Anthony J. Colozza

Analex Corporation, Brook Park, Ohio

Hydrogen Storage for Aircraft

Applications Overview

NASA/CR—2002-211867

September 2002
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Anthony J. Colozza

Analex Corporation, Brook Park, Ohio

Hydrogen Storage for Aircraft

Applications Overview

NASA/CR—2002-211867

September 2002

National Aeronautics and

Space Administration

Glenn Research Center

Prepared under Contract NAS3–00145
Available from

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Available electronically at http://gltrs.grc.nasa.gov
NASA/CR—2002-211867 iii 

Table of Contents

Introduction.....................................................................................................................................................1 

Gaseous Hydrogen Storage.............................................................................................................................2 

Pressure Tank .............................................................................................................................................2 

Metal Hydride.............................................................................................................................................9 

Carbon Nanotubes ....................................................................................................................................11 

Glass Microspheres ..................................................................................................................................11 

Cryogenic Hydrogen Storage........................................................................................................................13 

Gelled Hydrogen ......................................................................................................................................19 

Cryo-cooler for Boil-Off Compensation ..................................................................................................20 

Chemically Bound Hydrogen Storage...........................................................................................................22 

References.....................................................................................................................................................24 

Bibliography of Reviewed Materials........................................................................................................25 
NASA/CR—2002-211867 1 

	

Hydrogen is a very high energy density element that holds much promise as a potential 

fuel for aircraft. The energy density of hydrogen, which is around 120 MJ/kg, is more 

than double that of most conventional fuels (for example natural gas: 43 MJ/kg and 

gasoline 44.4 MJ/kg). The main issue with using hydrogen in aircraft is its very low 

density. At ambient conditions 1 liter of hydrogen contains only 10.7 KJ of energy. 

Even in its liquid state the volumetric energy density of hydrogen (8.4 MJ/liter ) is less 

then half that of other fuels (natural gas 17.8 MJ/liter, gasoline 31.1 MJ/liter). Storing a 

sufficient amount of it for use in most applications requires a large volume. Therefore, 

in order to make it practical for aircraft applications, the storage method utilized must 

increase the density of hydrogen. 

Conventional methods of storage are as a compressed gas or as a cryogenic liquid. 

Work is being done in these areas that would make them more applicable to an aircraft 

by reducing storage tank weight (carbon composite tanks) [1,15] or increasing the density 

of cryogenic hydrogen (gelled or slush hydrogen) [12,13]. In addition new storage 

methods are being devised that may be capable of storing hydrogen without the need for 

high pressure tanks or the need to manage a cryogenic liquid (carbon nanotubes, carbon 

fullerenes, and hydrides) [1,9]. 

Aside from storing hydrogen directly, it can also be stored as a component of a 

hydrocarbon fuel. Since most conventional fuels contain hydrogen, this method of 

storage can be utilized as long as there is a method of removing the hydrogen from the 

fuel that does not require more energy than that provided by the hydrogen itself in this 

case. A reformer would be used to break apart the hydrocarbon fuel and liberate the 

hydrogen. 

Regardless of the storage mechanism the first thing that must be known when 

determining which type of storage is most applicable is the amount of hydrogen that 

is required. For a polymer electrolyte membrane (PEM) fuel cell application, an estimate 

of the hydrogen requirement can be determined by the following basic analysis. 

The voltage of an individual cell in a PEM fuel cell stack is given by: 

 V = 1.2 ηfc [1] 

Where V is the cell voltage and ηfc is the electrical conversion efficiency of the cell. 

The power produced per plate (Pp) is given by equation 2 where I is the current density 

(Amps/ m2

) of the plate and A is the plate area: 

 P

p = VIA [2] 

For a given desired output power (Po) the number of plates (Np) can be calculated. 
NASA/CR—2002-211867 2 

 N

p

 = Po / (VIA) [3] 

The flow rate of hydrogen (FH) in moles of H2 per second needed to produce the given 

output power is 

 FH = N

p I A / (2)(96,500) [4] 

Combining equations 1,3 and 4 and changing the units from moles to kilograms 

(500 moles H2 per kg) yields the following. 

FH = Po /(1.2 ηfc (2)(96,500 ) (500)) = Po / (1.158E8 ηfc) [5] 

The total mass of hydrogen (MH) needed is based on the operation time (t) of the fuel 

cell (in hours). 

 MH = FH t = Po t / (32,167 ηfc) [6] 

Based on this quick method of calculating the mass of hydrogen needed for a given 

application, an estimate of the size of the storage system can be determined. 







	


High-pressure hydrogen storage is the most conventional type of hydrogen storage. As 

the storage pressure increases, the density of the hydrogen gas will also increase. This 

relationship is shown in figure 1. This figure represents the change in density (ρ) of 

hydrogen gas at various temperatures and pressures. This figure is based on the ideal 

gas law with a compressibility factor (Z) for the hydrogen gas[5

the gas pressure and temperature respectively and R is the gas constant for hydrogen 

(4157 N m / kg K) 

 ρ = P / ZRT [7] 

 Z = 0.99704 + 6.4149E-9 P [8] 

]. Where P and T are
NASA/CR—2002-211867 3 

0.01

0.1

1

10

100

100 200 300 400 500 600 700 800

101.3 kPa (14.7 psia)

689.4 kPa (100 psia)

6,894 kPa (1000 psia)

34,470 kPa (5000 psia)

68,940 kPa (10,000 psia)

Density (kg/m3

)

Temperature (K)

 

 Figure 1.—Hydrogen Density at Various Pressures and Temperatures 

The tradeoff with utilizing high density / high pressure storage is the increase in tank 

mass necessary to withstand the higher pressures. The wall thickness of the tank will 

increase with the increasing hoop stress due to the higher gas pressure. The calculation 

of the tank mass for hydrogen gas stored under pressure is a fairly straightforward 

analysis. This analysis is outlined below. 

For this analysis it is assumed that hydrogen will follow the behavior of an ideal gas 

represented by the equation of state, where mH is the mass of hydrogen; 

P VH = mH RT [9] 

Where R is the gas constant for hydrogen and has a value of 4157.2 (N m/K kg), 

P (Pa) is the pressure of the gas, VH (m3) is the volume and T (K) is its temperature. 

Using the gas constant given above, equation 9 can be restated as follows. 

VH = Z 4157.2 mH T / P [10] 
NASA/CR—2002-211867 4 

The tank radius (r) can be calculated from the volume determined in equation 10. The 

tank is assumed to be either a sphere or a cylinder. The sphere is actually a special case 

of the cylinder in which the length (L) is zero. Equation 11 can be solved for r through 

an iterative process. 

VH = 4 π r

3

 / 3+ π r

2

 L [11] 

With the radius and tank pressure known the required wall thickness (tw) can be 

determined based on the maximum allowable stress (σy) and a factor of safety (FoS). The 

following two equations represent the wall thickness for a spherical tank and a cylindrical 

tank with hemispherical end caps, respectively. 

 tw = P r FoS / σy [12] 

 tw = P r FoS / (2 σy) [13] 

From the wall thickness and the density of material used to construct the tank (ρt

), the 

mass of the tank (mt

) can be calculated. 

 mt

 = ρt (4/3) π (r + tw)

 3

 + π (r + tw) 2 L – VH [14] 
NASA/CR—2002-211867 5 

0

50000

100000

150000

200000

250000

300000

414.5 2073 4,145

Hydrogen Storage Mass (kg)

Carbon Sphere

Titanium Sphere

Steel Sphere

Carbon Cylinder

Titanium Cylinder

Steel Cylinder

 Figure 2.—Pressure Tank Mass for Various Materials, Configurations and Storage 

 Volumes 

Figure 2 shows how the mass of the tank will vary with different construction materials. 

The curves were generated for steel, titanium and carbon. The volumes of hydrogen used 

were calculated from the amount of hydrogen consumed by a fuel cell to produce 1 MW, 

5 MW and 10 MW of power for 8 hours. For comparison purposes the length of the 

cylindrical tank was assumed to be 3 m. Due to the compressibility effects of hydrogen, 

the change in storage pressure does have an effect on the mass of the tank. This is 

because the reduction in gas volume is not linear with the increase in pressure, as it 

would be if compressibility were not considered. The pressure tank mass and volume as a 

function of storage pressure for various amounts of stored hydrogen are shown in figures 

3 through 5. These figures are for the same hydrogen storage volumes that were given 

in figure 2. From these figures it can be seen that at low pressures the tank mass is 

fairly constant. However at higher pressures the tank mass rises. This is due to the 

compressibility effects of hydrogen gas. The Titanium tanks have a much greater 

mass then the carbon tanks. This is due to the low density and high strength of carbon 

composites. The carbon tanks however require a liner to prevent the hydrogen gas from 

leaking through the tank wall. Tank Mass (kg)
NASA/CR—2002-211867 6 

Figure 3.—Effect of Pressure on Mass of a Titanium Tank for Various Amounts 

 of Hydrogen Stored 

TABLE 1.—PRESSURE TANK MATERIAL PROPERTIES [19, 20] 

Material Density Yield Strength 

Steel (quenched and 

tempered alloy 

ASTM–A514 

7860 kg/m3

690 MPa 

Aluminum (4.4% Cu) 

2014–T6 

2800 kg/m3 410 MPa 

Titanium Alloy 

(6% AL, 4% V) 

4460 kg/m3 825 MPa 

Carbon Composite 1530 kg/m3 1900 MPa 

 

Titanium Tank

0

20000

40000

60000

80000

100000

120000

140000

160000

180000

3 7 14 34 55 69

Pressure (MPa)

414 kg H2

2073 kg H2

4145 kg H2

Tank Mass (kg)
NASA/CR—2002-211867 7 

 Figure 4.—Effect of Pressure on Mass of a Carbon Tank for Various Amounts 

 of Hydrogen Stored 

The material utilized in constructing the tank can be either a metal such as steel, titanium 

or aluminum, or a composite. Properties of these materials are given in table 1. Since 

most composites are porous to hydrogen, a composite tank will require a liner to prevent 

the hydrogen from migrating through the tank wall. Liner materials are usually a type of 

polymer or a metal such as aluminum. Also coatings are being investigated as a 

lightweight means of preventing hydrogen penetration through the tank. Composite tanks 

offer the best weight density of hydrogen. Tanks under development at Quantum 

Technologies have achieved up to 11.3% hydrogen by weight [4]. However a more 

realistic number for composite storage tanks under practical use is between 7.5 % and 

8.5% weight of hydrogen [4,15]. 

In addition to the stress placed on the tank due to the pressure loading, additional issues 

will arise when utilizing a high pressure tank over a period of time and cycle loadings. 

These issues include aging, creep, fatigue and hydrogen embrittlement. Depending on the 

tank material chosen, the effect of design life and the operating pressure on each of these 

factors would need to be investigated to determine their effect on a particular tank design. 

Carbon Tank

0

5000

10000

15000

20000

25000

3 7 14 34 55 69

Pressure (MPa)

414 kg H2

2073 kg H2

4145 kg H2

Tank Mass (kg)
NASA/CR—2002-211867 8 

 Figure 5.—Tank volume as a function of storage pressure for various masses of H2

To maximize the storage volume for a given tank placement, the ideal configuration 

would be to construct a conformal tank. A conformal tank can hold up to 20% more 

hydrogen in a given envelope and pressure then a cylindrical or spherical tank. This is 

based on the presumption that a conformal tank can utilize over 80% of its envelope 

volume. This increase in useable storage compared to a conventional cylindrical tank 

is shown in figure 6. The ability to construct a lightweight high pressure conformal 

tank is presently being investigated by Thiokol Propulsion under a Department of Energy 

contract [4]. 

1

10

100

1000

10000

69E4 (100) 35E5 (500) 69E5 (1000) 21E6 (3000) 48E6 (7000) 69e6 (10000)

Pressure Pa (psi)

415 kg H2

2073 kg H2

4150 kg H2

Volume (m3 )
NASA/CR—2002-211867 9 

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

1 1.5 2 2.5 3 3.5 4 4.5 5

Space Envelope Aspect Ratio

Cylindrical Tank

Conformal Tank

 Figure 6.—Effect of Conformal Tanks on Available Space Utilization [4] 




Metal hydrides are metallic alloys that absorb hydrogen. These alloys can be used as a 

storage mechanism because of their ability to not only absorb hydrogen but also release 

it. The release of hydrogen is directly related to the temperature of the hydride. Typically 

metal hydrides can hold hydrogen equal to approximately 1% to 2% of their weight. If 

active heating is supplied to remove the hydrogen this can increase to 5% to7% of the 

hydride weight. If the temperature is held constant the hydrogen is released at a constant 

pressure. The metal hydride tank can be used repeatedly to store and release hydrogen. 

The limiting factor on its ability to store hydrogen is the accumulation of impurities 

within the tank. These impurities fill the spaces that would normally store the hydrogen 

and thereby reduce the tanks capacity. 

Fraction of Available Space Utilized by the Tank
NASA/CR—2002-211867 10 

The key tradeoff to utilizing a metal hydride storage system is whether there is sufficient 

heat generated to extract the hydrogen from the hydride. The excess heat produced by the 

fuel cell (or other hydrogen consuming component) must be greater than that required by 

the metal hydride to liberate the hydrogen at the desired flow rate and pressure. If this is 

not achievable, the system will not be able to operate utilizing metal hydride storage. The 

heat available to the hydride must also take into account the inefficiencies associated with 

the heat transfer device used to move the heat from the source to the hydride. 

Another issue with hydride use is the time lag between initial heating and the release of 

hydrogen gas. Hydrides constructed of heavy metals such as vanadium, niobium and 

iron-titanium will release hydrogen at ambient temperatures and avoid this problem [6]. 

Other hydrides constructed of lighter materials will need to be heated from an auxiliary 

source until the temperature is sufficient to release hydrogen. 

Table 2 shows a list of potential metal hydride storage materials and the density of the 

hydrogen stored within the material. It should be noted that the density given in the table 

is for the hydrogen alone. This does not represent the hydride material or any other 

ancillary components needed for the storage system to operate. 

TABLE 2.—METAL HYDRIDES AND HYDROGEN DENSITY CAPABILITY [6] 

Metal Hydride Hydrogen Density (kg/m3

) 

Magnesium (MgH2) 109 

Lithium (LiH) 98.5 

Titanium (TiH1.97) 150.5 

Aluminum (AlH3) 151.2 

Zirconium (ZrH2) 122.2 

Lanthanum (LaNi5H6) 89 

Even though the storage density of hydrogen in a metal hydride is high the total mass of 

the system is large. The following table lists the specifications for commercially available 

state of the art metal hydride storage tanks. 

TABLE 3.—COMMERCIALLY AVAILABLE METAL HYDRIDE HYDROGEN 

STORAGE SPECIFICATIONS [7] 

Volume H2 (m2) Mass H2 (kg) Metal Hydride Tank 

Mass (kg) 

Percent Mass of H2

of Total Tank Mass 

0.042 0.0036 1 0.36% 

0.068 0.0058 0.86 0.68% 

0.327 0.0273 6.1 0.45% 

0.906 0.0767 16.78 0.46% 

1.274 0.1078 24 0.45% 

2.547 0.214 36 0.59% 
NASA/CR—2002-211867 11 

Other systems are under development that can offer higher mass fractions of hydrogen. 

These systems promise mass fractions up to 5% (6 kg hydrogen storage for a 120 kg 

system mass) [8]. 

 

		

Carbon nanotubes are tubular carbon structures on the order of 2 nanometers in size. 

These structures are theoretically capable of storing hydrogen within the tube structure. 

The storage mechanism is similar to that of metal hydrides except the amount of 

hydrogen capable of being stored is much greater. It is theorized that carbon nanotubes 

can store anywhere from 4% to 65% of their weight in hydrogen [1,4]. This technology 

is very new and still within the development stage. However if it can live up to its 

projected potential, this would by far be the lightest, most efficient way to store 

hydrogen. 




Glass microspheres store hydrogen in tiny hollow spheres of glass. If heated the spheres 

permeability to hydrogen will increase. This provides the ability to fill the spheres by 

placing the warmed spheres in a high-pressure hydrogen environment. The hoop stresses 

achievable for glass microspheres can range from 345 Mpa (50,000 psi) to 1,034 Mpa 

(150,000 psi) [9]. The corresponding maximum pressure sustainable by the microsphere 

is calculated in the same manner as that for any other sphere. This is given by equation 

12, which would be solved for P for a given will thickness (on the order of 0.68 μm). 

On the high end this is comparable to the stress achievable with carbon fiber tanks. Once 

cooled the spheres lock the hydrogen inside. The hydrogen is released by subsequently 

increasing the temperature of the spheres. This method of storage provides a safe, 

contamination resistant method for storing hydrogen. 

The fill rates of microspheres are related to the properties of the glass used to construct 

the spheres, the temperature at which the gas is absorbed (usually between 150 °C and 

40 °C) and the pressure of the gas during absorption. Fill and purge rates are directly 

proportional to the permeability of the glass spheres to hydrogen which increases 

with increasing temperature. At room temperature the fill / purge rate is on the order 

of 5000 hours, at 225 °C it is approximately 1 hour and at 300 °C it is approximately 

15 minutes [9]. This dramatic increase in hydrogen permeability with temperature allows 

the microspheres to maintain low hydrogen losses at storage conditions while providing 

sufficient hydrogen flow when needed. 

Engineered microspheres provide the greatest advantage for high density storage of 

hydrogen. The engineered microspheres can achieve the high hoop stress values 

suggested above. It is estimated that a bed of 50 μm diameter engineered microspheres 

can store hydrogen at 62 Mpa (9000 psi) with a safety factor of 1.5 and a hydrogen 

mass fraction of 10% [9]. This produces a hydrogen density of 20 kg / m3

. There is a 

trade off between storage pressure and mass fraction of hydrogen stored. At lower storage 
NASA/CR—2002-211867 12 

pressures the mass fraction of hydrogen stored increases but the overall hydrogen 

volumetric density decreases. This is caused by the increase in the glass sphere wall 

thickness needed to withstand the increase in storage pressure and maintain the same 

factor of safety. Figure 7 shows how the hydrogen mass fraction and volumetric density 

change for various storage pressures [9]. 

Although the storage capabilities of glass microspheres looks impressive there are a 

number of drawbacks to their use from a system standpoint. The main issue is that to 

get the hydrogen into and out of the spheres, they must be heated. This heating takes 

considerable energy and time to accomplish. The higher the heating, the quicker the 

hydrogen will purge from the spheres. But from a system standpoint this heating must 

be accounted for. For a fuel cell system it is conceivable that the waste heat from the fuel 

cell operation can be used to liberate the hydrogen from the microspheres. This scheme, 

although promising, requires a heat exchange loop between the fuel cell and storage tanks 

which can add significant weight to the system. 
NASA/CR—2002-211867 13 

Figure 7.—Glass Microsphere Hydrogen Storage Mass Fraction and Density for Various 

Storage Pressures 

	



To reduce tank mass and volume over high-pressure gas storage, cryogenic storage of 

hydrogen can be used. The properties of liquid hydrogen enable significant increases in 

density over high-pressure gas storage as well as reduced tank mass due to lower pressure 

operation. Liquid hydrogen is at a temperature of around –260 °C (–425 °F) and has a 

density (ρLH) of 71 kg/m3 (4.43 lb/ft3). To get a further increase in density above that of 

liquid hydrogen, a mixture of solid and liquid hydrogen can be produced. This mixture 

is called slush hydrogen. For a 50% solid / 50% liquid mixture the hydrogen density is 

80.9 kg/m3 [12]. 

6

8

10

12

14

16

6.89 13.79 20.68 27.58 34.47 41.37 48.26 55.16 62.05 68.95

Storage Pressure (MPa)

0

5

10

15

20

25

kg H2 / kg system *100

kg H2 / m3

Percentage of Hydrogen Mass of Total Storage Mass

Hydrogen Storage Density
NASA/CR—2002-211867 14 

Cryogenic storage maximizes the density of hydrogen but does impose some significant 

operational constraints on the fuel system. 

1. It requires an airtight insulation system to reduce the boil-off of the liquid 

hydrogen and maintain it at cryogenic temperatures. 

2. Liquid hydrogen handling requires specialized equipment and procedures. Also 

the storage of liquid hydrogen is time limited (due to boil-off) so the fuel usually 

needs to be manufactured on site or at a near by location. 

3. The fuel tanks need to be maintained at a constant pressure, usually around 

1.45×105 Pa (21 psia) to minimize boil-off. This requires a venting system and 

procedure to be implemented [10]. 

4. Liquid hydrogen tanks and lines must be sealed off from the atmosphere. If air 

enters the tanks it will freeze solid and can block the flow lines. Only helium 

can be used as a purge gas. 

The main components for storing liquid hydrogen are the tank and insulation. 

The tank is usually a thin-walled pressure vessel surrounded by a thick layer of 

insulation. The tank materials must be resistant to hydrogen embrittlement, impermeable 

to hydrogen gas and capable of structurally withstanding the temperatures of liquid 

hydrogen. Also, because of the great change in temperature when the tank is fueled or 

emptied, thermal expansion and contraction is a major concern. Therefore the attachment 

points of the tank to any structure must be capable of withstanding this movement as well 

as the tank structure itself. Because of this it is usually required that the tank be made out 

of one type of material. This also poses a significant problem with regard to lightweight 

strong materials, such as carbon, that will require a liner of different material in order to 

be made impermeable to hydrogen gas. 

The storage tank for holding the liquid hydrogen can be sized by the following analysis. 

The tank volume (Vt) required to hold the hydrogen is given by the following equation. 

 

Vt = MH (1+Vi)/ ρLH [15] 

The mass of hydrogen to be held in the tank is given by equation 6. There must be space 

left in the tank in order to maintain a constant pressure as well as provide space for 

boil-off. This excess volume is estimated to be around 7.2% (Vi = 0.072) of the total 

tank volume. 

From this the tank radius (r) can be calculated by solving the following equation for r. 

If the tank is a cylinder then this will have to be done iteratively. 

 Vt

 = 4 π r

3

 / 3+ π r

2

 L [16] 
NASA/CR—2002-211867 15 

The wall thickness for the tank is given by equations 12 and 13 for a cylindrical and 

spherical tank respectively. Utilizing the wall thickness and tank radius, the mass of 

the tank (mt

) can be calculated. 

mt

 = ρt (4/3) π (r + tw)

 3

 + π (r + tw) 2 L – Vt [17] 

The next main component of a liquid hydrogen storage system is the insulation that is 

needed surrounding the tank as well as any fuel lines or handling devices. The insulation 

serves a few purposes. It is necessary to reduce the amount of boil-off from the storage 

tanks. Without the insulation the boil-off rate would make the use of liquid hydrogen 

completely impracticable. Also the insulation prevents or reduces the frost buildup on 

the outer surface of the aircraft. Without proper insulation, the frost build up can pose 

a serious hazard to the aircraft’s ability to fly. The insulation must be impervious to air 

to eliminate the possibility of frozen air particles forming and it must be capable of 

withstanding the extreme thermal cycling to which it will be subjected. 

There are two main categories or types of insulation that can be used. The first is a 

vacuum-jacketed system consisting of a layers of Mylar (or a similar type of low 

emissivity, high reflectivity material) separated by thin fiberglass sheets, to maintain 

spacing, surrounded by an outer container capable of maintaining a low pressure within 

the insulation layers. The low-pressure environment minimizes the conductivity between 

the insulation layers and the layers act as a radiation barrier keeping heat out of the tank. 

The pressure within the vacuum-jacketed insulation is typically kept around 0.1 torr. 

The main drawback to the vacuum-jacketed insulation is that if the vacuum is lost the 

insulation will fail, causing a large and rapid boil-off of propellant [10]. 

The second type of insulation is a rigid closed-cell foam. This type of insulation can be 

applied to the outside of the tank. If needed, a thin metal-walled enclosure can be placed 

on the outside of the foam to maintain its integrity and protect it from damage. The foam 

type of insulation is much more resistant to catastrophic failure than the vacuum-jacketed 

type of insulation. However, the density and thermal conductivity of the foam insulation 

is greater. 

The following table lists a number of different insulation types and the effective 

conductivity and density [10,11]. The amount of insulation that is needed on the tank 

will vary depending on the insulation properties, tank size, allowable boil-off rate and 

overall allowable weight. In order to get an estimate of the insulation requirements, a 

brief analysis on the heat flow into the tank needs to be performed. The analysis is a one 

dimensional heat flow from the surrounding to the liquid hydrogen. The heat transfer 

mechanisms are shown in figure 8. 
NASA/CR—2002-211867 16 

TABLE 4.—TANK INSULATION PROPERTIES 

Insulation Type Density (kg/m3

) Thermal Conductivity 

(W/m K) 

Rigid closed cell 

polymethacrylimide 

35.3 0.0096 

Rigid open cell 

polyurethane 

32.1 0.0112 

Rigid closed cell 

polyvinalchloride 

49.8 0.0046 

Rigid closed cell 

polyurethane and 

chopped glass fiber 

64.2 0.0064 

Evacuated aluminum foil 

separated with 

fluffy glass mats 

40 0.00016 

Evacuated aluminum foil 

and glass paper laminate 

120 0.000017 

Evacuated silica powder 160 0.00017 

Figure 8.—One Dimensional Heat Transfer for Liquid Hydrogen Tank 

T

(Temperature of the surroundings)

Qradiation

convection Q conduction Q

Insulation

Ts (Insulation surface temperature)


NASA/CR—2002-211867 17 

Initially the outside surface temperature of the insulation (Ts) needs to be determined. 

This wall temperature is based on the heat flow into the insulation from convection and 

radiation and heat flow to the liquid hydrogen by conduction through the insulation. 

Qin = Q convection + Qradiation = h(T – Ts

) + εσ( T

4

– Ts

4

) [18] 

Qout = Qconduction = K (Ts – TLH2) /L [19] 

 Where ε is the emissivity of the insulation surface, σ is the Stefan-Boltzmann constant 

(5.67×10–8 W/m2 K

4

), h is the convection coefficient for the air surrounding the tank and 

K is the thermal conductivity of the insulation. It is assumed that the tank is in an isolated 

environment in which there is no active blowing of air surrounding it. Therefore the 

convection coefficient is based on natural convection of the air surrounding the tank. 

This coefficient can be represented by the following equation. 

 h = NUD K

g / D [20] 

The thermal conductivity of the air (Kg) is a property of the fluid at a given temperature 

and pressure and D is the tank diameter. The Nusselt number (NUD) is dependent on the 

geometry of the tank. Expressions for this are listed for spherical and cylindrical tank 

shapes [11]. 

For a sphere: 

 NUD = 2 + 0.589 Rad

1/4 / [1 + (0.469 /PR)9/16]4/9 [21] 

For a cylinder: 

 NUD = [0.60 + 0.387 Rad

1/6 / [1 + (0.559 /PR)9/16]8/27 ] 2 [22] 

Where the Rayleigh number (Rad) is given by the following equation in which g is the 

gravitational constant. 

 Rad = g β (T – Ts

) D3 / (ν α) [23] 

By assuming an ideal gas, the volumetric thermal expansion coefficient (β) is equal to 

the inverse of the gas temperature in degrees K. (β = 1/ T) 

The gas diffusivity and viscosity can be represented by the following data curve fits for 

air. 

 α = -3.119×10–6

+ 3.541×10–8 T + 1.679×10–10 T

2

 [24] 

 ν = -2.079×10–6 + 2.777×10–8 T + 1.077×10–10 T

2

 [25] 
NASA/CR—2002-211867 18 

Once the insulation outer surface temperature is known, the boil-off rate (M) in kg/s of 

the liquid hydrogen can be calculated. This is done through an energy balance between 

the heat flow through the insulation and the energy taken to boil the liquid hydrogen. 

The energy to boil the liquid is based on the latent heat of vaporization of liquid hydrogen 

(hfg = 446592 J/kg) 

 K A (Ts – TLH2) /L = M hfg [26] 

Using the above analysis, an estimate of how the total tank system mass is affected by the 

amount of insulation can be determined. For a storage duration of 8 hours, figure 9 shows 

how the total mass of the tank, including boil-off, is affected by the insulation thickness. 

Figure 10 shows the tank and boil-off mass as a percent of the liquid hydrogen used 

during an 8 hour period. 

 Figure 9.—Effect of Insulation Thickness on Overall Tank System Weight 

0

100

200

300

400

500

600

700

800

900

1000

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045

Insulation Thickness (m)

1660 kg of Hydrogen

830 kg of Hydrogen

210 kg of Hydrogen

40 kg of Hydrogen

Total Tank and Boil Off Mass (kg)
NASA/CR—2002-211867 19 

Figure 10.—Tank System Mass as a Percent of Useable Liquid Hydrogen 



	

To further increase the density of liquid hydrogen, it is possible to produce a gelled liquid 

hydrogen which produces an increase in density over conventional liquid hydrogen. It is 

estimated that gelled hydrogen can produce a 10% increase in the density over standard 

liquid hydrogen. 

Gelled hydrogen is produced by introducing a gellant into the liquid hydrogen. This 

gellant can be either another cryogenic material, such as solid ethane or methane, or silica 

particles [4]. Gelling agents constructed from other types of hydrocarbons such as ethyl 

alkoxides or hexyl alkoxides are also under development [12]. Figure 11 shows how the 

propellant density can increase with increasing gelling agent concentration. This figure is 

based on data obtained from reference 4 for methane as the gelling agent. 

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25 30

Tank Volume (m3)
NASA/CR—2002-211867 20 

Gelled hydrogen reduces the boil-off rate by 2 to 3 times compared to standard liquid 

hydrogen. There is also a significant safety benefit to using gelled hydrogen. It has an 

inherently smaller spill radius due to its higher viscosity than liquid hydrogen. This 

minimizes its effect if spilled. The increased viscosity reduces the leak potential by 

making the hydrogen more resistant to seeping through small openings. The higher 

viscosity also reduces the amount of hydrogen slosh within the storage tank reducing 

the chance for instabilities produced by the movement of the hydrogen [11]. 

Figure 11.—Theoretical Increase in Density with the Addition of Methane Gelling 

 Agent [13] 



		

Depending on the flight time and time on ground between filling and flight, the boil-off 

rate for the liquid hydrogen may become a significant issue. Therefore it is worth 

investigating whether a cryo cooler can be utilized as a means of maintaining zero 

boil-off within the hydrogen tanks. Work has previously been done on the method of 

eliminating boil-off in cryogenic systems for space applications [14]. 

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40 50 60 70 80

Percent Weight of Methane Added
NASA/CR—2002-211867 21 

The cryo-cooler is used to condense the hydrogen vapor that forms in the ullage (excess 

area in the tank not occupied by liquid hydrogen) of the tank. This vapor is generated 

by heat leakage into the tank through the insulation. By sizing the cryo-cooler to match 

the heat leakage, the system can effectively have no boil-off. A typical single stage cryocooler for reaching liquid hydrogen temperatures is shown in figure 12. 

Figure 12.—Single Stage 20 K Cryo-cooler [14]

The cryo-cooler needs to compensate for the leakage of heat into the tank through 

the insulation. The power consumption is based on the latent heat of vaporization of 

hydrogen and the rate of heat flow into the tank, previously given in equation 26. 
NASA/CR—2002-211867 22 

State of the art cryo-coolers require between 2.5 to 7.5 kW of power and provide 

cooling capacities of between 5 W and 18 W at 20 K. For this analysis the following 

specifications were used for the cryo-cooler sizing. The ratio of operating power to 

cooling power at temperature was estimated to be 0.45 kW/W. This was based on 

an average for the cryo-coolers examined. 

A single stage cryo-cooler, like that shown in figure 7, has a mass of 67 kg including 

the compressor unit (not shown in the picture). This cryo-cooler requires 1.6 kW of 

continuous power. The mass of the cryo-cooler per cooling capacity was estimated as 

0.0419 kg/W. 

A mass and power estimate for a zero boiloff system was made. The mass estimate 

included the tank mass, insulation mass, cryo-cooler mass and additional hydrogen mass 

needed to operate the cryo-cooler. The results of this analysis showed that the mass of a 

cryo-cooled zero boil-off system is much greater than that of a conventional cryo tank 

system that has boil-off. For all the cases examined the hydrogen required to produce the 

power needed to run the cryo-cooler was much greater then the boil off that would have 

occurred with out it. Also the mass of the cryo-coolers themselves increased the overall 

system mass by an order of magnitude. Based on these results it was obvious that a zero 

boil-off system was not beneficial for a system with an operating time on the order of 8 to 

12 hours or less. 

	




Another approach to storing hydrogen is to have it chemically combined with other 

elements to form a stable, easily handled (usually liquid) material that posses a relatively 

high weight fraction of hydrogen. The simplest method for this type of storage is to use a 

conventional hydrocarbon (such as gasoline or methane) as the storage compound. This 

greatly simplifies the transportation and storage of the hydrogen since there is extensive 

worldwide experience in handling and utilizing these compounds. The main issue with 

this type of hydrogen storage is that the hydrogen has to be released from the chemical 

compound in order to be used. This requires the consumption of energy to break the 

chemical bond as well as the potential for producing by-products that can be pollution 

causing. Even though energy needs to be added to liberate the hydrogen, the overall 

conversion efficiency of using the liberated hydrogen can still be higher than burning the 

compound directly. This is mainly due to the higher conversion efficiency of a fuel cell 

as compared to a conventional combustion engine. However, because this initial energy 

consuming step is required for this form of hydrogen storage, the complete system has to 

be analyzed (hydrogen liberation, consumption and conversion to the desired objective 

such as motion) to determine if this type of storage is more beneficial than utilizing the 

storage compound directly. 

The volumetric energy density of hydrogen within some common fuels is shown in 

table 5. 
NASA/CR—2002-211867 23 

TABLE 5.—HYDROGEN WEIGHT AND ENERGY DENSITY 

WITHIN COMMON FUELS [1] 

Fuel 

(in liquid state) 

Hydrogen Weight Fraction Hydrogen Volumetric 

Energy Density (MJ/liter) 

Hydrogen 1 8.4 

Methane 0.25 10.8 

Ethane 0.2 12.0 

Propane 0.18 10.6 

Gasoline 0.16 13.2 

Ethanol 0.13 12.3 

Methanol 0.12 11.9 

In addition to the fuels listed in table 4 other more non-conventional compounds can be 

utilized. Examples of these include ammonia and lithium hydride slurry. Ammonia is a 

non-flammable liquid that contains 1.7 times the hydrogen per unit of volume as liquid 

hydrogen. Also ammonia can be cracked easily into nitrogen and hydrogen in a single 

step conversion (2NH3 > 3H2 + N2) [2]. Lithium hydride slurry (LHS) provides 15.3% 

hydrogen by mass or 14.2 MJ/liter of energy storage. The advantage of LHS over other 

hydrocarbons is that it is storable at ambient pressure and temperature and the hydrogen 

within the slurry can be easily released by mixing it with water through a process called 

hydrolysis [3]. The hydrolysis is controlled by the addition of mineral oil to the mixture. 

This oil prevents runaway hydrolysis by limiting the hydride–water contact. This LHS 

storage system is presently under development, funded by the Department of Energy, by 

Thermo Technologies [3]. Operating systems utilizing this hydrogen storage concept 

have been demonstrated. In addition to lithium hydride, calcium hydride and magnesium 

hydride can also be used in a similar fashion to store hydrogen [4]. 

Hydrogen Reforming 

Reforming of most hydrocarbon fuels takes place through a reforming catalyst to produce 

hydrogen and by-product compounds such as carbon dioxide. Some of the more common 

fuels that have logistical advantages to their use include diesel and jet fuels. However 

these fuels present a difficulty in reforming due to their relatively high carbon and sulfur 

content which tend to saturate the reforming catalyst. This poisoning of the catalyst needs 

to be addressed to enable reformers to be able to utilize these types of fuels. 

The most common methods presently used for reforming hydrocarbon fuels is partial 

oxidation, steam reforming, thermal decomposition and exhaust gas reforming. Some of 

these methods however, are not applicable to the use of hydrogen as an on-board fuel. 

Of these partial oxidation and thermal decomposition would not be applicable methods. 

Partial oxidation requires more energy to produce the hydrogen than is available in the 

liberated hydrogen itself. Because of this, it would be more beneficial to utilize the stock 

fuel itself versus converting it to hydrogen through this method. Thermal decomposition 

requires the gasification of the hydrocarbon fuel and produces carbon solids, both of 

which are difficult to manage. The other methods, steam and exhaust gas reforming, are 

both potentially applicable to on-board hydrogen production. This is mainly because they 
NASA/CR—2002-211867 24 

can utilize the waste heat from the hydrogen consumption to augment the hydrogen 

production. This greatly enhances the overall efficiency of the system [16]. 

The most promising would be exhaust gas reforming which uses the waste heat from the 

hydrogen consumption and a catalyst bed to produce the hydrogen. A converter utilizing 

this method is being developed at Argonne National Laboratory [17]. This converter can 

be made very compact and potentially operate at lower temperatures then other hydrogen 

generating systems. It also has quick start-up and demand-following capabilities. The rate 

of hydrogen production is dependent on the rate of fuel and water flow into the reaction 

chamber. The reaction chamber consists of a catalyst filled cylinder. As fuel and water 

are sprayed in, an ignition source begins the reaction. This type of reformer can convert 

a wide variety of hydrocarbon based fuels to a hydrogen rich gas on the order of 60% 

hydrogen at temperatures of between 450 °C and 670 °C, depending on the type of fuel 

used. A test was run on an experimental version of this reformer using gasoline as the 

hydrogen source. It was capable of producing 40 L/min of hydrogen gas at standard 

atmospheric conditions for a 22 mL/min flow rate of gasoline, 24 mL/min flow rate of 

water and a 45 L/min flow rate of air [18]. This hydrogen production rate is sufficient to 

operate a 3 kW fuel cell stack. 





1. Thomas, G. “Overview of Storage Development DOE Hydrogen Program,”

Presentation—US DOE Hydrogen Program 2000 Annual Review, May 2000. 

2. Faleschini, G. et al., “Ammonia for High Density Hydrogen Storage,” 

3. McClaine A.W., Breault, R.W., Larsen, C., Konduri, R., Rolfe, J. and Becker, F., 

“Advanced Chemical Hydride Hydrogen-Generation/Storage System for PEM 

Fuel Cell Vehicles,” Thermo Technologies, DOE 2000 Progress Report Contract 

No. 98111. 

4. Euergentics Incorporated, “A Comprehensive Summary of Hydrogen R&D 

Technologies: Storage,” Final Report, U.S. Department of Energy contract 

DE–FG03–00SF22103, May 2001. 

5. Bolz, R.E. and Tuve, G.L. editors, Handbook of Tables for Applied Engineering 

Science, CRC Press 1973. 

6. Akyurtlu, A. and Akyurtlu, J.F., “Evaluation of On-Board Hydrogen Storage 

Methods for High-Speed Aircraft,” NASA/CR–187755, January 1991. 

7. Ergenics Corporation, Metal Hydride Tank Specifications, www.ergenics.com, 

June 2001. 

8. Energy Conversion Devices Inc., www.ovonics.com, June 2001. 

9. Rambach, G.D., “Hydrogen Transport and Storage in Engineered Glass Microspheres,” Lawrence Livermore National Laboratory, April 1994. 

10. Brewer, D.G., Hydrogen Aircraft Technology, CRC Press, 1991. 

11. Incropera, F.P. and DeWitt, D.P., Fundamentals of Heat and Mass Transfer, John 

Wiley and Sons 1990. 

12. Pelaccio, D.G., Palaszewski, B. and O’Leary R., “Preliminary Assessment of Using 

Gelled and Hybrid Propellant Propulsion for VTOL/SSTO Launch Systems”, NASA 

TM—1998-206306, February 1998. 
NASA/CR—2002-211867 25 

13. Wong, W., Starkovich, J., Adams, S. and Palaszewski, B., “Cryogenic Gellant and 

Fuel Formation for Metallized Gelled Propellants: Hydrocarbons and Hydrogen with 

Aluminum”, NASA TM–106698, June 1994. 

14. Plachta, D.W., “Hybrid Thermal Control Testing of a Cryogenic Propellant Tank,”

NASA TM—1999-209389, August 1999. 

15. Mitlitsky, F., Weisberg, A.H. and Myers, B., “Vehicular Hydrogen Storage Using 

Lightweight Tanks,” Proceedings of the 2000 US DOE Hydrogen Program Review, 

NREL/CP–570–28890. 

16. Wyszynski, J., “Onboard Generation of Hydrogen-Rich Gaseous Fuels-A Review”

International Journal of Hydrogen Energy, Vol 19, NO 7, pp 557–572, 1994. 

17. Argonne National Laboratory, Transportation Technology R&D Center, 

http://www.transportation.anl.gov/ttrdc/fuelcell/reformer.html , October 2001. 

18. Ahmed, S., Krumpelt, M., Kumar, R., Lee, S.H.D., Carter, J.D., Wilkenhoener, R. 

and Marshall, C., “ Catalytic Partial Oxidation Reforming of Hydrocarbon Fuels,”

1998 Fuel Cell Seminar, November 1998. 

19. Beer, F.P. and Johnston, E.R., Mechanics of Materials, McGraw Hill, 1981. 

20. Lovell, D.R., “Carbon Fiber Composite Reciprocating Guide Bar,” Carbon Fibers, 

Technology, Uses and Prospects, The Plastics and Rubber Institute, London, England, 

1986, pp 176–182. 


 !"


• Lynch, F.E. “Metal Hydrides—The Missing Link in Automotive Hydrogen 

Technology,” International Academy of Science—Hydrogen Tech Papers. 74001. 

• Billings. R.E., “Hydrogen Storage in Automobiles Using Cryogenics and Metal 

Hydrides,” International Academy of Science—Hydrogen Tech Papers. 74002. 

• Brewer, G.D., Morris, R.E., Davis, G.W., Versaw, E.F., Cunnington, Jr. G.R., 

Riple, J.C., Baerst, C.F., and Garmong, G., “Study of Fuel Systems for LH2 Fueled 

Subsonic Transport Aircraft”, Final Report, Lockheed-California Company, 

NAS 1–14614, December 1977. 

• Silverstein, A. and Hall, E.W., “Liquid Hydrogen as a Jet Fuel for High-Altitude 

Aircraft,” Research Memorandum, Lewis Flight Propulsion Laboratory, April 1955. 

• Fenn, D.B., Acker, L.W., Algranti, J.S., “Flight Operation of a Pump-Fed LiquidHydrogen Fuel System,” Technical Memorandum X–252, NASA Lewis Research 

Center, April 1960. 

• McKinney, C.D., Tarpley, W.B., “Gelling of Liquid Hydrogen, Final Report,”

Technidyne Incorporated, NAS 3–4186, NASA CR-54967, June 1966. 

• Brewer, G.D. and Morris R.E., “Study of LH2 Fueled Subsonic Passenger Transport 

Aircraft,” NASA CR–144935, January 1976. 

• Ewart, R.O., Dergance, R.H., “Cryogenic Propellant Densification Study,” NASA 

CR–159438, November 1978. 

• Hardy, T.L. and Whalen, M.V., “ Technology Issues Associated with Using Densified 

Hydrogen for Space Vehicles,” AIAA–92–3079, July 1992. 
NASA/CR—2002-211867 26 

• Lockheed Missiles and Space Company, “A Study of Hydrogen Slush and or 

Hydrogen Gel Utilization: Volume II Systems Optimization and Vehicle Application 

Studies,” Final Report, NAS 8–20342, July 1965. 

• Raman, Venki, “Research and Development of a PEM Fuel Cell, Hydrogen Reformer 

and Vehicle Refueling Facility,” 2001 DOE Hydrogen Program Review, NREL/CP–

570–30535. 
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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33

Hydrogen Storage for Aircraft Applications Overview

Anthony J. Colozza

Fuel tanks; Aircraft fuel systems; Fuel cells; Liquid hydrogen; Hydrogen

Unclassified -Unlimited

Subject Categories: 07 and 28 Distribution: Nonstandard

Analex Corporation

3001 Aerospace Parkway

Brook Park, Ohio 44142

Project Manager, Lisa Kohout, Power and On-Board Propulsion Technology Division, NASA Glenn Research Center,

organization code 5420, 216–433–8004.

Advances in fuel cell technology have brought about their consideration as sources of power for aircraft. This power

can be utilized to run aircraft systems or even provide propulsion power. One of the key obstacles to utilizing fuel

cells on aircraft is the storage of hydrogen. An overview of the potential methods of hydrogen storage was compiled.

This overview identifies various methods of hydrogen storage and points out their advantages and disadvantages

relative to aircraft applications. Minimizing weight and volume are the key aspects to storing hydrogen within an

aircraft. An analysis was performed to show how changes in certain parameters of a given storage system affect its

mass and volume.