Regenerative braking


in Geek stuff, Science, The environment

One distinct advantage of electric ground vehicles is that they can reduce their total energy consumption by converting forward kinetic energy back into stored electrical energy: a technique known as regenerative braking. This makes them draw less power per kilometre travelled (especially in stop-start city traffic) and increases the effective range of any particular battery pack. Some existing vehicles reduce their energy consumption by about 15% using this technology.

While flywheels could do something similar in vehicles powered by other means (such as biofuels), regenerative braking doesn’t require much extra hardware. Also, if electric vehicles end up using one motor per wheel, traction control capabilities could be easily incorporated.

A low-tech approach with similar properties is used in some subway systems. Stations are built at a higher level than tracks. As trains leave, they run downhill and gain speed as gravitational potential energy is converted into kinetic energy. As they approach the next station, they go uphill and make the conversion in reverse.

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{ 26 comments… read them below or add one }

Tristan September 26, 2009 at 1:37 am

I don’t think the low tech regenerative breaking technique you discuss is present in many subway stations. I’ve heard it discussed in the context of the Montreal subway – but as a way it was different from conventional subways (it uses rubber wheels – which enables steeper inclines and tighter corners).

. September 26, 2009 at 3:21 pm

“Braking energy can be stored as gravitational energy by driving the vehicle up a ramp whenever you want to slow down. This gravitational energy storage option is rather inflexible, since there must be a ramp in the right place. It’s an option that’s most useful for trains, and it is illustrated by the London Underground’s Victoria line, which has hump-back stations. Each station is at the top of a hill in the track. Arriving trains are automatically slowed down by the hill, and departing trains are accelerated as they go down the far side of the hill. The hump-back-station design provides an energy saving of 5% and makes the trains run 9% faster.”

. September 26, 2009 at 3:22 pm

“Regenerative systems using flywheels and hydraulics seem to work a little better than battery-based systems, salvaging at least 70% of the braking energy. Figure 20.17 describes a hybrid car with a petrol engine powering digitally-controlled hydraulics. On a standard driving cycle, this car uses 30% less fuel than the original petrol car. In urban driving, its energy consumption is halved, from 131 kWh per 100 km to 62 kWh per 100 km (20 mpg to 43 mpg). (Credit for this performance improvement must be shared between regenerative braking and the use of hybrid technology.) Hydraulics and flywheels are both promising ways to handle regenerative braking because small systems can handle large powers. A flywheel system weighing just 24 kg (figure 20.18), designed for energy storage in a racing car, can store 400 kJ (0.1 kWh) of energy – enough energy to accelerate an ordinary car up to 60 miles per hour (97 km/h); and it can accept or deliver 60 kW of power. Electric batteries capable of delivering that much power would weigh about 200 kg. So, unless you’re already carrying that much battery on board, an electrical regenerative-braking system should probably use capacitors to store braking energy. Super-capacitors have similar energy-storage and power-delivery parameters to the flywheel’s.”

Tristan September 26, 2009 at 7:09 pm

Are low-tech regenerative breaking techniques more efficient than the using the motors as breaks/generators?

Tristan September 26, 2009 at 7:10 pm

Sorry, I meant specifically in instances where there are no batteries – such as subways.

Milan September 26, 2009 at 8:01 pm

I would think so. The conversion of kinetic energy to gravitational potential and back is probably quite efficient.

Tristan September 27, 2009 at 2:55 am

I doubt its much better. Losses in electric motors are quite slight – totally unlike gasoline ones. (Gasoline motors have the added disadvantage of not seamlessly converting themselves into gasoline producing machines when the engine is under negative load).

Milan September 27, 2009 at 1:17 pm

Playing with Capsela shows that not all motors are terribly efficient generators.

You can use a battery to drive an electric motor, then use that motor to drive another one in reverse. If you connect the second motor to a light bulb, it is much less bright than it would be connected directly to the battery, demonstrating efficiency losses in the motor and ad hoc generator.

It would be good to see some efficiency figures for car-suitable electric motors being used as generators.

Milan September 27, 2009 at 1:19 pm

The quote above suggests that electrical regenerative braking systems are somewhat less than 70% efficient.

Where would the Victoria line hump-back system lose energy?

Tristan September 27, 2009 at 3:55 pm

The quote above refers to battery based systems. Subways don’t use batteries. The Skytrain’s motors don’t even have moving parts – it’s hard to see where large losses would be.

Tristan September 27, 2009 at 4:03 pm

Anyway, the most interesting thing here has to be using flywheels in production vehicles and non electric commuter trains. Breaking effectively wastes energy – so anything we can do to avoid breaking, is a good thing.

Milan September 27, 2009 at 4:29 pm

The Victoria line system would have some losses due to extra friction: there would be more between the wheels and the track when the train is going uphill.

Matt September 27, 2009 at 5:44 pm

The Skytrain’s motors don’t even have moving parts – it’s hard to see where large losses would be.

Completely true, but interestingly, the new Canada Line does not use the linear induction motors of Skytrain.

I don’t claim to know much about it, but I’ve read that linear induction motors are less efficient than conventional electric motors. The benefits,though, on the Skytrain lines are that they allow for steeper inclines because the wheels don’t propel the train, and that the fewer moving parts lessen maintenance.

I’ve also read that the trolley buses can regeneratively brake by feeding current back into the trolley wires, to be used elsewhere in the system (ie to drive other busses on the wire).

Finally, not related to trains, I’ve always wondered why a car company doesn’t make a diesel-electric hybrid. A Toyota Prius doesn’t get terribly better mileage than a diesel VW Jetta. If the two technologies were combined, though, that would probably yield a highly efficient car.

Tristan September 28, 2009 at 12:11 am

“The Victoria line system”

What is that?

“Completely true, but interestingly, the new Canada Line does not use the linear induction motors of Skytrain. ”

I know

Milan September 28, 2009 at 8:29 am

The system of hump-back stations used on the London Underground’s Victoria Line

Milan September 28, 2009 at 10:50 am

Apparently, the Crawler-transporters that carry the Space Shuttle are diesel-electric hybrids.

Tristan September 28, 2009 at 11:17 am

Sort of. “Hybrid” as refers to cars implies the presence of batteries. The Crawlers are hybrids in the same sense that all diesel locomotives are hybrids. In effect, the electric generators/motors are performing the role of a torque converter.

Milan September 28, 2009 at 12:31 pm

The second Wikipedia link – to the page on diesel hybrids – lists several concept automobiles.

Milan December 9, 2009 at 11:32 am

Regenerative braking for bicycles:

The Copenhagen Wheel

“A booth about the “Copenhagen wheel” shows a simple bicycle wheel, invented with MIT’s help, that can harness the energy generated when a bicycle brakes, and then release it when the bike needs extra power. (This in pursuit of Copenhagen’s aim to be the world’s first carbon-neutral capital, by 2025.)”

Tristan December 9, 2009 at 1:35 pm

I’m having trouble finding any information on this outside the linked sites – it doesn’t seem to be on wikipedia yet.

Are the batteries located in the hub?

Such technologies are very promising – especially since it can be added to any bike by replacing the front wheel (not exactly a difficult task). Continual improvements in battery technology should eventually allow electric bicycles to be as potent and as useful as limited speed motorcycles are today.

. December 9, 2009 at 1:55 pm

The regenerative battery and motor harvests energy when braking and releases it while cycling, a system similar to hybrid cars,” explained GreenWheel project leader Michael Lin, a graduate student at the Media Lab who also holds a Master of Science in Architecture and Urbanism.

All of the elements are housed in the rear wheel.

“I combined the battery into the hub, did the drawings, and sent out the plans for fabrication,” said Lin. “The motorized hub fits into any size and kind of bicycle.”

Other components include a wireless control module, reduction gearbox that reduces wheel speed and allows more torque, and an accelerometer, a chip that detects incline, decline, and acceleration.

. December 9, 2009 at 1:56 pm

“Finally, as part of the project, a prototype of a smart bicycle is being developed in collaboration with the MIT Media Lab’s Smart Cities Group at the MIT Media Lab, directed by William J. Mitchell, the Alexander W Dreyfoos (1954) Professor of Architecture and Media Arts and Sciences. This hybrid bicycle uses a regenerative motor to harvest the energy created when braking and release it while cycling, in a manner similar to hybrid cars. Everything, including the battery, is packed in the rear wheel, which becomes a self-contained element that could be retrofitted on most existing bicycles.”

Milan December 9, 2009 at 2:10 pm

That last linked page also has an exploded diagram of the wheel, over on the right side of the page.

. December 16, 2009 at 6:51 pm

MIT’s Copenhagen Wheel turns your bike into a hybrid, personal trainer

You really can’t fault MIT’s branding strategy here. Debuting at the biggest climate change conference since Kyoto, its Copenhagen Wheel is a mixture of established technologies with the ambition to make us all a little bit greener and a little bit more smartphone-dependent. On the one hand, it turns your bike into a hybrid — with energy being collected from regenerative braking and distributed when you need a boost — but on the other, it also allows you to track usage data with your iPhone, turning the trusty old bike into a nagging personal trainer. The Bluetooth connection can also be used for conveying real time traffic and air quality information, if you care about such things, and Copenhagen’s mayor has expressed her interest in promoting these as an alternative commuting method. Production is set to begin next year, but all that gear won’t come cheap, as prices for the single wheel are expected to match those of full-sized electric bikes. Video after the break.

Milan January 6, 2014 at 10:15 pm

Now available for pre-order:

The Copenhagen Wheel

Milan January 6, 2014 at 10:17 pm

See also:

Superpedestrian’s ‘Copenhagen Wheel’ Is Now For Sale

Born out of MIT, it helps give cyclists the boost they need to get up those tough-to-bike hills.

The wheel stores energy during a ride, and when cyclists need a boost, it helps give them an extra push to get where they are going. “The average non-professional rider puts out around 75 watts [when riding],” said Assaf Biderman, an MIT researcher and founder of Superpedestrian, the company that makes the wheel. “With the motor you get a continuous 250 watts extra. That’s 325 watts. That’s more than four times than your average person can put out. You are almost four times stronger.”

The wheel costs $700.

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