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Efficiency - the trolleybus versus diesel and hybrid diesel

The thermal efficiency (electricity out/fuel in) of modern combined cycle thermal power stations is essentially constant at around 60% (i.e. 40% gets lost to heat, not the 70% mentioned elsewhere), very large (marine) diesels do almost as well at around 55%, automotive diesels (bus engines) do about 40% at best but less at part load. In a diesel bus operating an urban duty cycle and unlike a trolleybus there are significant losses in fluid element automatic transmissions, while idling and moving at low speeds and operating at well below engine maximum efficiency levels, while burning fuel to regenerate particulate traps, etc. The efficiency of a diesel bus at the road wheels is a lot less than engine peak efficiency might suggest.

 

If one considers the relative efficiencies at the road wheels and starts from the basis that the net energy required to propel a diesel or trolley bus is about the same (around 2 kWh/km for an 18 tonne gross vehicle operating an urban duty cycle) and then follow the chain back to the 'fuel filler' - literally in the case of the diesel and the gas (Methane) pipe entry into a modern combined cycle power station, the numbers come out something like this.

Including all the grid, substation, overhead wiring, on-vehicle etc, losses, and regeneration gains for a trolley and all the transmission, cooling, idling and part load running losses, etc, for diesel, a diesel bus is somewhere around 25% efficient at the wheels based on typical MPG and a trolley around 40%.

 

A diesel-electric hybrid bus should do better than a straight diesel - fuel savings of 10% to 20% would be realistic in practice. Figures of 40% improvements in MPG (29% fuel savings) may be achieved on a test cycle against which a design has been optimised, but real service is different. If one took a mean figure of 15% savings, a diesel electric hybrid is about 30% efficient on the same basis as above.

If one allows that the demands on the environment to get gas into a power station are probably (much) less than those to get derv into a bus fuel tank, the above comparison is probably over kind to diesel. As gas contains a lot less carbon and a lot more hydrogen than derv, the CO2 emissions of the trolley are much better than the above efficiencies would suggest - around twice as good.

 

If one is going to propel an urban bus by burning a hydrocarbon fuel - it really does make more environmental sense to burn that fuel in a fixed power plant than in mobile one. If one looks to the future with an increasingly decarbonised grid, it doubly makes sense to power urban public transport vehicles, of whatever kind, electrically. Realistically the only way to decarbonise public transport is to electrify it.

 

In addition to the above, electric vehicles emit no noxious pollutants at all into the air people breathe on city streets. Absolutely guaranteed whatever the age or state of maintenance of a vehicle or how it is driven or anything else. It will never be possible to say that of a vehicle that burns a hydrocarbon fuel.

European (e.g. Salzburg) and other operators of trolleybuses find that savings in items like maintenance and energy costs, the better availability and reliability of electric vehicles and the greater attractiveness to passengers leading to sustained increases in numbers of passengers carried, mean that the economics of trolleybuses compare favourably with those of conventional diesels on urban networks.


email our technical / environmental / engineering query service -
Eur Ing Irvine Bell BSc CEng MIMechE CDipAF PGCE
 

'The UK Government believes that climate change is one of the gravest threats we face, and that urgent action at home and abroad is required. We need to use a wide range of levers to decarbonise the economy.' a quote from the
National Renewable Energy Action Plan
for the United Kingdom

The UK is committed to producing 30% of electricity,
12% of heat and 10% of transport energy,
a total of 15% of all energy provision, from non-fossil sources by 2020.

Trolleybus systems around the world, able to use renewable sources of energy such as hydro electricity, for instance in Vancouver, Zurich and Saltzburg, achieve efficiencies much greater those dependant on thermal generation. Water turbines can be 90% efficient for example. Renewably powered trolleybus systems are not only 100% free of on-street emissions but contribute no greenhouse gases whatsoever throughout a renewably powered system, economically and with well proven reliability.

the Trolleybus industry worldwide


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updated 9/10/12

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