**4. Performance evaluation of the AETEG**

The AETEGs are usually tested for their performance by three different methods viz. (a) using a laboratory built test rig with hot gas or air as the heat source (b) in a simulated driving condition using laboratory test rig with gasoline or diesel engine and dynamometer and (c) in actual road driving test. While most of the performance tests reported so far were carried out by first or second methods, testing in actual driving conditions are very few. Such real driving conditions of a vehicle can give more realistic assessment of issues associated with this technology and its commercial feasibility.

#### **4.1. TEG evaluation in test rig with a heat source**

This test method is the ideal way of validating the design of an AETEG optimized by analytical and numerical methods. It is also a simple method to evaluate TE module reliability under the typical engine operating conditions. In AETEG, apart from the module efficiency, the power produced is determined by multiple factors such as (1) location of the TEG in the exhaust line, (2) exhaust gas flow rate and temperature, (3) locations and arrangement of modules in the TEG, (4) area of coverage of the modules in the heat exchanger/s, (5) heat sink temperatures, (6) thermal conductance at various interfaces, and (7) scheme of the modules electrical connection. Several multiphysics simulations, combining fluid mechanics, heat transfer, and thermoelectric phenomena, have been carried out to predict the influence of some of the above mentioned factors on the performance and power output of the TEG. However, the experimental validation of these predictions is very limited [31, 32]. **Figure 6** shows the image of a test rig designed and developed by this author used for evaluating the AETEG performance using hot air/gas [33]. Testing of the TEG in this test rig offers complete performance details, that is, the efficiency of the TE modules, heat exchanger, and heat sink, which will be useful for further optimization of the design before evaluating their performance in the actual engine using established driving conditions. The test rig consists of the following sub systems:


higher than the other two for a given contact pressure particularly above 60 psi. The appropriate choice for the interface materials is the one with low hardness and high thermal conductivity. Such material will deform while applying pressure, make good contact between module and heat exchanger surfaces, and decrease the thermal contact resistance. **Figure 5** shows the

carried out using different interface materials [30]. Among the graphite, aluminum, tin and lead

The AETEGs are usually tested for their performance by three different methods viz. (a) using a laboratory built test rig with hot gas or air as the heat source (b) in a simulated driving condition using laboratory test rig with gasoline or diesel engine and dynamometer and (c) in actual road driving test. While most of the performance tests reported so far were carried out by first or second methods, testing in actual driving conditions are very few. Such real driving conditions of a vehicle can give more realistic assessment of issues associated with this

This test method is the ideal way of validating the design of an AETEG optimized by analytical and numerical methods. It is also a simple method to evaluate TE module reliability under the typical engine operating conditions. In AETEG, apart from the module efficiency, the power

modules as a function of ΔT in an investigation

Te3

**Figure 5.** Estimation of the power per modules as a function of ΔT with different interface materials [28, 30].

foils, the softest material lead is estimated to give lesser thermal contact resistance.

comparison of the estimated power per Bi<sup>2</sup>

172 Bringing Thermoelectricity into Reality

technology and its commercial feasibility.

**4.1. TEG evaluation in test rig with a heat source**

**4. Performance evaluation of the AETEG**


The hot air source is a blower and heater combined unit where a high pressure blower of 4000 lpm output capacity draws in air and passes it to air heater which can heat up the air up to 400°C. The pressure of the hot gas is measured at inlet and outlet of heat exchanger using

**Figure 6.** Photo image of the AETEG and the test rig developed at CAEM, ARCI.

a pressure transmitter and the back pressure due to the heat exchanger is measured by the differential pressure transmitter. The inlet and outlet temperatures of the hot air of the heat exchanger are measured using RTD sensors and K-type thermocouples are used for recording the temperatures on surface of heat exchanger. The data acquisition and integration unit collects the current and voltage signals from TE modules and also the pressure and flow rate signals and displays them in the display panel.

engine speed of 112.6 km/h with the hot side and coolant side of temperature around 300 and 80°C, respectively. Decreasing the cold side temperature to 15°C increased the power output to 229 W, which showed that decreasing the cold side temperature of the module appears to be more beneficial, unlike increasing the hot side temperature which may have the adverse

In a project partially funded by Swedish energy agency, with partners from Scania CV AB, Titan X, Eberspächer Exhaust Technology GmbH & Co. Germany, Swerea IVF, Gothenburg

designed, developed and tested in experimental hot gas test bench as well as in an actual engine exhaust [11]. The 224 modules which can be used up to 330°C are arranged in 14 modular TE units consist of hot exhaust gas and cooling water channels with counter crossflow arrangements. At the input gas parameters of 300°C temperature and 1000 kg/h flow rate, the TEG delivered power output of 416 W. In a similar work using 1.2 L gasoline engine, a TEG designed to a nominal power of 225.6 W has produced the maximum power of 189.3 W at the engine operating conditions of 80 Nm torque, 2600 RPM speed [10]. The TEG was fabricated using 24 numbers of commercial Ferrotech SCTB NORD thermoelectric modules (Code Name TMG-241-1.4-1.2) made of BiTe-based compounds with the maximum power rating of

Zhang et al. reported the development of high temperature, high power density TEG yielding 1002.6 W power when tested in a Caterpillar diesel engine exhaust [20]. The TEG was fabricated using modules made of half-Heuslers compounds of p-type (peak ZT of 1.0 at 500°C) and n-type (peak ZT of 0.9 at 700°C) compounds. Heat exchanger with 0.2 mm thick nickel fins and aluminum cold plate with coolant flow perpendicular (cross flow) to the exhaust was used to create the temperature difference between hot and cold side of the modules. The exhaust gas of 550°C with a flow rate of 1728 kg/h generated a temperature difference 339°C of between hot and cold side. The efficiency of the individual module was around 2.1%. Liu et al. investigated an automotive TEG designed, fabricated and tested in test rig with 2.0 L naturally aspirated engine with a dynamometer and in actual road test condition in a 3.9 L engine [36]. The TEG was made of 60 modules, a brass heat exchanger and aluminum water tank as the heat sink. The maximum power output of 335.8 W under the temperature difference of 235°C with the conversion efficiency of 0.9% was obtained in the test rig. Interestingly, combining the four of the same TEGs into a single system for road test could able to generate only 390 W power as under this test conditions the exhaust heat from engine appeared to be

Te3


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

175

Automotive Waste Heat Recovery by Thermoelectric Generator Technology

effect on the module durability.

is lesser.

and KTH, Stockholm, a TEG system using Bi<sup>2</sup>

9.4 W. The increase in overall engine efficiency was close to 0.2%.

inadequate and could able to create a temperature difference of 133°C only.

AETEG's have been investigated for the performance both in gasoline and diesel engines of various capacities and vehicles. Whether it is advantageous to use this technology for a particular kind of engine is a debatable topic. The amount of waste heat and the temperature of the exhaust gas usually is higher in the spark ignition (S.I) engine compared to compression ignition (C.I) engine. The maximum exhaust gas temperature for gasoline engines is about 700–800°C and for diesel engines is about 400–500°C at the exhaust manifold. A study carried out by Wojciechowski et al. in a single point injection 0.9 L Fiat spark ignition (gasoline) engine and 1.3 L diesel engine using BiTe-based AETEG suggests that it is more beneficial to use in spark ignition engine [16]. For a given engine power, the high output of gas flux from the diesel engine results in a low hot side temperature and hence the energy produced

#### **4.2. AETEG evaluation in test rig with engine exhaust**

The performance evaluation of AETEGs using the engine exhaust gas has been carried out by many research groups. This method uses a test rig consisting of either a petrol or diesel engine coupled to a dynamometer to apply variable load. The testing can be carried out either in the steady state or transient condition using various engine speed and torque combinations which allows the generation of exhaust gas of different flow rates and temperatures. IC engines of different sizes ranging from 0.8 L [34] to 14 L [12] have been used in this method. In most of the works, while the heat exchanger and heat sink designs are different, the overall configuration of the AETEG prototypes is similar to one another.

The earliest attempt of building an AETEG and testing in an engine exhaust was carried out by Neild in the year 1963 [3]. Subsequently, Serksnis [4] and Birkholz [5] developed a similar system for exhaust waste conversion. Takanose and Tamakoshi developed a TEG and demonstrated it in a passenger car exhaust [35]. The system generated 100–130 W of power under various driving conditions. During the same period, Bass and his coworkers developed a 1 kW unit using 72 units of HZ-13 modules and tested in a 1.4 L Cummins diesel truck engine [12]. The BiTe modules made by Hi-Z Technology Inc. USA were arranged in a nickel steel support structure with an octahedral cross-section through which exhaust gas flows close to its internal surface. The modules cold side was cooled using an aluminum heat sink. The TEG initially generated power output of 400 W. Subsequently with several modifications in the design which resulted in better heat transfer at hot side, the TEG generated 1068 W under the engine operating condition of 300 HP and 1700 RPM. Ikoma and his co-workers from Nissan Motor Corporation, Japan developed a SiGe based AETEG fitted to the exhaust of a gasoline engine [13]. The system was made using 72 modules arranged between the rectangular cross section exhaust pipe made of SS 304 and aluminum water-cooled jacket. The TEG produced the maximum power of 35.6 W under the engine condition of 60 km/h hill climb with overall power generation efficiency of 0.1% [13]. Matsubara and his team from Science University of Tokyo, Japan [15] developed a prototype TEG and tested in a 2.0 L Toyota Estima engine using inhouse manufactured segmented modules (skutterudites/Bi<sup>2</sup> Te3 ) and 4 HZ-14 modules made by Hi-Z Technology Inc., USA. The TEG produced an output power of 266 W under 60 km/h speed which is only half of the rated capability of the system. In both the abovementioned works, it was highlighted that the effectiveness of the heat exchanger and loss of heat at the various contact surfaces critically influence the power produced in the TEG.

The first qualitative assessment of the effect of AETEG system on the vehicle fuel efficiency and parasitic losses was carried but by Thatcher et al. in a 1999 model GMC Sierra light-duty pickup truck [9]. The study also emphasized the importance of the cold side temperature on the overall power output of the TEG. A 330 W capacity system built using 16 units of HZ-20 modules manufactured by Hi-Z Technology Inc., USA. The TEG produced 117 W under the engine speed of 112.6 km/h with the hot side and coolant side of temperature around 300 and 80°C, respectively. Decreasing the cold side temperature to 15°C increased the power output to 229 W, which showed that decreasing the cold side temperature of the module appears to be more beneficial, unlike increasing the hot side temperature which may have the adverse effect on the module durability.

a pressure transmitter and the back pressure due to the heat exchanger is measured by the differential pressure transmitter. The inlet and outlet temperatures of the hot air of the heat exchanger are measured using RTD sensors and K-type thermocouples are used for recording the temperatures on surface of heat exchanger. The data acquisition and integration unit collects the current and voltage signals from TE modules and also the pressure and flow rate

The performance evaluation of AETEGs using the engine exhaust gas has been carried out by many research groups. This method uses a test rig consisting of either a petrol or diesel engine coupled to a dynamometer to apply variable load. The testing can be carried out either in the steady state or transient condition using various engine speed and torque combinations which allows the generation of exhaust gas of different flow rates and temperatures. IC engines of different sizes ranging from 0.8 L [34] to 14 L [12] have been used in this method. In most of the works, while the heat exchanger and heat sink designs are different, the overall

The earliest attempt of building an AETEG and testing in an engine exhaust was carried out by Neild in the year 1963 [3]. Subsequently, Serksnis [4] and Birkholz [5] developed a similar system for exhaust waste conversion. Takanose and Tamakoshi developed a TEG and demonstrated it in a passenger car exhaust [35]. The system generated 100–130 W of power under various driving conditions. During the same period, Bass and his coworkers developed a 1 kW unit using 72 units of HZ-13 modules and tested in a 1.4 L Cummins diesel truck engine [12]. The BiTe modules made by Hi-Z Technology Inc. USA were arranged in a nickel steel support structure with an octahedral cross-section through which exhaust gas flows close to its internal surface. The modules cold side was cooled using an aluminum heat sink. The TEG initially generated power output of 400 W. Subsequently with several modifications in the design which resulted in better heat transfer at hot side, the TEG generated 1068 W under the engine operating condition of 300 HP and 1700 RPM. Ikoma and his co-workers from Nissan Motor Corporation, Japan developed a SiGe based AETEG fitted to the exhaust of a gasoline engine [13]. The system was made using 72 modules arranged between the rectangular cross section exhaust pipe made of SS 304 and aluminum water-cooled jacket. The TEG produced the maximum power of 35.6 W under the engine condition of 60 km/h hill climb with overall power generation efficiency of 0.1% [13]. Matsubara and his team from Science University of Tokyo, Japan [15] developed a prototype TEG and tested in a 2.0 L Toyota Estima engine using in-

Hi-Z Technology Inc., USA. The TEG produced an output power of 266 W under 60 km/h speed which is only half of the rated capability of the system. In both the abovementioned works, it was highlighted that the effectiveness of the heat exchanger and loss of heat at the various con-

The first qualitative assessment of the effect of AETEG system on the vehicle fuel efficiency and parasitic losses was carried but by Thatcher et al. in a 1999 model GMC Sierra light-duty pickup truck [9]. The study also emphasized the importance of the cold side temperature on the overall power output of the TEG. A 330 W capacity system built using 16 units of HZ-20 modules manufactured by Hi-Z Technology Inc., USA. The TEG produced 117 W under the

Te3

) and 4 HZ-14 modules made by

signals and displays them in the display panel.

174 Bringing Thermoelectricity into Reality

**4.2. AETEG evaluation in test rig with engine exhaust**

configuration of the AETEG prototypes is similar to one another.

house manufactured segmented modules (skutterudites/Bi<sup>2</sup>

tact surfaces critically influence the power produced in the TEG.

In a project partially funded by Swedish energy agency, with partners from Scania CV AB, Titan X, Eberspächer Exhaust Technology GmbH & Co. Germany, Swerea IVF, Gothenburg and KTH, Stockholm, a TEG system using Bi<sup>2</sup> Te3 -based commercial modules has been designed, developed and tested in experimental hot gas test bench as well as in an actual engine exhaust [11]. The 224 modules which can be used up to 330°C are arranged in 14 modular TE units consist of hot exhaust gas and cooling water channels with counter crossflow arrangements. At the input gas parameters of 300°C temperature and 1000 kg/h flow rate, the TEG delivered power output of 416 W. In a similar work using 1.2 L gasoline engine, a TEG designed to a nominal power of 225.6 W has produced the maximum power of 189.3 W at the engine operating conditions of 80 Nm torque, 2600 RPM speed [10]. The TEG was fabricated using 24 numbers of commercial Ferrotech SCTB NORD thermoelectric modules (Code Name TMG-241-1.4-1.2) made of BiTe-based compounds with the maximum power rating of 9.4 W. The increase in overall engine efficiency was close to 0.2%.

Zhang et al. reported the development of high temperature, high power density TEG yielding 1002.6 W power when tested in a Caterpillar diesel engine exhaust [20]. The TEG was fabricated using modules made of half-Heuslers compounds of p-type (peak ZT of 1.0 at 500°C) and n-type (peak ZT of 0.9 at 700°C) compounds. Heat exchanger with 0.2 mm thick nickel fins and aluminum cold plate with coolant flow perpendicular (cross flow) to the exhaust was used to create the temperature difference between hot and cold side of the modules. The exhaust gas of 550°C with a flow rate of 1728 kg/h generated a temperature difference 339°C of between hot and cold side. The efficiency of the individual module was around 2.1%. Liu et al. investigated an automotive TEG designed, fabricated and tested in test rig with 2.0 L naturally aspirated engine with a dynamometer and in actual road test condition in a 3.9 L engine [36]. The TEG was made of 60 modules, a brass heat exchanger and aluminum water tank as the heat sink. The maximum power output of 335.8 W under the temperature difference of 235°C with the conversion efficiency of 0.9% was obtained in the test rig. Interestingly, combining the four of the same TEGs into a single system for road test could able to generate only 390 W power as under this test conditions the exhaust heat from engine appeared to be inadequate and could able to create a temperature difference of 133°C only.

AETEG's have been investigated for the performance both in gasoline and diesel engines of various capacities and vehicles. Whether it is advantageous to use this technology for a particular kind of engine is a debatable topic. The amount of waste heat and the temperature of the exhaust gas usually is higher in the spark ignition (S.I) engine compared to compression ignition (C.I) engine. The maximum exhaust gas temperature for gasoline engines is about 700–800°C and for diesel engines is about 400–500°C at the exhaust manifold. A study carried out by Wojciechowski et al. in a single point injection 0.9 L Fiat spark ignition (gasoline) engine and 1.3 L diesel engine using BiTe-based AETEG suggests that it is more beneficial to use in spark ignition engine [16]. For a given engine power, the high output of gas flux from the diesel engine results in a low hot side temperature and hence the energy produced is lesser.
