Comparing the recovery performance of different thermoelectric generator modules in an exhaust system of a diesel engine both experimentally and theoretically

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Proc IMechE Part D: J Automobile Engineering 1–8

Ó IMechE 2019 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0954407019837786 journals.sagepub.com/home/pid

Comparing the recovery performance

of different thermoelectric generator

modules in an exhaust system of a

diesel engine both experimentally

and theoretically

Mehmet Akif Kunt

and Haluk Gunes

Abstract

In this study, a thermoelectric recovery system was designed to convert the exhaust waste heat of an internal combus-tion diesel engine directly to electric power and the performance was measured at different engine speeds in the unloaded state. The performances of two different thermoelectric generators were compared in a system designed using four modules. Maximum 0.92 W power was obtained for four modules at 3500 r/min, at an area of 0.0016 m2. Internal resistance of modules has increased according to the engine speed. The highest internal resistance obtained during the experiments is 11.69 O at engine speed of 3500 r/min. The characteristics of the overall thermoelectric generator perfor-mance is coherent with the analysis model. In the current graph according to engine speed, the maximum absolute error is calculated for modules TEG 12-8 and TEG1-199 as 0.010 and 0.044, respectively (at experimented 3500 r/min). To charge the battery under maximum power point conditions, 133 thermoelectric modules were required (TEG1-199). Maximum power transfer is obtained when the load resistor is connected in parallel at 10 O. It is seen that modular structure thermoelectric generators are more important alternative than Rankine cycle system in terms of waste heat recovery, despite thermoelectric system has low efficiency.

Keywords

Thermoelectric module, battery charge, waste heat, exhaust system, internal combustion engine

Date received: 21 January 2019; accepted: 21 February 2019

Introduction

Thermoelectric generators (TEGs) are totally environment-friendly in power generation as they use waste heat as input source, and they provide efficient usage of energy.1 Power generation from TEGs depends upon Seebeck effect. As the temperature dif-ference between TEG surfaces increases, the power gen-eration increases as well. As such temperature difference varies according to the source, voltage regu-lation is required in order to obtain fixed voltage.2As long as the need for energy increases constantly in the world and environmental concerns related to energy sources used continues, and as long as efficient usage of energy is taken into consideration, the need for TEGs, which are renewable energy sources, will con-tinue and it will be included in hot research subjects.3 Thermoelectric devices have no moving parts subject failure (with the possible exception of an exhaust

bypass valve), and therefore they have no need for complex control systems as may be required for turbo-compounding. Low efficiency and high cost are cur-rently problematic issues with thermoelectric devices.4 TEG is more advantages that other heat recovery tech-nologies due to its silence small size, scability and dur-ability. The main contribution of using TEG is minimizing maintenance resulting from wear and cor-rosion because it has no moving parts and chemical reaction. Rankine cycle waste heat efficiency is

Department of Motor Vehicles and Transportation Technology, Tavsanli Vocational Training School, Ku¨tahya Dumlupinar University, Ku¨tahya, Turkey

Corresponding author:

Mehmet Akif Kunt, Department of Motor Vehicles and Transportation Technology, Tavsanli Vocational Training School, Ku¨tahya Dumlupinar University, Ku¨tahya 43300, Turkey.

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relatively higher than TEG5but the efficiency is not the determining criterion as there is no cost on waste heat using TEG. Even if Rankine’s cycle seems to be more powerful, its cost is not advantageous compared with TEG technology in terms of the area covered and power/weight ratio.6 If energy distribution of internal combustion engines are analyzed, it is seen that only 25% of useful energy is obtained in the engine output shaft for the reason that some energy is lost because of exhaust, friction, and cooling.7 The system discharge 40% waste heat through exhaust system. To convert heat energy to electrical power brings measurable advantages.8Using TEGs for heat recovering in auto-mobile exhaust systems can lower fuel consumption from 5% to 10% depending on driving conditions.9,10 TEG technology was first tested in automotive waste heat recovery by Neild,11 at a later time by tests on modified cars/engines such as a Porsche 944, a 14 L Cummins Turbo-diesel engine truck,12 and a GM Sierra Pickup Truck.13 After these studies, different studies on recycling waste heat discharged through exhaust systems have also been done.14–27The empha-sis of these studies from the literature has varied among convection heat transfer, heat transfer in thermoelectric devices, and engine recycling performance. Gu¨rbu¨z and Akcxay28have experimentally examined the potential of recovery of waste heat from coolant and exhaust sys-tem in an engine running on LPG and spark plug. In experiments carried out with engine speeds between 1800 and 4000 r/min, stoichiometric mixture rate has derived maximum 63.18 W recovery under 1/2; throttle state. The recovery power derived has been used in order to meet the energy need of a PEM (proton exchange membrane) fuel cell used to enrich the mix-ture delivered to the internal combustion engine.28 Gu¨rbu¨z et al.29 proposed a theoretical model of a three-layer TEG converting exhaust waste heat energy to DC electric energy by means of a two-cylinder SI engine’s exhaust gas and engine coolant values (tem-perature and flow) in the range of 1500–4000 r/min, developed in MATLAB/Simulink program. The exhaust gas and coolant values of the engine have been compiled from the experimental work of the authors’ previous studies.28,30 The experimental results of stud-ies performed under the same conditions with the theo-retical model results obtained in this study are found to be in line with each other.

In this study, exhaust waste heat recovery of an internal combustion diesel engine in unloaded state and depending on engine r/min has been examined using two different TEGs, and their module performances have been compared experimentally and theoretical. The changes have been examined according to the internal exhaust temperature (TH= 383°K) obtained in all experimented engine r/mins. Moreover, the potential of the vehicle to charge its battery has been questioned according to obtained voltage and current values.

TEG

Thermoelectric module

Thermoelectric modules are composed by serial con-nection of n- and p-type semi-conductors. In addition, the thermoelectric modules have a thermally parallel structure. General structure of TEGs has been shown in Figure 1. By placing TEGs between ceramic plates, the best harmony among mechanical voltage, electrical resistance, and thermal conductivity has been obtained.

Mathematical module

In TEGs, hot zone heat (Qh), cold zone heat (Qc), elec-trical power (Pe), and open circuit voltage (Vocv) are stated by following relations31

Qh= 2aITh+ 2kA ‘ DT 1 2I 22r‘ A ð1Þ Qc= 2aITc+ 2kA ‘ DT+ 1 2I 22r‘ A ð2Þ Pe= Qh Qc= 2aIDT I2 2r‘ A ð3Þ Vocv= aDT ð4Þ

Here, a, r, k, A, and l refer, respectively, to Seebeck coefficient, electrical resistance, thermal conductivity, cross-sectional area, and pellet length. The current value, which has been obtained, is stated as follows on the basis of Ohm’s law

I= Vocv RI+ RL

= aADT

r‘(1 + m) ð5Þ

‘‘m’’ symbol shown in equation (5) is the ratio of load resistance (RL) to internal electrical resistance (RI). When equation (5) is written in equation (3), the fol-lowing relation is obtained with respect to electrical power

Pe=

2Nma2_DT2_A

r‘(1 + m)2 ð6Þ

In order to obtain maximum electrical power, m = 1 status should be provided. The obtained loaded voltage is V=Pe I = 2N m 1 + m aDT ð7Þ

Figure 1. General display of TEG module.31

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The connection between pellet and ceramic surfaces, selection of connection materials, state of electrical connections, and thermal contact thickness change per-formance of TEG modules significantly. In this regard, if equations (5) and (6) are re-formed considering con-tact status, current and electrical power relations are obtained as follows.32,33 Both thermal and electrical contact effect, an analysis of an optimized configura-tion, and convection and radiation heat transfer on the thermoelectric module free surfaces can be ignored because in practice the thermoelectric modules are cov-ered with thermal insulation material34

I= aADT

rl(1 + m) ð8Þ

Pe=

2Nma2_DT2_A

rl(1 + m)2 ð9Þ

Introduction of experiment equipment

Main body of recovery system is composed of CRS (Cold Rolled Steel) sheet profile. TEGs have been placed on sheet profile body, and rectangular wing alu-minum coolers have been used in order to cool the cold parts. By using tightening clamps, TEG modules and aluminum cooling wings have been mounted on the system body (Figure 2).

In the system, 4 pcs of TEG12-8 and 4 pcs of TEG1-199 thermoelectric modules have been used. Technical

specification of TEG modules used in experiments have been shown in Table 1. Geometric properties of the modules used were measured and added to the table. Thermoelectric features and generation factors and val-ues of the Pellet have been quoted from the bibliogra-phy. TEG12-8 module derives maximum 7.95 W power under temperature conditions of Th= 230°C, Tc= 50°C.35

While a part of exhaust fluid passing through the body is transferred on to TEGs, the cold part is cooled by an electrical blower. As cooling blower, SUN ROAD-A-MATIC XI used in chassis dynamometer has been used. Edges of the modules have been con-nected to each other serially; electrical voltage is obtained in line with the temperature difference occur-ring between cold and hot surfaces. In order to increase heat transfer between hot and cold ceramic surfaces, conductive paste has been applied on both surfaces. Experiment engine is 68HP Ford Fiesta engine of 2006 model and 1399 cc. Technical specification of experi-ment engine is shown in Table 2.

In order to measure and record open circuit voltage, loaded voltage, current, and power values, electronic load of CHROMA 6310A brand has been used. In order to measure temperatures of cold and hot parts, EL_IMKO E-TC15-1K1PT type thermo-couple has been used. Thermo-couples have been placed in input and output of exhaust recovery system in the canals between modules on cold part and aluminum cooler. The temperature values measured have been transferred to the computer after being scanned by EL_IMKO E680 scanner. Thermal paste was used to minimize thermal contact resistance between the main body surface, TEG modules, the heat exchanger surface, and thermocou-ples. The measurements have been carried out after fixed temperature conditions were obtained in all mea-surement points. General view of experiment mechan-ism is shown in Figure 3.

Experimental results and evaluation

In Figure 4, change of Th, Tc, and DT according to engine speed at TH= 383°K is shown. As the engine speed increases, mass of exhaust fluid passing through the exhaust in unit period increases and accordingly Th module surface temperature increases. When compared to increase of Thtemperature, increase of Tc tempera-ture is lower. Such situation increases the DT Figure 2. General view of recovery system.

TEG: thermoelectric generator.

Table 1. Technical specification of thermoelectric generator.36–38

Model Pellet’s thermoelectric properties Geometric properties Production factors

RI a P k lc l A N n r

O mV/K mO m W/mK mm mm mm2

TEG12-8 2.6 190 10.5 1.5 1.2 1.8 1.82 128 0.05 0.1

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temperature difference between module surfaces. This is a result of the decrease in thermal conductivity of ‘‘n’’ and ‘‘p’’ semi-conductors together with the increase in temperature. In the experiments, temperature differ-ences of DT = 65.3°K, 67.9°K, 69.6°K, 71.6°K have been obtained, respectively, at engine speeds of 2000, 2500, 3000, and 3500 r/min.

In Figure 5, loaded and open circuit voltage change of thermoelectric module of TEG-12-8 brand is shown. As the engine speed increases, Vocv voltage and V vol-tage increase too. The increase in engine speed has

caused an increase in temperature difference between surfaces of TEG modules. In the experiments, voltages of Vocv= 3.74, 4.59, 5.04, and 5.62 V have been obtained, respectively, at engine speeds of 2000, 2500, 3000, and 3500 r/min. Here again, Seebeck coefficient and temperature difference have increased due to the increase in engine speed, and accordingly loaded vol-tage (V) has increased too. In the experiments, volvol-tages of V = 2.14, 2.29, 2.51, and 2.78 V have been obtained, respectively, at engine speeds of 2000, 2500, 3000, and 3500 r/min. If Ohm’s law is written for a TEG with external load resistance, the output voltage equals to (V = Vocv IRL). In case of lack of external resistance, the voltage of system equals to Vocv. Such state results in difference between loaded and unloaded voltage values.

In Figure 6, loaded and open circuit voltage change of thermoelectric module of TEG1-199 brand is shown. As the engine speed increases, Vocv voltage and V vol-tage increase too. The increase in engine speed has caused an increase in temperature difference between surfaces of TEG modules. As the temperature differ-ence increases, Vocvvoltage increases too. In the experi-ments, voltages of Vocv= 3.53, 4.45, 4.83, and 5.41 V have been obtained, respectively, at engine speeds of 2000, 2500, 3000, and 3500 r/min. Here again, Seebeck Figure 3. General view of experiment mechanism: 1: cooling

blower; 2: engine; 3: waste heat recovery system; 4: data logger; 5: DC electronic load; 6: computer; 7: exhaust pipe.

Figure 4. Change of system temperatures according to engine speed (TH= 383°K).

Figure 5. Loaded (V) and open circuit voltage (Vocv) change

(TH= 383°K) depending on engine speed.

Figure 6. Loaded (V) and open circuit voltage (Vocv) change

(TH= 383°K) depending on engine speed.

Table 2. Technical specification of experiment engine.39

Brand Ford Fiesta

Engine volume 1399 cc

Maximum power 68 HP/4000 r/min Maximum torque 160 N m/1750 r/min

Valve structure DOHC

Number of cylinders 4 Diameter of cylinder 73.7 mm Compression ratio 17.9 Number of valves per cylinder 4

Fuel type Diesel

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coefficient and temperature difference have increased due to the increase in engine speed, and accordingly loaded voltage (V) has increased too. In the experi-ments, voltages of V = 2.1, 2.5, 2.71, and 2.89 V have been obtained, respectively, at engine speeds of 2000, 2500, 3000, and 3500 r/min.

In Figure 7, change of loaded voltage obtained from two different thermoelectric modules used according to engine speed has been shown. At the same temperature difference, more unloaded voltage has been obtained from TEG12-8 brand thermoelectric module which has higher Seebeck coefficient. In TEG12-8 thermoelectric modules, there are more thermoelectric couples (cou-ples of number). Moreover; the higher value of internal resistance of TEG12-8 thermoelectric modules, calcu-lated in experiments, has resulted in increase of (m=m + 1) value mentioned in equation (7) and affect-ing the loaded voltage value. In the experiments, the highest voltage has been obtained as 5.61 V for TEG-12-8 brand thermoelectric module at engine speed of 3500 r/min, and as 5.41 V for TEG1-199 brand thermo-electric module.

In Figure 8, change of internal resistance of modules depending on engine speed has been shown. Internal temperature change of the system at experimented 2000, 2500, 3000, and 3500 r/min engine speeds has been measured as DT = 338.3°K, 340.5°K, 342°K, and 343.2°K. As the temperature difference increases, loaded voltage, open circuit voltage, and current values increase too. According to RI= (Vocv VL)=I relation, internal resistance of modules has increased according to the engine speed. The highest internal resistance obtained during the experiments is 11.69 O for thermo-electric module of TEG12-8 brand and 7.2 O for ther-moelectric module for TEG1-199 brand at engine speed

of 3500 r/min. The reason why TEG1-199 brand ule shows lower internal resistance than the other mod-ule is the fact that it obtains higher current and power.

In Figure 9, comparison between the analysis and experimental results has been shown. As the engine speed increases, Seebeck coefficient and temperature difference increase too. Such a situation has increased the value of current obtained. TEG1-199 brand module greater pellet cross-sectional area has caused the higher current. In addition to internal resistance of TEG1-199 brand module that is lower than the other one included in the experiment, higher current has passed through the circuit. The coefficient of Seebeck was calculated in the experiments performed in mathematical model pre-pared depending on experimental conditions. The char-acteristics of the overall TEG performance is coherent with the analysis model. In the current graph according to engine speed, the maximum absolute error is calcu-lated for modules TEG12-8 and TEG 1-199 as 0.010 and 0.044, respectively (at experimented 3500 r/min). With the increase of engine speed, loaded voltage and current in both modules increase too. The power, obtained as a function of voltage and current, has increased as the engine speed increased. As internal resistance of TEG12-8 thermoelectric modules is higher than internal resistance of TEG 1-199 modules, the (1 + m)2 value included in power statement has decreased. This is one of the reasons for TEG12-8 ther-moelectric modules to obtain more power. In the power graph depending on the engine speed, the maximum absolute error is also calculated for modules TEG12-8 and TEG1-199 as 0.008 and 0.046, respectively (at experimented 3500 r/min). The maximum absolute error occurred at the same engine speed (at experimen-ted 3500 r/min) as the power obtained is a function of the current.

Figure 7. Open circuit voltage (Vocv) change graphic

(TH= 383°K) of thermoelectric modules depending on engine

speed.

Figure 8. Internal resistance (RI) change graphic (TH= 383°K)

of thermoelectric modules depending on engine speed.

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Charging process is carried out by means of charging system composed of alternator and regulator during operation of the vehicle. Such a process is a charging action carried out by fixed voltage. Charging voltage value changes between 13.8 and 14.2 V. Charging cur-rent is selected between 1/10 and 1/20 of the accumula-tor capacity. In the experiments, 4 pcs of TEG12-8 brand module have been used at 3500 r/min, and a maximum voltage of 2.78 V and current of 0.287 A have been obtained. Under same conditions, 4 pcs of TEG1-199 brand module have been used at 3500 r/min, and a maximum voltage of 3.01 V and current of 0.305 A have been obtained. In this regard, number of modules required to charge a battery of 12 V 40 A h (Lead-acid battery) and related cost details are given in Table 3. If 20 pcs of TEG12-8 modules are connected in parallel by seven serial cranks, 2 A current and 13.9 V voltage shall be obtained. Similarly, if 19 pcs of TEG1-199 modules are connected by seven serial cranks, 2.135 A and 14.3 V voltage shall be obtained.

Power-engine speed, internal resistance-engine speed, and performance graphic equations according to experimental results are shown in Table 4. The equa-tion has been formed logarithmic as V = a ln(x)6b.

Conclusion

In line with the increase in engine speed and mass of exhaust fluid passing through unit period, Th module surface temperature has increased too. When compared to increase speed of Thtemperature, increase speed of Tc temperature is lower. Such situation has increased

the DT temperature difference between the surfaces. With the increase in engine speed, the temperature dif-ference between surfaces of TEG modules has increased too. With the increase in temperature difference, Vocv voltage has increased too. Under same temperature dif-ference conditions, more unloaded voltage has been obtained from TEG-12-8 brand thermoelectric module which has higher Seebeck coefficient. As internal resis-tance of TEG1-199 brand is lower than two modules included in the experiment, higher voltage has passed through the circuit. In the experiments, 4 pcs of TEG12-8 brand module have been used at 3500 r/min, and a maximum voltage of 2.78 V and current of 0.287 A have been obtained. Under same conditions, 4 pcs of TEG-199 brand module have been used at 3500 r/min, and a maximum voltage of 3.01 V and cur-rent of 0.305 A have been obtained. Accordingly, in order to charge a battery of 12 V 40 A h, 140 pcs of TEG12-8 brand module or 124 pcs of TEG1-199 brand module are required. Usage of TEG1-199 brand mod-ule is suitable in terms of total cost for the same Figure 9. Comparison between the analysis and experimental results (TH= 383°K): (a) current output and (b) power output.

Table 3. Number of modules required to charge vehicle battery and related costs.

Module Number of modules Unit price ($) Total cost ($) Obtained battery charge TEG12-8 140 31 4340 13.9 V, 2.0 A TEG1-199 133 10 1330 14.3 V, 2.135 A

Table 4. Experimental performance graphic equations. n (r/min) Th(°C) DT (°C) P (W) (TEG12-8) equation P (W) (TEG1-199) equation RI(O) (TEG12-8) equation RI(O) (TEG1-199) equation 2000 100 65.3 P = 0:8954 ln xð Þ 6:4038 P = 0:7712 ln xð Þ 5:5095 RI= 0:5348 ln xð Þ + 5:5867 RI= 0:4945 ln xð Þ + 3:8936 2500 103.5 67.5 3000 106 69 3500 108 70.2

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capacity of battery charge. One of the important factors increasing recovery performance of TEGs is the engine load. As the engine is diesel and the experiments are carried out in unloaded state, the recovery values obtained have remained low. In a gasoline engine, much more recovery performance will be obtained under different load conditions.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publi-cation of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Mehmet Akif Kunt https://orcid.org/0000-0001-5710-7253

Haluk Gunes https://orcid.org/0000-0002-0915-0924

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