Clean combustion, but hard to handle: Hydrogen as a source of energy

From the very beginning of alternative drive system development in the late 1960s, Daimler-Benz had an eye on hydrogen as a possible source of energy for the future. Hydrogen has only about 25 percent of the energy density of conventional fuels. On the other hand, no harmful emissions whatsoever are produced during combustion: hydrogen reacts with the oxygen in the atmosphere to form pure water.

The first problem to solve in using hydrogen to power vehicles was that of storage. High-pressure tanks or storage in liquefied form could not be realized for use in vehicles back then. For one thing, the strictest safety precautions had to be observed since in the event of an accident the hydrogen could combine with atmospheric oxygen to create an explosive mixture. Adequately safe storage tanks to achieve even small ranges would have been extremely heavy. Cooling to minus 253°C, on the other hand, would have required an unreasonably large input of energy.

So in 1967 Daimler-Benz awarded a research contract to the Geneva-based Battelle Institute to develop a titanium hydride storage system suitable for vehicle drive purposes. Storage in the form of attaching the hydrogen to metal hydrides (adsorption) was the only safe variant for a vehicle drive system at that time. Initial results were recorded in the minutes of the technical meeting of November 13, 1967:

In 1975 it was finally ready: at the Frankfurt International Motor Show, Daimler-Benz introduced the world’s first hydrogen-powered minibus with a hydride storage tank. From practical testing of the vehicle the developers gained further valuable insights which were incorporated in the design of a new hydrogen minibus two years later.

Hydrogen-propelled city bus: Modified gasoline engine and TN chassis

At the 1977 Frankfurt Motor Show, Daimler-Benz presented the new city bus with hydrogen drive system. The basis was a vehicle from the new “Bremen” model series on a TN chassis. The modified gasoline engine developed 60 hp. Its range was 200 kilometers with tanks weighing 200 kilograms. “Since hydrogen operation must begin in conurbations for reasons of supply, we started with the city bus project on the basis of Mercedes-Benz vans,” a press release stated.

“Hydrogen - Drive System of the Future” is the title of an information sheet distributed at the 1977 Frankfurt Motor Show to explain the project: “With the limited availability of petroleum reserves in mind, Daimler-Benz investigates possibilities for the use of alternative fuels. In this context, hydrogen, together with the use of suitable metal hydride storage tanks, which have supplanted the previously known both voluminous and dangerous storage options (high-pressure cylinders and liquid hydrogen), gains increased importance. These hydride storage units, whose development is furthered by the Federal Ministry for Research and Technology, contain special metal alloys which absorb hydrogen and give off heat as they do. Waste heat from the engine then releases the hydrogen again for use in the drive system.”

An ingenious feature of the new city bus was that it integrated the heat produced during hydrogen adsorption and the heat required to release the hydrogen into a complex heat management system which made use of the waste heat of the engine, and also incorporated the heating and air conditioning system of the vehicle. For test purposes, five different hydride tanks were installed in the vehicle. They were based on the materials titanium/iron and magnesium/nickel and operated either as high-temperature accumulators at temperatures of 250 to 300°C, or as low-temperature hydrides between 80 and 90°C: “Fundamentally, various methods of releasing hydrogen are possible: using the engine coolant, the engine exhaust gases, or the heat contained in the air of the passenger compartment. This opens up interesting opportunities for vehicle climate control, heat recovery and exhaust gas cooling.”