Methods of hydrogen storage for subsequent use span many approaches including high pressures, cryogenics, and chemical compounds that reversibly release H2 upon heating. Underground hydrogen storage is useful to provide grid energy storage for intermittent energy sources, like wind power, as well as providing fuel for transportation, particularly for ships and airplanes.
Most research into hydrogen storage is focused on storing hydrogen as a lightweight, compact energy carrier for mobile applications.
Liquid hydrogen or slush hydrogen may be used, as in the Space Shuttle. However liquid hydrogen requires cryogenic storage and boils around 20.268 K (−252.882 °C or −423.188 °F). Hence, its liquefaction
imposes a large energy loss (as energy is needed to cool it down to
that temperature). The tanks must also be well insulated to prevent boil off but adding insulation increases cost. Liquid hydrogen has less energy densityby volume than hydrocarbon fuels such as gasoline
by approximately a factor of four. This highlights the density problem
for pure hydrogen: there is actually about 64% more hydrogen in a liter
of gasoline (116 grams hydrogen) than there is in a liter of pure liquid
hydrogen (71 grams hydrogen). The carbon in the gasoline also
contributes to the energy of combustion.
Compressed hydrogen, by comparison, is stored quite differently. Hydrogen gas has good energy density by weight, but poor energy density by volume versus hydrocarbons, hence it requires a larger tank to store. A large hydrogen tank
will be heavier than the small hydrocarbon tank used to store the same
amount of energy, all other factors remaining equal. Increasing gas
pressure would improve the energy density by volume, making for smaller,
but not lighter container tanks. Compressed hydrogen costs 2.1% of the energy content to power the compressor. Higher compression without energy recovery will mean more energy lost to the compression step. Compressed hydrogen storage can exhibit very low permeation.
Established technologies
net storage density of hydrogen
Compressed hydrogen
Compressed
hydrogen is a storage form where hydrogen gas is kept under pressures
to increase the storage density. Compressed hydrogen in hydrogen tanks
at 350 bar (5,000 psi) and 700 bar (10,000 psi) is used for hydrogen
tank systems in vehicles, based on type IV carbon-composite technology. Car manufacturers have been developing this solution, such as Honda or Nissan.
Liquid hydrogen
BMW has been working on liquid hydrogen tanks for cars, producing for example the BMW Hydrogen 7.
Japan have liquid hydrogen (LH2) storage at a tanker port in Kobe, and
are anticipated to receive the first shipment of liquid hydrogen via LH2
carrier in 2020.
Hydrogen is liquified by reducing its temperature to -253°C, similar to
liquified natural gas (LNG) which is stored at -162°C. A potential
efficiency loss of 12.79% can be achieved, or 4.26kWh/kg out of
33.3kWh/kg.
Proposals and research
Hydrogen
storage technologies can be divided into physical storage, where
hydrogen molecules are stored (including pure hydrogen storage via
compression and liquefaction), and chemical storage, where hydrides are
stored.
Chemical storage
Chemical
storage could offer high storage performance due to the strong binding
of hydrogen and the high storage densities. However, the regeneration of
storage material is still an issue. A large number of chemical storage
systems are under investigation, which involve hydrolysis reactions, hydrogenation/dehydrogenation reactions, ammonia borane and other boron hydrides, ammonia, and alane etc.
Storage in hydrocarbons may also be successful in overcoming the issue
with low density. For example, supercritical hydrogen at 30 °C and 500
bar only has a density of 15.0 mol/L while methanol has a density of 49.5 mol H2/L methanol and saturated dimethyl ether at 30 °C and 7 bar has a density of 42.1 mol H2/L dimethyl ether. These liquids would use much smaller, cheaper, safer storage tanks.
The most promising chemical approach is electrochemical hydrogen
storage, as the release of hydrogen can be controlled by the applied
electricity. Most of the materials listed below can be directly used for electrochemical hydrogen storage.
Metal hydrides
Metal hydride hydrogen storage
Metal hydrides, such as MgH2, NaAlH4, LiAlH4, LiH, LaNi5H6, TiFeH2 and palladium hydride, with varying degrees of efficiency, can be used as a storage medium for hydrogen, often reversibly.
Some are easy-to-fuel liquids at ambient temperature and pressure,
others are solids which could be turned into pellets. These materials
have good energy density, although their specific energy is often worse than the leading hydrocarbon fuels.
Most metal hydrides bind with hydrogen very strongly. As a
result, high temperatures around 120 °C (248 °F) – 200 °C (392 °F) are
required to release their hydrogen content. This energy cost can be
reduced by using alloys which consists of a strong hydride former and a
weak one such as in LiNH2, LiBH4 and NaBH4.
These are able to form weaker bonds, thereby requiring less input to
release stored hydrogen. However, if the interaction is too weak, the
pressure needed for rehydriding is high, thereby eliminating any energy
savings. The target for onboard hydrogen fuel systems is roughly
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).
An alternative method for reducing dissociation temperatures is doping with activators. This has been successfully used for aluminium hydride but its complex synthesis makes it undesirable for most applications as it is not easily recharged with hydrogen.
Currently the only hydrides which are capable of achieving the 9
wt% gravimetric goal for 2015 (see chart above) are limited to lithium,
boron and aluminium based compounds; at least one of the second-row
elements or Al must be added. Research is being done to determine new
compounds which can be used to meet these requirements.
New Scientist reported that Arizona State University is investigating using a borohydride solution to store hydrogen, which is released when the solution flows over a catalyst made of ruthenium. Researchers at University of Pittsburgh and Georgia Tech
performed extensive benchmarking simulations on mixtures of several
light metal hydrides to predict possible reaction thermodynamics for
hydrogen storage.
Non-metal hydrides
The Italian catalyst manufacturer Acta has proposed using hydrazine as an alternative to hydrogen in fuel cells.
As the hydrazine fuel is liquid at room temperature, it can be handled
and stored more easily than hydrogen. By storing it in a tank full of a
double-bonded carbon-oxygencarbonyl, it reacts and forms a safe solid called hydrazone.
By then flushing the tank with warm water, the liquid hydrazine hydrate
is released. Hydrazine breaks down in the cell to form nitrogen and hydrogen which bonds with oxygen, releasing water.
Silicon hydrides and germanium hydrides are also potential candidates
of hydrogen storage materials, as they can subject to energetically
favored reaction to form covalently bonded dimers with loss of a
hydrogen molecule.
Carbohydrates
Carbohydrates (polymeric C6H10O5) releases H2
in a bioreformer mediated by the enzyme cocktail—cell-free synthetic
pathway biotransformation. Carbohydrate provides high hydrogen storage
densities as a liquid with mild pressurization and cryogenic
constraints: It can also be stored as a solid powder. Carbohydrate is
the most abundant renewable bioresource in the world.
In May 2007 biochemical engineers from the Virginia Polytechnic Institute and State University
and biologists and chemists from the Oak Ridge National Laboratory
announced a method of producing high-yield pure hydrogen from starch and
water. In 2009, they demonstrated to produce nearly 12 moles of hydrogen per glucose unit from cellulosic materials and water.
Thanks to complete conversion and modest reaction conditions, they
propose to use carbohydrate as a high energy density hydrogen carrier
with a density of 14.8 wt%.
Synthesized hydrocarbons
An alternative to hydrides is to use regular hydrocarbon fuels as the hydrogen carrier. Then a small hydrogen reformer would extract the hydrogen as needed by the fuel cell. However, these reformers are slow to react to changes in demand and add a large incremental cost to the vehicle powertrain.
Direct methanol fuel cells
do not require a reformer, but provide a lower energy density compared
to conventional fuel cells, although this could be counterbalanced with
the much better energy densities of ethanol and methanol over hydrogen. Alcohol fuel is a renewable resource.
Solid-oxide fuel cells can operate on light hydrocarbons such as propane and methane
without a reformer, or can run on higher hydrocarbons with only partial
reforming, but the high temperature and slow startup time of these fuel
cells are problematic for automotive applications.
Aluminum
Aluminum has been proposed as an energy storage method by a number of researchers. hydrogen can be extracted from aluminum by reacting it with water. To react with water, however, aluminum must be stripped of its natural oxide layer, a process which requires pulverization, chemical reactions with caustic substances, or alloys. The byproduct of the reaction to create hydrogen is aluminum oxide, which can be recycled back into aluminum with the Hall–Héroult process, making the reaction theoretically renewable.
Liquid organic hydrogen carriers (LOHC)
Unsaturated organic compounds can store huge amounts of hydrogen. These Liquid Organic Hydrogen Carriers
(LOHC) are hydrogenated for storage and dehydrogenated again when the
energy/hydrogen is needed. Research on LOHC was concentrated on
cycloalkanes at an early stage, with its relatively high hydrogen
capacity (6-8 wt %) and production of COx-free hydrogen. Heterocyclic aromatic compounds (or N-Heterocycles) are also appropriate for this task. A compound that stands in the focus of the current LOHC research is N-ethylcarbazole (NEC) but many others do exist. More recently dibenzyltoluene,
which is already industrially used as a heat transfer fluid in
industry, was identified as potential LOHC. With a wide liquid range
between -39 °C (melting point) and 390 °C (boiling point) and a hydrogen
storage density of 6.2 wt% dibenzyltoluene is ideally suited as LOHC
material. More recently, formic acid (FA) has been suggested as a promising hydrogen storage material with a 4.4wt% hydrogen capacity.
Using LOHCs relatively high gravimetric storage densities can be
reached (about 6 wt-%) and the overall energy efficiency is higher than
for other chemical storage options such as producing methane from the hydrogen.
Cycloalkanes
Cycloalkanes reported as LOHC include cyclohexane, methyl-cyclohexane
and decalin. The dehydrogenation of cycloalkanes is highly endothermic
(63-69 kJ/mol H2), which means this process requires high temperature.
Dehydrogenation of decalin is the most thermodynamically favored among
the three cycloalkanes, and methyl-cyclohexane is second because of the
presence of the methyl group.
Research on catalyst development for dehydrogenation of cycloalkanes
has been carried out for decades. Nickel (Ni), Molybdenum (Mo) and
Platinum (Pt) based catalysts are highly investigated for
dehydrogenation. However, coking is still a big challenge for catalyst’s
long-term stability.
N-Heterocycles
Both hydrogenation and dehydrogenation of LOHCs requires catalysts.
It was demonstrated that replacing hydrocarbons by hetero-atoms, like
N, O etc. improves reversible de/hydrogenation properties. The
temperature required for hydrogenation and dehydrogenation of drops
significantly with increasing numbers of heteroatoms.
Among all the N-heterocycles, the saturated-unsaturated pair of
dodecahydro-N-ethylcarbazole (12H-NEC) and NEC has been considered as a
promising candidate for hydrogen storage with a fairly large hydrogen
content (5.8wt%).
The figure on the top right shows dehydrogenation and hydrogenation of
the 12H-NEC and NEC pair. The standard catalyst for NEC to 12H-NEC is Ru
and Rh based. The selectivity of hydrogenation can reach 97% at 7 MPa
and 130 °C-150 °C.
Although N-Heterocyles can optimize the unfavorable thermodynamic
properties of cycloalkanes, a lot of issues remain unsolved, such as
high cost, high toxicity and kinetic barriers etc.
Formic acid
In 2006 researchers of EPFL, Switzerland, reported the use of formic acid as a hydrogen storage material.
Carbon monoxide free hydrogen has been generated in a very wide
pressure range (1–600 bar). A homogeneous catalytic system based on
water-soluble ruthenium catalysts selectively decompose HCOOH into H2 and CO2 in aqueous solution.
This catalytic system overcomes the limitations of other catalysts
(e.g. poor stability, limited catalytic lifetimes, formation of CO) for
the decomposition of formic acid making it a viable hydrogen storage
material.
And the co-product of this decomposition, carbon dioxide, can be used
as hydrogen vector by hydrogenating it back to formic acid in a second
step. The catalytic hydrogenation of CO2 has long been studied and efficient procedures have been developed. Formic acid contains 53 g L−1
hydrogen at room temperature and atmospheric pressure. By weight, pure
formic acid stores 4.3 wt% hydrogen. Pure formic acid is a liquid with a
flash point 69 °C (cf. gasoline −40 °C, ethanol 13 °C). 85% formic acid
is not flammable.
Ammonia
Ammonia (NH3) releases H2
in an appropriate catalytic reformer. Ammonia provides high hydrogen
storage densities as a liquid with mild pressurization and cryogenic
constraints: It can also be stored as a liquid at room temperature and
pressure when mixed with water. Ammonia is the second most commonly
produced chemical in the world and a large infrastructure for making,
transporting, and distributing ammonia exists. Ammonia can be reformed
to produce hydrogen with no harmful waste, or can mix with existing
fuels and under the right conditions burn efficiently. Since there is no
carbon in ammonia, no carbon by-products are produced; thereby making
this possibility a "carbon neutral" option for the future. Pure ammonia
burns poorly at the atmospheric pressures found in natural gas fired
water heaters and stoves. Under compression in an automobile engine it
is a suitable fuel for slightly modified gasoline engines. Ammonia is a
toxic gas at normal temperature and pressure and has a potent odor.
In 2018, researchers at CSIRO in Australia powered a Toyota Mirai and Hyundai Nexo with hydrogen separated from ammonia using a membrane technology.
In September 2005 chemists from the Technical University of Denmark announced a method of storing hydrogen in the form of ammonia saturated into a salt tablet. They claim it will be an inexpensive and safe storage method.
Amine borane complexes
Prior to 1980, several compounds were investigated for hydrogen
storage including complex borohydrides, or aluminohydrides, and ammonium
salts. These hydrides have an upper theoretical hydrogen yield limited
to about 8.5% by weight. Amongst the compounds that contain only B, N,
and H (both positive and negative ions), representative examples
include: amine boranes, boron hydride ammoniates, hydrazine-borane
complexes, and ammonium octahydrotriborates or tetrahydroborates. Of
these, amine boranes (and especially ammonia borane)
have been extensively investigated as hydrogen carriers. During the
1970s and 1980s, the U.S. Army and Navy funded efforts aimed at
developing hydrogen/deuterium gas-generating compounds for use in the
HF/DF and HCl chemical lasers,
and gas dynamic lasers. Earlier hydrogen gas-generating formulations
used amine boranes and their derivatives. Ignition of the amine
borane(s) forms boron nitride (BN) and hydrogen gas. In addition to ammonia borane
(H3BNH3), other gas-generators include diborane diammoniate, H2B(NH3)2BH4.
Imidazolium ionic liquids
In 2007 Dupont
and others reported hydrogen-storage materials based on imidazolium
ionic liquids. Simple alkyl(aryl)-3-methylimidazolium
N-bis(trifluoromethanesulfonyl)imidate salts that possess very low
vapour pressure, high density, and thermal stability and are not
inflammable can add reversibly 6–12 hydrogen atoms in the presence of
classical Pd/C or Ir0 nanoparticle catalysts and can be used as
alternative materials for on-board hydrogen-storage devices. These salts
can hold up to 30 g L−1 of hydrogen at atmospheric pressure.
The phosphino-borane on the left accepts one equivalent of hydrogen
at one atmosphere and 25 °C and expels it again by heating to 100 °C.
The storage capacity is 0.25 wt% still rather below the 6 to 9 wt%
required for practical use.
Carbonite substances
Research has proven that graphene can store hydrogen efficiently. After taking up hydrogen, the substance becomes graphane. After tests, conducted by dr André Geim at the University of Manchester,
it was shown that not only can graphene store hydrogen easily, it can
also release the hydrogen again, after heating to 450 °C.
Metal-organic frameworks
Metal-organic frameworks
represent another class of synthetic porous materials that store
hydrogen and energy at the molecular level. MOFs are highly crystalline
inorganic-organic hybrid structures that contain metal clusters or ions
(secondary building units) as nodes and organic ligands as linkers. When
guest molecules (solvent) occupying the pores are removed during
solvent exchange and heating under vacuum, porous structure of MOFs can
be achieved without destabilizing the frame and hydrogen molecules will
be adsorbed onto the surface of the pores by physisorption. Compared to
traditional zeolites and porous carbon materials, MOFs have very high
number of pores and surface area which allow higher hydrogen uptake in a
given volume. Thus, research interests on hydrogen storage in MOFs have
been growing since 2003 when the first MOF-based hydrogen storage was
introduced. Since there are infinite geometric and chemical variations
of MOFs based on different combinations of SBUs and linkers, many
researches explore what combination will provide the maximum hydrogen
uptake by varying materials of metal ions and linkers.
In 2006, chemists at UCLA and the University of Michigan have achieved hydrogen storage concentrations of up to 7.5 wt% in MOF-74 at a low temperature of 77 K. In 2009, researchers at University of Nottingham reached 10 wt% at 77 bar (1,117 psi) and 77 K with MOF NOTT-112. Most articles about hydrogen storage in MOFs report hydrogen uptake
capacity at a temperature of 77K and a pressure of 1 bar because these
conditions are commonly available and the binding energy between
hydrogen and the MOF at this temperature is large compared to the
thermal vibration energy. Varying several factors such as surface area,
pore size, catenation, ligand structure, and sample purity can result in
different amounts of hydrogen uptake in MOFs.
Encapsulation
Cella Energy
technology is based around the encapsulation of hydrogen gas and
nano-structuring of chemical hydrides in small plastic balls, at room
temperature and pressure.
Physical storage
In
this case hydrogen remains in physical forms, i.e., as gas,
supercritical fluid, adsorbate, or molecular inclusions. Theoretical
limitations and experimental results are considered concerning the volumetric and gravimetric capacity of glass
microvessels, microporous, and nanoporous media, as well as safety and
refilling-time demands.
Activated carbons
Activated carbons are highly porous amorphous carbon materials with high apparent surface area. Hydrogen physisorption can be increased in these materials by increasing the apparent surface area and optimizing pore diameter to around 7 Å.
These materials are of particular interest due to the fact that they
can be made from waste materials, such as cigarette butts which have
shown great potential as precursor materials for high-capacity hydrogen
storage materials.
Cryo-compressed
Cryo-compressed
storage of hydrogen is the only technology that meets 2015 DOE targets
for volumetric and gravimetric efficiency.
Furthermore, another study has shown that cryo-compressed
exhibits interesting cost advantages: ownership cost (price per mile)
and storage system cost (price per vehicle) are actually the lowest when
compared to any other technology.
For example, a cryo-compressed hydrogen system would cost $0.12 per
mile (including cost of fuel and every associated other cost), while
conventional gasoline vehicles cost between $0.05 and $0.07 per mile.
Like liquid storage, cryo-compressed uses cold hydrogen (20.3 K
and slightly above) in order to reach a high energy density. However,
the main difference is that, when the hydrogen would warm-up due to heat
transfer with the environment ("boil off"), the tank is allowed to go
to pressures much higher (up to 350 bars versus a couple of bars for
liquid storage). As a consequence, it takes more time before the
hydrogen has to vent, and in most driving situations, enough hydrogen is
used by the car to keep the pressure well below the venting limit.
Consequently, it has been demonstrated that a high driving range
could be achieved with a cryo-compressed tank : more than 650 miles
(1,050 km) were driven with a full tank mounted on an hydrogen-fueled
engine of Toyota Prius. Research is still on its way in order to study and demonstrate the full potential of the technology.
As of 2010, the BMW Group has started a thorough component and
system level validation of cryo-compressed vehicle storage on its way to
a commercial product.
Carbon nanotubes
Carbon nanotubes
Hydrogen carriers based on nanostructured carbon (such as carbon buckyballs and nanotubes)
have been proposed. However, since Hydrogen usually amounts up to
~3.0-7.0 wt% at 77K which is far from the value set by US department of
Energy (6 wt% at nearly ambient conditions), it makes carbon materials
poor candidates for hydrogen storage.
Clathrate hydrates
H2 caged in a clathrate hydrate was first reported in 2002, but requires very high pressures to be stable. In 2004, researchers from Delft University of Technology and Colorado School of Mines showed solid H2-containing hydrates could be formed at ambient temperature and 10s of bar by adding small amounts of promoting substances such as THF. These clathrates have a theoretical maximum hydrogen densities of around 5 wt% and 40 kg/m3.
Glass capillary arrays
A
team of Russian, Israeli and German scientists have collaboratively
developed an innovative technology based on glass capillary arrays for
the safe infusion, storage and controlled release of hydrogen in mobile
applications. The C.En technology has achieved the United States Department of Energy (DOE) 2010 targets for on-board hydrogen storage systems.
DOE 2015 targets can be achieved using flexible glass capillaries and cryo-compressed method of hydrogen storage.
Glass microspheres
Hollow glass microspheres (HGM) can be utilized for controlled storage and release of hydrogen.
Stationary hydrogen storage
Unlike
mobile applications, hydrogen density is not a huge problem for
stationary applications. As for mobile applications, stationary
applications can use established technology:
'Available
storage technologies, their capacity and discharge time.' COMMISSION
STAFF WORKING DOCUMENT Energy storage – the role of electricity
Underground hydrogen storage is the practice of hydrogen storage in underground caverns, salt domes and depleted oil and gas fields. Large quantities of gaseous hydrogen have been stored in underground caverns by ICI for many years without any difficulties. The storage of large quantities of liquid hydrogen underground can function as grid energy storage. The round-trip efficiency is approximately 40% (vs. 75-80% for pumped-hydro (PHES)), and the cost is slightly higher than pumped hydro, if only a limited number of hours of storage is required.
Another study referenced by a European staff working paper found that
for large scale storage, the cheapest option is hydrogen at €140/MWh for
2,000 hours of storage using an electrolyser, salt cavern storage and
combined-cycle power plant. The European project Hyunder
indicated in 2013 that for the storage of wind and solar energy an
additional 85 caverns are required as it cannot be covered by PHES and CAES systems.
A German case study on storage of hydrogen in salt caverns found that
if the German power surplus (7% of total variable renewable generation
by 2025 and 20% by 2050) would be converted to hydrogen
and stored underground, these quantities would require some 15 caverns
of 500,000 cubic metres each
by 2025 and some 60 caverns by 2050 – corresponding to approximately one
third of the number of
underground gas caverns currently operated in Germany.
In the US, Sandia Labs are conducting research into the storage of
hydrogen in depleted oil and gas fields, which could easily absorb large
amounts of renewably produced hydrogen as there are some 2.7 million
depleted wells in existence.
Power to gas
Power to gas is a technology which converts electrical power to a gas fuel. There are two methods: the first is to use the electricity for water splitting and inject the resulting hydrogen into the natural gas grid; the second, less efficient method is used to convert carbon dioxide and hydrogen to methane, (see natural gas) using electrolysis and the Sabatier reaction. A third option is to combine the hydrogen via electrolysis with a source of carbon (either carbon dioxide or carbon monoxide from biogas, from industrial processes or via direct air-captured carbon dioxide) via biomethanation, where biomethanogens (archaea) consume carbon dioxide and hydrogen and produce methane within an anaerobic environment. This process is highly efficient, as the archaea are self-replicating and only require low-grade (60°C) heat to perform the reaction.
Another process has also been achieved by SoCalGas to convert the carbon dioxide in raw biogas to methane in a single electrochemical step, representing a simpler method of converting excess renewable electricity into storable natural gas.
The UK has completed surveys and is preparing to start injecting hydrogen into the gas grid as the grid previously carried 'town gas' which is a 50% hydrogen-methane gas formed from coal. Auditors KPMG found that converting the UK to hydrogen gas could be £150bn to £200bn cheaper than rewiring British homes to use electric heating powered by lower-carbon sources.
Excess power or off peak power generated by wind generators or solar arrays can then be used for load balancing in the energy grid. Using the existing natural gas system for hydrogen, Fuel cell maker Hydrogenics and natural gas distributor Enbridge have teamed up to develop such a power to gas system in Canada.
Pipeline storage of hydrogen where a natural gas network is used for the storage of hydrogen. Before switching to natural gas, the German gas networks were operated using towngas, which for the most part (60-65%) consisted of hydrogen. The storage capacity of the German natural gas network is more than 200,000 GW·h which is enough for several months of energy requirement. By comparison, the capacity of all German pumped storage power plants amounts to only about 40 GW·h. The transport of energy through a gas network is done with much less loss.
Automotive Onboard hydrogen storage
Targets for on-board hydrogen storage assuming storage of 5
kg of hydrogen
Targets were set by the FreedomCAR Partnership in January 2002 between the United States Council for Automotive Research (USCAR) and U.S. DOE (Targets assume a 5-kg H2 storage system). The 2005 targets were not reached in 2005. The targets were revised in 2009 to reflect new data on system efficiencies obtained from fleets of test cars. The ultimate goal for volumetric storage is still above the theoretical density of liquid hydrogen.
It is important to note that these targets are for the hydrogen
storage system, not the hydrogen storage material. System densities are
often around half those of the working material, thus while a material
may store 6 wt% H2,
a working system using that material may only achieve 3 wt% when the
weight of tanks, temperature and pressure control equipment, etc., is
considered.
In 2010, only two storage technologies were identified as having
the potential to meet DOE targets: MOF-177 exceeds 2010 target for
volumetric capacity, while cryo-compressed H2 exceeds more restrictive 2015 targets for both gravimetric and volumetric capacity.
The hydrogen economy is a proposed system of delivering energy using hydrogen. The term hydrogen economy was coined by John Bockris during a talk he gave in 1970 at General Motors (GM) Technical Center. The concept was proposed earlier by geneticist J.B.S. Haldane.
Proponents of a hydrogen economy advocate hydrogen as a potential fuel for motive power
(including cars and boats) and on-board auxiliary power, stationary
power generation (e.g., for the energy needs of buildings), and as an
energy storage medium (e.g., for interconversion from excess electric
power generated off-peak). Molecular hydrogen of the sort that can be
used as a fuel does not occur naturally in convenient reservoirs;
nonetheless it can be generated by steam reformation of hydrocarbons, water electrolysis or by other methods.
A spike in attention for the concept during the 2000s has been repeatedly described as hype by some critics and proponents of alternative technologies. A resurgence in the energy carrier is now underway, notably by the forming of the Hydrogen Council in 2017. Several manufacturers have now released hydrogen fuel cell
cars commercially, with manufacturers such as Toyota and industry
groups in China planning to increase numbers of the cars into the
hundreds of thousands over the next decade.
Rationale
Elements of the hydrogen economy
A hydrogen economy was proposed by the University of Michigan to solve some of the negative effects of using hydrocarbon
fuels where the carbon is released to the atmosphere (as carbon
dioxide, carbon monoxide, unburnt hydrocarbons, etc.). Modern interest
in the hydrogen economy can generally be traced to a 1970 technical
report by Lawrence W. Jones of the University of Michigan.
In the current hydrocarbon economy, transportation is fueled primarily by petroleum. Burning of hydrocarbon fuels emits carbon dioxide
and other pollutants. The supply of economically usable hydrocarbon
resources in the world is limited, and the demand for hydrocarbon fuels
is increasing, particularly in China, India, and other developing countries.
Proponents of a world-scale hydrogen economy argue that hydrogen
can be an environmentally cleaner source of energy to end-users,
particularly in transportation applications, without release of
pollutants (such as particulate matter) or carbon dioxide at the point
of end use. A 2004 analysis asserted that "most of the hydrogen supply
chain pathways would release significantly less carbon dioxide into the
atmosphere than would gasoline used in hybrid electric vehicles" and that significant reductions in carbon dioxide emissions would be possible if carbon capture or carbon sequestration methods were utilized at the site of energy or hydrogen production.
Hydrogen has a high energy density by weight but has a low energy density by volume. Even when highly compressed or liquified, the energy density
by volume is only 1/4 that of gasoline, although the energy density by
weight is approximately three times that of gasoline or natural gas. An
Otto cycleinternal-combustion engine running on hydrogen is said to have a maximum efficiency of about 38%, 8% higher than a gasoline internal-combustion engine.
The combination of the fuel cell and electric motor is 2-3 times more efficient than an internal-combustion engine.
Capital costs of fuel cells have reduced significantly over recent
years, with a modeled cost of $50/kW cited by the Department of Energy.
Previous technical obstacles have included hydrogen storage issues
and the purity requirement of hydrogen used in fuel cells, as with
current technology, an operating fuel cell requires the purity of
hydrogen to be as high as 99.999%. Hydrogen engine conversion technology
could be considered more economical than fuel cells.
Current hydrogen market
Timeline
Hydrogen production is a large and growing industry, as of 2004. Globally, some 57 million metric tons of hydrogen, equal to about 170 million tons of oil equivalent, were produced in 2004. The growth rate is around 10% per year. Within the United States, 2004 production was about 11 million metric tons (Mt), an average power flow of 48 gigawatts.
(For comparison, the average electric production in 2003 was some
442 GW.) As of 2005, the economic value of all hydrogen produced
worldwide is about $135 billion per year.
There are two primary uses for hydrogen today. About half is used in the Haber process to produce ammonia (NH3), which is then used directly or indirectly as fertilizer. Because both the world population and the intensive agriculture
used to support it are growing, ammonia demand is growing. Ammonia can
be used as a safer and easier indirect method of transporting hydrogen.
Transported ammonia can be then converted back to hydrogen at the bowser
by a membrane technology.
The other half of current hydrogen production is used to convert heavy petroleum sources into lighter fractions suitable for use as fuels. This latter process is known as hydrocracking.
Hydrocracking represents an even larger growth area, since rising oil
prices encourage oil companies to extract poorer source material, such
as tar sands and oil shale.
The scale economies inherent in large-scale oil refining and fertilizer
manufacture make possible on-site production and "captive" use. Smaller
quantities of "merchant" hydrogen are manufactured and delivered to end
users as well.
If energy for hydrogen production were available (from wind, solar, fission or fusion nuclear power etc.), use of the substance for hydrocarbon synfuel
production could expand captive use of hydrogen by a factor of 5 to 10.
Present U.S. use of hydrogen for hydrocracking is roughly 4 Mt per
year. It is estimated that 37.7 Mt/yr of hydrogen would be sufficient to
convert enough domestic coal to liquid fuels to end U.S. dependence on
foreign oil importation, and less than half this figure to end dependence on Middle East oil. Coal liquefaction
would present significantly worse emissions of carbon dioxide than does
the current system of burning fossil petroleum, but it would eliminate
the political and economic vulnerabilities inherent in US oil
importation before the commercialization of tight oil in North America.
As of 2004 and 2016, 96% of global hydrogen production is from fossil fuels (48% from natural gas, 30% from oil, and 18% from coal); water electrolysis accounts for only 4%.
The distribution of production reflects the effects of thermodynamic
constraints on economic choices: of the four methods for obtaining
hydrogen, partial combustion of natural gas in a NGCC
(natural gas combined cycle) power plant offers the most efficient
chemical pathway and the greatest off-take of usable heat energy.
The large market and sharply rising prices in fossil fuels have
also stimulated great interest in alternate, cheaper means of hydrogen
production.
As of 2002, most hydrogen is produced on site and the cost is
approximately $0.70/kg and, if not produced on site, the cost of liquid
hydrogen is about $2.20/kg to $3.08/kg.
Production, storage, infrastructure
Today's hydrogen is mainly produced (>90%) from fossil sources. Linking its centralized production to a fleet of light-duty fuel cell vehicles would require the siting and construction of a distribution infrastructure with large investment of capital.
Further, the technological challenge of providing safe, energy-dense
storage of hydrogen on board the vehicle must be overcome to provide
sufficient range between fillups.
Methods of production
Molecular hydrogen is not available on Earth in convenient natural reservoirs. Most hydrogen in the lithosphere is bonded to oxygen in water. Manufacturing elemental hydrogen does require the consumption of a hydrogen carrier such as a fossil fuel
or water. The former carrier consumes the fossil resource and produces
carbon dioxide, but often requires no further energy input beyond the
fossil fuel. Decomposing water, the latter carrier, requires electrical or heat input, generated from some primary energy source (fossil fuel, nuclear power or a renewable energy). Hydrogen can also be produced by refining the effluent from geothermal sources in the lithosphere. Hydrogen produced by zero emission renewable energy sources such as electrolysis of water using wind power, solar power, hydro power, wave power or tidal power is referred to as green hydrogen.
Hydrogen produced by non-renewable energy sources may be referred to as
brown hydrogen. Hydrogen produced as a waste by-product or industrial
by-product is sometimes referred to as grey hydrogen.
Current production methods
Hydrogen is industrially produced from steam reforming, which uses fossil fuels such as natural gas, oil, or coal.
The energy content of the produced hydrogen is less than the energy
content of the original fuel, some of it being lost as excessive heat
during production. Steam reforming leads to carbon dioxide emissions, in
the same way as a car engine would do.
A small part (4% in 2006) is produced by electrolysis using electricity and water, consuming approximately 50 kilowatt-hours of electricity per kilogram of hydrogen produced.
H2 production cost ($-gge untaxed) at varying natural gas prices
Hydrogen can be made via high pressure electrolysis, low pressure electrolysis of water, or a range of other emerging electrochemical processes such as high temperature electrolysis or carbon assisted electrolysis. However, current best processes for water electrolysis have an effective electrical efficiency of 70-80%, so that producing 1 kg of hydrogen (which has a specific energy
of 143 MJ/kg or about 40 kWh/kg) requires 50–55 kWh of electricity. At
an electricity cost of $0.06/kWh, as set out in the Department of Energy
hydrogen production
targets for 2015, the hydrogen cost is $3/kg. With the range of natural gas prices from 2016 as shown in the graph (Hydrogen Production Tech Team Roadmap, November 2017)
putting the cost of SMR hydrogen at between $1.20 and $1.50, the cost
price of hydrogen via electrolysis is still over double 2015 DOE
hydrogen target prices. The US DOE target price for hydrogen in 2020 is
$2.30/kg, requiring an electricity cost $0.037/kWh, which is achievable
given recent PPA tenders
for wind and solar in many regions. This puts the $4/gge H2 dispensed
objective well within reach, and close to a slightly elevated natural
gas production cost for SMR.
In other parts of the world, steam methane reforming is between
$1-3/kg on average. This makes production of hydrogen via electrolysis
cost competitive in many regions already, as outlined by Nel Hydrogen and others, including an article by the IEA examining the conditions which could lead to a competitive advantage for electrolysis.
Biological hydrogen can be produced in an algaebioreactor. In the late 1990s it was discovered that if the algae is deprived of sulfur it will switch from the production of oxygen, i.e. normal photosynthesis, to the production of hydrogen.
Biological hydrogen can be produced in bioreactors that use
feedstocks other than algae, the most common feedstock being waste
streams. The process involves bacteria feeding on hydrocarbons and
excreting hydrogen and CO2. The CO2 can be
sequestered successfully by several methods, leaving hydrogen gas. In
2006-2007, NanoLogix first demonstrated a prototype hydrogen bioreactor
using waste as a feedstock at Welch's grape juice factory in North East,
Pennsylvania (U.S.).
Biocatalysed electrolysis
Besides
regular electrolysis, electrolysis using microbes is another
possibility. With biocatalysed electrolysis, hydrogen is generated after
running through the microbial fuel cell and a variety of aquatic plants can be used. These include reed sweetgrass, cordgrass, rice, tomatoes, lupines, and algae
High-pressure electrolysis
High pressure electrolysis is the electrolysis of water by decomposition of water (H2O) into oxygen (O2) and hydrogen gas (H2) by means of an electric current being passed through the water. The difference with a standard electrolyzer is the compressed hydrogen output around 120-200 bar (1740-2900 psi, 12–20 MPa). By pressurising the hydrogen in the electrolyser, through a process known as chemical compression, the need for an external hydrogen compressor is eliminated, the average energy consumption for internal compression is around 3%.
European largest (1 400 000 kg/a, High-pressure Electrolysis of water,
acaline technology) hydrogen production plant is operating at Kokkola,
Finland.
High-temperature electrolysis
Hydrogen can be generated from energy supplied in the form of heat
and electricity through high-temperature electrolysis (HTE). Because
some of the energy in HTE is supplied in the form of heat, less of the
energy must be converted twice (from heat to electricity, and then to
chemical form), and so potentially far less energy is required per
kilogram of hydrogen produced.
While nuclear-generated electricity could be used for
electrolysis, nuclear heat can be directly applied to split hydrogen
from water. High temperature (950–1000 °C) gas cooled nuclear reactors
have the potential to split hydrogen from water by thermochemical means
using nuclear heat. Research into high-temperature nuclear reactors may
eventually lead to a hydrogen supply that is cost-competitive with
natural gas steam reforming. General Atomics
predicts that hydrogen produced in a High Temperature Gas Cooled
Reactor (HTGR) would cost $1.53/kg. In 2003, steam reforming of natural
gas yielded hydrogen at $1.40/kg. In 2005 natural gas prices, hydrogen
costs $2.70/kg.
High-temperature electrolysis has been demonstrated in a laboratory, at 108 MJ (thermal) per kilogram of hydrogen produced,
but not at a commercial scale. In addition, this is lower-quality
"commercial" grade Hydrogen, unsuitable for use in fuel cells.
Photoelectrochemical water splitting
Using
electricity produced by photovoltaic systems offers the cleanest way to
produce hydrogen. Water is broken into hydrogen and oxygen by
electrolysis—a photoelectrochemical cell (PEC) process which is also named artificial photosynthesis.
William Ayers at Energy Conversion Devices demonstrated and patented
the first multijunction high efficiency photoelectrochemical system for
direct splitting of water in 1983.
This group demonstrated direct water splitting now referred to as an
"artificial leaf" or "wireless solar water splitting" with a low cost
thin film amorphous silicon multijunction sheet immersed directly in
water. Hydrogen evolved on the front amorphous silicon surface decorated
with various catalysts while oxygen evolved off the back metal
substrate. A Nafion membrane above the multijunction cell provided a
path for ion transport. Their patent also lists a variety of other
semiconductor multijunction materials for the direct water splitting in
addition to amorphous silicon and silicon germanium alloys. Research
continues towards developing high-efficiency multi-junction cell
technology at universities and the photovoltaic industry. If this
process is assisted by photocatalysts suspended directly in water
instead of using photovoltaic and an electrolytic system, the reaction
is in just one step, which can improve efficiency.
Photoelectrocatalytic production
A
method studied by Thomas Nann and his team at the University of East
Anglia consists of a gold electrode covered in layers of indium
phosphide (InP) nanoparticles. They introduced an iron-sulfur complex
into the layered arrangement, which when submerged in water and
irradiated with light under a small electric current, produced hydrogen
with an efficiency of 60%.
In 2015, it was reported that Panasonic Corp. has developed a photocatalyst based on niobium nitride that can absorb 57% of sunlight to support the decomposition of water to produce hydrogen gas. The company plans to achieve commercial application "as early as possible", not before 2020.
Concentrating solar thermal
Very
high temperatures are required to dissociate water into hydrogen and
oxygen. A catalyst is required to make the process operate at feasible
temperatures. Heating the water can be achieved through the use of concentrating solar power. Hydrosol-2 is a 100-kilowatt pilot plant at the Plataforma Solar de Almería in Spain
which uses sunlight to obtain the required 800 to 1,200 °C to heat
water. Hydrosol II has been in operation since 2008. The design of this
100-kilowatt pilot plant is based on a modular concept. As a result, it
may be possible that this technology could be readily scaled up to the
megawatt range by multiplying the available reactor units and by
connecting the plant to heliostat fields (fields of sun-tracking mirrors) of a suitable size.
Thermochemical production
There are more than 352 thermochemical cycles which can be used for water splitting, around a dozen of these cycles such as the iron oxide cycle, cerium(IV) oxide-cerium(III) oxide cycle, zinc zinc-oxide cycle, sulfur-iodine cycle, copper-chlorine cycle and hybrid sulfur cycle are under research and in testing phase to produce hydrogen and oxygen from water and heat without using electricity. These processes can be more efficient than high-temperature electrolysis, typical in the range from 35% - 49% LHV
efficiency. Thermochemical production of hydrogen using chemical energy
from coal or natural gas is generally not considered, because the
direct chemical path is more efficient.
None of the thermochemical hydrogen production processes have
been demonstrated at production levels, although several have been
demonstrated in laboratories.
Hydrogen as a byproduct of other chemical processes
The industrial production of chlorine and caustic soda by electrolysis
generates a sizable amount of Hydrogen as a byproduct. In the port of
Antwerp a 1MW demonstration fuel cell power plant is powered by such
byproduct. This unit has been operational since late 2011. The excess hydrogen is often managed with a hydrogen pinch analysis.
Storage
Although molecular hydrogen has very high energy density on a mass basis, partly because of its low molecular weight,
as a gas at ambient conditions it has very low energy density by
volume. If it is to be used as fuel stored on board the vehicle, pure
hydrogen gas must be stored in an energy-dense form to provide
sufficient driving range.
Pressurized hydrogen gas
Increasing gas pressure improves the energy density by volume, making for smaller, but not lighter container tanks (see pressure vessel). Achieving higher pressures necessitates greater use of external energy to power the compression. The mass of the hydrogen tanks needed for compressed hydrogen
reduces the fuel economy of the vehicle. Because it is a small
molecule, hydrogen tends to diffuse through any liner material intended
to contain it, leading to the embrittlement,
or weakening, of its container. The most common method of on board
hydrogen storage in today's demonstration vehicles is as a compressed
gas at pressures of roughly 700 bar (70 MPa).
Liquid hydrogen
Alternatively, higher volumetric energy density liquid hydrogen or slush hydrogen may be used. However, liquid hydrogen is cryogenic and boils at 20.268 K (–252.882 °C or –423.188 °F). Cryogenic storage cuts weight but requires large liquification energies. The liquefaction process, involving pressurizing and cooling steps, is energy intensive.
The liquefied hydrogen has lower energy density by volume than gasoline
by approximately a factor of four, because of the low density of liquid
hydrogen — there is actually more hydrogen in a liter of gasoline
(116 grams) than there is in a liter of pure liquid hydrogen (71 grams).
Liquid hydrogen storage tanks must also be well insulated to minimize boil off.
Japan have a liquid hydrogen (LH2) storage facility at a terminal
in Kobe, and are expected to receive the first shipment of liquid
hydrogen via LH2 carrier in 2020.
Hydrogen is liquified by reducing its temperature to -253°C, similar to
liquified natural gas (LNG) which is stored at -162°C. A potential
efficiency loss of 12.79% can be achieved, or 4.26kWh/kg out of
33.3kWh/kg.
Storage as hydride
Distinct from storing molecular hydrogen, hydrogen can be stored as a chemical hydride
or in some other hydrogen-containing compound. Hydrogen gas is reacted
with some other materials to produce the hydrogen storage material,
which can be transported relatively easily. At the point of use the
hydrogen storage material can be made to decompose, yielding hydrogen
gas. As well as the mass and volume density problems associated with
molecular hydrogen storage, current barriers to practical storage
schemes stem from the high pressure and temperature conditions needed
for hydride formation and hydrogen release. For many potential systems
hydriding and dehydriding kinetics and heat management are also issues that need to be overcome. A French company McPhy Energy is developing the first industrial product, based on Magnesium Hydrate,
already sold to some major clients such as Iwatani and ENEL.
Adsorption
A third approach is to adsorb
molecular hydrogen on the surface of a solid storage material. Unlike
in the hydrides mentioned above, the hydrogen does not
dissociate/recombine upon charging/discharging the storage system, and
hence does not suffer from the kinetic limitations of many hydride
storage systems. Hydrogen densities similar to liquefied hydrogen can be
achieved with appropriate adsorbent materials. Some suggested
adsorbents include activated carbon, nanostructured carbons (including CNTs), MOFs, and hydrogen clathrate hydrate.
Underground hydrogen storage
'Available
storage technologies, their capacity and discharge time.' COMMISSION
STAFF WORKING DOCUMENT Energy storage – the role of electricity
Underground hydrogen storage is the practice of hydrogen storage in underground caverns, salt domes and depleted oil and gas fields. Large quantities of gaseous hydrogen have been stored in underground caverns by ICI for many years without any difficulties. The storage of large quantities of liquid hydrogen underground can function as grid energy storage. The round-trip efficiency is approximately 40% (vs. 75-80% for pumped-hydro (PHES)), and the cost is slightly higher than pumped hydro.
Another study referenced by a European staff working paper found that
for large scale storage, the cheapest option is hydrogen at €140/MWh for
2,000 hours of storage using an electrolyser, salt cavern storage and
combined-cycle power plant. The European project Hyunder
indicated in 2013 that for the storage of wind and solar energy an
additional 85 caverns are required as it cannot be covered by PHES and CAES systems.
A German case study on storage of hydrogen in salt caverns found that
if the German power surplus (7% of total variable renewable generation
by 2025 and 20% by 2050) would be converted to hydrogen
and stored underground, these quantities would require some 15 caverns
of 500,000 cubic metres each
by 2025 and some 60 caverns by 2050 – corresponding to approximately one
third of the number of
underground gas caverns currently operated in Germany.
In the US, Sandia Labs are conducting research into the storage of
hydrogen in depleted oil and gas fields, which could easily absorb large
amounts of renewably produced hydrogen as there are some 2.7 million
depleted wells in existence.
Power to gas
Power to gas is a technology which converts electrical power to a gas fuel. There are 2 methods, the first is to use the electricity for water splitting and inject the resulting hydrogen into the natural gas grid. The second (less efficient) method is used to convert carbon dioxide and water to methane, using electrolysis and the Sabatier reaction.
The excess power or off peak power generated by wind generators or
solar arrays is then used for load balancing in the energy grid. Using
the existing natural gas system for hydrogen Fuel cell maker Hydrogenics and natural gas distributor Enbridge have teamed up to develop such a power to gas system in Canada.
Pipeline storage
A natural gas network may be used for the storage of hydrogen. Before switching to natural gas, the German gas networks were operated using towngas,
which for the most part consisted of hydrogen. The storage capacity of
the German natural gas network is more than 200,000 GW·h which is enough
for several months of energy requirement. By comparison, the capacity
of all German pumped storage power plants amounts to only about 40 GW·h.
The transport of energy through a gas network is done with much less
loss (<0 .1="" a="" existing="" href="https://en.wikipedia.org/wiki/List_of_natural_gas_pipelines" in="" network="" of="" power="" than="" the="" title="List of natural gas pipelines" use="">natural gas pipelines0>
for hydrogen was studied by NaturalHy
Because of hydrogen embrittlement of steel, and corrosion
natural gas pipes require internal coatings or replacement in order to
convey hydrogen. Techniques are well-known; over 700 miles of hydrogen pipeline
currently exist in the United States. Although expensive, pipelines are
the cheapest way to move hydrogen. Hydrogen gas piping is routine in
large oil-refineries, because hydrogen is used to hydrocrack fuels from crude oil.
Hydrogen piping can in theory be avoided in distributed systems
of hydrogen production, where hydrogen is routinely made on site using
medium or small-sized generators which would produce enough hydrogen for
personal use or perhaps a neighborhood. In the end, a combination of
options for hydrogen gas distribution may succeed.
While millions of tons of elemental hydrogen are distributed
around the world each year in various ways, bringing hydrogen to
individual consumers would require an evolution of the fuel
infrastructure. For example, according to GM, 70% of the U.S.
population lives near a hydrogen-generating facility but has little
public access to that hydrogen. The same study however, shows that
building the infrastructure in a systematic way is much more doable and
affordable than most people think. For example, one article has noted
that hydrogen stations could be put within every 10 miles in metro Los
Angeles, and on the highways between LA and neighboring cities like Palm
Springs, Las Vegas, San Diego and Santa Barbara, for the cost of a
Starbuck's latte for every one of the 15 million residents living in
these areas.
A key tradeoff: centralized vs. distributed production
In
a future full hydrogen economy, primary energy sources and feedstock
would be used to produce hydrogen gas as stored energy for use in
various sectors of the economy. Producing hydrogen from primary energy
sources other than coal, oil, and natural gas, would result in lower
production of the greenhouse gases characteristic of the combustion of
these fossil energy resources.
One key feature of a hydrogen economy would be that in mobile
applications (primarily vehicular transport) energy generation and use
could be decoupled. The primary energy source would need no longer
travel with the vehicle, as it currently does with hydrocarbon fuels.
Instead of tailpipes creating dispersed emissions, the energy (and
pollution) could be generated from point sources such as large-scale,
centralized facilities with improved efficiency. This would allow the
possibility of technologies such as carbon sequestration, which are otherwise impossible for mobile applications. Alternatively, distributed energy generation schemes (such as small scale renewable energy sources) could be used, possibly associated with hydrogen stations.
Aside from the energy generation, hydrogen production could be
centralized, distributed or a mixture of both. While generating hydrogen
at centralized primary energy plants promises higher hydrogen
production efficiency, difficulties in high-volume, long range hydrogen
transportation (due to factors such as hydrogen damage
and the ease of hydrogen diffusion through solid materials) makes
electrical energy distribution attractive within a hydrogen economy. In
such a scenario, small regional plants or even local filling stations
could generate hydrogen using energy provided through the electrical
distribution grid. While hydrogen generation efficiency is likely to be
lower than for centralized hydrogen generation, losses in hydrogen
transport could make such a scheme more efficient in terms of the
primary energy used per kilogram of hydrogen delivered to the end user.
The proper balance between hydrogen distribution and
long-distance electrical distribution is one of the primary questions
that arises about the hydrogen economy.
Again the dilemmas of production sources and transportation of
hydrogen can now be overcome using on site (home, business, or fuel
station) generation of hydrogen from off grid renewable sources.
Distributed electrolysis
Distributed
electrolysis would bypass the problems of distributing hydrogen by
distributing electricity instead. It would use existing electrical
networks to transport electricity to small, on-site electrolysers
located at filling stations. However, accounting for the energy used to
produce the electricity and transmission losses would reduce the
overall efficiency.
Natural gascombined cyclepower plants,
which account for almost all construction of new electricity generation
plants in the United States, generate electricity at efficiencies of 60
percent or greater.[citation needed]
Increased demand for electricity, whether due to hydrogen cars or other
demand, would have the marginal impact of adding new combined cycle
power plants. On this basis, distributed production of hydrogen would be
roughly 40% efficient. However, if the marginal impact is referred to
today's power grid, with an efficiency of roughly 40% owing to its mix
of fuels and conversion methods, the efficiency of distributed hydrogen
production would be roughly 25%.
The distributed production of hydrogen in this fashion would be
expected to generate air emissions of pollutants and carbon dioxide at
various points in the supply chain, e.g., electrolysis, transportation
and storage. Such externalities as pollution must be weighed against the
potential advantages of a hydrogen economy.
Fuel cells as alternative to internal combustion
One of the main offerings of a hydrogen economy is that the fuel can replace the fossil fuel burned in internal combustion engines and turbines
as the primary way to convert chemical energy into kinetic or
electrical energy; hereby eliminating greenhouse gas emissions and
pollution from that engine. Although hydrogen can be used in
conventional internal combustion engines, fuel cells, being electrochemical,
have a theoretical efficiency advantage over heat engines. Fuel cells
are more expensive to produce than common internal combustion engines.
Some types of fuel cells work with hydrocarbon fuels,
while all can be operated on pure hydrogen. In the event that fuel
cells become price-competitive with internal combustion engines and
turbines, large gas-fired power plants could adopt this technology.
Hydrogen gas must be distinguished as "technical-grade" (five
nines pure, 99.999%), which is suitable for applications such as fuel
cells, and "commercial-grade", which has carbon- and sulfur-containing
impurities, but which can be produced by the much cheaper
steam-reformation process. Fuel cells require high-purity hydrogen
because the impurities would quickly degrade the life of the fuel cell
stack.
Much of the interest in the hydrogen economy concept is focused on the use of fuel cells to power electric cars. Current hydrogen fuel cells suffer from a low power-to-weight ratio.
Fuel cells are much more efficient than internal combustion engines,
and produce no harmful emissions. If a practical method of hydrogen storage is introduced, and fuel cells become cheaper, they can be economically viable to power hybrid fuel cell/battery
vehicles, or purely fuel cell-driven ones. The economic viability of
fuel cell powered vehicles will improve as the hydrocarbon fuels used in
internal combustion engines become more expensive, because of the
depletion of easily accessible reserves or economic accounting of
environmental impact through such measures as carbon taxes.
Other fuel cell technologies based on the exchange of metal ions (e.g. zinc-air fuel cells)
are typically more efficient at energy conversion than hydrogen fuel
cells, but the widespread use of any electrical energy → chemical
energy → electrical energy systems would necessitate the production of
electricity.
Since the 2003 State of the Union address, when the notion of the hydrogen economy came to national prominence in the United States,
there has been a steady chorus of naysayers. Most recently, in 2013,
Lux Research, Inc. issued a report that stated: "The dream of a hydrogen
economy ... is no nearer." It concluded that "Capital cost, not
hydrogen supply, will limit adoption to a mere 5.9 GW" by 2030,
providing "a nearly insurmountable barrier to adoption, except in niche
applications". Lux's analysis speculated that by 2030, PEM stationary
market will reach $1 billion, while the vehicle market, including
forklifts, will reach a total of $2 billion.
Use as an automotive fuel and system efficiency
An accounting of the energy utilized during a thermodynamic process,
known as an energy balance, can be applied to automotive fuels. With
today's technology, the manufacture of hydrogen via steam reforming
can be accomplished with a thermal efficiency of 75 to 80 percent.
Additional energy will be required to liquefy or compress the hydrogen,
and to transport it to the filling station via truck or pipeline. The
energy that must be utilized per kilogram to produce, transport and
deliver hydrogen (i.e., its well-to-tank energy use) is approximately
50 MJ
using technology available in 2004. Subtracting this energy from the
enthalpy of one kilogram of hydrogen, which is 141 MJ, and dividing by
the enthalpy, yields a thermal energy efficiency of roughly 60%.
Gasoline, by comparison, requires less energy input, per gallon, at the
refinery, and comparatively little energy is required to transport it
and store it owing to its high energy density per gallon at ambient
temperatures. Well-to-tank, the supply chain for gasoline is roughly 80%
efficient (Wang, 2002). Another grid-based method of supplying hydrogen
would be to use electrical
to run electrolysers. Roughly 6% of electricity is lost during
transmission along power lines, and the process of converting the fossil
fuel to electricity in the first place is roughly 33 percent efficient.
Thus if efficiency is the key determinant it would be unlikely hydrogen
vehicles would be fueled by such a method, and indeed viewed this way, electric vehicles
would appear to be a better choice. However, as noted above, hydrogen
can be produced from a number of feedstocks, in centralized or
distributed fashion, and these afford more efficient pathways to produce
and distribute the fuel.
A study of the well-to-wheels efficiency of hydrogen vehicles
compared to other vehicles in the Norwegian energy system indicates
that hydrogen fuel-cell vehicles (FCV) tend to be about a third as
efficient as EVs when electrolysis is used, with hydrogen Internal
Combustion Engines (ICE) being barely a sixth as efficient. Even in the
case where hydrogen fuel cells get their hydrogen from natural gas
reformation rather than electrolysis, and EVs get their power from a
natural gas power plant, the EVs still come out ahead 35% to 25% (and
only 13% for a H2 ICE). This compares to 14% for a gasoline
ICE, 27% for a gasoline ICE hybrid, and 17% for a diesel ICE, also on a
well-to-wheels basis.
Hydrogen has been called one of the least efficient and most
expensive possible replacements for gasoline (petrol) in terms of
reducing greenhouse gases; other technologies may be less expensive and
more quickly implemented.
A comprehensive study of hydrogen in transportation applications has
found that "there are major hurdles on the path to achieving the vision
of the hydrogen economy; the path will not be simple or
straightforward". Although Ford Motor Company and French Renault-Nissan cancelled their hydrogen car R&D efforts in 2008 and 2009, respectively,
they signed a 2009 letter of intent with the other manufacturers and
Now GMBH in September 2009 supporting the commercial introduction of
FCVs by 2015. A study by The Carbon Trust for the UK Department of Energy and Climate Change
suggests that hydrogen technologies have the potential to deliver UK
transport with near-zero emissions whilst reducing dependence on
imported oil and curtailment of renewable generation. However, the
technologies face very difficult challenges, in terms of cost,
performance and policy.
Hydrogen safety
Hydrogen has one of the widest explosive/ignition mix range with air of all the gases with few exceptions such as acetylene, silane, and ethylene oxide.
That means that whatever the mix proportion between air and hydrogen, a
hydrogen leak will most likely lead to an explosion, not a mere flame,
when a flame or spark ignites the mixture. This makes the use of
hydrogen particularly dangerous in enclosed areas such as tunnels or
underground parking. Pure hydrogen-oxygen flames burn in the ultraviolet color range and are nearly invisible to the naked eye, so a flame detector is needed to detect if a hydrogen leak is burning. Hydrogen is odorless and leaks cannot be detected by smell.
Codes and standards have repeatedly been identified as a major institutional barrier to deploying hydrogen technologies
and developing a hydrogen economy. To enable the commercialization of
hydrogen in consumer products, new model building codes and equipment
and other technical standards are developed and recognized by federal,
state, and local governments.
One of the measures on the roadmap is to implement higher safety standards like early leak detection with hydrogen sensors. The Canadian Hydrogen Safety Program concluded that hydrogen fueling is as safe as, or safer than, compressed natural gas (CNG) fueling. The European Commission has funded the first higher educational program in the world in hydrogen safety engineering at the University of Ulster.
It is expected that the general public will be able to use hydrogen
technologies in everyday life with at least the same level of safety and
comfort as with today's fossil fuels.
Environmental concerns
There are many concerns regarding the environmental effects of the manufacture of hydrogen. Hydrogen is made either by electrolysis of water, or by fossil fuel reforming.
Reforming a fossil fuel leads to a higher emissions of carbon dioxide
compared with direct use of the fossil fuel in an internal combustion
engine. Similarly, if hydrogen is produced by electrolysis from
fossil-fuel powered generators, increased carbon dioxide is emitted in
comparison with direct use of the fossil fuel.
Using renewable energy source to generate hydrogen by
electrolysis would require greater energy input than direct use of the
renewable energy to operate electric vehicles, because of the extra
conversion stages and losses in distribution. Hydrogen as transportation
fuel, however, is mainly used for fuel cells that do not produce
greenhouse gas emission, but water.
There have also been some concerns over possible problems related to hydrogen gas leakage. Molecular hydrogen leaks slowly from most containment vessels. It has
been hypothesized that if significant amounts of hydrogen gas (H2) escape, hydrogen gas may, because of ultraviolet radiation, form free radicals (H) in the stratosphere. These free radicals would then be able to act as catalysts for ozone depletion. A large enough increase in stratospheric hydrogen from leaked H2
could exacerbate the depletion process. However, the effect of these
leakage problems may not be significant. The amount of hydrogen that
leaks today is much lower (by a factor of 10–100) than the estimated
10–20% figure conjectured by some researchers; for example, in Germany,
the leakage rate is only 0.1% (less than the natural gas leak rate of
0.7%). At most, such leakage would likely be no more than 1–2% even with
widespread hydrogen use, using present technology.
Costs
In 2004,
the production of unit of hydrogen fuel by steam reformation or
electrolysis was approximately 3 to 6 times more expensive than the
production of an equivalent unit of fuel from natural gas.
When evaluating costs, fossil fuels are generally used as the
reference. The energy content of these fuels is not a product of human
effort and so has no cost assigned to it. Only the extraction, refining,
transportation and production costs are considered. On the other hand,
the energy content of a unit of hydrogen fuel must be manufactured, and
so has a significant cost, on top of all the costs of refining,
transportation, and distribution. Systems which use renewably generated
electricity more directly, for example in trolleybuses, or in battery electric vehicles
may have a significant economic advantage because there are fewer
conversion processes required between primary energy source and point of
use.
The barrier to lowering the price of high purity hydrogen is a
cost of more than 35 kWh of electricity used to generate each kilogram
of hydrogen gas. Hydrogen produced by steam reformation costs
approximately three times the cost of natural gas per unit of energy
produced. This means that if natural gas costs $6/million BTU, then
hydrogen will be $18/million BTU. Also, producing hydrogen from
electrolysis with electricity at 5 cents/kWh will cost $28/million BTU —
about 1.5 times the cost of hydrogen from natural gas. Note that the
cost of hydrogen production from electricity is a linear function of
electricity costs, so electricity at 10 cents/kWh means that hydrogen
will cost $56/million BTU.
Demonstrated advances in electrolyser and fuel cell technology by ITM Power
are claimed to have made significant in-roads into addressing the cost
of electrolysing water to make hydrogen. Cost reduction would make
hydrogen from off-grid renewable sources economic for refueling
vehicles.
Hydrogen pipelines are more expensive than even long-distance electric lines. Hydrogen is about three times bulkier in volume than natural gas for the same enthalpy. Hydrogen accelerates the cracking of steel (hydrogen embrittlement),
which increases maintenance costs, leakage rates, and material costs.
The difference in cost is likely to expand with newer technology: wires
suspended in air can use higher voltage with only marginally increased
material costs, but higher pressure pipes require proportionally more
material.
Setting up a hydrogen economy would require huge investments in
the infrastructure to store and distribute hydrogen to vehicles. In
contrast, battery electric vehicles,
which are already publicly available, would not necessitate immediate
expansion of the existing infrastructure for electricity transmission
and distribution. Power plant capacity that now goes unused at night
could be used for recharging electric vehicles. A study conducted by the
Pacific Northwest National Laboratory for the US Department of Energy
in December 2006 found that the idle off-peak grid capacity in the US
would be sufficient to power 84% of all vehicles in the US if they all
were immediately replaced with electric vehicles.
Different production methods each have differing associated
investment and marginal costs. The energy and feedstock could originate
from a multitude of sources, i.e. natural gas, nuclear, solar, wind,
biomass, coal, other fossil fuels, and geothermal.
Natural Gas at Small Scale
Uses steam reformation. Requires 15.9 million cubic feet (450,000 m3)
of gas, which, if produced by small 500 kg/day reformers at the point
of dispensing (i.e., the filling station), would equate to 777,000
reformers costing $1 trillion and producing 150 million tons of hydrogen
gas annually. Obviates the need for distribution infrastructure
dedicated to hydrogen. $3.00 per GGE (Gallons of Gasoline Equivalent)
Nuclear
Provides energy for electrolysis of water. Would require
240,000 tons of unenriched uranium — that's 2,000 600-megawatt power
plants, which would cost $840 billion, or about $2.50 per GGE.
Solar
Provides energy for electrolysis of water. Would require 2,500 kWh
of sun per square meter, 113 million 40-kilowatt systems, which would
cost $22 trillion, or about $9.50 per GGE.
Wind
Provides energy for electrolysis of water. At 7 meters per second
average wind speed, it would require 1 million 2-MW wind turbines, which
would cost $3 trillion, or about $3.00 per GGE.
Biomass
Gasification plants would produce gas with steam reformation.
1.5 billion tons of dry biomass, 3,300 plants which would require
113.4 million acres (460,000 km²) of farm to produce the biomass.
$565 billion in cost, or about $1.90 per GGE
Coal
FutureGen plants use coal gasification then steam reformation.
Requires 1 billion tons of coal or about 1,000 275-megawatt plants with a
cost of about $500 billion, or about $1 per GGE.
Several domestic U.S.automobile
manufactures have committed to develop vehicles using hydrogen. The
distribution of hydrogen for the purpose of transportation is currently
being tested around the world, particularly in Portugal, Iceland, Norway, Denmark, Germany, California, Japan and Canada, but the cost is very high.
Some hospitals have installed combined electrolyser-storage-fuel
cell units for local emergency power. These are advantageous for
emergency use because of their low maintenance requirement and ease of
location compared to internal combustion driven generators.
Iceland has committed to becoming the world's first hydrogen economy by the year 2050. Iceland is in a unique position. Presently, it imports all the petroleum products necessary to power its automobiles and fishing fleet. Iceland has large geothermal resources, so much that the local price of electricity actually is lower than the price of the hydrocarbons that could be used to produce that electricity.
Iceland already converts its surplus electricity into exportable
goods and hydrocarbon replacements. In 2002, it produced 2,000 tons of
hydrogen gas by electrolysis, primarily for the production of ammonia (NH3)
for fertilizer. Ammonia is produced, transported, and used throughout
the world, and 90% of the cost of ammonia is the cost of the energy to
produce it. Iceland is also developing an aluminium-smelting industry.
Aluminium costs are driven primarily by the cost of the electricity to
run the smelters. Either of these industries could effectively export
all of Iceland's potential geothermal electricity.
Neither industry directly replaces hydrocarbons. Reykjavík, Iceland, had a small pilot fleet of city buses running on compressed hydrogen,
and research on powering the nation's fishing fleet with hydrogen is
under way. For more practical purposes, Iceland might process imported
oil with hydrogen to extend it, rather than to replace it altogether.
The Reykjavík buses are part of a larger program, HyFLEET:CUTE,
operating hydrogen fueled buses in eight European cities. HyFLEET:CUTE
buses were also operated in Beijing, China and Perth, Australia (see
below). A pilot project demonstrating a hydrogen economy is operational
on the Norwegian island of Utsira. The installation combines wind power and hydrogen power. In periods when there is surplus wind energy, the excess power is used for generating hydrogen by electrolysis. The hydrogen is stored, and is available for power generation in periods when there is little wind.
The UK started a fuel cell pilot program in January 2004, the program ran two Fuel cell buses on route 25 in London until December 2005, and switched to route RV1 until January 2007.
The Hydrogen Expedition is currently working to create a hydrogen fuel
cell-powered ship and using it to circumnavigate the globe, as a way to
demonstrate the capability of hydrogen fuel cells.
Western Australia's Department of Planning and Infrastructure
operated three Daimler Chrysler Citaro fuel cell buses as part of its
Sustainable Transport Energy for Perth Fuel Cells Bus Trial in Perth.
The buses were operated by Path Transit on regular Transperth public
bus routes. The trial began in September 2004 and concluded in September
2007. The buses' fuel cells used a proton exchange membrane system and
were supplied with raw hydrogen from a BP refinery in Kwinana, south of
Perth. The hydrogen was a byproduct of the refinery's industrial
process. The buses were refueled at a station in the northern Perth
suburb of Malaga.
The United Nations Industrial Development Organization (UNIDO) and the Turkish Ministry of Energy and Natural Resources have signed in 2003 a $40 million trust fund agreement for the creation of the International Centre for Hydrogen Energy Technologies (UNIDO-ICHET) in Istanbul, which started operation in 2004.
A hydrogen forklift, a hydrogen cart and a mobile house powered by
renewable energies are being demonstrated in UNIDO-ICHET's premises. An
uninterruptible power supply system has been working since April 2009 in
the headquarters of Istanbul Sea Buses company.
Hydrogen-using alternatives to a fully distributive hydrogen economy
Hydrogen is simply a method to store and transmit energy. Various
alternative energy transmission and storage scenarios which begin with
hydrogen production, but do not use it for all parts of the store and
transmission infrastructure, may be more economic, in both near and far
term. These include:
Ammonia economy
An alternative to gaseous hydrogen as an energy carrier is to bond it with nitrogen from the air to produce ammonia, which can be easily liquefied, transported, and used (directly or indirectly) as a clean and renewable fuel. For example, researchers at CSIRO in Australia in 2018 fuelled a Toyota Mirai and Hyundai Nexo with hydrogen separated from ammonia using a membrane technology.
Hydrogen production of greenhouse-neutral alcohol
The methanol economy is a synfuel production energy plan which may
begin with hydrogen production. Hydrogen in a full "hydrogen economy"
was initially suggested as a way to make renewable energy,
in non-polluting form, available to automobiles. However, a theoretical
alternative to address the same problem is to produce hydrogen
centrally and immediately use it to make liquid fuels from a CO2 source. This would eliminate the requirement to transport and store the hydrogen. The source could be CO2 that is produced by fuel-burning power plants. In order to be greenhouse-neutral, the source for CO2 in such a plan would need to be from air, biomass, or other source of CO2 which is already in, or to be released into, the air. Direct methanol fuel cells are in commercial use, though as of August 2011 they are not efficient.
The electrical grid plus synthetic methanol fuel cells
Many of the hybrid strategies described above, using captive hydrogen
to generate other more easily usable fuels, might be more effective
than hydrogen-production alone. Short term energy storage (meaning the
energy is used not long after it has been captured) may be best
accomplished with battery or even ultracapacitor storage. Longer term
energy storage (meaning the energy is used weeks or months after
capture) may be better done with synthetic methane or alcohols, which
can be stored indefinitely at relatively low cost, and even used
directly in some type of fuel cells, for electric vehicles. These
strategies dovetail well with the recent interest in Plug-in Hybrid Electric Vehicles, or PHEVs, which use a hybrid strategy of electrical and fuel storage for their energy needs.
Hydrogen storage has been proposed by some
to be optimal in a narrow range of energy storage time, probably
somewhere between a few days and a few weeks. This range is subject to
further narrowing with any improvements in battery technology. It is
always possible that some kind of breakthrough in hydrogen storage or
generation could occur, but this is unlikely given that the physical and
chemical limitations of the technical choices are fairly well
understood.
Captive hydrogen synthetic methane production (SNG synthetic natural gas)
In
a similar way as with synthetic alcohol production, hydrogen can be
used on site to directly (nonbiologically) produce greenhouse-neutral
gaseous fuels. Thus, captive-hydrogen-mediated production of
greenhouse-neutral methane
has been proposed (note that this is the reverse of the present method
of acquiring hydrogen from natural methane, but one that does not
require ultimate burning and release of fossil fuel carbon). Captive
hydrogen (and carbon dioxide from, for example, CCS (Carbon Capture
& Storage)) may be used onsite to synthesize methane, using the Sabatier reaction.
This is about 60% efficient, and with the round trip reducing to 20 to
36% depending on the method of fuel utilization. This is even lower than
hydrogen, but the storage costs drop by at least a factor of 3, because
of methane's higher boiling point and higher energy density. Liquid
methane has 3.2 times the energy density of liquid hydrogen and is
easier to store compactly. Additionally, the pipe infrastructure (natural gas
pipelines) are already in place. Natural-gas-powered vehicles already
exist, and are known to be easier to adapt from existing internal engine
technology, than internal combustion autos running directly on
hydrogen. Experience with natural gas powered vehicles shows that
methane storage is inexpensive, once one has accepted the cost of
conversion to store the fuel. However, the cost of alcohol storage is
even lower, so this technology would need to produce methane at a
considerable savings with regard to alcohol production. Ultimate mature
prices of fuels in the competing technologies are not presently known,
but both are expected to offer substantial infrastructural savings over
attempts to transport and use hydrogen directly.
It has been proposed in a hypothetical renewable energy dominated
energy system to use the excess electricity generated by wind, solar
photovoltaic, hydro, marine currents and others to produce hydrogen by
electrolysis of water then combine it with CO2 make methane (natural gas). Hydrogen would firstly be used onsite in fuel cells (CHP) or for
transportation due to its greater efficiency of production and then
methane created which could then be injected into the existing gas
network to generate electricity and heat on demand to overcome low
points of renewable energy production. The process described would be
to create hydrogen (which could partly be used directly in fuel cells)
and the addition of carbon dioxide CO2 possibly from BECCS (Bio-Energy with Carbon Capture & Storage) via the (Sabatier reaction) to create methane as follows :
CO2 + 4H2 → CH4 + 2H2O.
Note: After combusting methane in CCGT the CO2 would again be captured, i.e., CCS and used to produce new methane.