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Electric bus
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Electric vehicles (EVs) operate on electrochemical energy stored in a battery and used to drive an electric motor. Cost is a primary concern followed by infrastructure and range.

Electric buses can use electricity by means of an overhead wire (trolley bus) or an on-board storage system, i.e. a battery. Of these two, the Info Pool currently only discusses battery electric vehicles (BEVs). Their general principle is relatively simple: electric energy - from any source - can be stored in batteries on-board the vehicle. When required, the energy is drawn from the batteries and converted to motive power by the use of an electric motor ( 1 ). When the stored energy is depleted, the batteries must be recharged ("refueled") by passing electricity into them. The current practice is to connect the buses to an electricity generation grid overnight ( 2 ).

BEVs have various advantages over other alternative drive systems, perhaps the most prominent one being that they are true local zero-emission-vehicles (ZEV), i.e. they produce no emissions at their point of use. This fact, together with their other technical characteristics, makes them particularly useful in public transport systems of polluted urban environments ( 1 ).

Advantages of battery electric buses include the following ( 1 ):

  • they produce zero emissions at point of use
  • they can utilize renewable energy (from power stations)
  • they are quiet and low vibration in operation
  • they have a higher energy efficiency in stop-start driving compared to ICE (internal combustion engine) vehicles
  • they can recover kinetic energy via regenerative braking.

Disadvantages on the other hand include:

  • high capital vehicle cost (e.g. due to high cost of batteries) ( 1 )
  • limited vehicle range due to the amount of energy which can be stored in batteries ( 1 )
  • typical battery recharge time of 6-8 hours (slow charge) ( 1 )
  • increased vehicle mass from the battery pack which increases vehicle mass by 300-900 kg and therefore energy consumption ( 1 )
  • potential problems with emissions from electricity production especially in old power plants fueled with coal.
  • Electromagnetic field.
  • Significant space requirement for battery pack.

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Battery electric vehicles emit no pollutants at their point of use, thus contributing to local air quality, which makes their usage ideal in highly populated urban areas. Thus the environmental impact of electric vehicles lies primarily in the electricity generation, through which, for instance, greenhouse gases are released. The exact nature and extent of these impacts depends on the means of electricity production: if renewable energy is used, then the overall environmental performance of electric buses will be better than if conventional fuels are used.
"Electric vehicles can be up to 40% more energy efficient than conventional ICE (internal combustion engine) petrol vehicles for congested urban city driving" ( 1 ). Regarding their life-cycle data, carbon monoxide and total hydrocarbons "are significantly reduced for the passenger car and bus. Particulate emissions are also reduced, for the passenger car and bus, although with an increase in SOx emissions. NOx emissions are significantly reduced for the bus ... " ( 1 ).

As noted earlier, full life cycle greenhouse gas emissions must be based on a local assessment carefully considering the source of the electricity. In general, if coal fired powerplants provide the electricity, one would expect very high greenhouse gases; however, if electricity is generated largely by hydropower, then one would expect very low greenhouse gases relative to conventional diesel buses. Anyway when calculating or assessing the environmental properties of electric vehicles the electricity should be seen as coming from marginal electricity production.

It should be noted that these local ecological advantages do not necessarily justify a choice in favor of electric buses. As with CNG buses, a careful analysis is advisable, assessing to what degree local emissions can be reduced using improved diesel buses (advanced or retrofitted) or CNG-buses, which may be the faster and more economic option. Nevertheless, all environmental improvements in electricity production (e.g. larger share of renewable energy) can also directly improve the environmental performance of the electric bus.

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As a result of limited experience with electric buses, it is as yet difficult to comment on the technical reliability of electric buses. As stated in ( 3 ): "very little maintenance cost data for battery-electric buses are reported in the literature. This may be because the power trains in many of the buses in service to date have been developmental and so have had maintenance requirements that are higher than would be expected in fully commercialized production vehicles and therefore are not comparable to production diesel vehicles."
According to ( 4 ), "recently, battery-electric buses have been built and operated in transit applications. The Santa Barbara Transit District and Regional Transit in Sacramento each operate 6 battery-powered buses. Sacramento reports that the battery powered vehicles are the least reliable buses in their fleet, and as a consequence are very expensive to operate".

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For a comparison of some cost data, ( 1 ) is a useful source, stating that "BEVs currently cost approximately 50-100% more than their conventional counterparts." According to ( 3 ), "the capital costs of battery-electric buses are substantially higher than those of similarly sized diesel transit buses. A 25-foot battery-electric shuttle bus is slightly more than twice as expensive as a comparable diesel model when the battery-electric bus is equipped with a lead-acid battery pack. With the larger 33-foot buses, the cost premium for battery-electric buses falls to approximately 33 percent."
Costs for battery-electric buses "that may differ from those of diesel motor buses include energy costs, maintenance costs, and the costs or savings associated with lower or higher vehicle availability. The energy costs per mile reported for battery-electric buses are similar to those for similarly sized diesel buses" ( 3 ).

Due to the relatively limited experience with electric buses so far, exact estimates for maintenance costs etc. are difficult to obtain ( 3 ).

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It is estimated that in 1999, there were only 150 full-sized electric-powered transit buses used in the United States (3). But research and development are ongoing in this field: "many U.S. companies have electric bus development projects. The current research focus for electric propulsion vehicles is in the area of battery development, where the goal is to develop batteries that have low initial cost, high specific energy, and high power density. Battery-electric buses currently in use are predominantly 22- to 30-foot buses, not full-sized buses.

Although full-sized battery-electric buses have been successfully operated in downtown shuttle routes with limited speed and range, their performance limitations make them impractical for conventional route service but quite appropriate for niche routes requiring only 22- to 30-foot vehicles and ranges of 100 or fewer miles" ( 3 ).

According to ( 2 ) also, "commercial battery-electric bus technology is currently limited to smaller buses, known as electric shuttles, that do not meet the gross vehicle weight rating classification for conventional urban buses (>33,000 pounds). These electric shuttles are in regular service in many transit districts nationwide. In California, about 30 percent of the Santa Barbara Municipal Transit District fleet is battery-electric shuttles, which are used primarily on waterfront and downtown routes. Electric shuttle utilization is constrained by range requirements, terrain, and climate.

Larger electric buses that would meet the definition of an urban bus are still in the developmental stage." According to ( 1 ) a recent FT Report predicts a "negligible world-wide demand for battery electric passenger cars in the mainstream car sectors over the coming decade. This leaves open the possibility that demand will increase for battery-powered commercial vehicles, buses and scooters - all sectors which have some characteristics that would make them more suitable for BEV applications."

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With regard to a future market penetration of battery electric buses, further technological development is desirable in order to improve upon their weak points such as limited range. According to ( 2 ), current development efforts are focusing on battery and recharging technology.

"Like all current efforts to build commercial electric vehicles, electric bus development is hampered by battery technology. Batteries today are heavy and expensive, and do not provide the range, top speed, or acceleration capabilities of liquid or gaseous fuels. As a result, hybrid buses are regarded as having greater potential than vehicles powered solely by electricity" ( 4 ).

Finally, it must be carefully analyzed whether the significant ecological advantage of zero local emissions justifies the additional expenditures in comparison to other options, such as CNG or improved diesel buses. In the future, electricity from renewable resources may contribute significantly to the environmental advantage of the electric bus.

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