Kenneth A. Burke

Glenn Research Center, Cleveland, Ohio

Ian Jakupca and Anthony Colozza

Analex Corporation, Brook Park, Ohio

Development of Passive Fuel Cell

Thermal Management Technology

NASA/TM—2010-216773

December 2010
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Kenneth A. Burke

Glenn Research Center, Cleveland, Ohio

Ian Jakupca and Anthony Colozza

Analex Corporation, Brook Park, Ohio

Development of Passive Fuel Cell

Thermal Management Technology

NASA/TM—2010-216773

December 2010

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2010-216773 1 

Development of Passive Fuel Cell Thermal Management Technology 

Kenneth A. Burke 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Ian Jakupca and Anthony Colozza 

Analex Corporation 

Brook Park, Ohio 44142 

Abstract 

The NASA Glenn Research Center is developing advanced passive thermal management technology to reduce 

the mass and improve the reliability of space fuel cell systems for the NASA exploration program. The passive 

thermal management system relies on heat conduction within the cooling plate to move the heat from the central 

portion of the cell stack out to the edges of the fuel cell stack rather than using a pumped loop cooling system to 

convectively remove the heat. Using the passive approach eliminates the need for a coolant pump and other cooling 

loop components which reduces fuel cell system mass and improves overall system reliability. Previous analysis had 

identified that low density, ultra-high thermal conductivity materials would be needed for the cooling plates in order 

to achieve the desired reductions in mass and the highly uniform thermal heat sink for each cell within a fuel cell 

stack (Ref. 1). A pyrolytic graphite material was identified and fabricated into a thin plate using different methods. 

Also a development project with Thermacore, Inc. resulted in a planar heat pipe. Thermal conductivity tests were 

done using these materials. The results indicated that lightweight passive fuel cell cooling is feasible. 

Introduction 

The purpose of this work was to test sample potential cooling plates for NASA’s fuel cells. The key parameters 

used to screen these samples were thermal conductivity, material density, and compatibility with the anticipated fuel 

cell stack environment. Materials that demonstrated sufficient promise as passive cooling plates would continue to 

be studied and further optimized with the goal of developing a light weight fuel cell cooling plate. 

Nomenclature 

A Cooling plate cross sectional area, m2

dT/dx Temperature gradient, K/m 

k Thermal conductivity, Watt/m-K 

Q Applied heat, W 

T Cooling plate temperature, K 

x Location of the temperature measurement on the cooling plate, m 

Background 

The heart of a fuel cell is an electrochemical “cell” that combines a fuel and an oxidizing agent, and converts 

the chemical energy directly into electrical power, water, and waste heat. The fuel cells used by NASA are 

hydrogen-oxygen fuel cells. The fuel cells under development for future NASA missions are acid-based Proton 

Exchange Membrane (PEM) hydrogen-oxygen fuel cells. An illustration of this type of cell is shown in Figure 1. A 

hydrogen molecule reacts at the anode to create a pair of protons and electrons. The proton ion exchange membrane 

conducts the protons which were generated at the anode from the anode to the cathode. The electrons which were 

also generated at the anode are conducted through the electrical load that is connected to the fuel cell and also reach 

the cathode. The hydrogen protons and the electrons are reacted at the cathode with an oxygen atom to produce a 

molecule of water. An illustration of a “stack” of cells connected electrically in series shown in Figure 1. 
NASA/TM—2010-216773 2 

T P

T P

Accumulator

Pump

Heater

Sensors

Fuel Cell 

System Hx

Fuel Cell 

Stack

Heater

Fuel Cell 

System Hx

Fuel Cell 

Stack

Thermostat 

Valve

Conventional 

Fuel Cell with 

Pumped Loop 

Thermal 

Management

Bypass 

Valve

Fuel Cell with 

Passive Thermal 

Management

Coolant 

from 

Spacecraft 

Cooling 

System

Coolant 

from 

Spacecraft 

Cooling 

System

Figure 2.—Fuel cell thermal management systems. 

Heat must be removed from the fuel cell stack to prevent the stack from overheating. Heat is typically removed 

from the fuel cells via cooling plates within the fuel cell stack that are located between the cells as shown in 

Figure 1. Typically a liquid coolant is circulated within the plate and heat is removed convectively as the coolant 

passes through each cooling plate and out of the fuel cell stack to a fuel cell system heat exchanger. A passive 

cooling plate must conduct the heat within the plane of the plate out to one or more of the edges of the plate so that 

the heat can be transferred to a heat exchanger external to the fuel cell stack. Figure 2 shows the difference between 

a conventional fuel cell thermal management system and a passive fuel cell thermal management system. Potential 

benefits of the passive approach include reductions in mass, system complexity, and parasitic power as well as 

improvements in system reliability. 

Figure 3 plots the fuel cell heat generation density (the heat generated per unit of cell area) versus the fuel cell 

output current density (Ref. 1). NASA’s fuel cell applications typically optimize in the lower current density range 

(400 mA/cm2

), so the heat generation expected from NASA’s fuel cells is generally 0.3 Watt/cm2. 

The key to making the passive approach workable is making the cooling plates light enough yet highly 

thermally conductive so that the heat can be effectively removed and also provide each cell in the fuel cell stack a 

thermally uniform heat sink. Analytical expressions were developed (Ref. 1) that relate the thermal performance of a 

passive cooling plate to its physical characteristics. There are two key metrics used in the evaluation of the thermal 

management system, the first is the maximum temperature difference, the T, over the chemically active area of 

each cell in the fuel cell stack. Ideally, a uniform temperature over the active area is desired because this maximizes 

the electrochemical performance throughout the stack. In practice, a T of zero is never achieved because the 

H2 2H+

2e-

1⁄2 O2

H2O

Anode Reaction

Overall Reaction

Cathode Reaction

H2 2H+ + 2e-

1⁄2 O2 +2H+ + 2e- H2Oliq

1⁄2 O2 + H2 H2Oliq

fH Enthalpy of Formation 2 98 = -285.83 kJ/mole

fG Gibbs Free Energy 298 = -237.14 kJ/mole

2H+ 2e- 2H+ 2e- 2e- 2H+

2eH2 1⁄2 O H2 H2 2 1⁄2 O2 1⁄2 O2



Single Cell n – Cells in Cell Stack

H2O H2O H2O

Cooling Plates 

Figure 1.—Proton exchange membrane fuel cell. 
NASA/TM—2010-216773 3 

Figure 3.—Fuel cell heat generation. 

Figure 4.—Cooling plate heat transmission distance, thermal 

conductivity, and plate specific gravity. 

process of removing waste heat from the fuel cell always requires a temperature differential. In practice, a T of 3 C 

has been acceptable, and therefore was used as a driving requirement. A second key metric is the mass of the thermal 

management system. The waste heat managed per unit mass of the thermal management system was the defined metric 

for mass evaluation. The thermal system mass was considered to be the mass of the cooling plates internal to the fuel 

cell stack and the mass of thermal system components external to the fuel cell stack such as the coolant, cooling lines, 

pump, accumulator, sensors, etc. The mass of the fuel cell system heat exchanger common to both approaches was 

excluded from the mass of the thermal system. The space shuttle fuel cell powerplant has a maximum heat rejection of 

7322 W and an overall mass of 115.7 kg (Ref. 2). The fraction of the overall shuttle fuel cell powerplant mass 

represented by the thermal system was not known but estimated to be 10 percent. This would result in a waste heat per 

unit mass of 633 W/kg. A value of 1000 W/kg was used as a goal for the passive thermal management system. 

Using the 1000 W/kg as the value of the metric, and 3 C as the T, the relationship between heat transmission 

distance, L, the specific gravity of the cooling plate, , and the thermal conductivity, k of the cooling plate is plotted in 

Figure 4 using the analytical relationships developed (Ref. 1). 

From the plot shown in Figure 4, it is apparent that for fuel cells which have to transmit the heat 4 cm, a 

thermal conductivity of 1000 W/m/K and a specific gravity of 4 gm/cc would be required to meet the metric 

targets. 
NASA/TM—2010-216773 4 

Advanced Cooling Plate Materials 

Table 1 lists candidate high thermally conductive materials (Ref. 3). These alternative materials are all carbon 

based: diamond, diamond composites, pyrolytic graphite, and graphite composites. Highly Oriented Pyrolytic 

Graphite (HOPG) has the best combination of high thermal conductivity and low specific gravity. HOPG appears to 

meet the criteria if the heat transmission distance is less than 7 cm. 

TABLE 1.—PASSIVE COOLING PLATE MATERIALS (REF. 3) 

Material Thermal conductivity, 

Watt/m-K 

Specific gravity, 

g/cm3

CVD diamond 1100 to 1800 3.5 

HOPG a1500 to 1700 2.3 

Natural graphite 500 ------------ 

Cont. CF/Cu a400 to 420 5.3 to 8.2 

Carbon/carbon a400 (690) 1.9 

Gr Flake/Al a400 to 600 2.3 

Diamond/Cu 600 to 1200 5.9 

Diamond/Al 550 to 600 3.1 a

In-plane isotropic value 

In addition to these materials, heat pipes, which use phase changes to absorb and reject heat are capable of 

extremely high effective thermal conductivities (50,000 Watt/m-K) (Ref. 4) to even “several orders of magnitude 

higher than aluminum or copper” (Ref. 5), and also are candidates for passive cooling plates. Heat pipes are 

probably the only candidate capable of passively moving the heat for large fuel cells (transmission distances of 

10 cm). 

Fabrication of Experimental Cooling Plates 

Samples of fuel cell cooling plate structures were fabricated using HOPG. In addition to these samples, planar 

heat pipes were fabricated. 

Thermal Pyrolytic Graphite (TPG) 

Graphite in general and HOPG in particular consist of a lamellar structure that are composed of stacked planes. 

These lamellar structures have much stronger forces within the lateral planes than between the planes, thus 

explaining the characteristic cleaving properties of all of these materials. The pyrolytic graphite used in these 

experiments was manufactured from thermal decomposition of hydrocarbon gas and is called Thermal Pyrolytic 

Graphite (TPG). The TPG was obtained from General Electric Advanced Ceramics (Ref. 6) in the form of thin 

(0.38 mm) planar sheets. In Table 2 are the technical specifications published by GE Advanced Ceramics. 

TABLE 2.—THERMAL PYROLYTIC GRAPHITE 

MANUFACTURERS SPECIFICATIONS

Characteristic Unit Value 

Thermal conductivity (In-plane) Watt/m-K 1500 

Thermal conductivity (Thru-plane) Watt/m-K <20 

Density Gram/cm3 2.26 

Tensile strength MPa Nil 

Compressive strength MPa Nil 

The TPG has little to no mechanical strength. It has very little abrasion resistance and also the surface is too 

rough for gas tight sealing within a fuel cell stack. Metal/TPG composite structures were fabricated that had 

improved strength, abrasion resistance and surface roughness characteristics. 

 
NASA/TM—2010-216773 5 

A diffusion bonded Steel/TPG composite cooling plate was fabricated by GE Advanced Ceramics. To make this 

sample, a thin TPG sheet (0.38 mm) was diffusion bonded on both sides to 1040 steel. The resultant diffusion 

bonded part was subsequently surface ground as thin as possible. Difficulty with this process prevented the 

fabrication of as thin a cooling plate as was desired, and the thickness of these plates still consisted of 2/3 steel and 

1/3 TPG. These plates were the thickest plates tested, had the highest density, and on a percentage basis contained 

the least TPG material. The plates were very stiff, and had a very smooth surface (no quantified measures of these 

characteristics were made). 

A 316SS/TPG composite cooling plate was fabricated at the Glenn Research Center. To make this sample, thin 

TPG sheets (0.38 mm) were laminated with 0.05 mm thick 316SS foils using a hot melt polyester film adhesive at 

125 to 135 C. This technique also produced very stiff plates with a very smooth surface, but were about half the 

thickness of the diffusion bonded cooling plates. More than half of the overall thickness of the cooling plate was 

from the TPG. 

Copper/TPG composite cooling plates were fabricated at the Glenn Research Center. To make these samples, 

thin TPG sheets (0.38 mm) were also laminated with 0.05 mm thick copper foils using two different electrically 

conductive adhesives, one adhesive designated as ATTA-LM2 and another adhesive designated as TP1. The 

electrically conductive adhesives were used to ensure an electrically conductive as well as thermally conductive 

cooling plate since the cooling plates will also need to conduct electricity between fuel cells. Both electrically 

conductive adhesives were obtained from Btech Corporation (Ref. 7). Table 3 listed some of the manufacturers 

published characteristics of these adhesives. Figure 5 shows the TPG cooling plate test samples that were tested. 

TABLE 3.—BTECH CORPORATION (REF. 7) 

ELECTRICALLY CONDUCTIVE ADHESIVES 

 Unit ATTA-LM2 TP1 

Thermal conductivity Watt/m-K 750 5 

Electrical resistivity Ohm-cm 9.8 <10–6

Tensile modulus GPa 0.03 to 0.09 0.06 

1

Mass, grams 7.73

Density,g/cc 2.03

Volume, cm3 3.81

Thickness, mm 0.38

Width, mm 100.0

Length, mm 100.0

Pyrolytic Graphite (PG)

Mass, grams 79.02

Density,g/cc 6.69

Volume, cm3 11.81

Thickness, mm 1.14

Width, mm 101.6

Length, mm 101.6

Steel Encased PG

Mass, grams 8.84 23.98

Density, g/cc 2.72 3.48

Volume, cm3 3.25 6.88

Thickness, mm 0.66 0.66

Width, mm 74.6 102.2

Length, mm 66.0 102.0

Adhesive ATTA TP-1

Cu Laminated PG

Mass, grams 17.97

Adhesive Polyester

Density, g/cc 2.82

Volume, cm3 6.37

Thickness, mm 0.61

Width, mm 102.2

Length, mm 102.3

SS Laminated PG

Figure 5.—Thermal pyrolytic graphite cooling plate test samples. 
NASA/TM—2010-216773 6 

Mass, grams 81.9

Density, g/cc 3.92

Volume, cm3 20.9

Thickness, mm 2.01

Width, mm 102.0

Length, mm 102.1

Cu Planar Heat Pipe

Mass, grams 27.9

Density, g/cc 2.56

Volume, cm3 10.9

Thickness, mm 1.19

Width, mm 95.5

Length, mm 95.5

Ti Planar Heat Pipe

Figure 6.—Heat pipe cooling plate test samples. 

Figure 7.—Cooling plate vacuum test chamber. 

Planar Heat Pipes 

Heat pipes are another possible approach to a high thermal conductance, low density fuel cell cooling plate. As 

the name, heat pipe implies, the geometry of heat pipes has been tubular. This geometry is not amenable to 

integration with a fuel cell stack which consists of planar shapes. Thermacore, Inc. (Ref. 8) was contracted with to 

develop prototype planar heat pipes. The first planar heat pipe produced was of an all copper construction. A second 

effort to produce a thinner and lighter prototype produced an all titanium planar heat pipe. Figure 6 shows the planar 

heat pipes tested, and some basic geometry and mass characteristics. 

Cooling Plate Testing 

The cooling plate testing consisted of applying a known heat rate at one edge of each plate and removing the 

heat from the opposite edge of each plate. Temperature measurements made on the face of each plate gave a 

measure of the overall plate temperature and the temperature gradient that developed between the hot and cold edges 

of the plates. These measurements allowed the cooling plate thermal conductivity to be determined. 

Cooling Plate Testing Facilities 

The cooling plates were tested in a vacuum chamber shown in Figure 7. The interior of the chamber showing 

one of the cooling plates and attached instrumentation is shown in Figure 8. Previous attempts to test the samples in 

an ambient laboratory environment resulted in data that needed corrections to account for convective heat losses. 

This approach complicated the analysis of the data and was abandoned in favor of testing in vacuum to eliminate 

convective heat losses. Radiative heat losses from the samples during the test were negligibly small in comparison to 

the heat being conducted through the plane of the cooling plate. 

Figure 8.—Cooling plate vacuum test chamber 

interior. 
NASA/TM—2010-216773 7 

Figure 9 illustrates the configuration of the cooling plate test article. Two copper bars containing heater 

elements were clamped around one edge of the cooling plate. Two copper bars containing cooling channels were 

clamped around the opposite edge of the cooling plate. Thermocouples were placed on both the heating bars and the 

cooling bars. Thermocouples were also placed on the face of the cooling plate between the heating and cooling bars. 

Figure 10 shows a top view of a cooling plate test article inside the vacuum chamber. 

Cooling lines were run from the test article to a feed through in the vacuum tank wall, and from there to a 

temperature controlled chiller bath located outside of the vacuum chamber. Power lines from the cartridge heaters 

were run to a feed through in the vacuum tank wall and from there to a dc power supply located outside of the 

vacuum chamber. The thermocouples were similarly run to a feed through and from there to a computer to record 

the temperature data. Figure 11 illustrates the overall cooling plate test rig. 

Figure 9.—Cooling plate test article (side view). 

Figure 10.—Cooling plate test article (top view). 

Figure 11.—Cooling plate test rig. 
NASA/TM—2010-216773 8 

Cooling Plate Testing Results 

The conducted heat flux Q/A through the plate is proportional to the temperature gradient as shown in 

Equation (1). 

dx

dT k

A

Q  (1) 

Where 

 Q Applied heat, Watts 

 A Cooling plate cross sectional area, m2

 k Thermal conductivity, Watt/m-K 

 T Cooling plate temperature, K 

 x Location of the temperature measurement on the cooling plate, m 

 dT/dx Temperature gradient, K/m 

The temperature data was recorded for a given power applied to the cartridge heaters. The power level was then 

changed, the temperatures allowed to come to steady state, and another set of temperatures recorded. The 

temperatures were plotted versus locations on the cooling plate, and the temperature gradient determined. The heat 

flux at each power level was determined by dividing the applied power by the cooling plate cross sectional area. The 

heat flux was then plotted versus the temperature gradient determined at each power level. The best fit slope of the 

line through the data was taken as the thermal conductivity. Table 4 lists all of the test samples that were tested. 

TABLE 4.—COOLING PLATE TEST SAMPLES 

Test sample Characteristics Test results 

316 SS 108 by 114 by 1.85 mm See Figures 12 and 13 

Diffusion bonded TPG See Figure 5 See Figures 14 and 15 

Copper 113.5 by 114.8 by 0.89 mm See Figures 16 and 17 

316 SS Clad TPG See Figure 5 See Figures 18 and 19 

Copper clad TPG TP1 See Figure 5 See Figures 20 and 21 

Copper clad TPG ATTA  See Figure 5 See Figures 22 and 23 

Copper clad TPG ATTA // See Figure 5 See Figures 24 and 25 

TPG See Figure 5 See Figures 26 and 27 

Copper planar heat pipe See Figure 6 See Figures 28 and 29 

Titanium planar heat pipe See Figure 6 See Figures 30 and 31 

A 316 stainless steel plate was tested because it is a commonly used metal in fuel cell stacks, and also because it 

had a published value for the thermal conductivity. Figure 12 plots the temperature data collected on a 316 stainless 

steel plate. Figure 13 plots the heat flux through the stainless steel plate against the temperature gradients 

determined from Figure 12. The slope of the line in Figure 13 is 28 Watt/m-K which is a higher thermal conductivity 

than its published value of 10 Watt/m-K. The inaccuracy of the measurement may be due to the limited number of 

power levels. 

A diffusion bonded TPG/steel composite cooling plate was tested. Figure 14 plots the temperature data 

collected on the diffusion bonded plate. Figure 15 plots the heat flux through the composite plate against the 

temperature gradients determined from Figure 14. The slope of the line in Figure 15 is 250 Watt/m-K which is a 

thermal conductivity much higher than ordinary steel. This demonstrates the ability of TPG to dramatically improve 

the thermal conductivity of a low conductance material. Despite this improvement, the conductance of this material 

is still much too below 1000 Watt/m-K to be suitable for a fuel cell cooling plate. 

A copper plate was tested because it was a material with a high thermal conductivity that was known and could 

serve to check our test methodology. Figure 16 plots the temperature data collected on the copper plate. Figure 17 

plots the heat flux through the copper plate against the temperature gradients determined from Figure 16. The slope 

of the line in Figure 17 is 405 Watt/m-K which is close to copper’s published thermal conductivity of 395 Watt/ 

m-K. This is still too low of a thermal conductance to be suitable as a cooling plate. 
NASA/TM—2010-216773 9 

 

 Figure 14.—Diffusion bonded TPG temperature gradient. Figure 15.—Diffusion bonded TPG thermal conductivity.

 Figure 16.—Copper plate temperature gradient. Figure 17.—Copper plate thermal conductivity.

 

y = 28.703x - 61.084

0

5000

10000

15000

20000

25000

30000

35000

700 750 800 850 900 950 1000 1050 1100 1150

Thermal Gradient C/m

Flux, Watt/m2

Figure 13.—316SS temperature conductivity.

Gradient = 773

Gradient = 993

Gradient = 1112

15

25

35

45

55

65

75

0.03 0.04 0.05 0.06 0.07 0.08

Sensor Location, meters

4.9 Watts

5.9 Watts

7.2 Watts

 Figure 12.—316SS temperature gradient.
NASA/TM—2010-216773 10 

A 316 stainless steel clad TPG composite cooling plate was tested. Figure 18 plots the temperature data 

collected on the stainless steel clad cooling plate. Figure 19 plots the heat flux through the composite plate against 

the temperature gradients determined from Figure 18. The slope of the line in Figure 19 is 962 Watt/m-K which is a 

thermal conductivity much higher than that measured for the pure stainless steel plate. This again demonstrates the 

ability of TPG to dramatically improve the thermal conductivity of a low conductance material. This improvement is 

sufficient to be an acceptable material for a fuel cell cooling plate where the heat does not have to be transmitted 

across the active area of a fuel cell more than 4 to 5 cm.

A copper clad TPG composite cooling plate was tested. This composite plate was made with the TP1 

electrically conductive adhesive. Figure 20 plots the temperature data collected on the copper clad cooling plate. 

Figure 21 plots the heat flux through the composite plate against the temperature gradients determined from 

Figure 20. 

The slope of the line in Figure 21 is 1105 Watt/m-K which is a thermal conductivity much higher than that 

measured for the pure copper plate. This again demonstrates the ability of TPG to dramatically improve the thermal 

conductivity of even a high conductance material like copper. This improvement is sufficient to be an acceptable 

material for a fuel cell cooling plate where the heat does not have to be transmitted across the active area of a fuel 

cell more than 4 to 5 cm.

 Figure 18.—316SS clad TPG temperature gradient. 

 Figure 20.—Cu clad TPG TP1 temperature gradient. 

 

y = 962.71x + 37273

0

100000

200000

300000

400000

500000

600000

0 50 100 150 200 250 300 350 400 450 500

Thermal Gradient C/m

Flux, Watt/m2

Figure 19.—316SS clad TPG thermo conductivity. 

y = 1105.7x + 13646

0

50000

100000

150000

200000

250000

300000

350000

0 50 100 150 200 250 300

Thermal Gradient C/m

Flux, Watt/m2

Figure 21.—Cu clad TPG TP1 thermal conductivity. 
NASA/TM—2010-216773 11 

Another copper clad TPG composite cooling plate was tested. This composite plate was made with the 

ATTA-LM2 electrically conductive adhesive. The ATTA adhesive has a greater thermal conductivity in one inplane direction than the other in-plane direction. For this sample the higher conductivity direction was oriented 

perpendicular to the heat flux direction. Figure 22 plots the temperature data collected on the copper clad cooling 

plate. 

Figure 23 plots the heat flux through the composite plate against the temperature gradients determined from 

Figure 22. The slope of the line in Figure 23 is 1149 Watt/m-K which is a thermal conductivity much higher than 

that measured for the pure copper plate. This copper clad composite plate had a lower density than the copper clad 

plate made with the TP1 adhesive. The lower density and the high thermal conductance makes this composite plate 

an acceptable material for a fuel cell cooling plate where the heat does not have to be transmitted across the active 

area of a fuel cell more than 5 to 6 cm.

Another copper clad TPG composite cooling plate was tested. This composite plate was also made with the 

ATTA-LM2 electrically conductive adhesive, but this sample was tested with the higher conductivity direction 

oriented parallel to the heat flux direction. Figure 24 plots the temperature data collected on the copper clad cooling 

plate. Figure 25 plots the heat flux through the composite plate against the temperature gradients determined from 

Figure 24. The slope of the line in Figure 25 is 1167 Watt/m-K which is a thermal conductivity slightly higher than 

that measured for the other composite plate made with the ATTA adhesive oriented in the perpendicular direction. 

This copper clad composite plate had a lower density than the copper clad plate made with the TP1 adhesive. 

The lower density and the high thermal conductance makes this composite plate an acceptable material for a 

fuel cell cooling plate where the heat does not have to be transmitted across the active area of a fuel cell more than 

5 to 6 cm. 

 Figure 22.—Cu clad TPG ATTA  temperature gradient. 

 Figure 24.—Cu clad TPG ATTA // temperature gradient. Figure 25.—Cu clad TPG ATTA // thermal conductivity.

 

y = 1149.3x + 53843

0

100000

200000

300000

400000

500000

600000

700000

800000

0 100 200 300 400 500 600

Thermal Gradient C/m

Flux, Watt/m2

 Figure 23.—Cu clad TPG ATTA  thermal conductivity. 

y = 1167.4x + 58157

0

100000

200000

300000

400000

500000

600000

700000

800000

0 100 200 300 400 500 600

Thermal Gradient C/m

Flux, Watt/m2
NASA/TM—2010-216773 12 

The unaltered TPG was tested. The thermal conductivity was measured to compare with the manufacturers 

published value and to help validate our testing apparatus. The measurement also served as a comparison to the 

various metal/TPG composites that were fabricated and tested. Figure 26 plots the temperature data collected on the 

TPG plate. Figure 27 plots the heat flux through the composite plate against the temperature gradients determined 

from Figure 26. The slope of the line in Figure 27 is 1426 Watt/m-K which falls only 5 percent lower than the value 

published by the manufacturer. The composites made with the TPG had thermal conductivity ranging from 962 to 

1167 Watt/m-K. The lower values represent the penalty for upgrading the undesirable characteristics of the TPG.

A copper planar heat pipe from Thermacore, Inc. (Ref. 8) was tested. This heat pipe was Thermacore’s first 

attempt to produce a heat pipe with a thin planar geometry. The design of the planar heat pipe drew from 

Thermacore’s experience in producing tubular copper heat pipes. Figure 28 plots the temperature data collected on 

the copper planar heat pipe plate. Figure 29 plots the heat flux through the heat pipe plate against the temperature 

gradients determined from Figure 28. The slope of the line in Figure 29 is 14050 Watt/m-K. This is an order of 

magnitude higher than any of the other cooling plate prototypes tested, and represents a major leap in thermal 

conductance. The data plotted in Figure 29 does not fit a linear relationship, but shows an increasing conductance as 

the heat flux and temperature gradient increase. The two-phase heat transfer mechanism in the heat pipe involves 

boiling the internal fluid at one location on the heat pipe plate and condensing the same internal fluid at another 

location on the heat pipe plate. This heat transfer behavior is fundamentally different than conductive heat transfer. 

As more heat is applied to the heat pipe, the boiling rate, hence the production of vapor inside the heat pipe 

increases. Because the interior space of the heat pipe is constant, the speed of the vapor movement inside the heat 

pipe increases. As the vapor transport speed increases, the thermal gradient across the heat pipe increases at slower 

rate, and can actually get smaller with increasing heat flux (i.e., a negative slope). Despite the extraordinary thermal 

conductivity, the thickness and mass of this heat pipe limits its utility for cooling in fuel cells. 

 Figure 26.—TPG temperature gradient. Figure 27.—TPG thermal conductivity gradient.

 Figure 28.—Copper heat pipe temperature gradient. Figure 29.—Copper heat pipe thermal conductivity.

 

y = 1426.3x + 13827

0

100000

200000

300000

400000

500000

600000

700000

800000

900000

0 100 200 300 400 500 600

Thermal Gradient C/m

Flux, Watt/m2
NASA/TM—2010-216773 13 

A titanium planar heat pipe from Thermacore, Inc. (Ref. 8) was also tested. This heat pipe was Thermacore’s 

second attempt to produce a heat pipe with a thin planar geometry. The choice of titanium was to reduce the mass of 

a planar heat pipe. The titanium metal is much lighter than copper, and its greater strength would allow the planar 

heat pipe to be made thinner as well. Thermacore, Inc. had some experience producing tubular titanium heat pipes, 

and drew on this experience to design and fabricate the planar titanium heat pipe. Figure 30 plots the temperature 

data collected on the titanium planar heat pipe plate. Figure 31 plots the heat flux through the heat pipe plate against 

the temperature gradients determined from Figure 30. The slope of the line in Figure 31 is –20447 Watt/m-K. Like 

the copper heat pipe this is an order of magnitude higher than any of the other cooling plate prototypes tested. The 

data plotted in Figure 31 does fit a linear relationship, but shows a negative slope, that is a negative thermal 

conductance. This behavior is obviously very different that the various positive slopes seen with the other cooling 

plate prototypes, and like the copper heat pipe is because the physics underlying the two phase heat transfer 

mechanism in the heat pipe is not the same as the physics underlying the solid conduction heat transfer in the other 

cooling plates tested. This heat pipe has the lowest density of any of the cooling plates tested except for the 

unaltered TPG. Coupled with the extraordinary thermal conductivity, this heat pipe was clearly the best alternative 

for a passive fuel cell cooling plate. 

Figure 32 shows the thermal conductivity plots of all the plates listed in Table 4. Looking at the plot, the data 

breaks into three groups, the highest conductivity group which are the heat pipes, the second highest group which 

are the TPG and TPG laminates, and the final group which is the diffusion bonded TPG and the metals. 

Figure 30.—Titanium heat pipe temperature gradient. 

Figure 32.—Cooling plate thermal conductivity comparison. 

y = -20447x + 894147

0

50000

100000

150000

200000

250000

300000

350000

400000

0 5 10 15 20 25 30 35 40 45

Thermal Gradient C/m

Flux, Watt/m2

Figure 31.—Titanium heat pipe thermal conductivity. 
NASA/TM—2010-216773 14 

Conclusions 

The efforts to develop passive fuel cell thermal management materials has lead to the following conclusions: 

1. Passive cooling plates can be fabricated that are capable of meeting the 3 °C maximum temperature 

difference and the 1000 Watt/kg mass target. 

2. There are two approaches to the construction of the passive cooling plates, use TPG thin sheets laminated 

with even thinner metal foils to improve TPG’s low mechanical strength. This approach will be satisfactory 

for smaller fuel cells where the heat transmission distance across the active area of the fuel cell is 6 cm or 

less. The second approach is to use planar titanium heat pipes. This approach will be satisfactory for all fuel 

cell sizes, and will also be lower in mass than the TPG approach. 

References 

1. Kenneth A. Burke, “Advanced Fuel Cell System Thermal Management for NASA Exploration Missions,” 

NASA/TM—2008-215426, AIAA–2008–5795, presented at AIAA 6th International Energy Conversion 

Engineering Conference, July 28–30 2008, Cleveland, Ohio. 

2. William E. Simon and Donald L. Nored, “Manned Spacecraft Electrical Power Systems,” Proceeding of the 

IEEE, Vol. 75, No. 3, March 1987. 

3. Zweben, Carl “New, Ultrahigh-Thermal-Conductivity Packaging Materials,” IEEE Components, Packaging & 

Manufacturing Technology Society - Santa Clara Valley Chapter, Sunnyvale, California, March 16, 2005. 

4. Geoffrey Thyrum and Ellen Cruse, “Heat Pipe Simulation,” Advanced Packaging Magazine, Dec. 2001. 

5. The Engineering Handbook, Second Edition, Richard C. Dorf, Editor-in-Chief, CRC Press, 2005. 

6. GE Advanced Ceramics, This company was bought and now is Momentive Performance Materials, Inc. 

22557 Lunn Road, Strongsville, Ohio 44149 (440) 878–5700. 

7. Btech Corporation, 8395 Greenwood Drive, Longmont, Colorado 80503 USA (303) 652–6418. 

8. Thermacore, Inc., 780 Eden Road, Lancaster, Pennsylvania 17601 USA (717) 569–6551. 
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Development of Passive Fuel Cell Thermal Management Technology 

5a. CONTRACT NUMBER

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Burke, Kenneth, A.; Jakupca, Ian; Colozza, Anthony 

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WBS 038957.04.06.02.02.03 

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National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

Cleveland, Ohio 44135-3191 

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14. ABSTRACT

The NASA Glenn Research Center is developing advanced passive thermal management technology to reduce the mass and improve the 

reliability of space fuel cell systems for the NASA exploration program. The passive thermal management system relies on heat conduction 

within the cooling plate to move the heat from the central portion of the cell stack out to the edges of the fuel cell stack rather than using a 

pumped loop cooling system to convectively remove the heat. Using the passive approach eliminates the need for a coolant pump and other 

cooling loop components which reduces fuel cell system mass and improves overall system reliability. Previous analysis had identified that 

low density, ultra-high thermal conductivity materials would be needed for the cooling plates in order to achieve the desired reductions in 

mass and the highly uniform thermal heat sink for each cell within a fuel cell stack. A pyrolytic graphite material was identified and 

fabricated into a thin plate using different methods. Also a development project with Thermacore, Inc. resulted in a planar heat pipe. 

Thermal conductivity tests were done using these materials. The results indicated that lightweight passive fuel cell cooling is feasible.

15. SUBJECT TERMS

Fuel cells; Cooling; Temperature control; Cooling systems; Thermal conductivity 

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