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B Traffic Turbines

  1. Jun 25, 2018 #1
    I saw a video on FaceBook today that showed traffic turbines. They supposedly generate "free" energy from the traffic that passes by them causing them to spin. Just because I'm curious, I was wondering if this energy isn't really free, but is in exchange of a minuscule drag put on the passing vehicles thereby transferring the energy cost (as small as it might be) to the drivers that pass by.
     
  2. jcsd
  3. Jun 25, 2018 #2

    berkeman

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    Thread closed briefly for evaluation...
     
  4. Jun 26, 2018 #3

    berkeman

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    After a Mentor discussion and a PM discussion with the OP, the thread is re-opened. The key here is energy recovery without placing an extra energy burden on the passing vehicles. It appears that this may be possible with some of the technologies that the OP @InquiringMind has been reading about. :smile:
     
  5. Jun 26, 2018 #4

    anorlunda

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    slask.jpg

    You can surf on the wake of a boat. So can you surf on the wake of a truck, or otherwise extract useful energy from it? The answer depends on scale. On a micro scale, this is ridiculous. On a mega scale it is simply weather and wind power.

    You can model a vehicle on the highway as H<<L (High pressure at the front, Low at the back, < shows direction of motion). Some vehicles are short H<L, some long H<<<L. So a highway with 4 vehicles might look like this.

    H<<L.......................................H<L................H<<L..............................H<<<L................


    If we put a pressure sensor on the side of the road, it would record a transient like this for each passing vehicle, with the time between high and low peaks roughly one half second.
    slask.jpg

    Can we extract useful energy from those H and L pressure fluctuations?
    • Micro scale: Could we extract useful energy from the random fluctuations in pressure of the air in a room? Clearly no. That is a the kind of question we don't allow here on PF. If you try really hard to analyze why it won't work, you will eventually come to Maxwell's Demon.
    • Mega scale: Think of those H and L symbols on the weather map of your continent. We can and do extract energy from those fluctuations; and we call it wind power. It does not change things that a highway is one dimensional while the surface of the Earth is two dimensional.
    • Intermediate scales: So where do we draw the line between mega/micro practical/crazy? If we were talking about hot/cold temperature fluctuations, then we have firm theoretical foundation, The Carnot Engine.
    I don't know of an equivalent to Carnot for pressure fluctuations, so I can't define where to draw the boundary. Nevertheless a boundary exists. Maybe some other PF member can provide an equivalent to Carnot's Theorem for this case.

    So where does a traffic turbine fall on the macro/micro scale. Imagine the blades of the turbine accelerating, then reversing direction in a half second. What do you think?

    [Note: I originally rejected this thread because I was thinking of the micro, which made this idea ridiculous. But that was hasty, and the better answer is that "it depends on scale."]
     
  6. Jun 26, 2018 #5

    sophiecentaur

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    It would help if you could give us the link to that video.
     
  7. Jun 26, 2018 #6

    berkeman

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    It looks like the OP is not online yet today, so here are his links from my PM conversation with him:
     
  8. Jun 26, 2018 #7

    sophiecentaur

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    I am always skeptical about energy harvesting as a concept because any proposals I see are very light on actual figures for recovered energy. the videos of turbines turning as buses go past them say nothing about the actual energy obtained by them. When we see leaves being swept along by a passing car, what actual energy was involved? Clearly it is finite but would it be worth while trying to 'harvest' it, compared with putting a tubing a bit higher up in the air and getting the wind to turn it? I do know that small scale wind turbines are pretty much a waste of time unless there is really no alternative supply for your remote piece of electronics. I had a small wind turbine which (mostly) topped up my boat start battery and nothing more.

    As usual, if you are hoping to obtain significant levels of power, there will be significant interaction with the vehicle which will increase the required power for the vehicle. What people are after is a way to use power that is lost anyway but vehicles are designed to minimise the lost energy. Wasted power will tend to be due to turbulence which is in an incoherent form and difficult to utilise.

    One identifiable form of 'coherent' energy that is wasted by transport is the massive wake that is set up by large passing ships. That always strikes me as a possible source of energy, waiting to be harvested. However. ships pass quite infrequently (only a few per hour in even the busiest shipping channels) and it would require a very wide collector to accumulate enough energy to be significant. The natural waves hitting the shore would actually constitute a more worthwhile source but wave power has only been shown to be viable in a few locations where the waves are actually pretty high most of the time. I think a similar objection also applies to harvesting 'truly' wasted energy from traffic.
     
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