It might sound like science fiction, but a public transport system powered by "green" energy could be a reality for the next generation. Instead of using the harmful and rapidly-depleting fossil fuels, engineers and scientists are already at work designing other ways to power Bristol. Their vision is a Bristol of the future kept running by hydrogen gas.

Schools, street lights, libraries, old people's homes - all could be electrified using hydrogen created by the power of wind, and also by recycling the tonnes of rubbish thrown away every day.

Already on the drawing board is a scheme which could quickly revolutionise Bristol's creaking public transport, a futuristic tram without the need for ugly overhead cables.

Instead, hydrogen used to power it would be on board the vehicle.

Mention the "tram" word in Bristol and you can expect to hear groans from supporters of a recent scheme to get one up and running in the city.

The supertram idea was effectively killed off by city councillors two years ago after they decided to use £1.5 million earmarked to develop it to keep council tax down.

Another blow came from the Department for Transport, which argued that the scheme was too expensive after the estimated cost soared from £70 million in 1998 to £200 million in 2003.

But the persistent engineers quietly working on the next generation of trams predict that costs would be far lower.

Colin Jefferson, 68, of Montpelier, is a technical director of Sustraco, a company which promotes light rail.

It grew out of the Bristol Electric Rail Bus, which ran a demonstration electric tram up and down the Bristol Harbourside several years ago.

Mr Jefferson, who was a lecturer at the University of the West of England for 27 years, is currently working on securing contracts for a hydrogen-powered tram in Liverpool and Leeds, as well as Bristol. Mr Jefferson, who is also a member of Bristol Civic Society, said: "What we are looking at is trams, and trams which do not need overhead wires.

"One of the reasons the trams idea bit the dust is the cost. That went through the roof, and one of the ways of cutting costs is to introduce on-board power."

So if the tram of the future will not need petrol, or overhead cables to power it with electricity, how will it work?

Like the Toyota Prius currently being marketed as a "green" car, the trams will be equipped with what are known as fuel cells.

A fuel cell is like a battery, powered by chemicals - in this case hydrogen. But it does not run out.

The hydrogen stored on the trams will be piped into the fuel cell, which in turn will power what is known as a "flywheel".

These devices, already in use on buses in Holland, are rotating machines which store and supply energy so that the vehicle becomes self-powered.

Hydrogen on the tram is used to top up the flywheel with energy.

But not only is the on-board power a greener alternative to petrol, the way that the hydrogen itself is produced is also more environmentally friendly.

The plan is to use wind turbines planned for Avonmouth to create the gas.

Huge new ovens in Avonmouth which "cook" waste can also produce hydrogen from ordinary household rubbish.

Mr Jefferson said: "At the moment, buses and trams use a lot of energy pulling up and stopping, because when the brakes are applied the energy of the momentum of the vehicle goes.

"In our system, we are promoting hydrogen propulsion.

"There would be an onboard generator which can keep the flywheel topped up, and instead of applying mechanical brakes you instead reverse the power flow so that it flows from the wheels back into the flywheel.

"They're building wind turbines at Avonmouth. When they are not at peak use, such as during the night, they could be used to produce hydrogen.

"Avonmouth will be awash with hydrogen. We represent a realistic opportunity to use that hydrogen in transport. It makes sense to put it all together. It dispenses with the need for overhead power and the need to insulate the track.

"What happens normally is that the power lines return current through the rail, and there is a huge cost to lay the rails and surround it with large volumes of concrete."

The simplest, and greenest, way to make hydrogen is to take it from water. It is then turned into electricity when it is passed through the fuel cell.

Mr Jefferson believes the cost of each tram would be about £200,000.

He said: "We could do everything the city council was proposing with the tram system before it was dropped, except that it would cost a lot less. The actual route would be up to the council.

"Our system means laying the track is fairly simple. You could do it anywhere. It could run as far as Portishead. The trams could be configured to suit different routes. I think people would end up abandoning their cars. It is well established that light rail encourages people out of cars as it is more attractive to ride than buses."

Research by the Chartered Society of Physiotherapy found that the city had one of the worst levels of air pollutants in the South West, as a result of the huge number of cars rumbling through.

Replacing street-choking buses with clean transport is, therefore, an attractive vision, and one being taken seriously by Bristol City Council.

Paul Isbell, energy manager for the council, said there are a range of green ways of producing hydrogen to power the city.

The council's plans for turbines near Avonmouth will, if they go ahead, provide electricity for Bristol's schools, libraries, old people's homes, and council offices. And there are also plans, still very much in the ideas phase, to create more hydrogen from crops at various locations around the city.

Firms have also started processing the rubbish which is thrown away into hydrogen.

Mr Isbell said all the discussions for a "hydrogen city" were at an early stage, but people could expect it to become the norm within several generations.

"With massive wind turbines, there are times when they are not used, such as at night," he said.

"Instead of turning them off, the turbines can be kept going to generate hydrogen. The wind power is used to split water, and the hydrogen can then be stored and piped to a fuel cell.

"What can also be used are crops, which are gasified, and the hydrogen is taken from that, making it a renewable source.

"My idea would be a grid around the city, with the hydrogen piped into things like bus depots, but also domiciles and industrial units."

Anyone caught in the city centre at rush hour will know how bad the pollution can get. And with carbon resources running out, it is only a matter of time before greener, now fanciful-sounding, schemes are the only option.