Web tool helps devise value for money electric bus lines

Switching from conventional fossil-fuel-powered buses to electric ones can be complex and costly. Not only do cities have to consider upfront costs of installation and recharging installations, as well as basic operation and electricity costs, they need to look at reliability and longevity, too.

Electric buses usually come with strings attached – literally. A dense web of overhead power lines has to be strung above the routes they service.
That grid of powerlines is expensive and unsightly – two factors that can be off-putting to municipalities considering installing electric bus systems. But advances in battery and charging technology may soon make them unnecessary.
Still, switching from conventional fossil-fuel-powered buses to electric ones can be surprisingly complex and costly. Not only do the authorities have to consider upfront costs of installation and recharging installations, as well as basic operation and electricity costs, they need to look at reliability and longevity, too.
Working in collaboration with Geneva-based ABB-Secheron and other local partners, researchers at EPFL in Lausanne have developed a decision-making tool that helps city transport authorities to evaluate the costs and benefits of implementing ABB's new cable-free electric trolleybus lines in their bus network.
Switching from diesel to batteries requires a complete rethink of the design of the bus line. A conventional city bus can easily run its route several times on a single tank of petrol. To do the same, an electric bus would need a battery weighing thousands of kilograms – leaving hardly any room for the passengers.
Companies across the globe are developing different solutions to provide sufficient power capacity for electric buses without wasting valuable passenger space.
ABB this year successfully completed a pilot trial for its electric bus. Instead of recharging massive batteries, it takes small "sips" of charge at regular stops along the way, in the time it takes passengers to get on and off. With small regular sips, a 133-passenger bus managed to run solely on electricity, using a battery just twice the size of those in standard electric cars.
The ABB electric buses rely on three different recharging technologies, each adapted to a specific context. At the garage, buses can be recharged slowly overnight. At either end of their service line they can be recharged in a matter of minutes. And at specific stops, buses can use flash charging to top up the batteries in seconds by extending a laser-guided arm from the bus's roof into a socket installed at the bus stop.
There are some trade-offs to be considered. Flash charging requires new infrastructure at bus stops, and it also reduces the lifespan of batteries. But it means those batteries can be much smaller, shedding weight and freeing space for passengers.
There are external factors, too, such as traffic, construction sites, or even the weather. Batteries are more efficient at cooler temperatures than under the blazing Middle East sun. But the UAE's abundant sunlight could be used to cheaply power the charging stations at bus stops.
Setting up an electric bus line, then, is a complex optimisation problem with no simple solution. Lines have to be individually tailored to their city, taking into account these and a number of other constraints imposed by the city planners.
In order to design an optimised electric bus line and compare its cost against existing transit systems, we developed a web app that lets city planners set up a virtual electric bus line. It takes into account more than 50 parameters. These include the price and lifespan of the batteries, the drivers' salaries, the time it takes passengers to get on and off the bus, typical congestion levels in specific parts of the city, as well as the local cost of electricity.
An optimisation algorithm finds the most cost-effective solution that uses an existing technology while satisfying the operational requirements of the city planners.
Geneva is planning to launch its first cable-free electric bus line by 2017.
In the meantime, other cities around the world have expressed their interest in the technology, seduced by affordable and sustainable public transport with no strings attached.
Professor Michel Bierlaire is the director of the transport and mobility laboratory at EPFL in Lausanne, Switzerland