These seem to crop up every now and again, with a press-release or two about how some company is interested in using them, and a website which subsequently vanishes.
However, today I saw
http://news.bbc.co.uk/1/hi/uk/8406331.stm
and you'd assume (or at least hope) the idea would have been checked out before getting that kind of positive media attention.
Generally speaking, I'm fairly skeptical about the overall idea, since unless someone is actually trying to slow down, a speed-absorbing speed bump is effectively consuming energy that ends up having to be put back into the vehicle by the engine, so environmentally they're pretty pointless in almost all places.
Also, the whole idea seems to have all kinds of hurdles to cross.
Intermittent small movements of large objects aren't ideal for generating electricity, and there must be a limit to how much energy can be absorbed from a vehicle by a bump of a given height.
I'd have thought that for a single vertically moving device, an upper bound on possible energy extracted would come from the vertical travel of a bump and the weight of the vehicle, and that's ignoring any losses from suspension damping, equipment efficiency, the need for different resistance for different axle-weight vehicles, etc.
For a travel of 10cm, (apparently the maximum legal height of a speed bump) the maximum energy extracted from a vehicle would seem to be ~1000J/ton, which for a 2-ton car is 2000J, or about half a Watt-hour.
A street light that was a current LED replacement for a regular sodium light would be somewhere in the 60-80W range, and so would be powered for <30 seconds, rather than 9 hours.
0.5 Watt-hours is 0.0005 electricity units, or ~0.0025p at 5p/unit
20000 cars/day (1 car every 7 seconds) would create 25p-worth of electricity per day, or £90/year.
And don't forget that's assuming zero losses.
For comparison, a busy urban road maxes out at about 1000-1500 vehicles/hour/lane, and any streets with traffic calming would be expected to have much lower traffic rates.
To get more energy out would seem to suggest either multiple devices, (multiplying the cost) or greater travel, and there must be a limit of travel beyond which a device would be too 'rough'.
Are there ways of mechanically extracting energy from a vehicle via a downward-deflected device which extract more energy than would be calculated from 'vertical-wheel-travel-distance * axle weight'?
... you'd assume (or at least hope) the idea would have been checked out before getting that kind of positive media attention.
Yes folks, he'll be at the comedy club tonight, 8pm. Admission is five bucks with a two drink minimum...
Interesting post. It's even an interesting idea, except for the fact that it does not appear that it will work very well, if at all.
The idea is supposed to have won a competition.
It would be odd if the ability for it to work in practice wasn't one of the first hurdles it had to cross in order to be even considered.
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