Hydrogen-powered bus. This bus is powered by a fuel cell that runs on hydrogen gas. Inside the proton exchange membrane (PEM) fuel cell, hydrogen is combined with air, without combustion, to form heat and water. The electricity generated drives the bus and the only emission is water vapour. This bus is part of the CUTE (Clean Urban Transport Europe) project, a European trial to test 27 pollution-free buses in 9 European cities. Photographed outside the Royal Exchange building, in Bank, London, UK, in 2005.