Trolleybuses vs. Fuel-cell busesA developing technology is seen in the much talked about Hydrogen Fuel Cell bus, engineered by such well-known companies as Ballard Power Systems and Daimler-Chrysler. The fuel cell bus is essentially an electric vehicle that uses a set of hydrogen fuel cells to generate its own electricity on board.
Typically hydrogen is stored in large tanks mounted on the vehicle?s roof, but it is also possible to produce hydrogen on board from fossil fuels using a special device called a "reformer". The electricity is stored in a battery pack, from which it is fed to an electric traction motor(s). The most recent engineering efforts have sought ways to eliminate the battery pack as it has been labelled a source of problems. Similar to CNG buses, hydrogen fuel cell buses must be equipped with leak detectors because hydrogen is extremely volatile. Difficulties in adapting fuel cells for practical use have severely limited their application, in particular as mobile fuel sources for transportation. It must be emphasised that the use of fuel cells for powering transit vehicles is absolutely experimental at the present time. While they may hold promise for the future, technologies with a proven track record will always be the most desirable choice for any transit authority simply for reasons of reliability and the relative predictability of costs.
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Hydrogen fuel cell buses face tough obstacles that must be overcome to make them truly viable transit vehicles:
Weight: A complete fuel cell powerplant for a transit vehicle has five to ten times the weight of an equivalent internal combustion powerplant. This means that without a single passenger on board, a 40 foot fuel cell bus has practically the weight of an equivalent diesel bus with a fully seated load. Because of vehicle weight limitations, a fuel cell vehicle could not currently handle the passenger volumes found on high patronage bus routes.
Performance and Reliability: Acceleration was cited as a problem on early fuel cell test buses in Vancouver and Chicago. Hill climbing proved problematic. Subsequent adjustments and modifications to the vehicle managed to overcome this, but to the detriment of fuel consumption. Fuel cells are extremely sensitive to impurities in the hydrogen. Even the best fuel cells degrade quickly if there is more than 10 ppm of carbon monoxide in the hydrogen. Whether the cells are fed by hydrogen stored in tanks on board the vehicle, or whether the hydrogen is manufactured from fossil fuels using an on-board reformer, impurities appear unavoidable and pose a serious problem for the fuel cell?s reliability and durability. Special ?clean-up treatments? to purify the hydrogen carry a substantial cost penalty and their effectiveness may be limited. The range and reliability of Vancouver's test vehicles was such that they were only permitted to remain in service for a maximum of 4.5 hours at a time. This contrasts sharply with a a trolleybus which has no refuelling requirements at all and can provide continuous service.
Cost: Vehicular fuel cell powerplants are extremely expensive, costing about ?3,500 per kW. Similar to CNG buses, fuel cell vehicles require expensive infrastructure. This may include equipment to produce and store hydrogen as well as special refuelling stations. The fuel costs alone for the operation of the Vancouver test buses were found to be at least three times those of a trolleybus. The investment of large sums of money in fuel cell infrastructure may be considered a questionable prospect at this time. In British Columbia, the provincial government offered an annual subsidy of $40 million for the operation of fuel cell buses. The transit authority believed such funds would be better spent on adding 40 km of overhead to its trolleybus system each year.
Greenhouse Gas Emissions: The operation of large numbers of fuel cell vehicles requires a large and steady supply of hydrogen. Currently, the most readily available and economical sources of hydrogen are fossil fuels. The hydrogen molecules are removed by a process called "stripping". Making hydrogen in this fashion creates considerable quantities of the greenhouse gas carbon dioxide and thus contributes to the greenhouse effect.
The amount of CO2 produced is only slightly less than for internal combustion vehicles operating on gasoline or diesel fuel. Using data gathered by Daimler-Benz, the chart above shows that hydrogen production for the operation of a subcompact car would yield around 77% of the CO2 emissions generated by the same car with a diesel engine. The prospect of reducing CO2 emissions from coal and gas-fired power plants using new technologies appears better than the most feasible methods of hydrogen extraction. While it is possible to generate hydrogen from electricity produced by renewable sources like biomass, wind, water and the sun, this process is not nearly as efficient as using this clean energy for powering transit vehicles directly from overhead lines.
Efficency: Fuel cell vehicles are practically a non-starter in terms of energy efficiency. Chartered Mechanical Engineer (Eur Ing) Irvine Bell estimates the overall energy efficiency of a fuel cell bus currently at around 11% - about the same as a steam locomotive. The most optimistic predictions of fuel cell development point to a potential total energy efficiency in vehicular applications of 20-30%, taking into account the hydrogen production process. This compares with a diesel bus at about 25-40%. A trolleybus driven by electricity generated by the latest high technology gas-powered turbines can achieve total efficiencies between 40 and 60%; trolleys can also be efficiently powered by renewable sources. Fuel cells would prove much more efficient in supplying electricity for public transit vehicles if installed in large stationary power plants than if used on board vehicles.
Fuel cell Bus It takes about 4 kWh of electricity to create, compress and transport 1 kWh worth of hydrogen. The engineering goal is to achieve 50% efficiency for the fuel cell unit itself; the electric motors on the bus are about 90% efficient. The end result is 9 kWh of electricity (however generated) for 1 kWh output at the wheels of a fuel cell bus. In theory regenerative braking is possible on fuel cell buses, using batteries or flywheels, but the fuel cell bus is already overweight and cannot carry more equipment.
Trolleybus Line losses in power transmission typically range between 10 - 20%. The vehicle would be 80% efficient, with 60% going to the motor and 20% going to auxiliary functions, such as air compressor, lighting and heating grids. This means it takes 1.56 kWH to produce 1 kWh tractive effort without regenerative braking. Thus, a trolleybus is about 6 times as fuel efficient as a fuel cell bus. If you factor in the reduced passenger capacity of a fuel cell bus, the trolleybus is probably closer to 12 times more energy efficient. Even the most optimistic projections for fuel cells and mass production of hydrogen are unlikely to reduce this below a factor of five.
Trolleybus (with regenerative braking) The trolleybus with regenerative braking can recover at times as high as 30% of the total energy used and put it back in the wires. It would then require about 1.1 kWh for every 1 kWh of tractive power. This applies only to trolleybuses with high levels of regenerative braking operating on a system with frequent headways where there are usually other buses in the vicinity to utilize the power returned to the lines. (i.e. a high likelihood of other buses in the vicinity able to use regenerated power).
Hybrid Bus A hybrid bus uses electric motors at the wheels and a smaller diesel engine which drives a generator. Connected to the generator and motor is a battery pack, (which adds to the vehicle's weight). The hybrid bus also has regenerative braking (putting energy back into the battery), a lower fuel consumption and a decrease in emissions of up to 46%.
APTA Electric Bus Subcommittee Meeting, November 8, 1999. Notes.
Bell, Irvine (Eur Ing) to Editor, Professional Engineering, re. "Warming, What Warming-reducing global warming through fuel cell vehicles", January 18, 2001.
Fisher, Ian. Electric Trolleybuses in Vancouver: Past, present and future alternatives. Transport 2000 BC Website (http://vcn.bc.ca/t2000bc/learning/_trolleybus/trolleybus_essay.html), 1997.
"How Green is your Hydrogen?" The Economist, April 2000, p. 74.
Kenward, Michael. "The Power to Clean Up," Professional Engineering, June 9, 1999.
Lythgo, Chris. Bus Technology Review. TransLink, September 1, 1999.
Monaghan, M. L. Fuel Cells for Passenger Cars-The Hydrogen Revolution. Chairman's Address, The Automobile Division of the Institution of Mechanical Engineers, January 2001.
Perkins, Terry. "Fuel Cells for Power Plants under Development," Office.com (www.office.com), September 1, 2000.
Reducing Greenhouse Gases and Air Pollution: A Menu of Harmonized Options. State and Territorial Air Pollution Program Administrators and Association of Local Air Pollution Control Officers, October 1999.
"The Fuel Cell, Trolley and the Hybrid Bus - Where Should We Invest?" Transport Canada 2000 Western Newsletter, November 2000.
Kevin Brown BA MA, Academic Advisor, Faculty of Science, University of Alberta in Edmonton, Canada.