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Wednesday, 17 June 2009

An uphill struggle

Yesterday saw the launch of a hydrogen car whose designs will be available online so the cars can be built and improved locally. The Riversimple car will reach 80km/hr (50mph) and travel 322km (200mi) per re-fuelling, with an efficiency equivalent to 300 miles to the gallon.

Sounds neat, and digging deeper the car achieves some of its efficiency from a novel electrical system. Instead of using batteries or driving the wheels directly from the motor, the hydrogen fuel cell is used to charge ultra capacitors which will power the engine. Why is this such a good idea? The instantaneous power required to accelerate a car is much higher than the power needed at "cruising speed" so the designers think that they can use the capacitors to give a boost of power when required, achieving an istantaneous power evel much higher than the output from the engine. Their fuel-cell motor has an output of 6kW, compared with the 50kW output of a Smart car.

Sounds great in principle, but cars don't need a 20 second power boost to accelerate. They also need energy to go uphill, and with a 500kg mass of driver plus car, an 80% efficient power system with no frictional losses could only go uphill at the rate of 1 metere per second, or about 15mph on a 15% slope , but probably less.

The car will be available in a number of cities. I would suggest as flat as possible, Lincolnshire or the Netherlands sound good, and avoid Bath, Edinburgh and Rome.

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