Tech Briefs
BMW's car for tomorrow

The BMW Group's CleanEnergy 7-series hydrogen car. Robotic refueling of BMW's hydrogen car.

If BMW is right, tomorrow's car will draw its power from the sun and water, rather than from oil. Using solar energy, water will be broken down into oxygen and hydrogen, and the hydrogen will then be transformed into fuel. Since 90% of the universe is composed of hydrogen, vehicular fuel supplies would be virtually endless. The new process that BMW is relying upon to change gas stations to hydrogen stations uses solar energy to induce the electrolysis that frees hydrogen from water. Then, in the car's engine, the hydrogen fuel is recombined with atmospheric oxygen to produce water once again.

"We feel that hydrogen, generated from a renewable resource such as sunlight, is the only viable long-term solution to providing an absolutely environmentally sound fuel," said Henrich Heitmann, Board Member for BMW AG, at the North American International Auto Show in January. "Use this fuel in a combustion engine, and you can provide the performance characteristics desired by those who seek the 'ultimate driving machine.'"

Nearly 88% of current energy needs are filled by fossil fuels: coal, oil, and natural gas. Almost half of the world's annual oil production is used to power its 500 million cars, and 170 million utility vehicles. BMW quotes estimates that predict the oil supply will run out in 50 years.

BMW began developing hydrogen-powered engines over 20 years ago, with the first actual road trials done in the late 1970s. In 1986, BMW combined forces with Solar-Wasserstoff-Bayern GmbH, a German company that specializes in research on the storage and application of hydrogen energy. Three years later, BMW became the first carmaker to bench-test hydrogen-powered engines.

Today, the fifth generation of hydrogen vehicles has taken to the road. It took much development work to reach this point. The normal state of hydrogen is gaseous. But when used to fuel vehicles, it has to be in a liquid state, otherwise not enough of it can be stored to provide an acceptable driving range. To liquefy, this colorless, odorless, and tasteless gas has to be cooled to -250°C (-419°F). It then shrinks to almost a thousandth of its gaseous volume. The fuel tank must be extremely well insulated. BMW's solution was to create a high-vacuum environment between double tank walls in which 70 layers of glass fiber mats alternate with aluminum foil. Although this insulation is only just over 25-mm (1-in) thick, the company says that it is as effective as a 4-m (13-ft) thick polystyrene shield in keeping away heat.

When hydrogen combines with oxygen in an enclosed space, a combustible mixture can result. To prevent this from happening, sensors monitor the fuel system. Should hydrogen manage to leak out, the car is automatically ventilated whether it is being driven or not so that a build-up of gas to critical levels will not occur. To prevent a major collision from exploding the fuel tank, BMW installed a safety valve and a built-in line of breakage to burn-off any escaped hydrogen in an upward direction.

Refueling techniques also had to be redesigned for BMW's hydrogen-powered cars, since conventional methods are not up to handling hydrogen at -250°C (-419°F). The first hydrogen refueling station for BMW's test cars was installed on the premises of Solar-Wasserstoff-Bayern GmbH in 1990. It took one hour to refuel a car. Liquid hydrogen entered the tank from below, while residual gaseous hydrogen was forced out at the top. Nowadays, supercooled hydrogen liquid is drizzled into the tank from above. The gaseous hydrogen remaining in the tank then condenses on the droplets rather than being forced out. This procedure eliminates fuel loss and reduces refueling time to less than three minutes. If there is no hydrogen refueling station in the vicinity, the car can be run on gasoline that is stored in a second, conventional tank.

Experts estimate that it will take 30 to 50 years to build the necessary infrastructure to support hydrogen stations. "When the infrastructure for hydrogen fuel is eventually realized," said Heitmann, "BMW will be well prepared to provide exciting products."

Until then, BMW believes that fuel cells could provide an alternative. Fuel cells enable electricity to be created through the chemical reaction of hydrogen with oxygen, essentially the opposite of electrolysis. The energy generated is harnessed for cars by means of electric motors. While BMW believes that current fuel cells are too big, too heavy, and too expensive to be a viable alternative to the combustion engine, the company says that fuel cells will play a large role in the company's future hydrogen-powered vehicles.

Jean L. Broge