May 2001

Hydrogen storage methods for motor vehicles

High-tech Tanks for Liquefied and Low-temperature Transport

As safe and easy to handle as conventional fuels

There are various ways to store hydrogen for use in motor vehicles, and they are all at different stages of development. The most promising methods include storing the hy-drogen in liquefied, frozen form, transporting it under high pressure as a gas and stor-ing the hydrogen atoms in metal hydrides. Researchers are also looking into ways of storing hydrogen in carbon nano-structures.

At present, the favored method at GM and Opel's Global Alternative Propulsion Center (GAPC) is that of liquefied storage at minus 253 degrees Celsius in a special stainless steel tank. Apart from the high level of safety and the advantage of being able to use proven technology, the so-called "cryogenic" tanks offer a high storage density in terms of both volume and weight. The HydroGen1 tank, for example, has a capacity of five kilograms of liquefied hydrogen and of gross volume of some 120 liters (contents: 75 liters). The complete tank system, including the necessary valves, heat exchanger, and mountings, weighs about 90 kilograms. On the road, these five kilograms of hy-drogen permit an operating radius of more than 400 kilometers.

Through further development phases such as the active cooling of the tank, the use of alternative materials such as aluminum, or through a space-saving design, the spe-cialists at GAPC see significant potential in the cryogenic technology. The medium-term goal is to raise the hydrogen capacity to seven kilograms and at the same time lower the weight of the system to around 55 kilograms. In this way, the range would go up to 700 kilometers. Intensive trials are also being carried out at GAPC on other hy-drogen storage systems, for example storing gaseous hydrogen in pressurized tanks. The biggest advantages of this technology are its reduced complexity and its broad usage in other branches of industry. However, compared with the cryogenic tank, the dimensions would have to be considerably larger, since hydrogen as a gas has a lower volumetric energy density. For safety reasons, too, the tank walls would have to be thicker and therefore heavier (even though they are made of modern composite mate-rials). The specialists at GAPC predict that, even with intensive further development using, the targeted operating radius of around 700 kilometers would require a weight of 110 kilograms for the pressurized tank.

Another method of storing hydrogen is to utilize the physical phenomenon exhibited by metal hydrides, which are capable of storing the hydrogen atoms within their crystalline structure. This technology has a number of advantages: in particular its simple con-struction, its high volumetric storage capacity and its innate, high level of safety. At the current stage of development, the available materials are still too heavy.

Looking further into the future, scientists can also envisage storing hydrogen in carbon nano-structures. In this case, the hydrogen would be encapsulated in the microscopically small spaces within the carbon structure providing a high storage density. At the same time, it would be easy to retrieve the hydrogen for use.

From a safety perspective, the systems available today – high pressure and liquefied storage as well metal hydride storage – are already regarded as entirely reliable. The tank technology used on board HydroGen1, for example, harbors no additional risks because the stainless steel tank is well protected in front of the rear axle. Its impact behavior has been tested in numerous computer simulations, as has the entire fuel cell system, showing that it can withstand acceleration forces of up to 30 g (1g = 9.81m/s&sub2;). Nor are there any additional risks in the day-to-day handling of this fuel. The pre-condition is that new practicable solutions are found for the hydrogen filling stations of the future: it is quite possible that robot systems will assume the job of filling up the tank in future.