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Efficiency Study of a Commercial Thermoelectric PowerGenerator (TEG) Under Thermal Cycling

E. HATZIKRANIOTIS,1 K.T. ZORBAS,1 I. SAMARAS,1 TH. KYRATSI,2,3

and K.M. PARASKEVOPOULOS1

1.Department of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece.2.Department of Mechanical and Manufacturing Engineering, University of Cyprus, Kallipoleos75, 1678 Nicosia, Cyprus.3.e-mail: kyratsi@ucy.ac.cy

Thermoelectric generators (TEGs) make use of the Seebeck effect insemiconductors for the direct conversion of heat to electrical energy. The possibleuse of a device consisting of numerous TEG modules for waste heat recoveryfrom an internal combustion (IC) engine could considerably help worldwideefforts towards energy saving. However, commercially available TEGs operateat temperatures much lower than the actual operating temperature range inthe exhaust pipe of an automobile, which could cause structural failure of thethermoelectric elements. Furthermore, continuous thermal cycling could leadto reduced efficiency and lifetime of the TEG. In this work we investigate thelong-term performance and stability of a commercially available TEG undertemperature and power cycling. The module was subjected to sequential hot-side heating (at 200C) and cooling for long times (3000 h) in order to measurechanges in the TEGs performance. A reduction in Seebeck coefficient and anincrease in resistivity were observed. Alternating-current (AC) impedancemeasurements and scanning electron microscope (SEM) observations wereperformed on the module, and results are presented and discussed.

Key word: Thermoelectricity, thermoelectric power generator, performancereliability of thermoelectric modules, cyclic thermal loading,alternating current impedance measurements

INTRODUCTION

The waste heat produced by an internal combus-tion engine generally accounts for a large percent-age of the input fuel energy. In gasoline-fueled ICengines, about 75% of the total fuel energy is

rejected to the environment.1,2 The recovery of 6% ofthe exhaust energy could lead to a 10% fuel saving.3

Thermoelectric modules are considered to behighly reliable components due to their solid-stateconstruction. For most applications they will pro-vide long-term trouble-free service. There have beenmany cases where TE modules have been usedcontinuously for 20 or more years, and the lifetimeof a module often exceeds that of the associatedequipment.4 For applications involving relatively

steady-state cooling where direct-current (DC)power is being applied to the module on a more orless continuous and uniform basis, thermoelectricmodule reliability is extremely high. However,thermal shocks and temperatures much higher than

the desired operating range could cause structuralfailure of the thermoelectric elements. This limita-tion is a significant difficulty when designing asystem for transferring waste exhaust gas heat tothermoelectric elements, since exhaust gas tem-peratures vary widely. Such a thermoelectric deviceshould be resistant to thermal shocks and to veryhigh exhaust gas temperatures (up to 900C,depending onthe type of engine and the location ofthe device),5 in different possible operating situa-tions and at temperatures of the external environ-ment that could range from 40C to 50C.Exposure of thermoelectric materials to such envi-ronments causes, generally, an increase in electrical

(Received July 10, 2009; accepted October 23, 2009;published online November 18, 2009)

Journal of ELECTRONIC MATERIALS, Vol. 39, No. 9, 2010

DOI: 10.1007/s11664-009-0988-8

2009 TMS

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resistivity, as well as a decrease in the materialsfigure of merit ZT.6,7 Furthermore, continuousthermal cycling could lead to reduced efficiency andlifetime of the TEG.

This work focuses on the investigation of the long-term efficiency of a commercial TEG, subjected tothermal shock for 3000 h, in order to simulate the

rough conditions dominating in automotive appli-cations. A measurement device and a theoreticalmodel have been developed,8 which allows the cal-culation of gained power and efficiency of a ther-moelectric generator device under different electricloads and temperature gradients. After the experi-ment, module distortions were checked and changesin the thermoelectric material were examined byusing Z-meter and AC impedance measurements.

EXPERIMENTAL PROCEDURES

Commercial 2.5 cm 9 2.5 cm Bi2Te3-based mod-ules (Melcor HT9-3-25) were used. The moduleconsists of N = 31 thermocouples. The heater ismade of copper and is attached directly to the top ofthe TEG module. In order to keep a constant coolingtemperature, a liquid heat exchanger was used as acooler. All pieces were bonded together with bolts ata pressure of 4 MPa on the TEG surfaces. In orderto reduce the thermal contact resistance, all sur-faces were lapped to a maximum roughness of about25 lm, and a thin layer of graphite thermal grease(Melcor GRF-159) was used.

For temperature monitoring, two K-type thermo-couples were mounted, one in a hole of 1 mmdiameter near the bottom surface of the heater and

the other in a thin copper plate on the bottom of themodule. The temperature of the heater was con-trolled by an Eliwell EWTR 910 temperature con-troller. For the electrical measurements, Agilent34401A and Metrahit multimeters were used asvoltmeter and ammeter, respectively.

The TEG module was subjected to 6000 sequen-tial heatingcooling cycles. Prior to and after theheating-cooling cycles the module was tested with aZ-meter device (type DX4065, RMT Ltd.9) as well asby AC impedance measurements using a ZahnerIM6ex impedance meter in the frequency rangefrom 1 mHz to 100 Hz.

SEM investigations were carried out using ascanning electron microscope with associated EDS(JEOL J.S.M. 840A, Tokyo, Japan).

RESULTS

Efficiency Study

A typical heatingcooling cycle is presented inFig.1. In total, 6000 sequential heatingcoolingcycles lasting about 3 9103 h were applied to theTEG module during the long-term efficiency test. ASiemens Logo RC230 PLC was used to control therepeated heatingcooling cycles. An ohmic resistorof 0.45 X (as near as possible to the value of the

TEGs internal resistance at a hot-side temperatureof 200C) was used as a load, in order to achieve themaximum power. The value of the external ohmicresistor was kept constant throughout the thermalcycling experiment. During the heating phase theexternal ohmic load was disconnected to reach thedesired temperature quickly, while the loadremained connected during the cooling phase forrapid cooling. Measurements ofPmaxwere taken themoment that the desired temperature was reached,when the external resistor was connected.

Figure2 presents the TEGs maximum gainedpower P and the electromotive force (EMF) U0 (theopen-circuit voltage of the TEG when the maximumtemperature was reached) during the efficiency test.As can be seen, both decrease with thermal cycling.The TEGs EMF is proportional to the Seebeckcoefficient STEG, and thus the change in the mate-rials Seebeck coefficient can be evaluated. Thechange of the TEGs resistance RTEG can be

HEATING

0

50

100

150

200

0 5 10 15 20 25 30 35

TIME (min)

TEM

PT1(C)

0

1

2

3

4

I(A

),U(V)

CONSTANT

OPERATION

COOLING

I_TEG

U_TEG T2=24C

T1

Fig. 1. Typical coolingheating cycle. The TEG module was sub-jected to 6000 sequential heatingcooling cycles. The heaterstemperature was fluctuated from 30C to 200C, while the cold sideremained at 24C. During the heating phase the external ohmic loadwas disconnected to reach the desired temperature quickly, while theload remained connected during the cooling phase for rapid cooling.

U0= 2.1591x-0.0129

P = 2.5482x-0.0468

1

1.2

1.4

1.6

1.8

2

2.2

2.4

0 1000 2000 3000 4000 5000 6000 7000

cycle

P_

TEG(W)

1.7

1.8

1.9

2

2.1

2.2

2.3

2.4

U0_

TEG(V)

Fig. 2. TEGs maximum gained power (P) and EMF (U0) during thereliability test. A total drop of about 14% in gained power and 3.3% inEMF to 6000 cycles was observed, which can be attributed tomaterial deterioration.

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determined from the slope of the linear trend,VTEG=U0 I

*RTEG, whereVTEGis the voltage afterthe external load is connected and I is the currentthat passes. The apparent change in resistivity (q)and Seebeck coefficient (a) can be evaluated fromthe values of RTEG and STEG, respectively (Fig.3).As can be seen from Fig.3, the value ofq increasedfrom 1.62 9103 X cm to 1.79 9103 X cm whilethe value ofa decreased from 205 lV/K to 197 lV/K.

Z-Meter Study

For the Z-study, a Z-meter device was used. Withthis device, the material figure of merit ZTcould bemeasured, along with the TEGs electrical resis-tance RTEG and the time constant s, which definesthe time period necessary for a module to reach thesteady state in response to switching of the current.Measurements, presented in Fig.4, were carriedout at room temperature, on the same TEG, prior toand after thermal cycling. The measuring currentwas set to 5 mA. As can be seen, the device electricalresistance changed from 0.31X before thermalcycling to 0.36 X afterwards. Consequently, thetime constant s changed from 6.01 s to 6.36 s. Thedevices figure of merit, ZT, was found to decreasefrom ZT= 0.74 (Z = 2.47 9103 K1) to ZT= 0.63(Z = 2.11 9103 K1) after 6000 thermal cycles.

AC Impedance Studies

AC impedance measurements are not commonlyused in the characterization of TEG modules. Veryfew studies are reported in the literature,1012 mainlyconcerning the study of pure elements (legs). ACimpedance measurements, presented in Fig. 5, werecarried out at room temperature, on the same TEG,prior to and after thermal cycling.

From the curve of Fig.5, two regimes can beobserved. The low-frequency regime is representa-tive of both Seebeck and ohmic contributions. Asfrequency increases, the heat cannot diffuse into thematerial and the temperature variation becomesless and less important. Then, at high frequencies,

the ohmic part of the voltage remains only. As canbe seen, a minimum of the phase is located around0.035 Hz, which comes from the competitionbetween the Seebeck and ohmic voltage. Indeed,while the ohmic term has zero phase for any fre-quency, the Seebeck voltage phase varies according tothe thermal diffusivity of the samples. At high fre-quencies, the remaining ohmic term has zero phase.

The AC impedance graph can be analyzed with a

simple RC circuit in series with an ohmic contribu-tion R0. The analysis yields R0 = 0.318 X andR0 = 0.330 X, prior to and after the thermal cycling,respectively. The corresponding values for the RCcomponent areRprior= 0.237 X andRafter = 0.214 X.The ZT values, calculated from the expressionZT= R0/R give ZTprior = 0.74 and ZTafter= 0.65,which are comparable to the values measured by theZ-meter.

DISCUSSION

As can be seen from Fig.2, after the first 50cycles the module exhibits a power drop of about

1.55

1.6

1.65

1.7

1.75

1.8

0 1000 2000 3000 4000 5000 6000 7000

cycle

(m.cm)

0.196

0.197

0.198

0.199

0.2

0.201

0.202

0.203

0.204

0.205

*1

0^3(V/K)

Fig. 3. Apparent change in resistivity (q) and Seebeck coefficient (a)during the reliability test.

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

0 10 20 30 40 50

time (s)

V(mV)

Vused

Vnew

IR drop

VS

Fig. 4. Typical V-meter output for a TEG prior to (new) and after(used) thermal cycling.

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

0.001 0.01 0.1 1 10

Frequency (Hz)

Z()

-14

-12

-10

-8

-6

-4

-2

0

(

)

used-Z

new-Z

used-

new-

Fig. 5. AC impedance measurements for a TEG prior to (new) andafter (used) thermal cycling.

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12.5% and an EMF drop of about 15%. This abruptchange can be attributed to the deterioration of thethermal grease, caused by the elevated tempera-ture. Based on our theoretical model,8 it waspossible to estimate an increase in the hot-sidethermal resistance from 0.15 K/W to 0.3 K/W. Thisincrease in contact thermal resistance results in a

decrease in the hot-side temperature, and thus thetemperature difference between the TEGs hot andcold side, TH TC, is decreased from 164 K to158 K. The average temperature is also decreasedfrom 385 K to 381 K.

After the initial 50 cycles, the maximum poweroutput presents a more gradual drop with a slope ofapproximately 3.8% up to 1000 cycles. After the first1000 cycles and up to 6000 cycles, a slighter drop isobserved with a tendency to stabilize. Ignoring theinitial power drop, a total power drop of about 14%up to 6000 cycles was observed, which can beattributed to material deterioration. Consequently,the EMF drops by 1.1% up to 1000 cycles and thenreveals a total drop of about 3% up to 6000 cycles.

The measurements with the Z-meter and ACimpedance show an initial TEG resistance of 0.31 X,while the resistance after 6000 cycles is 0.36 X,corresponding to an increase of 16.1%. This value isin good agreement with our measurements of TEGresistance (for the average temperature of 108C).

From the well-known equation Z = a2/qj, theaverage thermal conductivity for the p- and n-legsmay be calculated, and has been shown to decreasefrom 1.72 W/m K to 1.60 W/m K, resulting from a3.8% decrease in the Seebeck coefficient and a 16.1%increase in the material resistivity.

Z-meter measurements can be modeled by con-sidering the transient behavior of the metalsemi-conductor dual heterojunction formed from the coldand hot junctions. The beginning of the transient isdefined as the moment that the electric current isapplied, and the end of the transient is defined tooccur when the steady-state temperature DT

1 is

asymptotically approached. During the transient,the heat absorbed by the cold junction (or releasedfrom the hot junction) is not zero but is a function ofthe amount of material on either side of the junc-tion, their respective molar heat capacities at con-stant pressure, Cp, and the rate of change oftemperature with time. Use of the boundary condi-tions DT(0) = 0 and DT(1) = DT1 results in thefollowing simple equation for the non-steady-statetemperature behavior of a typical thermoelectriccooling device:13:

DTt DT1 1 exp KTEG I STEG

nCpt

; (1)

where KTEG is the total thermal conductance of theN couples in the module [KTEG = N(jM + jP)G,where G = area/length is the legs geometry factor]and STEG is the total Seebeck coefficient [STEG =N(aN + aP)]. The developed temperature difference

is related to the Seebeck voltage through theexpression

VSt STEGTAMB DTt ; (2)

whereTAMBis the ambient temperature. Therefore,the output signal may be fitted by

Vt I R DV1expt=s ; (3)

where the first term is the IR drop and DV is theSeebeck voltage generated on application of thecurrent. As can be seen, despite the simplicity ofthe model, the overall fit of the voltage output (solidlines in Fig.4) is perfect. Equation 3suggests that asimple RC circuit can be used to model the non-steady-state case. This suggests that the two

methods, namely the Z-meter and the AC imped-ance measurements, are complementary to eachother.

The reduction in the Seebeck coefficient and theincrease in resistivity are indicative of materialdeterioration caused by repeated thermal shock.The 3.8% decrease in the Seebeck coefficient couldbe attributed to interdiffusion of metal from thesolder to the p- or n-type blocks. Consequently, the16.1% increase in resistivity could be attributed tomicrocrack formation, caused by the elevated tem-perature. Indeed, microcracks were observed in thevicinity of the TE legsolder interface, at the end of

TEG leg

solder

connecting

metal

ceramic

plate

TEG leg

solder

connecting

metal

ceramic

plate

Fig. 6. SEM micrograph for a TEG before (a) and after thermalcycling (b). The development of microcracks in the vicinity of thelegsolder interface is evident.

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the experiment, as shown in the SEM micrograph inFig.6b. SEM observations on a new TEG do notindicate any microcracks at the solderTE leginterface (Fig.6a). Though it is not clear when theformation of microcracks happened because of therather smooth variation of TEG internal resistance(RTEG), it can be estimated that they should have

appeared soon after the beginning of thermalcycling.

CONCLUSIONS

In this work the long-term performance andstability of a commercially available TEG have beeninvestigated under temperature and power cycling.With the development of a measuring device and atheoretical model that takes into account the con-tact thermal resistances the changes in resistivityand Seebeck coefficient of the thermoelectric mate-rial are evaluated. Results indicate a 6.6% decreasein the average leg thermal conductivity, a 3.8%

decrease in the Seebeck coefficient, and a 16.1%increase in resistivity. The increase of TEG resis-tance may be mainly attributed to the formation ofmicrocracks in the vicinity of the leg-solder inter-face, caused by the elevated temperature, though anincrease in material resistivity cannot be excluded.Detailed studies on the electrical properties of eachleg in a disassembled TEG after thermal cycling arein progress.

ACKNOWLEDGEMENTS

Financial support from the project entitledApplication of Advanced Materials Thermoelectric

Technology in the Recovery of Wasted Heat fromAutomobile Exhaust Systems by the GreekSecretariat of Research and Development under thebilateral framework with Non-European countries(Greece-USA) is acknowledged.

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