Out of the water tidal power generation?

  1. Hello,

    I live in an area with large tides averaging 15 ft, twice daily. The city has a huge floating dock system in a large harbor held in place by fixed steel pilings. I have no idea what the weight is but it is a huge harbor housing a large fishing and personal use fleet. The docks are wood and concrete are obviously enormously heavy.

    Heres the question. It would take a lot of power to lift and lower this huge mass up and down 15 ft (4.72M) twice daily so could that energy conversely be harnessed?

    So far as I can tell, all ideas for tidal energy are essentially versions of underwater wind farms, with many inherent limitations such as salt water corrosion and marine wildlife mortality. What I am picturing here is a small power plant on each or many of the fixed pilings with all mechanics out of the water. The question is, is this physically possible harness electrical energy from a large mass moving at a VERY slow pace?

    Any ideas? Is this an impossible thought. I am a layperson (healthcare worker) so answers in dummy speak or direction to resources would be helpful. I fully expect someone to say this would require mile wide cog or something but I have to ask.

  2. jcsd
  3. Simon Bridge

    Simon Bridge 15,474
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    Short answer: no.
    All that power associated with the weight is (rate of) work against gravity.

    You could put a track along a piling and fix a generator to the deck so it's crack runs along the piling.
    This would allow you to, in principle, generate electricity from the up-down motion, but that has nothing to do with the weight of the dock. It would work as well for a polystyrene float moving up and down a pile.

    Since it is moving very slowly, the power generation would be very slight ... depending on the type of generator chosen. Imagine doing this with a bicycle dynamo - you'd just get a series of pulses.
  4. If it could be harvested, and there are tidal generators in existence, ask yourself where that energy is coming from. It's coming from the rotation of the earth. As the energy is extracted, the rotational period of the earth increases. It may not be noticeable in one lifetime but then again it may. It certainly is easier to extract energy from the rotation of the earth than it is to put it back again.
  5. Simon Bridge

    Simon Bridge 15,474
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    Interesting concern - have you tried doing the calculation: it's is not as difficult as is sounds.
    The hardest part is getting a decent ballpark function for the energy demand with time.
    You should be able to tell how long to increase the length of the day by, say, 1 second.
  6. Tidal forces are already slowing the rotation of the earth without any intervention. That energy is going towards friction in erosion processes. If you extract the energy you are probably stealing it from erosion and not rotation.

    If your docks are heavy then you could get a significant amount of torque on a generator.

    One way you could do this is run cables from the bottom of the pilings and another set of cables from the top of the pilings to a rotating drum and gearbox connected to a generator. Picture this: It's just past high tide and the water level is going down. If you hold the cables still the weight of the dock will slowly be transferred to the cables connected to the top of the pilings. Now if you release the cables and engage the generator the dock can lower in pace with the tide going out while maintaining some tension on the cables. At low tide you would do the opposite. You could hold the dock still as the water goes up and build some buoyancy. Then release the dock and use the tension in the cables to put some torque on a generator.

    In theory that would work but let's run some numbers first. Energy is mass X distance X gravity. Let's say that a dock section is 5,000 kilos. 5,000 X 4.5m X 9.8 m/s^2 = 220.5 kJ. Tides move up or down in rougly 6 Hours so we have Power = Energy/time = 220.5 kJ / (21600s) = 10.2 W. That's the limit of what you could get not what you would actually get once the system is designed and built.
  7. sophiecentaur

    sophiecentaur 14,712
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    I would disagree with that statement - without some qualification. The mass of the dock is directly related to the Force in the Force times Distance formula. The dock is already in existence - which I think is the point of the OP and you would need a massive piece of Expanded polystyrene to achieve the equivalent energy output.

    There is a lot of inefficiency built into an electromagnetic type of generator, working over a 12 hour cycle as you would need a vast gearing ratio to produce 50 / 60 Hz. The fact that the dock is floating is vital to its usefulness. The ships moored to it have to find it at the same level all the time so you couldn't delay its vertical movement without making it a totally useless floating dock. Unfortunately!
    It strikes me that a large pneumatic piston / cylinder arrangement could be used to drive a turbine - but where would you put all that stuff, in an operational dock? The total volume of air would have to be huge and the basic idea has already been tried, I believe.
  8. Baluncore

    Baluncore 3,305
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    If you did harness all that energy then the floating dock would have to stop going up and down with the tide.
    It would cease to be a dock.
  9. I think that's incorrect. you would have to completely suspend it just above the water on the way down and slightly submerge it on the way up to get the maximum force for the generator. Your longshoremen would hate you during a rising tide :).
  10. Baluncore

    Baluncore 3,305
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    If it must be just submerged when the tide is rising, and just above the surface while ebbing, then there must be a step transition twice per cycle as the tide turns, followed each change, by a delay for the tide to get ahead again. It is impossible therefore to recover all the energy.

    The energy flow would be proportional to the rate of change of the water level.
    The dock would certainly no longer go up and down in phase with the tide.
    I suspect the optimum energy recovery may be when the dock moves in quadrature with the tide.
  11. sophiecentaur

    sophiecentaur 14,712
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    Absolutely. Really no use at all as a dock.
    I guess the basic idea could be applied for a low power electrical supply to an isolated dock or pontoon - in the same way that PV and wind is used these days. It would have to be using a tiny fraction of what energy is available over the tidal cycle.
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