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B Is Hydroelectric power plant a perpetual motion machine?

  1. Feb 22, 2016 #1
  2. jcsd
  3. Feb 22, 2016 #2

    rcgldr

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    It's not, ultimately the sun is the source of power, evaporating water and raising it which later condenses (usually rain) to fall into rivers and lakes which in turn drive the hydroelectric dams.
     
  4. Feb 22, 2016 #3

    Drakkith

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    It's not a PMM at all. A hydroelectric plant converts the gravitational potential energy of the water into electrical energy. As rcgldr said, this is replenished by heat from the Sun, which is using up nuclear fusion fuel and will eventually cease to shine.
     
  5. Feb 22, 2016 #4
    It is definitely not PMM of the first kind.
    Why it is not a perpetual motion machine of the second kind, which is a machine which spontaneously converts thermal energy into mechanical work?
    eventually It converts solar thermal energy to mechanical work.
     
  6. Feb 22, 2016 #5

    SteamKing

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    What do you mean 'spontaneously'?

    The water from the reservoir behind the dam must be directed to flow through the turbines in order for any conversion of thermal energy to work to take place. That means someone has to push a button somewhere inside the power plant. Like all power plants, a hydroelectric plant does not run full blast all the time: portions of it may be shut down because the demand for its electricity is reduced or for maintenance of the generating equipment.
     
  7. Feb 22, 2016 #6
    'spontaneously' it is copy from wikipedia article https://en.wikipedia.org/wiki/Perpetual_motion#Classification

    I mean without Sun's thermal energy water behind the dam will be finished in 1 month or so. How water gets back behind the dam? Because Sun's energy leads to water evaporation from the ocean and this evaporated water goes back to the "behind the dam" from clouds.
     
  8. Feb 22, 2016 #7

    SteamKing

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    By that definition, any internal combustion engine is a PMM, because it converts the thermal energy stored in the gasoline or diesel fuel into mechanical work 'spontaneously'.

    I think you are overlooking some of the subtleties in the definition of the PMM contained within the Wiki article. Note too that all of these machines which we have discussed, i.e. hydroelectric plants and internal combustion engines, each have various thermodynamic inefficiencies built in. None of them is capable of converting 100% of the available thermal energy into its equivalent mechanical work.
     
  9. Feb 22, 2016 #8

    Drakkith

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    Because a hydroelectric dam does not directly convert thermal energy into anything. It converts gravitational potential energy into electrical energy by using pressurized water to turn a turbine.
     
  10. Feb 22, 2016 #9
    I got a good answer in another place.

    It's not a perpetual motion machine at all: it's a machine which converts energy from the Sun to electrical energy, indirectly:
    1. the sun heats water in the sea and elsewhere, which evaporates;
    2. this condenses out as rain and falls on high up bits of land;
    3. and flows down hill to the sea again;
    4. the hydroelectric plant gets in the way of this flow and captures some of the energy from the water.
     
  11. Feb 22, 2016 #10

    Dale

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    With that we will close this thread. PMMs are not discussed here.
     
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