**2. Thermoelectric generator in the automotive**

In transportation, TEG system turns thermal losses in the exhaust pipe into useful electric energy. This is usually the place where thermoelectric power generator can be installed. The technology can be used either on a hybrid vehicle or a conventional one wherever parasite heat loss from internal combustion engine (ICE) can be utilised properly. And it is targeted to produce electric energy either for batteries charging or for alternator starting so far. Last decades, many pioneering projects have seen such rapid developments from lab prototype to industrial demonstrators. However, it is still too early to conclude that the technology will be adopted by car manufacturers in near future. Moreover, such technology development is also shadowed by the government regulations and strategies of full electrification in this sector over next 15–20 years. Nevertheless, there is still space for innovation and development of thermoelectric materials to take advantage of their solid-state nature, scalability and environmental friendliness in the automotive industry.

in the 4.0 L V8 diesel engine. In terms of fuel economy, this equates to about a 2% savings in gas fuel consumption. This technology was inspired by the Radioisotope Thermoelectric

In the past decade, General Motors has completed numerous investigation about TEG system for automobile on TE materials, system-level thermal management, efficient TEG heat exchanger design and modelling. One of the most successful achievements is the skutterudite TEMs with the figure of merit, ZT = 1.6 at 850 K [2]. This kind of TEM has a multiple filled p-n skutterudites TE materials which have been synthesised by the GM-TE material research group. A prototype of the Skutterudite-based TEG system was assembled and the experimental teat was carried out on test rig. A maximum power output of 300 W was achieved under FTP city driving cycle and 5% of fuel economy improvement is expected from this TEG system under FTP-75 driving cycle (**Figure 3**). The feature in lightweight and highly integration is definitely welcomed by customers. A 3 kg compact TEG system was claimed by Faurecia, which made the system can be easily installed close to the engine for extreme waste heat recovery. The Faurecia TEG system claimed to be competitive among other existing systems because of faster warm-up and increased usage of the electric mode. The system can reduce the fuel consumption up to 7% as claimed.

Furthermore, this TEG system was based on a hybrid vehicle platform, Faurecia equipped the all-new Hyundai IONIQ Hybrid and plug-in Hybrid with this TEG system, offering up to 3%

fuel savings, as measured on U.S. EPA Federal Test Procedure (FTP20) (**Figure 4**).

off-cycle credits of 1.5 g/mile in US [3].

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Generator (RTG), which was first used in the 1960s on spacecraft by NASA (**Figure 2**).

**Figure 1.** A prototype of Ford's thermoelectric power generator with 350 W power output [1].

**Figure 2.** Thermoelectric power generator integrated with EGR cooler system.

Additionally, the technology is eligible for CO<sup>2</sup>

#### **2.1. Major players in automotive TEG application**

During the last decade, plenty of automotive manufacturers have invested their money on the research of waste heat recovery by using TEG systems. In this section, the TEG systems presented by some leading automotive manufacturers and component supplier will be discussed including Ford, BMW, General Motors, Faurecia, FAW China.

Ford has presented a 350 W-rated power TEG system with the central bypass functionality aiming to control the temperature on its Lincoln MKT platform with 3.0 L V-6 engine. Under US06 driving cycle, the average power output is 180 W [1], according to the test data. In addition to the experimental investigation, Ford engineers also conducted 1-D performance simulation for an existing 2.5 L gas-electric hybrid vehicle. The system simulation predicted that the potential power output for this hybrid vehicle is 300–400 W under US Environmental Protection Agency (EPA) highway driving cycle. The aim of Ford is to improve the structure of the heat exchanger to enhance the heat transfer and reduce the increment of backpressure at the same time (**Figure 1**).

After 2009, most of BMW's waste heat recovery research work focused on integrating a thermoelectric generator inside the exhaust gas recirculation (EGR) cooler system. This technology could harness up to 250 W of energy [1], which is half the on-board energy need of a 5 series Thermoelectric Power Generation for Heat Recovery in Automotive Industries http://dx.doi.org/10.5772/intechopen.75467 149

**Figure 1.** A prototype of Ford's thermoelectric power generator with 350 W power output [1].

industry as primary sector to improve the propulsion efficiency, and the development of exhaust heat recovery system become imminent, especially for massive light-weight vehicle on roads. The thermoelectric power generation (TEG) technology emerges as an alternative

conversion efficiency and thermal design of the TEG heat exchangers are eagerly needed, and it is essential that recover this part of waste heat effectively to contribute a higher thermal efficiency of automotive engine and better fuel economy and emission. Therefore, this chapter mainly focus on the introduction of recent TEG systems, which are developed by major car manufacturers. In addition, the discussions are introduced for longstanding problems of the

In transportation, TEG system turns thermal losses in the exhaust pipe into useful electric energy. This is usually the place where thermoelectric power generator can be installed. The technology can be used either on a hybrid vehicle or a conventional one wherever parasite heat loss from internal combustion engine (ICE) can be utilised properly. And it is targeted to produce electric energy either for batteries charging or for alternator starting so far. Last decades, many pioneering projects have seen such rapid developments from lab prototype to industrial demonstrators. However, it is still too early to conclude that the technology will be adopted by car manufacturers in near future. Moreover, such technology development is also shadowed by the government regulations and strategies of full electrification in this sector over next 15–20 years. Nevertheless, there is still space for innovation and development of thermoelectric materials to take advantage of their solid-state nature, scalability and environmental friendliness in the automotive industry.

During the last decade, plenty of automotive manufacturers have invested their money on the research of waste heat recovery by using TEG systems. In this section, the TEG systems presented by some leading automotive manufacturers and component supplier will be discussed

Ford has presented a 350 W-rated power TEG system with the central bypass functionality aiming to control the temperature on its Lincoln MKT platform with 3.0 L V-6 engine. Under US06 driving cycle, the average power output is 180 W [1], according to the test data. In addition to the experimental investigation, Ford engineers also conducted 1-D performance simulation for an existing 2.5 L gas-electric hybrid vehicle. The system simulation predicted that the potential power output for this hybrid vehicle is 300–400 W under US Environmental Protection Agency (EPA) highway driving cycle. The aim of Ford is to improve the structure of the heat exchanger to enhance the heat transfer and reduce the increment of backpressure at the same time (**Figure 1**). After 2009, most of BMW's waste heat recovery research work focused on integrating a thermoelectric generator inside the exhaust gas recirculation (EGR) cooler system. This technology could harness up to 250 W of energy [1], which is half the on-board energy need of a 5 series

emission reduction in this area. However, much effort on

solution to the challenge of CO<sup>2</sup>

148 Bringing Thermoelectricity into Reality

heat exchanger design hindering the full integration efforts.

**2. Thermoelectric generator in the automotive**

**2.1. Major players in automotive TEG application**

including Ford, BMW, General Motors, Faurecia, FAW China.

**Figure 2.** Thermoelectric power generator integrated with EGR cooler system.

in the 4.0 L V8 diesel engine. In terms of fuel economy, this equates to about a 2% savings in gas fuel consumption. This technology was inspired by the Radioisotope Thermoelectric Generator (RTG), which was first used in the 1960s on spacecraft by NASA (**Figure 2**).

In the past decade, General Motors has completed numerous investigation about TEG system for automobile on TE materials, system-level thermal management, efficient TEG heat exchanger design and modelling. One of the most successful achievements is the skutterudite TEMs with the figure of merit, ZT = 1.6 at 850 K [2]. This kind of TEM has a multiple filled p-n skutterudites TE materials which have been synthesised by the GM-TE material research group. A prototype of the Skutterudite-based TEG system was assembled and the experimental teat was carried out on test rig. A maximum power output of 300 W was achieved under FTP city driving cycle and 5% of fuel economy improvement is expected from this TEG system under FTP-75 driving cycle (**Figure 3**).

The feature in lightweight and highly integration is definitely welcomed by customers. A 3 kg compact TEG system was claimed by Faurecia, which made the system can be easily installed close to the engine for extreme waste heat recovery. The Faurecia TEG system claimed to be competitive among other existing systems because of faster warm-up and increased usage of the electric mode. The system can reduce the fuel consumption up to 7% as claimed. Additionally, the technology is eligible for CO<sup>2</sup> off-cycle credits of 1.5 g/mile in US [3].

Furthermore, this TEG system was based on a hybrid vehicle platform, Faurecia equipped the all-new Hyundai IONIQ Hybrid and plug-in Hybrid with this TEG system, offering up to 3% fuel savings, as measured on U.S. EPA Federal Test Procedure (FTP20) (**Figure 4**).

**Figure 3.** Skutterudite based TEG system with a rated power output of 330 W for FTP city drive cycle.

accidentally. Therefore, TEG modules should be thermally contacted other components but electrically insulated. All these issues should be carefully considered since sealing an assem-

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What makes the scaling up problematic is from not only TEG modules, but also the heat/cold source in the vehicle. They add extra difficulties to the final design process. As known, intermittent heat source from ICE makes heatsink temperature fluctuate. Moreover, in a typical exhaust heat recovery system, heat sink/source temperature also varies along the flow direction. Different thermoelectric materials along the exhaust flow direction may be applied in order to match each material's optimum operating temperature. The optimum operating temperature probably can be achieved by structural optimization in heatsink design. However, if the temperature varies significantly within the exhaust flow, the different types of materials should be applied to obtained higher efficiency. Additionally, sufficient thermal insulation within the whole system should be employed. Otherwise, heat leakage from the exhaust gas

For example, Gao et al. [5] assessed a flat TE module with an air space in-between, which is commercially available in the market. As shown in **Figure 6**, commonly a TEM consists of several positive-type (P-type) and negative-type (N-type) that are connected by conducting strips in serials to increase the total voltage, combined with the cold and hot ceramic plates. The space between the hot and cold ceramic plates is filled by air. This air space will cause a heat loss within the TEM, as there will be heat radiation from the thermoelectric legs to the air space, which is non-negligible, even though the heat radiation is rare in TEMs. However, when it comes to the system level, a series of TEMs scaled up together, this part of radiation will cause a considerable heat loss from the hot side to cold side. As a result, the design opti-

bly make the configuration process irreversible in most cases.

**Figure 5.** A heat pipe assisted and modulated TEG system offered by FAW China.

mization of TEM should be deliberated before the assembly of TEM.

to the coolant will cause efficiency decline.

**Figure 4.** Faurecia offers a compact TEG system with 3 kg weight.

China First Automobile Works (FAW), as the leading vehicle manufacturer in China, has also made great effort in the waste heat recovery technologies to keep pace with the research status worldwide. FAW has proposed a novel structured concentric cylindrical TEG system, which used annular shaped TEM and combined heat pipes to enhance the heat transfer in radial direction and total filling ratio of the whole system. According to some simulation results, the peak power output can be as high as 1.2 kW under New Europe Driving Cycle (NEDC), in the same time, the power density is 800 W/m [4]. The concentric cylindrical TEG system takes the advantages of heat pipes and acquires the uniformity with the shape of exhaust pipe to make the system much easier in matching with different platform of vehicles (**Figure 5**).

#### **2.2. The problems of scaling up**

Conventional single TEG module is usually simple in a square/rectangular shape with positive/ negative leads soldering on the cold side copper interconnectors. Although the common structure of thermoelectric module is rather simple, it is very difficult to configure the small modules into a large assembly. The assembling processes usually determine the working condition, the electric and thermal contacts and the final performance of the TEG. These also include the number of total TEG modules, the electric interconnections through them (serried, paralleled or hybrid), the direction of arrangement against the exhaust, the clamping method and the coolant tightness.

Usually, an electric assembly with proper sized cables is carefully designed before the structural system design. This is because that the proper electric insulation to TEG assembly is under risk of short-circuit by surrounding coolants, which may drain all the electric energy Thermoelectric Power Generation for Heat Recovery in Automotive Industries http://dx.doi.org/10.5772/intechopen.75467 151

**Figure 5.** A heat pipe assisted and modulated TEG system offered by FAW China.

China First Automobile Works (FAW), as the leading vehicle manufacturer in China, has also made great effort in the waste heat recovery technologies to keep pace with the research status worldwide. FAW has proposed a novel structured concentric cylindrical TEG system, which used annular shaped TEM and combined heat pipes to enhance the heat transfer in radial direction and total filling ratio of the whole system. According to some simulation results, the peak power output can be as high as 1.2 kW under New Europe Driving Cycle (NEDC), in the same time, the power density is 800 W/m [4]. The concentric cylindrical TEG system takes the advantages of heat pipes and acquires the uniformity with the shape of exhaust pipe to make the

Conventional single TEG module is usually simple in a square/rectangular shape with positive/ negative leads soldering on the cold side copper interconnectors. Although the common structure of thermoelectric module is rather simple, it is very difficult to configure the small modules into a large assembly. The assembling processes usually determine the working condition, the electric and thermal contacts and the final performance of the TEG. These also include the number of total TEG modules, the electric interconnections through them (serried, paralleled or hybrid), the direction of arrangement against the exhaust, the clamping method and the coolant tightness. Usually, an electric assembly with proper sized cables is carefully designed before the structural system design. This is because that the proper electric insulation to TEG assembly is under risk of short-circuit by surrounding coolants, which may drain all the electric energy

system much easier in matching with different platform of vehicles (**Figure 5**).

**Figure 3.** Skutterudite based TEG system with a rated power output of 330 W for FTP city drive cycle.

**2.2. The problems of scaling up**

150 Bringing Thermoelectricity into Reality

**Figure 4.** Faurecia offers a compact TEG system with 3 kg weight.

accidentally. Therefore, TEG modules should be thermally contacted other components but electrically insulated. All these issues should be carefully considered since sealing an assembly make the configuration process irreversible in most cases.

What makes the scaling up problematic is from not only TEG modules, but also the heat/cold source in the vehicle. They add extra difficulties to the final design process. As known, intermittent heat source from ICE makes heatsink temperature fluctuate. Moreover, in a typical exhaust heat recovery system, heat sink/source temperature also varies along the flow direction. Different thermoelectric materials along the exhaust flow direction may be applied in order to match each material's optimum operating temperature. The optimum operating temperature probably can be achieved by structural optimization in heatsink design. However, if the temperature varies significantly within the exhaust flow, the different types of materials should be applied to obtained higher efficiency. Additionally, sufficient thermal insulation within the whole system should be employed. Otherwise, heat leakage from the exhaust gas to the coolant will cause efficiency decline.

For example, Gao et al. [5] assessed a flat TE module with an air space in-between, which is commercially available in the market. As shown in **Figure 6**, commonly a TEM consists of several positive-type (P-type) and negative-type (N-type) that are connected by conducting strips in serials to increase the total voltage, combined with the cold and hot ceramic plates. The space between the hot and cold ceramic plates is filled by air. This air space will cause a heat loss within the TEM, as there will be heat radiation from the thermoelectric legs to the air space, which is non-negligible, even though the heat radiation is rare in TEMs. However, when it comes to the system level, a series of TEMs scaled up together, this part of radiation will cause a considerable heat loss from the hot side to cold side. As a result, the design optimization of TEM should be deliberated before the assembly of TEM.

**2.3. The potential integration of TEG system with mufflers or catalytic converters**

advantage of such integration is the cost reduction of manufacturing.

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

electric modules and systems in the context of automotive applications.

**3.1. Heat transfer in thermoelectric device**

re-evaluated under such circumstances.

Since the economics of thermoelectric power generation depends on the nature of the heat source, there is an increasing awareness of deep integration among undergoing research projects. Moving thermoelectric power assembly closer to the ICE will certainly enhance the TEG performance. Inevitably, stricter requirements of TEG have to match working condition both muffler and thermoelectric power generators when such integration happens. One apparent

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Mufflers are used for noise reduction emitted by the exhaust of an internal combustion engine. It usually consists of several perforated tubes inside the shell. The structure of muffler is similar to the common shell and tube heat exchanger. In that case, it inspire a natural thinking of integrating two separated components into one functional device. Therefore, the integration can be achieved by redesign the muffler structure with added TEG modules on the inner surface of the shell. Double layered shell may accommodate coolant loop to cool the TEG modules. Nonetheless, both noise reduction and TEG assembly performance have to be

At present, most of the TEGs installed in the exhaust pipe system are located after the catalyst converter. The simple reason is that the exhaust emission treatment is prior to energy recovery process. However, the high temperature will decrease roughly by 100 K through the catalyst converter. If TEG developer would like to harness higher-grade heat from exhaust, the integration with catalyst converter is sensible but difficult. Inside catalyst converter, there are two functional components including reduction and oxidation process. Both components are coated perforated structures with 600 mesh number or higher. Therefore, fundamental changes in combined structure are needed and effects of harmful gas on the TEG modules need investigation in future work.

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 thermo-

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

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

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

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