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English   Alternative drive systems in Buses of Daimler AG – Part II
30.12.2008 von admin

Two approximately 80 x 60 x 60 centimeter stacks with 980 fuel cells each were accommodated on the bus roof this time. They each developed 125 kilowatts and powered a central 225 kilowatt asynchronous motor. The consistently high torque made itself felt in a positive way for hill starts: 1027 Nm were already available at 500 rpm and rose to 1045 Nm at 1000 rpm. The use of carbon fiber in the casing permitted raising the pressure in the hydrogen storage tank cylinders to 350 bar. Nine 200-liter cylinders were arranged on the forward section of the roof and stored a total of 43 kilograms of hydrogen, adequate for a range of 200 to 300 kilometers.

On the occasion of the 2003 UITP Congress in Madrid, the first trio of buses took the start. Hamburg, Stuttgart, Amsterdam, Barcelona, London, Luxembourg, Porto, Stockholm and Reykjavik followed. One aim was to test the fuel cell drive system under practical conditions which differed as much as possible: from the flat-as-a-pancake downtown parts of Amsterdam and Hamburg to the hills of Stuttgart and Luxembourg, but also from the summer heat of Madrid to the Icelandic winter. Before the two-year test was over, Perth in Australia joined it with three vehicles; in 2005 Beijing followed suit. When the two-year testing period was complete, each of the ten participating cities had gained an average of about 7,000 operating hours or 100,000 kilometers of practical experience.

Special attention was paid in the test to the production of the hydrogen, by means of steam reformer or electrolysis, because the energy balance and lifecycle assessment for the drive system can be conclusively determined only on the basis of the complete system. If the bus operates free of emissions, but the production of the hydrogen consumes all the more energy and simultaneously gives rise to high emissions, then little has been gained in the end. In some cities the hydrogen was produced in distributed fashion, which led to unexpected technical difficulties in Stuttgart, for example. With central production, on the other hand, additional energy was lost during the subsequent transport to the hydrogen filling station.

First Mercedes-Benz Citaro regular-service bus with fuel cell drive is running in Madrid.

At the end of the two-year test, one thing was certain: with an availability rate of more than 90 percent the fuel cell drive system had given a better account of itself than expected. The new technology was extremely well received by local public transport operators, drivers and passengers so that seven cities promptly extended the test by another year. Hamburg even bought the vehicles from Stockholm and Stuttgart and now operates nine fuel-cell buses.

However, the fuel-cell bus is still far from being able to compete with the conventional diesel bus. The cost price of a single vehicle was 1.25 million euros. Despite its surprisingly good performance, the service life of the fuel cell is still far less than that of a diesel engine. The consumption of the test buses ranged between 15 and 30 kilograms of hydrogen per 100 traveled kilometers, the equivalent of the consumption of 50 to 100 liters of diesel fuel (diesel has a higher gross calorific value).

Unless service life and efficiency are enhanced significantly and the price is reduced at the same time, the new technology consequently will not be able to keep up with conventional drive systems. Moreover, considering the additional weight of the fuel-cell Citaro – some three tons – there is still major potential for reducing weight. Further development will therefore necessarily focus on the reduction of cost, weight and consumption, as well as on efficient energy management.

Finally, the definitive energy balance and lifecycle assessment for the drive system have not even been computed yet. It may make sense to operate emission-free locally, and the efficiency of the fuel cell itself is distinctly higher than that of an internal combustion engine. But if, for instance, more energy is lost during the production of hydrogen from natural gas than in the combustion of that gas in a natural gas engine, then little will be gained. And hydrogen production through electrolysis will in the end only help to protect the environment if the electricity required for electrolysis is generated from renewables like wind, water or solar energy.

Of course, questions like this cannot be answered in theory, but only through practical testing. Bearing this in mind, the large-scale test with the Citaro fuel-cell buses has furnished the most comprehensive data material so far.

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