**3. Automotive exhaust thermoelectric generator system**

The TEG for automobile exhaust heat conversion mainly consists of four parts, namely, TE modules, a heat exchanger to capture the heat from the flowing exhaust gas and transferring it to the hot side of the modules, heat sink to remove the heat from the cold side of the modules and assembly components. **Figure 2** shows the schematic of typical TEG arrangement for automotive exhaust waste heat conversion. In the exhaust pipe attached to the engine, the TEG is usually integrated after the catalytic converter. This is because positioning TEG before the catalytic converter can lower the exhaust gas temperature which will affect catalytic converter's performance. It is always preferable that the location of the TEG in the exhaust pipe is as close as catalytic converter since as we move away from it the temperature drops significantly. In most of the prototypes tested either in simulated or in actual driving conditions, only a TEG in the exhaust pipe alone is used. However, it is not uncommon to use two TEGs, one at the regular exhaust line and another one in the exhaust gas recirculation system (EGR) to maximize the power output.

#### **3.1. TE modules**

TE modules which are the main functional part of the AETEG are made of several pairs of p and n-type legs/elements of the thermoelectric compounds, which are connected electrically in series and thermally in parallel. The choice of the appropriate materials for modules mainly depends on (a) the optimal temperature range where zT is maximum (b) easy availability and (c) mechanical durability at the operating conditions of the TEG. The efficiency of the TE module given by.

$$\boldsymbol{\eta}\_{\rm TE} = \left[ (\mathbf{T}\_{\rm h} - \mathbf{T}\_{\rm c}) / \mathbf{T}\_{\rm h} \right] \cdot \left\{ \sqrt{(\mathbf{1} + \mathbf{z}\mathbf{T}) - \mathbf{1}} / \left[ \sqrt{(\mathbf{1} + \mathbf{z}\mathbf{T})} \right] + \mathbf{T}\_{\rm c} / \mathbf{T}\_{\rm h} \right\} \tag{1}$$

are some of the compounds showing promising results. However, the cost factor associated with these compounds because of the rare elements used makes it difficult in lowering of per Watt cost and the large-scale mass production. Apart from this, unlike the stationary or space

**Table 1.** Materials, modules specification, and performance of some of the AETEGs fabricated and tested in engine exhaust.

(under ΔT = 500°C)

**Materials Module specifications TEG performance Ref.**

~125 W

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at 112.6 km/h speed

1.0 kW [12]

35.6 W [13]

42.3 W [14]

266 W at 60 km/h driving in

2 L engine.

using 12 modules

(using 12 modules)

450 W (US06 driving cycle)

1 kW [20]

[9]

167

[15]

[16]

[18]

[19]

1 Fe-based compound — — [5]

(at ΔT = 300°C) ηmax. 4.5%

Pmodule-13 W

Pmax:1.2 W (at ΔT = 563 K)

(at ΔT = 563 K)

ηmod.-6–8%

Te3 Pnominal: 7 W at ΔT of 175°C 44 W

Te3 4 × 4 × 0.42 cm 350 W

10 Skutterudites — 700 W (steady state)

Mod. Size:0.24 cm<sup>2</sup> Pmax~18 W (at ΔT = 560°C)

Te3 0.4 × 0.4 cm 75 W [17]

Ge 20 × 20 cm

Te3

**Figure 2.** Schematic of the different type of TEG's arrangement [22, 36].

2 BiTe Pmax ~35 W

3 BiTe 5.3 × 5.3 cm

5 BiTe 6.2 × 6.2 × 0.5 cm

Te3 modules

11 Half Heusler 5.26 W/cm<sup>2</sup>

**Sl. No**

4 B and P doped Si<sup>2</sup>

6 Segmented Skutterudites and Bi<sup>2</sup>

and commercial Bi<sup>2</sup>

7 Bi<sup>2</sup> Te3 /Sb<sup>2</sup>

8 Bi<sup>2</sup>

9 Bi<sup>2</sup>

where Th, T<sup>c</sup> are the hot and cold side temperatures of the module, T is the average temperature given by (T<sup>h</sup> + Tc )/2, and zT is the figure of merit of the TE legs used.

To get the maximum efficiency in the modules, the operating temperature of the modules should be in the range where the zT of the leg materials is the highest.

Several materials with appropriate doping have been used in the modules which can be reliably employed in the typical exhaust gas temperature range which is 400–600°C for a diesel engine and 700–800°C for gasoline engine. **Table 1** gives the details of TE materials, modules and the performance of AETEG either in a simulated or actual road test conditions. In the temperature range of interest for AETEG, filled skutterudites, doped PbTe, and half-Heuslers Automotive Waste Heat Recovery by Thermoelectric Generator Technology http://dx.doi.org/10.5772/intechopen.75443 167

**Figure 2.** Schematic of the different type of TEG's arrangement [22, 36].

it reaches the customers. In high-temperature modules (i.e., operating temperature > 400°C), the use of rare earth elements along with intricate assembling and packaging process escalates the overall cost tremendously. Incorporation of suitable diffusion barrier layers between TE elements and metal interconnects, sealing of the complete assembly in inert gas to prevent the degradation at operating conditions etc. are some of the essential requirements for modules operating above 400°C. In spite of all these hurdles, the emergence of low-cost abundantly available materials

Si and MnSi<sup>2</sup>

The TEG for automobile exhaust heat conversion mainly consists of four parts, namely, TE modules, a heat exchanger to capture the heat from the flowing exhaust gas and transferring it to the hot side of the modules, heat sink to remove the heat from the cold side of the modules and assembly components. **Figure 2** shows the schematic of typical TEG arrangement for automotive exhaust waste heat conversion. In the exhaust pipe attached to the engine, the TEG is usually integrated after the catalytic converter. This is because positioning TEG before the catalytic converter can lower the exhaust gas temperature which will affect catalytic converter's performance. It is always preferable that the location of the TEG in the exhaust pipe is as close as catalytic converter since as we move away from it the temperature drops significantly. In most of the prototypes tested either in simulated or in actual driving conditions, only a TEG in the exhaust pipe alone is used. However, it is not uncommon to use two TEGs, one at the regular exhaust line and another one in the exhaust gas recirculation system (EGR) to maximize the power output.

TE modules which are the main functional part of the AETEG are made of several pairs of p and n-type legs/elements of the thermoelectric compounds, which are connected electrically in series and thermally in parallel. The choice of the appropriate materials for modules mainly depends on (a) the optimal temperature range where zT is maximum (b) easy availability and (c) mechanical durability at the operating conditions of the TEG. The efficiency of the TE module given by.

)/2, and zT is the figure of merit of the TE legs used. To get the maximum efficiency in the modules, the operating temperature of the modules

Several materials with appropriate doping have been used in the modules which can be reliably employed in the typical exhaust gas temperature range which is 400–600°C for a diesel engine and 700–800°C for gasoline engine. **Table 1** gives the details of TE materials, modules and the performance of AETEG either in a simulated or actual road test conditions. In the temperature range of interest for AETEG, filled skutterudites, doped PbTe, and half-Heuslers

{√(1 + zT) − 1/[√(1 + zT)

are the hot and cold side temperatures of the module, T is the average tempera-

∗

should be in the range where the zT of the leg materials is the highest.

showing promising features can give the

] + Tc /Th} (1)

such as tetrahedrites (Cu12-xM<sup>x</sup>

166 Bringing Thermoelectricity into Reality

**3.1. TE modules**

where Th, T<sup>c</sup>

ture given by (T<sup>h</sup> + Tc

ηTE = [(Th − Tc)/Th]

required breakthrough for commercialization.

Sb4

S13), Mg<sup>2</sup>

**3. Automotive exhaust thermoelectric generator system**


**Table 1.** Materials, modules specification, and performance of some of the AETEGs fabricated and tested in engine exhaust.

are some of the compounds showing promising results. However, the cost factor associated with these compounds because of the rare elements used makes it difficult in lowering of per Watt cost and the large-scale mass production. Apart from this, unlike the stationary or space applications where the TEGs showed very high durability, in the case of automotive application, the modules and materials are subjected to highly fluctuating thermal and mechanical conditions which can affect its longevity. As mentioned earlier, TE legs are electrically connected in series. Hence, the failure of either a leg or the joint between leg and metallic electrodes/interconnects can result in to a complete electrical breakdown of the module. The reliability of the modules, which is affected owing to such failures occurs predominantly due to thermo-mechanical stresses created at the interface by the coefficient of thermal expansion mismatch between TE elements and interconnects. Using a fault management system which can cut off the failed module from the rest can overcome this problem in AETEG. However, such arrangement makes the system more complicated and costly. The development of a particular material system-specific design using multiphysics simulations and experiments can help in improving the reliability of the leg-interconnect joints and the interface. Another problem severely affecting the durability of the module is the loss of materials from the TE legs by sublimation under prolonged exposure to high temperatures. Coating the surface with a stable thin layer of materials with comparable thermal coefficient of expansion (CTE) or casting the space between the TE legs with highly tortuous, extremely low thermal conductivity (<0.01 W/m·K) aero-gel could reduce the sublimation loss. However, all these processes substantially add up the overall cost of the modules.

multiple manufacturing routes could bring in design flexibility and result in reduction of overall AETEG cost. Stainless steel, aluminum and brass are some of the heat exchanger

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The heat transfer from the exhaust gas to the outer shell of the heat exchanger where the TE modules are placed occurs by the combination of the convection and conduction mechanisms. The thermal resistance (R) for the convective heat transfer is given by R = 1/(h.A) where h is the heat transfer coefficient and A is the area of the heat transfer surface. Any internal arrangement which enhances the heat transfer area (A) increases the convective heat transfer which subsequently improves the hot side temperature (TH). The thermal resistance for the convection mostly occurs in the boundary layer. Various kinds of fins with different shapes, dimensions, and arrangements are customarily set in the heat exchanger inside wall to enhance the turbulence resulting in the breakdown of the boundary layer. **Figure 3** shows some of the most commonly used internal arrangements in box type heat exchangers. Fishbone and inclined plate fin arrangements are some of the shapes showing high heat transfer rate from the exhaust gas with acceptable level of back pressure [22, 23]. Serial plate arrangement with the plate's direction perpendicular to the gas inlet showed the highest back pressure. Such arrangement gave backpressure as high as 190 kPa in a shell of 280 × 110 × 30 mm with inlet and outlet of 40 mm diameter [23]. An open shell metal foam filled plate heat exchanger also showed a very high efficiency of heat recovery 83.5% [24]. However, the high tortuosity of the

The temperature distribution in the heat exchanger along the exhaust flow direction usually tends to be lower in the downstream than the gas inlet region due to the heat loss to the TE module located close to the inlet [25]. Such nonuniformity in the temperature distribution reduces the power output of the modules placed beyond certain specified length in the downstream. Computational analysis carried out using different exhaust and coolant flow arrangements such as co-flow/parallel flow and counter flow suggest predicted a different overall power output [26]. However, it must be noted that a detailed experimental validation of these analyses only can confirm the preferred configuration that can maximize the overall

**Figure 3.** Different shapes of the internal arrangement for heat exchangers used in automotive TEG. (a) Empty cavity, (b) inclined plate, (c) parallel plate structure, (d) separate plate with holes, (e) serial plate structure and (f) pipe structure,

materials used so far and tested in both diesel and gasoline engines.

foam structure creates an unacceptable levels of back pressure.

power output.

(g) fish bone structure, and (h) accordion shape [23].

#### **3.2. Heat exchanger**

The heat exchanger is an essential element of in AETEG which determines its overall performance. Since its primary function is to extract heat from flowing exhaust gas, an optimum design and sizing are critical for delivering the maximum power output. The efficiency of the TEG (ηTEG) which is given by ηTEG=ηHE×ηTE×ε where ηHE is the heat exchanger efficiency, ηTE is the conversion efficiency of the cluster of TE modules and ε is the ratio of heat transfer from modules hot side to the cold side. An ideal heat exchanger should have low weight and high ηHE without causing severe backpressure to the flow of the exhaust gas. The back pressure will increase the parasitic losses in the vehicle.

Thermal efficiency (ηHE) of the heat exchanger mainly depends on three factors: (a) type, (b) internal geometrical shape and (c) materials of construction. The types of heat exchangers are classified according to the heat transfer mechanisms and the number of fluids. Construction type and flow arrangements are some of the other parameters used for further classifications. In automotive TEG, the type of heat exchangers used are mostly indirect, contact type with direct heat transfer between different medium such as gas and solid in the hot side and solid and liquid in the cold side of the system. According to the construction type and geometrical shape classification, the heat exchangers used are mostly either box type or tubular with extended surfaces such as fins or heat pipes depending upon the space available for the integration into the vehicle [10, 11, 21].

The choice of the materials for the heat exchanger fabrication is determined by their thermal conductivity, density, and fabricability. Since most of the heat exchangers are with extended surfaces, its shell outer temperatures are significantly influenced by the thermal conductivity of the materials used. The density of the materials used decides the overall heat exchanger weight and the parasitic loss associated with it. Materials which are easy to fabricate by

multiple manufacturing routes could bring in design flexibility and result in reduction of overall AETEG cost. Stainless steel, aluminum and brass are some of the heat exchanger materials used so far and tested in both diesel and gasoline engines.

applications where the TEGs showed very high durability, in the case of automotive application, the modules and materials are subjected to highly fluctuating thermal and mechanical conditions which can affect its longevity. As mentioned earlier, TE legs are electrically connected in series. Hence, the failure of either a leg or the joint between leg and metallic electrodes/interconnects can result in to a complete electrical breakdown of the module. The reliability of the modules, which is affected owing to such failures occurs predominantly due to thermo-mechanical stresses created at the interface by the coefficient of thermal expansion mismatch between TE elements and interconnects. Using a fault management system which can cut off the failed module from the rest can overcome this problem in AETEG. However, such arrangement makes the system more complicated and costly. The development of a particular material system-specific design using multiphysics simulations and experiments can help in improving the reliability of the leg-interconnect joints and the interface. Another problem severely affecting the durability of the module is the loss of materials from the TE legs by sublimation under prolonged exposure to high temperatures. Coating the surface with a stable thin layer of materials with comparable thermal coefficient of expansion (CTE) or casting the space between the TE legs with highly tortuous, extremely low thermal conductivity (<0.01 W/m·K) aero-gel could reduce the sublimation loss. However, all these pro-

The heat exchanger is an essential element of in AETEG which determines its overall performance. Since its primary function is to extract heat from flowing exhaust gas, an optimum design and sizing are critical for delivering the maximum power output. The efficiency of the TEG (ηTEG) which is given by ηTEG=ηHE×ηTE×ε where ηHE is the heat exchanger efficiency, ηTE is the conversion efficiency of the cluster of TE modules and ε is the ratio of heat transfer from modules hot side to the cold side. An ideal heat exchanger should have low weight and high ηHE without causing severe backpressure to the flow of the exhaust gas. The back pressure will

Thermal efficiency (ηHE) of the heat exchanger mainly depends on three factors: (a) type, (b) internal geometrical shape and (c) materials of construction. The types of heat exchangers are classified according to the heat transfer mechanisms and the number of fluids. Construction type and flow arrangements are some of the other parameters used for further classifications. In automotive TEG, the type of heat exchangers used are mostly indirect, contact type with direct heat transfer between different medium such as gas and solid in the hot side and solid and liquid in the cold side of the system. According to the construction type and geometrical shape classification, the heat exchangers used are mostly either box type or tubular with extended surfaces such as fins or heat pipes depending upon the space available for the

The choice of the materials for the heat exchanger fabrication is determined by their thermal conductivity, density, and fabricability. Since most of the heat exchangers are with extended surfaces, its shell outer temperatures are significantly influenced by the thermal conductivity of the materials used. The density of the materials used decides the overall heat exchanger weight and the parasitic loss associated with it. Materials which are easy to fabricate by

cesses substantially add up the overall cost of the modules.

increase the parasitic losses in the vehicle.

integration into the vehicle [10, 11, 21].

**3.2. Heat exchanger**

168 Bringing Thermoelectricity into Reality

The heat transfer from the exhaust gas to the outer shell of the heat exchanger where the TE modules are placed occurs by the combination of the convection and conduction mechanisms. The thermal resistance (R) for the convective heat transfer is given by R = 1/(h.A) where h is the heat transfer coefficient and A is the area of the heat transfer surface. Any internal arrangement which enhances the heat transfer area (A) increases the convective heat transfer which subsequently improves the hot side temperature (TH). The thermal resistance for the convection mostly occurs in the boundary layer. Various kinds of fins with different shapes, dimensions, and arrangements are customarily set in the heat exchanger inside wall to enhance the turbulence resulting in the breakdown of the boundary layer. **Figure 3** shows some of the most commonly used internal arrangements in box type heat exchangers. Fishbone and inclined plate fin arrangements are some of the shapes showing high heat transfer rate from the exhaust gas with acceptable level of back pressure [22, 23]. Serial plate arrangement with the plate's direction perpendicular to the gas inlet showed the highest back pressure. Such arrangement gave backpressure as high as 190 kPa in a shell of 280 × 110 × 30 mm with inlet and outlet of 40 mm diameter [23]. An open shell metal foam filled plate heat exchanger also showed a very high efficiency of heat recovery 83.5% [24]. However, the high tortuosity of the foam structure creates an unacceptable levels of back pressure.

The temperature distribution in the heat exchanger along the exhaust flow direction usually tends to be lower in the downstream than the gas inlet region due to the heat loss to the TE module located close to the inlet [25]. Such nonuniformity in the temperature distribution reduces the power output of the modules placed beyond certain specified length in the downstream. Computational analysis carried out using different exhaust and coolant flow arrangements such as co-flow/parallel flow and counter flow suggest predicted a different overall power output [26]. However, it must be noted that a detailed experimental validation of these analyses only can confirm the preferred configuration that can maximize the overall power output.

**Figure 3.** Different shapes of the internal arrangement for heat exchangers used in automotive TEG. (a) Empty cavity, (b) inclined plate, (c) parallel plate structure, (d) separate plate with holes, (e) serial plate structure and (f) pipe structure, (g) fish bone structure, and (h) accordion shape [23].

Using heat exchanger made of high thermal conductivity materials can improve the uniformity of the hot side surface to some extent. For example, in a study using steel and brass, Deng et al. observed better temperature uniformity in brass heat exchanger due to its higher thermal conductivity (κbrass-109 W/m·K) [22]. Similarly, in a study using three-dimensional model to optimize heat exchanger parameters, Kempf et al. showed that by using silicon carbide in the heat exchanger better temperature uniformity can be achieved compared to stainless steel of 444 (SS 444) grade [25]. However, from the fabrication point of view, it will be highly cost-effective and easy to use SS than silicon carbide. The silicon carbide components are in general fabricated either by slip casting or gel-casting followed by sintering at above 2000°C. Such processes can adds up the TEG's cost significantly. The brass has higher density than SS which will increase the overall TEG's weight and the parasitic loss associated with it.

system). While the separate system requires additional space and increases the overall weight of the vehicle, connecting the existing engine coolant circuit avoids these complexities. However, the cooling pump capacity and radiator size may have to be increased to accommodate the additional heat coming from the cold side of the TEG so that the overheating of the coolant can be prevented. In a combined simulation and experimental studies carried out in a 2.0 L 4 cylinder engine, Deng et al. observed that under certain vehicle operating conditions, temperature of

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The heat transfer between hot side heat exchanger and the cold side heat sink through TE modules depends on how well various components of the AETEG are assembled so that thermal contact resistance will be minimal between various interfaces. The thermal contact resistance depends on many factors: the applied contact pressure, surface roughness, the interface materials and its hardness are some of the important parameters to list. The assembly components are responsible for enforcing sufficient force over the modules sandwiched between the heat exchanger and the heat sink. The ΔT across the hot and cold side of the TE module increases typically with contact pressure as it generates more area of physical contact at the microscopic level resulting in more heat conductance through the interfaces. **Figure 4** shows the curve showing the typical variation

the integrated cooling system increases and exceeds the boiling point of coolant [28].

of ΔT with applied contact pressure in a TEG with and without interface materials. [29].

**Figure 4.** Variation of differential temperatures with contact pressure using different interface materials [29].

In the absence of any interface materials, it can be seen that the ΔT remarkably increases with applied pressure. With the use of thermal grease at the interface, the ΔT is almost invariant. However with graphite, the most common interface material in AETEG, the ΔT was significantly

**3.4. Assembly components**

#### **3.3. Heat sink or cold side heat exchanger**

The AETEGs can be operated either at the maximum power (Pmax) or maximum efficiency. In automotive exhaust application, it is always preferable to maximize the power output as it reduces the engine load which will improve the overall fuel efficiency. The Pmax of a AETEG not only depends on hot side temperature (TH) but the temperature difference (ΔT) between the hot and the cold side of the module. Hence, to maximize the power output, the cold side temperature (T<sup>c</sup> ) of the TE modules should be as low as possible. In most of the AETEGs designed, fabricated and tested in the laboratory testing conditions, the cold side temperature was controlled by using a water-cooled heat sink. However, in actual vehicles, it can be connected to the engine coolant circuit.

In the heat sink, the coolant flow is usually in the same (co-flow) or opposite (counterflow) direction to the exhaust gas flow as described in the previous section. Cross-flow and counter cross-flow arrangements are also used in few cases. The output power and the conversion efficiency of the AETEG for the various coolant flow arrangements mentioned above depends on the specific geometrical design of the cold side heat exchanger also. In the co-flow arrangement, the ΔT decreases along the streamwise direction of the exhaust gas flow as more sensible heat is absorbed and transferred to the cold side of the module tends to increase the coolant temperature. On the other hand, in counterflow arrangement, the ΔT decrease is lesser along the streamwise direction. However, to get the maximum power output in the TEG whether coflow or counter-flow is preferable depends on the temperature and flow rate of the exhaust gas which in turn depends on the type, capacity and operating conditions of the engine.

The output power and the conversion efficiency of the AETEG for the various coolant flow configurations mentioned above depends on the specific geometrical design of the heat exchanger and heat sink combination. Su et al. performed simulation studies on different configurations of cooler design for TEG system viz., plate-shaped, stripe-shaped and diamond-shaped designs [27]. They further validated the simulation outputs with experiments and concluded that diamond-shaped design gave the highest power for a given engine speed among the three configurations for the cold side of the TEG system. They also observed that the temperature of the cold side is relatively uniform ensuring that the deviation in ΔT between the individual modules is minimal.

The cold side heat exchanger in TEG system in actual vehicle can either be a separate TEG coolant circulation system or integrated to the existing engine coolant circuit (integrated cooling system). While the separate system requires additional space and increases the overall weight of the vehicle, connecting the existing engine coolant circuit avoids these complexities. However, the cooling pump capacity and radiator size may have to be increased to accommodate the additional heat coming from the cold side of the TEG so that the overheating of the coolant can be prevented. In a combined simulation and experimental studies carried out in a 2.0 L 4 cylinder engine, Deng et al. observed that under certain vehicle operating conditions, temperature of the integrated cooling system increases and exceeds the boiling point of coolant [28].

#### **3.4. Assembly components**

Using heat exchanger made of high thermal conductivity materials can improve the uniformity of the hot side surface to some extent. For example, in a study using steel and brass, Deng et al. observed better temperature uniformity in brass heat exchanger due to its higher thermal conductivity (κbrass-109 W/m·K) [22]. Similarly, in a study using three-dimensional model to optimize heat exchanger parameters, Kempf et al. showed that by using silicon carbide in the heat exchanger better temperature uniformity can be achieved compared to stainless steel of 444 (SS 444) grade [25]. However, from the fabrication point of view, it will be highly cost-effective and easy to use SS than silicon carbide. The silicon carbide components are in general fabricated either by slip casting or gel-casting followed by sintering at above 2000°C. Such processes can adds up the TEG's cost significantly. The brass has higher density than SS which will increase the overall TEG's weight and the parasitic loss associated with it.

The AETEGs can be operated either at the maximum power (Pmax) or maximum efficiency. In automotive exhaust application, it is always preferable to maximize the power output as it reduces the engine load which will improve the overall fuel efficiency. The Pmax of a AETEG not only depends on hot side temperature (TH) but the temperature difference (ΔT) between the hot and the cold side of the module. Hence, to maximize the power output, the cold side

designed, fabricated and tested in the laboratory testing conditions, the cold side temperature was controlled by using a water-cooled heat sink. However, in actual vehicles, it can be con-

In the heat sink, the coolant flow is usually in the same (co-flow) or opposite (counterflow) direction to the exhaust gas flow as described in the previous section. Cross-flow and counter cross-flow arrangements are also used in few cases. The output power and the conversion efficiency of the AETEG for the various coolant flow arrangements mentioned above depends on the specific geometrical design of the cold side heat exchanger also. In the co-flow arrangement, the ΔT decreases along the streamwise direction of the exhaust gas flow as more sensible heat is absorbed and transferred to the cold side of the module tends to increase the coolant temperature. On the other hand, in counterflow arrangement, the ΔT decrease is lesser along the streamwise direction. However, to get the maximum power output in the TEG whether coflow or counter-flow is preferable depends on the temperature and flow rate of the exhaust gas

which in turn depends on the type, capacity and operating conditions of the engine.

The output power and the conversion efficiency of the AETEG for the various coolant flow configurations mentioned above depends on the specific geometrical design of the heat exchanger and heat sink combination. Su et al. performed simulation studies on different configurations of cooler design for TEG system viz., plate-shaped, stripe-shaped and diamond-shaped designs [27]. They further validated the simulation outputs with experiments and concluded that diamond-shaped design gave the highest power for a given engine speed among the three configurations for the cold side of the TEG system. They also observed that the temperature of the cold side is relatively uniform ensuring that the deviation in ΔT

The cold side heat exchanger in TEG system in actual vehicle can either be a separate TEG coolant circulation system or integrated to the existing engine coolant circuit (integrated cooling

) of the TE modules should be as low as possible. In most of the AETEGs

**3.3. Heat sink or cold side heat exchanger**

nected to the engine coolant circuit.

between the individual modules is minimal.

temperature (T<sup>c</sup>

170 Bringing Thermoelectricity into Reality

The heat transfer between hot side heat exchanger and the cold side heat sink through TE modules depends on how well various components of the AETEG are assembled so that thermal contact resistance will be minimal between various interfaces. The thermal contact resistance depends on many factors: the applied contact pressure, surface roughness, the interface materials and its hardness are some of the important parameters to list. The assembly components are responsible for enforcing sufficient force over the modules sandwiched between the heat exchanger and the heat sink. The ΔT across the hot and cold side of the TE module increases typically with contact pressure as it generates more area of physical contact at the microscopic level resulting in more heat conductance through the interfaces. **Figure 4** shows the curve showing the typical variation of ΔT with applied contact pressure in a TEG with and without interface materials. [29].

In the absence of any interface materials, it can be seen that the ΔT remarkably increases with applied pressure. With the use of thermal grease at the interface, the ΔT is almost invariant. However with graphite, the most common interface material in AETEG, the ΔT was significantly

**Figure 4.** Variation of differential temperatures with contact pressure using different interface materials [29].

produced is determined by multiple factors such as (1) location of the TEG in the exhaust line, (2) exhaust gas flow rate and temperature, (3) locations and arrangement of modules in the TEG, (4) area of coverage of the modules in the heat exchanger/s, (5) heat sink temperatures, (6) thermal conductance at various interfaces, and (7) scheme of the modules electrical connection. Several multiphysics simulations, combining fluid mechanics, heat transfer, and thermoelectric phenomena, have been carried out to predict the influence of some of the above mentioned factors on the performance and power output of the TEG. However, the experimental validation of these predictions is very limited [31, 32]. **Figure 6** shows the image of a test rig designed and developed by this author used for evaluating the AETEG performance using hot air/gas [33]. Testing of the TEG in this test rig offers complete performance details, that is, the efficiency of the TE modules, heat exchanger, and heat sink, which will be useful for further optimization of the design before evaluating their performance in the actual engine using established driving conditions. The test rig consists of the following sub systems:

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The hot air source is a blower and heater combined unit where a high pressure blower of 4000 lpm output capacity draws in air and passes it to air heater which can heat up the air up to 400°C. The pressure of the hot gas is measured at inlet and outlet of heat exchanger using

• Heat source

• Flowmeter

• Heat exchanger

• Pressure transmitter

• Differential Pressure transmitter

• Data Acquisition and Integration unit

**Figure 6.** Photo image of the AETEG and the test rig developed at CAEM, ARCI.

**Figure 5.** Estimation of the power per modules as a function of ΔT with different interface materials [28, 30].

higher than the other two for a given contact pressure particularly above 60 psi. The appropriate choice for the interface materials is the one with low hardness and high thermal conductivity. Such material will deform while applying pressure, make good contact between module and heat exchanger surfaces, and decrease the thermal contact resistance. **Figure 5** shows the comparison of the estimated power per Bi<sup>2</sup> Te3 modules as a function of ΔT in an investigation carried out using different interface materials [30]. Among the graphite, aluminum, tin and lead foils, the softest material lead is estimated to give lesser thermal contact resistance.
