**5. The ways of ATEG efficiency increasing**

#### **5.1. Adapting the design to variable operating modes**

ATEG operation is accompanied by frequent volume flow differences and EG temperature changes, and vehicle electrical load is not constant. At the same time, it is necessary to ensure the optimum flow of EG through ATEG, when, on the one hand, there is sufficient intensive heat exchange in the ATEG, and on the other hand, the pressure drop EG is not too great.

Therefore, it is practical to use bypass for removing excessive EG through ATEG or a hot heat exchanger with variable hydraulic resistance of the flowing channel. For example, the patent [50] describes the construction of a heat exchanger using rotatable fins that allows intensifying heat transfer at low shaft speeds and reducing hydraulic resistance at high speeds. In **Figures 14**, 1- turning fins, 2- displacer, 3- TEM, 4- cold heat exchanger, 5- rotational blade-intensifier, 6 - control rod.

#### **5.2. Application of heat pipes**

The solution of operation modeling issue for the described system consists of thermal and

The heat problem is solved by finding the distribution of thermal fluxes and temperatures over the entire cross-section of the ATEG section. For this purpose, the heat balance equations are used [46]. The electric current, voltage and power generated by the TEM can be calculated as a function of the electrical load using the Kirchhoff rules in accordance with the proposed electrical connection circuit for the thermoelements [47]. The amount of generated electricity is calculated using classical methods for determining the efficiency of thermoelectric conversion [48]. All characteristic values, such as the Seebeck coefficient, heat and conductivity coef-

Since the relationship between vehicle operation modes, the generated heat by exhaust gases and the generated electricity by ATEG are nonlinear, it is necessary to dynamically model these processes. To simulate the operation of ATEG under non-stationary operating condi-

The ATEG model allows estimating transient processes which makes it possible to use it The New European Driving Cycle (NEDC) is most often used to test the new work models ATEG. There are numerous scientific publications presenting measurements and calculations of thermoelectric generators used in cars on the basis of the NEDC [49]. It is extremely important to consider the dynamic behavior of the thermoelectric system in order to make realistic

tions, the heat balance equations must be described in a differential form [45].

electric circuits calculations taking into account thermoelectric processes.

ficients, are taken into account.

206 Bringing Thermoelectricity into Reality

**4.3. Tests of models, city cycles**

**Figure 13.** Functional diagram of ATEG section.

predictions of the performance of ATEG in the vehicle.

One of the methods to increase the efficiency of ATEG can be the use of heat pipes in its structure [51, 52]. The reduction of the thermal resistance between exhaust gases and hot junctions allows increasing the hot junction temperature and also reduces the counter-pressure in the exhaust system in some cases. In addition, the heat pipes give the possibility of a more flexible approach towards the design of ATEG and there is no need to be limited to only the surface area of the hot heat exchanger. They also help to regulate the temperature of the hot junction by varying their length or by using variable conductance of heat pipes.

#### **5.3. About materials with a phase transition**

There is a need to select the operating point in the ATEG design. This fact connected with: variable heat flux through TEM under different operating conditions of the internal combustion engine, the dependence of the TEM semiconductor materials ZT on the junctions' temperature and the limitations on the peak temperature of the hot junction. Optimizing ATEG for obtaining maximum power at the extreme operating conditions of the internal combustion engine leads to low efficiency at low and medium rotational rates with low engine load. In the reverse situation, there is a need to use bypass for not overheating the TEM and not creating a counter-pressure in the exhaust pipe at high engine speeds. The solution to the problem of combining these two extreme situations and, consequently, increasing the efficiency of ATEG under different operating conditions of the ICE can be the use of materials with a phase transition that store heat at high loads on the ICE and give it to ATEG with a heat flux decrease in the exhaust line [53].

#### **5.4. Temperature rise of EG**

It is advisable to use EG at high temperatures for efficient ATEG operating which can be achieved through two technical solutions.

Firstly, installing the ATEG on ICE, which has the insulating combustion chamber with protective coatings. Such ICEs are increasingly being used in recent decades and are characterized as a high temperature of EG, low *CO* and residual hydrocarbons emissions, and fewer loads on the cooling system. Inside the ICE cylinders the operating gas reaches on its highest temperature which is about 2500°C. Mechanical work and temporary averaging of peak values reduce the temperature of the EG at the combustion engine output to 950–1000°C for gasoline internal combustion engines and 800–850°C for diesel engines.

Secondly, a precise thermal insulation of the collector and the exhaust pipe near the TEG location. For example, the authors of this chapter conducted experiments on ATEG which was connected to collector of 0.7 m long with a VAZ 21126 gasoline engine on a test stand. Its power was 72 kW and a volume of 1.6. The EG temperature measurements were made by using Kistler 4049B05DS1–2.0 sensor—installed at the entrance of the ATEG diffuser. It was noted that the insulation coating of the collector with basalt cloth increased the temperature of EG *Teg* by more than 300°C (**Figure 15**).

Thirdly, to obtain the maximum temperature of EG, it is desirable to install ATEG directly after the internal combustion engine, in front of the catalyst, turbocharger and silencer.

#### **5.5. Reduction of thermal resistance**

The thermal resistance of the heat exchanger contacts can greatly reduce heat flow through ATEG [25, 54]. It is shown in [25] that neglect of contact thermal resistances during calculation leads to an overestimation of the generated TEM power by 25–30%. To reduce contact resistance, all possible methods should be taken, including:


When operating ATEG on diesel engine, power may be reduced due to the deposition of soot in the hot heat exchanger. For example, Kajikawa describes an ATEG installed with a diesel engine [55]. During the first 100 h of operation, the heat flow through ATEG fell by 30%, reaching saturation levels due to soot formation, and further, remained almost unchanged for another 100 h. For gasoline engines, soot formation is much less critical.

It is possible to represent the TEM as a power source (*Vs*

*P* = *I*<sup>2</sup> ⋅ *RL* = (

and temperature consistency.

fabric.

Ohm's law maximum amount of power dissipates on load resistance (*RL*

value of the load resistance is exactly equal to the resistance of the power source (**Figure 17**).

**Figure 15.** Manifold of the engine influence: (a) thermography and photo without insolation; (b) manifold with basalt

**Figure 14.** Constructions of hot hexagonal ATEG with variable angle of inclination of heat exchange intensifiers [51].

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Obtaining required output characteristics from TEMs array is ensuring by connecting TEMs in series or in parallel. The serial connection type is used in the case of increasing the output voltage. The parallel connection allows increasing total current. However, for each ATEG system the choice of connection type should be done attentively, according to modules electrical

*<sup>V</sup>* \_\_\_\_\_*<sup>s</sup> RS* <sup>+</sup> *RL*)

2

) with an internal resistance (*Rs*

⋅ *RL* (3)

). By

) (Eq. (3)), when the

#### **5.6. Features of electrical load supply**

The task of ensuring efficient transmission of TEG power to electrical consumers is significant. Usually, TEG consists of separate TEMs, which can be considered as conventional direct current (DC) sources. As all DC sources, each type of TEM has its own maximum power transfer condition. This condition can be simply described using the equivalent circuit (**Figure 16**).

Prospects and Problems of Increasing the Automotive Thermoelectric Generators Efficiency http://dx.doi.org/10.5772/intechopen.76971 209

Firstly, installing the ATEG on ICE, which has the insulating combustion chamber with protective coatings. Such ICEs are increasingly being used in recent decades and are characterized as a high temperature of EG, low *CO* and residual hydrocarbons emissions, and fewer loads on the cooling system. Inside the ICE cylinders the operating gas reaches on its highest temperature which is about 2500°C. Mechanical work and temporary averaging of peak values reduce the temperature of the EG at the combustion engine output to 950–1000°C for

Secondly, a precise thermal insulation of the collector and the exhaust pipe near the TEG location. For example, the authors of this chapter conducted experiments on ATEG which was connected to collector of 0.7 m long with a VAZ 21126 gasoline engine on a test stand. Its power was 72 kW and a volume of 1.6. The EG temperature measurements were made by using Kistler 4049B05DS1–2.0 sensor—installed at the entrance of the ATEG diffuser. It was noted that the insulation coating of the collector with basalt cloth increased the temperature

Thirdly, to obtain the maximum temperature of EG, it is desirable to install ATEG directly after the internal combustion engine, in front of the catalyst, turbocharger and silencer.

The thermal resistance of the heat exchanger contacts can greatly reduce heat flow through ATEG [25, 54]. It is shown in [25] that neglect of contact thermal resistances during calculation leads to an overestimation of the generated TEM power by 25–30%. To reduce contact

• the use of graphite grease or high-temperature thermal grease for hot heat exchangers and

• installation of elastic expansion joints for temperature deformation to ensure a constant

When operating ATEG on diesel engine, power may be reduced due to the deposition of soot in the hot heat exchanger. For example, Kajikawa describes an ATEG installed with a diesel engine [55]. During the first 100 h of operation, the heat flow through ATEG fell by 30%, reaching saturation levels due to soot formation, and further, remained almost unchanged for

The task of ensuring efficient transmission of TEG power to electrical consumers is significant. Usually, TEG consists of separate TEMs, which can be considered as conventional direct current (DC) sources. As all DC sources, each type of TEM has its own maximum power transfer condition. This condition can be simply described using the equivalent circuit (**Figure 16**).

gasoline internal combustion engines and 800–850°C for diesel engines.

of EG *Teg* by more than 300°C (**Figure 15**).

208 Bringing Thermoelectricity into Reality

**5.5. Reduction of thermal resistance**

resistance, all possible methods should be taken, including:

• machining with minimal roughness and deviations from flatness.

another 100 h. For gasoline engines, soot formation is much less critical.

• elimination of possible corrosion of contact joints.

conventional thermal grease for cold ones.

**5.6. Features of electrical load supply**

clamping force [54].

**Figure 14.** Constructions of hot hexagonal ATEG with variable angle of inclination of heat exchange intensifiers [51].

**Figure 15.** Manifold of the engine influence: (a) thermography and photo without insolation; (b) manifold with basalt fabric.

It is possible to represent the TEM as a power source (*Vs* ) with an internal resistance (*Rs* ). By Ohm's law maximum amount of power dissipates on load resistance (*RL* ) (Eq. (3)), when the value of the load resistance is exactly equal to the resistance of the power source (**Figure 17**).

$$P = I^2 \cdot R\_\perp = \left(\frac{V\_\circ}{R\_\circ + R\_\perp}\right)^2 \cdot R\_\perp \tag{3}$$

Obtaining required output characteristics from TEMs array is ensuring by connecting TEMs in series or in parallel. The serial connection type is used in the case of increasing the output voltage. The parallel connection allows increasing total current. However, for each ATEG system the choice of connection type should be done attentively, according to modules electrical and temperature consistency.

**Figure 16.** Equivalent circuit schema.

**Figure 17.** TEM power curve.

Also, heat flux distribution of the ATEG is often not the same all over its surface. Consequently, temperature states of various TEMs may differ. Thereby, it is correct to combine TEMs with equal temperature differences in one electrical circuit.

In the case of steady-state operating conditions, it is natural to connect TEG modules directly to the electrical load. The main parameter, which should be taken into account, is a match between summary electrical resistance of TEMs and a resistance of the electrical load. It allows harvesting maximum output power from TEG, as explained previously.

adjustment of the electrical system resistance to keep it operating at the peak power point under varying conditions. There are many different methods for tracking power point of TEG system: the constant voltage method [61], the perturbation and observation (P&O) method [62], the incremental conductance method [63], the ripple correlation control method [64], the dichotomy method [65] and the gradient method [66]. There are also combinations of several methods, for example, the aggregated dichotomy and gradient (ADG) method [67]. Intelligent methods, such as fuzzy logic [68] and neural network algorithms [69] are also developed for performance enhancement. The appearance and modifying of MPPT algorithms aim to obtain more rapid and accurate tracking of the power point. Some methods are more effective than others; some are easier to implement, but their main goal remains same. In this way, the usage of the DC/DC converter with MPPT function is necessary for such systems as ATEG.

**Figure 18.** TEM power *P* versus hot-side temperature *T hot* and load resistance *R load* at cold side temperature *T cold* =

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50°C.

**Figure 19.** Conversion losses influence.

However, ATEG presents a transient system due to varying engine load conditions. Inconsistency in driving style, speed and torque can lead to changes in TEM temperature difference (**Figure 18**), which affects TEM internal resistance [56]. Consequently, a converter is required to continuously track ATEG power when the engine operates under various working conditions. The solution of this issue is connecting ATEG modules to the vehicle batteries and electrical loads through a DC/DC converter with maximum power point tracking (MPPT) algorithms.

MPPT method is mainly used in photovoltaic applications [57–60]. As solar panels, an operating point of TEM is rarely at peak power, as shown in **Figure 4**. MPPT technique executes continuous Prospects and Problems of Increasing the Automotive Thermoelectric Generators Efficiency http://dx.doi.org/10.5772/intechopen.76971 211

**Figure 18.** TEM power *P* versus hot-side temperature *T hot* and load resistance *R load* at cold side temperature *T cold* = 50°C.

**Figure 19.** Conversion losses influence.

Also, heat flux distribution of the ATEG is often not the same all over its surface. Consequently, temperature states of various TEMs may differ. Thereby, it is correct to combine TEMs with

In the case of steady-state operating conditions, it is natural to connect TEG modules directly to the electrical load. The main parameter, which should be taken into account, is a match between summary electrical resistance of TEMs and a resistance of the electrical load. It

However, ATEG presents a transient system due to varying engine load conditions. Inconsistency in driving style, speed and torque can lead to changes in TEM temperature difference (**Figure 18**), which affects TEM internal resistance [56]. Consequently, a converter is required to continuously track ATEG power when the engine operates under various working conditions. The solution of this issue is connecting ATEG modules to the vehicle batteries and electrical loads

MPPT method is mainly used in photovoltaic applications [57–60]. As solar panels, an operating point of TEM is rarely at peak power, as shown in **Figure 4**. MPPT technique executes continuous

allows harvesting maximum output power from TEG, as explained previously.

through a DC/DC converter with maximum power point tracking (MPPT) algorithms.

equal temperature differences in one electrical circuit.

**Figure 16.** Equivalent circuit schema.

210 Bringing Thermoelectricity into Reality

**Figure 17.** TEM power curve.

adjustment of the electrical system resistance to keep it operating at the peak power point under varying conditions. There are many different methods for tracking power point of TEG system: the constant voltage method [61], the perturbation and observation (P&O) method [62], the incremental conductance method [63], the ripple correlation control method [64], the dichotomy method [65] and the gradient method [66]. There are also combinations of several methods, for example, the aggregated dichotomy and gradient (ADG) method [67]. Intelligent methods, such as fuzzy logic [68] and neural network algorithms [69] are also developed for performance enhancement. The appearance and modifying of MPPT algorithms aim to obtain more rapid and accurate tracking of the power point. Some methods are more effective than others; some are easier to implement, but their main goal remains same. In this way, the usage of the DC/DC converter with MPPT function is necessary for such systems as ATEG.

However, ATEG electrical network has conversion losses, which decrease the total efficiency of the system. This fact should be considered while designing DC/DC converter. The function of switching on/off from general schema should be added to converter. When ATEG conditions nearby maximum power, converter with MPPT can be disconnected from ATEG and ATEG should transfer energy directly to the electrical load. It allows avoiding losses on the converter (**Figure 19**).

**Author details**

**References**

014001

1915

Alexey Osipkov1,2\*, Roman Poshekhonov1,2, Konstantin Shishov<sup>1</sup>

1 Bauman Moscow State Technical University (BMSTU), Moscow, Russia

2 Peoples' Friendship University of Russia (RUDN University), Moscow, Russia

[1] Srivastava DK, Agarwal AK, Datta A, Maurya RK. Advances in Internal Combustion

Prospects and Problems of Increasing the Automotive Thermoelectric Generators Efficiency

[2] Kavtaradze RZ. Theory of piston engines. Special chapters: Textbook for high schools.

[3] Yang J, Stabler FR. Automotive applications of thermoelectric materials. Journal of

[4] Copeland C, Pesiridis A, Martinez-Botas R, Rajoo S, Romagnoli A, Mamat A. Automotive

[5] Legros A et al. Comparison and impact of waste heat recovery technologies on passenger car fuel consumption in a normalized driving cycle. Energies. 2014;**7**(8):5273-5290

[6] Singh DV, Pedersen E. A review of waste heat recovery technologies for maritime appli-

[7] Noor AM, Puteh RC, Rajoo S. Waste heat recovery technologies in turbocharged automotive engine–a review. Journal of Modern Science and Technology. 2014;**2**(1):108-119

[8] Armstead JR, Miers SA. Review of waste heat recovery mechanisms for internal combustion engines. Journal of Thermal Science and Engineering Applications. 2014;**6**(1). DOI:

[9] Zhang QH et al. Thermoelectric devices for power generation: Recent progress and

[10] Hale LE. Thermo-electric battery for motor-vehicles. In: U.S. Patent No. 1,134,452. 6 Apr.

[11] Bauer RH. Auxiliary electric power for an automobile through the utilization of a thermoelectric generator: A critical examination. M of ME Thesis, Department of Mechanical

future challenges. Advanced Engineering Materials. 2016;**18**(2):194-213

Engineering, Clarkson College of Technology. 1963;**10**

\*Address all correspondence to: osipkov@bmstu.ru

Engine Research. Singapore: Springer; 2017

Moscow: Mashinostroenie Publ; 2008

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and Pavel Shiriaev<sup>1</sup>

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Also, the electrical network should be equipped with a vehicle battery. The battery provides a constant supply voltage in a range of 12–13 V. According to the principle that in a parallel circuit the voltage is same for all elements and that the voltage of vehicle electrical devices is also 12 V. Using the battery in the electrical network makes it possible to properly power all car systems.
