Power Plant Efficiency & Heat Dissipation Calculations

[Martinii]

[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:

a)
Pel=400 MW
η=40%=0.4
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.

 

[BoTemp]

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.

 

[Martinii]

Yeah , i think i got it! Thanks anyway :)