You'll never guess who we had in our cab this morning. It was Crave, at London's City Hall to see the new Zero Emission Fuel Cell Hybrid Taxi, a crud-free hackney that runs on hydrogen and produces no dirty carbon emissions.

The classic London double-decker recently had a climate-friendly makeover, so it's time for the iconic black cab to join in. The success of the hybrid vehicle depends on take-up by cabbies , but they may not have much choice: London mayor Boris Johnson has committed to see all London taxis operating with zero emissions from their tail-pipes by 2020. The first elements of a hydrogen infrastructure in place by 2012 -- whether there will be some sort of subsidy for the hybrid vehicles is as yet unknown.

Hydrogen allows for faster fuelling and gives a longer range than electricity: a 5-minute pit stop will run a cab 250 miles, or a full day's fares, even including airport runs. The disadvantage is that hydrogen doesn't yet have an infrastructure, unlike electricity. Storage and pumping stations are required, whereas you can just plug your 'leccy car into any mains socket. But infrastructure is less of a problem for a city taxi fleet than consumer vehicles, because they're concentrated in one area and already work on the principle of returning to base.

As well as the range and charging advantages, the hybrid hydrogen system is near-silent, more efficient from a cold start, and can hit 80mph. We should note however that, like electricity, hydrogen only offers zero emissions from the exhaust pipe of the vehicle itself. Carbon emissions are emitted during the production process, varying according to how the electricity and hydrogen is produced.

Crave visited the mayor's gaff in the shadow of Tower Bridge to witness the unveiling of the Zero Emission Fuel Cell Hybrid taxi. Flag us down, jump in and click 'Continue' as we race through photos of the cab to find out how it works. Follow that cab!

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London Deputy Mayor Kit Malthouse prepares to unveil the hybrid taxi, which has been developed by Intelligent Energy, Lotus, TRW Conekt and taxi-makers LTI.
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And here it is. The hybrid fuel-cell system is contained in the existing black-cab body, which significantly cuts the development costs.
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Under the bonnet, we find the hydrogen fuel tank and air intake. Hydrogen fuel is pumped into a twin-stack fuel cell, which charges a 14kWh li-polymer battery...
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...which is located under the middle of the car. Water vapour is released down there too. The battery powers the cab, with the fuel cell kicking in to help out when more grunt is required, in acceleration or going up hills or jumping amber lights. The fuel cell recharges the battery even as the cab is driving around, while regenerative braking can also pump juice back into it. In emergencies, the battery can be charged directly from the mains. Because it's rear-wheel drive, the taxi keeps its famous tight turning circle.
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Before taxis can be fitted with fuel-cell units like this one, the infrastructure needs to be in place to support them. There will be six hydrogen-charging stations on London's streets by 2012, which means you could be riding in a zero-emission black cab by the time of the Olympics. A fleet of twenty hybrid cabs is expected to be on the streets by 2012, and all cabs will be emission-free by 2020. Taxi!
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