WO2001081850A1

WO2001081850A1 - A hydrogen cooled hydrogen storage unit having maximized cooling efficiency

Info

Publication number: WO2001081850A1
Application number: PCT/US2001/011267
Authority: WO
Inventors: Stanford R. Ovshinsky; Rosa Young
Original assignee: Energy Conversion Devices, Inc.
Priority date: 2000-04-26
Filing date: 2001-04-06
Publication date: 2001-11-01
Also published as: EP1277021A1; AU2001251398A1; US6318453B1; TW496941B

Abstract

A hydrogen cooled hydrogen storage unit which employs excess hydrogen flow through flow channels (2) between hydrogen storage alloy plates (1) in order to provide convective cooling of the plates. The unit provides for high packing density of the storage materials and ease of expansion of storage capacity by merely adding more storage material plates (1). Since the hydrogen flows transversely between the plates and does not flow along the entire length of the stack, the cooling flow path of the hydrogen is short, and the temperature differential between any point of the stack and the hydrogen coolant is maximized, which maximizes the cooling efficiency of the unit.

Description

A HYDROGEN COOLED HYDROGEN STORAGE UNIT HAVING MAXIMIZED COOLING EFFICIENCY

FIELD OF THE INVENTION

The instant application relates to hydrogen cooled hydrogen storage units,

and more specifically to a unit which maximizes the cooling efficiency thereof, while

also providing for a high packing density of the storage materials therein, and ease

of expansion of storage capacity by merely adding more storage material plates.

BACKGROUND OF THE INVENTION

The instant patent application describes a hydrogen storage unit useful for

a hydrogen-based economy. The storage unit allows for fast and efficient cooling

and/or heating thereof using gaseous hydrogen as a direct, convective heat transfer

medium. The unit maximizes the cooling efficiency of the storage material therein

while providing for a high packing density of the storage materials and ease of

expansion of storage capacity by merely adding more storage material plates. The

instant storage unit is useful in a hydrogen-based economy.

An infrastructure for such a hydrogen-based economy is disclosed in U.S.

Application Serial No. 09/444,810, entitled "A Hydrogen-based Ecosystem" filed on

November 22, 1999 for Ovshinsky, et al. (the '810 application), which is hereby

incorporated by reference. This infrastructure, in turn, is made possible by

hydrogen storage alloys that have surmounted the chemical, physical, electronic

and catalytic barriers that have heretofore been considered insoluble. These alloys

are fully described in copending U.S. Patent Application Serial No. 09/435,497,

entitled "High Storage Capacity Alloys Enabling a Hydrogen-based Ecosystem", filed on November 6, 1999 for Ovshinsky et al. (the '497 application), which is

hereby incorporated by reference.

Hydrogen storage units have a number of requirements. First and foremost,

they are required to be able to store hydrogen. This bare minimal requirement is

met by many prior art storage units. However, to be commercially useful in a

hydrogen-based economy, the hydrogen storage unit requires many more

properties. One requirement is a high specific capacity hydrogen storage material.

Such materials were invented by the instant inventors and are disclosed in the '497

application. Another requirement is a unit which has a high volumetric and

gravimetric packing density of storage materials. One such unit is also disclosed

in the '497 application.

A further requirement is a unit that has the ability to be cooled a high rate.

This is required to be able to quickly charge hydrogen into the unit while maintaining

proper operating temperature by removing the heat of hydride formation. The

instant inventors have determined that maximal cooling using minimal hardware can

be achieved using excess hydrogen flow though the system to remove the heat. A

system which employs hydrogen cooling is also disclosed in the '497 application.

However, such a unit also requires high cooling efficiency. To achieve this maximal

efficiency, the temperature between the cooling hydrogen and the heated storage

materials must be maximized, while maintaining the storage material at its proper

operating temperature. This requires that the temperature rise of the cooling

hydrogen over the entire length of contact between the hydrogen and the storage

material be minimal. Prior hydrogen-cooled units fail to achieve this goal. In addition to being able to quickly and efficiently cool the hydrogen storage

materials within the storage unit, the hydrogen storage materials must be heated quickly and efficiently to release hydrogen therefrom during use. To accomplish

this, there must be efficient thermal transfer from the source thereof through the bulk

of the storage material. While most systems will transfer this heat, they do not do

so efficiently.

Finally, for many applications ease of capacity expansion is a must. While

many prior art systems do not allow for expansion, some do. However, with most

systems, expansion of the amount of hydrogen storage materials reduces the

cooling and heating efficiency.

Therefor, what is needed in the art is a high capacity hydrogen storage unit

having high volumetric and gravimetric storage capacity (i.e., high packing density

of storage materials), which is capable if being cooled at a high rate with maximal

efficiency, is capable of efficient heat transfer from an internal heat source

throughout the storage material, and is expandable without losing any of the aforementioned properties.

SUMMARY OF THE INVENTION

The instant invention is a hydrogen-cooled hydrogen storage unit. The unit

employs excess hydrogen flow through flow channels between hydrogen storage

alloy plates in the hydrogen storage unit in order to provide convective cooling of

the plates. The unit provides for high packing density of the storage materials and

ease of expansion of storage capacity by merely adding more storage material

plates. Also, because the hydrogen flows transversely between the plates, not along the entire length of the stack, the cooling flow path of the hydrogen is much

shorter, and the temperature differential between any point of the stack and the

hydrogen coolant is maximized, which maximizes the cooling efficiency of the unit.

The unit also allows for efficient radial thermal transfer of heat energy from a central

source of heat through the plates thereof during discharge of the stored hydrogen.

The hydrogen-gas-cooled hydrogen storage unit includes a stack of a

plurality of hydrogen storage alloy plates disposed within a storage unit casing. The unit also includes hydrogen flow channels between the hydrogen storage alloy

plates. The flow channels provide for transverse pathways between the plates to

allow for high speed hydrogen gas flow. A portion of the hydrogen is stored within

the storage material and releases its heat of hydride formation. The remainder of

the hydrogen flows through the hydrogen flow channels at a sufficient mass flow

rate to remove the heat of hydride formation.

The unit casing also includes a hydrogen inlet port, a hydrogen inlet

manifold, a hydrogen outlet manifold, a hydrogen outlet port and hydrogen

deflection walls, which are designed and integrated such that hydrogen: 1 ) flows

into the unit through the hydrogen inlet port, 2) is distributed throughout said

hydrogen inlet manifold, 3) is then forced though said hydrogen flow channels by

said hydrogen deflection walls, 4) is collected in said hydrogen outlet manifold, and

5) exits the unit though said hydrogen outlet port.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic depiction, not to scale, of a stack of hydrogen storage

plates and flow channels according to the instant invention; Figure 2 is a cross sectional view of a schematic depiction, not to scale, of

the hydrogen storage unit of the instant invention;

Figure 3 is another cross sectional view of the hydrogen storage unit of the

instant invention, specifically delineating the positioning of the hydrogen deflection

walls and the central opening wall;

Figure 4 is a stylistic depiction of a hydrogen refueling station;

Figure 5 shows a schematic representation of a hydrogen gas supply system

for powering an internal combustion engine vehicle; and

Figure 6 shows a schematic representation of a hydrogen gas supply system

for powering for a fuel cell vehicle.

DETAILED DESCRIPTION OF THE INVENTION

There is disclosed herein a hydrogen-cooled hydrogen storage unit. The unit

employs excess hydrogen flow through flow channels between hydrogen storage

the plates. The storage unit of the instant invention provides for very good packing

density of the storage materials and ease of expansion of storage capacity, by

merely adding more storage material plates. Also, the addition of more plates does

not interfere with the proper hydrogen cooling of the storage unit of the instant

invention because of its unique design. That is, because the hydrogen flows

transversely between the plates and does not flow along the entire length of the

stack, the cooling flow path of the hydrogen is much shorter, and thus the temperature rise of any of the cooling hydrogen is much smaller. Thus, the temperature differential between any point of the stack and the hydrogen coolant

is maximized, which maximizes the cooling efficiency of the unit.

Any alloy which safely and efficiently stores and releases hydrogen may be

used in the storage unit of the instant invention. Specifically useful are alloys such

as Ti-Zr based room temperature hydrogen storage alloys and high capacity

Mg-based storage alloys. The most useful alloys are the high-capacity, high-

kinetics hydrogen storage alloys of the '497 application. In general the alloys

contain greater than about 90 weight % magnesium, and contain at least one

modifier element. The at least one modifier element creates a magnesium based

alloy which is capable of storing at least 6.9 weight % hydrogen and is capable of

absorbing 80% of the full storage capacity of hydrogen in under 1.5 minutes at 300

°C. The modifier elements mainly include Ni and Mm (misch metal) and can also

include additional elements such as Al, Y and Si. Thus the alloys will typically

contain 0.5-2.5 weight % nickel and about 1.0-4.0 weight % Mm (predominantly

contains Ce and La and Pr). The alloy may also contain one or more of 3-7 weight

% Al, 0.1 -1.5 weight % Y and 0.3-1.5 weight % silicon.

Figure 1 is a schematic depiction, not to scale, of a stack of hydrogen storage

plates 1 and flow channels 2 according to the instant invention. During charging,

hydrogen flows through the flow channel regions 2. Some of the hydrogen is

adsorbed into the hydrogen storage alloy and the excess hydrogen passes through

the flow channel region and out the other side. As hydrogen is adsorbed into the

hydrogen storage material of the plates 1 , it releases it's heat of hydride formation,

which is carried away by the excess hydrogen that passes through the channel

regions 2. The stack of plates and flow channels include a central opening 3 into which a heater can be place to allow the stack to be heated during discharge, thereby providing the heat necessary to desorb the hydrogen.

Figure 2 is a cross sectional view of a schematic depiction, not to scale, of

the hydrogen storage unit of the instant invention. The unit specifically includes a

casing 4, which houses stack of storage material plates 1 and flow channels 2. The

casing 4 includes a hydrogen inlet port 6, though which hydrogen flows into the unit

during charging, and a hydrogen outlet port 7, through which hydrogen flows out of

the unit during charging and discharging. The unit also includes hydrogen inlet

manifold 8 region which distributes the incoming hydrogen among all of the flow

channel regions 2, and a hydrogen outlet manifold 9 which collects the excess

hydrogen flowing out of the flow channel regions 2. The unit also includes hydrogen

deflection walls 5, which extend the entire length of the casing 4. The deflection

walls 5 prevent hydrogen from passing directly from the inlet manifold 8 to the outlet

manifold 9, thus forcing the hydrogen to flow through channel regions 2. Finally, the

unit includes a central opening wall 10. This central opening wall 10 prevents

interaction between the hydrogen/storage plates 1 /flow channels 2 and the interior

of the central opening 3. Thus, hydrogen which is in the storage plates 1 and flow

channels 2 cannot escape into the central opening 3, and any gases or other

materials in the central opening 3 do not interact with anything within the unit. For

example, one of the heating units that may be placed withing the central opening

3 is a catalytic combustor. Such a heating unit combusts a fuel source (such as

hydrogen) to produce heat. The central opening wall prevents the fuel, oxygen and

exhaust gases from escaping from the central opening into the storage plates 1 and

flow channels 2. Figure 3 is another cross sectional view of the hydrogen storage unit of the instant invention. In this view the positioning of the hydrogen deflection walls 5 and

the central opening wall 10 is more clearly delineated. As can be seen, the

hydrogen deflection walls 5 prevent hydrogen from passing directly from the inlet

manifold 8 to the outlet manifold 9. It should be noted that while there is the

possibility that the hydrogen flow may be "short circuited" around the deflection walls 5 (though a very short path in the flow channels 2), measures can be taken

to prevent this from happening such as grading of the flow channel 2 porosity and

size and/or grading of the thickness of the storage palates 1.

The hydrogen storage plates 1 are compacted/sintered from hydrogen

storage alloy. The hydrogen storage alloy may be, if needed, bonded to a support

means. The support means may be formed from a variety of materials with the

appropriate thermodynamic characteristics that can help to provide heat transfer

into and out of said hydrogen storage alloy. The useful materials for the support

means include both metals and non-metals. Preferable metals include those from

the group consisting of Ni, Al, Cu, Fe and mixtures or alloys thereof. Examples of

support means that can be formed from metals include mesh, grid, matte, foil, foam

and plate. This support material may also store hydrogen, which may improve the

overall performance of the storage unit.

The hydrogen storage alloy material may be physically bonded to the support

means by compaction and/or sintering processes. The alloy material is first

converted into a fine powder. The powder is then compacted, optionally onto the

support means, to form plates of storage material. The compaction process causes the powder to adhere to itself and, if included, become an integral part of the support means. After compaction, the plates, including any support means, that

have been impregnated with alloy powder are preheated and sintered, if necessary.

The preheating process liberates excess moisture and discourages oxidation of the

alloy powder. Sintering is carried out in a high temperature, substantially inert

atmosphere containing hydrogen. The temperature is sufficiently high to promote

particle-to-particle bonding of the alloy material as well as the bonding of the alloy

material to any support means.

Preferably the hydrogen storage alloy is compacted in hydride form. This is

because the hydrogen storage alloys expand and contract as they are hydrided and

dehydrided, respectively. When the materials are compacted in the non-hydride

state, they expand during initial charging with hydrogen and then contract again

when the hydrogen is removed (although not to the original size). This expansion

during initial cycling causes the compacted material to disintegrate, risking

entrainment of the storage material in the hydrogen flow. However, when the alloy

is compacted in hydride form, all of the initial expansion is built in to the compact,

thus when the material is cycled the effects of expansion/contraction is minimized.

It should be noted that the compacts may be formed in the desired end shape by

this process, thereby minimizing additional machining to form the final shape of the

storage material. Also, any additional components of the system may be molded

directly into the shape. Thus, any flow channels, or heating elements, etc. may be

directly integrated with the formed shaped compact.

The formed hydrogen storage alloy compact may be encapsulated by a

porous encapsulant material which prevents entrainment of the hydrogen storage

material in the high flow rate hydrogen. The encapsulant sheet can be pressed onto the storage alloy compact during formation. While the porous sheet may be formed

from polymers it is preferably composed of material which is thermally conductive,

such as metals, or even thermally conductive non-metals. Examples of polymers

would be porous polypropylene sheet porous polytetrafluoroethylene sheet. The

metal may be capable of storing hydrogen also, thereby enhancing the overall

storage capacity of the system. Useful metals include Cu, Ni and Al and mixtures

or alloys thereof. Useful non-metals can include thermally conductive ceramics and

thermally conductive graphite materials. Once again, the non-metal may also store

hydrogen, thereby enhancing the storage capacity of the system.

Compacting and sintering the alloy material onto a support means increases

the packing density of the alloy material, thereby improving the thermodynamic and

kinetic characteristics of the hydrogen storage system. The close contact between

the support means and the alloy material improves the efficiency of the heat transfer

into and out of the hydrogen storage alloy material as hydrogen is absorbed and

desorbed. In addition, the uniform distribution of the storage alloy/support means

throughout the interior of the container provides for an even temperature and heat

distribution throughout the bed of alloy material. This results in more uniform rates

of hydrogen absorption and desorption throughout the entirety thereof, thus creating

a more efficient energy storage system.

Once the compacted storage material plates 1 are produced, they are

stacked with flow channel materials 2. The flow channel are preferably formed from

very porous materials. The flow channel regions 2 provide for high speed flow of

hydrogen through the storage bed, allowing for hydrogen cooling of the bed. Some

of the hydrogen is adsorbed into the alloy material, and the remainder of the hydrogen carries away the heat of hydride formation. The porous materials are preferably selected form foam, matte, mesh or expanded materials. They are

preferably formed from metals, but may also be formed from thermally conductive

non-metals. The materials from which the flow channels are formed should be non-

reactive with the storage unit materials, but they may also store hydrogen.

Exemplary materials are metals such as nickel, aluminum, and copper and thermally

conductive non-metals such carbon materials. The most preferred materials are

nickel or aluminum foams. The flow channels should be highly porous to allow for

high speed flow of hydrogen, but must still provide for mechanical support of the

storage plate stack. The foam flow channel regions are preferably between 0.1 and

2 mm thick.

As an alternative to using specific materials to form the flow channels 2, the

hydrogen storage plates may be corrugated on one or both surfaces such that when

they are stacked (preferably with the encapsulant material discussed above), the

stack will have incorporated, therewithin, hydrogen flow channels 2 due to the

corrugation. This alternative method of forming the flow channels 2 has the

advantage of reducing the overall weight of the storage unit, while maximizing the

storage capacity thereof.

The storage plates 1 may be of any size and shape, but preferably are in the

shape of disks. The disks are preferably between 5mm and 3cm thick, and have

diameters up to 12 inches. The disks preferably have a central annular opening 3,

into which (when incorporated into the stack) combustive or electric heaters may be

inserted to assist in release of the hydrogen, if needed for the end use applications.

The plates can be 100 percent dense (with respect to the hydride state) or can have porosity "built in" to compensate for expansion/contraction during cycling. Once the

plates are manufactured, adding capacity to any hydrogen storage unit is as simple

as packing the required number of plates into a single or multiple casings 4.

Figure 4 is a stylistic depiction of a hydrogen refueling station which

specifically shows how hydrogen is used to capture the heat of hydride formation

in the vehicles storage bed 11 and transfer that heat to the stations primary hydride

storage bed 12 to assist in the release of hydrogen from the primary storage bed.

Specifically, high flow rate hydrogen is dispensed from the "pump" 13 into the

vehicle's hydrogen storage bed 11 through cool hydrogen supply line 14a. Some

of the hydrogen is absorbed into the hydrogen storage material within the bed,

thereby releasing heat of hydride formation. This heat is removed by the excess

cool hydrogen. The now heated hydrogen leaves storage bed 11 and is transported

to the pump 13 via hot hydrogen return line 15a. The hot hydrogen is then

transported from the pump 13 to the stations primary hydrogen storage bed 12 via

hot hydrogen return line 15b. The hot hydrogen releases its heat into the hydrogen

storage material within bed 12 to assist in providing the required heat (heat of

dehydriding) to release the stored hydrogen therein. The released hydrogen, now

cooler, is supplied to the pump 13, via cool hydrogen supply line 14b, to ultimately

be sent again to the vehicles hydrogen storage bed 11. This set up allows for very

fast charging of a vehicle's storage bed 11 , and yet eliminates the loss of the

released heat and overheating of the bed.

Hydrogen Powered Internal Combustion Engine and Fuel Cell Vehicles

The instant storage unit is useful as a hydrogen supply for many applications.

One such application is the field of automobiles. Specifically, the storage unit can be used as a source of hydrogen for internal combustion engine (ICE) or fuel cell

(FC) vehicles.

Figure 5 shows a schematic representation of a hydrogen gas supply system

for an ICE vehicle, which is for supplying a hydrogen engine 21 with hydrogen gas.

The system has a hydrogen gas storage bed 11 and an engine waste heat transfer

supply passage 23 which leads engine waste heat (in the form of exhaust gas or

engine coolant) discharged from the engine 21 to the hydrogen gas storage bed 11.

The system also includes a return passage 24 for returning any engine coolant used

to heat the hydrogen storage material back to the engine 21 and an exhaust gas

vent 27 for releasing used exhaust gas. The system further includes a hydrogen

gas supply passage 25 which leads hydrogen gas from the hydrogen gas storage

bed 11 to the engine 21. The engine waste heat transfer supply passage 23 is

provided with a temperature regulating unit 26 which regulates the temperature of

the waste heat to be introduced into the hydrogen gas storage bed 11. With such

a system, waste heat generated within the ICE can be efficiently used to heat the

hydrogen storage material to release hydrogen therefrom for use in the ICE.

Figure 6 shows a schematic representation of a hydrogen gas supply system

for an FC vehicle, which is for supplying a fuel cell 28 with hydrogen gas. The

system has a hydrogen gas storage bed 1 1 and a fuel cell waste heat/hydrogen

transfer supply passage 29 which leads fuel cell waste heat and unused hydrogen

discharged from the fuel cell 28 to a hydrogen gas combustor 30. Waste heat from

the fuel cell may be in the form of heated gases or heated aqueous electrolyte. The

hydrogen combustor 30, heats a thermal transfer medium (preferably in the form of

the aqueous electrolyte from the fuel cell) utilizing waste heat from the fuel cell 28, and by combusting hydrogen. Hydrogen is supplied to the combustor 30 via unused hydrogen from the fuel cell 28, and via fresh hydrogen supplied from the hydrogen

storage bed 11 via hydrogen supply line 34. Heated thermal transfer medium is

supplied to the hydrogen storage bed 11 via supply line 33. The system also

includes a return passage 36 for returning any fuel cell aqueous electrolyte used to

heat the hydrogen storage material back to the fuel cell 28 and an exhaust gas vent 35 for releasing used combustor gas. The system further includes a hydrogen gas

supply passage 31 which leads hydrogen gas from the hydrogen gas storage bed

11 to the fuel cell 28.

While the invention has been described in connection with preferred

embodiments and procedures, it is to be understood that it is not intended to limit the invention to the described embodiments and procedures. On the contrary it is

intended to cover all alternatives, modifications and equivalence which may be

included within the spirit and scope of the invention as defined by the claims

appended hereinafter.

Claims

We claim:

1. A hydrogen gas cooled hydrogen storage unit comprising:

a storage unit casing,

a stack of a plurality of hydrogen storage alloy plates disposed within said

storage unit casing;

hydrogen flow channels provided between said hydrogen storage alloy

plates, said flow channels providing transverse pathways between said hydrogen storage alloy plates to allow for high speed hydrogen gas flow, a portion of said

hydrogen being stored within said storage material and releasing heat of hydride

formation and the remainder of said hydrogen flowing through said hydrogen flow

channels at a sufficient mass flow rate to remove said heat of hydride formation;

said unit casing including: a hydrogen inlet port; a hydrogen inlet manifold;

a hydrogen outlet manifold; a hydrogen outlet port and hydrogen deflection walls;

and

wherein said hydrogen inlet port, said hydrogen inlet manifold, said hydrogen

outlet manifold, said hydrogen outlet port and said hydrogen deflection walls are

designed and integrated such that hydrogen:

1 ) flows into the unit through the hydrogen inlet port, 2) is distributed

throughout said hydrogen inlet manifold, 3) is then forced though said hydrogen flow

channels by said hydrogen deflection walls, 4) is collected in said hydrogen outlet

manifold, and 5) exits the unit though said hydrogen outlet port.

2. The hydrogen gas cooled hydrogen storage unit of claim 1 , wherein said hydrogen storage alloy plates are formed from compacted powered hydrogen

storage alloy materials.

3. The hydrogen gas cooled hydrogen storage unit of claim 2, wherein said

hydrogen storage alloy plates are formed form compacted powered alloy physically

bonded to a support means.

4. The hydrogen gas cooled hydrogen storage unit of claim 3, wherein said

hydrogen storage alloy is physically bonded to said support means by compaction

and/or sintering.

5. The hydrogen gas cooled hydrogen storage unit of claim 3, wherein said

support means comprises at least one selected from the group consisting of mesh,

grid, matte, foil, foam and plate.

6. The hydrogen gas cooled hydrogen storage unit of claim 3, wherein said

support means is formed from a metal.

7. The hydrogen gas cooled hydrogen storage unit of claim 6, wherein said

support means is formed from one or more metals selected from the group

consisting of Ni, Al, Cu, Fe and mixtures or alloys thereof.

8. The hydrogen gas cooled hydrogen storage unit of claim 2, wherein said

storage alloy plates are formed from compacted hydrided powered alloy materials.

9. The hydrogen gas cooled hydrogen storage unit of claim 8, wherein said

storage alloy plates are formed into disks.

10. The hydrogen gas cooled hydrogen storage unit of claim 8, wherein said

disks are between 5mm and 3cm in thickness and are up to 12 inches in diameter.

11. The hydrogen gas cooled hydrogen storage unit of claim 1 , wherein said

hydrogen flow channels are formed from a porous material disposed between said

storage alloy plates.

12. The hydrogen gas cooled hydrogen storage unit of claim 11 , wherein said

porous material is a porous mesh, grid, matte, foam or expanded metal.

13. The hydrogen gas cooled hydrogen storage unit of claim 12, wherein said

porous material is formed from nickel, aluminum, copper or carbon.

14. The hydrogen gas cooled hydrogen storage unit of claim 13, wherein said

hydrogen flow channels are between 0.1 mm and 2mm thick.

15. The hydrogen gas cooled hydrogen storage unit of claim 1 , wherein said

hydrogen flow channels are formed by corrugating at least one surface of said

plates before forming said stack.

16. The hydrogen gas cooled hydrogen storage unit of claim 15, wherein said

plates are further surrounded by a porous encapsulating material.

17. The hydrogen gas cooled hydrogen storage unit of claim 16, wherein said

porous encapsulating material surrounding said hydrogen storage alloy plates is a

thin, porous sheet.

18. The hydrogen gas cooled hydrogen storage unit of claim 17, wherein said

thin, porous polymer sheet.

19. The hydrogen gas cooled hydrogen storage unit of claim 18, wherein said

thin, porous polypropylene sheet.

20. The hydrogen gas cooled hydrogen storage unit of claim 18, wherein said

thin, porous polytetrafluoroethylene sheet.

21. The hydrogen gas cooled hydrogen storage unit of claim 17, wherein said porous encapsulating material surrounding said hydrogen storage alloy plates is a

thin, porous metal sheet.

22. The hydrogen gas cooled hydrogen storage unit of claim 21 , wherein said

porous metal sheet is formed from at least one metal selected from Ni, Al, Cu, and mixtures or alloys thereof.

23. The hydrogen gas cooled hydrogen storage unit of claim 17, wherein said

thin, porous graphite sheet.