Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Velocity more efficient than volume?

  1. Sep 30, 2004 #1
    Here are two equations showing a nearly equivelant energy output for a given volume and velocity of water. Using the formula:

    EKin = M/2 x Vsquared

    250/2 x 15.34m/s x 15.34m/s = 29,414 KW (requires 5 times more volume)

    50/2 x 35m/s x 35m/s = 30,625 KW (requires only 2.3x more velocity)

    Since the velocity is squared, isn't it better to look to use velocity over volume? IF velocity can be acheived through another means other than water pressure via water depth, wouldn't that be the most efficient way to go?
  2. jcsd
  3. Oct 1, 2004 #2
    Mathematically, yes, it is.

    [tex]v \propto \sqrt{h}[/tex]
    where h is the height of the water and pressure
    [tex]p \propto \rho g h [/tex]
    This depends on desinty of water and height. If you want to double velocity, you'll 4x the height, which 4x the pressure...It'd be a challenge to find material that can withstand that...
  4. Oct 1, 2004 #3
    Water depth? You will still have to get the water to a sufficent height above the outlet in order to get the pressure. You will also have to refill this depth of water in order to maintain pressure. That means using energy to get all this water from the working level to the top.

    Are you thinking that you can just put a hose deep in the ocean and water will flow up to land with the pressure from the depth? I hope not
  5. Oct 1, 2004 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Why do you suppose they build dams as high as they can? The hydro electric system on the Umpqua river in Southern Oregon, does not have many high dams. They use a system of flumes to carry the water to top of a several hundred ft high cliff, then using huge pipes (8' to10' in diameter) drop the water at a high velocity into the generators and back into the river bed. This is a convenience of our local geography, and a relatively small river that drops rapidly from 5000' to near sea level in less then 50 mi.

    http://www.outstandingrivers.org/northumpqua.asp [Broken]
    Last edited by a moderator: May 1, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook