Mercedes unveils prototype fuel-cell hybrid bus

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Two fewer hydrogen tanks are needed for the prototype fuel-cell hybrid bus than for its fuel-cell-only predecessor. The seven hydrogen tanks carry a total of 35 kg (77 lb) of hydrogen.

Combine Mercedes-Benz experience with fuel-cell buses to date with the hybrid drive elements of the diesel-electric Citaro G BlueTec serial hybrid bus and what is the result? The prototype Citaro fuel-cell hybrid bus, shown at the recent UITP Congress (the World Congress of the International Association of Public Transport) in Vienna. Mercedes promises a short series production run, to be followed in 2010 by an extensive fleet trial of 10 buses in Hamburg, Germany.

Compared with the first NEBUS fuel-cell prototype bus from Mercedes, first seen 22 years ago, the company claims that hydrogen consumption for the latest prototype will be almost halved. In addition, Mercedes claims that the service life of the fuel cells is expected to be at least six years or 12,000 h. Maintenance requirements have also been reduced, with the company claiming that the batteries and drive motors are practically maintenance-free for life.

The electric hybrid drive system, shared with the diesel hybrid Citaro, is based around two water-cooled asynchronous hub motors, with a continuous output of 60 kW each. Mercedes claims a peak output of 80 kW each for starting. Transplanted from the diesel hybrid alongside the hub motors are the dc/ac converters and the water-cooled lithium-ion traction batteries. These are rated at 27 kW·h, enough to supply the 120 kW for the drive motors over a distance of 2 to 3 km (1.2 to 1.9 mi) without assistance from the fuel cells.

Most of the fuel-cell components are housed under the raised roof panel, forming a full-length compartment between the reinforced roof of the regular Citaro bus and the raised panel. The hydrogen tanks are located in a transverse row at the front of the compartment. Since the fuel cells are more efficient than in the previous Mercedes fuel-cell bus and the hybrid system improves efficiency further, less hydrogen is needed on board. Consequently, the number of tanks has been reduced from nine to seven, giving a total capacity of 35 kg (77 lb) of hydrogen gas.

The two Ballard fuel-cell stacks are located at the rear of the rooftop compartment behind the passenger compartment air-conditioning system. The two stacks function independently, each containing 396 individual fuel cells. Between them are the hydrogen metering systems and air feeds to the fuel cells.

Compared with the previous-generation Mercedes fuel-cell bus, the service life of the stacks has been extended by about 50%. Mercedes claims a life of at least six years. The company also claims greater efficiency for them—between 51 and 58%, compared with 38 to 43% for the first-generation stacks. Waste heat from the stacks is used either to heat the passenger compartment or, when that is not needed, four fans extract the hot air.

What would have been the engine compartment in a diesel Citaro now houses the electronic and electrical equipment that drives the ancillary components, sharing space with the two dc/ac converters. All ancillaries—such as the power steering pump, air-conditioning, and air compressors—are electrically driven and all are carried over from the diesel hybrid Citaro, along with the dc/ac converters and lithium-ion battery pack.

Mercedes claims that the regenerative braking system reduces hydrogen consumption by between 10 and 25%. Overall, the improved efficiencies of the fuel-cell system result in claimed hydrogen consumption of between 11 and 13 kg/100 km, compared with about 22 kg/100 km from earlier Mercedes fuel-cell bus prototypes.

Despite the addition of the hybrid drive system and lithium-ion battery pack, the effect of dispensing with the automatic transmission from this generation fuel-cell Citaro, lighter fuel-cell stacks, smaller cooling system, and reduced hydrogen storage is a considerable weight saving. Mercedes quotes a curb weight of approximately 13.2 t (14.5 ton), some 1000 kg (2205 lb) lighter than its predecessor.

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