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Is in-stream, low-flow, micro-hydro production of great power a fantasy?

  1. Jan 5, 2013 #1
    I put my question in this forum because, apparently, the answer is yes?

    Things I don’t understand:

    There must be a law of physics which prevents a principle from being scaled up or down with simple equivalent offsets. It probably has something to do with the introduction of variables or whatever, I don’t know.

    But, as a liberal arts lay person, I don’t understand it. I don’t know if I could, or even would want to understand it; I just want to make sure the physicists and engineers understand it and know for sure that law is valid. I hope they are not just accepting that law because that’s what their text books told them in school.


    Take a wind turban of a given size and configuration designed to generate electricity from cubic feet of moving air thrown against it. Why can’t the principles involved in the size and configuration simply be scaled to permit the generation of electricity from cubic feet of much heavier water thrown against it in an in-stream, low-flow micro hydro situation?

    Conversely, when people have argued the benefits of high head water turbans relative to the in-stream, low-flow micro hydro situation they convince me. But why can’t those principles be scaled up to the wind turban application? I have heard the wind turbans must actually be shut down during high wind situations to keep them from flying apart or generating too much electricity. Isn’t a high wind situation something like a high head situation, where you have more, bigger, better, faster? If it’s such a boon in the water context, then why waste it when there is a high wind situation?

    When comparing the four conditions (1. low head hydro; 2, high head hydro; 3. low wind; and 4. high wind), can’t all the various measurements and engineering principles (blade tilt, configuration, materials, size, etc.) be scaled with offsets to reach an ideal size and configuration based on the weight of that which is thrown against it, and creating an equivalent generation of electricity? In other words, if “low head air” is capable of generating X, then why can’t a much smaller low head water contraption generate X? If the lightweight nature of air requires a giant turban, why can’t the heavyweight of water generate the same amount of electricity with a much small, scaled-down and reconfigured contraption?

    I sit by my river and watch countless tons of water go by every second of every minute of every hour of every day of every year for many years; and it’s been doing that for hundreds of thousands of years. I am not allowed to dam it, and I am not allowed to divert it through a penstock or a waterwheel. However, I am allowed to affix something to the boulders on the bottom where it won’t interfere with rafters or kayakers. If a wind turban is 100 feet across generating X electricity with air that weighs 1 pound, why can’t I have a water turban that is 1 foot across to generate X electricity with water that weighs 100 pounds?

    On a side note, but with similar curiosity, and using the scaling referenced above, if a given solar panel is optimally configured as it is, couldn’t it be reconfigured smaller to handle sun brought to it after passing through a simple magnifying glass? We all know what can be done with a child, a magnifying glass and the sun.

    I’m just me, but if I were Bill Gates and willing to pay billions, or if I was an evil tyrant holding loved one’s hostage, and I said “Get it done!” Would it be done? Or is there a law of physics that would tell me to go pee up a rope?
  2. jcsd
  3. Jan 5, 2013 #2


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    Staff: Mentor

    Welcome to the PF.

    Thank you for being conservative and posting in the Science Fiction forum initially. I'm going to move this to General Physics for now, and the Physics Mentors are welcome to move it to the Engineering forums if they would prefer that.

    I too have thought about this energy generation mechanism. The energy available from all of our creeks and rivers is substantial. But as I thought about it more, the main problem that I see is the environmental impact on fish in the waterways. Just as we have found with wind power and the clobbering of birds, the efficient conversion of the water power in our waterways would probably clobber a lot of the fish as well.

    So, the main innovation we need is a way to lay long lines of water turbine mechanisms that do not impact the wildlife in the waterways. If we can figure out how to do that, we will have a great new source of energy, IMO.
  4. Jan 5, 2013 #3


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    Staff: Mentor

    Welcome to PF!
    Don't sell yourself short! This is a mechanical engineering question, and a good one.
    Good instinct: a low-flow/head water turbine looks much the same as a wind turbine. Consider under-sea turbines that work on low-velocity (~3mph) currents: http://www.guardian.co.uk/environment/2003/feb/10/energy.renewableenergy
    It's a cost-benefit problem. In order to extract energy when the wind is moving fast, the turbine has to be made stronger, the transmission with heavier gears, the generator with larger wires, and all of the downstream electrical infrastructure has to be larger. For what? 0.01% additional electricity generated due to the fact that that very high winds only occur a few hours, a few days a year? Not worth it.
    See link above: answer is it does work that way.

    There is a principle of scale-ability in fluid dynamics, quantified with the Reynolds number. Reynolds number mixes together velocity, viscosity and density of the fluid and the size of the object to give one number that determines the flow characteristics. Increasing density while decreasing the flow to give the same Reynolds number does indeed yield very similar looking flow. http://en.wikipedia.org/wiki/Reynolds_number

    So the issue with this (and your solar panel example) becomes one of cost (both financial and environmental) vs benefit. For an individual trying to extract energy from a large moving river, you won't have much of an impact on the river and you might be able to make the financial cost work out in your favor. Putting lots of turbines in a river will start to affect the flow and the infrastructure cost probably doesn't scale very well because of the large number of moving parts and small wires to connect the huge number of turbines. And that's assuming the river is even deep enough to put turbines in it without affecting shipping.
  5. Jan 5, 2013 #4


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    Staff Emeritus
    Science Advisor

    There are various programs looking at environmentally or ecologically compatible energy sources.

    http://www.zero.no/publikasjoner/small-scale-water-current-turbines-for-river-applications.pdf [Broken]


    It's only a matter of momentum/energy. One can have a low speed (low rpm) turbine and gear it up to an appropriate speed to produce power at 50 or 60 Hz.

    The Gorlov turbine was derived from the Darrieus wind turbine design.
    Last edited by a moderator: May 6, 2017
  6. Jan 5, 2013 #5


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    Gold Member

    I don't know of any such "Law of Physics" that doesn't allow scaling.
    You can build a 1 watt generator, or you can build a 5 million watt generator. The motive force can be any that you've mentioned: wind, water, solar, et al.

    bolding mine
    :bugeye: Herecy!

    Though I did put together a perpetual motion device once to figure out why it wouldn't work.

    And as far as I can tell, I do more "hobby" experiments than anyone else here at the forum.

    I don't think it's a matter of me not accepting the laws I learned in school, as much as it is me having forgotten everything.

    I sit and look at my river also:

    Toast Ants!

    But there are properties of solar voltaic panels which make this a less than optimal idea.
    On the other hand, what you've described is how most solar thermal panels operate.
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