Hydrogen-fuelled London taxi unveiled

8 June 2010 | By Siobhan Wagner

London’s Deputy Mayor for Policing Kit Malthouse, who chairs the London Hydrogen Partnership, announced that a fleet of up to 20 of these cabs will be on the streets by 2012. This falls into line with Mayor Boris Johnson’s call for zero emissions from all London taxis by 2020.

The London Hydrogen Partnership, which was created by former London Mayor Ken Livingstone to foster the deployment of hydrogen-based transport technologies, has stated these cabs will also be introduced with a new hydrogen infrastructure throughout the city. The group is currently scoping out locations for approximately six hydrogen pump stations.

Intelligent Energy, Lotus Engineering, LTI Vehicles and TRW Conek collaborated on the design and build of the new prototype cab, which relies on a fuel cell system hybridised with lithium polymer batteries. According to its specs, the cab is capable of top speeds of 80mph with a range of more than 250 miles on a full tank of hydrogen. A refuel takes approximately five minutes.

Ashley Kells, senior programme manager for Intelligent Energy, explained that hydrogen fuel cell technology was decided as the best green solution for London cabs because alternatives such as batteries would not be as reliable.

‘We’re talking about a London cab which will need to take trips to Gatwick and Heathrow,’ he said. ‘A fuel cell is more suited to this application than batteries because with batteries you would not be able to achieve the range required and also in terms of recharging it would take a lot longer if you’re using a traditional three pin plug.’

More importantly it was decided hydrogen would be pumped in and stored on board the fuel cell cab in its gaseous, and not liquid, form. Kells said gas was the right choice because others, mostly notably BMW, found that liquid hydrogen storage in vehicles is too energy consuming.

‘The problem with liquefied storage is the hydrogen needs to be stored at -271 Celsius, which is pretty much absolute zero,’ he said. ‘The bottom line is it boils off because you’ve got to insulate it and use energy to keep it at that temperature and after 10 days you’ve lost all your hydrogen.

‘So if you rocked up to Heathrow Airport went on holiday and came back you’d have no fuel.’

Intelligent Energy was charged with designing the cab’s fuel cell stack. Kells explained its hydrogen fuel cell design works the same as conventional ones except for a slight twist. Just like a conventional fuel cell, theirs generates electricity through reactions between hydrogen and air, which acts as an oxidant.

The difference is the way they manage the heat generated by reactions in the fuel cell. Kells said conventional designs use an external cooling loop and humidifier, but Intelligent Energy makes this extra hardware redundant.  

‘We use a patented evaporative cooling technique whereby water is injected straight into the fuel cell stack, he said.

The water evaporates because of the heat present within the fuel cell and it leaves the end of the cell configuration as a mixture of saturated air and liquid water.

Kells said this mixture is then passed through a heat exchanger similar to a car radiator. There the temperature is reduced by 20 degrees Celsius, turning the saturated air and remaining water into its original liquid. The water is then reintroduced into the fuel cell and continues its cycle.

The fuel cell engine was packaged into the cab by Lotus Engineering, which also designed control systems to optimise performance of the fuel cells and electric drive systems. 

When in motion the taxi uses an electric motor with a single gear ratio to provide propulsion to the rear wheels. This is a critical feature for cabs in the city of London because it helps maintain the tight turning circle of the vehicle.

The motor is powered by high voltage DC supply and the current from the fuel cell system is regulated via a DC DC converter. Hydrogen is provided by an onboard 35MPa storage tank and air is fed to the fuel cell from a blower. The electric power generated is fed to a lithium battery of the electric drive.

As in typical hybrid cars, during braking the motor acts as a generator and recharges the battery.

Kells noted that if required it can also be configured as a plug in hybrid and topped up from the mains.

TRW Conekt led a safety analysis programme of the prototype vehicle, which went through tests on braking and steering systems among other things on Lotus test track near Norwich. The cab will still need to be assessed by the Public Carriage Office before it can be used for commercial use on the road.