**3.1. Features of technical requirements for ATEG in comparison with stationary TEGs**

The increase in the electrical power of on-board equipment cannot always be achieved by installing a more efficient alternator due to the high density of the layouts of modern ICEs (this is especially true for small vehicles, for example, motorcycles). In this case, the TEG installation on the EG system can be the way out.

At the same time, in contrast to TEGs operating in stationary conditions with a constant heat flux, the effective use of TEGs for vehicles requires the solution of a number of specific tasks which must ensure:


The solution of these problems requires a rational arrangement of TEG, parametric optimization of the quantitative TEG models taking into account multiple accompanying physical processes as well as carrying out experimental studies for quantitative verification of models and prototype tests.

#### **3.2. General requirements for ATEG design**

As a rule, ATEG has a hot heat exchanger coming through the EG and one or more TEM sections, cold heat exchanger to drain the heat into the hydraulic cooling system or directly into the external environment, the voltage stabilization system for the on-board power supply system (12 V).

ATEGs must be connected to the exhaust, power and hydraulic cooling systems of the vehicle and effectively operate with them. However, there are challenges of all these points implementation during ATEG design.

When TEG and ICE are simultaneously considered, the quality characteristics are:

• increment of the ICE shaft power Δ *Wmech* while maintaining fuel consumption *qfuel*;

• the change in the content of toxic impurities in the EG;

• the change in the level of acoustic vibrations in the EG stream.

shaft power Δ *qfuel*;

vehicles.

**3.4. Hot heat exchanger**

high-temperature thermal grease.

fins 7 serve to intensify the heat exchange.

• measuring the fuel consumption of the internal combustion engine while maintaining the

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191

These values must be measured for certain operating modes of the ICE. For example, the fuel consumption and emissions into the atmosphere from passenger cars are regulated by the standards: In Europe NEDC, in US EPA Federal test FTP72/75 and JC08 in Japan. Recently, the transition to the unified standard Worldwide harmonized Light vehicles Test Procedure (WLTP) is planned. These standards describe the sequence of changes in the speed of a vehicle in typical operating modes ("Urban driving Cycle", "Extra-urban driving Cycle" and "Combined"), which is installed on a test stand. Similar standards exist for other types of

Much attention is paid to the design of the ATEG hot exchanger. As a rule, the resistance of convective heat transfer is the largest in the thermal circuit of ATEG or comparable to the resistance of TEM. For example, the thermal resistance of convective heat transfer was 39–76% of the total resistance in the thermal circuit of TEG, decreasing with the growth of engine rotation n<sup>e</sup> (**Figure 2**) in the mathematical model for TEG with hexagonal heat exchanger and longitudinal finning described in [25]. The design of the TAG is similar to the one described in article [26], but the contact resistance of the joints was also taken into account which are also very significant. To obtain acceptable contact resistance of joints, the following actions are required: increase of holding pressure on the surface area, surface treatment of heat exchanger with low roughness and shape deviations, elimination of corrosion and thick oxide films and application of

The hot TEG heat exchanger should have a large area for the installation of serial flat TEMs. Constructions with 2 [27], 4 [28] and 6 [25, 26, 29, 30] external faces are known. In **Figure 3**, the elements of TEG with such constructions are marked: 1 - hot heat exchanger; 2 - TEM; 3 - cold heat exchanger; 4 and 5 - diffuser and conceiver. Diffuser and conceiver are needed for uniform heat exchange and reduction of hydraulic resistance. The displacer 6 and the heat

There are known TEG versions made with several modular heat exchangers of a flat [33] or cylindrical [21] shape (**Figure 4**), the number of which must be selected for different cars, based on the consumption of EG. This greatly simplifies the design of the TEG, because the heat exchange module, with the hot and cold heat exchanger, and thermoelectric elements

General technical requirements for ATEG:


Technical requirements differ significantly in a wide range depending on the type of vehicle. For example, the limitation for mass and dimensions for trucks is much softer, while the requirement for air resistance is much stricter than for a motorcycle.

There are conflicts of design goals. The most challenging issue is that high efficiency of heat removal can be achieved due to fins and turbulators of the EG flow, but they increase the air resistance.

Below, in this section, the known technical solutions and the structural elements used in them and requirements to them are described.

#### **3.3. ATEG quality characteristics**

The integral TEG quality characteristics are


But these parameters can only be determined for a given engine with a selected operating mode or as the function of the flow, temperature of the EG, the flow and temperature of the liquid in the cooling system and the resistance of the electrical load *Rload* applied to the TEG.

When TEG and ICE are simultaneously considered, the quality characteristics are:


These values must be measured for certain operating modes of the ICE. For example, the fuel consumption and emissions into the atmosphere from passenger cars are regulated by the standards: In Europe NEDC, in US EPA Federal test FTP72/75 and JC08 in Japan. Recently, the transition to the unified standard Worldwide harmonized Light vehicles Test Procedure (WLTP) is planned. These standards describe the sequence of changes in the speed of a vehicle in typical operating modes ("Urban driving Cycle", "Extra-urban driving Cycle" and "Combined"), which is installed on a test stand. Similar standards exist for other types of vehicles.

#### **3.4. Hot heat exchanger**

ATEGs must be connected to the exhaust, power and hydraulic cooling systems of the vehicle and effectively operate with them. However, there are challenges of all these points imple-

Technical requirements differ significantly in a wide range depending on the type of vehicle. For example, the limitation for mass and dimensions for trucks is much softer, while the

There are conflicts of design goals. The most challenging issue is that high efficiency of heat removal can be achieved due to fins and turbulators of the EG flow, but they increase the air

Below, in this section, the known technical solutions and the structural elements used in them

But these parameters can only be determined for a given engine with a selected operating mode or as the function of the flow, temperature of the EG, the flow and temperature of the liquid in the cooling system and the resistance of the electrical load *Rload* applied to the TEG.

• electric power consumption *Wneed* for the operation of TEG auxiliary systems;

mentation during ATEG design.

190 Bringing Thermoelectricity into Reality

General technical requirements for ATEG:

• limitation of weight and dimensions;

and requirements to them are described.

The integral TEG quality characteristics are

• electric power supplied by the TEM *WTEM*;

• total electric power produced by TEG *Wteg*; • drop pressure of a hot heat exchanger Δ *pteg*;

**3.3. ATEG quality characteristics**

• the efficiency of TEM *ηTEM*; • the efficiency of TEG *ηteg*;

• TEG weight *mteg*.

• low hydraulic resistance;

resistance.

• technological design for series production;

• high efficiency of heat removal and heat transfer;

• no resonance frequencies in the range of 15–200 Hz.

• reliability with frequent changes in pressure and temperature of EG;

requirement for air resistance is much stricter than for a motorcycle.

Much attention is paid to the design of the ATEG hot exchanger. As a rule, the resistance of convective heat transfer is the largest in the thermal circuit of ATEG or comparable to the resistance of TEM. For example, the thermal resistance of convective heat transfer was 39–76% of the total resistance in the thermal circuit of TEG, decreasing with the growth of engine rotation n<sup>e</sup> (**Figure 2**) in the mathematical model for TEG with hexagonal heat exchanger and longitudinal finning described in [25]. The design of the TAG is similar to the one described in article [26], but the contact resistance of the joints was also taken into account which are also very significant.

To obtain acceptable contact resistance of joints, the following actions are required: increase of holding pressure on the surface area, surface treatment of heat exchanger with low roughness and shape deviations, elimination of corrosion and thick oxide films and application of high-temperature thermal grease.

The hot TEG heat exchanger should have a large area for the installation of serial flat TEMs. Constructions with 2 [27], 4 [28] and 6 [25, 26, 29, 30] external faces are known. In **Figure 3**, the elements of TEG with such constructions are marked: 1 - hot heat exchanger; 2 - TEM; 3 - cold heat exchanger; 4 and 5 - diffuser and conceiver. Diffuser and conceiver are needed for uniform heat exchange and reduction of hydraulic resistance. The displacer 6 and the heat fins 7 serve to intensify the heat exchange.

There are known TEG versions made with several modular heat exchangers of a flat [33] or cylindrical [21] shape (**Figure 4**), the number of which must be selected for different cars, based on the consumption of EG. This greatly simplifies the design of the TEG, because the heat exchange module, with the hot and cold heat exchanger, and thermoelectric elements have been optimized in advance. The designs with flat heat exchangers involve the use of serial TEMs and can easily be adapted to other exhaust gas temperatures by changing the TEM. The design with cylindrical modules requires manufacturing special thermocouples and is more difficult to be adapted to other temperatures. Constructions with cylindrical heat exchangers (**Figure 4b**) have a larger ratio of the area of the hot heat exchanger to the area of the cold heat exchanger, but its disadvantage is reduction of pressure in a contact area, in the case if this area is heated.

These drop pressures are quite large and can lead to a drop in the efficiency of the engine, which suggests the need for careful optimization of the flow channel taking into account its

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According to RANS turbulent model usage [27], it is defined that the presence of numerous dimples in comparison with a flat wall allows increasing the Nusselt number *Nu* by 1.17–1.4 times with an increase of the Reynolds number Re from 10,000 to 25,000, but at the same time fanning friction factor *f* grows in 1.05–1.25 times. A single quality parameter is selected in [27] taking into account the intensification of heat exchange and the increase in friction factor:

> *Nu*<sup>0</sup> ( *f* \_\_0 *f*) 1/3

wall. According to parameter P, the heat exchanger with dimples turned out to be better than

**Figure 3.** Constructions with flat walls of hot heat exchanger: (a) with two flat walls [31, 32]; (b) with four flat walls [28];

are Nusselt number and the friction factor flow for a heat exchanger with a flat

(1)

193

resistance to exhaust gas flow and its effect on the ICE.

*P* = \_\_\_\_ *Nu*

where *Nu* and *f*

(c) with six walls [25, 33].

0

two designs with straight fins.

To intensify convective heat transfer in a hot heat exchanger, longitudinal [25] or oblique [29] fins and a displacer [25, 29, 30] are used, although they increase the hydraulic resistance. To reduce the length of the thermal way, the fins must run perpendicular to the plane of the heat exchanger, but in terms of manufacturability, it is convenient to make the fins in the form of folded plates [34–36] (**Figure 5a**). The heat exchanger shell can be made from cast iron or by connecting steel billets. If the fins are made from individual blanks, they must be welded to reduce thermal resistances.

The EG temperature reduces as it moves inside the hot heat exchanger. If several sections of the same TEM are used in the TEG, in order to equalize their heat flux through the different sections, it is necessary to increase the heat flux for the TEM sections located closer to the outlet. To do this, increase the area and thickness of the heat fins located closer to the outlet (**Figures 3a** and **5a**) or include water cooling towards the EG flow.

Despite the fact that the models of gas heat exchangers have been thoroughly studied [37, 38], complex geometry of the structure and nonlinearity of heat transfer in turbulent motion require the optimization of the flow channel with the use of finite volume method [27, 29] for estimating the heat flow and hydraulic resistance.

The approximate values of the pressure differences for some of the considered TEG heat exchangers for passenger cars are given in **Table 1**. The length of heat exchanger L is with lengths of diffuser and conceiver. *Dp* is an inside diameter of the inlet branch pipe.

**Figure 2.** Resistance of the components of the thermal circuit in [25], HE, hot exchanger; TE, thermoelectric element; CE, cold exchanger.

These drop pressures are quite large and can lead to a drop in the efficiency of the engine, which suggests the need for careful optimization of the flow channel taking into account its resistance to exhaust gas flow and its effect on the ICE.

have been optimized in advance. The designs with flat heat exchangers involve the use of serial TEMs and can easily be adapted to other exhaust gas temperatures by changing the TEM. The design with cylindrical modules requires manufacturing special thermocouples and is more difficult to be adapted to other temperatures. Constructions with cylindrical heat exchangers (**Figure 4b**) have a larger ratio of the area of the hot heat exchanger to the area of the cold heat exchanger, but its disadvantage is reduction of pressure in a contact area, in the

To intensify convective heat transfer in a hot heat exchanger, longitudinal [25] or oblique [29] fins and a displacer [25, 29, 30] are used, although they increase the hydraulic resistance. To reduce the length of the thermal way, the fins must run perpendicular to the plane of the heat exchanger, but in terms of manufacturability, it is convenient to make the fins in the form of folded plates [34–36] (**Figure 5a**). The heat exchanger shell can be made from cast iron or by connecting steel billets. If the fins are made from individual blanks, they must be welded to

The EG temperature reduces as it moves inside the hot heat exchanger. If several sections of the same TEM are used in the TEG, in order to equalize their heat flux through the different sections, it is necessary to increase the heat flux for the TEM sections located closer to the outlet. To do this, increase the area and thickness of the heat fins located closer to the outlet

Despite the fact that the models of gas heat exchangers have been thoroughly studied [37, 38], complex geometry of the structure and nonlinearity of heat transfer in turbulent motion require the optimization of the flow channel with the use of finite volume method [27, 29] for

The approximate values of the pressure differences for some of the considered TEG heat exchangers for passenger cars are given in **Table 1**. The length of heat exchanger L is with

**Figure 2.** Resistance of the components of the thermal circuit in [25], HE, hot exchanger; TE, thermoelectric element; CE,

is an inside diameter of the inlet branch pipe.

(**Figures 3a** and **5a**) or include water cooling towards the EG flow.

estimating the heat flow and hydraulic resistance.

lengths of diffuser and conceiver. *Dp*

cold exchanger.

case if this area is heated.

192 Bringing Thermoelectricity into Reality

reduce thermal resistances.

According to RANS turbulent model usage [27], it is defined that the presence of numerous dimples in comparison with a flat wall allows increasing the Nusselt number *Nu* by 1.17–1.4 times with an increase of the Reynolds number Re from 10,000 to 25,000, but at the same time fanning friction factor *f* grows in 1.05–1.25 times. A single quality parameter is selected in [27] taking into account the intensification of heat exchange and the increase in friction factor:

$$P = \frac{N\mu}{N\mu\_0} \left(\frac{f\_0}{f}\right)^{1/3} \tag{1}$$

where *Nu* and *f* 0 are Nusselt number and the friction factor flow for a heat exchanger with a flat wall. According to parameter P, the heat exchanger with dimples turned out to be better than two designs with straight fins.

**Figure 3.** Constructions with flat walls of hot heat exchanger: (a) with two flat walls [31, 32]; (b) with four flat walls [28]; (c) with six walls [25, 33].

**Figure 4.** TEG constructions with several hot heat exchangers: (a) with flat modules [33]; (b) with cylindrical modules [21].

> In [29], eight designs of hexagonal heat exchangers with different combinations of longitudinal, oblique fins and also with dimpless are compared based on finite volume method quantification with the use of Lam-Bremhorst *k* − *ε* turbulence model. As a generalized criterion, a

1920 3650 1073 7550 Hexagonal heat exchanger

2360 3650 1073 7200 Hexagonal heat exchanger

form of pits [29] <sup>6420</sup> <sup>3650</sup> <sup>1073</sup> <sup>8640</sup>

*Nu*<sup>0</sup> *ε* \_\_<sup>0</sup> *<sup>ε</sup>* <sup>=</sup> \_\_\_\_ *Nu Nu*<sup>0</sup> *f* \_\_0

However, such approaches do not allow to take into account the actual drop in the engine power from the pressure drop, therefore it is necessary to introduce the ICE model and consider the mutual behavior of the internal combustion engine and ATEG, as was done in [21, 25, 30]. For this purpose, approximate dependencies of the power drop of the internal combustion engine on the volume flow EG and additional hydraulic resistance of the exhaust pipe was

Convection intensification of heat exchanger can provoke the oxidation of products due to incomplete fuel combustion (hydrocarbon and volatile organic compound), but the authors

of this section are not acquainted with the quantitative studies on this issue.

Thus, the technical requirements for the hot heat exchanger are:

*<sup>f</sup>* (2)

parameter of:

**Drop pressure Δ** *peg***, Pa**

constructed.

where *ε* is loss factor.

• low hydraulic resistance;

*P* = \_\_\_\_ *Nu*

**Table 1.** Comparison of pressure differentials in various works.

**Flow EG** *Qeg***, L/min Inlet EG** 

**temperature**  *TEG***, K**

100–700 1800 600 500–750 Rectangle 295 × 12

4800–5360 3920 570–870 — Rectangle 310 × 22

250–450 2730 350–760 500–4200 Hexagon (side 63)

960 3650 1073 5300 Hexagon (side 62)

**TEM heat flow Q, W**

**Cross-section of heat exchanger, mm**

Prospects and Problems of Increasing the Automotive Thermoelectric Generators Efficiency

(inner sizes) *L* = 625,

(outer sizes) *L* = 696,

 = 50 inner hexahedral dispergator (side 20, length 174)

=45 with

*Dp* =35

*Dp* =50

*L* = 408, *Dp*

*L* = 593 *Dp*

inner hexahedral dispergator (side 29, length 513)

**Methods of convection intensification**

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Flat heat exchangers with fins and dimples (**Figure 5c**) [27]

195

Flat heat exchangers with folded fins (**Figure 5a**) [35]

Hexagonal heat exchanger with longitudinal fins [39]

Hexagonal heat exchanger with flat walls [29]

with longitudinal fins [29]

with oblique fins and heat exchange intensifiers in the

**Figure 5.** Thermal heat exchange intensifiers: (a) fins in the form of folded plates [35]; (b) oblique fins [29]; (c) dimpled surface [27].


**Table 1.** Comparison of pressure differentials in various works.

In [29], eight designs of hexagonal heat exchangers with different combinations of longitudinal, oblique fins and also with dimpless are compared based on finite volume method quantification with the use of Lam-Bremhorst *k* − *ε* turbulence model. As a generalized criterion, a parameter of:

$$P = \frac{Nu}{Nu\_0} \frac{\varepsilon\_0}{\varepsilon} = \frac{Nu}{Nu\_0} \frac{f\_0}{f} \tag{2}$$

where *ε* is loss factor.

However, such approaches do not allow to take into account the actual drop in the engine power from the pressure drop, therefore it is necessary to introduce the ICE model and consider the mutual behavior of the internal combustion engine and ATEG, as was done in [21, 25, 30]. For this purpose, approximate dependencies of the power drop of the internal combustion engine on the volume flow EG and additional hydraulic resistance of the exhaust pipe was constructed.

Convection intensification of heat exchanger can provoke the oxidation of products due to incomplete fuel combustion (hydrocarbon and volatile organic compound), but the authors of this section are not acquainted with the quantitative studies on this issue.

Thus, the technical requirements for the hot heat exchanger are:

• low hydraulic resistance;

**Figure 5.** Thermal heat exchange intensifiers: (a) fins in the form of folded plates [35]; (b) oblique fins [29]; (c) dimpled

**Figure 4.** TEG constructions with several hot heat exchangers: (a) with flat modules [33]; (b) with cylindrical modules

surface [27].

[21].

194 Bringing Thermoelectricity into Reality


## **3.5. Thermoelectric materials and modules**

The TEM is a key element of ATEG, in which, through the Seebeck effect, there is a direct conversion of the heat into electricity. The output electric power of the TEM depends on the heat flux, the temperature difference of its thermocouples, the thermal and electrical resistances in its construction, the geometry of the thermoelements which must be optimized for the given temperature conditions, the consistency of the internal resistance with the load resistance and the ZT-factor of used semiconductor materials. Moreover, ZT under optimal design is a key parameter of TEM efficiency.

**Figure 6** shows the required (green line) and achievable electrical power of ATEG when it is installed in different parts of the exhaust system (close-coupled position or away from the engine). The data are given for different materials/modules for different types of cars with efficiency of the hot heat exchanger *η<sup>t</sup>* of 50, 66 and 75% (C/D - mid-range class, E/F - upper/ luxury class and M/J - multi-van, SUV, utilities). **Table 2** shows the efficiency of the most promising materials for use in ATEG.

efficiency of the heat exchanger and installing ATEG closer to the output manifold can reduce this requirement up to 5% or less. At the same time, on the basis of classical materials for which large-

Prospects and Problems of Increasing the Automotive Thermoelectric Generators Efficiency

requirements. In addition, the large-scale serial production also imposes on us the friendly environmental production, non-toxic, reliability and low cost of (\$/W energy) which unfortunately does not fully match with the materials above. The latter requirement is paramount. If the estimated the cost of current TEMs is 5 \$/W, then economically it is necessary for ATEG, to increase

Among the most promising materials—which are being actively developed and persistently

• mechanical strength in conditions of cyclic operation (heating/cooling, not less than

Existing developed ATEG mock-ups (prototypes) basically use TEMS with flat structure which thermoelectric elements are connected in series by conductor tabs commutation and clamped between two insulating ceramic or polymer plates (**Figure 7a**). Those plates are

being used in ATEG are the silicide, skutterudite or half-Heusler materials [40].

General technical requirements for TEMs used in ATEG can be presented:

, *PbTe*, *SiGe*), it is quite capable to satisfy these

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scale serial production is currently established (*Bi*<sup>2</sup> *Te*<sup>3</sup>

**Figure 6.** Necessary material/module efficiency [41].

the prospect of its commercialization, achieve 1 \$/W [40].

• efficiency (TEM should convert as much heat as possible);

• chemical stability under given temperature variables;

• shock and vibration durability in the range of 15–200 Hz;

• manufacturability and low production costs for serial production.

• compactness;

10,000 cycles);

The analysis of **Figure 6** shows that for achieving the required electrical power, the efficiency of the thermoelectric module, using a heat exchanger of 50%*η<sup>t</sup>* , should be at least 8.2%. Increasing the


**Table 2.** Efficiency of thermoelectric materials and modules on their basis [40].

Prospects and Problems of Increasing the Automotive Thermoelectric Generators Efficiency http://dx.doi.org/10.5772/intechopen.76971 197

**Figure 6.** Necessary material/module efficiency [41].

efficiency of the heat exchanger and installing ATEG closer to the output manifold can reduce this requirement up to 5% or less. At the same time, on the basis of classical materials for which largescale serial production is currently established (*Bi*<sup>2</sup> *Te*<sup>3</sup> , *PbTe*, *SiGe*), it is quite capable to satisfy these requirements. In addition, the large-scale serial production also imposes on us the friendly environmental production, non-toxic, reliability and low cost of (\$/W energy) which unfortunately does not fully match with the materials above. The latter requirement is paramount. If the estimated the cost of current TEMs is 5 \$/W, then economically it is necessary for ATEG, to increase the prospect of its commercialization, achieve 1 \$/W [40].

Among the most promising materials—which are being actively developed and persistently being used in ATEG are the silicide, skutterudite or half-Heusler materials [40].

General technical requirements for TEMs used in ATEG can be presented:

• compactness;

• ability to work at EG temperature up to 700–1000°C without corrosion and loss of load-

• it is desirable to have elastic expansion bends for temperature deformations in the heat

The TEM is a key element of ATEG, in which, through the Seebeck effect, there is a direct conversion of the heat into electricity. The output electric power of the TEM depends on the heat flux, the temperature difference of its thermocouples, the thermal and electrical resistances in its construction, the geometry of the thermoelements which must be optimized for the given temperature conditions, the consistency of the internal resistance with the load resistance and the ZT-factor of used semiconductor materials. Moreover, ZT under optimal design is a key

**Figure 6** shows the required (green line) and achievable electrical power of ATEG when it is installed in different parts of the exhaust system (close-coupled position or away from the engine). The data are given for different materials/modules for different types of cars with

luxury class and M/J - multi-van, SUV, utilities). **Table 2** shows the efficiency of the most

The analysis of **Figure 6** shows that for achieving the required electrical power, the efficiency of

of 50, 66 and 75% (C/D - mid-range class, E/F - upper/

, should be at least 8.2%. Increasing the

• possibility of clamping TEM with considerable efforts without heat bridges;

exchanger itself and (or) in the TEM system clamping.

bearing capacity;

196 Bringing Thermoelectricity into Reality

• high efficiency of convective heat sink;

**3.5. Thermoelectric materials and modules**

parameter of TEM efficiency.

efficiency of the hot heat exchanger *η<sup>t</sup>*

promising materials for use in ATEG.

the thermoelectric module, using a heat exchanger of 50%*η<sup>t</sup>*

**Table 2.** Efficiency of thermoelectric materials and modules on their basis [40].


Existing developed ATEG mock-ups (prototypes) basically use TEMS with flat structure which thermoelectric elements are connected in series by conductor tabs commutation and clamped between two insulating ceramic or polymer plates (**Figure 7a**). Those plates are

between thermoelement and conductive tab, due to the fact that during compression of such thermoelement between hot and cold heat exchangers minimum thermal resistances should

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ATEG for heat removal must be connected to the cooling system of the car or use its own

In the first case, the load on the regular car cooling system is increasing. In most cars, the air intake area cannot be increased without significant design changes. This circumstance limits the thermal power passing through the TEG and the electric power it generates. This obstacle can be overcome for new models of cars that would be developed taking into account the pos-

In the second case, the autonomic TEG cooling system can be designed in different ways.

An independent liquid cooling system with forced circulation is relatively complex and increases mass and dimensions. The air cooling of the ribbed cold heat exchanger with an oncoming airflow can be comparable to forced liquid cooling. So in the article [28] when considering TEG models for motorcycles and snowmobiles with Yamaha WR450F motorcycle engine (a volume 0.449 mL), the values of the generated electric power of 80–105 W have been obtained at speeds of more than 40 km/h. But the protruding air radiators will inevitably

Thermoelectric generators for internal combustion engines on water transport or snowmobiles can be installed without large and protruding cold heat exchangers due to the high

It should be noted that the use of an independent forced liquid cooling system requires additional energy costs for pumps, fans and valves operations. In the case of the ATEG integration into the car cooling system there is a need in the additional power for increasing cooling system productivity. In this way, the electricity generated by ATEG partly expend on the

The paper [21] presents the results of the evaluation of the ATEG various components and

F350 cars. The energy consumption of the ATEG cooling system on the BMW X3 (**Figure 8**)

In the same article [21], 23 concepts of ATEG integrating into the car cooling system are devel-

tor power of 19.8 W. The cooling system of FORD F350 requires 25 W (0.5 g *CO*<sup>2</sup>

increase) with an average generator power of 470 W under the same conditions.

emissions for two generators installed on BMW X3 and Ford

/ mil emission increase) with an average genera-

/mil emission

be ensured.

**3.6. Cooling system**

sibility of installing TEG.

degrade the aerodynamic characteristics of the vehicle.

efficiency of cooling by water and snow.

averages 5.7 W in the cycle US06 (0.2 g *CO*<sup>2</sup>

oped according to the following criteria:

• influence on transmission warm-up;

• influence on engine warm-up; • influence on engine cooling;

operation of the ATEG itself.

parameters influence on *CO*<sup>2</sup>

cooling system.

**Figure 7.** TEM designs: (a) with flat legs; (b) with cylindrical shape; (c) with disc shape; (d) with T-shaped conductive layers.

(cold and hot heat sides) perpendicular to heat flow. To protect from the sublimation process at high temperatures, the module can be placed in a sealed shell or coated with heat-resistant protective enamels. The use of such TEMs in ATEG presupposes the presence of flat surfaces on heat exchangers, and also requires careful study of the clamping design in order to ensure a constant and uniform effort throughout. The inability to provide such a clamp and, as a consequence, large thermal resistances forced the development of ATEG in the framework of the DOE No. DE-FC26-04NT42279 to move from a flat construction to a cylindrical one on its 4th stage [20].

The cylindrical design of ATEG with radial heat flow requires the use of tubular TEMs, in which the thermoelements can be utilized in the cylindrical (**Figure 7b**) or disc (**Figure 7c**) shapes [41]. Analogues of flat TEM with a cylindrical surface as heat exchangers can also be made. It should be noted that such TEMs will be considerably less technologically advanced than flat modules.

In view of the fact that ZTs of individual thermoelectric materials are significantly dependent on the temperature, there is a practice to use segmented thermoelements in TEMs for improving their efficiency [42, 43]. At the same time, the use of different materials with different physic-mechanical properties in the design of a thermoelement results in the need to harmonize, for example, the cross-sectional area for different materials and their thermal linear expansion coefficient (TLEC). The latter is especially important for ATEG operating under conditions of constantly changing heat flow, and also relates to the coordination of the TLEC of the p- and n-type elements and the conductive tabs for multi-segment TEMs.

An interesting design of the TEM, partially solving the problem of coordinating the material properties, was proposed in D.T. Crane et al. [44, 45]. This design with T-shaped conductive layers (**Figure 7d**), also playing the role of heat conductors, makes it possible to apply different thicknesses of elements and their different geometries, and to vary the thickness of individual segments, providing an optimal thermal operating mode for each material, therefore maximum efficiency. At the same time, attention is drawn to the stress state of the contact between thermoelement and conductive tab, due to the fact that during compression of such thermoelement between hot and cold heat exchangers minimum thermal resistances should be ensured.

#### **3.6. Cooling system**

(cold and hot heat sides) perpendicular to heat flow. To protect from the sublimation process at high temperatures, the module can be placed in a sealed shell or coated with heat-resistant protective enamels. The use of such TEMs in ATEG presupposes the presence of flat surfaces on heat exchangers, and also requires careful study of the clamping design in order to ensure a constant and uniform effort throughout. The inability to provide such a clamp and, as a consequence, large thermal resistances forced the development of ATEG in the framework of the DOE No. DE-FC26-04NT42279 to move from a flat construction to a cylindrical one on its 4th stage [20]. The cylindrical design of ATEG with radial heat flow requires the use of tubular TEMs, in which the thermoelements can be utilized in the cylindrical (**Figure 7b**) or disc (**Figure 7c**) shapes [41]. Analogues of flat TEM with a cylindrical surface as heat exchangers can also be made. It should be noted that such TEMs will be considerably less technologically advanced

**Figure 7.** TEM designs: (a) with flat legs; (b) with cylindrical shape; (c) with disc shape; (d) with T-shaped conductive

In view of the fact that ZTs of individual thermoelectric materials are significantly dependent on the temperature, there is a practice to use segmented thermoelements in TEMs for improving their efficiency [42, 43]. At the same time, the use of different materials with different physic-mechanical properties in the design of a thermoelement results in the need to harmonize, for example, the cross-sectional area for different materials and their thermal linear expansion coefficient (TLEC). The latter is especially important for ATEG operating under conditions of constantly changing heat flow, and also relates to the coordination of the TLEC

An interesting design of the TEM, partially solving the problem of coordinating the material properties, was proposed in D.T. Crane et al. [44, 45]. This design with T-shaped conductive layers (**Figure 7d**), also playing the role of heat conductors, makes it possible to apply different thicknesses of elements and their different geometries, and to vary the thickness of individual segments, providing an optimal thermal operating mode for each material, therefore maximum efficiency. At the same time, attention is drawn to the stress state of the contact

of the p- and n-type elements and the conductive tabs for multi-segment TEMs.

than flat modules.

layers.

198 Bringing Thermoelectricity into Reality

ATEG for heat removal must be connected to the cooling system of the car or use its own cooling system.

In the first case, the load on the regular car cooling system is increasing. In most cars, the air intake area cannot be increased without significant design changes. This circumstance limits the thermal power passing through the TEG and the electric power it generates. This obstacle can be overcome for new models of cars that would be developed taking into account the possibility of installing TEG.

In the second case, the autonomic TEG cooling system can be designed in different ways.

An independent liquid cooling system with forced circulation is relatively complex and increases mass and dimensions. The air cooling of the ribbed cold heat exchanger with an oncoming airflow can be comparable to forced liquid cooling. So in the article [28] when considering TEG models for motorcycles and snowmobiles with Yamaha WR450F motorcycle engine (a volume 0.449 mL), the values of the generated electric power of 80–105 W have been obtained at speeds of more than 40 km/h. But the protruding air radiators will inevitably degrade the aerodynamic characteristics of the vehicle.

Thermoelectric generators for internal combustion engines on water transport or snowmobiles can be installed without large and protruding cold heat exchangers due to the high efficiency of cooling by water and snow.

It should be noted that the use of an independent forced liquid cooling system requires additional energy costs for pumps, fans and valves operations. In the case of the ATEG integration into the car cooling system there is a need in the additional power for increasing cooling system productivity. In this way, the electricity generated by ATEG partly expend on the operation of the ATEG itself.

The paper [21] presents the results of the evaluation of the ATEG various components and parameters influence on *CO*<sup>2</sup> emissions for two generators installed on BMW X3 and Ford F350 cars. The energy consumption of the ATEG cooling system on the BMW X3 (**Figure 8**) averages 5.7 W in the cycle US06 (0.2 g *CO*<sup>2</sup> / mil emission increase) with an average generator power of 19.8 W. The cooling system of FORD F350 requires 25 W (0.5 g *CO*<sup>2</sup> /mil emission increase) with an average generator power of 470 W under the same conditions.

In the same article [21], 23 concepts of ATEG integrating into the car cooling system are developed according to the following criteria:


As a result, the authors choose the most optimal solution (**Figure 9**), the main idea of which is that the designer should use the extracted exhaust heat for accelerate engine warm-up, without influencing the original warm-up strategy of the cooling system.

When using forced liquid cooling (general or independent), it is necessary to provide for the regulation of the flow rate of the liquid in order to reduce the energy loss when pumping the liquid in the conditions of low thermal power passing through the TEG (for example, idling).

#### **3.7. Cold heat exchanger**

A cold heat exchanger must remove heat from the TEG into the water cooling system or directly into the external environment. The second option can be very simple and effective when cooling with snow (for snowmobiles) or with water (for hydrocycles) when the heat transfer coefficient *acold* > 1000 W/(m2 K). Cooling by external ambient air requires the installation of numerous ribs, since in this case *acold* = 20–100 W/(m2 K), and this strongly depends on the speed of the ambient air [28]. Installing an external ribbed air heat exchanger can increase the aerodynamic resistance of the vehicle. Even if it is a small increase in resistance—this phenomenon can offset the reduction in fuel consumption from electricity generation.

The geometry of the water heat exchanger is paid much less attention, since due to the high heat transfer coefficient there is very favorable heat removal conditions (**Figure 2**).

As an example, there are the results of ATEG calculations with air and forced water cooling, considered in the article [28] (**Figure 10a**, **b**) for Yamaha WR450F IC engine (volume 0.449 L, 9000 revmin−1). **Figure 10** shows: 1 – hot heat exchanger; 2 - TEM with the height *b te* of the thermoelement; 3 – cold heat exchanger. The air heat exchangers are considered in a motorcycle operating conditions (air flow temperature *Tair* = +20°C and speed *Vair*) and snowmachine mode (*Tair*= − 20°C and speed *Vair*).

The gas heat transfer calculations are performed with free air flow around ATEG without taking into account the geometry of the vehicle.

Technical requirements for the cold heat exchanger:

• amount of water should be precisely defined for required cooling;

• when using a heat transfer fluid, the possibility of boiling, leaks, and condensation on cold

**Figure 9.** Coolant system of X3 xDrive28i with automatic transmission and selected concept for coolant integration [21].

**Figure 8.** The results of the evaluation of the ATEG various components and parameters influence on *CO*<sup>2</sup>

Prospects and Problems of Increasing the Automotive Thermoelectric Generators Efficiency

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emissions for

201

• insignificant contact thermal resistance;

• lightweight construction;

BMW X3 (a) and Ford F350 (b) ATEGs [21].

• corrosion resistance and

parts should be avoided.

The different heights of thermoelements calculations allows choosing the optimal value *b te* = 17 mm for the developed ATEG (**Figure 10d**). The generated ATEG power *WTEG* greatly decreases with air speed *Vair* decreasing and air temperature *Tair* increasing.

At *Tair*= − 20°C and *Vair*= 100 km h−1 ATEG with air cooling has comparable power *WTEG* with water cooled ATEG (**Figure 10d**). In this case, averaged in time electric power which supplies cooling systems *Wneed = WTEM*−*WTEG* is about 10 W or about 10% of the power produced by the optimal design.

Prospects and Problems of Increasing the Automotive Thermoelectric Generators Efficiency http://dx.doi.org/10.5772/intechopen.76971 201

**Figure 8.** The results of the evaluation of the ATEG various components and parameters influence on *CO*<sup>2</sup> emissions for BMW X3 (a) and Ford F350 (b) ATEGs [21].

**Figure 9.** Coolant system of X3 xDrive28i with automatic transmission and selected concept for coolant integration [21].

Technical requirements for the cold heat exchanger:


• influence on transmission cooling;

• influence on warm-up of cabin heating;

As a result, the authors choose the most optimal solution (**Figure 9**), the main idea of which is that the designer should use the extracted exhaust heat for accelerate engine warm-up, with-

When using forced liquid cooling (general or independent), it is necessary to provide for the regulation of the flow rate of the liquid in order to reduce the energy loss when pumping the liquid in the conditions of low thermal power passing through the TEG (for example, idling).

A cold heat exchanger must remove heat from the TEG into the water cooling system or directly into the external environment. The second option can be very simple and effective when cooling with snow (for snowmobiles) or with water (for hydrocycles) when the heat transfer coefficient *acold* > 1000 W/(m2 K). Cooling by external ambient air requires the installation of numerous ribs, since in this case *acold* = 20–100 W/(m2 K), and this strongly depends on the speed of the ambient air [28]. Installing an external ribbed air heat exchanger can increase the aerodynamic resistance of the vehicle. Even if it is a small increase in resistance—this phe-

The geometry of the water heat exchanger is paid much less attention, since due to the high

As an example, there are the results of ATEG calculations with air and forced water cooling, considered in the article [28] (**Figure 10a**, **b**) for Yamaha WR450F IC engine (volume 0.449 L,

moelement; 3 – cold heat exchanger. The air heat exchangers are considered in a motorcycle operating conditions (air flow temperature *Tair* = +20°C and speed *Vair*) and snowmachine mode

The gas heat transfer calculations are performed with free air flow around ATEG without tak-

The different heights of thermoelements calculations allows choosing the optimal value

At *Tair*= − 20°C and *Vair*= 100 km h−1 ATEG with air cooling has comparable power *WTEG* with water cooled ATEG (**Figure 10d**). In this case, averaged in time electric power which supplies cooling systems *Wneed = WTEM*−*WTEG* is about 10 W or about 10% of the power produced by the optimal

= 17 mm for the developed ATEG (**Figure 10d**). The generated ATEG power *WTEG* greatly

*te*

of the ther-

nomenon can offset the reduction in fuel consumption from electricity generation.

heat transfer coefficient there is very favorable heat removal conditions (**Figure 2**).

9000 revmin−1). **Figure 10** shows: 1 – hot heat exchanger; 2 - TEM with the height *b*

decreases with air speed *Vair* decreasing and air temperature *Tair* increasing.

out influencing the original warm-up strategy of the cooling system.

• influence on TEG cooling;

200 Bringing Thermoelectricity into Reality

• electric power consumption;

• weight and

• expected costs.

**3.7. Cold heat exchanger**

(*Tair*= − 20°C and speed *Vair*).

*b te*

design.

ing into account the geometry of the vehicle.


#### **3.8. Choice of heat exchanger material**

Physical requirements for the material of heat exchangers:


Recommendations:

power *W TEM*(d).

Stainless steel AICI 304

**Density** *ρ***, kg·m-3**

• cold water and air heat exchangers can be made from most of aluminum alloys.

corrosion coatings) or cast iron (for example, Cast EN-JL1050).

by welding from stainless steels, for example AICI 304.

• the finned hot heat exchangers are preferably made of low-carbon steel (possibly with anti-

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203

• hot heat exchangers without fins or with short fins can be made by casting from cast iron or

**Figure 10.** Comparison of TEG power with air cooling (a) and water cooling (b); temperature influence *T air* and air speed *V air* on power *W TEG* generated by ATEG (c); the influence of the thermoelement height *b te* on TEM generated

8000 17 17–18 1400

**1/K**

**Thermal expansion** *α* **·106,** 

**Melt point (soludus forallow) T, K**

**Thermal conductivity** *λ***,W/**

Pure copper 8933 400 17 1358 Pure aluminum 2702 240 24 933 Aluminum alloys 2600…2850 110–190 21–24 775–993 Low-carbon steel 7850 40–60 11–13 1425–1540

Cast EN-JL 1050 7250 42–44 13 1426

**(m·K)**

**Table 3.** Characteristics of materials for heat exchangers at T range 573–873 K.


**Table 3** compares the properties of the materials used with pure copper, which has the greatest thermal conductivity among structural materials. Adding impurities to copper or aluminum greatly reduces thermal conductivity, but increases strength and often corrosion resistance. The density of the material *ρ* affects the weight and cost. The coefficient of thermal expansion *α* is important in calculating thermal deformations and stresses.

The thickness of metallic heat exchangers walls is usually a few millimeters and their thermal resistance is negligible. But the finned gas heat exchangers are characterized by a long thermal path, therefore a high thermal conductivity is desirable for them *λ*.

Copper cannot be used in a hot heat exchanger because of its low corrosive resistance in the air against *CO*<sup>2</sup> vapors and water, without protective coatings (for example, nickel). Its application could be justified for external heat exchangers, but the issue is not enough density and high cost.

Aluminum can be applicable only for cold heat exchanger due to its low melting temperature characteristic. Also aluminum is cheap and has excellent corrosion resistance. Thermal conductivity of pure technical Aluminum is very high, but even insignificant amount of impurities can reduce conductivity by 1.5–2 times. So it is necessary to carefully select the fins size for finned aluminum air cold heat exchangers.

Steel has a high melting point and density. Low-carbon steels are prone to corrosion, and stainless steels have very low thermal conductivity, limited weldability and high cost. It is possible to use special stainless steels, for example, AICI 304, used for the manufacture of mufflers and hot air ducts. But the low thermal conductivity of this steel requires the production of hot heat exchangers with short and thick fins.

Cast iron makes it possible to create complex parts with good corrosion resistance. For example, cast radiators for heating systems were widely used before. The thermal conductivity of cast iron is strongly dependent on the shape and orientation of graphite inclusions having a thermal conductivity comparable to copper. The thermal conductivity of cast iron decreases with decreasing carbon content and increasing alloying additives, especially those that reduce graphitization (*Mn*,*Cr*). Grinding the size of graphite additives increases the strength, but reduces the thermal conductivity. For the manufacture of heat exchangers by casting, EN-JL1050 and the like grades can be used. However, it is desirable to avoid overheating of parts of cast iron heat exchangers in order to avoid ferritic-austenitic transformations (723°C).

Recommendations:

**3.8. Choice of heat exchanger material**

• high thermal conductivity of the material;

• high corrosion resistance;

202 Bringing Thermoelectricity into Reality

(700–1000°C) is needed.

a shape;

against *CO*<sup>2</sup>

Physical requirements for the material of heat exchangers:

• low density and/or high durability for decreasing the weight.

is important in calculating thermal deformations and stresses.

for finned aluminum air cold heat exchangers.

tion of hot heat exchangers with short and thick fins.

mal path, therefore a high thermal conductivity is desirable for them *λ*.

• good machinability of obtaining thermal surfaces with small roughness and deviations of

• for hot heat exchangers, a melting point substantially higher than the temperature of EG

**Table 3** compares the properties of the materials used with pure copper, which has the greatest thermal conductivity among structural materials. Adding impurities to copper or aluminum greatly reduces thermal conductivity, but increases strength and often corrosion resistance. The density of the material *ρ* affects the weight and cost. The coefficient of thermal expansion *α*

The thickness of metallic heat exchangers walls is usually a few millimeters and their thermal resistance is negligible. But the finned gas heat exchangers are characterized by a long ther-

Copper cannot be used in a hot heat exchanger because of its low corrosive resistance in the air

could be justified for external heat exchangers, but the issue is not enough density and high cost. Aluminum can be applicable only for cold heat exchanger due to its low melting temperature characteristic. Also aluminum is cheap and has excellent corrosion resistance. Thermal conductivity of pure technical Aluminum is very high, but even insignificant amount of impurities can reduce conductivity by 1.5–2 times. So it is necessary to carefully select the fins size

Steel has a high melting point and density. Low-carbon steels are prone to corrosion, and stainless steels have very low thermal conductivity, limited weldability and high cost. It is possible to use special stainless steels, for example, AICI 304, used for the manufacture of mufflers and hot air ducts. But the low thermal conductivity of this steel requires the produc-

Cast iron makes it possible to create complex parts with good corrosion resistance. For example, cast radiators for heating systems were widely used before. The thermal conductivity of cast iron is strongly dependent on the shape and orientation of graphite inclusions having a thermal conductivity comparable to copper. The thermal conductivity of cast iron decreases with decreasing carbon content and increasing alloying additives, especially those that reduce graphitization (*Mn*,*Cr*). Grinding the size of graphite additives increases the strength, but reduces the thermal conductivity. For the manufacture of heat exchangers by casting, EN-JL1050 and the like grades can be used. However, it is desirable to avoid overheating of parts of cast iron heat exchangers in order to avoid ferritic-austenitic transformations (723°C).

vapors and water, without protective coatings (for example, nickel). Its application


**Figure 10.** Comparison of TEG power with air cooling (a) and water cooling (b); temperature influence *T air* and air speed *V air* on power *W TEG* generated by ATEG (c); the influence of the thermoelement height *b te* on TEM generated power *W TEM*(d).


**Table 3.** Characteristics of materials for heat exchangers at T range 573–873 K.

#### **3.9. Troubleshooting**

This section provides general recommendations for troubleshooting issues that may occur when operating the ATEG.

Electrochemical corrosion is excluded by the selection of contacting materials with equal electrochemical potentials, qualitative isolation of electrical systems, using protective, electrically insulating or corrosion-resistant coatings and observing the pH of the coolant.

Excessive thermal deformations can lead to plastic deformation of parts and loss of flatness of contacts. It is possible to reduce the pressure force with the help of selecting the suitable materials as per their thermal expansion coefficients and using the elastic compensators for thermal deformations.

Fretting-wear occurs under the action of periodic tangential shifts on contacts. This process has not been fully studied. It can be eliminated by reducing the contact pressure (see paragraph above), using hardening or antifriction coatings or thermal grease.

The thermal contact resistance can increase due to several factors which are: oxidation, degradation of the thermal grease or reduction of the pressure during heating or cooling. Using of resistant materials, protective coatings and adjusting loads pressure can eliminate it.

Cleaning and monitoring of the pH of the coolant, selection of corrosion-resistant materials or coatings prevent heat exchangers pollution from corrosion, water deposits and soot.
