**5. Automotive TEG for engine coolant waste heat conversion**

Similar to the exhaust waste heat conversion, the heat in the engine coolant can also be utilized to generate power using the TEG, as nearly 30% of the energy from the fuel combustion accounts for this loss. However, the important point to be noted here is that the waste heat available in the coolant is of low grade in nature. Unlike exhaust gas, in this case the temperature and flow rate are lesser and need better heat capturing technique. Kim and his coworkers demonstrated TEG for engine coolant heat conversion using a 2.0 L passenger car engine [17]. The TEG was made of 72 BiTe modules with the hot and cold side blocks used to recover and dissipate the heat from the coolant. The cold side block was integrated with heat pipes which enhances its efficiency. Under the engine idle condition, the maximum output power of 0.4 W/module was generated which increased to 1.04 W/module under 80 km/h driving mode. The higher power obtained in the driving mode can be attributed to the lesser cold side temperature rather than hot side temperature, which shows an improvement of only 5°C. Under driving mode, the cold side temperature decreases by 25°C due to the arrangement of the heat pipes which showed better cooling performance than the radiator.

heat sink, exhaust blow-down power loss, and the rolling resistance [31]. The dominant among these three is the rolling resistance loss due to the weight of the TEG system which tends to

ant, χ is the loss coefficient for coolant flowing through the TEG loop, Q is the coolant flow rate, and ηcp is the coolant pump efficiency. The loss coefficient χ depends on Reynolds number of the fluid flow. In a typical AETEG where the coolant circuit is connected to engine coolant loop, the flow rate through the TEG will be low relative to the flow through engine coolant jacket.

The exhaust blow-down power, which is the power required for the engine to drive the gaseous products of the combustion through the exhaust system can change because of the flow resistance introduced by the TEG system's heat exchanger. The blow-down power can be both positive and negative depending on the power gains obtained due to AETEG. If the power produced helps to decrease the shaft power, the net blow-down power will decrease.

running at a speed (ω), the volumetric flow rate of exhaust is given by V<sup>F</sup> = (π.ω.Np.S.b<sup>2</sup>

The rolling resistance is due to the weight of the AETEG system produces power loss given by PR = μRWTν/ηR, where μR is the rolling resistance coefficient, WT weight of the TEG system, ν is the velocity of the vehicle, and ηR driveline efficiency which is normally constant at around 0.9 [31]. An estimation in a 1.5 L car fitted with TEG tested under new European driving cycle showed that the weight penalty of TEG could be as high as 12 W/kg [47]. Therefore, the TEG requires very stringent design criteria in terms of the heat exchanger materials and TE module

In AETEG, the output voltage changes dynamically in a nonlinear way over a wide range with the fluctuation of flow rate and temperature of exhaust gas. Therefore, a proper power conditioning circuit similar to the ones used in photovoltaic power system with a maximum power point tracking (MPPT) control is essential between the TEG and the load. A number of different power conditioning circuits for AETEG to step-up or step-down the voltage have been proposed. Some of the prominent circuits are DC-DC converters such as Ćuk converter, SEPIC converter, and Boost-buck cascade converter. MPPT is an algorithm used to operate a power system at its maximum power capability under various operating conditions. Different MPPT techniques such as load matching method, incremental conductance technique, ripple correlation, perturbation and observation (PAO) method has been developed. The PAO method is the most commonly used one due to its simplicity and system independence.

In automotive exhaust waste heat conversion, MPPT can be used, for example, between the TEG system and a battery pack so that power flow is regulated to obtain the maximum power transfer. Eakburanawat et al. proposed an MPPT technique based on feedback from battery current alone assuming constant battery voltage [40]. However, all practical batteries have a significant change

where Np is the number of pistons, S is the piston stroke and b is the piston bore [25].

However, as the capacity of the TEG increases, the χ through it also will be significant.

heat exchanger (Δp) and the volumetric flow rate of the exhaust gas (V<sup>F</sup>

**8. Power conditioning and maximum power point tracking**

χQ/ηcp, where ρ<sup>f</sup>

Automotive Waste Heat Recovery by Thermoelectric Generator Technology

is calculated from pressure drop across the TEG's

is the density of the cool-

http://dx.doi.org/10.5772/intechopen.75443

). For a given engine

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increase with the vehicle speed.

The coolant pumping power (Pcp) is given by Pcp≈ρ <sup>f</sup>

The blow-down power given by P=Δp.V<sup>F</sup>

materials.

## **6. TEG for hybrid and electric vehicles**

Hybrid vehicles consume relatively less fuel than the petrol or diesel vehicle by efficiently combining a conventional I.C engine power with the electric motor/s. The power to the drive comes from either downsized engine, motor or both depending on the driving conditions. The improvement in fuel efficiency mainly comes from operating the engine in an optimized condition with less idling, regenerative breaking, and dual power sources. Depending on the kind of the power source, the hybrid vehicles can be classified as serial hybrid, series–parallel hybrid, and plug-in hybrid. In some hybrid systems, the engine is automatically shut off during idling and restart when accelerated by integrated starter generator (ISG) thereby reducing the fuel consumption. The regenerative braking system converts the kinetic energy from the moving vehicle into electrical energy and stores it in a battery. At present, though the hybrid cars consume less fuel than conventional vehicles, there are still CO<sup>2</sup> emissions, the level of which may have to be reduced further to meet the futuristic goal of the allowable limit. Though, unlike in conventional I.C engines, not much work has been carried out in AETEG for the hybrid vehicles. The computational and experimental work carried out so far suggests that a notable improvement in fuel efficiency can be achieved using TEG in hybrid cars [37, 38]. Since in hybrid vehicles, the engines used are downsized, the exhaust flow rate and the temperature are expected to be low and hence using heat pipe-assisted TEG will be more suitable to maximize the power output [39].
