By Doug Jack
The many innovations at the UITP conference and exhibition in June in Vienna, Austria, may have overshadowed the significance of the latest generation of fuel cell bus from Mercedes-Benz.
Hartmut Schick, head of Daimler Buses, emphasized the latest generation is not a prototype, but another important step on the path to zero-emission public transport, and an important element in the development of mobility solutions of the future. The Citaro FuelCELL-Hybrid bus is a response to growing calls for a drive system that was emission-free and helped to conserve resources at the same time.
The initial price of fuel cell vehicles is still a major barrier, compared with conventional diesel buses, but Mercedes-Benz has the major advantage of being able to share development and production costs over a wide product range. The service life and reliability of fuel cells will continue to improve and produce further efficiencies. As demand for fuel cells increases, we also can expect unit costs to come down.
In ten years time the operating cost per mile of fuel cell buses, compared with conventional diesel or gas-powered buses, might be much more attractive, especially when one factors in the environmental friendliness.
Mercedes-Benz has been working on alternative fuels and drive systems for more than 40 years, and presented its first hybrid city bus at the Frankfurt Motor Show in 1969. A 65hp four-cylinder 3.8-liter engine provided thermal power with electrical energy stored in five large blocks of batteries weighing more than three tons. The DC electric traction motor developed a steady 156hp and 204bhp peak output at low speeds — well ahead of the normal power of city buses at that time. One advanced feature was an electric brake to recoup braking energy.
Over the years Mercedes-Benz developed more hybrid prototypes as well as duo-buses that could run with standard diesel engines or power from overhead lines like a trolleybus. Its hybrid development culminated in November 2007 with the launch of the Citaro G BlueTec hybrid articulated bus powered by a small diesel engine or a fuel cell power pack.
In 1997 I visited the Mercedes-Benz bus and coach sales center in Frankfurt, Germany, where one of the senior managers showed me the very interesting prototype of the NEBUS, a low-floor city bus fitted with a first-generation Ballard fuel cell. This large unit under the floor behind the rear axle raised the rear of the bus four steps higher up than the low front section. The hydrogen stored in gas cylinders on the roof.
If I remember rightly, the fuel cell drove through a conventional fully automatic gearbox to the rear axle and the performance was quite astonishing and silent for a heavy bus. In the absence of the normal hard-working diesel engine, I suddenly became aware of other noises from the suspension, power-steering pump and particularly the tires.
It was a cold and blustery day in Frankfurt and the only emissions were steam emerging at roof level from a tail pipe located vertically behind the panels at the rear of the bus. When we stopped at traffic lights, a driver in a large truck pulled up behind us and shook his head sadly, obviously thinking that another silly bus driver was about to blow up his engine.
That research program encouraged Mercedes-Benz to continue developments with fuel cells. Costs might have been prohibitive if confined solely to city buses, but fortunately other product groups within Daimler were also interested in the future of fuel cells and shared costs. A number of fuel companies took part in the project to gain experience of various alternative methods of production of the hydrogen fuel.
In 2003 Mercedes-Benz unveiled its next generation of fuel cell buses, delivering 36 Citaro low-floor buses in batches of three, to 10 European cities as well as Perth, Australia and Beijing, China. The operating conditions of the chosen cities varied widely, from flat to hilly terrain. The cities also varied in climate from winters in Iceland and Sweden to summers in Spain and Portugal.
Although the Mercedes-Benz engineers knew they would obtain greater efficiency from a hybrid drive system, they chose to use a simpler drivetrain to easier identify and rectify any problem. On each bus the fuel cell provided electric current to a generator coupled to a standard ZF fully automatic gearbox and to the same type of portal rear axle widely used in conventional diesel-powered buses.
The packaging of the components on these buses was much better than on the original NEBUS prototype. The floor was only one step above the ground from the front entrance to the third door, behind the rear axle.
The participating cities and engineering teams kept in constant contact. All were aware of the safety implications of running such advanced buses in regular service. The fuel cell buses were part of a total system, with a major concern being the supply, storage and delivery of hydrogen. If a problem occurred at any one of the filling stations, they all shut down for safety reasons until they found a solution.
Taken out of service at the end of the trial period, the performance of the vehicles exceeded the expectations of the engineering team. Collectively they accumulated more than 1,250,000 miles over more than 135,000 hours of operation.
Their 90 and 95 percent availability provided an impressive demonstration of the suitability of fuel cell drive systems for everyday use in regular urban service. Ballard continued to develop more efficient and compact fuel cells.
Now Mercedes-Benz has launched the Citaro FuelCELL-Hybrid bus. Although a standard two-axle 40-ft low-floor city bus, many of the components other than the fuel cell are common with the earlier articulated diesel-electric hybrid.
Compared to previous models, the FuelCELL-Hybrid bus is around one ton lighter. New technology extends the service life of the fuel cells by around 50 percent to at least six years, as efficiency has increased from 51 to 58 percent, compared with 38 to 43 percent for the previous generation.
The hybrid drive system uses less hydrogen, reducing the number of roof-mounted fuel tanks from nine to seven.
Electrical energy stored in compact lithium-ion batteries supply the two water-cooled wheel hub motors with a constant 120kW — sufficient energy for the bus to run a few miles on battery power alone. Two stacks of 396 individual fuel cells fit into the full height compartment at the rear of the bus opposite the third door.
Heat exchangers to the rear of the stacks use the waste heat from the fuel cells to heat the passenger compartment. Four fans evacuate the hot air if heating is not necessary. The only exhaust emission is harmless water vapor.
Mercedes-Benz will build a small production run of the new FuelCELL-Hybrid buses this year, followed by 10 that will enter service in Hamburg next year. The German city is a great fan of fuel cell vehicles, having not only retained the three it took under the test program in 2003, but also a further six vehicles acquired after their trials in Stockholm and Stuttgart. The forward plans of Mercedes-Benz include further in-service testing in customer fleets at a European level.
Doug Jack is with Transport Resources in the United Kingdom.