How Efficient Is a Steam-Electric Power Plant That Delivers 900 MW?

[kboykb]

Homework Statement

(c13p30) A steam-electric power plant delivers 900 MW of electric power. The surplus heat is exhausted into a river with a flow of 6.29×10^5 kg/s, causing a change in temperature of 1.03 oC. What is the efficiency of the power plant (in % Don't enter a unit.)?

Homework Equations

Not sure with this one. Efficiency = 1 - Tc/Th, or Efficiency = W/Qh

The Attempt at a Solution

900MW = 900,000,000 J/s

900,000,000 J/s / 629,000 kg/s = 1430.8

1.03 Deg C = 274.18 K

274.18 K / 1430.8 = .19

So 19%

 

[Filip Larsen]

As I read the problem text, cooling water leaves the plant with the given mass flow rate and with the given temperature increase. Knowing the heat capacity of water you should be able to calculate how much power is lost via the cooling water, so the efficiency must be how much 900 MW are relative to the total output of the plant (900 MW + loss). Doing that calculation you should get around 25%.

 

[kboykb]

That was the answer, thanks :-)

Also after seeing what you said I figured out how to do it

I was factoring in the specific heat (4184) or using my formulas correctly.

Thanks for your help !