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Old May 23rd 2009, 07:55 AM   #1
Join Date: Apr 2009
Posts: 71
A powerplant near a river- entropy question

Hi, I am stuck on Termodynamics again (and I have an exam really soon)!

A typical coal-fired power plant generates 1000 MW of usable power at an overall thermal efficiency of 40%.

1. The river's temperature is 18.0 degrees celcius before it reaches the

power plant and 18.6 degrees celcius after it has received the plant's

waste heat. Calculate the river's flow rate, in cubic meters per second.

(the answer you should get is V =597 m^3/s )

2.By how much does the river's entropy increase each second?

(the answer is $\displaystyle \Delta S$ =5.15106 J/K ).

I don't know how to start 1, please help. I am in a similar position with 2.

Usually I use $\displaystyle \Delta S=\int_{T_1}^{T_2} \frac{mcdT}{T}$

for this type of problem (as I can't find any other relevant formulas in my notes)

but as I don't know the mass of the river I don't this is not an option. Am

I missing something really obvious?

Last edited by C.E; May 23rd 2009 at 07:58 AM.
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Old Jun 6th 2009, 03:19 AM   #2
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Location: Udupi, Karnataka, India
Posts: 292
Efficiency = (Q1 - Q2)/Q1
(Q1 - Q2) is the energy output. In the given problem energy output is 1000 MJ per second. Using efficiency as 0.4 calculate Q1 and Q2. Q2 is the amount of heat given to the cooling water. Rise in temperature of the water is given. Using Q = mcΔt. you can find mass of the water flowing per second. Using density of water you can find the volume of the water flowing out.
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entropy, powerplant, question, river

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