Development of a Variable-Conductance Heat-Pipe for a
Sodium-Sulfur (NAS) Battery
By Kenji Watanabe *, Atsushi Kimura *2, Kenya Kawabata *3,
Tomoki Yanagida *2 and Mamoru Yamauchi *2

In order to reduce the cost of electric power, efforts have been made to increase
utilization efficiency by means of load leveling. One way of accomplishing this iswith sodium-sulfur (NAS) batteries, but for these to provide stable long-term service, a temperature control mechanism that operates at high temperatures is required. The variable-conductanceheat-pipe (VCHP) described here meets this need. Unlike conventional heat-pipes the VCHP iscontrollable, and its heat-radiating structure requires no external power source or sensors, andhas no moving parts. Installing VCHPs provides a level of heat control ample for the NAS battery,
thereby contributing to improved charge/discharge efficiency. This paper describes a VCHP
developed for high-temperature applications.

ABSTRACT1.

INTRODUCTION

In order to reduce the cost of electric power, efforts havebeen made to increase utilization efficiency by means ofload leveling. Sodium-sulfur (NAS) batteries offer an effective way of accomplishing this, storing excess power atnight for use during the daytime.
To charge and discharge efficiently, NAS batteries must
be kept above 300°C, but if they become too hot due toresistance loss, etc. their lifetime is reduced. Thus toobtain efficient operation over the long term they must be
provided with suitable means of controlling temperature.
The on-off temperature control available from ordinary
heaters, including auxiliary heaters, and heat radiationfrom the container (casing), however, are inadequate.
The authors therefore set about to provide a heat-radiating mechanism suited to the characteristics of the NASbattery. As a result they have developed this high-temperature variable-conductance heat-pipe (VCHP) for NAS
batteries, which provides ample temperature control capability and improved charge/discharge efficiency. This
paper provides a description of the VCHP.

2.1 VCHP Structure and Operation
2.1.1 Comparison with conventional heat-pipe
Figure 2 shows a structural comparison of a conventional
heat-pipe and the variable-conductance heat-pipe(VCHP). In the conventional type, a small volume of working fluid is sealed into an evacuated metal container. The
working fluid repeatedly vaporizes and condenses as aresult of the small temperature difference (or temperature
gradient) between the evaporator and condenser sections,and heat is transferred by the latent heat of the working
fluid.
The heat-pipe comprises an evaporator section, an adiabatic section and a condenser section, and a wick ormesh is provided in the container to facilitate the circulation of the working fluid. The maximum heat transfer rate,
which is the measure of heat-pipe performance, is determined by the wick, the volume of working fluid, and so on,while the working temperature is determined passively by
the external heat source, the temperature of the heat radiator (condenser) section, and so on. When the heat-pipe
Battery
enclosure
VCHP

2.

Cells

DESIGNING THE VCHP

Figure 1 shows a conceptual drawing of VCHPs installedon an NAS battery.

Aluminum heat-equalizing plate*Tokyo Electric Power Co., Ltd.
Power Cable Accessories Div.
*3 Environment and Energy Laboratory*2

Figure 1

Conceptual drawing of a VCHP installed on an NAS
battery71

is actually used as the heat-transfer device of an NAS battery, however, it becomes necessary to keep the temperature of the evaporator section constant, or to use on-off
operation based on a preset reference temperature.
For these reasons the authors embarked on the development of the VCHP as a heat-transfer device for temperature control of NAS batteries, which, unlike the conventional heat-pipe, is "controllable" and has no need ofexternal power, sensors or moving parts.
Figure 3 compares the heat-radiation characteristics ofconventional and variable-conductance heat-pipes. Unlike
the conventional heat-pipe, in which the heat radiation
rate has a constant gradient relative to temperature, theradiation rate in the VCHP increases rapidly from a given
temperature (the radiation onset temperature) until theradiation limit is reached. When the radiation limit isexceeded, it traces a constant gradient, like a conventional heat-pipe. The term "variable conductance" has its origin in this characteristic, and the slope of the line between
the radiation onset point and the radiation limit point (hereinafter referred to as the radiation gradient) is an importantcharacteristic of the VCHP.
2.1.2 Operation of the VCHP
Figure 4 is a conceptual diagram illustrating VCHP operation. The VCHP comprises noncondensable gas enclosed
in a heat-pipe structure and has a reservoir for the gas. Its
operation may be explained in terms of steps (1) and (2)below:

Gas reservoir

ContainerConse dens
cti er Ad
oniWorkingse abat
cti ic
fluid
on
E
va
Co
p
n
se oratsa dencti orVatio on
po
n
r fl
Liq
ow
uid
H
f
low
rad eat
iat
Ev
ion
ap
ora
tio
n
θ
He
inp atut

Vg

Tg, PgHeat
radiation

Condenser
sectionTc, PcAdiabatic
section

Heat
input

Evaporator
sectionWicka) Conventional

Figure 2b) VCHP

Structural comparison of conventional and variableconductance heat-pipes(1) The evaporator section is heated by an external heatsource and when the working fluid vaporizes thevapor begins to flow toward the condenser section,
which is at a lower temperature.
(2) The enclosed noncondensable gas is forced to theend of the condenser (toward the adiabatic section)and reaches equilibrium at saturated vapor pressure
for a given working temperature.
The boundary surface (actually a diffusion layer) formed
between the working fluid and the noncondensable gas in
Step 2 plays an important role in determining the heatradiation rate and the radiation onset temperature. Whenthe boundary surface lies nearer to the condenser section
than to the evaporator, the internal heat transfer to thecondenser that is characteristic of heat-pipe operation
does not occur, and the only radiation is that transferredby the container--a tiny fraction of the maximum heattransfer rate.
In Figure 4 (a), a gradual increase in working temperature is accompanied by a rapid increase in saturated
vapor pressure of the working fluid. In contrast there is
only a slight increase in the temperature on the noncondensable gas and, since the relationship between pressure and cubic capacity is constant, the boundary surface
is pushed out by the working fluid so that the "condenser
section that is effective for radiation with admixture ofworking fluid" migrates toward the gas reservoir. In Figure
4 (b), the region occupied by the working fluid is reduced,
and the internal heat transfer rate is still small. If the temperature is further increased, the condenser section that is
effective for radiation becomes larger, and the heat radiation rate increases. At the point of maximum heat radiation, the noncondensable gas is totally contained within
the gas reservoir. In Figure 4 (c), if the heat radiation rateis greater than the maximum heat input to the evaporator
section by the heat source, sufficient heat radiation can beobtained and the temperature of the evaporator section
will not increase further.
From the above it can be seen that if the container isfilled with a fully adequate volume of working fluid and the
optimum volume of noncondensable gas, and is provided
with a condenser (radiator) section adequate for heat radiation, the VCHP can deliver appropriate heat radiation

500
Noncondensable gas

Heat radiation (W)

QH
400

Radiation limit point

300
Conventional
heat-pipe

200

QH/QL = 8.8

VCHP

100
QL

Radiation onset point
240

260

280

300

320

340

360

72

Condenser
Heat section (max.)
radiation

Heat
radiation

Evaporator
section

Heat
input

Heat
input

Heat
input

380

Evaporator temperature (°C)

Figure 3

Effective
condenser
sectionAdiabatic
section0
220

Condenser
sectionHeat-radiation characteristics of conventional andvariable-conductance heat-pipesa) QL: No heat radiation

Figure 4b) Boundary surface
migrationc) QH: Maximum heat transfer

Heat transfer during VCHP operation

Furukawa Review, No. 20

2001

T0

T0

ng

n0

Table 1
Item

Specification

Saturated (max.)
Vc

nc
P1

Tc

T1

T1 < T2
P2

Vg
P1
=
Vc
P2 - P1

Figure 5

T0
(1)
Tc

heat transfer rate

1000 W (at maximum temperatures shown below)
Average evaporator

T2

temperaturen0 =a) QL: Onset of heat radiation

VCHP Specifications

P1 P2
P2 - P1

Vc
(2)
R Tc

Temperature
controlTemperature rangeb) QH: Maximum heat transfer

Other

Thermal model of a VCHP

Length of evaporator

Structure

+3°C
-1°C

Temperature rise ∆ t = not morethan 28°C
Dryout does not occur

sectionHeight of main unit

between the upper and lower limits of its temperature
range.
In considering applications to NAS batteries, if the
VCHP is designed so that its lower limit temperature corresponds to the minimum operating temperature (radiation
onset point) of the battery and its upper limit temperature
to the battery's maximum operating temperature (radiation
limit point), it is possible to configure an efficient temperature control mechanism that requires no external powersource, sensors or moving parts, and at the same timeminimize battery aging.

285

approx. 1400 mm
approx. 900 mm
Battery contact by means of

Otherheat-equalizing plate

2.2 VCHP Design
2.2.1 Design model
Figure 5 shows a design model. As described above, theoperational sequence involves changes of the volumes
and pressures of various substances in a sealed container, and is fundamentally governed by Boyle's law:

(1)
(2)

where:
Vg is the volume of the gas reservoir,
Vc is the volume of the condenser section
P1 is the radiation onset pressure (evaporator section)
P2 is the maximum radiation pressure (evaporator
section)T1 is the radiation onset temperature (evaporator
section)T2 is the maximum radiation temperature (evaporator section)
Tc is the gas temperature in the condenser section
T0 is the temperature of the gas reservoir
ng is the number of moles of vaporized working fluid
nc is the number of moles of gas in the condenser
sectionn0 is the number of moles of mixed gas
It is then possible, by substituting into the above equation design conditions--T1 or T2, Vc, P1 or P2 and so on, toestablish the volume of the gas reservoir, volume of working fluid, and initial gas charge. Specifically the ratio of

7
14
9
2

7

14

9
4

6

4
3

8

3
12
1

2

Approx. 150 mm

n0

P1 . T0
P2 - P1
Tc
P1 . P2 . Vc=
P2 - P1
R . TcApprox. 850 mm

Vg
=
Vc

1

14
13
12
9
8
7
5
4
3
2
1
No.
13

Gas
Working fluid
Heat-conductive grease
Tank mounting hardware
φ 6 SUS tube (B)φ 6 SUS tube (A)Wick (mesh)
Radiator fin B
Radiator fin A
Tank
Pipe
Name of component
5

Approx. 2400 mm

Figure 6

Schematic of a VCHP

volumes Vg/Vc contributes to the temperature control properties, which improve as Vg becomes greater than Vc.
Table 1 shows the specifications of the VCHP.
2.2.2 Materials and structure
Figure 6 is a schematic of a VCHP.
(1) Working fluid
The main differences between a VCHP and a conventional heat-pipe are that its working temperature andthe maximum temperature at saturation exceed
300°C. VCHP operation under such conditions owes
much to the operating temperature range of the work-73

ing fluid. Accordingly it was decided to use as working
fluid a mixture of diphenyl and diphenyl ether, equivalent to DOWTHERM A.
(2) Noncondensable gas
Although nitrogen, helium and xenon were all possible
candidates for use as the noncondensable gas, it was
decided, for reasons of safety, low thermal conductivity and cost, to use argon. The volume of the gas
charge was determined from the model, and the gaspressure was monitored.
(3) Container
If the combination of working fluid and container is not
appropriate, reactive gas may be generated within thecontainer degrading control properties and leading
eventually to a drop in the maximum heat transfer
rate. It is also impossible, due to the temperature
range employed, to use the copper that is used forconventional heat-pipes, and it was decided, for environmental and other reasons, to use stainless steel.
The model was also used to determine the volumeof the gas reservoir and the diameter of the pipe, andthe overall configuration of the VCHP. The container
was designed as a pressure vessel and pressureresistance calculations were used to determine wall
thickness.
The structure also inhibits the entry of working fluid
into the reservoir due to bumping, and, in the eventthat entry should occur, capillary tubes are provided
linking the gas reservoir and the condenser section, toallow recirculation. Figure 7 shows the mechanism forpreventing working fluid backflow.
(4) Wicks
The design provides for a basically gravity-driven
method of working fluid circulation, but to facilitate
return to the condenser section after heat radiationand to compensate for the greater distance of theevaporator section, a wick is provided to equalize
heat and guard against excess localized heat input.
(5) Radiator finsTo allow for the required heat radiation rate and air

cooling by natural convection, and because of thetemperature difference between absorption and radiation of heat, a compact fin configuration was adopted,
providing a suitable heat-dissipating area. The finsare attached to the VCHP by bolts.
(6) Parts interfaces
Heat-conductive compound was used at the interfaces between the fins and the VCHP, and between
the VCHP and the heat-equalizing plate. It would
have been possible to use welded connections at theinterfaces but this method was preferred from thestandpoint of facilitating removal and replacement
during transportation and inspection. Since the prime
constituent of the compound was oil, there was a
problem in terms of maintaining thermal conductivity,
plasticity and adhesion at high temperatures, but it
was possible to obtain a compound that satisfied performance requirements.

3.

VERIFICATION OF VCHP PERFORMANCE

3.1 Testing Equipment
Figure 8 is a block diagram of the testing equipment, simulating the VCHP attached to the battery unit in a designthat takes account of heat radiation to the outside of thedevice. The input of heat to the VCHP was set so as toobtain a maximum heat transfer rate of 1000 W. Heatmeasurement was carried out at several points on theevaporator, adiabatic and condenser sections, and pressure measurements were carried out on the gas reservoir.
3.2 Test Results
3.2.1 Temperatures and gradients
Figure 9 shows temperature distribution in VCHPs under
test. At a heat input of 90 W, achieving the radiation onset
temperature of 285°C, the condenser (heat radiator) section was at about 20°C, or around ambient temperature.
As heat input increased, condenser temperature rose, andat a heat radiation rate of 1000 W, temperatures reachedabout 120°C at the top end of the condenser and about
90°C at the bottom end. From this it can be seen that thePressuregauge
Gas
reservoir

Pressure monitoring Temperature monitoringP

Pen recorder
Pressure sensor

Terminal
Data logger

Radiator
fins
Thermocouple

Heat-equalizing plate

Gas entry path
Watt-hour meterfor heater

AC 100 V

Capillary tube

Watt-hour meter
for auxiliary heater

AC 100 V

Figure 7

74

Mechanism to prevent working fluid backflow

Figure 8

HeaterAuxiliary heater +heat insulation* Heat insulating compound is applied to contact surfaces

Block diagram of testing equipment

Furukawa Review, No. 20

2001

VCHP was operating normally. There was also an
increase in the temperature gradient as heat input
increased, but this shows that condenser temperaturerose gradually from the evaporator side, and demonstrates the stability of its characteristics.
3.2.2 Heat radiation characteristics
Figure 10 shows the heat radiation characteristics foundwhen heat input and heat radiation were in equilibrium.
This test was conducted on three samples for verification
purposes, and all three came within specification for radiation onset temperature. The radiation gradient was also
high, with all three samples exhibiting a temperature control range ∆t of not more than 10°C at saturation (maximum) heat transfer rate, an excellent result that satisfied
specifications. From this we may conclude that control
performance and heat radiation characteristics are stableand uniform across product samples.
3.2.3

Relationship between working fluid volume andgas charge volume
The following tests were performed to verify the differencein characteristics due to errors in the volume of gas
charged.
(1) Changes in volume of working fluid
Figure 11 shows changes in the heat radiation characteristics when the volume of the working fluid waschanged plus or minus 10% with respect to the stan-

dard volume, maintaining constant gas charging pressure. When working fluid volume was low there wasno problem with the radiation gradient, but there wasa rise in the radiation onset temperature. When there
was an excess of working fluid, on the other hand,
there was no problem with respect to radiation onset
temperature, but as the rate of radiation increased
there was a tendency for evaporator temperature to
rise. This is due to the fact that the volume of gas
charged varied with increases and decreases in thevolume of working fluid, since the charging pressureof the gas was constant. In practice charging error isin the neighborhood of 1%, and there is no change incontrol characteristics due to error in the volume ofworking fluid charged.
(2) Volume of gas charge
Figure 12 shows changes in the heat radiation characteristics when the volume of the gas charge waschanged plus or minus 2.5% with respect to the standard volume, maintaining constant working fluid volume. When the gas charge volume was large theradiation onset temperature rose. As was explained in
Section 2 above, the increase in gas pressure had theeffect of pushing the boundary surface toward thecondenser. In practice charging error is in the neighborhood of 0.1%, and there is no change in control
characteristics due to error in the volume of gas
charged.

350

Heat radiation rate (W)Temperature (°C)

300
250
Evaporator
section200

Adiabatic
sectionCondenser Gas
section reservoir

90 W
200 W
400 W
600 W
800 W
1000 W
1200 W
1300 W

150
100
50
0
0

500

1000

1500

2000

2500

3000

3500

1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
0

Heat radiation rate of 1 kW(∆t = 25°C)
Radiation onset
temperature:
285°C -1, +3

280

285

290

Distance from end of evaporator (mm)Temperature distribution in VCHPs under test

1400
1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
0

Radiation onset
temperature:
285°C -1, +3

Heat radiation rate of 1 kW(∆t = 25°C)

Average temperatureof evaporator:
No. 1
No. 2
No. 3
∆t = 28°C or less(target: 28°C)
280

285

290

295

300

305

310

Temperature (°C)

Figure 10 Heat radiation characteristics

300

Working fluid volume:
10% down
Working fluid volume:
as specified
Working fluid volume:
10% up

305

310

315

320

Temperature (°C)

Figure 11 Influence of working fluid volume on heat radiation
characteristics

Heat radiation rate (W)Heat radiation rate (W)Figure 9295

Gas pressure: as specified

315

1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
0
280

Heat radiation rate of 1 kW(∆t = 25°C)
Radiation onset
temperature:
285°C -1, +3

285

290

295

300

Working fluid volume:
as specified
Gas pressure:
2.5% down
Gas pressure:as specified
Gas pressure:
2.5% up

305

310

315

320

Average evaporator temperature (°C)

Figure 12 Influence of gas charging pressure on heat radiation
characteristics

75

Battery life is affected

Conventional
With VCHPs

360

Temperature (°C)

Heat radiation rate (W)

380

1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
0

Heat radiation rate of 1 kW(∆t = 25°C)
Initial values
After 1 week
After 2 weeks
After 3 weeks
After 4 weeks
After 5 weeks
After 6 weeks

Radiation onset
temperature:
285°C -1, +3
Working fluid volume:
as specified
Gas pressure:as specified

280

285

290

295

300

305

310

315

340
320
300
Heater operation

280
260

Discharge

Rest

ChargeRest Temperature control

1 cycle

240
Start
Stop
discharging discharging

320

Start
charging

Stop
charging

Start
discharging→Time course

Average evaporator temperature (°C)

Figure 13 Results of life tests

Figure 14 Comparison of time courses of NAS battery temperature with and without VCHPs

3.2.4 Long-term reliability
Figure 13 shows the results of life tests, which involved
operating the apparatus for six weeks. Based on actual
battery operation, a duty cycle of five days on and twodays off was adopted. The radiation gradient was constant, and long-term reliability was obtained. Variations incharacteristics are thought to be a matter of variations intemperature measurement; the characteristics themselves
do not change significantly. Life tests of actual apparatus
were carried out for from three months to as long as one
year.

ture is determined by the design, so that at temperatures
below this value it has a low heat transfer rate attributable
to the container itself but above the onset temperature theradiation rate rises abruptly and does not diminish even in
the high-temperature range.

4.

4.3 Battery Life and Efficiency
NAS batteries provided with VCHPs were maintained
within the limiting temperatures for aging even when operated continuously at the initial radiation rate. And the target level of efficiency was achieved even when operating
at the terminal period of its service life (when the radiation
rate was increased above the rated value).

EFFECT WITH NAS BATTERIESThe following material makes clear the advantages ofusing VCHPs with NAS batteries. Figure 14 shows thechanges in the time course of NAS battery temperature
due to the use of VCHPs.
4.1 Temperature Control Characteristics
Temperatures were controlled within a narrower range
than was specified in the design values. Results of measurements at various points demonstrated that the temperature of the evaporator section exhibited outstanding
uniformity in the length direction.

5.

CONCLUSION

As a result of efforts to provide a radiation mechanism tocontrol the temperature of NAS batteries, the authors
have developed a VCHP for high-temperature applications
which has ample temperature control capability resulting
in improved charge/discharge efficiency. Work is going
forward to further reduce the cost of the apparatus, and toextend applications to other controllable heat radiation
mechanisms.
Manuscript received on May 22, 2000.

4.2 Reaction to heat input
Radiation characteristics (when heat input is increased,raising the temperature of the evaporator section) form thebasis for determining whether VCHPs can be applied totemperature control for NAS batteries. As noted in Section
3, the VCHP is superior in that its radiation onset tempera-

76

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2001