# Homework Help: Thermo, heat transfer rate, power plant

1. Mar 23, 2013

### Confusedby

1. The problem statement, all variables and given/known data
A power plant generates 1300MW and operates at 315 °C a 20°C heat sink is available.
what is the minimum rate at which heat must be discarded?

2. Relevant equations
not really sure how to begin this one, have a feeling it is related to dq=Tds at constant pressure giving (dq/dT)p=T(dS/dT)p=(dH/dT)p=Cp but that seems to go no where. tried relating it to Enthalpy Changes along the path from 315->100 then condense to liquid water and go 100->20 but none of these seem to use the power out put. honestly completely lost on this one.

3. The attempt at a solution

2. Mar 23, 2013

### TSny

Hi, Confusedby. Welcome to PF!

This looks like a basic heat engine problem. Have you studied the concept of efficiency of a heat engine and how the second law implies a maximum possible efficiency for given hot and cold reservoir temperatures?

3. Mar 24, 2013

### Confusedby

Thank You

Well I feel a little silly, for some reason "maximum Thermal efficiency" became "maximum residual heat". If I were to use this arrangement:
(W/e)-W=q residual. were e is the efficiency and W is the work. it should give the minimal heat flow if I assume Carnot Efficiency correct?

4. Mar 24, 2013

### TSny

Yes. q is the heat discarded rather than the rate at which heat is discarded. But if you let W be the work done each second, then q will be the heat discarded each second.