Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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0 SELF-HEATING METAL-HYDRIDE HYDROGEN STORAGE SYSTEM
FIELD OF THE INVENTION
The present invention deals with metal-hydride hydrogen
storage system. More specifically the present invention deals
with a self-heating metal-hydride hydrogen storage system.
BACKGROUND OF THE INVENTION
Growing energy needs have prompted specialists to take
cognizance of the fact that the traditional energy resources,
such as coal, petroleum or natural gas, are not inexhaustible,
or at least that they are becoming costlier all the time, and
that it is advisable to consider replacing them gradually with
other energy sources, such as nuclear energy, solar energy, or
geothermal energy. Hydrogen, too, is coming into use as an
energy source.
Hydrogen may be used, for example, as fuel for
internal-combustion engines in place of hydrocarbons. In this
case it has the advantage of eliminating atmospheric pollution
through the formation of oxides of carbon or of sulfur upon
combustion of the hydrocarbons. Hydrogen may also be used to
fuel hydrogen-air fuel cells for production of the electricity
needed for electric motors.
One of the problems posed by the use of hydrogen is its
storage and transportation. A number of solutions have been
proposed. Hydrogen may be stored under high pressure in steel
cylinders, but this approach has the drawback of requiring
hazardous and heavy containers which are difficult to handle
(in addition to having a low storage capacity of about 1o by
weight). Hydrogen may also be stored in cryogenic containers,
but this entails the disadvantages associated with the use of
cryogenic liquids; such as, for example, the high cost of the
containers, which also require careful handling. There are
also "boil off" losses of about 2-5o per day.
Another method of storing hydrogen is to store it in the
form of a hydride, which then is decomposed at the proper time
to furnish hydrogen. The hydrides of iron-titanium,
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lanthanum-nickel, vanadium, and magnesium have been used in
this manner, as described in French Pat. No. 1,529,371.
The MgH2-Mg system is the most appropriate of all known
metal-hydride and metal systems that can be used as reversible
hydrogen-storage systems because it has the highest percentage
by weight (7.650 by weight) of theoretical capacity for
hydrogen storage and hence the highest theoretical energy
density (2332 Wh/kg; Reilly & Sandrock, Spektrum der
Wissenschaft, Apr. 1980, 53) per unit of storage material.
Although this property and the relatively low price of
magnesium make the MgH2-Mg seem the optimum hydrogen storage
system for transportation, for hydrogen-powered vehicles that
is, its unsatisfactory kinetics have prevented it from being
used up to the present time. It is known for instance that
pure magnesium can be hydrided only under drastic conditions,
and then only very slowly and incompletely. The dehydriding
rate of the resulting hydride is also unacceptable for a
hydrogen storage material (Genossar & Rudman, Z. f. Phys.
Chem., Neue Folge 116, 215 [1979], and the literature cited
therein).
Moreover, the hydrogen storage capacity of a magnesium
reserve diminishes during the decomposition-reconstitution
cycles. This phenomenon may be explained by a progressive
poisoning of the surface, which during the reconstitution
renders the magnesium atoms located in the interior of the
reserve inaccessible to the hydrogen.
To expel the hydrogen in conventional magnesium or
magnesium/nickel reserve systems, temperatures of more than
250°C. are required, with a large supply of energy at the same
time. The high temperature level and the high energy
requirement for expelling the hydrogen have the effect that,
for example, a motor vehicle with an internal combustion
engine, cannot exclusively be operated from these stores.
This occurs because the energy contained in the exhaust gas,
in the most favorable case (full load), is sufficient for
meeting 500 of the hydrogen requirement of the internal
combustion engine from a magnesium or magnesium/nickel store.
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Thus, the remaining hydrogen demand must be taken from a
hydride store. For example, this store can be titanium/iron
hydride (a typical low-temperature hydride store) which can be
operated at temperatures down to below 0°C. These
low-temperature hydride stores have the disadvantage of only
having a low hydrogen storage capacity.
Storage materials have been developed in the past, which
have a relatively high storage capacity but from which
hydrogen is nevertheless expelled at temperatures of up to
about 250°C. U.S. Pat. No. 4,160,014 describes a hydrogen
storage material of the formula Ti~l - X~Zr~X~Mn~2 - y - Z~Cr~y~V~Z~,
wherein x = 0.05 to 0.4, y = 0 to 1 and z - 0 to 0.4. Up to
about 2 o by weight of hydrogen can be stored in such an alloy.
In addition to this relatively low storage capacity, these
alloys also have the disadvantage that the price of the alloy
is very high when metallic vanadium is used.
Moreover, U . S . Pat . No . 4, 111, 68 9 has disclosed a storage
alloy which comprises 31 to 46% by weight of titanium, 5 to
33o by weight of vanadium and 36 to 53o by weight of iron
and/or manganese. Although alloys of this type have a greater
storage capacity for hydrogen than the alloy according to U : S .
Pat. No. 4,160,014, hereby incorporated by reference, they
have the disadvantage that temperatures of at least 250°C are
necessary in order to completely expel the hydrogen. At
temperatures of up to about 100°C., about 800 of the hydrogen
content can be discharged in the best case. However, a high
discharge capacity, particularly at low temperatures, is
frequently necessary in industry because the heat required for
liberating the hydrogen from the hydride stores is often
available only at a low temperature level.
In contrast to other metals or metal alloys, especially
such metal alloys which contain titanium or lanthanum,
magnesium is preferred for the storage of hydrogen not only
because of its lower material costs, but above all, because of
its lower specific weight as a storage material. However, the
hydriding
Mg + H2 ~ MgH2
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is, in general, more difficult to achieve with magnesium,
inasmuch as the surface of the magnesium will rapidly oxidize
in air so as to form stable Mg0 and/or Mg(OH)2 surface layers.
These layers inhibit the dissociation of hydrogen molecules,
as well as the absorption of produced hydrogen atoms and their
diffusion from the surface of the granulate particles into the
magnesium storage mass.
Intensive efforts have been devoted in recent years to
improve the hydriding ability of magnesium by doping or
alloying it with such individual foreign metals as aluminum
(Douglass, Metall. Trans. 6a, 2179 [1975]) indium (Mintz,
Gavra, & Hadari, J. Inorg. Nucl. Chem. 40, 765 [1978]), or
iron (Welter & Rudman, Scripta Metallurgica 16, 285 [1982]),
with various foreign metals (German Offenlegungsschriften 2
846 672 and 2 846 673), or with intermetallic compounds like
Mg2Ni or Mg2Cu (Wiswall, Top Appl. Phys. 29, 201 [1978] and
Genossar & Rudman, op. cit.) and LaNi5 (Tanguy et al., Mater.
Res. Bull. 11, 1441 [1976]).
Although these attempts did improve the kinetics
somewhat, certain essential disadvantages have not yet been
eliminated from the resulting systems. The preliminary
hydriding of magnesium doped with a foreign metal or
intermetallic compound still demands drastic reaction
conditions, and the system kinetics will be satisfactory and
the reversible hydrogen content high only after many cycles of
hydriding and dehydriding. Considerable percentages of
foreign metal or of expensive intermetallic compound are also
necessary to improve kinetic properties. Furthermore, the
storage capacity of such systems are generally far below what
would theoretically be expected for MgH2.
Traditional ambient temperature metal hydrides suffer
from low gravimetric hydrogen storage densities of normally
less than 2 weight percent. Potential storage alloys which
have gravimetric storage densities of greater than 3 weight
percent tend to require high temperatures (>200°C) for
desorption. Magnesium-based alloys are considered to be very
promising for storage alloys due to their high potential
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gravimetric storage densities and the low cost of Mg. However
these alloys normally have poor properties such as slow
kinetics, intolerance to surface poisoning and require high
temperatures, typically around 300°C.
Energy Conversion Devices, Inc. has investigated and
developed Mg-based hydrogen storage alloys for improved
storage properties, such as rapid kinetics, high cyclability
and tolerance to surface poisoning. Formation of these alloys
by a mechanical alloying process has been found to be
successful. The mechanical alloying produces a fine multi-
phase powdered alloy which can be readily activated under mild
conditions, has rapid sorption kinetics and long cycle-life
with tolerance to surface poisoning. However these multi-
phase alloys still require temperatures of 250-350°C to desorb
all of the stored hydrogen at atmospheric pressure and have
strong enthalpies of formation (OHf), typically in the range
of about -60 to -75 kJ per mol of H2.
The high temperatures and heat of desorption cannot be
provided by most hydrogen-use applications. Even internal
combustion engines (ICE), especially when highly optimized,
may not be able to provide sufficient heat to desorb the
hydrogen at the required rates. Therefore another method of
heating the hydrides is needed to successfully use these Mg-
based alloys as practical hydrogen storage materials. As
noted the ~Hf are -60 to -75 kJ/mol H2. The higher and lower
heats of combustion for H2 are: 286.6 and 242.3 kJ/mol H2
respectively. Three to four times the amount of heat is
released from the combustion of hydrogen than is absorbed by
the desorption of hydrogen from the Mg-based hydrides.
Therefore it is possible to design a system which uses a
portion of the stored hydrogen to provide the necessary heat
of desorption. The apparent gravimetric H2 density of the
system would be approximately 2/3 the actual H2 density. For
instance, an alloy with 6 wt.o H2 density would have an
apparent H2 density of about 4 wt. o. The instant invention
provides such a system.
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SUMMARY OF THE INVENTION
An objective of the present invention is to provide a
metal-hydride hydrogen storage system that can be used to
store hydrogen in a high-temperature, high-capacity metal
s hydride hydrogen storage material.
This objective is satisfied by a self-heating metal-
hydride hydrogen storage system comprising: a primary metal-
hydride storage container housing a metal-hydride hydrogen
storage material, the primary storage container having a
hydrogen outlet port; a hydrogen combustor housed within the
primary metal-hydride storage container and surrounded by the
high-temperature storage material, the combustor having a
hydrogen inlet port gaseously connected to the hydrogen outlet
port of the primary storage container; and means for supplying
hydrogen gaseously connected to the hydrogen inlet port of the
combustor.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a highly stylized view of the metal-hydride
hydrogen storage system of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
A detailed embodiment of the invention is shown in Figure
1. As shown in Figure 1, the high-temperature metal-hydride
hydrogen storage system of the present invention comprises a
primary metal-hydride hydrogen storage container 5. The
metal-hydride hydrogen storage container 5 is preferably made
from a thermally nonconductive material. Heat is transferred
away from a metal hydride material during the hydrogen
absorption process (exothermic) and transferred into the metal
hydride material during the hydrogen desorption process
(endothermic).
Housed within the primary storage container 5 is a metal
hydride hydrogen storage material. Preferably, the metal
hydride hydrogen storage material housed within the primary
storage container 5 is a high-temperature metal-hydride
hydrogen storage material. As used herein, a high-temperature
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metal-hydride hydrogen storage material is a metal-hydride
hydrogen storage material which will have a 1 atmosphere
equilibrium plateau pressure at temperatures at or above
100°C. The high-temperature storage material may be a
magnesium-based hydride alloy (also known as an "Mg-based"
hydride alloy) such as MgH2. Other examples of Mg-based
hydrides that can be used as the high-temperature storage
material are discussed in commonly assigned U.S. Patent
Application No. 08/730,274, the disclosure of which is
incorporated by reference herein.
The metal-hydride hydrogen storage material housed within
the primary storage container may be physically bonded to a
support means. Generally, the support means can take the form
of any structure than can hold the hydridable material.
Examples of support means include, but are not limited to
mesh, grid, matte, foil, foam and plate. Each may exist as
either a metal or a non-metal. The support means may be
formed from a variety of materials with the appropriate
thermodynamic characteristics that can provide the necessary
heat transfer mechanism. These include both metals and non-
metals. Preferable metals include those from the group
consisting of Ni, A1, Cu, Fe and mixtures or alloys thereof.
Examples of support means that can be formed from metals
include wire mesh, expanded metal and foamed metal.
The metal-hydride hydrogen storage material may be
physically bonded to the support means by compaction and
sintering processes. The hydridable material is first
converted into a fine hydridable power. The hydridable powder
is then compacted onto the support means. The compaction
process causes the hydridable powder to adhere to and become
an integral part of the support means. After compaction, the
support means that has been impregnated with hydridable powder
is preheated and then sintered. The preheating process
liberates excess moisture and discourages oxidation of the
hydridable powder. Sintering is carried out in a high
temperature, substantially inert atmosphere containing
hydrogen. The temperature is sufficiently high to promote
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particle-to-particle bonding of the hydridable material as
well as the bonding of the hydridable material to the support
means.
The support means/hydridable material can be packaged
within the primary storage container in many different
configurations. Examples are provided in commonly assigned
U.S. Patent Application 08/623,497, the disclosure of which is
incorporated by reference herein. The hydrogen that is
stored in the hydridable metal-hydride storage material may
flow out of the primary storage container 5 via the hydrogen
outlet port 8. This port permits the flow of hydrogen gas
into and out of the primary container 5.
The primary metal-hydride hydrogen storage container of
the present invention also comprises at least one hydrogen
combustor that is housed within the primary storage container
5. In the embodiment shown in Figure 1, the primary metal-
hydride hydrogen storage container houses a single hydrogen
combustor 30. The hydrogen combustor 30 has a hydrogen inlet
port 31 which is gaseously connected to the hydrogen outlet
port 8 of the storage container 5 via an external manifold 23.
The hydrogen combustor 30 is surrounded by the metal-hydride
storage material and provides an internal heat source for
heating the material. The combustor 30 comprises a nonporous
chamber 32. The chamber 32 is made from a "nonporous"
material which does not permit the flow of either oxygen,
hydrogen or water between the interior of the combustor and
the metal-hydride material. Preferably, the chamber 32 is
formed from a nonporous, thermally conductive material.
Housed within the chamber 32 is a catalyst 30. Generally, the
catalyst 30 is one which can lower the activation energy of
hydrogen combustion. Preferably, the catalyst 30 comprises
Pd. More preferably, the catalyst 30 is Pd which is supported
on alumina. Most preferably, the catalyst 30 is a mixture of
(1) 50o by volume of 5o Pd supported on gamma-alumina mixed
with (2) 50o by volume of gamma-alumina. Also housed within
the chamber 32 is a hydrogen distribution tube 35. The
hydrogen tube 35 is positioned so that it is surrounded by the
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catalyst 34. Furthermore, the hydrogen distribution tube 35
is gaseously connected to the hydrogen inlet port 31. Hence,
hydrogen gas from the metal-hydride material within the
primary storage container 5 is transported into the hydrogen
distribution tube 35 via the external manifold 23.
The hydrogen distribution tube 35 is formed from a
material which permits the passage of hydrogen gas into the
high-temperature storage material. Preferably, the hydrogen
distribution tube 35 is formed from a porous metal such as a
porous stainless steel. Most preferably, the hydrogen
distribution tube is adapted to have alternating areas of high
and low porosity, thereby providing for a more uniform
distribution of hydrogen gas into the metal-hydride hydrogen
storage material. In the one embodiment of the invention, the
hydrogen distribution tube 35 is formed from porous stainless
steel. This tube is coated with spiral bands of a high-
temperature steel-like sealant (i.e., Thermo Seal) thereby
forming the desired alternating pattern of high-porous and
low-porous areas on the distribution tube. The embodiment of
the hydrogen combustor 30 shown in Figure 1 includes an oxygen
inlet port 37 which provides a means of allowing oxygen to
enter the combustor 30. More specifically, the oxygen inlet
port 37 is positioned so that oxygen can enter the region of
the combustor 30 housing the catalyst 34. The embodiment of
the combustor 30 also includes an exhaust outlet port 39.
Combustor exhaust collects in the exhaust region 38 of the
combustor 30 and exists the exhaust outlet port 39. The
exhaust is transported from the combustor and out of the
primary metal-hydride container via the exhaust tube 25 which
is gaseously connected to the exhaust outlet port 39. In the
embodiment shown in Figure 1, the exhaust tube 25 is a short
and straight piece of tubing. However, in another embodiment
of the invention, the exhaust tube 25 is adapted so that it is
coiled and wound through the metal-hydride material. This
permits the heat from the exhaust to transfer into the metal-
hydride material in the primary container. Further, in
another embodiment of the invention, the exhaust tube is
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additionally wound around the outside of the primary storage
container after exiting said primary storage container.
It is noted that more than one hydrogen combustor may be
housed within the interior of the primary metal-hydride
storage container. In specific embodiments of the present
invention, the primary metal-hydride storage container houses
one, two, three, four, and more than four hydrogen combustors.
The metal-hydride hydrogen storage system of the present
invention further comprises means for supplying hydrogen which
is gaseously connected to the hydrogen inlet port 31 of the
hydrogen combustor 30. Generally, the means for supplying
hydrogen can be any source of hydrogen that can supply the
necessary quantity of hydrogen to the combustor. Typically,
the "necessary" quantity of hydrogen is that amount sufficient
to heat the metal-hydride storage material of the primary
storage container to a temperature enabling hydrogen
desorption from said material. Examples of the means for
supplying hydrogen include, but are not limited to hydrogen
storage as a gas in a high-pressure hydrogen storage tank,
hydrogen storage as a liquid in a cryogenic container, and
hydrogen storage in a solid in the form of a metal-hydride.
Preferably, the means for supplying hydrogen is a
secondary metal-hydride hydrogen storage container 50 which
houses a second metal-hydride hydrogen storage material.
Preferably, the second metal-hydride storage material housed
in the secondary storage container is a "low-temperature"
metal hydride hydrogen storage material. As used herein a
low-temperature metal-hydride hydrogen storage material is a
metal-hydride hydrogen storage material which will have a 1
atmosphere equilibrium plateau pressure at temperatures below
100°C. Examples of compounds that may be used to form low-
temperature metal hydrides include, but are not limited to,
the alloys from the AB5 (e.g. LaNi5), AB (e.g. TiFe), and AB2
(e. g., ZrV2 and ZrMn2) families. Using these compound, metal
hydrides can be formed readily and reversibly (i.e.
hydrogenated and dehydrogenated) in the vicinity of ordinary
temperatures and at modest hydrogen pressures. The second
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metal-hydride hydrogen storage material may be physically
bonded to the same type of support means described above in
reference to the primary metal-hydride storage container.
The secondary storage container is gaseously connected to
the hydrogen inlet port 31 of the hydrogen combustor 30.
Hence, hydrogen gas can flow from the secondary container 50
and into the combustor 30. Furthermore, as shown in Figure l,
the secondary storage container 50 is gaseously connected to
the hydrogen outlet port 8 of the primary container 5 via the
external manifold 23, thereby permitting the flow of hydrogen
gas from the primary container 5 to the secondary container
50.
As shown in Figure 1, hydrogen from the secondary storage
container 50 (or from any other embodiment of the means for
supplying hydrogen) enters the hydrogen inlet port 31 of the
combustor 30. The hydrogen passes through the porous chamber
35 and into the catalyst 34. Compressed air, which includes
oxygen, is passed into the catalyst via the oxygen inlet port
37. The Pd catalyst catalyzes the reaction between the
hydrogen and the oxygen in the compressed air. The reaction
is
Pd
(1) H2 + 1/2 O2 ----------> H20 + heat AHD = 242.3kJ/mol H2
catalyst
The heat generated from the hydrogen combustion is
initially used to heat the high-temperature metal-hydride
storage material in the primary storage module up to a
temperature at which the hydride material starts to desorb
hydrogen. This temperature is typically greater than or equal
to about 250°C. At this point, the heat produced by the
hydrogen combustor 30 does two things: (1) it maintains the
temperature of the metal-hydride storage material (i.e.,
compensates for heat transfer out of the material), and (2)
provides the heat of desorption of hydrogen from the metal-
hydride material. The reaction describing said desorption of
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hydrogen is
(2) MHX + heat (~Hdesorption) ---------~ M + x/2 H2
For the Mg-based hydride alloys, ~Hdesorption is about 75kJ/mol
H2. Therefore, if the system were ideal, with no heat losses,
the desorption rate would be equal to OH~/~Hdesorption times the
rate of hydrogen to the combustor. For the system described
above, the desorption rate would be equal to 242.3 divided by
75 = 3.2 times the rate of hydrogen to the combustor.
Hence, at hydrogen is desorbed from the hydrogen storage
material, is exits the hydrogen outlet port 8 and enters the
manifold 23. Most of the hydrogen exists the manifold outlet
25 while a portion is recycled by the manifold 23 to the
hydrogen inlet port 31 and into the combustor 30. The
recycled hydrogen is used by the combustor to maintain the
temperature of the system and provide the heat necessary to
the continued desorption of hydrogen.
Comparing the OHM from equation ( 1 ) with the OHdesorption
from equation (2), shows that about three to four times the
amount of heat is released from the combustion of hydrogen
than is absorbed by the Mg-based hydrides due to desorption of
hydrogen from the hydrides. There only a portion of the
stored hydrogen is needed to provide the necessary heat of
desorption. The apparent gravimetric H2 density of the system
would be approximately two-thirds the actual H2 density. For
example, an alloy with 6 wt o H2 density would have an
apparent H2 density of about 4 wt o.
It is noted that the self-heating metal-hydride hydrogen
storage system of the present invention may further comprise
temperature sensors for detecting the temperature of the
hydrogen storage materials in both the primary and the
secondary storage containers. As well, the storage system of
the present invention may further include prssure sensors for
detecting the pressure of hydrogen released from the primary
and well as the secondary storage containers.
While the invention has been described in connection with
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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.
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