**2. ATEG research review**

One of the first published works on a thermoelectric generator for vehicles was done in 1913 [10] where a thermoelectric cell based on a thermocouple wire powers lighting and ignition systems. This generator was meant to eliminate the need for a standard electromechanical alternator, thus making the electric power supply system less complex and expensive.

The first research in this area was conducted in 1961–1963 at Clarkson University (USA) by Bauer [11] and Tomarchio [12]. Bauer considered the possibility of using *PbTe*-based thermoelectric generators to recover heat in the automobile cooling system and came to the conclusion that a noticeable electric power requires a lot of thermoelectric material. Tomarchio investigated the possibility of replacing an alternator with a thermoelectric generator in the automobile exhaust system and concluded that the desired power levels in the 20–50 mph (32–80 km/h) speed range. Speeds below 20 mph require new, more efficient materials.

In the mid-1950s, research by Goldsmid [13] and Ioffe [14], the subsequent development of thermoelectric materials based on bismuth telluride solid-state solutions achieved significant development in the applications of thermoelectric effect. However, these technologies were adopted in automobile generators only in the late 1980s and early 1990s, which was apparently caused by the changes in environmental policies of different countries (the EU environmental guidelines were formulated in 1987 in The Single European Act (SEA)).

In 1988, Birkholt et al. [15]. developed automotive thermoelectric generator (ATEG) and tested on Porsche 944. It had a peak power of 58 W at 800°C temperature for hot-side heat exchanger. Temperature difference between hot and cold side of heat exchanger was 490°C. *FeSi*<sup>2</sup> was used as the thermoelectric material.

the exhaust gases. Since then, the environmental standards are progressively becoming more stringent (Euro-6 standard is in force since 2015). In addition, the international efforts to reduce the rate of global warming resulted a new law Standard No 443/2009 by European

till 2015 and up to 95 g/km by end of 2021. With the passage of time every year, new restrictions are being advocated by tax policies which take into account a vehicle's environmental performance level, encourage automakers to work towards finding effective solutions to

The activity in this area takes two directions: optimization of processes in a combustion chamber and neutralization of exhaust gases [1]. However, these developed methods can partially solve the problem of reducing harmful emissions. At the same time, the average fuel consumption for vehicle operation is 35% [2], while approximately 37% is dissipated as EG [3],

At present numerous technical solutions are being developed for heat recovery systems. Among the most promising are the Rankine cycle, thermoelectric generator and a turbo-compound engine. All these solutions have their own merits and drawbacks [4, 5]. Numerous authors believe [6–8] that up to now the thermoelectric generator is the least developed technology. However, due to the absence of moving parts, potentially high reliability, compactness and the prospects in new thermoelectric materials over the past 20 years [9], this

One of the first published works on a thermoelectric generator for vehicles was done in 1913 [10] where a thermoelectric cell based on a thermocouple wire powers lighting and ignition systems. This generator was meant to eliminate the need for a standard electromechanical alter-

The first research in this area was conducted in 1961–1963 at Clarkson University (USA) by Bauer [11] and Tomarchio [12]. Bauer considered the possibility of using *PbTe*-based thermoelectric generators to recover heat in the automobile cooling system and came to the conclusion that a noticeable electric power requires a lot of thermoelectric material. Tomarchio investigated the possibility of replacing an alternator with a thermoelectric generator in the automobile exhaust system and concluded that the desired power levels in the 20–50 mph (32–80 km/h) speed range. Speeds below 20 mph require new, more efficient materials.

In the mid-1950s, research by Goldsmid [13] and Ioffe [14], the subsequent development of thermoelectric materials based on bismuth telluride solid-state solutions achieved significant development in the applications of thermoelectric effect. However, these technologies were adopted in automobile generators only in the late 1980s and early 1990s, which was apparently caused by the changes in environmental policies of different countries (the EU environ-

In 1988, Birkholt et al. [15]. developed automotive thermoelectric generator (ATEG) and tested on Porsche 944. It had a peak power of 58 W at 800°C temperature for hot-side heat exchanger.

technology has great practical potential as an exhaust heat recovery system for ICEs.

nator, thus making the electric power supply system less complex and expensive.

mental guidelines were formulated in 1987 in The Single European Act (SEA)).

which means that great potential is available for heat recovery technology.

up to 130 g/km for new passenger cars

Union, which prescribes the reduction emission of *CO*<sup>2</sup>

reduce harmful emissions into the atmosphere.

186 Bringing Thermoelectricity into Reality

**2. ATEG research review**

Hi-Z Technology, Inc. (Hi-Z) funded by the U.S. Department of Energy (DOE) began a largescale of research in this direction in 1987 with the aim of obtaining sufficient electric power to eliminate the need of engine driven alternators in trucks. Different locations were purposed for the TEG installation to enable heat recovery; among them were the exhaust manifold, the internal combustion engine, the intercooler and the lubrication system. The conclusion was made that the exhaust gases have the best potential for ATEG [16]. Thermoelectric materials were analyzed in terms of their effectiveness in solving this problem and the optimal materials were selected. In 1994, Hi-Z presented test results for 1 kW ATEG installed on the Cummins NTC 325 and NTC 30 engines [17]. The obtained power was 1068 W for a 300 hp. engine at 1700 rpm. The ATEG used 72 thermoelectric modules (Hi-Z-13) based on bismuth telluride with 4.5% efficiency, the hot and cold side temperatures constituted 230 and 30°C, respectively. The authors concluded that special attention must be paid to the heat exchanger configuration because of the increased sensitivity to the temperature difference between its various components and their mean temperature.

In 1998, Nissan Motor group presented ATEG test results for a 3000 mL gasoline engine with *SiGe*-based thermoelectric modules that demonstrated maximum power 35.6 W at 60 km/h on hill-climb mode with 1141°C exhaust gas temperature [18]. The total efficiency *η<sup>t</sup>* of heat exchanger constituted 11% and the heat flow through the modules was converted into electricity with generated power *η<sup>p</sup>* 0.9% efficiency. It was noted that by increasing 50% *η<sup>t</sup>* and 5% *ηp* , the alternator's power would reach 950 W. It is worth to mention that a bypass was used to regulate the heat flow during the experiment.

In 1999, this group created and tested generator for 2 and 3 L gasoline engines with Hi-Z-14 modules based on Bi2 Te<sup>3</sup> . This system demonstrated maximum power 193 W under the same operating conditions; with *η<sup>t</sup>* constituting 37% and *η<sup>p</sup>* 2.9% [19].

The early research in ATEG was devoted to the following issues: the total heat to electricity conversion efficiency, the dependence of the output power on the engine rpm (obtaining the maximum power at the maximum rpm), and the key directions for further work. Along with the development of more efficient thermoelectric materials, special emphasis is laid on the intensification of heat transfer processes in ATEG, especially the heat recovery in a gas heat exchanger.

In 2012, Amerigon (now Genterm) in collaboration with BMW and Ford, and the financial support from DOE presented results of their project in ATEG launched in 2004 [20]. A prototype ATEG for two passenger cars (BMW X6 and Lincoln MKT) was constructed and installed into the exhaust system behind the catalytic converter. The project focused on the effect the ATEG with an integrated bypass had on various car systems. Static and dynamic experiments were carried out, including the US06 drive cycle. Fuel efficiency was found to be 1.2% at 110 km/h. The maximum peak power for BMW X6 at 125 km/h in the stationary and dynamic modes constituted 605 and 450 W, respectively.

Between 2011 and 2015, Genterm in cooperation with BMW and Tenneco, continued work on ATEG. The aim of the new project was to achieve 5% reduction in fuel consumption over US06 cycle with the potential for efficient commercialization [21]. In the end, the average fuel saving was 1.2% (9.2 g/mil CO2 emission reduction) for a Ford F350 (6.2 L SOHC V8 flex fuel engine) with maximum 1160 W and average 470 W generator power over US06 cycle. At the same time, CO2 emission increased by 0.2 g/mil for a BMW X3 28i with the ATEG. CO2 emission increasing was explained by automobile weigh increasing and requiring more power for cooling system. These results confirm the necessity of TEG and all car systems optimization.

Over the past 5 years, China has become the leader in the number of publications in this field, with Wuhan University of Technology being the most active at ATEG research. In terms of publications total, the US holds the first place, with Caltech having the highest number of publications. The following organizations are playing an active part in the ATEG research: German Aerospace Center (DLR), TU Berlin, Loughborough University (UK), AGH University of Science and Technology (Poland) and Chungbuk National University (Korea). In Russia, Bauman Moscow State Technical University and Moscow Polytech are working on ATEG.

Prospects and Problems of Increasing the Automotive Thermoelectric Generators Efficiency

http://dx.doi.org/10.5772/intechopen.76971

189

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

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

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

• durability of device under variable thermal and mechanical loads associated with frequent

• minimization of electric losses during various operating conditions of internal combustion

• effective convective heat transfer of TEG hot heat exchanger with a limited heat exchange

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

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

**3. ATEG design**

which must ensure:

and prototype tests.

system (12 V).

installation on the EG system can be the way out.

engine and varying electric load;

surface area with low hydraulic resistance.

**3.2. General requirements for ATEG design**

changes in engine operating modes and vehicle vibrations;

• technological solutions must be compatible with mass production;

• optimal layout of ATEG with certain mass and dimensional constraints;

**TEGs**

ATEG with *Bi*<sup>2</sup> *Te*<sup>3</sup> -based TEMs was developed as part of the HeatReCar [22, 23] project with Fiat and Chrysler collaboration. The system was installed onto an IVECO Daily light-duty truck with a 2.3 l diesel engine, and showed a 2.2% increase in fuel efficiency (6.7 g/km reduction in *CO*<sup>2</sup> emission) over NEDC cycle and 3.9% increase in fuel efficiency (9.6 g/km reduction in *CO*<sup>2</sup> emission) over WLTP cycle. The project also developed and tested skutterudite-based TEM for high-temperature applications.

The RENOTER project, launched in 2008 by Renault Trucks and Volvo, was aimed at creating ATEG for diesel (100–300 W depending on the driving cycle) and gasoline (up to 500 W) passenger cars, as well as for large trucks (up to 1 kW) with 0.3–1.3 \$/W cost of generated electricity [24]. Apart from the heat exchanger design optimization, the project focused on the development of effective, cheap and reliable thermoelectric materials and their installation onto generators (*Mg*<sup>2</sup> *Si*- and *MnSi*1.77-based materials).

The analysis of the ATEG projects over the past 15 years reveals the drive to reduce fuel consumption or *CO*<sup>2</sup> emission, to make the design solutions economically efficient and to meet the reliability indicators. This has the following implications: ATEG must be investigated in dynamic as well as in stationary modes; tests over various operating cycles must be conducted, cheaper; less toxic materials must be used for TEM; reliability must be evaluated and maintained. Even though the projects do not always achieve their goals in full, the great potential in optimizing individual design and technological solutions is obvious.

The analysis of publications shows that the number of papers dealing with various aspects of the ATEG development is growing every year (**Figure 1**).

**Figure 1.** Research papers in ATEG (SCOPUS).

Over the past 5 years, China has become the leader in the number of publications in this field, with Wuhan University of Technology being the most active at ATEG research. In terms of publications total, the US holds the first place, with Caltech having the highest number of publications. The following organizations are playing an active part in the ATEG research: German Aerospace Center (DLR), TU Berlin, Loughborough University (UK), AGH University of Science and Technology (Poland) and Chungbuk National University (Korea). In Russia, Bauman Moscow State Technical University and Moscow Polytech are working on ATEG.
