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Is Hydrogen the Fuel of the Future? Hydrogen generated from non-polluting sources can supply energy without significant effects on the planet. It can be burned as a fuel, and electricity can be generated in fuel cells. But there are serious cost, safety, and technical problems. This page covers the generation of hydrogen, its storage and transport, its use for fuel cells, examples of projects currently underway, and the general issues of its economics, safety and environmental problems. It ends with a short discussion of some general points and conclusions. |
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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. 
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. 
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:
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. 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. 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:
Either way, the infrastructure for obtaining hydrogen where it is needed does not yet exist. 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:
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. 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. 
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. 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:
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. 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:
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. 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. |
