Why the Trolleybus?
This article is an update of one previously published in UIT magazine in July 2002.
The author Eur Ing Irvine Bell BSc CEng MIMechE CDipAF PGCE was a professional automotive engineer with over 25 years experience in the commercial vehicle manufacturing and operating industries.
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The electric trolleybus is a long established form of urban public transportation. It had a heyday in the 1930s and 1940s as replacement for street tramways. Its fortunes began to decline in the 1950s as diesels became as cost effective and the 'inflexibility' of a fixed infrastructure became to be perceived as a disadvantage. At that time, decreasing public transport ridership was often just accepted as inevitable and environmental issues were of little concern. As equipment wore out, many trolleybus systems were replaced by diesels. Falling markets for trolleybuses and their equipment increased costs and accelerated the decline.
Major oil supply crises in the 1970s reversed the process. Since then the need to improve urban public transport and increasing awareness of environmental issues have generated renewed interest in trolleybus technology. Existing systems have expanded and re-equipped and new systems opened. The announcement of a new trolleybus system for Rome and the opening of a new system in Landskrona in Sweden are examples. Rome, which already has metro and tramway systems, is investing in a 300 vehicle, 10 route trolleybus system, with off wire operation in the city's historic centre.
Fig 1 First of Romes new Solaris/Ganz articulated trolleybuses
Substantial numbers of trolleybuses have been ordered by or recently delivered to Athens [Fig 2], Arnhem, Basel, Bern, Bologna, Boston, Dayton, Esslingen, Genoa, Grenoble, Lausanne, Linz, Lyon, Naples, St. Etienne, Salzberg, San Francisco, Sao Paulo, Seattle and Solingen, amongst other places.
Fig 2 One of Athens new Neoplan articulated trolleybuses
Interest has developed in places like Hong Kong, where there was no previous experience of trolleybuses. The Stagecoach subsidiary Citybus in Hong Kong has recently converted a Dennis double deck diesel bus to a trolleybus and set up as test circuit.
Current worldwide situation
From a handful of installations in the early 1900s, the number of trolleybus systems in the world open grew to a maximum of about 400 in the mid 1950s, dropped back to below 300 in the early 1970s and has since grown again to over 350. The world distribution [Murray, Alan] in 2000 is shown in the table below:
The West European distribution is shown below-
The country with the largest number of trolleybuses is Russia with 89 systems and 14110 vehicles. The world's largest system is Moscow with 2032 vehicles. The largest West European system is Athens with 315 vehicles.
[Data from Alan Murray's World Trolleybus Encyclopaedia 2000 ISBN 0 904245 18 1]
Modern Trolleybus technology
Established trolleybus systems generally operate at about 600V DC, but new systems at 750V DC. The 25% higher voltage permits lighter, neater and cheaper overhead. 'Elastic' overhead and modern designs of 'switches' or 'points' or 'frogs' permit trolleybuses to operate without having to slow down to avoid dewirement.
Modern low mass high stiffness trolley booms with modern overhead reduce the possibility of dewirements to rare events. Automatic lowering in the event of dewirement reduces the chance of a damaging dewirement even further.
The latest trolleybus designs feature individual motors at each driven wheel. These permit much better accessible low floor layouts than can be achieved with diesel bus mechanical transmission systems. An example of an Irisbus Cristalis 18m trolleybus interior with a wide low flat floor possible with individual wheel drive is shown in Fig 5. The drive configuration of an Irisbus Civis, which shares the same hub motor drive arrangement, is shown in Fig 10.
Fig 5 Interior of Irisbus Cristalis 18m trolleybus showing wide low flat floor possible with electric drive. The Irisbus Civis is similar.
Fig 10 Civis Hub mounted motors on the two rear axles, turning special 'super-single' tyres, provide a low platform height of 320mm
Modern trolleybuses typically feature auxiliary power units [APUs] to permit operation away from overhead. In conjunction with automatic lowering of trolley booms, a modern trolleybus can divert from route without having to stop. Automatic rewiring can be achieved with 'pans' fitted to the overhead at appropriate locations.
The most popular choice of APU is a 50kW to 75kW diesel alternator unit, permitting operation at up to 15 to 20 MPH. Alternative battery or flywheel APUs have the advantage of zero emissions but performance and / or range are more limited.
Heating and / or air conditioning require substantial amounts of power, which can be more readily found from the overhead on electric vehicles rather than from auxiliaries on diesel engines.
Modern trolleybuses have around double the power to weight ratio of similar diesel designs, permitting comfortable acceleration rates [around 3 mph/second or 1.3 m/s/s] to be maintained up to around double the speeds diesels can. An Athens Van Hool - Alstom A300T trolleybus can maintain about 3 mph/second [1.3m/s/s] up to about 16mph [26 kph] and fully laden can cover 0.37miles [600 m] from a standing start and stop again in 50 seconds - an average speed 27 mph [43 kph] without exceeding 40 mph [65 kph] or accelerating or braking at more than 3 mph/second [1.3 m/s/s]. To attempt to match that performance, a similar 12m diesel bus would need an engine of at least 300 kW. Trolleybus hill climbing performance is also very much better than diesels. If not constrained by congestion, a route can be operated faster and with fewer vehicles by trolleybuses than by diesels.
Modern trolleybus electrical equipment is based on AC motor technology. [See my article in 'Trolleybus Magazine' July - August 1999]. As well as being very reliable and long lived, it is almost maintenance free compared with diesel engines and transmissions. Most service braking can be achieved electrically meaning that foundation brake maintenance is very much less than with conventional diesel designs. Regenerative braking in modern trolleybuses - putting the energy that would otherwise be dissipated as heat back into the contact line - can give energy savings of the order of 30%. The better availability and reliability and lower maintenance and energy requirements of modern trolleybuses and higher performance, mean smaller operational fleets than diesel designs and significantly lower maintenance and energy usage costs.
The benefits of electrical individual wheel drives are finding their way into diesel designs. Products such as the Irisbus Civis [Fig 6] are also available in diesel electric versions. A diesel electric version of Civis has been proposed for the Cambridge 'superCam' guided busway project.
Fig 6 Irisbus Civis trolleybus
In reality, these are trolleybuses with large APUs and possibly without current collection equipment. With current collection equipment, they can operate in either electric or diesel modes. An example of such a vehicle is the Lausanne Neoplan, [Fig 7], which incorporates a 330kW diesel generator.
Fig 7 Lausanne Neoplan diesel electric hybrid trolleybus
Earlier [1980s] hybrid trolley / diesel designs attempted to integrate electrical driveline components in with a retained mechanical transmission. Perhaps not surprisingly, these designs proved to leave much to be desired. The new generation of all electric driveline diesel / trolley hybrids avoid the former design compromises. They open up the possibility of electric networks with infrastructure on only those parts of a network where the volume of traffic and / or environmental considerations dictate the most benefit. Such networks can be developed incrementally, from diesel only to whatever level of electric operation is deemed appropriate.
There has been considerable interest, particularly in France, in developing trolleybus technology into 'trams on tyres'. New rubber tyred [partly] guided electric or diesel electric vehicles are going into service in for example Nancy [Fig 4] and Caen and being trialled in Paris.
Fig 4 (a) Bombardier single rail guided trolley vehicle(s) in Nancy
Trolleybuses and the environment - the twin problems of air pollution and sustainability
Air quality is a problem in most urban areas. In 1999 more people died from air pollution in London than from road traffic accidents.
In major urban centres, the most significant emission source is internal combustion engine [IC] vehicles. Two pollutants - nitrogen oxides [NOx] and fine particles [particulates]- are the greatest cause for concern. Over half of emissions of NOx and over two thirds of particulates come from IC engines, particularly diesels.
In the longer term and being dependent [in the main] on fossil fuel, diesel or any other combustion engine vehicles can never be a significant part of a sustainable low carbon road transport system.
Environmentally superior alternatives to conventional diesel traction are a necessity. At the same time however, any such alternatives have to compete with the practicability and the proven cost effectiveness of diesel traction. The trolleybus has the capability to do this.
Trolleybuses provide true zero street emissions. This is guaranteed under all circumstances, including idling, cold running, transient conditions, sub optimal maintenance, etc., throughout the life of the vehicle. This is impossible with diesel or other combustion engines.
Electric vehicles indirectly introduce pollutants into the environment as a whole. The nature and level of these pollutants depend on the electric power source. However modern power stations produce far lower levels of pollutants than vehicle engines and the pollutants from power stations are not pumped directly into the air to be breathed on the streets.
As well as NOx and particulates, Greenhouse gases [GHGs] are a serious concern. The main GHG is Carbon Dioxide [CO2]. Burning fossil fuels in modern power stations is more efficient than in vehicle engines and less CO2 results.
A typical modern combined cycle thermal power station has a conversion efficiency of up to 60%. The best that the most efficient diesel bus engine can achieve is about 40% [other combustion engines are much less efficient]. But the bus engine operates under varying load reducing its average efficiency below 30%. In contrast, the power station operates under constant efficiency conditions. After allowing for transmission line losses [9% in the UK Grid] and factoring in gains from regeneration, a trolleybus is up to twice as energy efficient as a combustion engine bus. The trolleybus may only be responsible for half the GHGs therefore.
The authors of the report 'New Concepts for Trolley Buses in Sweden' describe a recent environmental study on the trolleybuses in Arnhem. This compared the emissions from 18m articulated trolleybuses with equivalent Euro 3 diesels, based on the Netherlands power generation mix, which is about 45% coal, 45% natural gas and with only about 10% from non fossil fuel sources.
The authors extrapolated this data to Swedish power production mix, which is about 90% hydroelectric and other non-fossil sources. Comparative emissions for trolleybuses with both power mixes against diesels are shown in the table below:
As can be seen trolleybus emissions into the environment as a whole are generally minute compared with even the best diesel technology. Euro 4 or Euro n IC engines or whatever will not change this.
The bulk of renewable energy resources such as wind, wave, solar, and waterpower, burning biomass, etc., generate electricity. Therefore, electricity just has to be the green flexible environmentally friendly sustainable 'fuel' of choice for urban transport systems for the future. Trolleybuses could have a major role in a greener and sustainable future.
Economics of trolleybuses
The economics of trolleybuses, compared with diesels, depend on balancing the greater capital cost of trolleybuses and the infrastructure, against savings in maintenance costs, energy usage costs, reductions in fleet sizes due to better performance and better reliability and availability of electric vehicles, and longer life of electric vehicles and their major components. These savings are most apparent on demanding heavily trafficked urban trunk routes The economics also depend on balancing benefits such as increased ridership and revenue, plus whatever [financial] importance is attached to health costs and environmental issues.
Every situation is different and needs to be evaluated individually. A situation such as operating in a subway may be very clear-cut - savings in ventilation equipment may easily repay the costs of electric operation. A situation may have a significant but subtle aspect. For example, it has been reported that round the clock operation of trolleybuses in Hong Kong, with quiet night time operation and needing fewer visits to depot than diesel buses, would mean significant savings in depot size and costs.
The main capital cost difference between a trolleybus and an otherwise similar diesel is due to the replacement of a high volume production diesel engine and transmission by a low volume production electric traction package.
Information published [article by the author 'Trolleybus Magazine' July - August 1999] indicates that the extra cost of a trolleybus over a diesel of the same basic design, is around ?100k, if purchased in a batch of about 100 from a West European manufacturer.
Many orders for trolleybuses are for considerably less than 100 vehicles. It can benefit operators to join together to place orders, as Arhem and Solingen have recently done. Arhem, work on the assumption that a trolleybus will cost them about double the cost of a similar diesel, in a 50 vehicle fleet.
A further complication is what kind of 'diesel' the comparison is made against. The above figures are based on trolleybus designs derived from diesel designs with mechanical transmissions and sharing similar final drive arrangements A trolleybus using the latest individual wheel motor drive technology such as a Cristalis cannot strictly be compared with a conventional diesel with a mechanical transmission. The trolleybus has a very much better accessible low floor layout, which can only be achieved in a diesel with an electric transmission. In such comparisons, the trolleybus is cheaper than the comparable diesel.
Figures which have been given to the Electric Tbus Group, which are indicative only, for diesel electric / trolleybuses with individual wheel drive, are ?400k for a 12m and ?600k for an 18m vehicle.
As with vehicle costs, it is difficult to be pedantic about infrastructure costs. The most expensive single item is the cost of manufacturing and planting the support poles. Once planted and given suitable corrosion protection, although depreciated in accounting terms over perhaps 20 years, these can well have a life of over 50 years or more. Curved sections of routes are more expensive than straight sections as they need more poles. 'Special work' - junctions and crossings - adds to the cost of overhead.
Figures obtained by the Tbus Group in connection with the proposed East London Transit [ELT] scheme indicate current infrastructure costs [including sub stations] of around ?400k per kilometre in the UK.
A problem with trolleybus costs is that they vary significantly between contexts. The costs mentioned above have been quoted in connection with proposed UK projects sourced from European suppliers. Much lower quoted prices can be obtained from East European or Russian suppliers, for example, although the comparisons may not be like for like. And unlike a diesel bus network, the unit costs of setting up and operating a trolleybus network decrease significantly for larger networks. If contemplating trolleybuses, it can pay to think big.
Comparing diesel and trolleybus economics is not straightforward, as one is comparing a low capital cost - higher operating cost scenario against a high capital cost - lower operating cost plus other benefits scenario. One needs to look into the future and make assumptions about parameters such as the lives of vehicles and infrastructure, relative diesel and electricity costs, likely increased ridership and revenue from trolleybus operation and the value placed on environmental benefits and so on.
Regarding ridership, the evidence is that trolleybuses attract higher riderships than diesels. For example, in Salzburg, ridership increases of 16% have justified further conversion of diesel routes to trolleybus. In Arnhem during the period 1999-2000 trolleybuses increased ridership 17% on routes converted from diesel and maintained market share while the trend in other cities in the Netherlands was a declining market share for public transport. The Arnhem operator Connexion have carried out market research that indicates by implementing their "Trolley 2000" strategy, they will get 21% more over five years compared with substituting the best type of diesel buses.
North American experience indicates that trolleybuses can attract 10% to 20% more rider ship than diesels as the chart below indicates.
Ridership increases, Seattle and San Francisco, Booz, Allen & Hamilton, Trolleybus Study for RTC and LACTC, 1992, Wil Teunissen, 2004, Salzburg AG, 2004
The public consultation exercise in connection with the Transport for London [TfL] proposed ELT system, a 53 km bus based transit scheme in East London, showed a 2:1 public preference for trolleybuses over diesels ['Tramways and Urban Transit' magazine February 2002]. TfL expect that trolleybuses will generate 24% higher revenue than diesels. ['East London Transit - Summary Report' published by TfL July 2001].
The only practicable way to compare alternative trolleybus and diesel proposals is by whole life benefit costing methods. In 1999 Vancouver reported in conjunction with proposals to renew and expand their trolleybus fleet that the expected costs per vehicle over a 20 year period were in C$ millions 1.7 for diesel, 1.9 for trolley and 2.9 for compressed natural gas [CNG]. These figures are costs and exclude benefits such as increased trolleybus ridership or environmental benefits. [Figures from article by Millar, Brown and the author in July 2000 'Buses' magazine].
Looking at cost - benefits, TfL expects trolleybuses to show a 14% greater cost benefit for ELT over diesels. This figure does not attempt to quantify environmental benefits of trolleybuses such as health costs. The Swedish report mentioned earlier estimated the 'social cost' of emissions of a diesel bus over trolleybus at about ?6K / year / vehicle.
London Buses, in "Cleaner Air for London - London Buses leads the way", estimate the pollution from their diesel buses to a mean figure, which equates to a cost of 13 pence per kilometre as the cost of health care caused by the pollution. Trolleybuses tend to be employed on intensive services and can be expected to run 60,000 km / year. Based on London Buses health cost, the benefit of using a trolleybus instead of a diesel would be ?8K / year per vehicle. As more is learned about the effects of IC engine exhausts on health, estimates of the financial costs are bound to rise.
As noted earlier, individual situations need to be assessed individually, but it is probably a truism for a network, that trolleybus lifetime costs will be of the same order as diesel. The environmental benefits therefore effectively come free. One can contrast this with, for example, experience reported by major North American operators of CNG where CNG has been found to be considerably more expensive than diesel. Technologies such as battery / combustion engine hybrid and fuel cell buses have yet to prove themselves, but the likely hood is that they will struggle to prove cost effective compared with diesel or trolleybuses. Both hybrid and fuel cell buses feature electric drives and will always tend to be more expensive than trolleybuses. Hybrid and fuel cell buses tend to have problems with unladen weight, limiting carrying capacity. Battery replacement costs may well be a substantial item in the long-term costs of hybrids [and fuel cells]. One fleet of 50 hybrid buses needs battery replacement every couple of years, according to an industry source. North American experience has shown that overall energy efficiency of fuel cells is so low that around a dozen equivalent electric trolleybuses can be operated for the primary energy consumption of one fuel cell bus ['Transport 2000 Canada Western Newsletter' November 2000].
Where to deploy trolleybuses?
While there will always continue to be 'niche' applications, such as subway running that may dictate the use of trolleybuses, the circumstances that are generally most likely to economically and operationally favour trolleybuses are intensive urban trunk route networks. And the larger the sale of deployment, the more favourable the economics of trolleybuses become.
Around the world there are increasing numbers of very high quality bus routes and networks being implemented or proposed. These are characterised by considerable investment in infrastructure such as bus lanes, bus ways, bus guide ways and other priority measures for buses, high quality raised platform stops for level entry, accessible low floor vehicles, real time information systems, etc. Such investment can easily amount to ?1 million/km and up to as much as ?5 million/km in the case of the proposed ELT [Fig 9].
Fig 9 - Artist's impression of proposed East London Transit scheme
Particular consideration is worth giving to trolleybuses for such schemes. High quality diesel electric buses, which have similar unit capital costs to trolleybuses, may well be the preferred diesel choice for such schemes. The additional costs of trolleybus infrastructure will be a small part of the investment and can pay for itself in energy usage costs, vehicle maintenance and other savings. Ridership with trolleybuses will be higher. The trolleybus infrastructure, which once was seen as 'inflexible' is now seen as 'commitment'. Potential riders will factor the existence of a stable electric high quality public transport system into their decisions about where to work, where to live, whether they need an extra car in the family, etc., generating long term ridership growth.
If one is looking for an environmentally responsible and cost effective alternative to the diesel for operating intensive urban bus networks, then the trolleybus is the only general current contender and is likely to remain so. Apart from anything else, alternatives such as CNG, hybrid combustion engine/battery or fuel cell vehicles are not likely to be cost competitive. And trolleybuses will bring benefits such as higher ridership. Employed in high quality bus schemes, trolleybuses have the potential to bring the ambience, functionality and benefits of modern light rail, but at lower capital cost and to a wider travelling public.
Eur Ing Irvine Bell BSc CEng MIMechE CDipAF PGCE