Special Issue

Automobile Electronics

Thermal Management System with Heatpipe
for in Vehicle Electric Devices
Takeshi Hirasawa*1, Masami Ikeda*1, Chiyoshi Sasaki*2

A variety of electrical devices such as a lithium ion battery (hereinafter LIB) andetc. are implemented in the next generation vehicles such as the plug-in hybrid
vehicle (hereinafter PHV), electric vehicle (hereinafter EV) and etc. for which we are expecting animportant growth in the future.
This paper describes the results of examining the benefits obtained by applying the heatpipe
system which has an excellent heat uniformity to the temperature management system for invehicle electronic devices to be mounted on these vehicles. It was found that with respect to thethermal management system for LIB, the heatpipe system has an excellent performance for thecooling and for the temperature uniformity. Although the volume is reduced by 88% in comparison to the air cooling system, if the 20 cells with ambient temperature of 35˚C and a heatamount of 3 W is cooled by the cooling source of 35˚C, the heatpipe system is able to suppress
the highest cell temperature to 38.1˚C and the temperature difference between cells to 1.3˚C.
Thus the excellent cooling performance and the temperature uniformity of the heatpipe systemwere confirmed. This paper also discusses the applicability of the heatpipe system to the cooling for other electric devices such as automotive inverters…etc.

ABSTRACT1.

INTRODUCTION

The further expansion of the eco-car which the motor isused as main or auxiliary power is foreseen in the future.
In 2020, it is forecasted that at least, about 20% of thevehicle sales will be of these eco-cars that have the motor
drive engine1). In addition, the rate of not only the hybridcar (hereinafter HV) which was the leading this category,
but also PHV which has a battery as main power source,
EV and etc. will increase and in the same year, the sales
will be of 2 M cars for each of the PHVs and the EVs.
Many in-vehicle electric devices such as LIB, inverter andetc. are installed in these eco-cars, but the heat amount
generated by each electric component is increasing
because of the improvement of the vehicle performance.
So it is predicted that the proposal for a cooling methodthat is efficient and saves the space is required. This
paper will report the result of examining the implementation of the heatpipe system which has an excellent heat
uniformity as a cooling technology for the in-vehicle electric devices such as LIB and inverters.

*1 Automotive System & Device Laboratories,
Research & Development Division*2 Thermal Management Solution & Product Div.

2.

THE CHARASTRISTICS OF LIB

2.1 The Problems at High Temperature
Because the vehicles with battery as the main powersource have a low cost to drive and a low emission, it isexpected that the number of sales will be expanding andthey will be the main stream of the vehicles. But one of
the hurdles for them to become popular is the distancethat can be reached per individual charge. Currently, thedistance that can be covered using only the battery
charge is about 40-60 km in case of the PHV and 100-230
km in case of the EV (Except for some sport type vehicle)
and it is much shorter in comparison to the 500 km of thetypical Internal Combustion Engine (ICE) vehicle. In addition, it is estimated that in practice, the use of electricalequipment such as air conditioning and headlights, will
reduce the actual travelling distance by 20-40% from thestated value. So the traveling distance is even shorter.
Also, if the capacity of the battery is increased to cover alonger travelling distance, this distance will not increase
proportionally because of the weight increase of the battery itself. Therefore, it is necessary to install the battery
with a large charge capacity per unit volume and in the
recent, the chances of these LIBs being mounted onthese eco-cars are increasing.
However, since the charging capacity is reduced
because of the storage in a fully charged state at hightemperature and because of the repeated charging andFurukawa Review, No. 44

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Thermal Management System with Heatpipe for in Vehicle Electric Devices

discharging, the charging will be controlled to be completed below the fully charged state. In general, it is said
that the used capacity range is about 50% in case of HVand 80% in case of PHV if the fully charged state is set as100% and the completely discharged state is set as 0%.
Therefore, additional battery has to be installed in order
to avoid the actual problems even if the charging capacityof the battery is reduced to some extent. So the situationis that the electric mileage will worsen and the living
space will shrink.
On the other hand, the capacity degradation of LIB isdue to the chemical reaction and it is stated that the degradation rate follows Arrhenius equation2). This meansthat the battery life will be longer if the battery operating
temperature is lowered or the heat generation during
rapid charging is suppressed.
That is, if the battery temperature can be controlled
appropriately, the capacity reduction due to the degradation can be suppressed while utilizing the battery up to aregion close to the full charge and the number of theinstalled battery can be reduced.
2.2 The Problems at Low Temperature
Also, because the moving speed of Lithium ions is dropping and the battery cell reaction becomes harder at a
low temperature well below 0˚C, a voltage drop will occur
and the discharge capacity will be reduced in most of the
LIB. Because this voltage drop will be considerable athigh output (high current discharge), it can be the causeof the drop of the output (acceleration force) and/or maximum speed.
Therefore, it is necessary to heat the battery quickly at a
low temperature and to take advantage of the originallyavailable charge capacity.

3.

In this method, the main components required for thetemperature management system are only the case forforming the duct and the fan for feeding the air. So it isvery simple system. On the other hand, in case of theeco-cars such as PHV or EV that number of cells is large,
the gaps in the duct and between cells are required andmore spaces for circulating air are also required. It is stated that as mentioned earlier, for PHV and HV that canreceive the economic efficiency by driving with only theelectricity of the battery (hereinafter EV-drive), the longer
EV-drive distance is required. On the other hand, because
the mounting space is limited, it is necessary to minimize
the volume for other than the battery cells as much as
possible. Therefore, in the air cooling system, it is necessary to minimize the ducts and the gap between the cells
as much as possible, but if it becomes too small, theimpedance of air path will becomes large and a sufficient
air flow won’t be obtained. So the downsizing has a limitation.
3.2 The Heatpipe System
Currently, we are now considering the temperature management system with the heatpipe. The heatpipe is madeby sealing the operating liquid into the container (vessel)
and it is the device to transport the heat from the hightemperature section to the low temperature section fastby removing the heat at the heat input section by theevaporation latent heat of the operating liquid and by
releasing the heat at the cooling section when the vapor
returns to the operating liquid. Also, in order to transport
the operating liquid from the cooling section to the heat
input section, the section called wick that generates capillary force is provided in the inner surface of the container
and the cycle that the operating liquid is continuously
supplied to the heat input section is formed. (Figure 2)

ABOUT THE BATTERY TEMPERATURE
MANAGEMENT SYSTEM

3.1 The Air Cooling System
Currently, the temperature management system in use in
the LIB-based vehicles, especially for HV, is the air cooling system. This construction consists of LIB cells (unitbattery) placed at a certain separation and they areplaced in the case together with other equipment. There
are the gaps between the case and the cells and by circulating the air into the gap (duct section), the air for temperature control flows through the gap between cells andexit on the opposite side. (Figure 1)
Duct

Cooling air

ExhaustFigure 1 Schematic of a conventional air cooling system.

Heat input(evaporation)sectionAdiabatic section
Heat dissipating(condense)
sectionSteam flow

Wick

Heat input

Container
Heat
outputReturn of condensed operating liquid
Liquid flow
Reduced pressure

Figure 2 Schematic of heatpipe working.

Because the chemical reaction occurs depending on
the combination of the operating liquid and the containerand degrades the performance of the heatpipe, in general, their combination is fixed. The temperature management system proposed in this paper, the heatpipe withthe combination of copper for the container and water forthe operating liquids is used. Since water has a large specific heat, the amount of the heat transported by the same
size of heatpipe is large so it contributes to the size
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Thermal Management System with Heatpipe for in Vehicle Electric Devices

reduction of the system.
If this heatpipe is thermally bonded to the side surface
or the bottom surface of the rectangular cell that is thegeneral shape in the LIB and is partially attached to theheating cooling source, it is possible to manage the celltemperature. Further, from the operation principle, theheatpipe has the function of not only simply transporting
the heat, but also keeping the heat uniformity of the components that contact thermally. Therefore, for example, ifmany cells are attached to the same heatpipe, not only
the temperature management can be done collectively,
but also the uniformity of the heat between cells can beobtained. This time, in consideration of the assemblingproperty, the case where the heatpipe system is attachedto the bottom surface of the cell will be examined.
3.3 The Comparison of the Volume of the Air Cooling
System and the Heatpipe Cooling System
For comparison of the air cooling system and the heatpipe system, the volume was examined in the following
model.
In a cell size of W120 mm×H100 mm×D20 mm, assuming the battery pack that is made of a total cell numbersof 200 and one module of 20 cells, Type A, the duct thickness of 30 mm and the gap between cells of 3 mm andType B, the duct thickness of 20 mm and the gap
between cells of 2 mm, are examined. (Both of verticalgaps are 0 mm.) (Figure 3) Without considering the duct
section other than the side surface of the cell, if the total
volume of the cell and the duct of the cell side is calculated simply, it will be 80 L in case of Type A and 69 L incase of Type B.

Inlet
182 mmBattery cells(Gap between cells 3 mm)

tened to 4 mm thickness, and then sandwiched by thealuminum plate of 2 mm thickness (Figure 4) and thecross sectional shape is 120 mm width and 8 mm thickness.

Heat exchange section
120 mm

400 mmBattery cells

101 mmHeatpipe

8 mm
Aluminum cover
Aluminum case
Figure4

Schematic of heatpipe system for CAE.

There is a section where some of the heatpipe are overlapped, but because the transferred heat capacity is
reduced in inverse proportion to its length, the heat of thetotal system is transferred by providing the overlapped
section as appropriate depending on the system configuration and by connecting the short heatpipes. (Figure 4)
For the heatpipe system, when the total volume of thesection that contacts to the bottom of the cell and thecells is calculated in the same way, it was 53 L. The volume of the heatpipe system itself was only 4 L and thepercentage of occupation in the total system was 7%.
(Figure 5)

Duct(Width 30 mm)
35

102 mmVolume (L)30

465 mmOutlet30 mm
Figure 3 Schematic of air-cooling system for CAE.
(Computer Aided Engineering)

Further, the occupation percentage of the volume of theduct and the gap between cells in the entire system is40% and 30% respectively.
On the other hand, the heatpipe system has a configuration where the heatpipe with a diameter of 8 mm is flat-

25

－88%

20
15

－82%

10
5
0

Air cooling system Air cooling system
Duct section
Duct section
20 mm
30 mm
Gap between cells Gap between cells
3 mm
2 mm

Heatpipe
systemFigure 5 Volume comparison of each system.

3.4 The Comparison of the Cooling Performance
Next, the comparison of the cooling performance of theconventional air cooling system and the heatpipe system
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Thermal Management System with Heatpipe for in Vehicle Electric Devices

is explained.
The air cooling system is Type A and inlet and outlet ofduct is placed in the opposite side of the cell module.
(Figure 3)
Assuming that the air is circulated into these 10 modules by the large fan, the air flow rate is set to 0.6 m3/min
per one module.
On the other hand, in case of the heatpipe system,
assuming that the temperature control is done by therefrigerant of an air conditioner or a water cooling pipe,
the 100 mm block as the heat exchange section is provided and the heatpipe is modeled that the temperature ofthis section is controlled as a constant. It was assumed
that the heat is transferred between the heatpipe systemand the battery cells in the thermal sheet of 1 mm thickness (Thermal conductivity 1 W/mk).
At first, assuming the case that the cooling is required,
the analysis condition is set. The ambient temperature,
the air temperature of the cooling system and the temperature of the heat exchange section of the heatpipe system
is set to 35˚C. In the case where the cell generates theheat of 3 W (an assumed condition for charging cycle),
the thermal analysis confirmed the maximum cell temperature and the temperature difference between the cells in
the module (the number of cell is 20.) for each.
Note that the distribution of temperature within the cells
is not taken into consideration and the comparison isbased on the average value of each cell, but the heat
capacity is set to be same as the aluminum of the same
volume.
Table 1 shows the results.

Table 1

The cooling performance comparison between air
cooling system and heatpipe system.
Max. temperatureof cells（℃）

Temperature difference
between cells（℃）

Air cooling system

44.6

5.4

Heatpipe system

38.1

1.3

In this way, temperature of the heatpipe system waslower by more than 6.5˚C compared to the one of the air
cooling system and also the temperature difference
between cells was 1.3˚C.
It further indicates that if there is a temperature difference, the extra energy is consumed because the othercells are cooled so much to cool the highest temperaturecell to the predetermined temperature. Especially in thetime of charging, the temperature of the cooling source
has to be held to low temperature for relatively long timeso from this point, it is seen that the heatpipe system isuseful to improve the electricity mileage and the driving
distance.
3.5 The Comparison of the Heating Performance
Next, the cases where the LIB is heated from a low temperature, in both systems, are compared.

The configuration of the cooling system is same as inparagraph 3.4 and the temperature of the entire system
including cells is set to -10˚C. When the air with the temperature of 20˚C is circulating into this system at the rateof 0.6 m3/min, the time when all the cell temperaturereaches to 10˚C and the temperature difference between
the cells at the time were confirmed by the thermal transit
analysis.
On the other hand, the configuration of the heatpipe
system is same as in paragraph 3.4 and in this case thetemperature of the heat exchange section is set to 20˚C
and the time when all cell temperature reaches to 10˚C
and the temperature difference between cells at the time
were confirmed by the same analysis as well.
The results are shown in Table 2.
Table 2Elapsed time reached to 10 degree Celsius andtemperature variation between battery cells.
Elapsed timereached to 10˚C
（sec.）

Temperature
difference between
cells（℃）

Air cooling system

2140

2.8

Heatpipe system

1010

6.0

In this way, the temperature difference between thecells was 2.8˚C in the case of the air cooling system andwas 6˚C in the case of the heatpipe system. 2140 seconds were required to heat to 10˚C for all cells in thecase of the air cooling system, but it can be seen that itwas 1010 seconds (about the half of the air cooling system) in the case of the heatpipe system. Because theenergy to control the temperature is consumed for only
the time while heating, the temperature difference
between the cells won’t be an issue and the shortness ofthe heating time becomes essentially important.
Note that because water is used for the operating liquid, the heatpipe won’t work when the system is stopped
below freezing point. However, if the heatpipe size andthe number of heatpipes were set appropriately, the operating liquid will melt from the heat input section even if theambient temperature is below the freezing point and themelting portion will start the heatpipe operation again.
Then gradually, the operating range will cover all of theheatpipe. In the end, the heat can be transfer to the terminal section of the heatpipe. This is the reason why theheatpipe heatsink with the operating liquid of water isused for such as railway where the ambient temperature
is below the freezing point.
In addition, in this examination, the melting time isneglected because the occupation percentage of theoperating liquid in the volume of the heatpipe is very little
and it is estimated that there is small impact to the analysis results.

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Thermal Management System with Heatpipe for in Vehicle Electric Devices

IN-VEHICLE ELECTRIC DEVICES

4.1

The Possibility of the Heatsink for the Inverter
Element
Next, the applicability of the heatpipe heatsink for thecooling of the inverter element that generates a large
amount of heat will be described.
4.2 The Heat Amount of Inverter and the Various
Cooling Method
Currently, the output of the drive motor is around 20 kW
for small HV car and around 100 kW for HV system of luxury car. In order to supply the electricity to this motor efficiently, the inverter circuit is used. The circuit of current
inverter element is configured on the Si wafer and it isstated that the heat loss is about 5%.
Therefore, if the heat amount of the inverter predictedfrom these is summarized in each size of car, it is listed inTable 3.
Table 3Model

HV・PHVEV

Assumed motor power heat loss of inverter.
Size

Motor output (kW)Calorific value of
inverter (kW)

Full

100

5.0

Medium60

3.0Compact20

1.0

Medium80

4.0Compact50

2.5

On the other hand, many combined elements are usedby matching its output and it is stated that the heat density is around 30 W/cm2. In the future, in the case of thehighly heat resistant element such as SiC, it reaches thelevel of 100 W/cm 2. Also the heat per one element isaround 200-300 W and the predetermined output isobtained using multiple elements. From the size of theheat amount and the heat density points of view, the air
cooling using aluminum die-casting or extruded heatsink
cannot achieve the cooling if the generated heat amountis not less than the value shown in the Table 3 or the heatdensity is not quite low. Therefore, currently the watercooling method is the common approach.
The water cooling system has a great benefit that canbe used in the compartment (hood) where the ICE for HV,
PHV and etc. is installed and the ambient temperature
exceeds 100˚C. On the other hand, the piping is required
and the manpower is needed for assembling. Also theweight of the system is heavy and in order to control thecooling temperature of the inverter element precisely, theelectric water pump is needed and there is a disadvantage that electric power is consumed for other than theradiator fan.
For the vehicle where the temperature of engine compartment is not high such as EV, Fuel Cell Vehicle (FCV)and etc., the air cooling becomes possible if the heatsink
becomes of high performance and therefore the weight

reduction and the low electricity consumption will be possible. Also, even for the HV or PHV where engine isinstalled, if the inverter system can be moved from theengine compartment to the trunk space or the bottom
floor, it can be installed at the location where the environmental temperature is relatively low and the air coolingwill be possible. Also if the downsizing of ICE is proceeding, the air cooling of the ICE will become possible as
before. So it is estimated that the air cooling of entire
vehicle would not be impossible.
Heatpipe heatsink is already used for not only the cooling of the railway inverter, but also for the cooling of theinverter of high speed elevator or of CPU server. The heatamount for the railway inverter reaches several tens kW
and the heat density of the CPU server sometimes reaches 60 kW/cm2. But the air cooling by heatpipe heatsink isimplemented for both cases. Therefore, for example, it isestimated that the air cooling can be done by installingthe heatsink and the fan as shown in Figure 6 for the invehicle inverter.

wind velocity 3 m/s
Temperature rise (ﾟC)

4.

Time (min.)Figure 6 Heatpipe heatsink for inverter used for elevator and its
performance.

4.3

The Cooling System for the Other In-Vehicle
Electric Devices
For the cooling of the control unit of the in-vehicle electric
devices such as the power control unit (PFU), the DC-DC
converter, the electric power steering control unit (EPS)
and etc., the aluminum die-casting and the extruded heatsink are being used.
Because of the processing limit, the fin with thicknessof more than 0.6 mm is attached. On the other hand incase of such as the natural air cooling, the cooling effiFurukawa Review, No. 44

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Thermal Management System with Heatpipe for in Vehicle Electric Devices

ciency of the fin itself won’t change even if the thickness
is thinner, for example 0.3 mm. So while maintaining the
distance between the fins, the fin pitch can be narrow andmore number of fins can be attached in the same volume.
It leads to the weight reduction. In order to manufacture
thin fins, it is better to use sheet metal processing plateand our company has the technology of the crimped fin(Figure 7) and the eco fin. By using these technologies, it
is possible to achieve the light weight and the high performance of the control unit for the electric devices. At thesame time it leads to the cost reduction.

5.

CONCLUSION

In this paper, as an initiative of the cooling device proposed to the in-vehicle device of our company, the examples of the application of the heatpipe system to suchcomponents as LIB and etc. were introduced. By utilizing
the heatpipe system to the temperature control device ofthe battery, the heating or cooling can be done more efficiently than the air cooling system. Also, by using theheatpipe heatsink, the air cooling system of the inverter
can be realized. Further, by using the sheet metal processed heatsink for the cooling of in-vehicle electric
devices or by installing the heatpipe on the base plate,the downsizing, weight reduction and cost reduction canbe realized and it leads to the improvement of the gas
mileage and the electricity mileage. We think that morecost reduction and more energy saving of eco-cars canbe achieved by applying the heatpipe system.
We will continue to work on the research and development to extend the applicable range. It’s to our pleasure if
our technologies contribute to the earth environment
even if only slightly.

REFERENCES
Figure 7 Crimped fin heatsink.

Furthermore, by embedding the heatpipe or installing itto the aluminum base, the thermal conductivity can be
improved significantly and by defusing the local heat tothe entire base, the highest temperature can be reduced.
In this way, by applying the heatsink that is combined
well with sheet metal processing technology and heatpipe to various devices, the weight of the vehicle can be
reduced and it leads to the improvement of the gas mileage or the electricity mileage.

1) Mizuho Corporate Bank, Mizuho Industry Focus Vol.79 (2010), 10.
(in Japanese)
2) Masahiro Ichimura, The Life of Small Size Lithium Ion Battery
NTT facilities Research Institute (2005) (in Japanese)

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