You could say we are married to fossil fuels. Despite the damage to our health, the ruination of our environment and the fact that these fuels will ultimately run out, this really does seem to be a case of "'til death us do part".

In the glimpses of the future offered by science fiction, the humdrum fuels like oil and gas on which we now depend are transformed into something a little bit more exotic - dilithium crystals, warp drives and crystalofusion.

They may be technically impossible, but they do point to our strong desire for clean, renewable and cheap sources of energy

However, hard cash dictates where our loyalties. Only when a barrel of crude starts to get very expensive will we seriously consider the greener alternatives.

Magic figure

"Once demand exceeds supply, then the price will begin to go up," says Dr David Fulford from Reading University's Energy Group. "In one sense, those in the renewable energy field are looking forward to that time. Once oil goes up to $30 a barrel - a sort of magic figure - all these renewable energies will begin to look economic and reasonable."

Vehicles of the future will have to find alternatives to oil

The UK countryside has seen a sprouting of wind turbines in recent years, and they are sure to be joined by other technologies producing the new "eco-tricity".

"We have the deep green technologies - solar, hydro and wind," says Dale Vince, Managing Director of the Renewable Energy Company, "and they will be there for our children's children's children - they're not going to run out. Then we have the slightly paler green landfill gas.

"We don't advocate that this is the way to deal with rubbish, but while it is in the ground it produces methane - which is 30 times more potent as a greenhouse gas than CO2 - so it must be collected and burnt. And while we do that we make electricity as a by-product."

Consumer choice

The UK Government is now giving the consumer some say in choosing how electricity is generated. By next May, customers all over the country should be able to purchase green electricity if they want it.

Solar is among the greenest of all the alternatives

And there is also some evidence that electricity companies are keen to offer greener products - a power station that burns willow was recently opened in Yorkshire. The willow, or short rotation coppice as it is called, offers a number of benefits. It is quick to grow, relatively efficient at making electricity and could be a viable crop for the hard pressed farming industry, says Dr David Fulford.

His university has developed a diesel engine that runs on vegetable oil. Some studies have suggested that to overcome our dependence on fossil fuels, we will need to grow fuel crops such as willow for electricity or sugar cane for alcohol to burn in cars. However, this is likely to pose its own dilemmas.

"To use acres and acres of land to grow crops to make fuel would not be sensible," says John Hickman from the Transport Research Laboratory. In humanitarian terms, it would not be a good thing to devote lots of food-bearing land to growth of petrol for our cars."

Serious business

One of the big drawbacks for alternative fuel sources is that they have often been produced on a small scale and have been unable to meet the demands of consumers brought up on a seamless supply and keen pricing.

Use it to cook, use it to power an engine

If green energy campaigners are to shake off that image of hapless hippies trying to save the planet, they know they have to get hard-headed.

"Commercially we are very effective and it is necessary to be so," says Dale Vince. "We are not asking for a subsidy from our customers and we're not asking for subsidy from the government - we have to make the numbers work.