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U.S. Department of Energy
Hydrogen Program
www.hydrogen.energy.gov
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U.S. Department of Energy
Hydrogen Programwww.hydrogen.energy.gov
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Hoe & O Ee Fe
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p.1
Hydrogen An Overview .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p.3
Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p.5
Delivery. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p.15
Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .p.19
Application and Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p.25
Saety, Codes and Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p.33
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p.35
Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p.39
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Introduction
Hydrogen holds the potential to provide clean, sae, aordable,
and secure energy rom abundant domestic resources.
In 2003, President George W. Bush announced the Hydrogen Fuel Initiative to
accelerate the research and development o hydrogen, uel cell, and inrastruc
ture technologies that would enable hydrogen uel cell vehicles to reach the
commercial market in the 2020 timerame. The widespread use o hydrogen
can reduce our dependence on imported oil and benet the environment by
reducing greenhouse gas emissions and criteria pollutant emissions that aect
our air quality.
The Energy Policy Act o 2005, passed by Congress and signed into law by
President Bush on August 8, 2005, reinorces Federal government support or
hydrogen and uel cell technologies. Title VIII, also called the Spark M. MatsunagaHydrogen Act o 2005 authorizes more than $3.2 billion or hydrogen and uel
cell activities intended to enable the commercial introduction o hydrogen uel
cell vehicles by 2020, consistent with the Hydrogen Fuel Initiative. Numerous
other titles in the Act call or related tax and market incentives, new studies,
collaboration with alternative uels and renewable energy programs, and broad
ened demonstrationsclearly demonstrating the strong support among members
o Congress or the development and use o hydrogen uel cell technologies.
In 2006, the President announced the Advanced Energy Initiative (AEI) to
accelerate research on technologies with the potential to reduce near-term oiluse in the transportation sectorbatteries or hybrid vehicles and cellulosic
ethanoland advance activities under the Hydrogen Fuel Initiative. The AEI
also supports research to reduce the cost o electricity production technologies
in the stationary sector such as clean coal, nuclear energy, solar photovoltaics,
and wind energy.
Energy Security
The increasing demand or gasoline and diesel uel used to power our cars and
trucks makes our nation more dependent on oreign sources o oil. More thanone-hal o the petroleum consumed in the United States is imported, and that
percentage is expected to rise to 68% by 2025.1 The U.S. transportation sector
relies almost exclusively on rened petroleum products, accounting or over
two-thirds o the oil used.2 Our increasing dependence on oreign sources o
oil makes us vulnerable to supply disruptions and price fuctuations that occur
outside o our country.
) u.S. depame o Ee, Ee
Iomaio Amiisaio, AalEe Olook 005, (Feba
005), 7.
) Oak rie naioal Laboao,
taspoaio Ee daa Book:
Eiio , (decembe 00), table
. Wol Peolem Pocio,
97-00, -.
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In the short term, conservation and the use o highly ecient hybrid-electric
vehicles (HEVs) can slow the overall rate o growth o oil consumption
Hybrid-electric vehicle technology is becoming commercially competitive in
Introduction
) u.S. Eviomeal Poecio
Aec, Ai tes: Six Picipal
Pollas, (..), eieve
Sepembe 5, 005, om hp://
www.epa.ov/aies/sixpoll.hml;
a u.S. Bea o he Cess global
Poplaio Pole 00, table A-
Poplaio b reio a Co:
950-050, 5.
todays consumer market, and additional research is ocused on making hybrid
batteries, electronics, and materials more aordable. But over the long term
when projected increases in the number o cars on the road and vehicle-miles
traveled are taken into account, HEV use alone will not reduce oil consumption
below todays level.
In addition to the increased use o biouels, like ethanol, and plug-in hybrid
vehicles technology, using hydrogen as an energy carrier can provide the United
States with a more ecient and diversied energy inrastructure. Hydrogen is a
promising energy carrier in part because it can be produced rom dierent and
abundant resources, including ossil, nuclear, and renewables. Using hydrogenparticularly or our transportation needs, will allow us to diversiy our energy
supply with abundant, domestic resources and reduce our dependence on
oreign oil.
Environmental Benefts
Air quality is a signicant national concern. The U.S. Environmental Protection
Agency estimates that about 50% o Americans live in areas where levels o
one or more air pollutants are high enough to aect public health and/or
the environment3. In addition, the combustion o ossil uels accounts or themajority o anthropogenic greenhouse gas emissions (primarily carbon dioxide
or CO2) released into the atmosphere. The largest sources o CO
2emissions are
the electric utility and transportation sectors.
Fuel cells use hydrogen to create electricity, with only heat and water as byprod
ucts. When used to power a vehicle, the only output rom the tailpipe is water
vaporno greenhouse gas or criteria emissions. To realize the environmental
benets o hydrogen, however, we must consider the ull uel cycle (also called
well-to-wheels)rom energy source to hydrogen production to end-use
Producing hydrogen rom renewable sources or nuclear energy yields virtuallyzero greenhouse gas emissions. Hydrogen produced rom coal, when combined
with capture and sequestration o the byproduct carbon dioxide, also results in
virtually no greenhouse gas emissions.
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Hydrogen An Overview
Hydrogen can power cars, trucks, buses, and other vehicles, as
well as homes, ofces, actories, and even portable electronic
equipment, such as laptop computers.
What is Hydrogen?
Hydrogen, chemical symbol H, is the simplest element on earth. An atom o
hydrogen has only one proton and one electron. Hydrogen gas is a diatomic
moleculeeach molecule has two atoms o hydrogen (which is why pure
hydrogen is commonly expressed as H2). At standard temperature and pres
sure, hydrogen exists as a gas. It is colorless, odorless, tasteless, and lighter
than air.
Like electricity, hydrogen is an energy carrier(not an energy source), meaning
it can store and deliver energy in an easily usable orm. Although abundant
on earth as an element, hydrogen combines readily with other elements and is
almost always ound as part o some other substance, such as water (H2O), or
hydrocarbons like natural gas (which consists primarily o methane, with the
chemical ormula, CH4). Hydrogen is also ound in biomass, which includes all
plants and animals.
How is Hydrogen Currently Used?
The United States currently produces about nine million tons o hydrogen per
year.4 This hydrogen is used primarily in industrial processes including petroleum
rening, petrochemical manuacturing, glass purication, and in ertilizers. It is
also used in the semiconductor industry and or the hydrogenation o unsaturated
ats in vegetable oil.
Only a small raction o the hydrogen produced in the United States is used
as an energy carrier, most notably by the National Aeronautics and Space
Administration (NASA). This could change, however, as our nations leaders
look to increase our energy security by reducing our dependence on imported
oil and expanding our portolio o energy choices. Hydrogen is the optimum
choice or uel cells, which are extremely ecient energy conversion devices
that can be used or transportation and electricity generation.
Hydrogen molecule (H2)
How Mch is nie Millio
tos o Hoe?
Eoh o el appoximael
millio hoe cas
Soce: dOE reco 50, www.hoe.
ee.ov/poam_ecos.hml
) naioal Acaem o Scieces/naioal reseach Cocil, the
Hoe Ecoom: Oppoiies,
Coss, Baies, a r&d nees,
pae 7.
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U.S. Department o Energy (DOE) HydrogenProgram: Implementing the Presidents
Hydrogen Fuel Initiative
Hydrogen - An Overview
Under the Presidents Hydrogen Fuel Initiative, the DOE Hydrogen Program
works in partnership with industry, academia, national laboratories, and other
ederal and international agencies to do the ollowing:
Overcome technical barriers through the research and development o
hydrogen production, delivery, and storage technologies, as well as uel cel
technologies or transportation, distributed stationary power, and portable
power applications;
Address saety concerns and acilitate the development o model codes andstandards;
Validate hydrogen and uel cell technologies in real-world conditions; and
Educate key target audiences who can acilitate the near-term use o hydrogen
as an energy carrier.
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Hydrogen can be produced using diverse, domestic resources at
both central and distributed production acilities.
Production
Basics
Although hydrogen is the most abundant element in the universe, it does not
naturally exist in its elemental orm on Earth. Pure hydrogen must be produced
rom other hydrogen-containing compounds, such as ossil uels, biomass, or
water. Each production method requires a source o energy, i.e., thermal (heat),
electrolytic (electricity), or photolytic (light) energy. Researchers are developing a
wide range o technologies to produce hydrogen in economical, environmentally
riendly ways so that we will not need to rely on any one energy resource. The
great potential or diversity o supply is an important reason why hydrogen is
such a promising energy carrier.
The overall challenge to hydrogen production is cost reduction. For transportation,
hydrogen must be cost-competitive with conventional uels and technologies
on a per-mile basis to succeed in the commercial marketplace. This means
that the cost o hydrogen (which includes the cost o production as well as
delivery to the point o use) should be between $2.00 - $3.00 per gallon gasoline
equivalent (untaxed).
Distributed and Central ProductionHydrogen can be produced in large central plants several hundred miles rom
the point o end-use; in smaller, semi-central plants within 20-100 miles o the
point o end-use; or in small distributed generation acilities located very near
or at the point o end-use.
Distributed production may be the most viable approach or expanding the use
o hydrogen in the near term because it requires less capital investment and less
delivery inrastructure. Natural gas or renewable liquid uel reorming and small-
scale water electrolysis at the point o end-use are two distributed technologies
with potential or near-term development and commercialization.
Large central hydrogen production acilities that take advantage o economies o
scale will be needed in the long term to meet the expected hydrogen demand.
Central production can use a variety o resourcesossil, nuclear, and renewable
but an inrastructure will be required to eciently and cost-eectively deliver
the hydrogen to vehicle reueling stations and other points o end-use.
Wha is a gallon
gasoline equivalent?
taspoaio els ae oe
compae base o hei
eqivalec o asolie. the
amo o el wih he ee
coe o oe allo o
asolie is eee o
as a allo asolie
eqivale (e).
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Hydrogen production technologies all into three general categories
Thermal Processes
Electrolytic Processes
Production
Pesse Covesio Facos
1 amosphee (am) =
14.7 pos pe sqae ich (psi)
1 am =
29.92 iches o mec (i H)
1 ba =
14.5 psi
1 mea Pascal (MPa) =
10 ba =
145 psi
Seam reomi reacios
Methane:
CH
+ HO (+heat)CO + H
Propane:
C H + H O (+heat)CO + 7H 8
Gasoline:(using iso-octane and toluene as example
compounds rom the hundred or more
compounds present in gasoline)
C H + 8H O (+heat)8CO + 7H8 8
C7H
8+ 7H
O (+heat)7CO + H
Wae-gas Shi reacio:
CO + HOCO
+ H
(+small
amount o heat)
Photolytic Processes
Thermal Processes
Thermal processes use the energy in resources including natural gas, coal, or
biomass to produce hydrogen. Some thermochemical processes use heat
combined with closed chemical cycles to produce hydrogen rom eedstocks
such as water.
Thermal processes include
Natural gas reorming
Gasication
Renewable liquid uel reorming
High temperature water splitting
Natural Gas Reorming
Natural gas contains methane (CH4) that can be used to produce hydrogen via
thermal processes including steam methane reorming and partial oxidation.
Steam Methane Reforming
Hydrogen can be produced via steam reorming o uels including gasoline
propane, and ethanol (see renewable liquid reorming on page 10). But about
95% o the hydrogen produced in the United States today is made via steam
methane reorming, in which high-temperature steam (700 1000C) is used
to produce hydrogen rom a methane source such as natural gas. The methane
reacts with steam under 3-25 bar pressure in the presence o a catalyst to
produce hydrogen, carbon monoxide, and a relatively small amount o carbon
dioxide. The carbon monoxide and steam are then reacted using a catalysto produce carbon dioxide and more hydrogen. This is called the water-gas
shit reaction. In the nal process step, called pressure-swing adsorption,
carbon dioxide and other impurities are removed rom the gas stream, leaving
essentially pure hydrogen.
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Production
Well-to-WheelsGreenhouseGasEmissions(g
/mile)
Current G
V
(GasolineVehicle)Future G
V
(GasolineVehicle)
CurrentGasoline HE
V
FutureGasoline HE
V
FutureNatural Gas
DistributedHydrogenFCV
FutureCentral Win
d
ElectrolysisHydrogenFCV
FutureCentra
l
Biomass HydrogenFCV
0
100
GREENHOUSE GAS EMISSIONS
200
300
400
500
Well-to-WheelsGreenhouseGasEmissions(g
/mile)
Current G
V
(GasolineVehicle)
Future G
V
(GasolineVehicle)
CurrentGasoline HE
V
FutureGasoline HE
V
FutureNatural Gas
DistributedHydrogenFCV
FutureCentral Win
d
ElectrolysisHydrogenFCV
FutureCentra
l
Biomass HydrogenFCV
0
100
GREENHOUSE GAS EMISSIONS
200
300
400
500
Fie . Well-o-Wheels Ee use Fie . Well-o-Wheels geehose gas Emissio
Well-to-WheelsEnergyUse(Btu/mile)
CurrentGV
(GasolineVehicle)FutureGV
(GasolineVehicle)
CurrentGasolineHEV
FutureGasolineHEV
FutureNaturalGas
DistributedHydrogenFCV
FutureCentralWind
ElectrolysisHydrogenFCV
FutureCentral
BiomassHydrogenFCV
0
1000
2000
TOTAL ENERGY USE PETROLEUM ENERGY USE
3000
4000
5000
6000
A el cell vehicle si hoe poce om aal as wol cosme 50% O a well-o-wheels basisaccoi o eehose ases emie i exaci
less ee ha a coveioal asolie vehicle a eal 5% less ee ha ei, aspoi, a si a ela el cell vehicle si hoe om
a asolie hbi elecic vehicle o a well-o-wheels basiswhich accos o aal as wol poce 50% less eehose as emissios ha a coveio
ee cosme i exaci, ei, aspoi, a si a el. asolie vehicle a 5% less ha a asolie hbi elecic vehicle.
Soce o boh es a : Department o Energy Hydrogen Program 2006
Annual Progress Report, novembe 00.Partial Oxidation
In partial oxidation, the methane and other hydrocarbons in natural gas are
reacted with a limited amount o oxygen (typically rom air) that is not enough
to completely oxidize the hydrocarbons to carbon dioxide and water. With less
than the stoichiometric amount o oxygen available or the reaction, the reac
tion products contain primarily hydrogen and carbon monoxide (and nitrogen,
i the reaction is carried out with air rather than pure oxygen), again with a
relatively small amount o carbon dioxide and other compounds. The product
gas is oten reerred to as synthesis gas or syngas, rom which hydrogen can
be separated or use.
Unlike steam reorming, which is an endothermic process that requires heat,
partial oxidation is an exothermic process that gives o heat. Typically, it ismuch aster than steam reorming and requires a smaller reactor vessel. As
illustrated in the chemical reactions (see sidebar), partial oxidation initially
produces less hydrogen per unit o the input uel than is obtained by steam
reorming o the same uel. As in steam reorming, to maximize the amount o
hydrogen produced, partial oxidation usually includes a water-gas shit reaction
to generate additional hydrogen rom the oxidation o carbon monoxide to
carbon dioxide by reaction with water (steam).
Paial Oxiaio reacios
Methane:
CH
+ OCO + H
(+heat)
Propane:
CH
8+ O
CO + H
(+heat)
Ethanol:
CH
5OH + O
CO+H
(+heat)
Gasoline:(using iso-octane and toluene as example
compounds rom the hundred or more
compounds present in gasoline)
C H + O 8CO + 9H (+heat)8 8
C7H
8+ O
7CO + H
(+heat)
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HydrOgEn & Our EnErgy FuturE
Most o the hydrogen produced today in the United States is made rom natura
gas in large, central acilities. Natural gas will continue to be an important
resource or near-term hydrogen production or many reasons. It has a high
Production
Soce: u.S. depame o Ee, Oce o
Fossil Ee, hp://www.ossil.ee.ov/
poams/powessems/asicaio/
howasicaiowoks.hml,
accesse Ma 9, 00.
Coal gasicaio reacio
Chemicall, coal is a complex
a hihl vaiable sbsace. I
bimios coal, he cabo a
hoe ma be appoximael
epesee as 0.8 aoms o
hoe pe aom o cabo.
Is asicaio eacio ca be
epesee b his (balace)
eacio eqaio:
CH + O + H O 0.8
CO + CO
+ H
+ ohe species
hydrogen-to-carbon ratio (it emits less carbon dioxide than other hydrocarbons
per unit o hydrogen production) and it has an existing pipeline-based transmis
sion, distribution, and delivery inrastructure. But because natural gas resources
in the United States are limited, producing large amounts o hydrogen rom
natural gas is unsustainable or the long term, as it would essentially trade U.S
dependence on imported oil or U.S. dependence on imported natural gas.
Although natural gas reorming is relatively mature compared to other hydrogen
production technologies, the capital equipment and operation and maintenance
costs associated with distributed natural gas reorming must be reduced to make
hydrogen cost-competitive with the conventional uels we use today.
Gasifcation
Gasication is a thermal process that converts coal or biomass into a gaseous
mixture o hydrogen, carbon monoxide, carbon dioxide, and other compoundsby applying heat under pressure and in the presence o steam. Adsorbers
or special membranes can separate the hydrogen rom this gas stream, and
additional hydrogen can be generated by reacting the carbon monoxide in a
separate unit with water (orming carbon dioxide and producing hydrogen)
To be commercially viable, the capital costs o the gasier and the equipmen
to separate and puriy hydrogen must be reduced.
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Production
Coal
Coal is an abundant and relatively inexpensive domestic resource. The United
States has more proven coal reserves than any other country in the worldabouthal o the electricity produced in the United States today is generated rom
coal. It is important to note that hydrogen can be produced directly rom coal
via gasication, rather than by using coal-generated electricity to produce
hydrogen. The carbon dioxide created as a byproduct o coal gasication can
be captured and sequestered so that the process results in near-zero greenhouse
gas emissions. Coal gasication research, development, and demonstration
activities seek to ensure that coal can produce clean, aordable, reliable and
ecient electricity and hydrogen in the uture (see below).
Wha is Cabo Cape
a Seqesaio?
I hoe is o be poce om
coal, he cabo ioxie ha is also
poce i he pocess ms be
hale i a eviomeall espo
sible mae. Cabo cape a
seqesaio echoloies ae bei
evelope o sepaae cabo ioxi
a he poi o pocio a soe
(i eep eo eoloic om
ios, o example, as aal as is
soe oa) whee i ca be moi
oe a pevee om escapi.
Fege
Aoce i Feba 00, Fege is a $ billio, 0-ea
iiiaive o emosae he wols s coal-base, ea-zeo
amospheic emissios powe pla o poce elecici a
hoe. this dOE eo, alhoh o pa o he Hoe
Fel Iiiaive, sppos poam oals hoh is objecive o
esablish he echical a ecoomic easibili o co-poci
elecici a hoe om coal while capi a
seqesei he cabo ioxie. Fege will se asicaio
echolo o poce hoe iecl, which is a moe
ecie pocess o he cealize pocio o hoe ha
coal-o-elecici-o-hoe eeaio.
www.ossil.ee.ov/ee
Biomass
Biomass includes crop or orest residues; special crops grown specically or
energy use like switchgrass or willow trees; and organic municipal solid waste.
Because biomass resources oten consume carbon dioxide in the atmosphereas part o their natural growth processes, producing hydrogen through biomass
gasication may release near-zero net greenhouse gases. Improved agricultural
handling practices and breeding eorts, as well as advancements in biotechnol
ogy, can reduce the cost o biomass eedstocksand thereore reduce the cost
o hydrogenin the uture.
Biomass gasicaio reacio
Biomass, like coal, is a sbsace
o hihl vaiable chemis a
complexi. Celllose is a majo
compoe i mos biomass, alo
wih liis a ohe compos.
Celllose is a polsacchaie
ha ma be epesee o he
asicaio pocess b lcose as
a soae:
C H O + O + H O
CO + CO
+ H
+ ohe species
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Renewable Liquid Fuel Reorming
Renewable liquid uels, such as ethanol or bio-oils made rom biomass resources
can be reormed to produce hydrogen in a distributed systemat reueling
Maki Liqi Fels
om Biomass
Production
Ethanol is mae b covei
he sach i co io sas a
emei he sas o poce
ehaol. Sas ca be exace
om ohe biomass esoces like
cop o oes esies hoh
a pocess ivolvi mil aci o
seam a ezme iesio.
these sas ae he emee,
poci ehaol.
Bio-oil is a oil-like liqi simila oiesel el. Whe eae wih hea,
biomass beaks ow o poce
a bio-oil.
reewable Liqi
Fel (e.., Ehaol)
reomi reacio
CH
5OH + H
O (+heat) CO + H
Hih tempeae Wae
Splii reacio Zic/
Zic Oxie Ccle Example
ZO + hea Z + O
Z + HO ZO + H
stations or stationary power sites. Biomass resources, when converted to ethanol
bio-oils, or other liquid uels, can be transported at relatively low cost to the
point o use and reormed onsite to produce hydrogen.
Reorming renewable liquids is similar to reorming natural gas. The liquid
uel is reacted with steam at high temperatures in the presence o a catalyst to
produce a reormate gas composed o mostly hydrogen and carbon monoxide
Reacting the carbon monoxide with steam in a water-gas shit reaction produces
additional hydrogen and carbon dioxide (see p.6).
Since renewable liquids consist o larger molecules with more carbon atomsthey can be somewhat more dicult to reorm than the methane in natural gas
Better catalysts will improve hydrogen yields and selectivity. And like natura
gas reorming, operation, maintenance and capital equipment costs must be
reduced and the energy eciency must be improved or the hydrogen produced
to be cost-competitive with conventional uels.
High Temperature Water Splitting
High-temperature water splitting, a longer-term technology in the early stages
o development, uses high temperatures to drive a series o chemical reactions
that produce hydrogen rom water. The chemicals are reused within each cycle
creating a closed loop that consumes only water and produces hydrogen and
oxygen. For both o the thermochemical processes described below, scientists
have identied cycles appropriate to specic temperature ranges at which hea
is available and are researching these systems in the laboratory.
High-Temperature Water Splitting Using Solar Concentrators
A solar concentrator uses mirrors and a refective or reractive lens to capture
and ocus sunlight to produce temperatures up to 2,000C. Researchers have
identied more than 150 chemical cycles that, in principle, could be used withheat rom a solar concentrator to produce hydrogen; more than a dozen o
these cycles are under active development.
One such pathway is the zinc/zinc oxide cycle, in which zinc oxide powder passes
through a reactor that is heated by a solar concentrator operating at about 1,900C
At this temperature, the zinc oxide dissociates to zinc and oxygen gas. The zinc
is cooled, separated, and reacted with water to orm hydrogen gas and solid zinc
oxide. The zinc oxide can be recycled and reused to create more hydrogen.
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High-Temperature Water Splitting Using Nuclear Energy
Similarly, a nuclear reactor produces heat that can drive a series o chemical
reactions to create hydrogen by splitting water and recycling the chemical
Hih tempeae Wae
Splii reacio
Sl-Ioie Ccle Example
Production
constituents. The next-generation nuclear reactors under development could
generate heat at temperatures o 800C to 1,000C. These temperatures are much
lower than those produced by a solar concentrator, and as such, a dierent set
o chemical reactions would be used to produce hydrogen.
The sulur-iodine cycle is among the thermochemical cycles being investigated
or this purpose. Suluric acid, when heated to about 850C, decomposes to
water, oxygen, and sulur dioxide. The oxygen is removed, the sulur dioxide
and water are cooled, and the sulur dioxide reacts with water and iodine to
orm suluric acid and hydrogen iodide. The suluric acid is separated and
removed; the remaining hydrogen iodide is heated to 300C, where it breaksdown into hydrogen and iodine. The net result is hydrogen and oxygen,
produced rom waterthe suluric acid and iodine are recycled and used to
repeat the process.
Electrolytic Processes
Electrolytic processes use electricity to split water into hydrogen and oxygen in
a unit called an electrolyzer. Like uel cells (see page 25), electrolyzers consist o
an anode and a cathode separated by an electrolyte. The electrolyte determines
the type o electrolyzer and its operating conditions.
In a polymer electrolyte membrane (PEM) electrolyzer
Water at the anodereacts to orm oxygen and positively charged hydrogen
ions (protons) and electrons that fow through the external circuit;
The hydrogen ions move across the PEM to the cathode;
At the cathode, hydrogen ions combine with electrons rom the external
circuit to orm hydrogen gas.
Alkaline electrolyzers are similar to PEM electrolyzers, but instead use an alkaline
solution (o sodium or potassium hydroxide) as the electrolyte. Both PEM and
alkaline electrolyzers operate at relatively low temperatures 80-100C and
100-250C, respectively.
A solid oxide electrolyzer, which has a solid ceramic electrolyte, generates
hydrogen in a slightly dierent way and must operate at higher temperatures
HSO
+ hea a 850C
HO + SO
+ O
H O + SO + I H SO + HI
HI + heat at 300CI
+ H
Elecolsis reacios
Anode Reaction:
HOO
+ H+ + e
Cathode Reaction:
H+ + eH
CATHODE ANODE
PEM
+
Elecici i he
uie Saes
the aveae mix o ee
esoces se oa o eeae
elecici i he uie Saes is:
50% coal
20% nuclear
18% natural gas
9% renewable
3% petroleum
Soce: Ee Iomaio Amiisaio,
e eeaio, 005 aa
u.S. depame o Ee Pocio |
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Other Variable Costs (including utilities) ($/kg of H )
Fixed O&M ($/kg of H )
2
2
Capital Costs ($/kg of H2) 0.33
0.04
Electricity Costs ($/kg) 0.330.04
0.58
0.33
0.040.57
0.33
0.040.55
0.33
0.040.54
0.33
0.040.53 2.81
3.28
0.52 1.88
2.35
0.94
1.41
HydrOgEn & Our EnErgy FuturE
(500-800C) or the membranes to unction properly. Solid oxide electrolyzers
can eectively use heat available at these elevated temperatures (rom various
sources, including nuclear energy) to decrease the amount o electrical energy
Production
needed to produce hydrogen rom water. In a solid oxide electrolyzer
Water at the cathodecombines with electrons rom the external circuit to
orm hydrogen gas and negatively charged oxygen ions;
The oxygen ions pass through the membrane and react at the anodeto orm
oxygen gas and give up the electrons to the external circuit.
PEM and alkaline electrolyzers can be smaller, appliance-sized equipment, and
well suited or distributed hydrogen production. The source o the required
electricityincluding its cost and eciency, as well as emissions resulting rom
electricity generationmust be considered when evaluating the benets o
hydrogen production via electrolysis.
Fie . Eec o Elecici Pice o disibe HoePocio Cos (assumes 1,500 GGE/day, electrolyzer systemat 76% efciency, capital cost o $250/kW)
0.04
0.33
0.59
3.75
Two electrolysis pathways with near zero greenhouse gas emissions are o
particular interest or wide-scale hydrogen production
Electrolysis using renewable sources o electricity
Nuclear high-temperature electrolysis
DistributedHydrogenCost$(Year2005/GGE)
$5.00
$4.50
$4.00
$3.50
$3.00
$2.50Soces: Solar and Wind Technologies or Hydrogen Production, Report to Congress
(ESECS EE-00), decembe 005.
Hydrogen, Fuel Cells & Inrastructure Technologies
Program, Multi-Year Research, Development and
Demonstration Plan, Planned program activities
or 2003-2010, techical Pla - Hoe
Pocio, upae novembe 00.
$2.00
$1.50
$1.00
$0.50
$0.00$0.02 $0.03 $0.04 $0.05 $0.06 $0.07 $0.08
Electricity Price ($/kWh)
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www.hoe.ee.ov
Production
Among the challenges or cost-eective electrolytic production o hydrogen are
reducing the capital cost o the electrolyzerespecially lower cost anodes and
cathodesand improving the energy eciency.
Renewable Electrolysis
Electricity can be generated using renewable resources such as wind, solar,
geothermal, or hydro-electric power. Hydrogen production via renewable
electrolysis oers opportunities or synergy with variable power generation. For
example, though the cost o wind power has continued to drop, the inherent
variability o wind impedes its widespread use. Co-production o hydrogen
and electric power could be integrated at a wind arm, allowing fexibility and
the ability to shit production to match the availability o resources with the
systems operational needs and market actors.
nclea Hoe Aciviies
Haessi Hea o Poce Hoe Fom Wae
the u.S. depame o Ee is woki wih paes o emosae he
commecial-scale pocio o hoe om wae si hea om a clea
ee ssem. I aiio o he emissio-ee elecici poce b clea
eacos, some avace clea eaco esis opeae a ve hih empeaes,
maki hem well-sie o ive hihl ecie hemochemical a elecolic
hoe pocio pocesses. Majo poam elemes icle he caiae
pocio pocesses a hih empeae ieace echoloies ivolve i
copli a hemochemical o hih empeae elecolsis pla o a
avace hih-empeae eaco. Fo moe iomaio, please visi
www.e.oe.ov/hoe/hoe.hml.
Nuclear High-Temperature Electrolysis
In high-temperature electrolysis, heat rom a nuclear reactor generates high-
temperature steam that is electrolyzed to produce hydrogen and oxygen. The
higher temperature reduces the amount o electricity required to split the watermolecules. Because heat is a byproduct o nuclear electricity generation and
no greenhouse gases are emitted in the process, nuclear high-temperature
electrolysis can be an ecient method or producing hydrogen rom water.
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HydrOgEn & Our EnErgy FuturE
Photolytic Processes
Photolytic processes use light energy to split water into hydrogen and oxygen
These processes are in the very early stages o research but oer long-term
Production
Scientists are researching ways in
which green algae can produce
hydrogen via photobiological
processes.
potential or sustainable hydrogen production with low environmental impact
Photolytic processes include:
Photobiological Water Splitting
Photoelectrochemical Water Splitting
Photobiological Water Splitting
In this process, hydrogen is produced rom water using sunlight and specialized
microorganisms including green algae and cyanobacteria. Just as plants produceoxygen during photosynthesis, these microbes consume water and produce
hydrogen as a byproduct o their natural metabolic processes. Photobiologica
water splitting is a long-term technology. Currently, the microbes split water at
rates too low to be used or ecient commercial hydrogen production. Scientists
are researching ways to modiy the microorganisms as well as to identiy other
naturally occurring microbes that can produce hydrogen at higher rates.
Photoelectrochemical Water Splitting
In this process, hydrogen is produced rom water using sunlight and photo-
electrochemical materialsspecialized semiconductors that absorb sunlight and
use the light energy to dissociate water molecules into hydrogen and oxygen
Currently, there are materials that can split water eciently and others tha
are durable, but to produce hydrogen or widespread use, a material must be
developed that is both ecient and durable.
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www.hoe.ee.ov
The widespread use o hydrogen will require a national
inrastructure or transportation, distribution, and delivery.
Delivery
Basics
Inrastructure is required to move hydrogen rom the point o production to the
dispenser at a reueling station or stationary power site. Options and trade-os or
hydrogen delivery rom central, semi-central, and distributed production acilities
to the point o use are complexthe choice o a hydrogen production strategy
greatly aects the cost and method o delivery. For example, larger, centralized
acilities can produce hydrogen at relatively low costs due to economies o
scale, but the delivery costs are high because the point o use is arther away.
In comparison, distributed production acilities have relatively low delivery
costs, but the hydrogen production costs are likely to be higherlower volume
production means higher equipment costs on a per-unit-o-hydrogen basis.
Hydrogen has been used in industrial applications or decades, but todays
delivery inrastructure and technology are not sucient to support widespread
consumer use o hydrogen. Because hydrogen has a relatively low volumetric
energy density, its transportation, storage, and nal delivery to the point o
use comprise a signicant cost and results in some o the energy inecien
cies associated with using it as an energy carrier. Key challenges to hydrogen
delivery include reducing delivery cost, increasing energy eciency, maintaining
hydrogen purity, and minimizing hydrogen leakage. Further research is needed toanalyze the trade-os between hydrogen production and delivery options taken
together as a system. Building a national hydrogen delivery inrastructure is a
big challenge. It will take time to develop and will likely include combinations
o various technologies. Delivery inrastructure needs and resources will vary
by region and type o market (e.g., urban, interstate, or rural). Inrastructure
options will also evolve as the demand or hydrogen grows and as delivery
technologies develop and improve.
PipelinesThere are approximately 700 miles o hydrogen pipelines operating in the United
States (compared to more than one million miles o natural gas pipelines).
Owned by merchant hydrogen producers, these pipelines are located where
large hydrogen reneries and chemical plants are concentrated (or example,
in the Gul Coast region).
Inrastructure is he em se
o escibe he pipelies, cks,
ailcas, ships, a baes ha
elive el, as well as he aciliies
a eqipme eee o loa
a loa hem.
Liquid hydrogen is delivered to the
Caliornia Fuel Cell Partnership hydrog
ueling station in Sacramento.
Photo courtesy o CaFCP
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hydrogen pipeline delivery inrastructure. Research is ocused on overcoming
technical concerns related to pipeline transmission, including the potential or
hydrogen to embrittle the pipeline steel and welds; the need to control hydrogen
permeation and leaks; and the need or lower cost, more reliable, and more
durable hydrogen compression technology.
One possibility or rapidly expanding the hydrogen delivery inrastructure
is to adapt part o the natural gas delivery inrastructure. Converting natura
gas pipelines to carry a blend o natural gas and hydrogen (up to about 20%
hydrogen) may require only modest modications to the pipeline; converting
existing natural gas pipelines to deliver pure hydrogen may require moresubstantial modications.
Another possible delivery process involves producing a liquid hydrogen carrier,
such as ethanol, at a central location, pumping it through pipelines to distributed
reueling stations, and processing it at the station to produce hydrogen or
dispensing. Liquid hydrogen carriers oer the potential o using existing pipeline
and truck inrastructure technology or hydrogen transport.
Trucks, Railcars, Ships, and Barges
Trucks, railcars, ships, and barges can be used to deliver compressed hydrogen
gas, cryogenic liquid hydrogen, or novel hydrogen liquid or solid carriers.
Today, compressed hydrogen can be shipped in tube trailers at pressures up
to 3,000 psi (about 200 bar). This method is expensive, however, and it is
cost-prohibitive or distances greater than about 100 miles. Researchers are
investigating technology that might permit tube trailers to operate at higher
pressures (up to 10,000 psi), which would reduce costs and extend the utility
o this delivery option.
Currently, or longer distances, hydrogen is transported as a liquid in super-insulated, cryogenic tank trucks. Gaseous hydrogen is liqueed (cooled to below
-253C/-423F) and stored at the liqueaction plant in large, insulated tanks. The
liquid hydrogen is then dispensed to delivery trucks and transported to distribu
tion sites, where it is vaporized and compressed to a high-pressure gaseous
product or dispensing. Over long distances, trucking liquid hydrogen is more
economical than trucking gaseous hydrogen because a liquid tanker truck can
HydrOgEn & Our EnErgy FuturE
Transporting gaseous hydrogen via existing pipelines is currently the lowest-cos
option or delivering large volumes o hydrogen, but the high initial capital
cost o new pipeline construction constitutes a major barrier to expanding
Delivery
Hydrogen is delivered by barge,
rail, and truck.
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www.hoe.ee.ov
Delivery
hold a much larger mass o hydrogen than a gaseous tube trailer. But it takes
energy to liquey hydrogenusing todays technology, liqueaction consumes
more than 30% o the energy content o the hydrogen and is costly. In addition,
some amount o stored liquid hydrogen will be lost through evaporation, or
boil-o, especially when using small tanks with large surace-to-volume ratios.
Research to improve liqueaction technology, as well as improved economies
o scale, could help lower costs (todays liqueaction units are small to meet
minimal demand). Larger liqueaction plants located near hydrogen production
acilities would also help reduce the cost o the process.
In the uture, hydrogen could be transported as a cryogenic liquid in rail cars,
barges, or ships that can carry large tanksand the larger the tank, the less
hydrogen lost to evaporation. Marine vessels carrying hydrogen would be similar
to tankers that currently carry liqueed natural gas, although better insulationwould be required to keep the hydrogen liqueed (and to minimize evaporation
losses) over long transport distances.
Additional research and analysis are needed to investigate novel liquid or solid
hydrogen carriers that can store hydrogen in some other chemical state, rather
than as ree molecules. Potential carriers include ammonia, metal hydrides (see
p.23), and carbon or other nanostructures.
Bulk Storage
A national hydrogen inrastructure may require geologic (underground) bulk
storage to handle variations in demand throughout the year. In some regions,
naturally occurring geologic ormations like salt caverns and aquier structures
might be used. In other regions, specially engineered rock caverns are a possi
bility. Geologic bulk storage is common practice in the natural gas industry.
The properties o hydrogen are dierent rom natural gas, however (hydrogen
molecules are much smaller than natural gas, or example). Further research
is needed to evaluate the suitability o geologic storage or hydrogen and to
ensure proper engineering o the storage site and hydrogen containment.
Interace with Vehicles
Technologies or storing hydrogen on-board vehicles aect the design and
selection o a hydrogen delivery system and inrastructure. To maximize overall
energy eciency, designs must avoid unnecessary energy-intensive steps, such
as liqueaction and compression, and deliver pressurized hydrogen gas directly
rom a dispenser at a reueling station.
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vehicle receptacle beore any hydrogen can fow. Hydrogen dispensers are
also equipped with saety devices including breakaway hoses, leak detection
sensors, and grounding mechanisms. These controls provide additional saety
measures in the case o human error, such as a driver trying to drive away
while the dispenser is still connected to the vehicle. In todays demonstration
HydrOgEn & Our EnErgy FuturE
Reueling a vehicle with hydrogen is very similar to reueling a vehicle with
compressed natural gas (CNG). Like CNG reueling systems, hydrogen vehicle
reueling is a closed-loop system. The dispensing nozzle must lock on to the
Delivery
Reueling a hydrogen vehicle is similar programs, most hydrogen vehicles are reueled by trained personnel. This wilto reueling a natural gas vehicle. become less common as the number o demonstrations increases and hydrogen
Photo courtesy o Daimler Chrysler vehicles become available to consumers.
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Storage
Compact on-board hydrogen storage systems or light-duty
vehicles are critical to the widespread use o hydrogen as an
energy carrier. Vehicles must carry enough hydrogen to travelmore than 300 miles between reuelings, without compromising
vehicle cargo capacity or passenger space.
Basics
Hydrogen has the highest energy content per unit weightbut not per unit
volumeo any uel. Its relatively low volumetric energy content poses a
signicant challenge or storage.
Under most scenarios, stationary hydrogen storage systems have less stringentrequirements than vehicular storage. Stationary systems can occupy a relatively
large area, operate at higher temperatures, and compensate or slower reuel
ing times with larger storage systems. Storage systems or vehicles, however,
ace much tougher challenges. They must operate within the size and weight
constraints o the vehicle, enable a driving range o more than 300 miles (gener
ally regarded as the minimum or widespread driver acceptance based on the
perormance o todays gasoline vehicles), and reuel at near room temperature
and at a rate ast enough to meet drivers requirements (generally only a ew
minutes). Because o these challenges, hydrogen storage research is ocused
primarily on vehicular applications.
Many consider on-board hydrogen storage to be the greatest technical chal
lenge to widespread commercialization o hydrogen uel cell vehicles. Current
approaches (see below) include compressed hydrogen gas tanks, liquid hydrogen
tanks, and hydrogen storage in materials. Researchers in government, industry,
and academia are working toward technical targets specic to the entire storage
systemwhich includes the tank, storage media, valves, regulators, piping, added
cooling capacity, and other balance-o-plant components. Research is ocused
on improving the weight, volume (gravimetric and volumetric capacity), and
cost o current hydrogen storage systems, as well as identiying and developing
new technologies that can achieve similar perormance, and at a similar cost,
as gasoline uel storage systems.
Compressed Gas
Compressing any gas allows more compact storage. Hydrogen used by industry
today is usually stored at pressures up to about 2,000 pounds per square inch
How Much is a Kilogram
o Hydrogen?
Oe kiloam o hoe coais
appoximael he same ee
as oe allo o asolie. to
achieve a 00-mile ae i he
ll specm o lih- vehicles,
appoximael 5- kiloams
(k) o hoe ms be soe
o-boa. (this ae epes
pimail o vehicle weih a
el ecoom).
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HydrOgEn & Our EnErgy FuturE
(psi) in steel cylinders or tubes on trailers. The tanks used on demonstration
hydrogen vehicles, however, must carry more hydrogen in less space and require
compressed gas storage at higher pressures.
Storage
Hoe ca be soe
as a as o liqi
Hoe molecle (H) =
Hoe aom (H) =
Compesse gas
Coeic Liqi
Lighter weight composite materials that hold hydrogen at pressures higher than
2,000 psi are being developed to reduce the weight o storage systems. These
composite tanks have an inner liner, such as a polymer, that is impermeable
to hydrogen. The liner is surrounded by a ber shell that provides strength or
high-pressure load-bearing capacity and an outer shell that provides more impact
resistance. Composite tanks or hydrogen gas at pressures o 5,000 psi (~350 bar)
and even 10,000 psi (~700 bar) are used in prototype vehicles, but they are still
much larger and heavier than what is ultimately desired or light-duty vehicles.
Two approaches are being pursued to increase the gravimetric and volumetric
storage capacities o compressed hydrogen gas tanks. The rst approach
involves cryo-compressed tanks and is based on the principle that gases, such
as hydrogen, become denser as temperature decreases. By cooling gaseous
hydrogen (or example, at 5,000 psi) rom room temperature to -196C (or
-321F, the temperature o liquid nitrogen), its volume will decrease by a actor
o about three. This reduction is somewhat oset by the increased volume o
insulation and other system components.
The term cryo-compressed also reers to a hybrid tank concept combining high-
pressure gaseous and cryogenic storage. Insulated hybrid pressure vessels are
relatively lightweight and can be reueled with either liquid hydrogen or highpressure hydrogen gas. High pressure cryogenic tanks have lower evaporative
hydrogen losses than conventional cryogenic hydrogen tanks.
A second approach involves developing conormable tanks as an alternative
to cylindrical tanks, which do not package well in light-duty vehicles. The
gasoline tanks in todays vehicles are highly conormable and take advantage
o available space. The walls o non-cylindrical high-pressure tanks, however
typically have to be thicker and heavier to contain pressure. New concepts and
geometries are being evaluated to develop structures that are capable o saely
containing the high pressures while more eciently tting into an automotivespace. In general, a key area o research required or high pressure hydrogen
tanks, including conormable tanks, is cost reduction.
Cryogenic Liquid
Liqueed hydrogen is denser than gaseous hydrogen and thereore has a higher
energy content on a per-unit-volume basis. To convert gaseous hydrogen to
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www.hyrogen.energy.gov
Storage
a liquid, it must be cooled to -253C (-423F) and must be stored in insulated
pressure vessels to minimize hydrogen loss through evaporation, or boil-o.
Hydrogen vapor that results rom boil-o is released rom the tank through
saety valves and vented into the atmosphere. Liquid hydrogen tanks can
store more hydrogen in a given volume than compressed gas tanks, but there
are trade-os. The energy requirement or hydrogen liqueaction is high, and
boil-o must be minimized (or eliminated) or the system to be cost-eective
and energy efcient.
Storage in Materials
Hydrogen also can be stored within the structure or on the surace o certain
materials, as well as in the orm o chemical compounds that undergo a reactionto release hydrogen.
Hydrogen atoms or molecules are tightly bound with other elements in a
compound (or storage material), which may make it possible to store larger
quantities in smaller volumes at low pressure and near room temperature.
Storing hydrogen in materials can occur via absorption, in which hydrogen
is absorbed directly into the storage media; adsorption, in which hydrogen is
stored on the surace o storage media; or chemical reaction. Materials used
or hydrogen storage can employ one or more o these mechanisms and may
be grouped into our general categories:
Metal HydridesCarbon-based Materials or High Surace Area SorbentsChemical Hydrogen StorageNew Materials and ProcessesMetal Hydrides
Very broadly, a hydride is a chemical compound containing hydrogen and
at least one other element (e.g., nickel). Metal hydrides can store hydrogen
via absorption.
Metal compounds with simple crystal structures orm simple metal hydrides
through absorption. Hydrogen, dissociated into hydrogen atoms, can be incor
porated into the crystal lattice ramework o atoms in the metal compound. For
example, hydrogen atoms ft in the gaps between the lanthanum and nickel
atoms in lanthanum nickel (LaNi5) to make lanthanum nickel hydride (a metal
hydride, LaNi5H
6). By adding heat, the hydrogen atoms will break loose rom
the crystal lattice and be releaseda process called desorption.
Hyrogen in tanks on
the srface of solis
(by adsorption) or withinsolis (by absorption)
In adsorption (A), hyrogen can be
store on the srface of a material
either as hyrogen molecles (H2) or
hyrogen atoms (H). In absorption
(B/C), hyrogen molecles issoci
ate into hyrogen atoms that are
incorporate into the soli lattice
framework this metho may make
it possible to store larger qantities
of hyrogen in smaller volmes atlow pressre an at temperatres
close to room temperatre. Finally,
hyrogen can be strongly bon
within moleclar strctres, as
chemical compons containing
hyrogen atoms (D).
A) Srface Asorption
B) Intermetallic Hyrie
C) Complex Hyrie
d) Chemical Hyrie
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HydrOgEn & Our EnErgy FuturE
Advanced metal hydrides are under development with potential to store more
hydrogen than conventional or simple metal hydrides. The reactions are revers
ible, and heat or energy is released when hydrogen reacts to orm the meta
hydride. Adding heat to the hydride reverses the reaction and releases hydrogen
Sodium alanate (NaAlH4), which can store and release hydrogen, is an example
o a complex metal hydride.
Metal hydride storage materials are primarily in powder orm to increase the
surace area available or absorption. They oer great promise, but additiona
Storage
Solid Metal Hydride with
Carbon Fiber Storage Vessel.
Photo courtesy o ECD Ovonics
Ke reqiemes
o Hoe Soae
O-Boa a Vehicle
High gravimetric
and volumetric densities
(lih i weih a cosevaive i space)
Fast kinetics
(qick pake a elease)
Appropriate thermodynamics
(e.., avoable heas o hoe
absopio a esopio)
Long cycle lie or hydrogen
charging and release, durability,
and tolerance to contaminants
Low system cost, as well
as total lie-cycle cost
Minimal energy requirementsand environmental impact
Saety is a ihee
assmpio a eqieme
research is required to overcome several critical challenges including low
hydrogen capacity, slow uptake and release kinetics, and high cost. Therma
management during reueling is also a challenge, as the signicant amount o
heat released must be saely rejected or captured or use.
Carbon-Based Materials or High Surace Area Sorbents
Carbon-based materials store hydrogen via adsorption, in which hydrogen
molecules or atoms attach to the surace o the carbon nanostructure. This
category o materials, which can exist as solids and potentially as powders or
pellets, includes a range o carbon-based materials including nanotubes and
nanobers, aerogels, and ullerenes (cage-like spheres).
Carbon-based materials are still in the early stages o investigation. The ocus
o research is metal-carbon hybrid systems. Scientists are working to better
understand the mechanisms and storage capabilities o these materials andimprove the reproducibility o their measured perormance, in order to estimate
the potential to store and release adequate amounts o hydrogen under practica
operating conditions.
Metal organic rameworks (MOFs) are new, cage-like, highly porous materials
composed o metal atoms as well as organic linkers. Recent studies have shown
that certain MOFs can store hydrogen by adsorption at low temperatures. A key
ocus area or uture research is to tailor materials so hydrogen can be stored
within them at room temperature.
Chemical Hydrogen Storage
Chemical hydrogen storage describes technologies in which hydrogen is stored
and released through chemical reactions. Common reactions involve reacting
chemical hydrides with water or alcohols. Chemical hydrides can exist as liquids
or solids, and the hydrogen atoms can be bonded to metal or non-metal species
The chemical reactions needed to release the hydrogen produce a byproduct
that can be regenerated or recharged o-board the vehicle, usually by adding
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Storage
heat, hydrogen, and perhaps other reactants. Chemical hydride storage systems
require management o the heat produced in the hydrogen-releasing reaction, as
well as removal o the byproducts, or spent uel, created when the hydrogen
is released. Hydrolysis reactions, hydrogenation/dehydrogenation reactions,
and several new chemical approaches are under investigation.
Hydrolysis Reactions
Hydrolysis reactions involve the oxidation reaction o chemical hydrides with
water to produce hydrogen. The reaction o sodium borohydride (NaBH4)
is among the most well-studied hydrolysis reactions to date. In this system,
sodium borohydride and water react in the presence o a catalyst to produce
hydrogen as needed, along with sodium borate as the byproduct. The sodium
borate can be converted back to sodium borohydride using heat, water, andhydrogen in an o-board regeneration process. Regeneration eciency and
energy requirements are issues still to be resolved.
Another process that unctions by hydrolysis is the reaction o magnesium hydride
with water to orm magnesium hydroxide and hydrogen. Similar to the sodium
borohydride approach, water must be carried on-board the vehicle in addition
to a slurry, and the magnesium hydroxide must be regenerated o-board.
Hydrogenation/Dehydrogenation Reactions
Hydrogenation describes the process o bonding or adding hydrogen to acompound; dehydrogenation describes the removal o hydrogen. Hydrogena
tion/dehydrogenation reactions do not require water as a reactant and instead
use catalysts and heat to release hydrogen. For example, when exposed to
heat in the presence o a catalyst, decalin (C10H
18) releases hydrogen and leaves
the byproduct naphthalene (C10H
8) at roughly 200C. Research is underway to
identiy chemical hydrides that dehydrogenate at lower temperatures.
New Chemical Approaches
Other new chemical approaches are being evaluated to identiy alternatives with
higher hydrogen storage capacities, such as ammonia borane (NH3BH3). Anotherexample is the concept o reacting lightweight metal hydrides, such as LiH, NaH,
and MgH2, with methanol and ethanola process called alcoholysis. Alcoholysis
reactions could provide controlled and convenient hydrogen production (and
storage) at room temperature and below. A disadvantage, however, is that the
alcohol also needs to be carried on-board.
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HydrOgEn & Our EnErgy FuturE
Among the challenges to chemical hydrogen storage on-board vehicles are
waste removal and reuelingbyproducts must be purged and resh chemical
hydrides and other reactants must be put back into the vehicle. Developing
Storage
chemical hydride regeneration systems at reueling stations or transporting the
byproducts to a central regeneration site will also be a challenge. Although the
materials hydrogen storage capacity can be high and reueling with a liquid is
attractive, issues related to regeneration energy requirements, cost, and lie-cycle
impacts are key technical barriers currently under research.
New Materials and Processes
Scientists are also exploring several new materials that may have potential or
reversible hydrogen storage, including conducting polymers. Initial studies
have indicated that a signicant amount o hydrogen can be incorporated intoconducting polymer structures. These and other new concepts are yet to be
explored or their viability to vehicular hydrogen storage applications.
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Application and Use
Hydrogen can be converted into usable energy through uel
cells or by combustion in turbines and engines. Fuel cells now in
development will not only provide a new way to produce power,but will also signifcantly improve energy conversion efciency,
especially in transportation applications.
Overview
Hydrogen is a versatile energy carrier that can be used to power nearly every
end-use energy need. The uel cellan energy conversion device that can
eciently convert and use the power o hydrogenis key to making it happen.
Stationary uel cells can be used or backup power, power or remote locations,
distributed power generation, and cogeneration (in which excess heat releasedduring electricity generation is used or other applications). Fuel cells can power
almost any portable application that uses batteries, rom hand-held devices to
portable generators. Fuel cells can also provide primary and auxiliary power or
transportation, including personal vehicles, trucks, buses, and marine vessels.
Why Fuel Cells?
Fuel cells directly convert the chemical energy in hydrogen to electricity and
heat. Inside a uel cell, hydrogen electrochemically combines with oxygen rom
the air to create electricity, with pure water and potentially useul heat as the
only byproducts.
Hydrogen-powered uel cells are not only pollution-ree, but also can have two
to three times the eciency o traditional internal combustion technologies.
A conventional combustion-based power plant typically generates electricity
at eciencies o 33 to 35%, while uel cell systems can generate electricity
at eciencies o up to 60% (and even higher with cogeneration). In normal
driving, the gasoline engine in a conventional car is less than 20% ecient in
converting the chemical energy in gasoline into power that moves the vehicle.
Hydrogen uel cell vehicles, which use electric motors, are much more energyecient and use up to 60% o the uels energy. This corresponds to more than
a 50% reduction in uel consumption, compared to a conventional vehicle with
a gasoline internal combustion engine.6 In addition, uel cells operate quietly,
have ewer moving parts, and are well suited to a variety o applications.
Photo courtesy o General Motors
) u.S. depame o Ee,
Ee Iomaio Amiisaio,
Spplemeal tables o he Aal
Ee Olook 005, taspoaio
dema Seco, table 7, (005), 5
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Fuel Cell Basics
A single uel cell consists o an electrolyte sandwiched between two electrodes
an anode and a cathode. The power produced by a uel cell depends on severaactors, including the uel cell type, size, temperature at which it operates, and
Application and Use
pressure at which gases are supplied to the cell. A single uel cell produces 1
volt o electricity or less. To generate more electricity, individual uel cells are
combined in a series to orm a stack. (The term uel cell is oten used to reer to
the entire stack, as well as to the individual cell). Depending on the application
a uel cell stack may contain hundreds o individual cells layered together. This
scalability makes uel cells ideal or a wide variety o applications, rom laptop
computers (50-100 Watts) to homes (1-5kW), vehicles (50-125 kW), and central
power generation (1-200 MW or more).
Types o Fuel Cells
In general, all uel cells have the same basic congurationan electrolyte
sandwiched between two electrodesbut there are dierent types o uel cells
classied primarily by the kind o electrolyte used. The electrolyte determines
the kind o chemical reactions that take place in the uel cell, the operating
temperature range, and other actors that aect the applications or which the
uel cell is most suitable, as well as its advantages and limitations.
Fie . Fel Cell Compaiso Cha
Fuel Cell Common Operating System ElectricalType Electrolyte Temp. Output Efciency Applications Advantages Disadvantages
PolmeElecoleMembae(PEM)*
Soli oaic polmepol-pefoosloicaci
50 00C F
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Application and Use
PEM Fuel Cells
Polymer electrolyte membrane uel cells (also called proton exchange membrane
or PEM uel cells) use a solid polymer as the electrolyte and porous carbonelectrodes containing a platinum catalyst. PEM uel cells require hydrogen,
oxygen, and water to operate. They are typically ueled with hydrogen supplied
rom storage tanks and oxygen drawn rom the air.
Compared to other kinds o uel cells, PEM uel cells operate at relatively low
temperatures, around 80C, which allows them to start quickly (less warm-up
time is needed) and results in less wear on system components. For these and
other reasons, including the ability to produce sucient power within the size
and weight constraints o a vehicle, PEM uel cells are widely regarded as the
most promising uel cells or light-duty transportation applications.
How Do They Work?
Like all uel cells, a single PEM uel cell consists o two electrodes separated
by a membrane. The polymer electrolyte membrane (PEM) is a solid organic
compound, typically the consistency o plastic wrap and thinner than a single
sheet o paper.
On one side o the cell, hydrogen gas fows through channels to the electrically
negative electrode, or anode. The anode, which is porous so that hydrogen
can pass through it, is composed o platinum (catalyst) particles uniormlysupported on carbon particles and surrounded by a thin layer o proton-conduct
ing ionomer. Hydrogen uel on the anode side moves through the electrode
and encounters the platinum catalyst, which causes the hydrogen molecules to
separate into protons and electrons. This is an oxidation reaction (see sidebar).
The PEMthe key to this uel cell technologyallows only the protons to pass
through it. While the protons are conducted through the ionomer and PEM to
the other side o the cell, the stream o negatively-charged electrons ollows
an external circuit to the cathode. This fow o electrons through the external
circuit is electricity that can be used to do work, such as power a motor.
On the other side o the cell, oxygen gas, typically drawn rom ambient air,
fows to the electrically positive electrode, or cathode. Like the anode, the
cathode is made o platinum particles uniormly supported on carbon particles
it is porous and oxygen can move through it. A reduction reaction, involving
the gaining o electrons, takes place at the cathode (see sidebar). When the
electrons (which have traveled through the external circuit) return rom doing
work, they react with oxygen and the hydrogen protons (which have moved
Chemical reacios
All elecochemical eacios,
icli hose ha ake place i
a el cell, cosis o wo sepaae
eacios: a oxiaio hal-eacio
a he aoe, a a ecio hal-
eacio a he cahoe. Oxiaio
ivolves a loss o elecos b oe
molecle, a ecio ivolves a
ai o elecos b aohe. Boh
oxiaio a ecio occ
simlaeosl a i eqivale
amos i a eacio
ivolvi eihe pocess.
Oxidation hal reaction
HH+ + e
Reduction hal reactionO
+ H+ + eHO
Fuel cell reaction
H
+ OH
O
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Fel Cells geeae
Elecici, Hea, a Wae
B How Mch Wae?Hoe el cell vehicles (FCVs)
through the PEM) at the cathode to orm water. Most o the water is collected
and reused within the system, but a small amount is released in the exhaust as
water vapor (how much water?...see sidebar). Heat is generated rom this union
Application and Use
(an exothermic reaction) and rom the rictional resistance o ion transer through
the membrane. This thermal energy can be used outside the uel cell.emi appoximael he same
amo o wae pe mile as vehicles
si asolie-powee ieal
combsio eies (ICEs).
How ar will one gallon go and
how much water will it produce?
Gasoline ICE Vehicle
5 miles pe allo (mp)
allo asolie = .7 k o el
(may be represented approximately as CH2)
CH + / O CO + H O (water)
.7 k + 9. k8.5 k + .5 k
.5 k wae/5 miles =
0. k wae/mi
Hydrogen FCV
0 miles pe allo asolie
eqivale (mpe)
allo o asolie eqivale o
hoe ve eal eqals .0 k H
H
+ / OH
O (wae)
.0 k + 8.0 k9.0 k
9.0 k wae/0 miles =
0.5 k wae/mi
noe: the calclaios above assme ha aasolie ICE vehicle aveaes 5 miles pe al
lo a a hoe FCV aveaes 0 mpe
o hoe. (Oe e o hoe has he
same ee coe as a allo o asolie
[i ems o hei lowe heai vales] a is
appoximael eqal o oe k o hoe.)
the el cell is a valable ssem becase i
is . imes as ee ecie as aiioal
combsio ssems, achievi . imes as
ma miles pe allo o asolie eqivale.
Soce: Aoe naioal Laboao
H2H2
H2
Anode
Hydro
genIN
Oxyge
nIN
Heat
OUT
Polymer Electrolyte Membrane (PEM)
O2
H2
H+
H+
e
ee
H2O
Cathode
eee-
e-
H+
Water OUT
O2
O2
O2
ee
H+
ee
H2O
H2O
e
Oxygen Molecule
Hydrogen Molecule
Hydrogen Atom
Electron
Water Molecule
A Closer Look at the Details
Catalyst
Platinum-group metals are critical to catalyzing reactions in a PEM uel cell
The uel cells typically use a catalyst made o platinum powder very thinly
coated onto carbon paper or cloth. The catalyst is rough and porous to expose
the maximum surace area o the platinum to the hydrogen or oxygen. The
use o platinum adds to the cost o the system, however. Platinum catalysts
are also sensitive to poisoning by contaminants such as carbon monoxide or
sulurmolecules that can bind to the platinum and prevent it rom reacting
with hydrogen at the anode, or example. This sensitivity requires high purity
hydrogen or additional purication steps, which can add to the cost.
Membrane Electrode Assembly and Bipolar Plates
The electrodes, catalyst, PEM, and gas diusion layers (GDLs) together orm
the membrane electrode assembly, or MEA. The GDLsone next to the anode
the other next to the cathodeare usually made o a porous carbon paper
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Application and Use
or cloth about as thick as 4-12 sheets o paper. The layers must be made o aFel Cells
material like carbon that can conduct electrons leaving the anode and enteringSpecial Applicaios
the cathode. Its porous nature helps ensure eective diusion o both hydrogen
and oxygen to the entire surace area o the catalyst layers on the MEA.
The GDLs also help manage water in the uel cell. The membrane must remain
moist to operate eectively; the diusion material allows the right amount o
water vapor to reach the MEA and keep the membrane humidied. The GDL
is oten coated with Tefon to ensure that some pores in the carbon cloth (or
carbon paper) do not clog with water.
The outer-most layer o each uel cell, pressed against the surace o each GDL,
is the bipolar plate. Channels ormed into the ace o the plate provide a fow
eld, carrying the hydrogen and oxygen gases to where they are used in the
uel cell. Bipolar plates, which can be carbon composites or metal, also act as
current collectors. Electrons produced by the oxidation o hydrogen must (1)
be conducted through the anode and GDL, along the length o the uel cell
stack, and through the plate beore they can exit the cell; (2) travel through an
external circuit; and (3) re-enter the cell at the cathode.
Challenges
Reducing cost and improving durability are the two most signicant challenges
to uel cell commercialization. Fuel cell systems must be cost-competitive with,
and perorm as well or better than, traditional power technologies over the lieo the system.
Materials and manuacturing costs are currently too high or catalysts, bipolar
plates, membranes, and gas diusion layers. Today, the costs or automotive
internal combustion engine power plants are about $25-$35/kW; or trans
portation applications, a uel cell system needs to cost approximately $30/kW
(at volumes o 500,000 units per year) or the technology to be competitive.
For stationary systems, the acceptable price point is considerably higher
$400-$750/kW is considered necessary or widespread commercialization
($1,000/kW or initial early adopter applications). (Data current as o March
2007. For the most up-to-date numbers, please reer to the Hydrogen Program
Multi-Year Research, Development, and Demonstration Planwww.eere.energy.
gov/hydrogenanduelcells/mypp/)
In addition to cost-related challenges, adequate uel cell system durability has
not been established. To be considered ready or widespread use in transporta
tion, uel cell power systems must demonstrate durability and reliability similar
Fel cells o ceai special o
iche applicaios ae commeciall
available oa. Fel cells se i
o-aspoaio applicaios
ca help evelop maaci
capabiliies a a spplie base while
aspoaio el cells ae e
evelopme. these applicaios
icle powe eeaio:
I emoe locaios whee he
elecici i is o accessible
Whee poable powe is eee
a coscio sies, isea o
eeaos, a campos,
o fexible a moveable powe
Fo ok lis isie
waehoses a ohe
ioo eviomes
Fo smalle applicaios,
sch as lapops a cellphoes, ha picall
se baeies
Fo axilia powe applicaios,
sch as powei he
cabi o a 8-wheele
while he eie is
o i
Fo isa emeec backp
powe o sppleme he i,
whee cosa powe is eqie
a places sch as baks, hospials
a elecommicaios owes
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Hbi elecic a el cell
vehicles shae echoloies
to todays automotive engines. Specically, this means a 5,000-hour operating
liespan (the equivalent o 150,000 miles) and the ability to perorm over the
ull range o dynamic vehicle operating conditions, including temperatures
Application and Use
techoloies evelope o oas
asolie hbi-elecic vehicles ca
help acceleae he evelopme
o el cell vehicles. Fo example,
hbi vehicles se elecic moos
o poplsio a eqie avace
ive cool elecoics a
sowae, mch like el cell vehicles.
Fhe eseach will impove he
aoabili a peomace o
hbi echolo available o
cosmes i he ea-em aalso avace el cell echolo
evelopme o he lo-em.
Photo courtesy o CaFCP
and climates (various degrees o humidity and ambient temperatures o -40C
to +40C, or example). Requirements or stationary applications are slightly
dierent because the associated duty cycles are less demanding. To compete
with other distributed power generation systems, stationary uel cells mus
demonstrate more than 40,000 hours (nearly 5 years) o reliable operation
in ambient temperatures o -40C to +50C to be considered ready or the
mainstream market.
For both transportation and stationary applications, research is needed to identiy
and develop new materials to reduce the cost and extend the lie o uel cel
stack components. Low cost, high volume manuacturing processes will alsohelp to make uel cell systems competitive with traditional technologies.
the Soli Sae Ee Covesio Alliace
Focsi o Soli Oxie Fel Cell r&d
the Soli Sae Ee Covesio Alliace (SECA) is a joi oveme-is
eo le b he u.S. depame o Ee o ece cos a achieve echolo
beakhohs i soli oxie el cells (SOFCs), pimail se o elecic ili a
saioa applicaios. SECA aims o evelop -0kW SOFCs b 00 ha ca be
mass-poce i mola om (he echolo will he be scale-p o seve as
bili blocks o Fege-pe plas o moe o Fege, see pae 9).
SECA el cells will be se o a wie ae o applicaios, icli axilia
powe a combie hea a powe. Fo eaile poam oals, eqiemes,
milesoes, a aciviies, please visi www.seca.oe.ov.
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Application and Use
Frequently Asked Question: When Can I Buy One?
The answer to that question depends on what you want to buy.
The widespread use o hydrogen as an energy carrier requires a combination
o technological breakthroughs, market acceptance, and large investments in a
national hydrogen energy inrastructure. This evolutionary process will happen
over decades, wherein hydrogen will phase in as the technologies and their
markets are ready.
Fuel cells or some portable and small stationary applications are available to
commercial customers today. Fuel cell vehicles and hydrogen ueling stations
are also operating in certain parts o the country as part o demonstration
programs, although hydrogen uel cell vehicles ace greater technical challenges
that extend the timeline or their commercial market introduction.
Government, industry, and others continue to research, develop, and validate
hydrogen and uel cell technologies to meet specic technical targets based
on customer requirements or cost and perormance. As uel cells or portable
and stationary applications begin to penetrate the market, experience will be
gained in uel cell manuacturing, standards or hydrogen transport and storage
containers, and integration with the electricity grid that will support the expanded
use o hydrogen in all sectors o the economy and help integrate hydrogen into
our nations energy portolio.
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Application and Use
techolo Valiaio o a Lae Scale: the naioal Hoe Leai demosaio
I Mach 005, he u.S. depame o Ee (dOE) aoce he naioal Hoe Leai demosaio, a iqe
collaboaio o aomobile a ee is paes, hei spplies, a he eeal oveme o evalae hoe
el cell vehicle a iasce echoloies oehe i eal-wol coiios a assess poess owa echolo
eaiess o he commecial make. Is is povii hal he cos o he emosaio.
the leai emosaio will valiae hoe el cell echolo aais aes o el cell abili a eciec,
vehicle ae, a hoe el cos. I will also help ese seamless ieaio o vehicle a iasce ieaces.
Learning Demonstration Projects
the naioal Hoe Leai demosaio cel ivolves he wok o he ollowi o eams opeai el cell
vehicles a hoe eli saios o collec aa boh i coolle es coiios a o he ope oa i a vaie o
eoaphic aeas a climaes.
Chevo Copoaio is bili hoe eli saios i ohe a sohe Calioia a i Michia; Hai-Kia Moo
Compa is woki i paeship wih Chevo o es vehicles wih el cells maace b uie techoloies Copoaio.
daimleChsle is esi vehicles wih Balla Powe Ssems el cells; he vehicles will eel a hoe saios bil b
pojec pae BP i ohe a sohe Calioia a i Michia.
Fo Moo Compa is esi vehicles wih Balla el cells; he vehicles will eel a hoe saios bil b pojec
pae BP i ohe Calioia a Michia, as well as Floia.
geeal Moos Copoaio is esi vehicles wih is ow el cell echolo i paeship wih Shell Hoe, LLC,
which is bili hoe eli saios i seveal locaios: new yok, deoi, Calioia, a Washio, d.C.
Project Outcomes
the naioal Hoe Leai demosaio will help dOE ie is hoe a el cell compoe a maeialseseach a ma also cove ew echical challees ha have o e bee cosiee. Becase compoe aa
ahee i he emosaio will be obaie om a ieae ssem a e eal opeai coiios, sakeholes
will have he iomaio he ee o valiae dOE ecoomic, ee, a eviomeal moels/aalses, as well as
echolo sas impoa o csome eqiemes (vehicle el ecoom, el cell eciec/abili, eeze sa abili,
hoe cos, a opeai ae). B ivolvi majo sakeholes who will be esposible o bii vehicle a
ee echoloies o cosmes, he emosaio also povies a eal eviome o he shai o sae, coes, a
saas isses acoss eams, as well as a oppoi o has-o pblic ecaio ha ca ioce cosmes o hei
e ee opios.
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Saety, Codes and Standards
The United States saely produces and uses more than nine million
tons o hydrogen each year, primarily in controlled industrial
environments. Hydrogen can be used saely in consumer environments as well, when users understand its basic properties and
observe guidelines.
Hydrogen Saety
Like gasoline or natural gas, hydrogen is a uel that must be handled appropriately.
The characteristics o hydrogen are dierent rom those o other uels, but it
can be used as saely as other common uels when guidelines are observed.
Hydrogen has a number o properties that are advantageous with regard tosaety. It is much lighter than air and has a rapid diusivity (3.8 times aster
than natural gas), which means that when released in an open environment,
it disperses quickly into a non-fammable concentration. Hydrogen rises at a
speed o almost 20 meters per second, twice as ast as helium and six times
aster than natural gas.
With the exception o oxygen, any gas can cause asphyxiation in high enough
concentrations. But because hydrogen is buoyant and disperses rapidly, it is
less likely to be conned than other gases and thereore poses less risk as
an asphyxiant.
Hydrogen is non-toxic and non-poisonous. It will not contaminate groundwater
and is a gas under normal atmospheric conditions with a very low solubility
in water. A release o hydrogen is not known to contribute to atmospheric
pollution or water pollution.
Hydrogen is odorless, colorless, and tasteless, and thus undetectable by human
senses. By comparison, natural gas is also odorless, colorless, and tasteless,
but industry adds a sulur-containing odorant to make it readily detectable by
people. Odorants are not added to hydrogen because currently, there is no
known odorant light enough to travel with hydrogen or disperse in air atthe same rate. Odorants also contaminate uel cells. Industry considers these
properties when designing structures where hydrogen is used or stored and
includes redundant saety systems that include leak detection sensors and
ventilation systems.
Photo courtesy o CaFCP
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Hydrogen burns with a pale blue, nearly invisible, fame. Hydrogen fames also
have low radiant heat compared to hydrocarbon fames. Detection sensors buil
into hydrogen systems can quickly identiy any leak and eliminate the potential
Saety, Codes, and Standards
Wha ae Coes
a Saas?
Moel bili coes ae
ielies o he esi o he
bil eviome (bilis a
aciliies, o example). Coes
ae eeall aope b local
jisicios, heeb achievi
oce o law. Coes oe ee o
o ivoke saas o eqipme
se wihi he bil eviome.
Saas ae les, ielies,
coiios, o chaaceisics o
pocs o elae pocesses, a
eeall appl o eqipme ocompoes. Saas achieve a
elao-like sas whe he
ae eee o i coes o hoh
oveme elaios.
or undetected fames. Since hydrogen fames emit relat
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