"Green" cars, powered by electricity from either rechargeable batteries or hydrogen fuel cells, are already on the market, but still too expensive for most consumers.

While big car manufacturers are doing what they can to make electric vehicles more attractive to buyers, small companies, such as Riversimple from Powys in Wales, are working not only to build affordable eco-friendly cars but also to make us change the way we think about owning a car.

Riversimple plans to make its two-seater Rasa available to drivers by 2018, but it won't sell them. Instead, the Rasa will be available by subscription.

The car is powered by compressed hydrogen, which, in combination with oxygen from the air, produces electricity in the car's fuel cell. The only byproduct is water vapor.

Besides being eco-friendly, the fuel cell cars get excellent mileage.

"If you're looking at hydrogen, this allows us a very, very long range in comparison to batteries," said Christophe Mazur, a chemical engineer at Imperial College London. "So here, when we're using hydrogen cars, we can reach the ranges we have in normal petrol cars and fossil-fuel-driven cars — so, about 600, 700 kilometers."

With 1.5 kilograms of hydrogen, Rasa will be able to cover approximately 480 kilometers, with a top speed of just under 100 kilometers per hour.

But fuel cell cars still face one major obstacle.

"As with all these things, it's a lack of infrastructure," said Darren Moss, editor of Autocar magazine. "The government has pledged to build 12 new fueling stations, but if you think of how many petrol stations there are across the UK, and that's thousands, it's still a small fraction of what's needed."

As the number of charging stations for battery-powered cars keeps growing, makers of fuel-cell cars hope the same will happen with pumps for their vehicles.