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SAFE and DEPENDABLE HYDROGEN STORAGE

By ROY E. McALISTER
PRESIDENT
AMERICAN HYDROGEN ASSOCIATION

Abstract

Hydrogen is 14 times lighter than air and poses difficult storage problems. Popular approaches towards densification of hydrogen include pressurization, cryogenic liquefaction, cryogenic solidification, adsorption, alloying and compounding with other elements along with various combinations of these processes. Several new proprietary approaches may provide hydrogen energy volumetric storage densities on par with gasoline. The American Hydrogen Association has an experimental investigation concerning production of hydrogen by electrolysis at sufficient pressure to directly load such dense energy storage systems. Excellent safety records have been developed for hydrogen storage. The present natural gas infrastructure can be utilized for safe storage and transportation of hydrogen. Illustratively, steel pipeline materials have been tested and found to provide excellent compatibility in more than six decades of continuous exposure to hydrogen. Most natural gas formations bave been found suitable for storing hydrogen for millions of years. Therefore, the existing natural gas and electricity grids are able to provide widespread and convenient transmission and/or storage of hydrogen for allowing the U.S. to be convert from a wealth-depletion to a wealth-expansion economy.




Virtually since its discovery hydrogen has been recognized as a difficult substance to store. Paracelsus (1493-1541) noted that when iron reacts with sulfuric acid that ?an air arises which bursts forth like the wind.? In 1700, Lemery demonstrated combustion of this invisible

rising wind? when it was ignited with another flame. By 1766, Cavendish called the rising wind ?inflammable air? and determined many of the true physical properties. Cavendish
ublished many of the findings from his long term research on hydrogen in 1781 noting that drogen and air or oxygen combine to ?produce nothing but water.? In 1783, Lavoisier ed the element ?hydrogen? meaning water producer in Greek.

GASEOUS METAL:

Hydrogen is about fourteen times lighter than air. It moves faster than any other at standard temperature and pressure conditions. It is smaller than any other atom
even in the diatomic molecular state which is normal it takes up less space than the next molecule which is helium. Like other metals, hydrogen gives up electrons in
with non-metals such as oxygen and the halogens. These energy-releasing reactions


are the basis of hydrogen combustion and fuel-cell batteries..

Hydrogen forms alloys with other metals. At temperatures above the boiling point of 20.40K, hydrogen is a gas. Prom 20.40K to about 13.80K, hydrogen is a liquid. Cooling below


13.80K produces solid hydrogen. Solid metallic hydrogen has been reported to have much greater electrical conductivity than other solid elements.? Hydrogen holds the world record for heat capacity with a specific heat (Cp) of 3.44 cal/g K from O-200C.


NUCLEAR SPIN PROPERTIES:

The prevalent hydrogen isotope has a mass of l and is the simplest atom consisting of one proton and one electron. Hydrogen molecules consist of two atoms and exist in two isomeric forms, oriholtydrogen and parahydrogen. In otthohydrogen the two atomic nuclei are

spinning 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 (20.4 K), the equilibrium composition is 99.8% parahydrogen. Because equilibrium takes time to develop it is


common to liquify hydrogen with about 75% orthohydrogen present which is called normal
H2.

STRANGE HEAT RELEASE:

Orthohydrogen in normal H2 liquid will slowly change to the parahydrogen isomer in an exothermic process. The exothermic process releases about 168 cal/g and will cause consider able evaporation of the liquid hydrogen even if it is perfectly isolated from all external heat sources. In order to preserve storage of hydrogen as a cryogenic substance it

is imperative to convert the orthohydrogen to parahydrogen. Early investigators were surprised to discover a strange heat release from some but not all batches of liquified

hydrogen.
Closer examination ultimately allowed investigators to find the two isomeric forms of

H2. The author believes that careful examination of the reported hydrogen ?cold fusion?
phenomenon may reveal heat releases due to conversion of orthohydrogen to parahydrogen. One fact that suggests this explanation is the catalytic role of pyrex.

or The cold fusion exotherm? seems to require the close presence of pyrex glass ware ground silicate additions to the electrolyte. Running cold fusion experiments in

fluorocarbon polymer ware seems to inhibit (or fail to catalyze) production of an exotherm.

COLDER and DENSER STORAGE:




Catalysts such as hydrated ferric oxide gel, ruthenium, and nickel silicate are effective in promoting equilibrium conditions by converting orthohydrogen to parahydrogen. Controlled conversion of orthohydrogen to parahydrogen has found commercial applications. Where long term storage of cryogenic liquid or solid hydrogen is desired catalysts are utilized to assure that only parahydrogen enters storage.

In instances that relatively rapid release of gaseous hydrogen from cryogenic normal H2 storage is desired, such catalysts can be used with great reduction in the requirement for application of external heat and have served the aerospace industry throughout the U.S. space exploration program.

This same approach may soon assist in the gasification of hydrogen from liquid storage for power systems such as gas turbines, piston engines, and fuel cells. Europe has opted for importation of liquid hydrogen which has been produced in areas with surplus renewable energy such as Canadian hydroelectricity and solar electricity from Saudi Arabia. Future European transportation equipment including cars, trucks, buses, and air planes will have weight reduction advantages based on liquid hydrogen.



Gaseous hydrogen at 0C and I atmosphere pressure has a density of 0.0899 g/liter. Liquid parahydrogen has a density of 70 gAiter at 20.4K. If parahydrogen is cooled below the liquid boiling state to the melting point at 13.81K it starts to become a solid with a density of 86.47 g/liter.

The denser solid hydrogen that forms sinks in liquid hydrogen. The liquid-solid slush
be easily pumped at solid-phase concentrations of 55% At this concentration, the slush requires about 14% less volume than the liquid. As more heat is removed, the slush finally jifies into a hexagonal crystalline structure.

EVEN GREATER DENSITY:

A greater amount of hydrogen can be stored in certain hydrides at room temperature the same volume will store as liquid hydrogen. There are at least four types of hydrides ?ding: ionic hydrides such as alkali metal hydrides (LiH, NaH, KH, CaH2, MgH2, and H2); covalent hydrides such as AIH3, SiH4, and CH4; transition metal hydrides such as those
by exothermic absorption through interstitial hydrogen expansion of the crystalline lattice of metals like titanium (TiH2), zirconium (ZrH2), vanadium hydride (VHLS), niobium
(NbH 0.94), and tantalum hydride (TaH 76); and complex hydrides such as lithium borodride (LiBH4), sodium borohydride (NaBH4), cuperous borohydride (CuBH4), lithium aluminum hydride (LiAAlH4), and sodium aluminum hydride (NaAlH4).



Because these hydrides form by exothermic processes the hydrogen may only be released by adding heat or through reactions that produce a lower energy state. A popular transition metal hydride is formed by alloying 48% titanium with 48% iron, 3% copper, and some rare earth metal. This alloy will store more hydrogen than liquid hydrogen and the hydrogen can be released when the alloy is heated by sources typical to the coolant or exhaust temperatures of heat engines.

An extremely safe storage system can be designed around this type of transition metal hydride. If the vessel that stores this host alloy and hydrogen is penetrated, it will only leak hydrogen at a rate that is dependent upon the rate that heat is added. If no heat is added it will not leak.

The main problems with this type of storage are: the heavy weight of the host alloy and the requirements to remove heat when it is filled and to add heat when hydrogen is needed.

CARBON STORAGE:

Carbon can store hydrogen in many ways. Carbon forms covalent compounds such as the paraffins (CnH2n+2) including CH4, C2H6, C,H8, and C,H10. Gasoline is a mixture of more than 1000 compounds of covalent hydrides. The main problem with this type of hydrogen storage is that it takes a red hot condition to release the hydrogen and it is very difficult and inefficient to re-compose the covalent hydrides. Photosynthesis has been responsible for producing virtually all of the covalent hydrides of commercial importance. Photosynthesis is generally less than 1% efficient.

Carbon can be made into fibers that are ten times stronger than steel. Carbon fiber reinforced tanks can store hydrogen at 250 atmospheres and are available in light-weight composite designs that will safely pTovde 100,000 cycles from full storage pressure to empty pressure, withstand the blast of a full stick of dynamite, withstand an attack with a .357 magnum pistol, and withstand 700C surface temperature in a bonfire test. The American Hydrogen Association has used such tanks for fifteen years to safely store hydrogen.

Still another type of carbon provides capillary storage of hydrogen. Very small carbon particles that are formed in geometries that resemble whiskers or popcorn provide very high surface areas. Generally called ac:ivawd carbon this type of storage shows great promise.

Surface areas of more than 3,000 sq.meters/gram, have been achieved in proprietary materials which provide a specific capacity of 2.4 times more hydrogen than the same volume of compressed gas at 3,000 psi. After 3,000 fill cycles, only a minor loss of about 5 percent, has been observed in adsorptive capacity, and no evidence of mechanical damage from cyclic swelling has been seen. The carbon does not appear to comminute (degrade to a fine powder) which has been a typical problem with other activated carbons.




A specific capacity of nearly 8 grams of hydrogen per 100 grams of carbon at 200 atmospheres and provides good range in tanks that are suitable for most transportation applications. This density corresponds to a volumetric capacity of about 3.1 lb of hydrogen per cu ft of carbon. Volumetric capacity of such activated carbon pressurized storage can be compared to the density of cryogenic liquid hydrogen of about 4.4 lb/cu ft.

NANOFIBER CARBON STORAGE:

Recent announcements have been made by Nelly M. Rodriquez, and R. Terry K. Baker, concerning oriented carbon whiskers with the projected capacity of storing 30 liters of hydrogen per gram of carbon.25,26,27 This new hope for denser storage of hydrogen is based on the theoretical optimization of capillary storage in which several layers of hydrogen condense between carbon crystal platelets. Electron interactions allow the hydrogen to condense in several layers within the 0.34 nanometer gap between carbon crystals.

Hydrogen shrinks from an effective gas diameter of 0.26 nanometers to about 0.128 nanometers while under the influence of capillary forces. The Rodriquez-Baker team have concentrated on making measurements using minute amounts of the new material but large scale production is predicted to produce a cost-effective material.

Production of the crystals is based on reactions of carbon monoxide on bi- or trimetallic nickel or iron based catalytic particles. The particles are acid washed to remove the catalyst, dried, and heated to over 900C in a vacuum to remove gases. Activation and storage is completed by adding hydrogen at around 120 atmospheres during a 4- to 24-hour charge cycle. The pressure is maintained at 40 atmospheres to retain the hydrogen in capillary suspension. Releasing the pressure allows hydrogen to be extracted without idditions of heat said Baker in a recent interview with the author. According to Rodriquez,
nanofiber whiskers can be refilled to original capacity at least 4 or 5 times.

If the Rodriguez-Baker measurements and scale-up calculations are representative commercial production of the nanofibers, it may be possible to manufacture a storage

system with more than 50% weight yield of hydrogen. This compares to other hydrides that to 12% hydrogen and which must be heated to release the hydrogen.

Several other new proprietary approaches may provide hydrogen energy volumetric densities on par with gasoline. The American Hydrogen Association has an
experimental investigation concerning production of hydrogen by electrolysis at sufficient ure to directly load such dense energy storage systems.



Hydrogen has been shipped in low-alloy steel cylinders with 2640 in3 volume under psi pressure since 1898. Such cylinders are painted red to signify hydrogen as
nable gas.


The author has such cylinders that were first pressure tested and stamped for approved service in 1916 and 1917 and which have routinely passed every safety test with 5/3 hydrostatic pressure (at 3360 psi) to prove the strength and resilience characteristics.

These cylinders were in service for more than 20 years before the infamous Hindenberg fire and have passed every safety test in their 80 years of faithful service. Such steel cylinders can remain in service so long as they pass visual and hydrostatic safety tests.


In addition to having very safe storage cylinders, the industry has adopted standards for preventing over-pressurization due to filling errors, fires, and other heat source

exposures, and mechanical impact. Cylinder control valves are equipped with pressure relief devices or PRDs which relieve pressure if the tank is exposed to a fire or other heat source.
In most instances that steel cylinders are removed from service it is because of corrosion of the steel or other damages to the outside surface. Another reason that cylinders are removed from service is because the threaded bore in the neck becomes damaged or enlarged due to overtightening of the control valve.

TRANSPORTATION AND STORAGE OF HYDROGEN IN NATURAL
GAS SYSTEMS:

Successful long term storage of hydrogen in steel cylinders suggests that many existing

natural gas pipelines could be used to transport and store hydrogen. In many instances
hydrogen has been found to be a substantial constituent of natural gas which has been transported to market for many decades through natural gas pipelines. Utilization of

depleted natural gas formations to store hydrogen will enable long-term storage at a very low cost. Transportation to and from such storage can be by existing steel pipelines.


CONCLUDING REMARKS:

Europe has opted for importation of liquid hydrogen which has been produced in areas with surplus renewable energy (Canadian hydro and Saudi solar). European transportation equipment including cars, trucks, buses, and air planes will have weight-reduction advantages based on liquid hydrogen.

Exciting discoveries are pacing the development of much denser hydrogen storage systems. Composite storage cylinders can be designed and manufactured from carbon, glass, and polymer fibers for even greater corrosion resistance, durability, and safety than provided by the extremely safe steel storage vessels.

The present natural gas infrastructure can be utilized for safe storage and transportation of hydrogen. Steel pipeline materials have been found to provide excellent

compatablity in more than six decades of continuous exposure to hydrogen. Natural gas formations have been found suitable for storing hydrogen for millions of years. Existing natural gas and electricity grids are able to provide widespread and convenient transmission

and/or storage of hydrogen for allowing the U.S. to be convert from a wealth~depletion to a wealth-expansion economy.


138




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