San Francisco Chronicle: They look like perfectly ordinary cars – a gaggle of sedans and SUVs, slinking silently around a Livermore parking lot, water vapor occasionally dribbling from their tail pipes. Water vapor? Yes indeed, because water vapor is about the only thing that comes out of the exhaust system of this particular – and rare – kind of car, powered by a fuel cell stack, essentially a sandwich of plates that generates electricity by combining hydrogen and oxygen in a complex process. A …