0.125 ºC/W for 20 ºC of ambient temperature to 0.079 ºC/W for ambient temperature 35 ºC. This leads to an improvement in the dissipation by 23.8 % at 20 ºC, and 51.4 % at 35 ºC of ambient temperature, with respect to the values obtained with a similar commercial dissipator. This heat exchanger was installed in a prototype thermoelectric refrigerator and the COP increased by 21.3 % for ambient temperature 19 ºC, and 36.5 % for ambient temperature 30 ºC.

As indicated before, the major advantage of thermoelectric technology with respect to vapour compression refrigeration lies on the reduction in the number of moving parts, since no compressor needs to be installed. However, the thermosyphon TSF does need a fan. Further designs present optimized thermosyphons that require no fans at all, thus removing all the moving parts, such as the Bosch-Siemens patented thermosyphon called TSV (Astrain

surface, it cools down, then condensates and finally gravity makes it go down to the bottom of the thermosyphon. As a result, the fluid forms a cycle completely closed and self-

Fig. 6. Phase-change thermosyphon for the hot end of a Peltier module.

is transferred to the ambient. A fan enhances the heat transfer.

and 36.5 % for ambient temperature 30 ºC.

The heat flux emitted by the module (*Qc*) is uniformly distributed along the base area of the finned dissipator, as can be seen at the bottom of Figure 5, thus increasing significantly the efficiency of the system. Likewise, the heat flux produced by the condensation process (*Qh*)

Figure 7 presents experimental values of thermal resistances of a prototype thermosyphon (called TSF) attached to a commercial 40 mm x 40 mm Peltier module, for different ambient temperatures, along with the thermal resistance of a similar-in-weight commercial dissipator (Astrain et al., 2003). It can be seen that the thermal resistance of this TSF decreases as the ambient temperature increases, owing to the fact that the boiling and condensation coefficients improve with temperature. This thermosyphon attains a thermal resistance ranging from 0.125 ºC/W for 20 ºC of ambient temperature to 0.079 ºC/W for ambient temperature 35 ºC. This leads to an improvement in the dissipation by 23.8 % at 20 ºC, and 51.4 % at 35 ºC of ambient temperature, with respect to the values obtained with a similar commercial dissipator. This heat exchanger was installed in a prototype thermoelectric refrigerator and the COP increased by 21.3 % for ambient temperature 19 ºC,

As indicated before, the major advantage of thermoelectric technology with respect to vapour compression refrigeration lies on the reduction in the number of moving parts, since no compressor needs to be installed. However, the thermosyphon TSF does need a fan. Further designs present optimized thermosyphons that require no fans at all, thus removing all the moving parts, such as the Bosch-Siemens patented thermosyphon called TSV (Astrain

sufficient. Figure 6 describes the process.

Fig. 7. Thermal resistance of a real thermosyphon and a similar finned dissipator versus ambient temperature.

et al., 2006a). Figure 10 shows a sketch of a thermoelectric device that incorporates a cylindrical TSV for the hot side of the Peltier module. Its basic concept is similar to that behind the TSF, so that a deposit for the liquid and a condensation zone must be included in the design. The latter represents the major difference with respect to the TSF, since it must be cylindrical now, thus increasing the heat transfer surface area, which makes TSV work properly with natural convection. Experimental values of TSV's thermal resistances are showed in Figure 8, where they are compared with those obtained with a TSF for both natural and forced convection.

## **3.3 Thermosyphon and capillarity lift for the cold end of a Peltier module**

At the cold side of a Peltier module, the problem remains similar to that at the hot side, though in this case the heat flux is not emitted but absorbed by the module, and the objective is to improve the heat transfer between the thermoelectric module and the refrigeration chamber of a thermoelectric refrigerator.

Like in the previous case, the most used heat exchanger is a finned dissipator due to its low cost. However, new designs combining thermosyphon and capillarity lift technologies have been proposed, such as the Bosch-Siemens patented thermosyphon TMP (Astrain et al., 2006b), which improves significantly the thermal resistance of this heat exchanger. The TMP is installed in the refrigerator so that one face is attached to the cold end of the module, and the opposite face is inside the refrigeration chamber. This thermosyphon increases the heat transfer surface area from the small surface of the Peltier module to the significantly bigger surface area of a finned dissipator, taking advantage of the high heat transfer inherent to

Heat Exchangers for Thermoelectric Devices 301

*Q <sup>C</sup>* .

Liquid

TSV

TSV

Assembly system

Liquid fluid return

g

Peltier module

Cold side of Peltier device

> Hot side of Peltier device

Fig. 9. Performance of the TMP.

Fins

Heat flow from the refrigerated

Heat

room

Porous material Capillary ascension

exchange Steam

Fig. 10. Thermoelectric device with the heat exchangers TSV and TMP.

TPM

Peltier module TPM

Fig. 8. Thermal resistances of TSF and TSV for natural and forced convection.

phase-change processes, capillarity lift through porous materials and gravity pulling down condensed liquids.

As can be seen in Figure 9, the TMP basically consists of a watertight compartment and a porous layer attached to one of its inner faces. When heat is absorbed from the refrigerated chamber, the liquid evaporates and transfers this heat to the cold end of the Peltier module. The porous layer makes the fluid at the bottom of the TMP ascend by capillarity, surmounting gravity, thus making use of all the surface area of the TMP for the evaporation process. Vapour formed ascends by natural convection, condenses near the cold face of the Peltier module and goes down as liquid pulled by gravity, thus forming a completely closed and self-sufficient cycle.

Subsequently, this TMP was incorporated into a prototype of thermoelectric refrigerator, which served to assess the improvement attained with respect to a similar thermoelectric refrigerator including a finned dissipator for the cold side of the Peltier modules (Vian & Astrain, 2009b). The TMP had a thermal resistance of 0.323 K/W when a small fan with 0.75 W of electric power consumption was installed in the refrigeration compartment. In the same conditions, a finned dissipator similar in size to the TMP provided a significantly higher thermal resistance of 0.513 K/W. Likewise, it was experimentally proved that the COP of the thermoelectric refrigerator endowed with a TMP increases by 32 % with respect to the COP of this refrigerator but including a finned dissipator at the cold side of the Peltier module.

Fig. 8. Thermal resistances of TSF and TSV for natural and forced convection.

condensed liquids.

and self-sufficient cycle.

module.

phase-change processes, capillarity lift through porous materials and gravity pulling down

As can be seen in Figure 9, the TMP basically consists of a watertight compartment and a porous layer attached to one of its inner faces. When heat is absorbed from the refrigerated chamber, the liquid evaporates and transfers this heat to the cold end of the Peltier module. The porous layer makes the fluid at the bottom of the TMP ascend by capillarity, surmounting gravity, thus making use of all the surface area of the TMP for the evaporation process. Vapour formed ascends by natural convection, condenses near the cold face of the Peltier module and goes down as liquid pulled by gravity, thus forming a completely closed

Subsequently, this TMP was incorporated into a prototype of thermoelectric refrigerator, which served to assess the improvement attained with respect to a similar thermoelectric refrigerator including a finned dissipator for the cold side of the Peltier modules (Vian & Astrain, 2009b). The TMP had a thermal resistance of 0.323 K/W when a small fan with 0.75 W of electric power consumption was installed in the refrigeration compartment. In the same conditions, a finned dissipator similar in size to the TMP provided a significantly higher thermal resistance of 0.513 K/W. Likewise, it was experimentally proved that the COP of the thermoelectric refrigerator endowed with a TMP increases by 32 % with respect to the COP of this refrigerator but including a finned dissipator at the cold side of the Peltier

Fig. 9. Performance of the TMP.

Fig. 10. Thermoelectric device with the heat exchangers TSV and TMP.

Heat Exchangers for Thermoelectric Devices 303

 Two thin film heating resistors with dimensions 80 mm x 80 mm x 0.5 mm, each one capable of providing 150 W at 200 ºC, connected in series to a controllable DC power source. These elements serve to generate a controllable and measurable heat flux, and

 A 220 mm x 160 mm x 32 mm aluminium plate composed of two pieces screwed to each other, the bottom one endowed with two similar cavities, wherein the heating resistors

Four Peltier modules Kryotherm TGM-287-1.0-1.5, with dimensions 40 mm x 40 mm x

 An aluminium finned dissipator composed of a square base plate, with side length 155 mm and height 12 mm, and 23 fins with dimensions 155 mm x 23 mm x 1.5 mm. One rectangular aluminium prism is installed between the modules and the dissipator in order to separate the device from the dissipator and avoid thermal bridges between them, which would decrease the efficiency of the system. This element is 55 mm long

This prototype served to conduct several experimental tests in order to study the thermal resistance between the heat source and the ambient, and compare it to that obtained when only the dissipator was mounted over the heat source (no modules, no fan), and finally compare it to the thermal resistance between the heat source and the ambient when no cooling system was mounted. Figure 13 shows the comparison between these three thermal resistances as functions of the heat flux generated by the heating resistors. As expected, the highest thermal resistance is achieved when no cooling system is attached to the device. More interesting is the fact that the TSC system always outperforms the dissipator alone, especially when the heat flux generated by the device exceeds 130 W. For lower values of heat flux, the electric power generated by the Peltier modules does not suffice to operate the fan. However for heat fluxes higher than 130 W, the electric power generated by the

A DC fan type SUNON KDE1208PTS1-6, and a wind tunnel over the dissipator.

represent the heat source that must be cooled.

3.8 mm, and capable of working at 225 ºC.

and has a squared base area with side length 80 mm.

Fig. 12. Sketch of a thermoelectric self-cooling system.

are installed.

Figure 10 shows the sketch of a prototype thermoelectric refrigerator including the two types of thermosyphon explained along this section, for either end of the Peltier module.

Likewise, Figure 11 provides two photographs of this prototype, indicating the cited heat exchangers. This prototype served to conclude that including the developed thermosyphons (TSV and TMP) in a thermoelectric refrigerator, the COP increased by 66% with respect to that obtained with a similar thermoelectric refrigerator endowed with finned dissipators (Vian & Astrain 2009b).

Fig. 11. Photographs of the prototype with heat exchangers TMP and TSV.
