[Robert A Moeser]

Robert A Moeser  asked the Naked Scientists:
   
Hi Scientists!

A young European man is being celebrated recently for devising a method for taking the energy of a car going over a speed-bump object and producing electricity for lighting the road.

Besides the fact that this is being worked on by many and so is nothing new new, I am troubled by the total omission of any discussion of the laws of thermodynamics, or as we put it "TANSTAAFL": there ain't no such thing as a free lunch.

Far from the green tech that this is promoted as ("saving energy"), it seems quite a bad idea. To the extent that any significant energy for lighting is "recovered", it is coming from the gasoline burned in the car, a relatively inefficient source. And it would be more accurately billed as "stolen"!

The cost of producing and installing these generator speed bumps (compared to regular lights) further reduces the total efficiency.

Only in places where mains current is unavailable would this be a good choice.

I would like to hear your comments and analysis.

Better invention: street lights that know whether anyone is around to benefit from their output!

Rob

What do you think?

[RD]

Don't forget to include the financial and ecological cost of the battery.

[Don_1]

Let's not forget that speed bumps also have a cost in wear & tear on vehicles regardless of their speed and cause unnecessary fuel consumption. They are a pain in the arse, quite literally sometimes.

[LeeE]

This isn't really a something-for-nothing solution as the car has to use the energy to get over the speed bump anyway.  Getting energy from a speed bump then, is just recovering some of the energy that would otherwise just be lost.

There are also a few different ways in which the efficiency of this type of scheme can be assessed.

In absolute terms, this type of scheme would not be very efficient in terms of recovering all of the energy put into the speed bump by the car, but in terms of recovering some of the energy then it is very efficient, as it represents a vast improvement upon zero recovery.  However, probably the most relevent way to assess it is in terms of economy; does the value of the energy recovered outweigh the cost of recovery over the lifetime of the system?

(Remember that if the car has got to go over a speed bump anyway, then the cost to the car will be unchanged and so isn't a factor)

[Karsten]

I guess one should then design a speed bump that requires just as much energy to pass over than current non-energy-recovery bumps. Might not be a bump. Could be something else. As long as it slows down the car and gets some energy out of it. On the other hand, maybe slowing down the cars without creating more energy use for the car should be the ultimate goal. Why force a car/driver to slow down while wasting some energy and then try to recover this energy if the could slow down without all that "energy back and forth"? Such as a small car that does not want to go fast. Or a car that knows where is and cannot go faster in certain areas. But back to the speed bumps...

 I do not know how those latest speed bumps designs look like but I was under the impression that a car would use slightly more energy to pass over the speed bumps that are designed to "create" energy from being driven over. Is that true?

[techmind]

Ultimately the energy is coming from the car engines - and yes, "energy recovery" speedbumps would by definition require slightly more energy from the engine than ordinary speedbumps. You can bet that a purpose-designed power station is a far more efficient electricity generator than a car engine combined with some kludge of a speedbump.

As others have pointed out, all that speeding up and slowing down for speedbumps, dumping loads of good kinetic energy as low-grade heat in the brakes is horribly wasteful anyway. "Speedbumps" and "energy efficiency" do not belong in the same sentence!

For the cost of such a bump, its raw materials, the maintainance of it, and the relatively small amount of energy you'd extract, I really can't see such a thing being cost-effective - which is also a further sign that it probably isn't environmentally effective either.


I don't want to discourage the youngster, but it's unfortunate when people are encouraged in misguided efforts.

[alto]

I thought about it a bit more after posing my question. You all make good points.

Speedbumps are not installed to slow cars down, they are there to make sure cars go over them slowly! So after the third time the damn speedbump jars the dentures loose one starts, er, behaving.

To the extent that the speedbump does its real job, the energy that can be recovered will be quite low.

I like the idea of other mechanisms to slow a car down, at least in theory. But I do not think we want to illuminate the McDonald's parking lot because we hurtle in at top velocity and get our kinetic energy "recovered" by some big magnetic coil or such like!

[Karsten]

All this science talk aside,...

... engineering those speed bumps, manufacturing them, selling them, distributing them, installing them, maintaining them, making replacements parts for them, fixing the cars that go over them, etc. is all great for our economy and could make us feel real good about ourselves and our "green" efforts. Especially if we make them from bamboo.  []

[LeeE]

...but the main purpose of speed-bumps is to get motorists to slow down, in which case some money has to be spent anyway; it's just the additional cost of the energy recovery components that need to be factored against the the value of the energy recovered, and not the overall cost of the complete installation.

[Erik Moeser]

Here is my bottom line.  The energy recovery, inefficient at best, is not the answer.  Don't spend it in the first place.  A modern car could have its computer receive a signal in certain neighborhoods limiting the top speed in that area.  Wait a minute, we already have that, a computer (human brain), a speed limit sign (visual signal), and ability of the computer (human brain) to do the correct thing.  The energy used to create the speed bumps and recovery systems could be put to direct use elsewhere.