A gas turbine (can assume an ideal Brayton cycle) has a pressure ratio of 10. It
ID: 1857293 • Letter: A
Question
A gas turbine (can assume an ideal Brayton cycle) has a pressure ratio of 10. It is connected to an electric generator with an efficiency of 95% which produces 200MW of electricity. The gas turbine burns natural gas with a heating value of 100,000 btu/therm. The waste heat from the gas turbine is used to generate steam in an ideal Rankine cycle. The efficiency of the Rankine cycle is 30%. The turbine of the Rankine cycle is attached to an electric generator with an efficiency of 95%. What electric output would you estimate for this electric generator? This system is called a combined cycle where you combine a gas turbine with a Rankine cycle (called the bottoming cycle in this case) to achieve an overall higher efficiency.
Explanation / Answer
Work = 300 / 0.95 = 315 .78 KW
thermal efficiency of brayton cycle= 1-(1/rp^y-1/y)
=0.482
heat added by gas turbine= workdone/thermalefficiency
=315.78/0.482=655 kW
waste heat= 655.145-315.78
==339.365 kw
COP =Q2/(Q1-Q2)
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