A novel thermal management for electric and hybrid vehicles

468 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 54, NO. 2, MARCH 2005

A Novel Thermal Management for Electric andHybrid Vehicles

Chakib Alaoui and Ziyad M. Salameh, Senior Member, IEEE

Abstract—Assuring the right temperature in battery compart-ments of an electric or hybrid vehicle is crucial for the safe op-eration and the achievement of optimal performance of the bat-teries. This paper is about the design, fabrication, and testing ofa novel system for thermal management for electric/hybrid vehi-cles. This system is based on Peltier-effect heat pumps. The exper-iment results show the applicability of this type of technology forthe thermal management for this type of vehicles.

Index Terms—Coefficient-of-performance (COP), electric vehi-cles, hybrid vehicles, Peltier-effect heat pump, thermal manage-ment.

I. INTRODUCTION

ONE of the main problems to be considered in an electricvehicle is the way to insure and maintain a good thermal

comfort in the passenger compartment and to keep the temper-ature of the battery compartment within certain limits to insureoptimum performance of the batteries. There are several bat-tery performance problems associated with lack of thermal man-agement. Cold weather causes loss of range in electric vehicles(EVs) due to the effects of low temperature on batteries. Forinstance, General Motors estimates that “Impact,” its most pop-ular electric vehicle, would lose 85% of its range in a 0 C en-vironment if no thermal management system is used [1]. Hightemperatures also have devastating effects on the batteries: theirlives can be significantly shortened if they get too hot. For leadacid batteries, this degradation in cycle life can start as low as50 C. Unregulated battery pack can easily reach 80 C duringperiods of sustained discharge or rapid charging [2]. This paperpresents a novel approach to solve problems related to high andlow temperature effects: the use of Peltier-effect thermoelectricmodules for thermal management for both the passenger and thebattery compartments.

In the first part, the general effects of temperature on bat-tery performance are discussed. The second part presents anoverview of the different technology solutions to the problemof thermal management with their main advantages and disad-vantages. Finally, the third part describes both the design andthe evaluation of the thermal management using Peltier-effectmodules.

Manuscript received July 17, 2004; revised September 8, 2004. The reviewof this paper was coordinated by Prof. A. Emadi.

C. Alaoui is with the School of Science and Engineering, Al Akhawayn Uni-versity, Ifrane, Morocco.

Z. M. Salameh is with the Department of Electrical and Computer Engi-neering, University of Massachusetts, Lowell, MA 01854 USA.

Digital Object Identifier 10.1109/TVT.2004.842444

II. OVERVIEW OF EV THERMAL MANAGEMENT SYSTEMS

In order to optimize the efficiency of the batteries, a thermalmanagement system must be installed in the electric and hy-brid vehicles. This system must keep the battery temperature asclose as possible to an optimal range of values generally givenby the battery manufacturer. For instance, the lead acid temper-ature must be close to 30 C in order to yield the maximum per-formance. Moreover, the driver and passengers need to be keptat an acceptable level of comfort. Deicing and defrosting mustalso be included in the design for the safety of the passengers.Several heating/cooling systems have been proposed in the pastdecade [3], [4]. These systems have different performances de-pending on their technologies; they are evaluated by calculatingtheir coefficient-of-performance (COP). It describes system ef-ficiency as a ratio of the cooling effect produced divided by theenergy input expressed on the same basis. The following is anoverview of the different solutions.

A. Fuel Fired Heaters

One proposed way of solving the problem is the use of fuel-fired heaters [5], [6]. This solution represents a viable solutionfor the electric vehicles in the cold weather range regions. Theseheaters allow 100% of the stored energy to propel the vehicle.The fuel fired heater includes an air fan, a blower/motor, fuel,and fuel-circulating pump. As fuel, gasoline or metal hydridehydrogen are used. However, emission from fuel-fired heatersis a concern for the environment, especially for commercialelectric vehicles whose main attribute is for being nonpollu-tants. Another disadvantage is the need for additional systemsfor cooling the vehicle when operating in hot weather.

B. Electric Resistance Heaters

Electric resistance heaters work with the Joule effect ,where is the resistance value of the heater and the supplycurrent. While the efficiency of the onboard resistance heater ishigh (COP 1), there is a large penalty incurred in the range ofthe vehicle since these heaters are fed from the batteries. Also,this solution solves only for heating.

C. Mechanical Heat Pumps

This technology does not rely on fuel combustion and there-fore does not produce pollutants such as hexachloroethane-zinc(HC), carbon monoxide (CO), nitrogen oxide (NOx), or particu-late matter (PM). There has been also considerable developmentand integration of heat pump technologies in electric and hybridvehicles produced by the major automakers. But heat pumps donot operate efficiently at temperatures below 0 C. Also, this

0018-9545/$20.00 © 2005 IEEE

ALAOUI AND SALAMEH: THERMAL MANAGEMENT FOR ELECTRIC AND HYBRID VEHICLES 469

Fig. 1. Peltier cooler.

system has many components such as the compressor, motor,condenser, evaporator, and hose system and therefore reducesthe accessibility and serviceability of the EV. Their COP rangesbetween 0.8 and 1.6 at 10 C [5], [6]. Finally, this system usesFreon as a refrigerant, which is harmful to the environment.

D. Insulation/Ventilation

Better retention of heat in the passenger cabin and batterycompartments may improve the vehicle’s temperature in coldweather. But the insulation provided to keep batteries warm inthe winter might result in overheated batteries in the summer.

III. PELTIER-EFFECT HEAT PUMP

In 1834, Peltier discovered that when a current flows acrossthe junction of two metals, heat is absorbed at the junction whenthe current flows in one direction, and liberated if the current isreversed. This heat is not related to the Joule heating effect .

The electromotive force (EMF) across the junction of two dis-similar materials is a function of the conduction electron ener-gies of the materials making up the junction. In the case of metals,the energy difference is small and therefore the EMF is small. Inthe case of semiconductors the electron energy difference maybe much greater, resulting in a higher EMF at the junction. Heatis liberated when a current does work in overcoming the EMF ata junction, and is absorbed when the EMF itself does the work.Fig. 1 shows the construction of a Peltier-effect heat pump. Thewarmer surface is clamped to a suitable heat sink while the coldface has the component to be cooled mounted in contact with it.Typical size for such a unit is on the order of 5–30 mm [7]–[9].

Like conventional refrigeration, Peltier modules obey thelaws of thermodynamics. If compared with conventional refrig-eration, the refrigerant in both liquid and vapor form is replacedby two dissimilar conductors. The cold junction (evaporatorsurface) becomes cold through absorption of energy by theelectrons as they pass from low energy level to high energylevel. The compressor is replaced by an EMF power sourcethat pumps the electrons from one semiconductor to another. Aheat sink replaces the conventional condenser fins, dischargingthe accumulated heat energy from the system. Table I outlinesthe differences and similarities between the Peltier and theconventional refrigeration:

The EMF produced by any couple with the junction at any twotemperatures may be obtained from a thermoelectric diagramsuggested by professor Tait [9] (see Fig. 2). This diagram rep-resents the thermoelectric line for any metal, the ordinate repre-

TABLE ICOMPARISON BETWEEN CONVENTIONAL AND PELTIER REFRIGERATION

Fig. 2. Thermoelectric diagram of two metals.

sents the thermoelectric power (defined as the rate of change ofEMF acting round a couple with the change of temperature ofone junction), and the abscissa represents the temperature.

If lines a and b represent the thermoelectric lines for twometals A and B, then the EMF around the circuit formed bythe two metals, when the temperature of the cold junction is t1and that of the hot junction is t2, will be the difference in theareas of triangles A1B1D and A2B2D. Tn is called the neutraltemperature, and must be obtained experimentally.

The area of the triangle is

A1B1D A1B1 ED

A2B2D A2B2 FD

EMF A1B1 ED A2B2 FD

Since triangles A1B1D and A2B2D are similar triangles, thesides A1B1 and A2B2 are proportional to ED and FD, respec-tively.

Therefore, EMF ED FD and

ED and FD

So the EMF

or EMF (1)


where is a constant that together with tn must be obtained ex-perimentally for any pair of metals. Equation (1) shows that theEMF in any couple is proportional to the difference of tempera-ture of the junctions and also to the difference between the neu-tral temperature and the average temperature of the junctions.

Peltier thermoelectric modules have numerous advantages[10]:

• small size and weight;• ability to cool below 0 C;• ability to cool and heat with the same device;• precise temperature control;• high reliability;• electrically quiet operation;• convenient power supply;• environmentally friendly (no freon needed).

IV. MECHANICAL DESIGN OF EV THERMAL

MANAGEMENT SYSTEM

A. The Electric Vehicle

The thermal management system was designed to fit Solec-tria Electric Vehicle of type “force.” This vehicle was manufac-tured by Solectria Corporation.1 The following are some speci-fications:

• conversion of Geo Metro of 1992;• seating capacity: two adults;• front wheel drive;• range: 50 to 60 miles;• maximum speed: 65 miles per hour;• 12 batteries of type Hawker 12 V, 38 AH;• motor: AC induction.Five of the 12 batteries are located in the front battery box,

and the seven remaining are located in the rear box in order tohave an even weight distribution. This electric vehicle comesequipped with two standard electric resistance heaters in thepassenger compartment and a small conventional air condi-tioner. The system has a compressor/motor combination, amotor controller, a condenser, a fan, an evaporator, and a hosesystem. The compressor/motor combination is located on thetop of the front battery compartment, which makes it difficultto service the batteries. This motor/compressor system wasremoved, and then replaced by a system made of Peltier-effectthermoelectric heat pumps.

B. Design of the Peltier Thermal Management System

The “hot” surface of the Peltier-effect module must be at-tached to a heat sink that is capable of carrying away both theheats pumped by the modules plus the generated by the Jouleeffect. The “cold” surface is also attached to another heat sinkthat will carry away the cold air, decreasing the differential tem-perature , and then making the Peltier-effect modules moreefficient.

“Spacer blocks” are also put between the modules and theheat sinks. Their thicknesses separate the “hot” heat sink fromthe cold one, which yields to a maximum heat transfer. One

1www.solectria.com.

Fig. 3. Heat sink with fins along the (a) width and (b) length.

Fig. 4. Basic thermal unit setup.

spacer block dimensions are 3.6 cm 6.6 cm 1 cm and it ismade from aluminum.

As shown in Fig. 3, the heat sinks have 35 cm 6.5 cm 4cm, and weight 2.5 kg each. Two types were used in this project:a) heat sink with fins along their width that are used to expel thewarm temperature to ambient in the cooling mode (or expel thecold temperature when heating the vehicle) and b) heat sinkswith fins along their length to channel the cold air into the ve-hicle in the cooling mode (or channel the warm air into the ve-hicle when heating).

For better performance, cooling air is supplied by fans andforced through the length of the heat sinks. The air blown intothe face of the heat sink creates greater turbulence, which resultsin improved heat transfer. The performance of the heat sink isimproved by a factor of three to four by the fans. (See Fig. 4.)

A basic unit in this thermal management consists of 12Peltier-effect units sandwiched between two heat sinks asshown in Fig. 2. Blowers were used to force the ‘cooled’ or‘heated’ air into the electric vehicle. Three similar units werefabricated:

1) for the front battery compartment;2) for the rear battery compartment;3) for the passenger compartment.Fig. 5 shows the block diagram of the system that consists of

the following main sections:

1) three thermoelectric units (as of Fig. 4);2) thermoelectric controller that supplies the electric power

and controls the temperature by regulating the current;3) hose system that distribute the treated air to the different

compartments.


Fig. 5. Thermal management block diagram.

Fig. 6. Installation of the thermal units (TUs) in EV.

In order to install the thermal management, the mechanical airconditioner had to be removed from the EV used for this project(Solectria force). This increased the limited space available. Thethree units were installed in the EVs shown in Fig. 6.

V. POWER SUPPLY DESIGN FOR EV THERMAL

MANAGEMENT SYSTEM

A. Power Circuit

The Peltier-effect modules operate directly from dc power,and suitable power sources can range from batteries to simpleunregulated dc power supply. For this application, a dc powerof V V was available from the electric vehiclebatteries. The power circuit must be capable of the following:

• reversing the voltage across the modules, thus makingit possible for the system to cool or the heat the EV;

Fig. 7. Power circuit for EV thermal management.

• being controlled by an analog circuit that maintains thetemperature inside the EV compartments by control-ling the current flowing in the Peltier-effect modules,and automatically regulating the temperature by meansof feedback loop.

The proposed power circuit for this project is shown in Fig. 7.It is made of two N-MOSFETs T1 and T2 (APT 5020BNFR)and two P-MOSFETs (IXYS 1XTH12P25) T3 and T4 con-nected as shown.

• When heating the EV, positive signal with adjustableduty cycle must be sent to the gate of T1 and a constantdc signal to the gate of T2. In this case the current flowsthrough T1 and T2 when T1 is conducting, and throughT2 and D4 when T1 is not conducting.

• When cooling the EV, negative signal with adjustableduty cycle must be sent to the gate of T3 and a con-stant negative dc signal to the gate of T4. Then the cur-rent flows through T3 and T4 when T3 conducts andthrough T4 and D2 when T3 is not conducting.


Fig. 8. Control circuit.

• It should be noted that neither pair (T1, T4) nor (T2,T3) should conduct at the same time, to avoid shortingthe EV batteries.

• The inductance H was calculated to achievea continuous current conduction.

• A capacitor of 60pF was put at the end of the trans-mission line connecting the dc-to-dc converter to thebatteries. It acts as a low-pass filter that attenuates thecurrent oscillations that occur when D2 or D4 are con-ducting.

• A diode was placed between the batteries and thedc-to-dc converter to protect the MOSFETs if the po-larity of the battery pack were accidentally inversed.

• Two strings of thermoelectric units were put in parallel.Each string consisting of 18 units connected in series.This configuration was chosen to maximize the effi-ciency of the modules.

B. Control Circuit

To control the power circuit, the analog circuit of Fig. 8 wasused. It must read the dc offset voltages coming from both amanual potentiometer and a temperature transducer AD592, andgenerate a pulse-width modulated signal that will control thedc-to-dc converter.

First, a triangular wave generator was built with a Schmitttrigger circuit (op-amp A1) and an integrator (op-amp A2). Thefrequency of oscillations KHz.

Then, the offset voltage coming from the temperature trans-ducer AD592 was amplified with a gain of 35 (op-amp A5) andshifted by 12 V (op-amp A6)

The electric vehicle user could adjust the desired tempera-ture by varying a thermostat. This was accomplished with theop-amp A3.

The differential amplifier made by op-amp A4 outputs thedifference between the voltage from the thermostat circuit andthe voltage from the temperature transducer circuit. So theop-amp A4 acts as a linear feedback loop that corrects forthe temperature. When heating the electric vehicle, this offsetvoltage is positive, and when cooling the vehicle, this offset isnegative.

The next step was to add the dc voltage coming from thedifferential amplifier to the triangular signal. This was achievedby the op-amp A7 circuit.

Op-amp circuits A8 and A9 are comparators. They will com-pare the triangular signal with two constant dc voltages, and thengenerate a rectangular signal with a variable duty cycle, and thatcan be either positive or negative. This signal will be sent tooptocouplers that drive the MOSFETs T1 and T3 of the powercircuit.

Finally, the op-amps A10 and A11 generate logic signals thatcan be either Vsat or Vsat. These signals are connected tothe gates of the MOSFETs T2 and T4 to allow a continuouscurrent conduction.

The power and control circuits along the thermal units wereinstalled and test in the electric vehicle. The system as a wholewas stable. The theoretical study of its stability will be presentedin a future publication.

VI. THERMAL MANAGEMENT MODELING

Fig. 9 illustrates the equivalent electrical circuit that reflectsthe behavior of the thermal management unit in the linear regionmode of operation with:

thermal capacitance from the heat sinks;thermal capacitance from the battery compart-ment;


Fig. 9. Model for the thermal management unit.

thermal resistance of the heat-sink (conduc-tive heat flow);

, thermal resistance through the forced convec-tion.

This system is described by the following state variable:

The characteristic polynomial of the system is the following:

(2)

Experimental tests were conducted to measure the values ofthe components of the model (refer to the next section). Theresults are the following:

m

This model was used to predict the results that may be ob-tained when applying different input current values. The theo-retical amount of heat from Peltier effect was evaluated by usingthe data sheets from the manufacturers. For instance, an inputcurrent of 2 A and an input voltage of 5 V give a max tempera-ture of 45 C. (See Fig. 10.)

This model was simulated using the computer-aided designprogram SPICE [11].

VII. THERMAL MANAGEMENT SYSTEM EVALUATION

A. Coefficient of Performance Theoretical Values

In order to evaluate the thermal performance of this system,the COP must be evaluated. It describes system efficiency as aratio of the amount of energy moved to the amount of energyrequired to do the moving. The COP is obtained by dividing the

output in watts (i.e., heat moved) by the input in watts (i.e., elec-trical energy consumed). The second law of thermodynamicsallows the finding of the maximum coefficient of performancethat any refrigeration and heat pump cycles can have while op-erating between reservoirs at temperatures Th and Tc [12], [13]:

For any system undergoing a reversible refrigeration cyclewhile operating between the two reservoirs, the maximum the-oretical COP

(3)

Similarly, for any system undergoing a reversible heat pumpcycle while operating between the two reservoirs, the maximumtheoretical COP

(4)

Equations (3) and (4) were compared with the actual results.

B. Measurement of the COP

The thermal output power of the thermal unit was calculatedas follows:

(5)

withH gained energy (joules);m mass of the heat sinks;C specific heat (for aluminum 0.963 J/g C);DT temperature differential C;H 2388 J.and

(6)

The input voltage and current were measured to evaluate theinput electric power. It should be noted that the input currentmeasured includes both the current absorbed by the Peltier-ef-fect heat pumps and the fans.

VIII. EXPERIMENT RESULTS

In order to test this thermal management system, several inputcurrent values were injected in the Peltier-effect modules, andthe values of the temperature variation inside the battery com-partment were recorded, in heating and cooling mode of opera-tions.

The temperature measurements were taken from two differentplaces:

• A-Position: at the frontal battery box and near thePeltier-effect modules;

• B-Position: at the rear battery compartment, far fromthe Peltier-effect devices.


Fig. 10. Thermoelectric module specification.

In order to evaluate the efficiency of the EV thermal man-agement system, the time of the experiment was recorded alongwith the input current. The battery capacity that was used by thesystem was calculated as follows:

(7)

The obtained data were used to calculate the COP. It shouldbe noted, however, that at the beginning, the outside losses wereneglected, and the heat gained by the heat sinks was consideredas a linear function of time (only during the first few minutes).

Some of the heating and cooling experimental results werealso compared with the results obtained from the model of thethermal management units.

A. Heating Tests

Several current values were injected to the thermal manage-ment system. The data were recorded and compared with the re-sults obtained from the model. The following are some sampledata.

Fig. 11 shows a linear increase of the temperature for thefirst 6 min. Then saturation starts happening due to the lossesto ambient temperature. A maximum temperature of 37 C wasachieved at the thermal units and 29 C inside the frontal batterybox. The 8 C difference was lost to ambient at the hose systems.

Fig. 12 shows that an input capacity of 1.5 AH was neededto warm up the EV batteries to a temperature of 29 C. The EVis equipped with Hawker batteries that carry 60 AH initially. Ofthe batteries, 2.5% initial capacity was used to warm up the EV.

B. Cooling Tests

The same experiment was conducted for the cooling tests.The following is a sample of the results.

Fig. 11. A 4 A heating test: time (minutes) versus temperature ( C).

Fig. 12. A 4 A heating test: used capacity (AH) versus temperature ( C).

Fig. 13 shows a linear decrease of the temperature for thefirst 5 min. Then saturation starts happening due to the lossesto ambient temperature. A minimum temperature of 9.5 C was


Fig. 13. A 10 A cooling test: used capacity (AH) versus temperature ( C).

Fig. 14. A 10 A cooling test: used capacity (AH) versus temperature ( C).

TABLE IIHEATING PERFORMANCE

achieved at the thermal units and 13 C inside the frontal bat-tery box. The 3.5 C difference was lost to ambient at the hosesystems.

Fig. 14 shows that an input capacity of 2.6 AH was neededcool down the EV batteries to a temperature of 9 C. The EVis equipped with Hawker batteries that carry 60 AH initially. Ofthe batteries, 4.34% initial capacity was used to cool down theEV.

Tables II and III summarize the results for heating and forcooling, respectively.

The differences between the actual and the maximum coeffi-cient of performance suggest that there may be some potentialfor improving the thermodynamic performance of the system.However, the limited onboard space on the EV allocated to thissystem was very limited, and an increase in size or complexityof the system was not possible.

TABLE IIICOOLING PERFORMANCE

IX. CONCLUSION

A new method for thermal management for electric and hy-brid vehicles was introduced in this paper. It is based on Peltier-effect thermoelectric heat pumps. The results show the feasi-bility of this type of technology. A maximum of temperature of52 C was reached in heating mode, while a minimum temper-ature of 9.5 C in cooling mode of operation. Also, an averageCOP of 0.65 was obtained in heating mode and 0.23 in coolingmode. This solution presents many advantages, mainly becausethe Peltier-effect heat pumps are in solid-state.

• The system is reliable and its lifetime is long, as nospecific chemical reaction or other absorbing materialis used.

• The maintenance costs are very small.• The system is environmentally friendly (no Freon

needed), which meets with the objectives of EVs.• The system is relatively light in weight.• The system is more effective and efficient when used

for cooling than for heating.

The proposed model reflects well the performance of thisthermal system.

REFERENCES

[1] Hybrid Electric Vehicle Program, “Thermal management,”, http://www.ott.doe.gov/hev/thermal_management.html.

[2] M. Maja, G. Morello, and F. Spinelli, “A Model for Simulating FastCharging of Lead-Acid Batteries,” Power Sources, pp. 8–9, 1992.

[3] A. S. Keller and G. Whitehead, Thermal Characteristics of Electric Ve-hicle Batteries: Electrotek Concepts, 1991. Ref. 911 916.

[4] D. Linden, Handbook of Batteries and Fuel Cells. New York: Mc-Graw-Hill, 1984, pp. 67–69.

[5] V. Pomme, “Reversible heat pump system for an electrical vehicle,” Soc.Auto. Eng., Feb. 1997.

[6] E. Jelinski and P. Olsen, “Design, manufacturing and operating experi-ence with an electric vehicle: Cold climate experience,” Soc. Auto. Eng.,Mar. 1997.

[7] T. G. Bekwith, Mechanical Measurements, 5th ed. Reading, MA: Ad-dison-Wesley, 1993, pp. 676–677.

[8] D. G. Alciatore and M. B. Histand, Introduction to Mechatronics andMeasurement Systems. New York: McGraw-Hill, pp. 349–356.

[9] B. E. Noltingk, Instrumentation Reference Book, 2nd ed. London: But-terworth Heinemann, pp. 2-27–2-36.

[10] ITI Ferrotec, Thermoelectric Product Catalog and Technical Refer-ence Manual Nashua, NH, 1998. Catalog 100. Available: [Online]www.ferrotec.com.

[11] Silvaco International, SmartSpice User’s Manual, vol. 1.[12] M. J. Moran and H. N. Shapiro, Fundamentals of Engineering Thermo-

dynamics, 4th ed. New York: Wiley, pp. 84–97.[13] P. Atkins, Physical Chemistry. New York: W. H. Freeman, 1994, pp.

67–77.


Chakib Alaoui was born in Tangiers, Morocco, in1972. He received the Ph.D. degree from the Univer-sity of Massachusetts, Lowell, in 2001.

He joined Al Akhawayn University, Ifrane, Mo-rocco, in September 2003, where he is an AssistantProfessor. His interests are in the area of powerelectronics, digital electronics, analog electronics,solid state electrical drive systems, energy storage,and electric vehicle technology.

Ziyad M. Salameh (S’78–M’82–SM’88) was bornin Agraba, Jordan, on May 4, 1944. He receivedthe B.S. degree in electrical engineering fromMoscow Power Engineering Institute, Russia, in1974 and the M.S. and Ph.D. degrees from theUniversity of Michigan, Ann Arbor, in 1980 and1982, respectively.

He taught at Yarmouk University, Irbid, Jordan,from 1982 to 1985. In 1985, he joined the Facultyof the University of Massachusetts, Lowell, wherehe is currently a Professor of electrical engineering.

His areas of interests are power electronics, solid-state electrical drive systems,modeling of electrical machines, wind and photovoltaic energy conversion sys-tems, batteries, and electric vehicle technology. He has authored or coauthoredmore than 100 research papers.

Prof. Salameh is a member of the IEEE Power Engineering, Industrial Appli-cations, and Power Electronics Societies.

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A novel thermal management for electric and hybrid vehicles