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Homework Help: Power plant

  1. Sep 13, 2007 #1
    [SOLVED] Power plant

    Hi! Here i have a problem : A power plant generates 400 MW of electric power.It has an effieciency of 40%.
    a) At what rate does it dissipate heat?
    b)If the waste heat is carried away by a river, and the temperature rise of the river must not exceed 3°C, what flow rate is required in the river?

    Here is what i have tried:

    Pel=400 MW
    Pth=Pel/η=1000 MW

    b) Pwaste=W/t Q=W=m*c*ΔT m=ρ*V

    Pwaste=(ρ*V*c*ΔT)/t V/t= Volume Flow = V°= Pwaste/(ρ*c*ΔT)

    Pwaste= Pth-Pel=600 MW and then calculations

    I am not quite sure whether this way is correct or not.I will be happy if someone can tell me his opinion.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 14, 2007 #2
    Seems right to me, although there is at least one assumpution which is implied:
    1. All of the wasted energy is lost as heat.
    Fairly reasonable, there's bound to be some light and sound etc. but the vast majority will definitely be heat.

    I don't fully understand your notation for Pwaste, but it looks pretty good. I guess one assumes that the river isn't below 0 C, and you can look up the specific heat of water, so that's no biggy. Seems like you got it.
  4. Sep 15, 2007 #3
    Yeah , i think i got it! Thanks anyway :)
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