CCGT How It Works

A simple-cycle gas turbine exhausts at 500-600 degrees C and calls it waste heat. A combined-cycle plant captures that exhaust and runs a steam turbine with it, pushing total efficiency past 60%. Same fuel input, 50% more electricity. This is the most efficient fossil fuel technology in commercial operation.

A combined-cycle gas turbine (CCGT) pairs two thermodynamic cycles using a heat recovery steam generator (HRSG). The gas turbine (Brayton cycle) burns natural gas at 1,400-1,600 degrees C, producing electricity at 38-42% efficiency. Its hot exhaust (not waste, in this configuration) flows through the HRSG, which boils water and superheats steam. The steam drives a steam turbine (Rankine cycle) at roughly 33% efficiency on the remaining heat. Total system efficiency: 55-63%.

Trace the energy. 100 MW fuel input. Gas turbine produces 40 MW electricity. Exhaust carries 60 MW of heat to the HRSG. Steam turbine converts 33% of that: 20 MW. Total: 60 MW from 100 MW input. 60% efficiency.

Explain why it's not 40% + 33% = 73%. The steam cycle operates only on the gas turbine's waste heat (60 MW), not on the original fuel input. 33% of 60 MW is 20 MW, not 33 MW.

Could you add a third cycle to capture the steam turbine's waste heat?

Diminishing returns. The steam turbine exhausts at roughly 30-40 degrees C. The Carnot limit between 40 degrees C and ambient is only a few percent. There is not enough temperature difference to drive a useful third cycle. Two cycles capture most of what physics allows.

CCGTs are the benchmark against which every other thermal technology is measured, and their 55-63% efficiency is the primary reason gas displaced coal in U.S. electricity markets.

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