**3. Heat transfer enhancement options in vehicle**

Unlike common circumstances, thermoelectric power generator in the vehicle has to work under a constant moving condition. Even if thermoelectric materials with high ZT are developed, there are still many system-level challenges to implement them into automotive applications. Especially, only ambient air is available as the eventual cold source for the thermoelectric power generator no matter what intermediate coolant adopted. Thermal systematic design is the key to match optimum heat flux between heatsinks and thermoelectric modules. Key innovations are urgently needed from not only material development but also the holistic system design. In this section, we discuss heat transfer related issues in thermoelectric modules and systems in the context of automotive applications.

#### **3.1. Heat transfer in thermoelectric device**

As previous researches discovered that the power output of TEG system was influenced by temperature gradient and temperature uniformity of the system significantly. Furthermore, the power output and efficiency could be improved by increasing the convection heat transfer coefficient of the high-temperature-side, but it was pricey due to the limited installation space and back pressure increment of the exhaust. Therefore, a well-designed heat exchanger will contribute to the improvement of the performance of TEG system effectively. Wang et al. [6] from Wuhan University of Technology have completed a number of investigation on the heat transfer enhancement in flat heat exchanger for TEG system [6]. The studies consisted a series of different structured heat exchanger with fins, deflectors or grooves. According to the results, by inserting fins, the heat transfer in heat exchanger could be enhanced. However it would also result in a large unwanted back pressure increment which went against to the efficiency of the engine. A heat exchanger containing cylindrical grooves on the interior surface of heat exchanger could increase the heat transfer area and enhance the turbulence intensity, meanwhile there was no additional inserts in the fluid to block the flow. Compared to flat surface, cylindrical grooves in exchanger could decrease the thermal resistance and enhance the power generation of TEG with nearly the same back pressure. Fins could greatly enhance the heat transfer and power genera-

tion of TEG, but the additional pressure loss was also great (**Figure 7**).

**Figure 6.** Mathematical models of thermoelectric module regarding the heat transfer.

152 Bringing Thermoelectricity into Reality

**Figure 7.** Topology of exhaust heat exchanger with different heat enhancement methods.

Layers in thermoelectric modules are thermal resistances such as interconnectors, solders and electric insulators. Considerations of minimising them without losing mechanical and electronic performance will benefit the overall system design. In exhaust pipeline, exhaust temperature ranges from 150 K to over 800 K, the added thermal protection from thermoelectric layers

In order to enhance heat transfer between the exhaust gas and the hot side of TEM, Li et al. [9] have applied foam metal to fill in the space of the exhaust pipe. By filling foam metal within the exhaust pipe, the convective heat-transfer coefficient is increased by 4 times, meanwhile,

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Another innovation may come from polymer with function of flexibility. The pipeline shape and exhaust assembly method make such demands speciously since car manufacturers are reluctant to fundamentally change the overall looking of the pipe, and it may involve the

When considering the installation of thermoelectric modules to the vehicle, the complexities of material configuration, clamping methods, heat sink structures, installation positions, flow resistance to the backpressure and overall cost make major car manufacturers hesitant to fully embrace this heat recovery technology. All these issues are related to the heat transfer in a powertrain system, a major research area that system specialist make enormous efforts to solve. To make use of scalable feature for large energy demands in the vehicle, careful system

When developing a thermoelectric system including heat exchangers, heat flux through the thermoelements should be large enough to maintain the appropriate temperature difference.

It could be achievable that we can deliver this level of heat flux by concentrating the heat at hot and cold side of TEM with fins or any other heat transfer enhancement methods. Therefore, the structural optimisation of TEM and heat exchanger is critical and both of them should be

In addition to the previous work, Kumar et al. [10] have completed more investigation on the influence of heat exchanger and thermoelectric module configurations to achieve optimization of the TEG system. As shown in figure, they presented four different structured heat sinks to compare the performance between them. The topologies having a rectangular

, assuming that the height of

the back pressure of the exhaust system do not boost significantly (**Figure 10**).

adjustment of chassis to accommodate irregular thermoelectric modules.

design are required including technical and cost considerations.

The general heat flux through thermoelectric legs is 100 kW/m<sup>2</sup>

developed to match the high density of heat flux.

thermoelectric leg is 1 mm, then the temperature gradient should be 100°C.

**3.2. Heat transfer in vehicle system**

**Figure 9.** Radial TEG arrangements for exhaust heat recovery.

**Figure 8.** Schematic of TEG device configuration with plate fin heat exchanger.

inevitably become the source of parasitic loss, which leads to reduced system power output and conversion efficiency. High temperature durability for thermal layers is the key to position thermoelectric modules closer to the engine.

Kumar et al. [7] have investigated the overall heat transferred, the electrical power output, and the associated pressure drop for given inlet conditions of the exhaust gas and the available TEG volume by using a rectangular configuration TEG system. In this system each TEM is mounted on the top and the bottom surface and arranged uniformly over 80% of total surface area. The remaining 20% area and the lateral walls are thermally insulated to minimise heat leakage. As shown in figure, the plate-fin heat exchanger is applied in the TEG system and there are several transverse fins distribute along the hot channel of heat exchanger. Moreover, the inlet and outlet of the hot channel of heat exchanger are connected to the engine exhaust pipe. According to the results, TEG power output is observed to have strong relation with the mass flow rate and inlet exhaust temperature. It was found that, at the average inlet conditions, up to 64% of the inlet energy can be transferred through the thermoelectric modules, resulting in a power output of 552 W, approximately 3.33% of the inlet power (**Figure 8**).

Zhou et al. [8] proposed a newly designed TEG with cylindrical shell and straight fins to overcome the common defects of conventional TEG system. They established a two-dimensional heat transfer numerical model under steady-state conditions and utilised this model to predict the performance of the TEG system in different working conditions. As shown in figure, the newly designed TEG system is compact in structure and can be arranged between catalytic converter and the muffler to make it effective in the recovery of waste heat. The TEMs and heat transfer fins are in direct contact, avoiding the exhaust tube structural transformation, and it will not cause any influence on the engine exhaust back pressure. The cooling tubes are branches of the engine cooling system, and the engine coolant flows into the tubes to cool down the cold sides of TEM (**Figure 9**).

**Figure 9.** Radial TEG arrangements for exhaust heat recovery.

In order to enhance heat transfer between the exhaust gas and the hot side of TEM, Li et al. [9] have applied foam metal to fill in the space of the exhaust pipe. By filling foam metal within the exhaust pipe, the convective heat-transfer coefficient is increased by 4 times, meanwhile, the back pressure of the exhaust system do not boost significantly (**Figure 10**).

Another innovation may come from polymer with function of flexibility. The pipeline shape and exhaust assembly method make such demands speciously since car manufacturers are reluctant to fundamentally change the overall looking of the pipe, and it may involve the adjustment of chassis to accommodate irregular thermoelectric modules.

#### **3.2. Heat transfer in vehicle system**

inevitably become the source of parasitic loss, which leads to reduced system power output and conversion efficiency. High temperature durability for thermal layers is the key to posi-

Kumar et al. [7] have investigated the overall heat transferred, the electrical power output, and the associated pressure drop for given inlet conditions of the exhaust gas and the available TEG volume by using a rectangular configuration TEG system. In this system each TEM is mounted on the top and the bottom surface and arranged uniformly over 80% of total surface area. The remaining 20% area and the lateral walls are thermally insulated to minimise heat leakage. As shown in figure, the plate-fin heat exchanger is applied in the TEG system and there are several transverse fins distribute along the hot channel of heat exchanger. Moreover, the inlet and outlet of the hot channel of heat exchanger are connected to the engine exhaust pipe. According to the results, TEG power output is observed to have strong relation with the mass flow rate and inlet exhaust temperature. It was found that, at the average inlet conditions, up to 64% of the inlet energy can be transferred through the thermoelectric modules, resulting in a power output of 552 W, approximately 3.33% of the inlet power (**Figure 8**).

Zhou et al. [8] proposed a newly designed TEG with cylindrical shell and straight fins to overcome the common defects of conventional TEG system. They established a two-dimensional heat transfer numerical model under steady-state conditions and utilised this model to predict the performance of the TEG system in different working conditions. As shown in figure, the newly designed TEG system is compact in structure and can be arranged between catalytic converter and the muffler to make it effective in the recovery of waste heat. The TEMs and heat transfer fins are in direct contact, avoiding the exhaust tube structural transformation, and it will not cause any influence on the engine exhaust back pressure. The cooling tubes are branches of the engine cooling system, and the engine coolant flows into the tubes

tion thermoelectric modules closer to the engine.

154 Bringing Thermoelectricity into Reality

**Figure 8.** Schematic of TEG device configuration with plate fin heat exchanger.

to cool down the cold sides of TEM (**Figure 9**).

When considering the installation of thermoelectric modules to the vehicle, the complexities of material configuration, clamping methods, heat sink structures, installation positions, flow resistance to the backpressure and overall cost make major car manufacturers hesitant to fully embrace this heat recovery technology. All these issues are related to the heat transfer in a powertrain system, a major research area that system specialist make enormous efforts to solve. To make use of scalable feature for large energy demands in the vehicle, careful system design are required including technical and cost considerations.

When developing a thermoelectric system including heat exchangers, heat flux through the thermoelements should be large enough to maintain the appropriate temperature difference. The general heat flux through thermoelectric legs is 100 kW/m<sup>2</sup> , assuming that the height of thermoelectric leg is 1 mm, then the temperature gradient should be 100°C.

It could be achievable that we can deliver this level of heat flux by concentrating the heat at hot and cold side of TEM with fins or any other heat transfer enhancement methods. Therefore, the structural optimisation of TEM and heat exchanger is critical and both of them should be developed to match the high density of heat flux.

In addition to the previous work, Kumar et al. [10] have completed more investigation on the influence of heat exchanger and thermoelectric module configurations to achieve optimization of the TEG system. As shown in figure, they presented four different structured heat sinks to compare the performance between them. The topologies having a rectangular

**Figure 10.** Heat enhancement method by filled metal foam into exhaust pipe.

length of a single TEM, the TEG system will obtain the highest electrical power output with

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**Figure 12.** Simplified heat pipe assisted exhaust heat recovery system for concentric TEG device.

Heat pipe-assisted heat enhancement method is approved to be an effective way to improve the TEG performance. Encapsulated heat pipes arrays in the radial direction of the exhaust pipe help to enhance heat effectively from an external fluid stream. The features of heat pipes such as temperature flattening, temperature control and thermal diode, may help TEG modules for autonomous, maintenance-free operation under fluctuating heating sources in the future. The spatial distribution of the temperature rise is considerately responsive by the

In order to get a better filling ratio, Bo et al. [11] from University of Nottingham have presented a concentric cylindrical TEG system consists of a series of repeat units that are conjugated along the exhaust stream to shape of the exhaust pipe. As shown in the figure, the repeat unit is made up of four concentric TEMs, three hot plates and two cooling plates including 12 heat pipes. The exhaust stream interacts with heat pipes and transfers the heat into the TEG in the radial direction of the exhaust stream. Comparing with some commonly used TEG system, the concentric cylindrical TEG system gain a better filling ratio by such configuration. Furthermore, in this system, a compact and lightweight heat sink which is assisted by heat pipes is introduced. The merits of utilising heat pipes in the system are explored regarding the improvement of heat transfer in radial direction and the simplicity of system integration. Besides, the combination of heat pipes reduce the weight of the TEG system as well, conse-

Lu et al. [12] investigated the effect of distribution consistency of the interceptor on the performance of heat transfer enhancement for the TEG system. The results showed the non-uniform configuration of the interceptor can lead to approximately doubled power output than smooth channel heat exchanger. However, the pressure drop which governs the pumping power of

Liu et al. [13] built a test bench for examining the performance of TEG system which is assembled into a prototype vehicle. Through the revolving drum test bench, the characteristics of

lower pressure drop among other designs.

variation of heating condition from exhaust streams.

quently improving the fuel economy (**Figure 12**).

heat exchanger need to be concerned carefully (**Figure 13**).

**Figure 11.** Various Heat exchanger structures for exhaust pipe adjustment.

box-like shape are grouped as rectangular topology. The names of the model—longitudinal and transverse—are derived from the way the TEMs are placed with respect to the exhaust flow direction. Two types of circular configuration: regular hexagon and cylinder are also presented for comparison. These models are similar to the longitudinal model except the crosssection is hexagonal or cylindrical (**Figure 11**).

All topologies behave somewhat similarly at lower numbers of TEMs in terms of electrical generation. However, the performance of the hexagonal and cylindrical topologies suffers when the number of TEMs exceeds 40 owing to large pressure drops. Overall, the transverse design shows better results in the heat enhancement and power output comparing with longitudinal designs. Furthermore, if the width of the transverse heat exchanger equals to the

**Figure 12.** Simplified heat pipe assisted exhaust heat recovery system for concentric TEG device.

length of a single TEM, the TEG system will obtain the highest electrical power output with lower pressure drop among other designs.

Heat pipe-assisted heat enhancement method is approved to be an effective way to improve the TEG performance. Encapsulated heat pipes arrays in the radial direction of the exhaust pipe help to enhance heat effectively from an external fluid stream. The features of heat pipes such as temperature flattening, temperature control and thermal diode, may help TEG modules for autonomous, maintenance-free operation under fluctuating heating sources in the future. The spatial distribution of the temperature rise is considerately responsive by the variation of heating condition from exhaust streams.

In order to get a better filling ratio, Bo et al. [11] from University of Nottingham have presented a concentric cylindrical TEG system consists of a series of repeat units that are conjugated along the exhaust stream to shape of the exhaust pipe. As shown in the figure, the repeat unit is made up of four concentric TEMs, three hot plates and two cooling plates including 12 heat pipes. The exhaust stream interacts with heat pipes and transfers the heat into the TEG in the radial direction of the exhaust stream. Comparing with some commonly used TEG system, the concentric cylindrical TEG system gain a better filling ratio by such configuration. Furthermore, in this system, a compact and lightweight heat sink which is assisted by heat pipes is introduced. The merits of utilising heat pipes in the system are explored regarding the improvement of heat transfer in radial direction and the simplicity of system integration. Besides, the combination of heat pipes reduce the weight of the TEG system as well, consequently improving the fuel economy (**Figure 12**).

box-like shape are grouped as rectangular topology. The names of the model—longitudinal and transverse—are derived from the way the TEMs are placed with respect to the exhaust flow direction. Two types of circular configuration: regular hexagon and cylinder are also presented for comparison. These models are similar to the longitudinal model except the cross-

All topologies behave somewhat similarly at lower numbers of TEMs in terms of electrical generation. However, the performance of the hexagonal and cylindrical topologies suffers when the number of TEMs exceeds 40 owing to large pressure drops. Overall, the transverse design shows better results in the heat enhancement and power output comparing with longitudinal designs. Furthermore, if the width of the transverse heat exchanger equals to the

section is hexagonal or cylindrical (**Figure 11**).

**Figure 11.** Various Heat exchanger structures for exhaust pipe adjustment.

**Figure 10.** Heat enhancement method by filled metal foam into exhaust pipe.

156 Bringing Thermoelectricity into Reality

Lu et al. [12] investigated the effect of distribution consistency of the interceptor on the performance of heat transfer enhancement for the TEG system. The results showed the non-uniform configuration of the interceptor can lead to approximately doubled power output than smooth channel heat exchanger. However, the pressure drop which governs the pumping power of heat exchanger need to be concerned carefully (**Figure 13**).

Liu et al. [13] built a test bench for examining the performance of TEG system which is assembled into a prototype vehicle. Through the revolving drum test bench, the characteristics of

Unfortunately, it is not the option for most of car manufacturers. Moreover, the TEG heat recovery system should be ideally designed in a cylindrical shape as much same as the

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In order to minimising the extra coolant loop or heat sink, extra coolant pumping power should be considered when routing the loop in/out of the TEG system. A proportional electric gas valve is usually integrated with TEG system in order to adjust the bypass flow of the exhaust. In some cases, a DC-DC converter or a DC-AC converter has to be positioned close

Inevitably, all the added up functional components mentioned here occupy significant space. To squeeze such TEG system into a highly-packed and harsh environment is not always an

Power density is usually the main concerns that car manufacturers focus on. Every car manufacturer pursues highest power output in a certain weight. At present, the threshold of power output in automotive market is 1 kW per unit. In other way, the thermoelectric have to obtain at least 10 kW heat energy from the exhaust given that most of current system efficiency are less than 10%. It is a dilemma for TE material scientist and thermal system designer. To maintain such power density, system designer have to find ways to deliver sufficient heat to the TE material. Material scientist have to tune the TE into best power output capacity within a defined space.

The lifespan of a common passenger car can reach over 25 years or more. Although no moving parts in the TEG system, TEG system exposes to an extremely varying thermal cycling condition. Tiny cracks between soldering layers and metalized layer will decrease the efficiency and cause mismatched resistive load. In addition, the clamping is prone to failure under a shock or vibration. Unfortunately, there is little research in this area but it should be included

Nonetheless, considering major services in certain intervals and payback time for the customer, it is crucial for TEG developer to define the business model for selling this technology

In this chapter, the latest progress on TEG exhaust heat recovery is introduced. The technology is still the favourite solution for lightweight vehicles until final phase-out of fuel-powered combustion engine in the middle of the twenty-first century. It is worth mentioning that automotive engineering is a process of system engineering, all the potential applicable technology should be evaluated in the context of specific vehicle platform. There is no exception for the TEG technology. Due to the nature of internal combustion engine, the heat source varies from

to the TEG system with extra electric cabling and connection work.

exhaust pipe.

easy task for the system designers.

**4.2. Power density**

**4.3. Reliability**

in any road test in future.

**5. Conclusion**

in term of installation cost and repair cost.

**Figure 13.** Fin arrangement studies for heat transfer enhancement in TEG heat exchanger.

**Figure 14.** Multi cross flow arrangements for TEG heat exchanger.

the TEG system can be measured, and a maximum power output of 944 W was obtained, which can fulfil the power for some accessories in automotive application (**Figure 14**).
