WO2001013032A1 - Gas storage on an adsorbent with exfoliated laminae - Google Patents

Abstract

Gases may be maintained in compact form, e.g. at energy densities comparable to that of gasoline in the case of fuel gases such as hydrogen, methane or natural gas, by storage in a relatively motion-arrested state between the exfoliated laminae of exfoliated laminar crystalline structures, e.g. exfoliated hexagonal crystalline structures such as graphite or boron nitride.

Description

GAS STORAGE ONAN ADSORBENTWITHEXFOLIATED LAMINAE

This invention relates to the storage of gases in compact form, more particularly to the preparation and use of exfoliated laminar crystalline materials as gas storage means .

Cryogens such as hydrogen, methane, oxygen, nitrogen and helium present difficult storage problems since they must be cooled to very low temperatures in order to be stored at atmospheric pressure. Correspondingly, storage of desired quantities of such gases at ambient temperature, e.g. for commercial, transportation or novelty applications, requires very high pressures.

Previous proposals for compact gas storage include the storage of acetylene in acetone and the storage of natural gas on activated carbons. However, even the use of carbon materials with high surface area:weight ratios, such as fullerenes (e.g. buckyballs) , fullerides, fulleroids, spherical crystals, carbon nanotubes, carbon whiskers, carbon scrolls, carbonized aerogels, carbonized foams and carbonized natural fibers such as spider webs, is incapable of permitting storage of gases such as natural gas or hydrogen at densities capable of competing with liquid-state storage. Problems involved in the use of such materials for gas storage include limited efficiency resulting from their surface-to-volume characteristics and the extremely limited possibilities for heat transfer to and from them.

The present invention is based on the finding that exfoliated laminar crystalline structures are capable of storing gas in a relatively motion-arrested state between the exfoliated laminae. Typically one or more monolayers of gas may be adsorbed on or otherwise bound to each of the exposed surfaces of the laminae; further gas may enter spaces between such monolayers, transfer energy to the laminae and so exhibit a reduced effective vapor pressure. Accordingly the gas molecules may be present as multiple layers between exfoliated laminae which may themselves have a thickness of only one atom, so that fuel gases such as hydrogen, methane, natural gas and landfill gas may be storable at energy densities approaching that of gasoline. Moreover, many exfoliable laminar crystalline structures have relatively high thermal conductivity in a direction perpendicular to the direction of exfoliation, i.e. have relatively high intralaminar thermal conductivity. As discussed in greater detail hereinafter, this permits the design of gas storage systems capable of efficient heat transfer, e.g. to allow rapid heat removal during application of gas to be stored.

Thus according to one aspect of the present invention there is provided a gas storage system comprising exfoliated laminar crystalline material capable of retaining two or more molecular layers of gas between at least a proportion of adjacent laminae.

The term "gas" as used herein includes substances which are gaseous under standard conditions of temperature and pressure, as well as volatile fluids, e.g. having a boiling point not exceeding 40 °C at one atmosphere pressure; it will be appreciated that in its substantially motion-arrested stored state the interlaminar gas may exhibit essentially liquid characteristics. In some applications of the invention the gas may be fed to the storage system in liquefied form; this has the advantage that the liquefied gas may act as a heat sink for heat generated by adsorption processes which may occur as the gas goes into storage, thereby allowing rapid storage without the need for substantial external heat removal, followed by controlled (and typically more gradual) release of gas at a desired time and pressure, e.g. once the system reaches ambient temperature. This may, for example, facilitate long distance bulk shipment of liquefied cryogenic gases and subsequent rapid delivery to local transportation vehicles or other small quantity requirements, such as recreational apparatus or agricultural implements.

Gases which may be stored in systems according to the invention include hydrogen, oxygen, nitrogen, helium and hydrocarbons such as methane, ethane, propane, butane, natural gas, landfill gas or liquefied petroleum gas (LPG) .

The exfoliated laminar crystalline material may be derived from any appropriate planar crystalline material comprising strongly bonded planes linked by relatively weak interplanar bonding. Examples thus include silicates, micas, vermiculite and, more preferably, hexagonal crystal structures, for example hexagonal crystalline structures comprising a single element, two or more elements in stoichiometric proportions, or two or more elements in non-stoichiometric proportions. Such hexagonal crystal structures, e.g. graphite or boron nitride, exhibit advantageously high intraplanar/intralaminar thermal conductivity. The use of oxide-free materials may also be of advantage in minimising the possibility of unwanted high temperature oxidation reactions with stored gases such as hydrocarbons .

Graphite crystals may be produced from saturated molten metals, e.g. from carbon-saturated iron-carbon melts, or from other saturated solutions which precipitate graphite sheet material upon slow cooling or other changes of state. They may also be formed by agglomeration from smaller particles. Single graphite crystals may be deposited as pyrolytic graphite upon a suitable substrate which provides a desired shape.

Materials suitable as molds for depositing pyrolytic graphite in a desired shape include boron nitride, tungsten carbide, titanium carbide, boron carbide and refractory metals. It is also possible to machine single crystals of boron nitride or pyrolytic graphite to the desired shapes from larger blanks. Single graphite crystals may be produced from coal or other carbon sources by, for example, grinding, cleaning, sizing, mixing for dense compaction (with or without addition of a suitable binder such as a hydrocarbon tar or phenolic resin) , pressing into a desired shape and heating to 2 , 000-3 , 000°C in a zone refining apparatus, thereby eventually producing a purified single crystal.

The growth of single graphite crystal preforms is preferably effected by deposition, especially by pyrolytic deposition, of carbon from landfill methane, natural gas or coal sources. Pyrolytic graphite is produced by dissociation of methane and other hydrocarbons at ≥1700°C and 25-150 mm Hg pressure according to reactions such as :

CH, + 2H,

The hydrogen produced by such dissociation of hydrocarbons may be recovered for use as an engine fuel . EP-A-0793772 discloses a particularly efficient method for burning hydrogen in internal combustion engines. It is preferred to burn hydrogen as engine fuel and to utilize the engine's shaft power to drive an electrical generator. Pyrolytic graphite may, therefore, be produced by a natural gas- or methane-fuelled cogeneration plant. On-site use of or sales of electricity are contemplated as important opportunities for reducing the cost of pyrolytic graphite. This is an important example of how to produce much greater value from existing fossil and waste hydrocarbons, as it facilitates widespread applications of hydrogen and other renewable fuels to produce a wealth expansion economy as described in "Solar Hydrogen: Powering the New Millenium", The World & I magazine, January 1999, pages 164-171.

Pyrolytic graphite deposits may form a single crystal having the shape of the surface upon which it is grown, with the basal (0001) planes parallel to the surface of deposition. The hexagonal lattice interatomic distance is about 1.415 A and the distances between 0001 planes is about 3.4 to 3.5 A rather than 3.35 A as in natural graphite. This accounts for a slightly lower density and lower electrical and thermal conductivities in the perpendicular direction compared to natural graphite.

The van der aals bonding energy between basal planes is about 1.3 to 1.6 Kcal/g-atom in natural graphite. The bonding strengths in basal planes are virtually equal in natural and pyrolytic graphite at 150-170 Kcal/g-atom.

Several methods may be used to exfoliate laminar crystalline materials such as graphite, including impregnation with exfoliation agents such as sulfuric acid, nitric acid, mixtures of sulfuric and nitric acids, mixtures of nitric acid and potassium chlorate, halogens, silver, aluminum, manganese, active metals, iron, zinc, ammonia, pyridines and ketones . Active metal impregnates may contribute electrons to graphite crystals, whereas non-metallic ions and atoms may withdraw electrons from the aromatic carbon rings. The resulting chemical bonds define the electron configuration and location of the impregnate and the crystal-layer spacing characteristics. Contribution of electrons to the graphite crystals strengthens the crystals, so that separated basal planes may be strengthened by chemical bonds with impregnants in the spaces between the said planes/laminae.

In a preferred embodiment of the invention, however, the exfoliated laminar crystalline material is prepared by a vacuum shock treatment in which the exfoliation agent is an exfoliation fluid which is applied to one or more exfoliable laminar crystals at elevated temperature and pressure whereby a uniform concentration of the fluid diffuses into the crystal (s) , whereupon the pressure is rapidly released. This causes the fluid to expand in regions of least dense packing and so exfoliate the laminae. The degree of exfoliation (i.e. the interlaminar separation distance) may be controlled by, for example, appropriate selection of the amount of exfoliation fluid applied and the temperature at the start of expansion, by any physical constraint placed on the size of the expanded crystal (s) , and/or by the successive use of exfoliation fluids with progressively larger molecular sizes, in order to generate appropriate separation distances between the laminae. Exfoliation fluids which may be employed to achieve increased separation distances include helium, water, hydrocarbons, halocarbon refrigerants (e.g. CC1₂F₂, CC1₃F, CHC1F₂, CC1F₂.CC1F₂ or CCl₂F . CC1F₂) , ammonia, iodine and other halogens, sulfuric acid, nitric acid, phosphoric acid, potassium sulfide, potassium chlorate, zinc chloride, and alkali, alkaline earth and transition metals. Such further exfoliation may, for example, be effected by impregnating the interlaminar space with exfoliation fluid and heating the crystal (s) to a temperature in the range 400- 1,800°C, with or without the aid of a vacuum, to cause sudden vaporization and expansion of the fluid and so to force further separation of the laminae.

Following expansion, the exfoliated laminar crystalline material may, if desired, be compacted so as partially to reduce the separation distances between the laminae, for example to customize storage and strength characteristics.

In the case of graphite crystals, for example, exfoliation with hydrogen at e.g. 260°C and 2 atmospheres pressure may optionally be followed by further exfoliation cycles using successively larger molecules such as methane, ethane, propane and butane. Where an exfoliation fluid with larger molecular size then hydrogen is initially employed it may be necessary to employ higher processing temperatures to expand the crystal structure sufficiently to ensure adequate diffusion of fluid therein. As noted above, pyrolytic graphite has a somewhat higher spacing between the 0001 planes than does natural graphite, and is therefore a preferred substrate since exfoliation fluids may more easily penetrate the crystal structure.

In order to enhance penetration of hydrogen into graphite crystals prior to exfoliation the graphite may advantageously be coated with one or more materials which catalyze conversion of molecular hydrogen to atomic hydrogen, which may more readily diffuse into the structure. Representative catalysts include the platinum metal group, rare earth metals, palladium- silver alloys, titanium, and iron-titanium, iron- titanium-copper and iron-titanium-copper-rare earth metal alloys. In general it will be sufficient to apply very thin coatings of such materials by vapor deposition, sputtering or electroplating techniques. After the coatings have served their purpose of facilitating entry of hydrogen into the graphite they may be removed for reuse .

Diffusion of exfoliation fluids such as hydrogen into exfoliable laminar crystals such as graphite may also be facilitated by excitation of the crystals, for example by application of energy such as inductive heating, radiative heating or ultrasound. Similar application of energy at the time of pressure reduction may likewise improve exfoliation uniformity. Exfoliation of graphite may also be achieved by diffusing hydrogen into a host crystal of graphite followed by elevation of the temperature sufficiently to cause reaction of the hydrogen with carbon atoms which are particularly susceptible to reaction. Highly probable reactions are with carbon in higher free energy states between the 0001 planes. The reaction:

C + 4 H > CH

provides clean-up of mislocated and other high free energy carbon atoms and improves the order of the resulting storage matrix. Homogeneous distribution of hydrogen followed by rapid heating to form methane between 0001 planes may thus cause generation of internal pressure in the desired locations for exfoliation as the methane molecules expand to occupy much larger volumes than the interstitial hydrogen and solid carbon atoms. This embodiment is particularly suited for high volume production from substrates ranging from compact powders to pyrolytically deposited graphite . Hydrogen may emulate alkali metals or halogens in its ability to donate or accept an electron depending upon the polarity of any applied electropotential field. Another approach to exfoliation is therefore to control the electric charge on laminae such as the basal planes of graphite. Adding electrons to the basal planes thus assists in separation of such planes by development of like-charge repulsive forces.

In general the interlaminar separation distances generated by exfoliation are preferably at least 5A, and may be chosen to allow retention of two or more layers of gas between adjacent laminae. By way of example, separation distances of 12 to 15 A allow methane to form two or more dense monolayers on each face of adjacent exfoliated laminae. Capillary states result when the separation distances are about 15 A or larger, whereby the spaces between adsorbed monolayers are filled with molecules which transfer kinetic energy to the crystal laminae and tend to occupy about the same molecular volumes as adsorbed gases, liquids or solids. Depending on the intended application, separation distances of 5- 15A, 15-50A, 50-360A or greater than 360A may be appropriate .

By way of example pyrolytic graphite has a density of about 2.26 grams per centimeter and may be grown or machined to desired shapes. By complete exfoliation, a cubic centimeter of pyrolytic graphite may produce about 9,680 square meters of new surfaces, as shown below:

1 cm³ graphite 2.26 moles

12

,23

12

1.13 x 10²³ atoms

Number of atoms ; i . l3 x 10" 23)\ 1 /3 atoms per edge of cm³

48 , 399 , 539 atoms

Total area of both 48 , 399 , 539 x 2 cm²/cm³ surfaces of all parallel planes

9 , 679 . 9 m²/cm³

9.679.9 m/g of graphite 2.26

4,283 m²/g of graphite

It will be appreciated that such large surface area expanses are orders of magnitude greater than the surface areas presented by structures such as fullerenes, nanotubes and scrolled whiskers, which have defining walls with a thickness of one atom.

The actual surface area per unit volume will vary with the selected separation distance, which may be customized for storage of particular gases. In practice such customization may conveniently be achieved by using the gas intended for storage as the (final) exfoliation fluid in embodiments where the exfoliated laminar crystal material is prepared by a vacuum shock method. The apparent area may be significantly higher than the actual surface area as a result of capillary action, whereby many additional layers of gas molecules may be arrested between adjacent exfoliated laminae.

As an alternative to maximizing the surface area it may be useful to exfoliate only every other lamina, every third lamina, every fourth lamina and so forth, for example in order to customize the density, specific heat, thermal conductivity, structural and other properties to specific applications. This may, for example, be achieved by controlling the concentration of the exfoliation agent or the magnitude of an applied electric charge so as to produce an average exfoliation of only 50% or 33.3% or 25% etc. of the theoretical maximum .

Control of parameters such as the concentration of exfoliation agent, heat addition and stress distribution in the substrate crystals may provide particular customizations of exfoliated crystals. Heat input by radiation, inductive generation of eddy currents in the laminae, and resistive heating with current substantially perpendicular to the laminae while the crystal is held in compression prior to allowing the exfoliation agent to separate the laminae illustrate representative combinations for various customization purposes. Subjection of a pyrolytic graphite crystal to eddy current heating in each lamina may allow control of the resulting separation distances by accentuating the chemical and/or physical effects of the exfoliation agent .

Inductive heating with control of the frequency, current level and rate of travel along the crystal, in conjunction with control of the time of introduction and amount of exfoliation fluid, may likewise enable customization of an exfoliated crystal. Inductive heater-induced eddy currents which travel at rates varying from slow to very fast from one end of a crystal to the other may provide additional degrees of control over customized properties.

Further possibilities for customization include the incorporation of microstructures such as scrolled whiskers, nanotubes, nanoscrolls and fullerenes (e.g. buckyballs) between the exfoliated laminae. This has the combined advantages that the microstructures may stabilize separation of the laminae and strengthen the overall crystal matrix, whilst the intralaminar thermal conductivity of the crystal may greatly enhance thermal conduction from and to the microstructures during gas loading and unloading procedures .

Alternatively, interstitial atoms may be introduced, for example between or within the laminae of crystalline structures such as graphite or boron nitride. This may permit modification of the crystal to exhibit customized surfactant, optical, specific heat and many other properties, by selection of interstitial atoms with appropriate size and elsctron donor or electron acceptor properties. Replacement of atoms of the crystalline structure by electron donor or electron acceptor atoms may alternatively or additionally be used to provide further customization.

Exfoliation processes such as vacuum shock treatment may, for example, be carried out in purpose- designed exfoliation chambers, whereafter the exfoliated crystals may be filled loosely into an appropriate container to form a sealable gas storage system. Alternatively, a container may be formed around one or more exfoliated crystals by suitable encasement techniques. Thus, for example, a series of appropriately shaped crystals mounted on a central tube, rod, bar or like support may be exfoliated in an exfoliation chamber, then coated on their outer edges with an adhesive or diffusion braze and affixed to an appropriate encapsulating container material, e.g. a wrapped, deep-drawn or spin-formed metal or a plastics material. If desired, further coatings and/or reinforcement fibers etc. may be applied to enhance properties such as tensile load bearing strength, burst pressure and thermal or electrical conductivity or resistance; thus, for example, a thermally insulating foam may be applied to enhance thermal isolation. In such embodiments the orientation of the exfoliable laminar crystals is preferably such that the direction of exfoliation is axial with reference to the central support; the crystals may advantageously initially be spaced along the support so that adjacent crystals expand to meet each other during exfoliation. The exfoliated laminae therefore extend radially from the central support to the container material . Accordingly, by virtue of the high intralaminar thermal conductivity of laminar crystalline materials such as graphite and boron nitride, heat may readily be transferred to or from such a system, e.g. by continuous or intermittent application of heat transfer fluids to the central support and/or the container material, for example using appropriate material incorporating channels adapted for circulation of heat exchange fluid. Moreover, the high intralaminar bonding strength of the basal planes of such laminar crystalline materials means that the radially extending exfoliated laminae may act as reinforcement disks for the container material affixed thereto, thereby substantially strengthening the overall construction. Considerable weight reductions may also be achieved, since the container may be formed from a thin membrane which is supported radially by the exfoliated laminae and axially by high strength fibers; the thickness of pressure vessel walls may thus be reduced by about 50% compared to systems which do not employ such radial support . In an alternative embodiment the mounted series of appropriately shaped crystals is exfoliated within a preformed and fitted container, for example comprising a metal, glass or plastics material, which may if desired be strengthened with higher strength fiber coatings etc. Again the orientation of the crystals is preferably such that the direction of exfoliation is axial with reference to the central support .

Following exfoliation it may be desirable to take steps to arrest further movement of the exfoliated laminae, a process hereinafter referred to as "staking". Whether or not this is necessary in a given application will depend on factors such as the size of the crystals, the interlaminar separation distances and the nature of the gas to be stored. In this last context fixation of the laminae may be particularly desirable where hydrogen is to be stored, given the strong contracting van der Waals forces which hydrogen may generate between the laminae of exfoliated laminar crystalline materials such as graphite. It will be appreciated that staking may be unnecessary when the outside edges of the exfoliated laminae have been directly affixed to a container material .

Where the crystals are mounted on a perforated central support, staking may, for example, be effected by injecting a staking compound such as a thermoplastic resin (e.g. a polyolefin, polyfluoroolefin, polyester or vinyl polymer) or a thermosetting mixture such as an epoxy resin through the support so as to immobilise the inner edges of the exfoliated laminae; the latter technique may be advantageous in producing less stress on the exfoliated laminae. A self-rising foam such as a polyurethane foam may similarly be employed. Alternatively an expandable central support and/or a contractable container material may be used to achieve staking through mechanical constrain .

The burst strength of any container surrounding the exfoliated laminar crystalline material should preferably be such as to permit high pressure loading of gas into the storage system. Loading may be accompanied or followed by removal of heat from the system whereby the effective vapor pressure of motion-arrested interlaminar gas may be reduced to nominal values . Heat generated in this way may be used in applications such as heating the cab of a motor vehicle equipped with a gas storage system in accordance with the invention. Application of a positive charge of appropriate voltage so that electrons are removed from the exfoliated laminae while gas is being stored may increase the storage density of gases such as hydrogen and hydrogen-containing gases (e.g. paraffins or ammonia) , and may also reduce the amount of heat generated during storage. Without wishing to be bound by theoretical considerations it is believed that such removal of electrons tends to create attraction between atoms or molecules of the laminae and atoms or molecules of the gas being stored.

In such embodiments it is preferred to provide electrical conduction paths to the exfoliated laminae by plating their edges with a conductive membrane made from substances such as precious metals, aluminum, nickel, conductive epoxy or conductive ink. This conductive membrane may then be insulated with a suitable isolating dielectric such as a fluoropolymer or thermoset polymer to provide a high resistance barrier to electron transfer except where an electrical contact is provided. Where high electrical resistance coupled with high thermal conductivity is needed, materials such as beryllium oxide, diamond, glassy carbon or boron nitride may be used as the isolating dielectric. Further layers of composite material may be added to provide the degree of impact, thermal transfer control and pressure containment strength desired.

In order to release stored gas from a storage system in accordance with the invention it will generally be necessary to apply some form of excitation, for example heat, electric charge or vibrational energy. Very little gas release will occur in the absence of such excitation; this provides a very important safety feature since gas release in the event of accidental puncture of a storage container will therefore be minimal in the absence of excitation. It will be appreciated that the rate of release of stored gas may be controlled by appropriate control of the rate of excitation.

Thermally-induced gas release may, for example, be achieved by heating the laminae to a temperature in the range 50-150°C, e.g. about 120°C. In cases where the stored gas is a fuel gas the source of applied heat may conveniently be waste heat from a combustion process or engine driven by the fuel .

Electrical charge-induced gas release may, for example, be achieved by application of a negative voltage in the range 250-750 volts, e.g. about 500 volts .

Vibrational energy may be continuously or intermittently applied, for example by attaching an ultrasonic driver such as a piezoelectric driver to the storage system. This type of stored gas release may be especially beneficial in instances where waste heat is not available from other sources and when it is not desired to generate heat or to incur a thermal signature. Sonic release may be used in combination with electric charge control and with heat transfer to produce appropriate release rates under a wide variety of application conditions. Embodiments in which vibrational energy is removed from the system, e.g. during gas storage, may also be useful. Alternatively the storage system may be used as an acoustic dampener of unwanted noise and vibration while providing energy conversion for desired releases of stored gases, e.g. for purposes such as forming a pressure source or, depending upon the type of gas in storage, providing a fuel supply, an oxidant supply or an inert gas supply. This embodiment of the invention may be particularly useful in quietening difficult noise sources such as a kinematic engine or other machinery with relative-motion components.

Where storage systems in accordance with the invention are used for storage of gases such as hydrogen or methane it may be advantageous initially to "plate" the exfoliated laminae with a relatively lower vapor pressure substance such as propane or butane. It may also be possible to store mixtures of gases such as hydrogen and methane at greater energy storage densities than is possible for either gas stored separately. it is believed that these features may result from more efficient volumetric packing densities achievable in multiple layers of arrested molecules which assume more or less crystalline arrangements between the exfoliated laminae.

With regard to the storage of hydrogen it will be appreciated that hydrogen molecules consist of two atoms and exist in two isomeric forms, namely orthohydrogen and parahydrogen . In orthohydrogen the two atomic nuclei spin in the same direction (parallel spin) while in parahydrogen the two nuclear spins are in opposite (antiparallel) directions. At and above ambient temperature the equilibrium composition is about 75% orthohydrogen. As hydrogen is cooled, the equilibrium shifts towards increased parahydrogen. At liquid nitrogen temperature (77.4°K), about 52% orthohydrogen would exist at equilibrium. At the boiling point of hydrogen (20.4°K), the equilibrium composition is 99.8% parahydrogen. Because equilibrium takes time to develop it is possible to liquefy hydrogen with about 75% or more orthohydrogen present .

Orthohydrogen quickly cooled to liquid state will slowly change to the parahydrogen isomer in an exothermic process. The exothermic process releases about 168 cal/g and will cause considerable evaporation of the liquid hydrogen even if it is perfectly isolated from all external heat sources. In order to prepare hydrogen as a cryogenic substance it is more efficient to convert the orthohydrogen to parahydrogen before liquefaction or solidification. Certain catalysts, including hydrated ferric oxide gel, ruthenium and nickel silicate, are effective in promoting equilibrium conditions by converting orthohydrogen to parahydrogen. When long-term storage of arrested hydrogen according to the present invention is desired, such catalysts may be used in conjunction with the exfoliated laminar crystalline material to ensure that predominantly parahydrogen enters storage. It is therefore preferred to use diatomic hydrogen that has been through cryogenic conditioning or to catalytically process hydrogen with hydrated ferric oxide gel, ruthenium, nickel silicate or similar catalysts to produce parahydrogen for storage in the exfoliated laminar crystal material.

Brief description of the drawings

In the accompanying drawings, which serve to illustrate the invention without in any way limiting the same :

Fig. 1 is a longitudinal sectional view of a gas storage system prior to exfoliation;

Fig. 2 is a sectional end view of the embodiment of Fig. 2;

Fig. 3 is a longitudinal sectional view of the embodiment of Fig. 1 following exfoliation;

Fig. 4 is a sectional end view of the embodiment of Fig. 3; Fig. 5 is a schematic representation of apparatus which may be used to effect exfoliation;

Fig. 6 is a longitudinal sectional view of a further gas storage system;

Fig. 7 is a sectional end view of the embodiment of Fig. 6;

Fig. 8 is a longitudinal sectional view of a still further gas storage system;

Fig. 9 is a sectional end view of the embodiment of Fig. 8;

Fig. 10 is a sectional view of a yet still further gas storage system; Fig. 11 is a sectional end view of the embodiment of Fig. 10;

Fig. 12 is a magnified view of a portion of the embodiment of Fig. 10;

Fig. 13 is a schematic representation of an embodiment for cooking and production of purified water; and

Fig. 14 is a schematic representation of an embodiment for generation of electricity and production of purified water. Referring to the drawings in further detail, Figs. 1 and 3 illustrate a cross-section of a compact gas storage system. Impermeable pressure vessel liner 2 is preferably manufactured as a thin walled vessel which is supported by higher strength filament windings 4, thereby forming a tank or container. Suitable vessel liners include those manufactured from a variety of materials such as steel, aluminum, titanium, glass and plastics materials. Vessel 2 is preferably fitted with suitable connections 8 and 10 at its ends, as shown. Tube 6 may, for example, be porous, slotted or made of wire cloth, and has the function of supporting exfoliable laminar crystals such as 12, 14, and 16. It is preferred to use single crystals, or stacks of crystals with the same crystalline orientation, of exfoliable laminar crystalline materials such as graphite or hexagonal boron nitride; a particularly useful orientation for hexagonal single crystals is with the closely packed 0001 planes substantially perpendicular to tube 6.

Single crystals of desired shapes are bored to allow free insertion of support tube 6. Liner 2 may be a deep-drawn two piece assembly, a spin formed part, or a longitudinally seamed assembly; it is assembled over the single crystals and welded or joined to fittings 8 and 10 as shown.

The assembly is placed in suitable tooling anvils (not shown) which support the outside surface of liner 2. A heated exfoliation fluid such as hydrogen is then admitted through fitting 8 and the assembled single crystals are warm soaked in the fluid until a uniform concentration thereof has diffused into each crystal . Sudden pressure release causes the hydrogen or other exfoliation fluid to expand into areas of least-dense packing and to cause exfoliation of each 0001 plane. Additional separation may be accomplished by repeating the exfoliation cycle with successively larger molecules such as methane, ethane, propane and butane. By controlling the amount of fluid which enters the crystals, the temperature at the start of expansion and the physical limits of crystal growth, controlled separation of the 0001 planes may be achieved. During exfoliation, the precursor crystals 12, 14, 16 grow in the direction perpendicular to the 0001 planes to fill the space available. It is preferred to use several crystals spaced as shown in Fig. 1, in order to achieve uniform separation distances. It is desirable to adhere or otherwise lock the exfoliated laminae to membrane 2 or to stake them to conduit 6 shortly after exfoliation. This, for example, may be effected by:

1. Injecting a thermoplastic resin through fitting 10 to produce a molded interference or "staking" fit. Thermoplastic polymer molecules are usually many times larger than the optimal spacing between 0001 layers. Suitable thermoplastics include polyolefins, fluoro-olefins, polyesters and vinyl polymers. It is preferred to use tube 6 as an equiaxed-flow distributor of the staking resin, whereby injected thermoplastic first fills tube 6 and then uniformly passes radially under hydrostatic pressure to stake the exfoliated planes in place .

2. Injecting a thermosetting mixture such as an epoxy resin. This technique has the advantage of producing less stress on the staked planes than using injection molded thermoplastic.

3. Substituting an expandable mechanical collet for tube 6. Suitable materials include thermoplastics, aluminum, magnesium, and copper alloys. 4. Using a self-rising foam 24 such as polyurethane or a reaction-injection-molded foam to create a radial loading against the exfoliated planes.

Fitting 8 may be of any suitable configuration including designs with internal or external straight or tapered threads, quick-coupling types, o-ring sealed fittings and flange-gasket systems. The functions of fitting 8 are to provide a high strength port through which escaping gases such as hydrogen may quickly exit during the exfoliation process and to provide access for introduction of staking compound. After introduction of the staking compound and sealing of the chamber, fitting 8 may be utilized as a tie point to mount the tank as desired.

Fitting 10 may be of any suitable configuration, including designs similar to fitting 8. One function of fitting 10 is to provide flow to and from the space within tank liner 2. Holes 18 allow flow to and from the space within the tank. Further functions of fitting 10 are: to provide a high strength port through which escaping hydrogen may quickly exit during exfoliation; to allow for plugging off of tube 6 beyond the crystalline material after exfoliation; and to allow flow to and from the storage media after the staking operation.

Fig. 3 shows a cross-section of the exfoliated crystals which have been staked by a suitable compound. Port 8 is plugged and port 10 has been fitted with filter body 22. Plug 20 may be of any suitable design, including a set screw, a wire cloth form, a crushed gauze or a sintered metal filter 22, such that it prevents passage of staking materials but allows filtration of gases passing into and out of tank 4 through holes 18.

The shape of the container represented^' by lining 2 and windings 4 may be varied in accordance with intended usage. Thus, for example, long thin tubular containers may be useful for streamlined applications such as torpedo propulsion fuel storage, whilst spherical containers may be particularly suited to least-weight fuel storage systems.

Fig. 5 schematically illustrates apparatus for exfoliating laminar crystals by a vacuum shock method. Tank assembly 30, comprising container 32, shaped crystals 34, 36, etc., fittings 38 and perforated tube 40, is prepared as for Fig. 1. Pressurized exfoliation fluid such as hydrogen is delivered from accumulator 50 to pressure regulator 28 and then to heat exchanger 52. Heated exfoliation fluid is then passed through valve 44 into tank 32 to charge the crystals. Container 32 may be heated by any suitable means, including the use of heated anvils that conform to its surface. After a sufficient time the exfoliation fluid becomes uniformly diffused throughout the crystals.

Pressurization of container 32 at pressures higher than its normal operating pressure may be achieved using conformal anvil tooling to limit strain thereon. Heated conformal anvil tooling may also be used for stress relieving container 32 before and after the exfoliation process . While developing desired concentrations of exfoliation fluid in the crystals, valve 44 is closed. Valves 46 are opened and vacuum tanks 58 are evacuated by pumping system 56. Hydrogen evacuated from tank(s) 58 is transferred through pumping system 56 and is stored in accumulator 50.

Exfoliation is accomplished by rapidly opening solenoid valves 42 and allowing exfoliation fluid in the crystals to migrate to low packing efficiency areas and form expanding gaseous layers. The gaseous exfoliation fluid escapes to tanks 58 leaving exfoliated layers of two dimensional crystal laminae.

Following exfoliation to suitable spacing between the densely packed 0001 layers, tank assembly 30 is heated to bake-out the exfoliation fluid. Depending upon the materials of construction and the choice of adsorption media, the vacuum bake-out temperature may, for example, be from 120 to 1,600°C. After bake-out the tank is cooled to ambient temperature and back filled to ambient pressure, preferably with the gas which will be stored.

Tank assembly 30 is then disconnected from the exfoliation circuit at fittings 38, filter strainer 22 is inserted, as shown in Fig. 3, and the staking compound is injected through tube 40 to retain the exfoliated laminae. In instances where the selected staking compound produces gaseous by-products it may be preferred to provide a tooling vent through fitting 38 in order to prevent contamination of the exfoliated surfaces by gases from the staking compound.

In the embodiment shown in cross-section in Fig. 6, foil strips or tapes 62 of graphitic composition are wound in a helical spiral around a central mandril 64. Suitable graphite materials include graphite fabrics in plain or satin weaves such as those provided by Hercules Incorporated P.O. Box 98, Magna, UT 84044, USA and "grafoil" ribbon from Union Carbide Corporation, Old Ridgebury Rd, Danbury, CT 06817, USA.

Mandril 64 may be a solid wire or bar stock of aluminum, steel, titanium, or magnesium alloy. In the embodiment shown, however, mandril 64 is a perforated tube and thus serves as a support for graphite spirals 62 and as a gas inlet and outlet manifold. Perforations or holes 66 allow free circulation of gases into and out of graphite spirals 62. Fitting 68 provides flow to and from the tank assembly. Spiral (s) 62 are loosely wound from controlled lengths of graphite foils or tape in order to form best fits for given tank geometries as shown. Spiral (s) are preferably wound with sufficient spacing between each layer to provide room for expansion upon exfoliation. Exfoliation is accomplished as detailed above with reference to Figs . 1 and 5. The assembled tank is loaded with heated exfoliation fluid such as hydrogen or helium, hot aged to expedite diffusion of the fluid throughout the crystals of the spiral media, and vacuum shocked to cause exfoliation of each crystal.

Because of the multi-crystalline nature of the graphite in spirals 62, their exfoliation results in varying amounts of growth in all directions. Tape and foil preparation provide for greatest exfoliation growth in directions perpendicular to the length axis of the foil, whilst woven graphite yarns provide greatest growth perpendicular to the axis of the yarn fibers.

Fig. 8 illustrates an embodiment of the invention which utilizes the characteristic physical properties of the basal planes of graphite and like hexagonal crystals to radially reinforce a pressure vessel while facilitating control of heat transfer processes. Pyrolytic graphite or boron nitride single crystals are grown or machined to desired shapes such as those shown in Fig. 1. A hole is bored through the center of each crystal to accommodate a suitable perforated or wire cloth central tube 78. The functions of the central tube are to hold the crystals in place during exfoliation, to provide longitudinal reinforcement to the eventual tank assembly and to circulate gases through perforations 86 into and out of the layers of exfoliated graphite or boron nitride . After assembly of the central tube within the single crystals, fitting 84 is welded in place. The distance between flanges of fittings 82 and 84 is designed to allow the crystals to exfoliate to desired basal plane spacings.

Exfoliation is accomplished by impregnation as outlined above or by loading the crystals and tube assembly into an exfoliation chamber (not shown) with provisions for heating, changing atmosphere and rapid vacuum treatment . The assembly is hot soaked in an exfoliation fluid such as hydrogen or helium and suddenly depressurized (vacuum shocked) to cause exfoliation of the crystals. The assembly is then baked to remove residual exfoliation fluid. Outside "edge" surfaces of the exfoliated crystals 88 are then coated with a suitable high strength adhesive or diffusion braze formula and encased within a suitable low-permeability membrane 90. The exfoliated basal planes form a high strength radial reinforcement to the membrane, so that a very low weight, high strength structure results.

Adhesives suitable for this purpose include thermoset resins such as epoxies, phenol-formaldehydes, melamine- formaldehydes, silicones and addition polyimides, including those containing siloxanes; and thermoplastics such as aromatic polyesters, unsaturated polyesters and polyetherimides . The outside edges may also be coated to enhance diffusion bonding of crystals 88 to membrane 90, for example with soldering, brazing or diffusion bonding materials. Carbon deposits such as those described in "Dual Ion Beam Deposition of Carbon Films with Diamond Like Properties" NASA TM- 83743 (N31512/NSP) may also be useful as aids for joining crystals 88 to membrane 90.

Suitable membranes 90 include wrapped, deep-drawn or spin formed titanium, aluminum, stainless steels or electro- formed nickel, as well as composite membranes such as metallized thin films of polyethylene terephthalate, ethylene-chlorotrifluoroethylene copolymers, polyvinylidene fluoride and polyolefins. Metallizing materials which may be useful in this last embodiment include iron, aluminum, titanium, chromium, nickel and sputtered alloys.

Basal planes of hexagonal crystal structures such as boron nitride and pyrolytic graphite have high thermal conductivities. Thus by joining the circumferential surfaces of exfoliated basal planes to an outside membrane which provides high heat transfer rates, it is possible readily to control heat exchange to and from gases stored within the exfoliated crystalline structure. Coatings used to enhance diffusion bonding between the edges of exfoliated crystals 88 and membrane 90 may be selected so as to optimize heat transfer.

Controlled heat transfer may also be facilitated by incorporating an extended surface (e.g. corrugated) metal foil fin 92 over impermeable membrane 90. Such a fin 92 may be covered by an insulating membrane 94 to produce a honeycomb of passageways 96 through which a heat transfer fluid may be circulated or stagnated for purposes of heat transfer control . Representative fluids for heat transfer include hydrogen, air, water, engine exhaust etc. Materials suitable for membrane 94 include thermoplastics and thermosetting resins, which may be reinforced or unreinforced.

In weight-sensitive applications it may be advantageous to form corrugated fins 92 over the spherical ends of membrane 90 and to bond the contact areas metallurgically to membrane 90. Insulating membrane 94 may, for example, be a composite of a flexible polymer foam and a shrink tube of polyvinylidene fluoride. Heat transfer fluids such as hydrogen, helium, air, water, ethylene glycol and hydraulic oils may be useful in such embodiments.

In transportation applications, filtered ambient temperature air may be circulated through passageways 96 to remove heat from the exfoliated planes as fuel gases are loaded into storage. In embodiments such as the system of Fig. 8, any desired reinforcement in the direction transverse to the radial reinforcement provided by the exfoliated laminae may be accomplished by use of high strength fibers (e.g. rovings or yarns) applied over membrane 90. In embodiments where longitudinal corrugations such as the heat transfer fins 92 are employed, it may be preferred to apply axial reinforcement rovings 98 over the corrugated surface of 92 as shown in Fig. 9; this allows the corrugated surface of 92 to serve as a load spreader against membrane 90 while avoiding interference with heat exchange between membrane 90 and fin 92. Axial fibers may, for example, be anchored to the flanges of fittings 82 and 84 on the ends of tube 78, or may be wrapped and secured around the neck of tube 78. Suitable high strength reinforcement yarns, cables etc. may, for example, be made from boron, boron nitride, carbon, graphite, glass, silicon carbide, refractory metals or ceramic fibers, and may if desired be protected, for example by coating with epoxy or polyamide varnishes or other appropriate adhesion or matrix resins .

It will be appreciated that the central tube 78 may also be used to impart strength in a direction orthogonal to the radial reinforcement provided by the exfoliated laminae.

By using the exfoliated laminae of crystals 88 to provide light weight radial strength reinforcement and support for the membrane 90 it may be possible to reduce the thickness of pressure vessel walls comprising membrane 90 and any reinforcement fibers to about one half the thickness that would be required without such radial support, thereby permitting significant weight savings to be made .

Depending on desired properties and characteristics, further weight savings may be achieved by separating the exfoliated laminae to distances of the order of 350 A or more, e.g. using exfoliation fluids with appropriately large molecular size. This reduces the exfoliated crystal bulk density of pyrolytic graphite or boron nitride from 2.26 grams per cubic centimeter to 0.02 grams per cubic centimeter or less. Consequently a tank 20 cm in diameter and 120 cm long with spherical ends could contain about 700 grams of such exfoliated graphite or boron nitride, providing radial reinforcement on 360 A centers. A 0.025 cm thick diffusion bonded titanium skin would contribute a weight of 480 grams, and axial reinforcement with graphite yarn would add a weight of 800 grams. The resulting tank assembly would have a weight of about 2,300 grams, a burst pressure of over 40.2 MPa, and be capable of delivering more than 35,000 cubic centimeters of gas, showing the great advantage in utilizing the exfoliated laminae in tensile load bearing strengthening of the containment membrane.

Similarly, a tank with an assembled weight of less than 16,000 grams may safely withstand gas storage pressures of more than 335 MPa, thereby allowing gases such as hydrogen or methane to be quickly loaded at high pressures and at an energy density comparable to that of gasoline, followed by heat transfer out of the laminae to reduce the pressure to nominal values. In summary, it is possible in accordance with this embodiment of the invention to prepare extremely robust tank assemblies capable of withstanding external forces and forces produced by, for example, rapid gas loading pressures, inertia loading and gas unloading pressures. This is facilitated by the ability to provide circulation of heat exchange fluids to allow rapid heat transfer for loading and unloading the system.

Another embodiment 100 of the invention is shown in Fig. 10. The storage vessel is preferably spherical as shown, although it may be constructed in any other appropriate desired shape. Shaped single crystals of graphite or boron nitride are exfoliated to create exfoliated laminae 102, which act as reinforcement disks and are diffusion bonded to a thin membrane 104. Membrane 104 is reinforced on its outside surface by high strength films 106 which provide good protection against fire impingement and point loading. Flow into and out of the storage vessel is through perforated tube 108, which is hermetically bonded to membrane 104. Tube 108 may be terminated as desired with fittings and flanges 112 for mounting purposes. Single crystals of graphite or boron nitride are prepared in the desired shapes by pyrolytic growth or by machining techniques. A hole is bored through each crystal to accommodate perforated tube 108. The assembly is loaded on appropriate tooling fixtures into an exfoliation chamber and a primary exfoliation fluid such as hydrogen is diffused into the single crystals, which are exfoliated upon sudden depressurization. A secondary exfoliation fluid such as CC1₂F₂, CC1₃F, CHC1F₂, CC1F₂.CC1F₂, or CC1₂F.CC1F₂ is used to pressure saturate the exfoliated layers and then to further separate the exfoliated layers upon sudden pressure release.

The fully exfoliated single crystals are encased within the thin-walled membrane 104. Suitable materials for membrane 104 include spin-formed aluminum or titanium and deposited polymers such as polyvinylidene chloride, polyvinylidene fluoride and ethylene- chlorotrifluoroethylene copolymers. Polymer membranes may be metallized with, for example, vapor-deposited aluminum to produce an impermeable composite membrane. High-strength exterior coatings may be applied to provide reinforcement and scratch protection. Coating methods which may be used include deposition of diamondlike carbon films by two-stage ion beam deposition as described in US-A-4490229 , deposition by partial oxidation, and various sputtering techniques for providing diamond- like properties from deposited carbon films. Such diamond-like coatings, including deposits of carbon, boron, boron carbide, boron nitride, silicon carbide, titanium boride and refractory metal carbides, may be deposited to form films with very high tensile strengths. Methods for effecting such deposition include radio frequency, plasma and ion beam techniques and chemical vapor deposition.

In the instance of diamond-like carbon deposition by high frequency multiple ion sources, the resulting coating is chemically inert, about as hard and strong as diamond, and optically clear with an index of refraction of about 3.2. Coatings 2 to 4 microns thick may provide substantial reinforcement without causing point loading or stress-risers. Scratch and abrasion resistance may approach that of diamond. Several layers of thin coatings with different properties may be applied. Thus, for example, alternating metallic and transparent dielectric layers may be used to produce very high thermal isolation capabilities. In this way apparatus such as that of Figs. 8 and 10 may be used as cryogenic liquid storage vessels. In the apparatus of Fig. 10, inlet and outlet tube 108 may be constructed to control heat exchange and membrane 104 may be polished to a very high reflectance, coated with transparent diamond-like carbon 106 to a thickness of several thousand angstroms, and then alternately coated with additional layers of high reflectivity materials 109, 112, 116, 120, 124, 128, 132 etc., each of which is isolated by highly transparent dielectric layers 104, 106, 110, 114, 118, 122, 126, 130, 134 etc., as shown in Fig. 12.

Substantially complete thermal isolation may, for example, be achieved using 5 to 10 layers of reflective material with 98% or higher reflectance value. The intervening dielectric layers, e.g. comprising material such as amorphous carbon, may prevent oxidation or tarnishing while providing diamond-like tensile strength to reinforce membrane 104. Thermal isolation of vessel 100 may equal the best vacuum thermos technology, while burst strengths and payloads may be much higher than current pressure vessels because of the diamond-like strength of exfoliated laminae/reinforcement disks 104 and layers 106...134.

Fig. 13 shows a basic embodiment 200 for cooking, heating and production of purified water. A gas such as hydrogen or a "Hy-Boost" formula mixture of hydrogen and a hydrocarbon or hydrogenous fluid such as ammonia is stored in reservoir 202, which is constructed in accordance with the present invention. Gaseous fuel is released by heating of the exfoliated storage crystals in reservoir 202 by heat transfer from vapors from catalytic combustion in combined burner/heat exchanger 204. Such vapors travel first through countercurrent heat exchanger 243; this has a primary reactor circuit which delivers fluids from accumulator tube 215 to valve 245, and a secondary circuit which delivers vapors/fluids from burner/heat exchanger 203 and/or burner tube 203 to heat exchanger 208 in reservoir 202. Heat exchange from the secondary circuit permits endothermic and/or catalyst-induced reactions, e.g. the following, to be performed in the primary circuit:

CH₄ + H₂0 + HEAT > CO + 3H₂ Equation 1

2NH₃ > N₂ + 3H₂ Equation 2 After further cooling by heat transfer into the contents of 202 through heat exchanger 208 and considerable condensation of the water vapor, liquid water is collected in reservoir 210. As shown in Equations 3 and 4, one kilogram of hydrogen can produce about nine kilograms of water upon condensation:

H₂ + 0.5O₂ > H₂0 + HEAT Equation 3

1 kg H₂ + 8 kg 0₂ > 9 kg H₂0 Equation 4

The heat removed from the water is applied for cooking or heating at burner/heat exchanger 204. Heat is transferred from cooling water vapor to fuel coming from storage and hydrocarbons are converted into hydrogen and carbon monoxide by partial oxidation and by endothermic reactions such as those shown above in the primary circuit of heat exchanger 243. In partial oxidation of hydrocarbons the following type of reaction occurs :

CxHy + 0.5XO₂ > XCO + 0.5YH₂ + HEAT Equation 5

Heat released from the reaction of Equation 5 may be used to supplement heat needed to drive reactions such as those of Equations 1 and 2, particularly at times before sufficient heat is delivered through heat exchanger 243 by vapors from burner/heat exchanger 204. The amount of heat delivered by the exothermic reaction of Equation 3 may be modulated by control of the amount of air or other oxygen donor added through solenoid operated valve 222 to meet requirements as determined by electronic controller 234. Additional oxygen donor may be added through check valve 217 to ensure complete combustion within burner/heat exchanger 204. Pump 214 is preferably a diaphragm type pump driven by linear motor armature 216, which reciprocates under the attractive electromotive force of solenoid coil 220 and the repulsive force of spring 218. Oxygen donor entering pump 214 passes through filter 226 and check valve 224 to enter the chamber swept by cyclic movement of diaphragm 219 as shown. Check valve 217 ensures flow from 214 into accumulator tube 215, which is preferably large enough to provide substantially steady pressure to solenoid valve 222, which thereby effectively operates as a pressure regulator for delivery to the primary circuit of heat exchanger 243. Check valve 230 prevents entry of oxygen donor into tank 202.

Valve 245 allows fuel from 202 to be burned in oxygen donor which enters burner tube 203 when collection of condensed water is not desired and/or when it is desired to add humidity to the area of operation by opening valve 246, which allows exhaust to the surrounding atmosphere from burner tube 203 or burner/heat exchanger 204. Valve 245 provides four-way flows including flow of fuel from heat exchanger 243 to burner/heat exchanger 204 or to burner tube 203 and oxygen donor flow into burner tube 203. In instances where it is preferred not to collect water, it is preferred to operate burner tube 203 as a hydrogen distributor and to burn hydrogen which exits from small orifices in open air to produce the heat needed. In this mode of operation, valve 245 simply routes hydrogen from reservoir 202 and heat exchanger 243 to burner tube 203.

A particularly useful embodiment for weight-saving applications such as back-packing is to utilize storage canister 202 with valve 245 and heat exchanger 208 to provide cooking and heating. In this application, hydrogen is the preferred fuel and is metered by valve 245 into burner tube 203 with sufficient momentum to ingest air for oxidizing the hydrogen to form water vapor. Water vapor is condensed in heat exchanger 208 to provide heat for endothermic release of stored hydrogen. Water leaving heat exchanger 208 may be collected in reservoir 210 as shown. Opening valve 246 to vent a portion of the water vapor allows steaming or humidification functions by this embodiment.

Fig. 14 is a schematic view of an embodiment 300 for generating electricity, providing heat for cooking, space heating or other applications, and production of purified water. In operation a thermoelectric generator or heat engine 302 such as one operating on a Stirling, Brayton, Otto or Diesel cycle powers a suitable linear or rotary generator. Exhaust gases from heat engine 302 are cooled in heat exchanger 304 and fuel ingredients such as a hydrocarbon and water are heated and reacted in heat exchanger 318 to form hydrogen and carbon monoxide according to reactions such as those shown in Equations 1, 2 and 5, selected to meet specific application conditions and needs. It should be noted that these generally refer to reactions in which a hydrogenous fuel constituent is reacted with an oxygen donor to form hydrogen and carbon monoxide. Equation 6 shows the case for various hydrocarbons in endothermic reactions with water as the oxygen donor:

C_xH_y + XH₂0 + HEAT --> XCO + (X + 0.5Y) H₂ Equation 6

Exhaust gases are routed from heat exchanger 304 to heat exchanger 330 for purposes of heating food in an oven or cooktop provision. Oven 332 is preferably provided with a circulation fan 334 to enhance heat transfer to food in oven 332 and to provide space heating if oven doors 336 are opened as shown for circulation of room air through the unit to heat exchanger 330.

Exhaust gases are then routed to fuel storage heat exchangers such as 308 to provide endothermic heat for releasing fuel gas or gases stored in storage system 306, which is constructed according to the present invention. Released fuel gas or gases are delivered from storage in 306 by the tube shown to check valve 316 to be mixed with an oxygen donor such as water which is delivered by pump 346, or with air which is filtered by filter 348 and delivered by pump 312. Air may be used to provide exothermic conditions in reactor 318 at times when insufficient heat is available from thermoelectric generator or heat engine 302 to operate heat exchanger 318 at desired rates. When water is chosen as an oxygen donor for endothermic reactions in heat exchanger 318, pump 346 may be operated in accordance with adaptive algorithms from controller 324 to supply heat exchanger 318 with water in proportion to the fuel delivered from storage system 306.

Distilled quality water condensed in heat exchanger 308 or in delivery tube 340 is delivered to water collector reservoir 342. Pump 346 may be operated by controller 324 to deliver water from reservoir 342 to heat exchanger/reactor 318 as needed. Solenoid valve 314 is actuated by controller 324 to proportion the oxygen donor (s) as required to provide efficient operation of heat exchanger/reactor 318 and to meet the fuelling requirements of thermoelectric generator or heat engine 302. Fuel constituents produced in heat exchanger/reactor 318 are delivered by tube 320 to the fuel metering system 322. If storage system 306 is loaded with hydrogen it is preferred to by-pass heat exchanger/reactor 318 by opening four-way valve 350 to deliver from valve 316 to line 320 as shown.

Final metering and ignition of fuel by metering system 322 for operation of the thermoelectric generator or heat engine 302 is preferably achieved using a SmartPlug as disclosed in EP-A-0793772. In instances where a SmartPlug is utilized for external combustion applications such as a thermoelectric generator or Stirling engine, it is preferable to operate the fuel delivery and combustion on a pulse combustion or an intermittent duty cycle to produce sound waves which enhance heat transfer. In instances where it is utilized with an internal combustion engine such as a piston or Wankel engine, it is preferred to provide stratified charge fuel delivery and ignition operations as disclosed in the aforesaid EP-A-0793772.

In operation, it is preferred to charge storage system 306 through fueling port 352 with fuel at times when the supply system is relatively unused, to allow optimum utilization of the natural gas delivery system. Storage system 306 is typically sized to last several hours or days under normal usage conditions. In camping, emergency support and military applications it may be preferred to charge the storage system with hydrogen to maximize production of distilled quality water.

Claims

Claims

1. A gas storage system comprising exfoliated laminar crystalline material capable of retaining two or more molecular layers of gas between at least a proportion of adjacent laminae.

2. A gas storage system as claimed in claim 1 wherein the laminar crystalline material has a hexagonal crystalline structure.

3. A gas storage system as claimed in claim 2 wherein the laminar crystalline material comprises graphite or boron nitride .

4. A gas storage system as claimed in claim 3 wherein the laminar crystalline material comprises pyrolytic graphite, graphite precipitated from molten metal or saturated solution, or graphite agglomerated from smaller particles.

5. A gas storage system as claimed in any of the preceding claims wherein substantially all of the laminae are separated by substantially the same separation distance.

6. A gas storage system as claimed in any of the preceding claims wherein the exfoliated laminae are separated by an interlaminar separation distance of at least 5A.

7. A gas storage system as claimed in any of the preceding claims wherein the exfoliated laminar crystalline material is contained within a container comprising a substantially impermeable barrier membrane.

8. A gas storage system as claimed in claim 7 wherein said impermeable barrier membrane is coated with reinforcing material .

9. A gas storage system as claimed in claim 7 or claim 8 wherein said impermeable barrier membrane is coated with alternate layers of high reflectivity material and layers of transparent material having low thermal conductivity in a direction perpendicular to said layers .

10. A gas storage system as claimed in any of claims 7 to 9 wherein said impermeable barrier membrane has material incorporating channels adapted for circulation of heat exchange fluid associated therewith.

11. A gas storage system as claimed in any of claims 7 to 10 wherein said impermeable barrier material has thermally insulating foam associated therewith.

12. A gas storage system as claimed in any of the preceding claims wherein the exfoliated laminar crystalline material is positioned around a central support .

13. A gas storage system as claimed in claim 12 wherein the exfoliated laminae extend radially between said central support and a substantially impermeable barrier membrane as defined in any of claims 7 to 11.

14. A gas storage system as claimed in claim 13 wherein said exfoliated laminae are affixed to said impermeable barrier membrane .

15. A gas storage system as claimed in claim 13 or claim 14 wherein said central support imparts tensile strength to said impermeable barrier membrane .

16. A gas storage system as claimed in any of the preceding claims wherein the exfoliated laminae are positionally fixed by application thereto of a staking compound .

17. A gas storage system as claimed in any of the preceding claims including heat transfer means whereby heat may be continuously or intermittently supplied to or removed from said exfoliated laminae.

18. A gas storage system as claimed in any of the preceding claims including means whereby as electric charge may be applied to said exfoliated laminae.

19. A gas storage system as claimed in any of the preceding claims including means whereby vibrational energy may be continuously or intermittently applied to or removed from said exfoliated laminae.

20. Use of exfoliated laminar crystalline material for storage of gas in a gas storage system.

21. Use as claimed in claim 20 wherein the gas storage system is as defined in any of claims 1 to 19.

22. Use as claimed in claim 20 or claim 21 wherein the stored gas is selected from hydrogen, helium, oxygen, nitrogen, methane, ethane, propane, butane, natural gas, landfill gas and liquefied petroleum gas.

23. A process for the preparation of an exfoliated laminar crystalline material which comprises applying an exfoliation agent to one or more exfoliable laminar crystals at a temperature and pressure such that said agent diffuses into said crystal (s) to form a uniform concentration so as to exfoliate the crystal laminae.

24. A process as claimed in claim 23 wherein a sequence of exfoliation steps is performed using successive exfoliation agents with progressively larger molecular size .

25. A process as claimed in claim 23 or claim 24 wherein exfoliation is induced by rapid pressure reduction.