BMW is prepping a fleet of hydrogen capable 7-series limousines to go on sale in 2007. In 2000, BMW showed off its first 7-series concept using a zero-emission system called CleanEnergy that allows the cars to use either hydrogen or petrol. The V12 engine is expected to develop 231hp and can switch between the different sources of fuel on-the-fly once it detects the tanks are empty.

British mag Auto Express has caught the 7-series test cars while testing in Germany, which were easily spotted because of the massive hydrogen tank in the boot. It’s expected the first batches will be leased out to small fleets that will give BMW feedback on the ownership experience. Mazda has begun a similar campaign with modified RX-8 hydrogen cars in Japan. It's possible an official announcement about this will be made at the LA Motor Show towards the end of the year.

BMW's CleanEnergy Website

Click ahead to see a video of a BMW official explaining CleanEnergy.