Flywheels in buses – not utopian
In the basement of Ångström Laboratory, hidden in a maze of corridors, lies one of the labs for electricity research – a workshop full of cords, cables, magnets, and batteries. Here major strides are being taken in the quest for renewable electricity – with flywheels as a power source.
A flywheel is a rotating disc linked to an energy source, like a battery and a power motor. The battery energy can be stored in the disc, which can function as a motor or a generator.
‘What’s good about a flywheel is its ability to serve as an output buffer’, says Janaina Goncalves, a researcher at the Department of Engineering Sciences.
‘The average output for the acceleration and braking of a car is normally just 2.2 kilowatts; but peak output is much higher – up to 40 kilowatts. The problem is that the battery can’t cope with these peaks. With a flywheel the technical efficiency is higher, plus the fact that you can invest in cheaper batteries, which nevertheless have longer service lives. This yields major savings.’
Two long desks are covered with her constructions, including a complete power train adapted to an electric bus or motor vehicle. Together with her colleague Johan Abrahamsson, she’s working on several prototypes. A computer screen shows the flow of the system’s outputs for analysis.
‘The goal is to make use of braking energy and store up to one kilowatt/hour. This can be achieved with a larger flywheel or a more rapidly rotating motor of the same size’, says Johan Abrahamsson.
He designs and crafts the prototypes, where the rotor is held in place by magnetic bearings, all enclosed in a vacuum chamber to minimise air friction losses. The weight of the whole flywheel system for a bus amounts to around 100 kilos. A composite shell holds the parts together and needs to be able to cope with heavy loads and high rotation speeds.
‘This composite has an extremely long service life, longer than the vehicle it’s used in. What’s more, it offers advantages in terms of both energy storage and safety, so it’s the most suitable solution for a bus in city traffic’, says Johan Abrahamsson.
‘The system improves the output of so-called “stop-and-go”, such as when a refuse lorry or a bus has to stop and start in an urban environment’, says Janaina Goncalves.
Considering all the interest in renewable energy, not least for the transportation systems of the future, sponsors should be beating a path to their door. But the two researchers say that the system needs to be fully scaled up and implemented before it can be commercially interesting. Johan Abrahamsson sums up the dream easily:
‘For Uppsala to have a city bus driving around with a flywheel – that would be super cool!’
Anneli Björkman