Hydrogen would appear to be the ideal fuel. It is readily available, non-toxic and the residue of burning
hydrogen is water. It thus can 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 the BMW Hydrogen 7 luxury car.
A much fuller discussion can be found in Hyweb - a hydrogen and fuel cell information
system site.
As hydrogen is not a fossil fuel it needs to be created. Ideally this would be from a renewable resource, or possibly
by nuclear power. However there can still be benefits from using hydrogen even if it is generated using other fuels.
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 revolution has been slow in coming. Nevertheless it remains
the most attractive option for combining a long term solution to the greenhouse effect with the freedom to expand energy usage
to meet human demands.
In one vision industrial installations in many tropical countries will use the energy of the sun to extract hydrogen from water.
The hydrogen will be transported by pipeline or ship to where it is needed. Here it may be converted into electrical energy,
used for space heating or cooking, or packaged for use in vehicles and for mobile requirements.
The vision is inspiring, but the practical problems are formidable.
Hydrogen is a very energetic fuel by weight, yielding around 33 kWh/kg. Three times the energy of the same weight of kerosene.
However it is a light gas - 11.2 litres weigh just 1 gram. It also has a very low boiling point close to absolute zero
-252.8 Deg. C. or 20 Deg K.
Storage is thus a problem. Energy may be needed to compress the gas, and typically even more energy is needed to liquidise it
and keep liquid hydrogen cool.
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.
Fuel cells allow the direct conversion of hydrogen energy into electricity with much greater efficiency than through
burning the hydrogen in an engine to drive a generator.
Low Temperature Electrolysis
The most straightforward method of generating hydrogen from electricity is by electrolysis of water. 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 as 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.
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.
Generation from Fossil Fuels
Enough hydrogen to supply about 1% of the world's energy needs is already generated, mainly for the production of fertilisers
such as Ammonium Nitrate. Currently this hydrogen is produced from fossil fuels such as natural gas. One process mixes the gas
- mainly methane CH4 - with steam H2O in the presence of a nickel catalyst to
generate hydrogen, carbon monoxide and carbon dioxide. Another, the Kvaerner process, uses a high temperature plasma to
convert methane to hydrogen and carbon. These generate hydrogen with much greater efficiency than if the gas was first used to
generate electricity and the electricity used to electrolyse water. It is possible to gain the heat necessary to drive
these processes from the spare heat which arises in power stations.
Hydrogen is also produced as a byproduct of oil refining.
Very High Temperature Separation
At temperatures of 3000 Deg. Celsius water molecules separate to the constituent hydrogen and oxygen.
The necessary temperature can be reached by
solar furnaces, and the gasses can then be separated by diffusion through a suitable membrane.
This method of separation is currently only experimental, there are no commercial installations.
After its generation, unless hydrogen is just to be burned, it is usually purified to
remove dust, Sulphur, Carbon Dioxide and other impurities which could damage the catalysts used in later processing.
Most Fuel Cells require clean hydrogen, and small scale systems to clean the hydrogen just
before it is used are a major current interest.
If hydrogen is stored at atmospheric pressure, a volume of 1 cu metre could release about 3 kWh.
Methane in the same volume could release about 10 kWh. Thus gasometers with the same energy reserves
need to be about three times the size if hydrogen is the fuel.
To reduce the volume needed for storage, hydrogen is often stored in liquefied or compressed states.
NASA stores liquid hydrogen in spherical tanks each holding 3,000 cubic metres.
The cheapest method of storing hydrogen is in suitable underground caverns. ICI stores hydrogen at 50 atmospheres pressure
in old salt mines at Teesside. Stationary storage above ground uses similar pressures.
Liquid hydrogen can provide 2.36 kWh per litre, while hydrogen at 50 atmospheres pressure holds about 0.15 kWh per litre.
The low density of hydrogen means that pipeline distribution of compressed hydrogen, though it is used,
is a less efficient method of transporting energy than a similar natural gas pipeline.
More energy is required for the same energy delivered.
Well established pipeline installations in the USA and Germany transport hydrogen over distances of up to 40 miles.
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.
Hydrogen can be transported in pressurised containers. Pressures of up to 250 atmospheres pressure are used.
At this pressure 100 litres will hold 65 kWh. The gas may weigh only about 2.5 kilograms, but it requires a strong container
weighing perhaps 30 kg. By comparison 7.5 litres of gasoline holds the same 65 kWh of energy and weighs about 5 kg,
but the container can be much lighter.
Hydrogen can also be stored for transport absorbed by metal hydride.
Compared with compressed hydrogen this method of storage provides a higher energy density of around 3.18 kWh per litre, at the
expense of greater weight. Hydrogen delivering 65 kWh will require about 110 kg of hydride, in a fairly light container,
the whole weighing perhaps four times that of the compressed hydrogen cylinder holding the same amount of hydrogen.
However the hydride store need only occupy 20 litres, a fifth of the volume needed for compressed hydrogen storage.
Compared with the equivalent gasoline tank, the hydride store is about three times the size and twenty times as heavy.
Charging the metal hydride store with hydrogen generates heat, and heat must be applied to the container to release the hydrogen.
Other methods of storage for transport are under development.
Electricity can be generated in the conventional way used for fossil fuels by burning the hydrogen to generate steam, and
then by using the steam to drive turbines. As in other such conversions the process is not more than 40% efficient, making
the overall efficiency of electricity to hydrogen and back to electricity unattractive.
Fuel Cells provide a way of converting more of the energy in the hydrogen into electricity. They can achieve very high
efficiencies of conversion. Fuel Cells were invented in 1839 by
a Welsh physician, Sir William Robert Grove, but were first used in space craft in the 1960s. Since 1990 there has been
intensive development for commercial applications. There are several different
kinds of Fuel Cell with different applications:
Alkaline Fuel Cells were the original type used on space craft. They are simple and cheap to make, but must
be fed with pure Hydrogen and Oxygen since Carbon Dioxide degrades the electrolyte.
The Proton Exchange Membrane Fuel Cell - PEMFC - is very light, it is very efficient and requires only atmospheric
oxygen instead of pure oxygen. However these fuel cells are very sensitive to carbon monoxide. This is the favoured design
of Fuel Cell at present. Commercial applications are expected in vehicles, in portable electricity supplies
such as for camping, portable computers and mobile phones. PEMFCs are also likely to be used in combined heat and power plant
for domestic or office use. In this case the hydrogen is likely to be generated locally from a mains gas supply.
The Phosphoric Acid Fuel Cell - PAFC - is already in use for commercial combined heat and power applications.
It is specific to large installations as the cell becomes unuseable if its temperature ever drops below 42 Deg .C.
The American company ONSI sells units with an electric power of 200 kW and a thermal power of 220 kW.
The Molten Carbonate Fuel Cell - MCFC - operates at around 600 Deg .C. It can accept a variety of fuels including
natural gas, and it is not harmed by impurities.
Hydrogen can be burned in a similar way to natural gas, indeed it was a significant component of town gas generated from coal.
However the burners used for natural gas are not suitable without adjustment. Heat can also be generated by fuel cells.
Engines that run on hydrogen have been available for over 60 years.
However Fuel Cells driving electric motors can convert much more of the energy in the Hydrogen into motive power,
and are believed to be the more appropriate technology.
Hydrogen technology is too new and in too little use to be directly competitive with the technologies which have
many years of experience and a mass market to support them. Thus its first practical applications are in cases where
the current technologies do not provide a suitable solution. Prime examples may 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
- in vehicles where the absence of polluting gases provides a unique benefit.
An early difficulty in introducing the Hydrogen Economy is the popular linkage of the term with the Hydrogen Bomb.
There is of course no radioactive implication in the use of hydrogen being proposed.
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. 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 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.
- 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, typically making them Brittle.
No fuel is entirely without problems for the environment.
Loss of Hydrogen from the Earth
If hydrogen is released to the atmosphere it can escape from the earth as the atoms are so light.
Thus using hydrogen can result in depleting the total hydrogen on earth. However only the hydrogen which is not used may be lost,
and this should be less than 1 percent of the whole, the rest is recombined with oxygen.
Even if the entire energy requirements of the world were met by
hydrogen, the total hydrogen used in a year might be a 30 billionth (3 * 10-8) of the hydrogen component of the
water in the oceans. The loss of 1 percent of this per year is unlikely to be significant.
Water is a Greenhouse Gas
The volume of water in the atmosphere created by burning hydrogen would be negligible compared with that generated by
natural processes. The concern would be the deposit of water at high altitudes by aeroplanes burning 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.
Hydrogen is certain to have some role in energy supplies in the future. The plans in Iceland are most advanced
partly as they can use hydrogen generated by spare geothermal energy.
The world’s first commercial hydrogen station opened on April 24, 2003.
Every city bus is expected to run on hydrogen fuel within a decade.
It is then proposed to convert every personal vehicle in the country to hydrogen.
A demonstration for a fuel cell powered ocean vessel is expected to be completed in 2006, and the plans call for
a complete conversion to hydrogen of Iceland’s 2,500 ship fishing fleet, beginning in 2015.
Local use of hydrogen fuel will not replace the use of mains electricity, where mains electricity is appropriate.
Generating hydrogen using electricity,
transporting the hydrogen and then generating electricity from the hydrogen cannot be as efficient as an electrical grid covering
reasonable distances. Though transporting electricity by power lines over thousands of kilometers, e.g. from the sahara,
would also lead to massive energy losses.
Thus any hydrogen generation from electrical power stations would be from electricity which was not needed at that time.
It is also unlikely that all the fuel needed for transport and heating in the UK will be met using locally
generated hydrogen from renewable resources.
The demand might be an average of about 120GW. A wind turbines rated at 2 MW, which
with variable winds and the losses in conversion to hydrogen, might on average generate 500 kW in hydrogen.
If this was the only source we would need 240,000 wind turbines, or 2.7 for every square mile of the UK and Northern Ireland.
This would be in addition to any turbines used for direct electricity generation.
It would be technically feasible, though probably not welcomed, to generate 120GW of hydrogen fuel from new nuclear power
stations.
Nevertheless in principle it would be possible to supply all the world’s energy needs via hydrogen.
Solar radiation in the tropics can reach 35 MJ/sq. metre/day. Let us assume an average over suitable sites
of 10 MJ/sq.metre/day and that we can convert 1 MJ/sq metre/day into hydrogen,
e.g. by a conversion to electricity very inefficiently at say 12.5% followed by electrolysis at about 80% efficiency.
On these cautious assumptions - with 3.6 MJ/kwh and 24 hours a day
- 100 sq metres of tropical land might generate hydrogen at an average rate of about 1 kW stored energy.
1 sq km could then generate 10 MW. If we estimate the world requirement for artificial energy at about 10,000 GW
and wish to gain all this as hydrogen from solar sources, we might need to devote a maximum of 1 million sq. km
of tropical land to its generation. The Sahara alone occupies about 4.6 million sq. km.
It therefore seems technically possible to generate enough hydrogen from the sun if we choose to do this.
Blewbury Energy Initiative
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