THE POTENTIAL OF THERMOELECTRIC GENERATOR FOR ENGINE EXHAUST HEAT RECOVERY APPLICATIONS

Transcription

1 ISSN: (P), (O), Japan, DOI: ijcst1 Special Issue n Science, Engineering & Envirnment THE POTENTIAL OF THERMOELECTRIC GENERATOR FOR ENGINE EXHAUST HEAT RECOVERY APPLICATIONS *Nyman Sugiartha 1 and I Putu Sastra Negara 2 1,2 Department f Mechanical Engineering, Pliteknik Negeri Bali, Indnesia *Crrespnding Authr, Received: 28 Dec. 217, Revised: 23 Mar. 218, Accepted: 13 April. 218 ABSTRACT: In this study, a thermelectric generatin (TEG) unit was designed and cnstructed t evaluate the technical ptential f exhaust heat recvery system fr autmbile applicatins. The TEG system was made up f a cmmercial thermelectric mdule f TEC mdel, an aluminum duct, an aluminum fin heat sink, an electric heater, a cling fan, AC and DC pwer supply, an external resistive lad and data cllectin devices. The heater was used as an exhaust heat surce simulatr t supply heat energy t the ht side f the TEG mdule. The cling fan was used fr simulating airflw and maintaining unifrm heat rejectin frm the heat sink t the ambient. The experimental investigatin had been cnducted t characterize the TEG s perfrmance. The tests were undertaken based n the measurements f temperature at bth ht and cld sides f the TEG mdule, pen circuit vltage, and utput vltage and current under varius external resistive lads. The test results shwed that the highest temperature difference was 4.1 C. This revealed a peak utput pwer f mw, a lad vltage f mv, a lad current f 22 ma and a cnversin efficiency f.24%. The thermelectric generatin frm waste heat energy f a simulated autmbile exhaust is technically feasible, thugh the utput pwer and the cnversin efficiency f the TEG unit seem lw. In general, it is fund that the temperature difference between the ht and cld sides f the thermelectric mdule has the mst significant impact n the TEG s perfrmance. Keywrds: Thermelectric generatr, Heat recvery, Exhaust heat, Autmbile, Feasibility 1. INTRODUCTION In the autmtive sectr, mre efficiently use f nn-renewable fuel energy resurces has been a great interest in the present day as well as cnserving envirnmental issues, such as air pllutin and glbal warming. Internal autmtive cmbustin engines (IACEs) are prminent fssil fuel cnsumer, which invlved irreversibility energy cnversin prcess thrugh discharging a by-prduct in the frm f waste heat t the envirnment. Besides the useful mechanical wrk gained, the waste heat cntributes a significant amunt f the IACEs energy balance. Up t 45% and 35% f fuel cmbustin energy is lst frm exhaust system fr Ott and Diesel engines, respectively. Hence, it is gd ptential t reuse the exhaust heat which therwise wasted as an engine water clant and exhaust gas. If we can generate electrical energy thrugh a temperature difference between waste gases and the envirnment, which can be restred in the battery f the autmbile, bth an imprvement f efficiency and decrease f fuel cnsumptin wuld be achieved [1]. The reuse f waste heat by means f the heat recvery frm heat rejectin f the engine culd be initiated via radiatr using a water clant with temperature distributin ranges frm 9 C t 95 C r thrugh exhaust gas system. Fr instance, a typical BMW diesel engine has exhaust gas temperatures at full lad arund 6 C and 24 C at near its exhaust manifld and rear muffler, respectively [2]-[5]. Thermelectric generatr (TEG) is ne f the green energy surces fr directly cnverting heat int electrical energy. The TEG system t recver energy lst frm dispersed heat is cnsidered useful n an autmbile. Thrugh the effect f a temperature difference between the tw f the heat surce and heat sink, thermal waste energy can be directly cnverted int electricity frm tw ptential areas, i.e. engine exhaust pipe and radiatr [6]-[8]. Thermelectric technlgy is presently becming prfitable in the autmtive industry because f the increased cncerns ver vehicle fuel ecnmy and exhaust gas emissins. TEG theretically ffers advantages when cmpared with cnventinal electric pwer generatrs, i.e. mre rbust, flexible and reliable since it requires neither mving parts nr wrking fluids, fewer maintenance requirements, capable f elevated temperature peratin, n scale effect, silent peratins, psitin-independent, light in weight and small in size, and envirnmentally friendly [9]-[13]. Frmer researches abut engine waste heat recvery system prgressed prf f cncepts f the TEG applicatins in the autmtive field. Kim 1

2 et al. [9] investigated a TEG device using engine water clant f passenger vehicles. They attached 72 bismuth telluride (Bi 2 Te 3 ) thermelectric mdules n the test vehicle. Their experimental results shwed that the maximum utput pwer was 75 W, the calculated thermelectric mdule efficiency f the TEG was 2.1%, and the verall efficiency f electric pwer generatin frm the waste heat f the engine clant was.3% in the driving mde at 8 km/h. The TEG test bench built by Liu et al. [15] rughly generated a maximum pwer f 944 W and system efficiency f 1.85%. The TEG system emplyed fur identical TEGs, which cntain 24 thermelectric mdules and a water-cled system. Lw-temperature thermelectric mdules f up t 36 C f ht side temperature were used in the TEG. Chung et al. [16] cnstructed a prttype f the thermelectric waste heat recvery system. The main feature f their study was t use a hightemperature cnditin up t 2 C t ensure the reliability f the thermelectric generatin system, especially fr the case f diesel vehicles which discharge exhaust gas f as high as arund 2-3 C at the utlet f the catalyst filter. Thrugh experiments n an autmtive diesel engine, Zhang et al. [17] fund that the TEG system generated 12.6 W f electricity and 2.1 % heat t electrical efficiency using exhaust waste heat at an average temperature difference f 339 C between the TEG ht and cld surfaces at a 55 C exhaust temperature. The cmplete TEG system in their wrk included 4 thermelectric mdules with a high-efficiency nanstructure material. The use f thermelectric cler (TEC) mdules fr autmbile exhaust heat recvery was reprted by Ca et al. [18]. They built a prttype f heat pipe assisted TEG that cmprises 36 pieces f thermelectric mdules f TEP1-142T3 and a water-cled heat sink. In their system, exhaust gas was simulated by ht air. The TEG system generated maximum pen circuit vltage f 81.9 V. The crrespnding pwer utput and cnversin efficiencies were 13.8 W and 2.58 %, respectively. Orr et al. [19] develped a bench type mdel f the TEG system fr exhaust heat recvery applicatins in typical passenger cars. The system was intended t be a prf f cncept f pwer prductin by the TEC using heat pipes and ht engine exhaust gases. Eight pieces f the TEC mdules were used. The heat pipes with water as a wrking fluid were als used t cl the TEC mdules. The experimental results shwed that the system prduced 6.3 W f electrical pwer with a heat t electricity cnversin efficiency f 1.43%. Maneewan and Chindaruksa [2] cnstructed a TEG system fr recvery f waste heat frm bimass dryer. Twelve thermelectric mdules f TEC were used in this system. Their experimental results shwed that the pwer utput was abut 22.4 W (14 V, 1.6 A) with an air flw f 9.62 m 3 /s achieving 4.8% cnversin efficiency. The purpse f this wrk is t evaluate the technical feasibility f the thermelectric generatin frm waste heat energy f a simulated autmbile exhaust. A Peltier thermelectric mdule f TEC mdel, which riginally designed fr cling purpses was used and experimentally tested t functin as a TEG device thrugh the reverse prcess. In additin, its merit f lw cst and wide availability cmpared t the basic TEG becmes the ther cnsideratins. Furthermre, it is expected that the thermelectric mdule culd be applied t autmbile Diesel engines, particularly n the rear muffler f the engine exhaust pipe system. 2. WORKING PRINCIPLE OF THE TEG Fig.1 shws a schematic diagram f an perating principle f the TEG. The basic perating principle f the TEG is based n its thermelectric effects, i.e. Seebeck effect, Peltier effect, and Thmsn effect. It als accmpanies with Julean and Furier effects. The Seebeck effect states that an electrical ptential is generated in an pen circuit frmed by tw dissimilar cnductrs when their junctins are bunded at tw places with different temperatures. The Peltier effect states that heat can be absrbed r liberated at the junctin f tw dissimilar cnductrs when a current is passed. The Thmsn effect states that heat can be liberated r absrbed in a single hmgeneus cnductr when an electric current flws in the presence f temperature gradient [21]. Ht Junctin P-type Heat Surce N-type Cld Junctin Ambient Heat Sink Lad Resistance Insulatr Cnductr Current I Fig.1 Schematic diagram f an perating principle f the TEG [6] Typically, the structure f the TEG mdule cnsists f a thermelectric element (which 2

3 cmpsed f ceramic substrates, electrical insulatrs, electrical cnductrs and N-type/ P-type semicnductr blck), a heat surce and a heat sink. By applying different temperature n bth sides in which ne side was heated (as a heat surce) while the ther side was kept at a lwer temperature (as a heat sink), electric current is induced because f the thermelectric effect [6]. 3. METHODOLOGY 3.1 Experimental Setup Digital multimeter (Vc, I, V, RL) + - Digital thermmeter (Tcld, Tht) Cling fan External lad (resistrs) Heat sink TEG mdule DC Pwer supply Heater Ducting AC Pwer supply (a) (b) Fig.2 (a) Schematic f system setup (b) phtgraph f the experimental system An arrangement f the experimental system is shwn in Fig.2. The system was made up f a thermelectric mdule, a duct, a heat sink, a heater, a cling fan, AC and DC pwer supply, an external resistive lad and data cllectin devices. A cmmercial Peltier cler mdule f TEC1 3

4 1276 mdel was used as a generatr thrugh thermelectric cnversin. The thermelectric mdule has 127 cuples f bismuth telluride semicnductr and alumina ceramic cver with a physical size f 4 mm x 4 mm x 3.8 mm. An aluminum duct with the size f 4 mm x 6 mm x 1.2 mm was utilized t represent the engine exhaust pipe. The duct was attached directly t the bttm surface f the thermelectric mdule as a ht side while an aluminum fin heat sink was munted n the tp f it as a cld side f the thermelectric mdule t maintain cnstant temperatures. A thermal paste was used t maintain surface thermal cntact between the mdule cver and bth the ducting surface and the heat sink thus reducing thermal resistivity. A 22 VAC/35 W heater made frm cpper was used as an exhaust heat surce simulatr t supply heat energy t the ht side f the thermelectric mdule and lcated inside the duct at abut 1 cm gap t the tp surface. A cling fan f 12 VDC/.15 A was used fr simulating airflw and maintaining unifrm heat rejectin frm the heat sink t the ambient. Fr temperature mnitring, tw K-type thermcuple sensrs were attached n bth sides f the mdule in which ne n the bttm f the heat sink surface and the ther n the tp surface f the duct plate. The data cllectin system emplyed a digital thermmeter (Krisbw KW6-283) and tw digital multimeters, i.e. Krisbw KW6-272 and Refc X-475. Table 1 shws range and accuracy f the measuring instruments assciated with the experiments. Table 1 Range and accuracy f the measuring instruments Instrument Range Accuracy Temperature sensr: (K-type thermcuple) -13 C ±.75% Temperature reading: C ±.5%±1 C (Krisbw KW6-283) Multimeter reading: (Krisbw KW6-272) ~ Ammeter (DC A) -2m 1±(1.5%±3d) ~ Vltmeter (DC V) -1 ±(.5%±2d) ~ Ohmmeter (Ω) -4M ±(1.%±2d) Multimeter reading: (Refc X-475) ~ Ammeter (DC A).1μ-1 ±(1.%±2d) ~ Vltmeter (DC V).1m-1 ±(.5%±2d) ~ Ohmmeter (Ω).1-4M ±(1.%±5d) 3.2 Test Prcedure The prcedures applied t evaluate technical perfrmance f the TEG mdule were as fllws: a. The TEG was tested t characterize pen circuit vltage (V c ) and temperature difference ( T = T ht -T cld ) between the ht side and cld sides with respect t the time. The electric heater was kept pwered cnstantly at 11 VAC, while the cling fan was pwered at 3 VDC, 6.5 VDC, and 7 VDC. The crrespnding V c and temperature difference ( T) readings were then recrded every 1 minute. b. The T f the varied cling fan supply vltage, i.e. at 31.9 C, 37 C, and 4.1 C was set at cnstant heat flw fr different value f resistive lad by cntrlling the cling fan and heater using analg cntrl (i.e. by switching the pwer supply n r ff). The 1 set f external resistive lad, i.e. 1 Ω, 1.2 Ω, 2.2 Ω, 3.8 Ω, 5.8 Ω, 7.8 Ω, 1 Ω, 15 Ω, 25 Ω and 33 Ω were cnsecutively cnnected t the thermelectric utput. The data f lad vltage and lad current were taken fr respective lad resistance and T setting in a 1-minute interval. The utput pwer can be calculated accrdingly using Eq Perfrmance Evaluatin The indicatrs t evaluate technical perfrmance f the TEG mdule include utput pwer, lad current, lad vltage and pen circuit vltage as given by Eqs. (1)-(3) belw: 2 V P = RL (1) P I = V (2) V V c 2 (3) where P is utput pwer in watt, V is lad utput vltage in vlt, R L is lad resistance in hm, I is lad current in ampere and V c is pen circuit vltage in vlt. 4. RESULTS AND DISCUSSION The tempral variatin f temperature difference ( T) and pen circuit vltage (V c ) between the ht and cld sides f the TEG mdule at varius fan supply vltage is depicted in Fig. 3. As it can be seen in the Fig.3, the T and V c fr the three values f the fan supply vltage are almst stable within 15-2 minutes. The average temperature differences are 31.9 C, 37 C and 4.1 C (Fig.3a), while the average pen circuit vltages are 131 mv, 15 mv and 161 mv (Fig. 3b) fr the fan supply vltage f 3 VDC, 6.5 VDC, and 7 VDC, respectively. The higher the cling fan supply vltage the higher bth the T and V c. By increasing the fan supply vltage, it 4

5 means that the fan rtatin speeds up as well as the TEG heat transfer rate, hence the heat rejectin frm the heat sink t the ambient increases. The pen circuit vltage at the utput f the thermelectric mdule increases linearly with the temperature difference. Furthermre, the set pint f temperature difference culd be fixed by cntrlling n/ff f the cling fan speed at a certain value f the fan supply vltage. In additin, the maximum temperature achieved at the ht side f the thermelectric mdule during the test was arund 111 C. V c (mv) T ( C) (a) (a) (b) Fan-3V Fan-6V Fan-7.5V Time (min) Fan-3V Fan-6V Fan-7.5V Time (min) Fig.3 (a) Tempral prfiles f temperature difference ( T) (b) pen circuit vltage f the TEG mdule at varius fan supply vltage The characteristic f lad vltage versus lad current f the TEG as a functin f temperature difference between the ht and cld sides f the TEG mdule is shwn in Fig. 4. Frm the figure, it can be nted that the lad vltage is linearly prprtinal t the lad current but in almst the same negative slpes. The higher value f the lad vltage crrespnds t the lwer value f the lad current. It als indicates that the slpe f each line which represents the internal electrical resistance f the TEG is still cnstant in varying T and lad peratins as gverned by the basic Ohm s law. Fr instance, the maximum lad vltage f abut mv is revealed at the lad current f 8 ma at the highest T f 4.1 C. Bth the lad vltage and current increases with the temperature differences but the slpe f the change f the temperature difference is almst independent f the lad current [22]. Lad vltage (mv) T = 37. ⁰C T = 4.1 ⁰C Lad current (ma) Fig.4 Lad vltage versus lad current in varying T Lad vltage (mv) T = 37. ⁰C T = 4.1 ⁰C Lad resistance (Ω) Fig.5 Lad vltage versus lad resistance in varying T By varying the lad resistance frm 1 t 33 Ω, the lad vltage f the TEG mdule versus the lad resistance under different T is recrded as depicted in Fig. 5. As can be bserved frm the Fig. 5, the lad vltage increases with increasing the temperature difference at varius applied external resistive lad. The rise f the temperature difference results in raising the lad vltage. As a cnsequence, the lad current will increase fr a given lad resistance. The lad vltage increases drastically with the resistive lad value frm 1 Ω t 1 Ω and then increases almst smthly at the 5

6 higher resistive lad up t 33 Ω. The lad vltage f the TEG reaches the maximum at mv with T f 4.1 C. Fig. 6 depicts the relatinship between utput pwer and the applied lad resistance. It can be seen in Fig. 6 that the utput pwer increases sharply reaching peak value as high as mw then decreases gradually as the lad resistance increases. Subsequently, the crrespnding temperature difference and the lad resistance at peak utput pwer are 4.1 C and 5.8 Ω, respectively. As the temperature difference increases, the lad vltage increases fr certain value f the lad resistance. As the lad vltage is high, the high will be the utput pwer. The utput pwer increases with increasing the temperature difference at different lad resistance. The graph seems t frm parablic curve since the utput pwer f the TEG mdule is btained by dividing the square f lad vltage by the lad resistance. The maximum utput pwer f the TEG mdule is achieved under the lad matching cnditin when the lad resistance matches the internal resistance f the mdule [22], [23]. Output pwer (mw) Fig.6 Output pwer versus lad resistance under different T Pwer utput (mw) T = 37. ⁰C T = 4.1 ⁰C Lad resistance (Ω) T = 37. ⁰C T = 4.1 ⁰C Lad vltage (mv) Fig.7 Output pwer versus lad vltage under different T Figs. 7 and 8 illustrate the variatin f the utput pwer with the lad vltage and the lad current fr varius T. The utput pwer f the TEG mdule depends n the lad vltage which is cntrlled by the lad resistance, and this relatinship changes fr varius T. The highest T delivered the highest utput pwer. The maximum utput pwer achieved in this experiment is abut mw. An electrical pwer f mw is generated with a current f 22 ma and a cnversin efficiency f.24% when T is 4.1 C, s there is a temperature difference f up t 4.1 C, when the ht side reaches 111 C, resulting in perfrmance f mw at pen circuit vltage f 161 mv. It is fund that the temperature difference has the mst significant impact n the TEG s perfrmance. Output pwer (mw) Lad current (ma) Fig.8 Output pwer versus lad current under different T 5. CONCLUSIONS T = 37. ⁰C T = 4.1 ⁰C A simple TEG unit has been designed and cnstructed fr simulated autmbile exhaust heat recvery system. A single Peltier thermelectric mdule (TEC1 1276) was used t functin as a TEG device. The experimental investigatin f the TEG unit had been cnducted t evaluate the technical ptential f autmbile applicatins. The tests were perfrmed based n the measurements f temperature at bth ht and cld sides f the TEG mdule, pen circuit vltage, utput vltage and current under the influence f different external lads. The highest temperature difference tested was 4.1 C. This revealed a peak utput pwer f mw, a lad vltage f mv, a lad current f 22 ma and a cnversin efficiency f.24%. The thermelectric generatin frm exhaust heat recvery is technically feasible, thugh the utput pwer and the cnversin efficiency f the TEG unit seem lw. By increasing number f the 6

7 TEG mdule cnnected in series might meet the electrical pwer demand fr a particular applicatin. Furthermre, design and calculatin prcedures fr system s sizing were fllwed. As the ht side temperature tested was as high as 111 C, which was belw the exhaust gas temperature f Diesel engines (2-3 C), the use f a heat exchanger at upstream f the TEG unit is recmmended t reduce the exhaust gas temperature. The maximum allwable perating temperature f the tested thermelectric mdule is 138 C based n the manufacturer s material data. 6. ACKNOWLEDGMENTS The authrs wuld like t thank ur lab assistant I Wayan Sutarsa frm the Labratry f Instrumentatin and Cntrl at the Department f Mechanical Engineering, Pliteknik Negeri Bali fr prviding instruments and services during the experimental prcess. Nyman Sugiartha wrked n the technical details and measurements, analysis and interpretatin f data, and was writing the manuscript. I Putu Sastra Negara set up an experimental system and cnducted the measurements. 7. REFERENCES [1] Ya D-J., Yeh K-J., Hsu C-T., Yu B-M. and Lee J-S., Efficient reuse f waste energy, IEEE Nantechnl Mag, Vl. 3, 29, pp [2] Serksnis A.W., Thermelectric Generatr fr Autmtive Charging System, in Prc. 11th Intersciety Cnversin Engineering Cnference, 1976, pp [3] Talm, H.L. and Beyene, A., Heat recvery frm the autmtive engine, J. Applied Thermal Engineering, Vl. 29, 29, pp [4] Anatychuk L.I., Luste O.J. and Kuz R.V., Theretical and experimental studies f a thermelectric generatr fr vehicles, Jurnal f Electrnic Materials, Vl. 4, N. 5, 211, pp [5] LaGrandeur J., Crane B. and Eder A., Vehicle Fuel Ecnmy Imprvement thrugh Thermelectric Waste Heat Recvery, in DEER Cnference, 25, pp [6] Hsia Y.Y., Chang W.C. and Chen S.L., A mathematic mdel f the thermelectric mdule with applicatins n waste heat recvery frm an autmbile engine, Energy, Vl. 35, 21, pp [7] Baatar N. and Shihi K., A Thermelectric Generatr Replacing Radiatr fr Internal Cmbustin Engine Vehicles, Telkmnika, Vl. 9, N. 3, 211, pp [8] Meng J.-H., Wang X.-D., and Chen W.-H., Perfrmance investigatin and design ptimizatin f a thermelectric generatr applied in autmbile exhaust waste heat recvery, Energy Cnversin and Management, Vl. 12, 216, pp [9] Niu X., Yu J. and Wang S., Experimental study n the lw-temperature waste heat thermelectric generatr, J. f Pwer Surces, Vl. 188, Issue 2, 29, pp [1] Wang Y., Dai C., and Wang S., Theretical analysis f a thermelectric generatr using exhaust gas f vehicles as a heat surce, Applied Energy, Vl. 112, 213, pp [11] Meng J.-H., Zhang X.-X., and Wang X.-D., Characteristics analysis and parametric study f a thermelectric generatr by cnsidering variable material prperties and heat lsses, Internatinal Jurnal f Heat and Mass Transfer, Vl. 8, 215, pp [12] Liu C., Pan X., Zheng X., Yan Y., anad Li W., An experimental study f a nvel prttype fr tw-stage thermelectric generatr frm vehicle exhaust, Jurnal f the Energy Institute, Vl. 89, Issue 2, 216, pp [13] Champier D., Thermelectric generatrs: A review f applicatins, Energy Cnversin and Management, Vl. 14, 217, pp [14] Kim S., Park S., Kim S. and Rhi S.H., A thermelectric generatr using engine clant fr light-duty internal cmbustin enginepwered vehicles, Jurnal f Electrnic Materials, Vl. 4, Issue 5, 211, pp [15] Liu X., Deng Y.D., Li Z., and Su C.Q., Perfrmance analysis f a waste heat recvery thermelectric generatin system fr autmtive applicatin, Energy Cnversin and Management, Vl. 9, 215, pp [16] Chung J.H., Kim W.C., Lee J.H. and Yu T.U., Experimental Study f a High-Temperature Thermelectric Generatin System using Waste Heat Recvery, in Prceedings f the KSME 27 fall annual meeting, 27, pp [17] Zhang Y., Cleary M., Wang X., Kempf N., and Meda L., High-temperature and highpwer-density nanstructured thermelectric generatr fr autmtive waste heat recvery, Energy Cnversin and Management, Vl. 15, 215, pp [18] Ca Q., Luan W. and Wang T., Perfrmance enhancement f heat pipes assisted thermelectric generatr fr autmbile exhaust heat recvery, Applied Thermal Engineering, Vl. 13, 218, pp [19] Orr B., Singh B., Tan L., and Akbarzadeh A., Electricity generatin frm an exhaust heat 7

8 recvery system utilizing thermelectric cells and heat pipes, Applied Thermal Engineering, Vl. 73, Issue 1, 214, pp [2] Maneewan S. and Chindaruksa S., Thermelectric Pwer Generatin System Using Waste Heat frm Bimass Drying, Jurnal f Electrnic Materials, Vl. 38, N. 7, 29, pp [21] Haidar J.G. and Ghgel, J.I., Waste heat recvery frm the exhaust f lw-pwer diesel engine using thermal electric generatrs, in Prc. 2 th Int. Cnf. Thermelectrics, 21, pp [22] Kim S., Analysis and mdeling f effective temperature differences and electrical parameters f thermelectric generatrs, Applied Energy, Vl. 12, 213, pp [23] Rwe D.M. and Min G., Evaluatin f thermelectric mdules fr pwer generatin, Jurnal f Pwer Surces, Vl. 73, 1998, pp Cpyright Int. J. f GEOMATE. All rights reserved, including the making f cpies unless permissin is btained frm the cpyright prprietrs. 8