Summary
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Hydro, hybrid and electric busses
Summary
Fossil fuels are becoming scarce and the greenhouse effect is already effecting the planet. On top of that the conventional transit bus produces many harmful local emissions. Although only a small percentage of all vehicles in the world are urban buses, they impact disproportionately on public health.
To reduce the contribution in use of fossil fuels, the greenhouse effect and the local emissions in the bus sector, hydrogen, hybrid and electric busses are being developed. There are a number of reasons why transit buses are one of the best proving grounds for advanced propulsion technologies and alternative fuels:
− They operate in urban areas where air pollution is considered a problem − They are highly visible for the public
− There is space available for the alternative drive trains and testing equipment (both in volume and weight)
− They are fuelled and coordinated centrally
Alternatives for the conventional fossil fuel driven bus exist and will someday replace the fossil fuel powered buses completely. To better understand these alternatives a review and comparison has to be made.
This report will review and compare the hydrogen, hybrid and electric alternative systems for buses in use today. A literature study has been done to investigate all the hydrogen, hybrid and electric systems in use. The study only compiled of hydrogen, hybrid and electric systems for buses in use today and their facilities needed. Busses or applicable techniques still in the early stages of development, trends and alternative fuels aren’t discussed.
To make a good comparison an overview with hydro, hybrid and electric alternative systems has been made. For each alternative important criteria are gathered to assess their capabilities and compare the current alternative drive systems.
For hydrogen propulsion methods two types currently exist: fuel cells and hydrogen Internal Combustion Engines (ICEs). The fuel cell technology enables Zero or Ultra Low Emission Vehicles (ZEVor ULEV) with increased comfort due to very low noise levels and the possibility to use fully electric, stepless, drivelines. Hydrogen ICE makes use of a proven concept, namely the internal combustion engines, and reduces harmful local emissions and greenhouse gases.
Hybrid technology has been applied, because of the potential to reduce fuel economy, in different driveline configurations: diesel-electric, fuel cell-electric and hydrogen ICE-electric. Fuel savings between 20-35% were found for internal combustion engine buses depending on the average speed. For fuel cell buses the regeneration extends efficiency and range with 24-28% when using hybrid propulsion.
Electric buses, although many different kind of configurations are possible, only exist of two categories: battery powered buses and trolleybuses. Battery powered buses store electrical energy in battery packs. Trolleybus systems make use of overhead wires to transport electricity, so weight is reduced considerably increasing passenger capacity.
Facilities also need to be included in the comparison, when upstream production is included the overall system could emit significantly more greenhouse gases than conventional diesel engines.
Hydrogen fueling stations are key building blocks of a hydrogen transportation infrastructure. Hydrogen fuel for vehicles can be acquired in many different ways: on-site, trucked-in and on-board. On-site production facilities are small scale production plants at the fueling station, most widely used are: steam reformer and electrolyser. Trucked-in hydrogen is produced at industrial production plants and trucked in liquid or gaseous state towards the refueling station. On-board production uses very small scale production on the bus itself, because efficiencies are very low and weight is increased these production facilities aren’t examined during this study.
Hybrid facilities for diesel-electric drive consist of conventional infrastructure. For diesel production petrochemical industries are available.
Summary
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Technical University Delft
charging and fast charging. Slow charging charges the whole fleet over night and the bus simply stays in the depot, thus using off-peak hours and a whole fleet can easily be recharged without disrupting the electricity network. Fast charging recharges within two hours, but impacts battery lifetime and the electricity grid negatively. Battery replacement systems swap a whole battery pack after service hours, so valuable charging time is reduced dramatically. Overhead wiring is used in trolleybuses, but batteries can be installed to travel up to 10km without wiring to increase flexibility.
To compare current alternative drive systems for transit buses standardized criteria need to be analyzed such as: well to wheel efficiency; fuel consumption; greenhouse gases; local emissions; fuel costs and investment costs. Passenger capacity is highly influenced by alternative drive systems, due to the increased weight introduced by some alternative drive systems. Greenhouse gases per passenger, when the maximum passenger capacity is occupied, will be significantly higher for a low passenger capacity bus with the same emissions. Therefore, to asses alternatives buses, the passenger capacity has been taken into account.
Many alternative drive system developments have been made to reduce the contribution in use of fossil fuels, greenhouse effect and local emissions. The variety of alternative drive systems are hard to compare due to many variables, which influence costs, local and global emissions and fuel consumption. When considering application of an alternative drive system a thorough feasibility study has to be preformed, since many variables are depending on the location. After the study a pilot project is advised, due to many unknowns during operation.
For a standardized 12 meter two axle European transit bus using current hydrogen, hybrid or electric drive systems the following can be concluded:
- Hydrogen fuel cell technology can only be viable when renewable energy is used for production. Otherwise greenhouse gas emission can be up to 14 times higher than conventional diesel buses. The biggest advantage is that local emissions are no longer emitted.
- Hydrogen Internal Combustion Engines (ICE) have got similar fuel consumption compared to diesel buses, but efficiencies are low and local emissions still occur. Hydrogen ICE could become the first step towards a hydrogen infrastructure, because the same components as conventional diesel engines are used.
- Diesel-electric buses lower fuel consumption and greenhouse gases and also increase efficiencies, but relative to passenger space improvements aren’t high. Nevertheless, when passenger capacity will not be fully utilized often, a relatively minor adjustment could increase performance and decrease fuel consumption and greenhouse gases, still maintaining the current infrastructure.
- Hybridization increases fuel efficiency with 20-30% for conventional internal combustion engines and 24-28% for fuel cell buses.
- Battery powered buses have got high potential to become the most energy efficient transit bus. However passenger capacity is low and much improvement in battery technology needs to occur. The use of an extra axle could solve these shortcomings, but will increase fuel consumption.
- Trolleybuses are currently the most fuel efficient transportation alternative for buses, fuel costs per passenger space are lower than conventional diesel buses and trolleybus investment costs are only three times that of a diesel bus. Greenhouse gas emissions are low, when a good power mix or renewable energy is used, due to high efficiencies. The biggest disadvantage are high infrastructure costs and needed political effort for overhead wiring. It is hard to asses which alternative drive system is the best, due to many complex variables depending on the location of use and fluctuating by passenger capacity. Currently diesel-electric hybrid drive systems are a good alternative to conventional diesel buses. Mainly due to low investment costs, an increase in efficiency and a decrease in fuel consumption and greenhouse gases, still maintaining the current infrastructure.
However, in the future, when local emissions and greenhouse gases need to be addressed different alternative systems are more capable. Three axle full electric battery powered buses could someday replace conventional diesel buses, because infrastructure investment costs for fuel cell buses and trolleybuses are significantly higher. The use of diesel-electric buses would stimulate battery technology making a battery powered bus more feasible in the future.
