Munich  - Car makers are discovering solar power as an effective additional energy source at the same time lowering fuel consumption and the emission of pollutants.

Solar power is still a long way from providing enough energy to run an ordinary vehicle, but car makers are finding more ways of tapping into the sun's free energy, said Thomic Ruschmeyer from the German Association for Solar Mobility (BSM)

So far car makers such as Audi have only made use of solar power for comfort functions. According to Audi spokesman, Udo Ruegheimer, the Audi A8 and A6 can be ordered with a sunroof complete with integrated solar cells that provide enough energy to power the fan and keeping the interior aired in summer.

But in several concept studies engineers are in fact looking at other ways of using solar power as an alternative fuel. At the recent Geneva Motor Show the Saab 9-X BioHybrid featured a roof fitted with a solar generator. The car transforms solar energy into electric power whether it is being driven or parked. The energy is stored in the lithium-ion batteries linked to the hybrid drive system.

BMW also makes use of carbon-neutral energy in its Vision Efficient Dynamics concept based on the X5. The Sports Utility Vehicle (SUV) has a one-square metre solar roof that generates one kilowatt hour of energy on a sunny day.

It can also be used to load a telephone or to keep drinks cool. The energy can also be used to warm the transmission oil, reducing fuel consumption by about 1 per cent, said BMW spokesman Wieland Bruch.

But it will be a while before the technology can be used for production cars, says Bruch. "We are working on a list of possible options. Those that benefit the customer the most and cost the least amount of money are the priority," he adds.

In addition a solar roof carries much more weight than a conventional steel roof. Even a normal glass roof has less weight, Bruch cautions.

Opel's CEO Hans Demant is watching the technology "with great interest" but he sees little use at present apart from utilising it for caravans and for the car air fan.

"Together with our partners we are working on the further development of the lithium-ion battery that can store energy in electric engines," Demant says.

At present engineers are still working on increasing the performance of the lithium-ion battery and on reducing costs.

But even solar lobbyists such as Ruschmeyer see little direct use of solar energy in the car.

"Cars used in the solar rallies in Australia meanwhile reach average speeds of 100 kilometres an hour," Ruschmeyer points out "but these racing cars have about as much similarity with a normal passenger car as a Formula 1 Ferrari with a Fiat 500."

But Ruschmeyer sees other possibilities. Solar panels on the roof of a house can be linked with a cable to an electric car in the garage, recharging the battery with energy from the sun.

In addition a solar charger for less than 50 euros (78 dollars) can be used for classic cars or other vehicles that spend most of the year in the garage. The charger is simply plugged into the cigarette lighter, preventing the battery from running flat. (dpa)