Question

A combined-cycle power plant employs a gas-turbine system with a thermal efficiency of 38% and 200 MWt of combustion-produced heat. The gas-turbine discharge is directed into a heat recovery steam generator (HRSG). Find: (i) the thermal efficiency and (ii) the net output power of the combined cycle plant for two cases: [12 pts] (a) no heat augmentation in the HRSG with a resulting steam-power cycle thermal efficiency of 20%, and (b) HRSG heat supplementation of 50 MWt, which raises the steam cycle thermal efficiency to 27%.

Answer 1

Answer:

a) The thermal efficiency is 50.4% and the net output power is 100.8 MW.

b) The thermal efficiency is 49.2% and the net output power is 123 MW.

Explanation:

a) To find out the thermal efficiency, we must calculate how much of the thermal energy is converted to mechanic work:

$\eta=\frac{\mbox{Net power out}}{\mbox{Heat in}}$

We know that the gas turbine delivers 38% of the 200 MWt in power to the generator. The remaining 62% of the heat is left in the escape gas, of which the HRSG is able to recover another 20%:

$\eta=38\%+62\%*20\%=50.4\%$

Knowing the efficiency and the thermal power delivered, we calculate the output power:

$\mbox{Net power out}=200MWt*50.4\% = 100.8 MW$

b)Now, we calculate how much power is recovered, knowing that we add 50 MWt to the original 62% in the HSRG:

$\mbox{Net power out}=200MWt*38\% + (200MWt*62\%+50MWt)*27\%=123 MW$

Finally, we calculate the efficiency, knowing that total thermal power added is 250 MWt (200 MWt in the gas turbine plus 50 MWt in the HRSG)

$\eta=\frac{\mbox{123MW}}{\mbox{250 MWt}}=49.2\%$