A Rankine heat engine uses water as the operating fluid. The steam leaving the boiler is saturated vapor at P = 1 MPa. The fluid leaving the turbine is at P = 0.2 MPa, and the turbine efficiency is 75 %. The fluid leaving the condenser is saturated liquid at P = 0.2 MPa. The pump work is 2 kJ/kg. What is the overall efficiency of the cycle?

Question

A. 9.4
B. 0.10
C. 0.13
D. 0.12
E. Insufficient information to solve

Answer 1

A. Incorrect. This is a coefficient of performance calculated as though the system was a refrigerator, which it’s not. You have the correct values if you got this, you just used the wrong index of performance.
B. Correct. As low as this may seem, it is not unlikely for a heat engine to “only” have an efficiency of 10 % or less in real life.
C. Incorrect. You did not correct the turbine’s shaftwork with the efficiency. This is for a reversible turbine.
D. Incorrect. You can get this if you assume the fluid exiting the turbine is saturated water, which it is not. Check Example 5-1 for an example that should show you how to do this.
E. Incorrect. There is enough information here. Note that you do not need to know about the liquid between the pump and boiler. If you have three of the four energy flows you can do a steady-state energy balance.