The loudest thing you’ll hear from behind the wheel of a fuel-cell car driving up the Californian coast is the hum of the tyres on the concrete interstate stretching north from Los Angeles towards Silicon Valley and San Francisco. There’s no rumble of a big American engine from under the bonnet, no growl from the exhaust. These are things of the past. The only emission this hydrogen-powered Hyundai Tucson produces is water — so pure…