The year is 2050 and traffic no longer makes a noise. The air is clean and fresh because what is coming from the vehicles' exhaust pipes is not black, sooty, choking gases but a stream of warm water vapour. This is because cars no longer run on petrol or diesel. Instead, when you pull into the filling station and insert the nozzle into your fuel tank, what comes out of the pump is hydrogen.
Under your car's bonnet is a device called a fuel cell, which combines the hydrogen with oxygen in the air to produce electricity that quietly drives the wheels. The sole by-product is nothing more noxious than water. Meanwhile, around the country scores of small power stations use fuel cells to consume piped-in hydrogen that generates electricity to feed hospitals, schools and local communities. Power stations that burn coal, oil or gas have been consigned to history.
This is the utopian vision of the so-called hydrogen economy. The concept is as seductive as it is simple, and most of the technologies that would be required are, theoretically, achievable. For hydrogen enthusiasts, the arguments are straightforward: hydrogen is abundant - each molecule of water contains two atoms of the stuff - and when you recombine the gas with oxygen in a fuel cell to make electricity, there are no polluting by-products.
The fuel cell consists of two flat electrodes, usually made from platinum-coated carbon, separated by a polymer membrane. Hydrogen gas is presented to one of the electrodes, where it diffuses into the pores of the platinum and splits into an electron and a proton. The proton travels across the polymer membrane to the other electrode, where it combines with oxygen in the air to make water. The electron passes around an external circuit as an electric current, performing useful work - such as driving a motor.
Hydrogen fuel cells are commercially available; many demonstration vehicles have been produced. It is widely acknowledged, however, that the cost of the cells must come down and their efficiency increase if they are to become economically viable.
However, obtaining the vast volumes of hydrogen gas that would be needed to drive the world's transport systems and feed remote power stations is still a massive challenge. And given that the whole concept of a hydrogen economy revolves around reducing pollution, the gas must be made in a way that does not belch yet more carbon into the atmosphere.
Currently, most of the world's hydrogen is manufactured for the chemical and oil-refining industries. It is largely made from natural gas by a process called steam reforming. However, this requires raw ingredients based on carbon and the process releases carbon dioxide, so it does not meet the relevant environmental standards. Hydrogen can also be made from coal, but the problems of using a fossil fuel as feedstock and the generation of carbon dioxide remain.
Perhaps the simplest way to make hydrogen is by passing an electric current through water - the process of electrolysis. The question then is where the electricity comes from. A hydrogen economy would require the electricity to be sustainable and renewable. Nuclear power may be one option, but there is deep public - and hence political - antipathy towards the industry. It is also expensive and produces radioactive waste.
Many supporters of the idea of a hydrogen economy point to the most mature renewable energy technology, wind power. An organisation called the Clean Energy Educational Trust runs a website (
www.hydrogen.co.uk) that carries a cogent argument for the use of wind power to provide the electricity to make hydrogen for transport.
The Trust envisages offshore wind farms feeding electricity to hydrogen-producing factories. It calculates that a single wind turbine capable of generating 2MW of electrical energy can produce sufficient hydrogen to run 18 large buses or 864 cars operating under city-driving conditions.
If you scale this up, a facility of 5,000 such turbines sited in shallow waters off the coast of the UK could manufacture enough gas to run 4.32 million small- or medium-sized hydrogen-fuelled cars. However, in 1998 there were some 22 million cars licensed in the UK, and the motor industry estimates that by 2020 this number will have grown to more than 30 million. To run this number of cars on hydrogen, based on the Trust's calculations, would require something in the region of 35,000 wind turbines.
Professor Ian Fells, the chairman of the New and Renewable Energy Centre in Blyth, Northumberland, says: "To make hydrogen for a hydrogen economy there won't be anything like enough renewable energy. If, over the next 30 years, you wanted to change all road vehicles to hydrogen, you would have to double the size of the electrical generation industry."
He adds: "I think that there is a place for hydrogen. It could be useful in vehicles such as buses in areas where air pollution is a problem."
Professor Andrew Oswald, an economist at the University of Warwick, and the energy consultant Jim Oswald have examined the energy requirements of a hydrogen economy. They say that there are many good reasons to consider switching vehicles from oil to hydrogen to reduce emissions and the country's reliance on importing oil. However, according to Jim Oswald, "the enormity of the green challenge is not understood."
The Oswalds have calculated that about 100,000 wind turbines would be required to provide all the hydrogen necessary to run the UK's road vehicles. If these were sited offshore, there would be a 10km-deep strip of turbines encircling the entire coastline of the British Isles. Moreover, they say, if nuclear power were to be used, we'd need 100 new nuclear power stations.
Mike Koefman, secretary of the Campaign for a Hydrogen Economy, disputes the Oswalds' figures on wind turbines. Koefman says that larger, more efficient 5MW turbines are being developed that would substantially reduce the numbers required. Koefman believes that, in the longer term, photovoltaic technology, which converts sunlight directly into electricity, will provide the bulk of the energy needed to power a hydrogen economy around the world, with hot, sunny regions such as North Africa exporting energy to northern Europe. Koefman says that a concerted, worldwide effort by governments could produce the advances in technology needed. "I do not see why the whole world should not have sufficient renewably generated hydrogen within 50 years," he asserts.
Such a scenario, however, would require significant strides in the development of so-called organic photovoltaic systems, which would be cheaper and easier to mass-produce than current solar cells, which rely on semiconductors such as silicon. Most experts accept that organic photovoltaics are at only an embryonic stage of development, however.
Earlier this year, the energy consultancy E4tech produced a comprehensive review for the Government of how hydrogen could contribute to the country's energy mix. "There is a kind of fuzzy aura about hydrogen being a generally good thing, but we wanted to ask a more structured question about how the UK should engage with the hydrogen economy to achieve maximum benefit," says the E4tech director Adam Chase. "We concluded that using hydrogen in transport could have strong prospects for reducing carbon dioxide, but that it was not an especially good option for stationary power or heat."
The report noted that hydrogen could be usefully made by renewable energy and nuclear power, as well as from biomass (which can include municipal waste and rapidly growing trees such as willow), natural gas and coal. Any carbon dioxide produced should be captured and stored. There are also other methods of producing hydrogen that are at an early stage of research, such as by fermentation and by using the energy of sunlight to split water. "We concluded that all these options should be pursued, and that none was likely to provide a solution on its own," Chase says.
The report calculated that for nuclear power to be used to generate one-fifth of the hydrogen needed for transport by 2030, a generating capacity of 9GW should be devoted solely to the electrolysis of water. This is equivalent to seven new Sizewell B reactors. "This is big, but not beyond the realms of possibility," says Chase. "Though it is clear that the primary energy requirements are not trivial."
The utopian hydrogen visionremains a distant prospect.
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