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Homework Help: Energy in an ocean current vs energy in an air current

  1. Nov 19, 2005 #1
    This isn't actually a homework question, but it does relate to a research project I'm doing relating to wind/ocean turbines. What I'd like to be able to do is state that:
    There is as much energy in an ocean current moving at "X" knots as there is in an air current moving at "Y" kph.
    I tried using P=1/2(ryo)(v^3)(pi)(r^2), where P is the power produced by the turbine, ryo is the density, v is the velocity, and r is the radius of the turbine.
    Assuming P is equal in either case, I set the equations (one of water, one for air) equal to each other. I also assumed r was equal in either case, so r, pi, and 1/2 cancel, leaving:
    (ryo_water)(v_water^3)=(ryo_air)(v_air^3)
    Using:
    ryo_air = 1.275kg/m^3
    ryo_water = 1025kg/m^3 (seawater)
    I end up with v_water = 0.1075(v_air)
    So if I have my conversions right an 8 knot current would have as much energy as a 137.8kph wind. This seems reasonable to me, but it is very low compared to data I can find on the net.
    Do anyone have any insight on this? Thanks in advance!
     
  2. jcsd
  3. Nov 19, 2005 #2

    Astronuc

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    Considering mass flow rate as [itex]\rho[/itex]VA and the specific kinetic energy as V2/2, and the same turbine area, the numbers seem about right.

    What number is considered low?
     
  4. Nov 19, 2005 #3
    I think it's low because of info found http://www.bluenergy.com/technology.html".
    To quote them: "Sea water is 832 times denser than air and a non-compressible medium, an 8 knot tidal current is the equivalent of a 390 km/hr wind."
     
    Last edited by a moderator: Apr 21, 2017
  5. Nov 19, 2005 #4

    Bystander

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    Don't confuse energy with power.
     
  6. Nov 19, 2005 #5

    Astronuc

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    well if one uses the relationship - (ryo_water)(v_water^2)=(ryo_air)(v_air^2), then one would get closer to 390 km/h.

    However, using 8 knots = 14.8 km/h and the ratio of densities of seawater to air = 832, I get more like 426 km/h for the wind speed.

    I'd have to give this more thought.
     
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