Hydrogen gas could be the ultimate clean fuel, because when it is mixed with oxygen and burned, it produces only energy and water vapour, which can be condensed into liquid water to prevent it becoming a greenhouse gas. Hydrogen can also be made from water, but the process uses a lot of electrical energy. The gas cannot be carried in a normal fuel tank unless it is turned into a liquid by chilling it to the amazingly low temperature of -253°C (-423°F). Despite this, hydrogen-powered vehicles like this bus are being tested, mostly based on"fuel cells"that use the hydrogen to produce electricity that powers electric motors.

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