**2. Electricity from automotive exhaust waste heat**

In an internal combustion (I.C) engine, only one-third of the total heat produced in the fuel combustion is utilized for the propulsion of the vehicle while the remaining two-third goes as waste heat mainly through the exhaust gas and the engine coolant. The exhaust gas, usually at a higher temperature compared to the engine coolant which absorbs heat from engine walls, is let out in the atmosphere and the engine coolant is recirculated after cooling in the radiator. In some of the engines, particularly in diesel fuelled ones, to get better efficiency, part of the exhaust gas is cooled and mixed with air in exhaust gas recirculation (EGR) system to reduce the NO<sup>x</sup> emissions. Turbo-charging is another technology utilizing the heat from the exhaust gas to improve the engine power. However, in all these technologies, only a small fraction of the exhaust gas or its energy is converted into useful work and remaining is let out to the atmosphere. Improving the engine performance by making use of this exhaust waste heat has been a subject of intense research in the field of energy recovery systems, exhibiting promising outcomes in the recent past.

Automotive exhaust thermoelectric generator (AETEG) technology involves converting the waste heat available in the exhaust gas into electricity that can be stored and utilized for various electrical inputs of a vehicle so that the fuel efficiency can be improved. The first such system was developed in 1963 by Neild [3] followed by Serksnis [4] in 1976. Later, Birkholz et al. in 1988 [5] and Bass et al. in 1990 [6] demonstrated AETEG using thermoelectric (TE) modules made of Fe-based and BiTe materials, respectively. Although the earliest AETEG was developed more than 50 years ago, a surge in research activities in this field has been occurring only in the past 15 years, which is evident from **Figure 1** showing the number of publications on this subject over the past five decades. Such exponential increase in the research output in recent years is mainly due to some of the path-breaking outcomes in the thermoelectric materials' properties which improved the TE figure of merit (zT) value which was ˂1 over a long period to more than 1. In recent years, zT ≥2 were also reported in few materials, which were achieved by engineering the microstructure of materials in different length scales [7, 8].

reported in energy technology perspective 2015, the number of light duty vehicles in the roads is expected to go up from present 900 million to 2 billion by 2050 [2]. With the global power sector moving towards clean technologies using renewable energy, the current 38% utilization of the global oil production for automotive use can increase to a significant extent. Though the advancement of electric vehicle (EV) technology is making a steady progress on one side (expected to reach 56 million passenger cars on road from the present 2 million by 2030), still it is far from making any drastic reduction in the emissions level due to transportation sector unless a radical innovation is made in the battery technology. Policies such as better urban planning that can increase the use of collective transportation and innovative technologies that can reduce the individual's vehicle need can make consider-

investment, and hence it is difficult to implement worldwide particularly in low and middle income countries. Implementing innovative technologies for improving automobile engine efficiency or innovations in the field of hybrid/low emissions vehicles can improve the fuel efficiency and thereby emissions can be reduced to a greater extent. Several recent developments in the engine, transmission and few ancillary systems of the vehicles show promising results. Converting a part of heat energy produced in the engine, released to the atmosphere via exhaust gas as waste heat into electricity by a thermoelectric generator (TEG) is one technology gaining a lot of attention in the past one decade though it is well explored long time back itself due to its inherent simplicity. This chapter discusses the various salient features

In an internal combustion (I.C) engine, only one-third of the total heat produced in the fuel combustion is utilized for the propulsion of the vehicle while the remaining two-third goes as waste heat mainly through the exhaust gas and the engine coolant. The exhaust gas, usually at a higher temperature compared to the engine coolant which absorbs heat from engine walls, is let out in the atmosphere and the engine coolant is recirculated after cooling in the radiator. In some of the engines, particularly in diesel fuelled ones, to get better efficiency, part of the exhaust gas is cooled and mixed with air in exhaust gas recirculation (EGR) system to reduce the NO<sup>x</sup>

sions. Turbo-charging is another technology utilizing the heat from the exhaust gas to improve the engine power. However, in all these technologies, only a small fraction of the exhaust gas or its energy is converted into useful work and remaining is let out to the atmosphere. Improving the engine performance by making use of this exhaust waste heat has been a subject of intense research in the field of energy recovery systems, exhibiting promising outcomes in the recent past. Automotive exhaust thermoelectric generator (AETEG) technology involves converting the waste heat available in the exhaust gas into electricity that can be stored and utilized for various electrical inputs of a vehicle so that the fuel efficiency can be improved. The first such system was developed in 1963 by Neild [3] followed by Serksnis [4] in 1976. Later, Birkholz et al. in 1988 [5] and Bass et al. in 1990 [6] demonstrated AETEG using thermoelectric (TE) modules

emissions. However, this requires substantial

emis-

able contributions to the reduction of the CO<sup>2</sup>

164 Bringing Thermoelectricity into Reality

and the progress made so far in this technology.

**2. Electricity from automotive exhaust waste heat**

The thermoelectric figure of merit (zT), a dimensionless parameter indicating the thermoelectric performance of the material is defined as zT = [(S<sup>2</sup> σ).T]/κ where S is the Seebeck coefficient (V/K), σ is the electrical conductivity (S/m), κ is the thermal conductivity (W/m.K) of the material and T is the absolute temperature (K). From the mid of last decade, almost all the major automobile manufacturers in the world are associated with R&D programmes involving design, development and testing of AETEG in collaboration with the TE device manufacturers and research institutes. However, the outcomes reported so far indicates that the improvement in the fuel efficiency obtained are of very low values and even negative in some cases due to the parasitic losses associated with the TEG [9, 10]. The commercialization of AETEG, which have been projected to be feasible with improvement in efficiency of >5%, is yet to be achieved. The major bottlenecks to achieve this target are non-availability of bulk TE materials with high zT and the high cost of the currently available modules. The main contributions to the high cost of AETEG at present mostly arise from the TE modules, which are still very high compared to the practically acceptable price of less than \$1/Watt. The presently available commercial modules made of Bi<sup>2</sup> Te3 are mostly manufactured by processes involving substantial manual operations resulting in high cost when

**Figure 1.** Number of papers published in every decade from 1965 onwards on the subject of thermoelectric generator". [data sourced from www.isiknowledge.com using the key word of 'thermoelectric generator'].

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 such as tetrahedrites (Cu12-xM<sup>x</sup> Sb4 S13), Mg<sup>2</sup> Si and MnSi<sup>2</sup> showing promising features can give the required breakthrough for commercialization.
