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Introduction
Hydrogen would appear to be the ideal fuel. It is available in great
abundance, is non-toxic, and the residue from burning pure hydrogen is simply
water, without other pollution. It could be used as a fuel for power stations,
for heating, and for use in land vehicles, aeroplanes and ships. It has already
been used to power space rockets, and in prototype buses and cars.
The advantages of using hydrogen to replace oil, gas and coal have been recognised
since at least 1975, and a great deal of research has been devoted to the subject.
However, the hoped-for hydrogen economy has been
slow in coming. Nevertheless, hydrogen remains a potentially very attractive
option for combining a long-term solution to the greenhouse effect with the
freedom to expand energy usage to meet human demands.
Much of the publicity gives the impression that hydrogen can simply be used
as a clean replacement for fossil fuel as soon as some challenging
technical problems are solved. But this misses an important point. Unlike
fossil fuel, which already exists, hydrogen has to be produced, and that uses
energy. This means that in deciding how useful hydrogen is we must take account
of the method used to produce it, as well as how it is distributed and used.
At present almost all hydrogen is produced from fossil fuel. The ideal is
to produce it from water, and to do so using renewable
resources such as hydro, wind or solar
electricity. It can also be produced by nuclear fission
power (with low carbon emissions) and possibly in the distant future by
nuclear fusion.
Hydrogen could be used to store energy from sources which don't
work all the time, such as solar photovoltaic panels, or which generate energy
when it is not needed, for example from nuclear power stations which are
more efficient if run continuously.
Hydrogen could also be used as a means for transporting energy. In one
scenario, industrial installations in tropical countries would use the energy
of the sun to extract hydrogen from water. The hydrogen would then be transported
by pipeline or ship to where it is needed; this is discussed further elsewhere.
It could then be converted into electrical energy, used for space heating or
cooking, or packaged for use in vehicles and for mobile requirements.
In many cases, an alternative to the use of hydrogen as a fuel
is the use of batteries. It seems unlikely that the use of batteries will satisfy
some applications
– for example powering aeroplanes. However, the future importance of
battery solutions versus solutions based on hydrogen is difficult
to predict, and depends on how the technologies develop. It may also prove
more economic to create other fuels rather than hydrogen.
On this page we will outline the situation and outlook. More detailed
discussions on the hydrogen economy can be found
in the Wikipedia,
Netinform and
a report to
the US Department of Energy.
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Hydrogen as a Fuel
Hydrogen is a very energetic fuel by weight, yielding around 33 kilowatt-hours
per kilogram. This is two and a half to three times the energy of the same
weight of conventional fuels, such as natural gas or petrol. However,
hydrogen has a very low molecular weight, and as a result it is a very light
gas
– one kilogram at atmospheric pressure occupies 11,200 litres.
Liquid hydrogen is course denser, but petrol at room temperature is roughly
10 times denser still, so by volume liquid
hydrogen actually has only about one-quarter of the energy density of petrol.
This means that a basic problem with hydrogen is storage.
If stored as a gas it needs to be compressed, and energy is needed to do that.
And as a liquid hydrogen must be kept very cold since its boiling point is –252.8 °C,
or 20 K, and even more energy is needed to liquefy
it and keep it cold.
Hydrogen burns with a light blue flame which is not easy to see. To avoid
people touching the flame accidentally, it is desirable to mix the hydrogen
with a small proportion of a fuel such as methane to colour the flame. Burning
hydrogen in air is not entirely benign, as some nitrous oxide is also produced.
More serious is the problem of explosion. A wide range of hydrogen concentrations
in air can explode if ignited, so care must be taken.
Fuel cells (discussed below) allow the direct conversion
of hydrogen energy into electricity. This is much more efficient than simply
burning the hydrogen in a conventional engine.
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Production of Hydrogen
Hydrogen is currently used in large quantities as an industrial chemical,
to make ammonia for fertiliser and in processing crude oil to make fuel. Almost
all of this hydrogen is produced from natural gas, oil and coal, as in the
photo. Producing hydrogen from natural gas is fairly efficient partly because
waste heat from power stations is used in the process. However, the fossil-fuel
sources are not renewable, and carbon dioxide is also produced as a by-product.
Electrolysis
of water, currently used to produce only about 4% of industrial hydrogen, could
be far cleaner and more sustainable.
Low temperature electrolysis
Electric current is passed between two electrodes immersed in water. Oxygen
appears at the positive electrode and hydrogen at the negative electrode.
The diagram illustrates the basic principle of electrolysis. The
water has to be pure as impurities can degrade the electrodes. The advantages
of this process are that:
- it is static and simple, and so can run for long periods without attention,
- it generates very pure hydrogen, desirable for use in fuel cells, and
- the hydrogen can be generated at high pressure – this saves the considerable
energy that can be required to pressurise hydrogen for ease of storage and
transport.
About 80% of the electrical energy can be stored as hydrogen. The remaining
energy appears as heat, which must be removed.
High temperature electrolysis
If electrolysis takes place at a higher temperature, or even on steam, more
of the energy used to create the hydrogen can be thermal energy,
which is less expensive than electrical energy. However, the need for high
temperatures rules out solar photovoltaic and wind energy. Heating by solar
concentrators or by nuclear power would be possible. If a lot of nuclear power
(which is opposed by many people) is available, this could help in utilising
the inflexible always-on operation of nuclear power stations, since they could
be making hydrogen at times of low electricity demand.
Thermochemical production
Thermochemical processes have been demonstrated which produce hydrogen and
oxygen from water and heat, without using electricity. These could be more
efficient than processes involving electrolysis, since the input energy is
only heat. However, none has been implemented on a commercial scale.
Very high temperature separation
At temperatures of 3000 °C water molecules separate into their constituent
hydrogen and oxygen. The necessary temperature can be reached by
solar furnaces, and the gases can then be separated by diffusion through a suitable
membrane. This method of separation is currently only experimental; there
are no commercial installations.
The energy for making hydrogen should ideally come from a cheap, clean,
renewable source – we are still a long way from this goal.
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Storage of Hydrogen
If hydrogen is stored at atmospheric pressure, its low density means that
there is only about one-third as much energy per cubic metre as in natural
gas.
A gasometer holding the same energy reserves needs to be three times
the size if hydrogen is the fuel. To reduce the volume needed for storage,
hydrogen must be stored in a compressed or liquefied state, or in
some sort of chemical form. Hydrogen at 50 atmospheres pressure provides
about 0.13 kilowatt-hours per litre, and liquid hydrogen provides
2.36 kilowatt-hours per
litre.
As a gas
At present, temporary storage of bulk hydrogen for later use is most cheaply
done in suitable underground caverns. ICI stores hydrogen
at 50 atmospheres pressure in old salt mines at Teesside. Stationary storage
above ground uses similar pressures.
If hydrogen is to be used as a transport fuel it is very important to provide
a sufficient driving range. It takes energy to compress the gas, and
although compressing the gas improves the energy density, smaller
tanks are not necessarily lighter than big ones because they must be stronger,
and heavy tanks are not good for fuel economy.
For example, a 100 litre tank of hydrogen at 250 atmospheres
pressure holds about 2.2 kg of hydrogen
providing 72 kilowatt-hours of energy. Such a tank might weigh 30 kg,
far more than the hydrogen itself. By
comparison, for 72 kilowatt-hours of petrol only 5.7 litres is needed
– this weighs about 4 kg plus a tank weighing a fraction
of a kilogram. These figures also indicate that such a hydrogen tank would
not provide very much driving range – well under 100 miles in today's
most efficient cars. To match the driving range of a typical petrol car in
this way would require a hydrogen tank of about 1 cu.metre.
Hydrogen is a small, energetic molecule, so it tends to diffuse through any
material intended to contain it. This causes embrittlement, or weakening, of
tanks and pipes.
As a liquid
Liquid hydrogen presents more problems. An energy-intensive liquefaction
process is needed to get to the required –252.8 °C, and then
cryogenic storage must be used. Liquid hydrogen storage tanks must be
well insulated to minimise boil-off. Due largely to the use of liquid hydrogen
fuel for space rockets, good low-mass liquid hydrogen tanks have been developed.
However, they will still be heavier and bulkier than petrol tanks, and for
the same energy yield the hydrogen volume must be more than four times greater.
The photo shows the liquid hydrogen fuel tank in the boot of a car.
As a solid
Instead of storing pure hydrogen, it can also be stored as a chemical
hydride or some other hydrogen-containing compound. Hydrogen gas is reacted
with 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. Charging the metal
hydride store with hydrogen generates heat, and heat must also be applied to
the container to release the hydrogen. Barriers to practical storage schemes
include the high pressure and temperature conditions needed first to form the
hydride and then to release the hydrogen. Compared with compressed
hydrogen, this storage method provides a higher energy density: 72 kilowatt-hours
of hydrogen would need about a 20 litre lightweight container, but the
hydride would weigh about 110 kg. Compared
with the equivalent petrol tank, the hydride store is about three times the
size and twenty times as heavy.
Another approach is to absorb molecular hydrogen into a solid storage material.
Unlike hydrides, the hydrogen does not combine chemically in the storage system,
and hence does not suffer from some of the limitations of hydride storage
systems. Hydrogen densities similar to liquefied hydrogen can be achieved with
appropriate absorption media.
However, the most common method of on-board hydrogen storage in today's demonstration
vehicles is still the simple option of compressed gas, at pressures of
roughly 700 atmospheres.
The low density of hydrogen presents major
unsolved problems for storage.
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Transport of Hydrogen
The low density of hydrogen means that pipeline distribution of compressed
hydrogen gas is a less efficient method of transporting energy
than a similar natural gas pipeline. As mentioned above, to get the same energy
as natural gas three times the volume of hydrogen must be delivered, and this
uses more energy. In addition, hydrogen accelerates
the cracking of steel (embrittlement), which increases maintenance costs, leakage
rates, and material costs.
Well-established pipeline installations in the USA and Germany do exist to
transport hydrogen over distances of up to 40 miles. However, hydrogen
pipelines are more expensive than even long-distance electric lines.
Pressurised containers can also be used to transport gaseous hydrogen, at
pressures up to 250 atmospheres, on lorries, trains, and ships.
Liquid hydrogen can be transported in standard shipping containers which include
the necessary cooling and insulation. Each container holds about 40 cubic
metres of liquid hydrogen. Large-scale world-wide distribution of liquid hydrogen
would require ships equipped with cryogenic tanks and cooling facilities.
Hydrogen can also be stored for transport in the form of metal hydride,
as described above, balancing convenience with a big weight and energy penalty.
The problems of transport raise the important question of whether it is better
to:
- use
centralised renewable energy facilities to produce the hydrogen, which
then needs to be distributed using infrastructure that does not yet exist,
or to
- produce the hydrogen locally, which would probably be less efficient
but would eliminate some of the problems of transporting it.
Either way, the
infrastructure for obtaining hydrogen where it is needed does not yet exist.
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Fuel Cells and
Electricity
Combustion vs. fuel cells
The low-tech option of simply burning hydrogen in
place of fossil fuels could be used for a variety of applications:
- in power stations, to generate
steam which then drives turbines. This would be very inefficient, since electricity
would be used to produce the hydrogen in the first place, and then the hydrogen
would be used to generate electricity. The only benefit would be in having
a clean fuel to generate electricity at times of high demand when renewable
methods are not available.
- to heat buildings, in a similar way
to natural gas. (Indeed, hydrogen was a significant component of the town
gas, generated from coal, that was used before natural gas.) However, as
indicated elsewhere on this page, distribution presents serious problems,
and also the burners used for natural gas are not suitable without adjustment.
- for transport, where a portable energy supply
is needed to drive cars, buses and aeroplanes. (Trains would do better to
use electricity directly where possible.) Internal-combustion engines that
run on hydrogen have been available for over 60 years.
Transport is probably the main area of interest for hydrogen. However, fuel
cells driving electric motors can convert much more of the energy of
the hydrogen into motive power than an internal combustion engine, and so
fuel cells are generally felt to be a more appropriate
technology than simply burning hydrogen.
Another area where fuel cells might become widely used, if they are more efficient
and cost-effective than batteries or portable generators, is to supply electricity
in places where there is no mains electricity supply.
Fuel cell basics
Fuel cells provide a very efficient way of converting the energy in
the hydrogen into electricity. They were first made in about 1842 by a Welsh
scientist, Sir William Robert Grove, and were first used in spacecraft in the
1960s. Since 1990 there has been intensive development for commercial applications.
A fuel cell is an electrochemical conversion
device. It produces electricity from fuel
and an oxidant, which react in the presence
of an electrolyte. The fuel and oxidant flow into the cell, and the reaction
products flow out of it, while the electrolyte remains inside. Fuel cells
can operate virtually continuously as long as the necessary flows are maintained.
The difference between fuel cells and batteries is that fuel cells must have
external supplies of fuel and reactant, e.g. hydrogen and oxygen
(from air), whereas batteries internally store electrical energy chemically.
Other combinations of fuel and oxidant are possible. Other fuels include
hydrocarbons and alcohols, and oxidants include chlorine and chlorine dioxide.
In a hydrogen fuel cell the electrons and protons making
up hydrogen are separated by a catalyst, typically platinum or a similar metal
or alloy. The protons pass through an electrolyte. The electrons, however,
are forced to travel through an external circuit, thus producing electrical
power. Another catalytic process takes the electrons back in, combining them
with the protons and the oxidant to form water, the waste product. Other fuels
than hydrogen produce carbon dioxide as well as water as the waste. The diagram
illustrates this. In normal use a number of fuel cells are stacked in order
to generate more electrical power.
Types of fuel cells
There are quite a few different kinds of fuel cells, with different applications.
Alkaline Fuel Cells were the original type used on spacecraft.
They are simple and cheap to make, but must be fed with pure hydrogen and
oxygen since carbon dioxide degrades the electrolyte.
Proton Exchange Membrane Fuel Cells (PEMFC) are very light, very
efficient, and require only atmospheric oxygen instead of pure oxygen. A membrane
allows the protons through and into the electrolyte, but forces the electrons
to flow through the external circuit before they can combine with oxygen to
form water. However, these fuel cells are very sensitive to carbon monoxide.
The photo shows a stationary stack of PEMFCs with a rated output of 5 kilowatts. This
is the favoured type of fuel cell at present for small-scale and portable applications,
but the cost is still very high and needs to be reduced.
Phosphoric Acid Fuel Cells (PAFC) are already
in use for commercial combined heat and power applications. They are specific
to large installations, as the cell becomes unusable if its temperature ever
drops below 42 °C.
Fuel cells with an electric power of 200 kilowatts and
a thermal power of 220 kilowatts are available.
Molten Carbonate Fuel Cells (MCFC) operate at around 600 °C.
They can accept a variety of fuels including natural gas, and are not harmed
by impurities.
There is a fuller discussion, including many other types of fuel cells, in
the
Wikipedia.
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Examples of pilot
projects and use for transport
Iceland
Due to abundant and cheap hydroelectric and geothermal energy sources,
virtually the only use of fossil fuels in Iceland is for vehicles and
fishing boats. (Photo shows a geothermal power station.) Therefore, it is not
surprising that Iceland's plans for using hydrogen are the most advanced, especially
as they can use hydrogen generated cleanly by geothermal energy. In 1998 Iceland
committed to becoming the world's first hydrogen economy, by the mid-21st century.
In 2003 the world's
first hydrogen station, like a petrol station, opened and some
buses using fuel cells fed by compressed hydrogen started running in
Reykjavík.
These buses are part of a larger programme,
in a number of cities including London – see buses below.
Every city bus in Reykjavík is expected
to run on hydrogen within less than a decade, followed by a switch to hydrogen-driven
cars. Fuel cell-powered ships are being developed as well, with the aim of
converting Iceland's 2500 fishing boats to hydrogen by 2015.
There are, however, some
reports that the programme is progressing more slowly than planned in getting
beyond the prototype bus stage.
Small islands
Pilot projects demonstrating a hydrogen economy
are under way or starting up on a number of small, isolated islands. For example,
on the Norwegian island of Utsira the installation combines wind
power and hydrogen power. In periods when there is surplus wind energy, the
excess is used for generating hydrogen by electrolysis. The hydrogen
is stored, and is available for power generation in periods when there is little
wind.
Buses – London and
elsewhere
Two Mercedes Citaro buses powered by hydrogen fuel cells operated
in London for three years, starting in 2004. This was part of a European programme
in nine cities called Clean Urban Transport for Europe (CUTE);
Perth and Beijing also participated. The aim was to test first-generation fuel
cell-powered buses.
Ten hydrogen-powered buses of a different model, five using fuel cells
and five using internal combustion engines, are apparently on order in London
for 2010. They reportedly cost eight times as much as a diesel bus.
Cars and other vehicles
Many of the main car manufacturers have produced a variety of hydrogen-fuelled
cars. A list can be found here.
Most of these are prototypes or one-off demonstrations.
Some, such as the BMW Hydrogen 7 (100 produced) use internal
combustion engines – the
BMW can burn petrol as well as hydrogen (which is stored in liquid form). Others,
such as the Mercedes F-cell, use fuel cells fed by compressed hydrogen
to drive electric motors. Some of the concept cars have also been hydrogen-driven
electric/petrol hybrids. However, the main interest seems to be in fuel
cells.
The first hydrogen car to go into
small-scale production beyond the level of a few prototypes is the Honda FCX
Clarity, shown at left. Limited marketing began in June, 2008 in California,
closely followed by Japan. However, mass marketing is only foreseen by 2018.
It is powered by 57 litre,
100 kilowatt hydrogen
fuel-cell stack. Electricity is stored in 288 volt lithium ion battery.
As noted above, a number of hydrogen-powered buses are in use in various
cities, and this is allowing designs to be tested and refined. Prototype lorries
and vans are also being developed. A very few, small prototype boats have
been built.
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Economics
Hydrogen technology is too new, and too little used at present, to be directly
competitive with technologies which have many years of experience and a mass
market to support them. Thus its first practical applications would likely
be cases where current technologies do not provide a suitable solution. Prime
examples might be:
- The supply of electricity in modest quantities away from an electricity grid.
Fuel cells and hydrogen supply can be much lighter than batteries supplying
the same energy, and might be more efficient and less polluting than portable
internal combustion-driven generators.
- In vehicles, where the absence of polluting gases provides a unique benefit.
However, this must be shown to be more efficient and practical than, for example,
electric cars with batteries recharged using renewable electricity.
Currently,
fuel cells are much more expensive to produce than internal combustion engines,
but are becoming cheaper as new technologies and production systems develop.
Fuel cells do not all need to use hydrogen as their energy source, but the
vision is to use hydrogen. If the cost of fuel cells could be reduced to the
point where some important applications can use them effectively, mass production
could then drive the cost down further as new techniques are developed.
For use in cars, there are possible attractions beyond clean emissions.
Fuel cells driving electric motors are lighter in weight and much more efficient
than conventional engines. However, as noted already, hydrogen storage is a
major problem, and so is the cost of making the fuel cells. At present it also
takes more than twice as much energy to make a hydrogen fuel cell than it produces
during its lifetime.
Fossil fuels such as oil and gas are often used as the
cost reference, but this is not a fair comparison. No allowance is made
for the millions of years it took to make these fuels in the earth, yet the
energy used for hydrogen production does get counted, even if it comes from
a renewable source such as solar energy. It is better to compare hydrogen
with other systems using renewable energy.
One company claiming progress in reducing the cost and efficiency of proton
exchange membrane fuel cells is ITM
Power,
which is developing fuel cell systems using hydrogen produced by off-grid renewable
resources. They have developed much cheaper, platinum-free membranes.
Converting to a hydrogen economy might well require a lot of new infrastructure
for distribution and storage, depending on how and where the hydrogen is generated
and used. On the other hand, electric vehicles using batteries can use the
existing electricity grid or local renewable generation for recharging, and
much of this could be done at night when the grid is not heavily used.
Hydrogen
must be shown to be more efficient, practical and cost effective than other
renewable technologies.
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Safety
An early difficulty in introducing the hydrogen economy is the popular
linkage of the term with the hydrogen bomb. There are, of course, no radioactivity
problems in the uses of hydrogen being proposed here. A totally
different use of hydrogen, nuclear fusion, is
discussed on another page.
More realistic concerns include the following:
- Mixtures of hydrogen with air can explode, and if an explosive mixture
is present very little energy is needed to trigger the explosion. Hydrogen
has the widest explosive-ignition mixture range with air of all gases
except acetylene, and because it diffuses easily its storage containers have
to be sealed extremely well. The argument of those promoting hydrogen is that
the same is true of natural gas and petrol vapour, yet we use these reasonably
safely with appropriate care.
- Hydrogen may spill in accidents, and then catch fire. This again
is similar to the problem with petrol. The advantage of hydrogen is that it
is much lighter than air and rises, so the area affected should be limited,
while petrol and oil spread across the ground spreading the fire at the same
time. A disadvantage is that a hydrogen flame is nearly invisible, so it is
more difficult to avoid.
(The photo shows the Hindenburg, a hydrogen-filled
passenger airship which caught fire and was destroyed within one minute at
the end of its first transatlantic voyage in 1937. The fire may or may not
have started in the hydrogen, but it did fuel it.)
- Hydrogen is colourless and odourless, and if released permeates the air very
rapidly. This has the advantage that any spill dissipates rapidly, but the
disadvantage that hydrogen can build up rapidly throughout a confined space.
- Hydrogen can affect many materials it comes in contact with, including its
containers, typically making them brittle.
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Possible Environmental Problems
Carbon dioxide produced during production of hydrogen
If hydrogen is produced from directly from fossil fuels, or by electrolysis
of water using fossil-fuel generated electricity, carbon dioxide will be produced
that will add to global warming unless it is captured and stored.
Loss of hydrogen from the earth
If hydrogen is released to the atmosphere by leaking from storage vessels
or during use, it can escape from the earth as the molecules are so light.
This would deplete the total hydrogen on earth. However, even if the entire
energy requirements of the world were met by hydrogen, the total hydrogen used
in a year might be only a 30-billionth
of the hydrogen component of the water in the oceans. This is very unlikely
to be significant.
Ozone-layer depletion
A recent paper hypothesized that escaped hydrogen gas in the stratosphere
might form free radicals due to ultraviolet radiation. These could act as catalysts
for ozone depletion. It is felt that the amount of leakage from even widespread
use of hydrogen is probably too little to be a problem, but this underlines
the need to minimise leakage, which is necessary anyway on safety grounds.
Water is a greenhouse gas
The unavoidable product of using hydrogen is water, so would water vapour
from the exhausts of millions of vehicles add to global warming? In fact, the
volume of water in the atmosphere created by burning hydrogen would be negligible
compared with that generated by natural processes. The main concern might be
the deposit of water at high altitudes by aeroplanes fuelled by hydrogen.
Disposal of the fuel cells
Fuel cells have a limited life, and there may be difficulties in recycling the materials from which they are made.
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Discussion
Hydrogen is fairly likely to have some role in energy supplies in the future
– the question is how big a role, and what it will be.
Large-scale electricity generation – The
main use of hydrogen would probably be to provide electricity at times when
demand becomes very high and other renewable sources are not available.
This is because producing hydrogen using electricity, transporting and storing
the hydrogen, and then generating electricity from the hydrogen cannot be as
efficient as using an electrical grid covering reasonable distances.
The use
of solar power in distant tropical areas is another matter. Very long transmission
lines may prove to be much more expensive than producing and shipping hydrogen.
This is discussed in more detail on our Green
Energy page.
Space heating - In the long term, better buildings
can greatly reduce the consumption of energy for space heating. Hydrogen might
have a role to play in providing clean heating, but there are other technologies
that might prove to be more cost effective, such as heat pumps and wood burners.
Fuel for industrial processes - Many processes,
notably industrial ones (for example processing of metals), require high temperatures.
A clean fuel source is needed, and hydrogen may well be the solution in place
of such fuels as oil and gas.
Transport - This seems to be the most likely
application of hydrogen, at least in the next few decades. Although both direct
burning and fuel cells are being tried, fuel cells driving electric motors
seem to be the better choice. The alternative of using batteries, recharged
with mains electricity from renewable sources, is also being developed. At
least for cars, buses and lorries (and possibly ships) it is not yet clear
which will turn out to be more effective. For trains, renewable mains electricity
seems to be a better solution. For aeroplanes batteries seem to be too heavy,
so if hydrogen tanks can be made light enough hydrogen is a possible fuel source.
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