Weight Penalty Incurred in Thermoelectric Recoveryof Automobile Exhaust Heat

D.M. ROWE,1,3,4 J. SMITH,2 G. THOMAS,2 and G. MIN1

1.—BABROW Consultants Ltd., South Glamorgan, UK. 2.—Calsonic Kansai plc., Llanelli, UK.3.—Cardiff University, Cardiff, UK. 4.—e-mail: roweDM1@cardiff.ac.uk

Thermoelectric recovery of automobile waste exhaust heat has been identifiedas having potential for reducing fuel consumption and environmentallyunfriendly emissions. Around 35% of combustion energy is discharged as heatthrough the exhaust system, at temperatures which depend upon the engine’soperation and range from 800�C to 900�C at the outlet port to less than 50�C atthe tail-pipe. Beneficial reduction in fuel consumption of 5% to 10% is widelyquoted in the literature. However, comparison between claims is difficult dueto nonuniformity of driving conditions. In this paper the available wasteexhaust heat energy produced by a 1.5 L family car when undergoing the newEuropean drive cycle was measured and the potential thermoelectric outputestimated. The work required to power the vehicle through the drive cycle wasalso determined and used to evaluate key parameters. This enabled an esti-mate to be made of the engine efficiency and additional work required by theengine to meet the load of a thermoelectric generating system. It is concludedthat incorporating a thermoelectric generator would attract a penalty ofaround 12 W/kg. Employing thermoelectric modules fabricated from low-density material such as magnesium silicide would considerably reduce thegenerator weight penalty.

Key words: Waste heat recovery, thermoelectrics, automobile weight penalty

INTRODUCTION

Fossil-fueled vehicles have an efficiency level ofapproximately 20% to 30%. This results in up to70% of the fuel energy being lost to the atmosphereas waste heat. One of the principal routes for theseheat losses occurs through the vehicle’s exhaustsystem. Recovering and reusing some of this heatwould improve the overall efficiency of the vehicleby decreasing fuel consumption with the addi-tional benefit of reducing environmentally harmfulemissions.

Thermoelectric generators (TGs) are able to con-vert waste heat energy to electrical energy,1–7 andmore than 25 years ago the possibility of utilizingautomobile exhaust waste heat was demonstrated.8

More recently increasing concern over global

warming and environmentally unfriendly emissionshas focused effort to develop this technology furtherfor wide-scale motor vehicle applications.9–16 Esti-mates of the electrical power that potentially can begenerated by thermoelectrics range from a couple ofhundred of watts electric17 to more than 3 kW18

with fuel savings of between 5% and 10%.However, there is a problem with comparing

efficiency improvements: Vehicle performance isassessed under different driving conditions. In thispaper the available heat from a family-size car ismeasured over a recognized standard drive cycle(new European drive cycle, NEDC), and the poten-tial for generating electricity employing thermo-electrics estimated. The work required to power asmall family car through the NEDC drive cycle wasalso determined and used to evaluate key parame-ters, which enabled an estimate to be made of theadditional work required by the engine to meet theextra load of the thermoelectric generating system.

(Received May 25, 2010; accepted January 27, 2011;published online March 22, 2011)

Journal of ELECTRONIC MATERIALS, Vol. 40, No. 5, 2011

DOI: 10.1007/s11664-011-1571-7� 2011 TMS

784

DRIVE CYCLE

Governments employ standard drive cycles toestablish fuel consumption and CO2 emissions to setappropriate taxation levels. By law, this informa-tion is a prerequisite before a vehicle can be sold.Evidently a vehicle’s performance over a drive cyclewould serve as a standard for evaluation of theenergy benefit of incorporating a TG. Drive cycleprofiles differ in different countries, and Table Idisplays drive cycle data for Europe, the USA, andJapan.

The NEDC profile is displayed in Fig. 1. It com-prises two cycles: an urban cycle and an extra-urban

cycle. In addition, motorway driving condition is ata constant 130 km/h.

MEASUREMENTS

The energy available for recovery in the exhaustgases depends on several factors related to the effi-ciency of the vehicle, the drive cycle, and theambient temperature. Temperature and staticpressure measurements were taken of the exhaustgases of an average size family car (A NissanQuasqai 1600 kg vehicle with a 1.5 L diesel engine)driven through the NEDC cycle in a climatic windtunnel. Temperature measurements were made atfour points: (1) the engine exhaust manifold, (2)after the catalytic converter, (3) at the silencer box,and (4) at the exhaust tail. The results are displayedin Fig. 2.

AVAILABLE HEAT ENERGY

At positions 1 and 2 supplementary tests utilizinga Venturi meter provided the approximate pressureof the exhaust gas mass flow under different drivingconditions. The exhaust gas energy estimates for

Table I. Drive cycle data

Europe USA Japan

Description NEDC FTP 75 10–15 modeCycle time (s) 1180 1874 660Cycle length (km) 11.007 17.77 4.16Max. speed (km/h) 120 70Average speed (km/h) 18.7 34.1 22.7

Veh

icle

spe

ed k

m/h

Time (min)

0

20

40

60

80

100

120

140

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00

Urban cycleTime = 1180s

Extra - urban cycleTime = 400s

Ambient temp. = 20~30oCAv. speed = 12 km/h

1 cycle

Av. speed = 39 km/h

Urban cycleTime = 1180s

Extra - urban cycleTime = 400s

Ambient temp. = 20~30oCAv. speed = 12 km/h

1 cycle

Fig. 1. New European standard drive cycle.

Tem

pera

ture

C

0

50

100

150

200

250

300

350

400

0 2 4 6 8 10 12 14 16 18 20

Ambient temperature = 20 o C

Time (min)

Fig. 2. Temperatures recorded at points 1–4 (reading plots from top to bottom).

Weight Penalty Incurred in Thermoelectric Recovery of Automobile Exhaust Heat 785

this vehicle’s engine obtained from mass flow andtemperature during the NEDC are shown in Fig. 3.

During the urban drive the thermal heat energyaverages at 1.5 kW thermal (t). This increases to7 kW(t) over the extra-urban cycle. For motorwaydriving (130 km/h) the exhaust heat energy isaround 30 kW(t).

POTENTIAL FOR THERMOELECTRICGENERATION

Within the urban drive cycle there is an averageof 1500 W(t) available. Assuming an exhaust gasheat exchanger efficiency of 60% (the upper limit ofcurrent technology), this would equate to an aver-age heat energy recovery of 900 W(t) available forthermoelectric recover. The TEG efficiency dependsupon the exhaust gas temperature, which varieswith driving conditions. A nominal value of 5% was

assumed for all three driving conditions, whichequates to 45 W(e) of generated electricity.

This increases to 210 W(e) and 315 W(e) underextra-urban and motorway driving, respectively.Figure 4 displays the generated output as a functionof drive conditions. The results are broadly inkeeping with published data. Researchers at BMWobtained 200 W(e) from a TG comprising 24 Bi2Te3

modules in a 3 L engine driven at 130 km/h.17

General motors report achieving 350 W(e) and600 W(e) when driving a Chevy Suburban undercity and highway conditions, respectively, althoughan average of 15 kW(t) of heat energy was availableover the drive cycle.19

WORK DONE IN MOVING THROUGH THEDRIVE CYCLE AND WEIGHT PENALTY

A vehicle driven through any of the drive cyclesmust provide sufficient energy to move the vehicle

Wat

ts

0

2000

4000

6000

8000

10000

12000

14000

16000

0 2 4 6 8 10 12 14 16 18 20

Urban cycle

Ex Urban cycle

Average gas energy ~ 1.5kW

Average gas energy ~ 7kW

Ambient = 20 o C

Time (min)

Fig. 3. Available waste exhaust heat when operating under the NEDC.

Fig. 4. Output (W) as a function of drive conditions and TEG efficiency.

Rowe, Smith, Thomas, and Min786

to meet the required speed/time profile. These forcesand the distances moved by the vehicle determine thework done by the vehicle through the cycle, mea-sured in Joules (J). The forces generated in the cyclecan be calculated, and the overall amount of work perkilogram can be established for each cycle. Hence, fora given mass of a TEG system, the additional loadimposed upon the engine to complete the drive cyclecan be estimated. A vehicle efficiency of 29% wasobtained from the fuel consumption over the drivecycle. This efficiency represents the ability of thevehicle system to convert fuel energy to useful workof motion. The work done by the engine in moving1 kg through the NEDC cycle was 12.08 W/kg.Figure 5 shows the same calculations performed on17 different diesel vehicles. The average work donefor the 17 vehicles is 13.8 kJ/kg (12 W/kg). Assumingthat the generator weight is 13 kg, then 156 W(e)needs to be generated to meet its weight penalty.

CHOICE OF MATERIALS

Evidently, the power-to-weight ratio and in par-ticular the specific figure of merit20 given by ZT/density, is a crucial parameter in transport appli-cations. Commercially available thermoelectricmodules are based on bismuth telluride technologywhich limits the temperature of operation to around250�C. Figure 6 displays the figure of merit ofsemiconductor materials suitable for exhaust heatrecovery at higher temperatures. Evidently, mag-nesium silicide with a ZT of around unity is a goodcandidate and has the additional attraction of hav-ing a very low density. For example, HI-Z 20 mod-ules operate at a power-to-weight ratio of around200 W/kg. Assuming that the same value of ther-moelectric figure of merit is obtained in Mg2Si-basedsystems, the power-to-weight ratio could be verysubstantially increased.

CONCLUSIONS

Evaluation and reporting of vehicle exhaustthermoelectric recovery systems under standard

drive conditions would allow real performancecomparisons to be made. The results indicate that,in the small family-size 1.5 L car under test,315 W(e) was recovered from the exhaust energyusing thermoelectrics under motorway driving(130 km/h), and for a 3 L engine this would increaseto around 600 W(e).

The weight of the thermoelectric generating sys-tem imposes a weight penalty, and on average11.93 W/kg must be generated from the recoverydevice to achieve a breakeven value for the vehicle.Assuming the TG plus pumping support require-ments weigh around 13 kg, a weight penalty ofaround 150 W(e) would be incurred. Employing alow-density material such as magnesium silicide inthe module technology would reduce the generatorweight penalty considerably.

ACKNOWLEDGEMENT

The authors acknowledge Calsonic Kansai plc.,Llanelli for financially supporting the project and

Fig. 5. kJ/kg versus vehicle mass through the NEDC.

Fig. 6. Figure of merit of magnesium silicide compounds.21

Weight Penalty Incurred in Thermoelectric Recovery of Automobile Exhaust Heat 787

providing engine test facilities and the technicalstaff for NEDC performance evaluation.

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