Next year, BMW plans to produce 100 luxury sedans that can burn liquid hydrogen in addition to gasoline. The hardest part of the job? Manufacturing the hydrogen tank (shown at left with the hydrogen-pump nozzle attached). The tank eats up half of a car’s trunk space and weighs 167 kilograms. To keep hydrogen liquid–which ensures there’s enough in the tank for a long drive–you need to chill it to -253 °C. So the tank has two walls of stainless steel separated by a vacuum and multiple layers of insulation.
Despite the insulation, the liquid inevitably turns into a gas. After 17 hours, pressure rises sufficiently to make a valve open, venting hydrogen gas that a catalytic converter oxidizes into water vapor. A half-tank of hydrogen in an undriven car will largely “boil off” in nine days. The tank also has a second system that quickly vents hydrogen through the roof of the car if the tank is damaged in an accident. To be cautious until more testing can be done, BMW asks users not to park its hydrogen cars in enclosed garages. The company is seeking new technologies for automotive storage of liquid hydrogen, but for now, its tank seems to be the state of the art.