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A revolution in public transport has started with the presentation of the
world's first fuel cell bus.
Based on a Mercedes-Benz 0 405 N regular-service bus, the NEBUS ('new electric bus'),
represents a further milestone in the many years of intensive research carried out by
Daimler-Benz towards a fuel cell suitable for automotive application.
The new technology will bring about a significant reduction in emissions in inner city
areas.
The NEBUS was preceded by "NECAR I", based on a Mercedes-Benz van and presented
in 1994, and "NECAR II", running on a Mercedes-Benz people carrier platform and
presented in 1996. With these three prototypes, Daimler-Benz is the first world's
automotive manufacturer to prove that road vehicles - whether as passenger cars or
commercial vehicles - fitted with alternative drive systems represent a viable option for
the future.
This fact, with its major significance for the mobility of mankind, is also a result of
Daimler-Benz worldwide unique expertise in research into alternative drive systems and
automotive fuels. To date, it has tested and analysed more than 200 prototypes
incorporating the most diverse system solutions and drive systems.
The energy carriers and drive systems tested include vegetable and methanol-based fuels,
mixed fuels, hydrogen, natural gas, electric vehicles, hybrid vehicles, wheel hub motors
for buses as well as the recently developed fuel cell with its major potential for the
future.
Fuel cell technology opens up new opportunities for replacing the battery as a medium of
on-board storage of electrical power, with its inherent disadvantages of short range and
heavy weight, by a system of on- board power generation.
The principle of the fuel cell is very simple and has been known for more than 150 years.
Water, electrical energy and heat are generated in a controlled reaction of hydrogen and
oxygen. This reaction, which occurs in the fuel cell at low temperatures, does not produce
any pollutants, especially no nitrogen oxides, The only substance produced is pure water.
This direct conversion is characterised by a high degree of efficiency, since - quite in
contrast to combustion processes - it is not subject to the thermodynamic limitations of
the "Carnot" cycle (a reversible vapour compression cycle with maximum thermal
efficiency). Another advantage of the application of fuel cell technology in bus design is
the generation of sufficient heat for the vehicle's heating system.
The idea of generating electrical energy from hydrogen and oxygen was developed by the
British physicist Sir William Grove. However, this idea was not able to gain the upper
hand over the dynamos which were becoming popular in those times; the principle thus fell
into oblivion, to be revitalised only with the progress of energy technology for the
American space programs.
The advantages of the fuel cell are its high degree of efficiency, the complete lack of
any pollutant emission, its low noise levels, the lack of any moving parts and its modular
design; this feature in particular allows the output to be adjusted to the specific
requirements of individual vehicle types and dimensions, as is borne out by the three
existing Mercedes-Benz prototypes.
From today's point of view, the intensive research activities carried out for the fuel
cell project at the Daimler-Benz Research Centre in Ulm indicate that this drive system
will be ready for practical and economic use in buses by the beginning of the next
millennium.
A revolution in bus design
The highly compact design of the fuel cell will usher in a genuine revolution in
bus design, since a large number of components which from today's point of view are
absolute necessities, such as engine unit, axles, gearbox, cardan shafts, alternators or
fuel tanks, will be replaced by electrical cables and wires. There will thus be more space
for passengers.
Ride comfort for the passengers will noticeably improve, as shifting operations will be
dispensed with entirely. The use of the low-floor driven axle from ZF, with integrated
wheel hub motors driven by the fuel cell, already makes a 12-metre low-floor bus feasible
even today.
Each end of this axle features an air-cooled asynchronous motor with a maximum peak output
of 75 kW. 150 kW are thus available for powering a single vehicle. This power,
corresponding to more than 200 HP, is comparable to that delivered by a diesel engine in a
conventional city bus.
The basic NEBUS vehicle, a conventional 0405 N city bus, is 12 metres long, 2.5 metres
wide and 3.5 metres high and accommodates 34 seated and a further 24 standing passengers.
The only difference to the standard production vehicle is the adaptive damping system,
which supports positive handling. The front and central exits have no steps at all; the
rear section - in contrast to the current standard production vehicle - has two steps,
which will no longer be required for future vehicles.
The gas unit, power electronics and cooling system are located on the roof behind and
under the coverings, as in the gas-driven buses. The power electronics include the AC
converter, the pulse-width-modulated inverter from ZF, both of which are installed on the
rooftop as control elements for the wheel hub motors, as well as water-cooled brake
resistors, which convert the electrical braking energy into heat which is then dissipated
by the cooling water. The roof system itself consists of seven 150-litre cylinders at a
pressure of 300 bar.
These supply the fuel cell with approximately 45,000 litres of hydrogen. Depending on
application profile, the NEBUS in this configuration has an operating range of up to 250
km.
A completely independent system
NEBUS, the prototype of the Mercedes-Benz fuel cell bus, is convincing proof of
the practicality of this future-oriented technology; so far, Daimler-Benz has been the
only manufacturer to implement this technology in an absolutely independent, dynamic
system. No additional on-board battery power needs to be installed to provide the
permanent driving power required by a bus.
The fuel cell of the NEBUS consists of ten stacks of 25 kW each, resulting in a gross
output of 250 kW. Following deduction of the power required for the fuel cell itself, an
output of 190 kW is left for driving and for the auxiliaries such as the on-board
electrical system and the air- conditioning unit.
Generation of electrical energy by the fuel cell takes place in the space which in a
conventional bus is occupied by the engine and gearbox.
The fuel cell runs on hydrogen and oxygen; the only emission discharged by the exhaust
pipe is pure water vapour.
Another ecological advantage of the NEBUS is its solar-energy roof unit, which converts
the electrical power generated by the solar cells for operation of the air conditioning
and ventilation ducts. The solar system installed in NEBUS works even without the main
electrical system having been activated.
More high tech instead of "back to nature"
Due to its economic efficiency, its high availability and considerable potential
for further reduction in pollutant and noise emission, the diesel engine is the dominating
drive system worldwide.
However, the increasing legal requirements in force throughout the world are placing
increasing pressure on manufacturers to build vehicles producing no exhaust and noise
emissions at all. According to European forecasts, the number of vehicles on the roads
will have doubled by the year 2030.
On the one hand, this strong increase reflects the enormous demand for mobility in the
industrialised countries and in the emerging markets of Asia and South America. On the
other hand, this high degree of mobility leads to increasing environmental problems.
For this reason, the road to the future cannot end in demands along the lines of
"back to nature". More sophisticated "high tech" solutions are
required. Fuel cell technology contributes to a long term solution of energy constraints
and environmental problems without causing any major new problems of its own.
There is a bright future in store for the electric fuel-cell city bus developed by
Daimler-Benz, with its alternative zero-emission drive system. Customers can be offered
environmental compatible drive systems which keep them mobile and which are at least on a
par with today's buses in terms of functionality and handling comfort.
How the fuel cell works
In ordinary electric cars, the electric energy first of all has to be generated at
a power station, then it is stored in a battery in the vehicle. The high costs, high
weight, limited durability and long charging times of these batteries are problems to
which no satisfactory answer has yet been found.
The aim is therefore to find a process which obviates the need for intermediate storage
and generates the electric current on board the vehicle as required.
A particularly promising avenue being explored is the fuel cell. The fuel
- hydrogen and oxygen - does not have to be combusted but is
transformed directly into electrical energy and water vapour in a "cold"
reaction.
The principle of the fuel cell
As most school children know from their chemistry lessons, igniting a mixture of
hydrogen and oxygen produces a loud detonation.
How can this reaction be used to produce electrical energy? The Daimler-Benz researchers
came up with a neat trick. A special foil, forming an "electrolyte", separates
the two gases and prevents the "hot reaction". An electrochemical process in the
thin foil ensures that only protons, ie. positively charged hydrogen ions, can pass
through while the hydrogen electrons are left behind. On the other side, the hydrogen ions
react with the oxygen.
The excess electrons on the hydrogen side and the electron deficit on the oxygen side
create plus and minus poles, from which electric energy can be obtained. The energy for
this "charging pump" is obtained from the reaction of the hydrogen (H2) with the
oxygen (02) which produces electrical energy and pure water vapour (H20).
The electrolyte in this PEM" (Proton Exchange Membrane) fuel cell consists of a
polymer foil a few tenths of a millimetre thick and coated on both sides with a catalyst
containing platinum. The electrolyte assists the ionisation of the hydrogen and the
reaction of the hydrogen ions with the oxygen.
Bipolar plates, which guide the hydrogen and air along the catalyst surfaces in a
labyrinthine channel system, enclose the cell on both sides. They also dissipate the heat
produced by the reaction and provide the electrical connection to the neighbouring cell.
By combining a large number of cells in so-called stacks, the necessary energy is provided
for driving the vehicle.
By regulating the supply of hydrogen, the amount of energy can be precisely controlled and
varied.
Facts about the fuel cell
With the presentation of the NEBUS vehicle, Daimler-Benz remains the front-runner
in research into the fuel cell, the most environmentally compatible automotive drive
system. This revolutionary unit is characterised above all by genuine "zero
emission" pure water vapour issues from the exhaust pipe; no-other substances are
emitted whatsoever, even in minimal quantities.
In noise emission levels, too, the fuel cell undercuts all its competitors: it operates in
almost complete silence. The fuel cells used by Daimler-Benz in NEBUS use hydrogen as a
fuel. On board the new vehicle, pressurised gas cylinders serve as a fuel tank.
The most important arguments for the fuel cell
Emissions: The hydrogen-powered fuel cell vehicle produces only chemically pure
water vapour,
Efficiency: The efficiency of the internal combustion engine - the extent to which the
energy contained in the fuel is harnessed for propulsion - is subject to specific
thermodynamic limits ("Carnot principle"). The purely electrochemical process
which takes place in the fuel cell is not subject to such limits. The NEBUS vehicle can
thus attain a degree of efficiency superior to that of a bus powered by a combustion
engine.
History of the fuel cell
Back in 1839, the British scientist William Grove was able to produce electricity
from hydrogen and oxygen. There were repeated attempts to use this "gas battery"
technology - for example in 1959 in an Allis Chalmers tractor, in 1963 in the Gemini space
missions, from 1968 in the Apollo flights to the moon, in 1965 in the Siemens boat
"eta", in 1983 in an 11 megawatt power station in Tokyo and since 1991 in the
Solar- Wasserstoff GmbGH demonstration power plant in Neunburg vorm Wald.
With "NECAR I", Daimler-Benz already demonstrated in May 1994 the potential for
powering a vehicle with a PEM fuel cell. Only two years later, this power unit was
integrated into a passenger car, "NECAR II", for the first time. Since the
presentation of 'NECAR II' in May 1996, several research activities have been publicised
in the field of fuel cell technology.
Applications
Fuel cells made their debut in the field of space and submarines, since they allow
relatively large supplies of energy to be carried on board in gaseous form and consumed as
required. Batteries are too heavy to store sufficient electricity for longer missions.
By the end of the decade, the trend towards decentralised power supply could open up new
horizons for this technology. Connected up to already existing gas infrastructures, fuel
cells can meet local electricity requirements with high efficiency, minimal emissions and
without losses due to long transport distances.
With its high efficiency and very low or, depending on the technology used, non-existent
pollutant emissions, big hopes are being pinned on the fuel cell for use in mobile
applications. However, the requirements in terms of space, weight, robustness and costs
are much more stringent than in the case of local energy supply.
The Fuel Cell Technologies
Five different fuel cell technologies are currently competing to serve the market
of the future: Proton Exchange Membrane Fuel Cells (PEMFC) - are the best suited to mobile
applications due to their working temperature of 20 to 100 degrees Centigrade and high
performance to weight ratios. They can be powered by air and hydrogen. That makes it
necessary either to have a hydrogen supply infrastructure or to use an on-board gas
generation system with a reformer producing hydrogen from, say, methanol. The Daimler-Benz
vehicle on show uses a PEM fuel cell.
Alkaline Fuel Cells (AFC) - use caustic potash as an electrolyte. Although they are the
most efficient type of fuel cell, they can only run on pure hydrogen and pure oxygen,
since carbon dioxide destroys the electrolyte. They are seen as having only limited
potential for future application.
Phosphoric Acid Fuel Cells (PAFC) - in this type of fuel cell, phosphoric acid with a
temperature of 200 degrees Centigrade forms the electrolyte. Decentralised heating and
power plants are the main type of application and such fuel cells are already in use in a
number of 200 kW demonstration facilities. However, their efficiency is limited.
Molten Carbon Fuel Cells (MCFC) - operate at temperatures of 650 degrees Centigrade with
molten carbonates providing the electrolyte. Amongst other things they are suitable for
use with coal gas, which will be of great advantage in future for decentralised energy
generation. This type of fuel cell will be ready to go on the market in a few years' time.
Solid Oxide Fuel Cells (SOFC) - are based on a solid electrolyte (zirnpn oxide). With
their working temperature of 1000 degrees Centigrade, they promise to be particularly
efficient in combined heating and power generation. On the other hand, the high
temperatures will necessitate considerable further development work.
Possible applications
With their efficiency and low emissions, fuel cells have a string of potential
applications in the energy sector, from emergency power supply at hospitals to use at
decentralised power stations in the kilowatt to megawatt range and at central power
stations. Other potential uses range from fuel cell cars, buses, rail vehicles and ships
to satellites and space stations.
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