Brayton Cycle

A jet engine and a natural gas power plant use the same thermodynamic cycle. Air goes in, gets compressed, fuel burns, hot gas spins a turbine, exhaust goes out. The Brayton cycle is the operating principle behind every gas turbine in aviation and electricity generation.

The Brayton cycle has three open-loop stages. A compressor draws in ambient air and squeezes it to 15-30 times atmospheric pressure, raising its temperature. Fuel (natural gas or jet fuel) is injected and burned in the compressed air, heating it to 1,200-1,600 degrees C. The hot, high-pressure gas expands through a turbine, spinning the shaft that drives both the compressor and the electrical generator. The critical detail: the compressor consumes 50-60% of the turbine's gross output. Only the remaining 40-50% is net power.

Trace the energy. A gas turbine receives 100 MW of fuel energy. Combustion converts 98% to heat. The turbine extracts 65 MW of gross mechanical work, but the compressor takes 35 MW. Net output: 30 MW. Simple-cycle efficiency: 30%.

Raise the firing temperature. Modern turbines use single-crystal nickel superalloy blades with internal cooling channels to withstand 1,600 degrees C. Higher inlet temperature raises the Carnot ceiling and increases net efficiency to 38-42%.

Why does the compressor consume such a large fraction of the turbine's output?

Understand the compression penalty. Compressing air from 1 atm to 20 atm requires enormous work because gas is compressible, unlike the liquid water in a Rankine cycle pump (which uses less than 3% of turbine output). This compression penalty is why simple-cycle gas turbines are less efficient than combined-cycle systems.

The Brayton cycle's hot exhaust (500-600 degrees C) is too valuable to waste, which is why combined-cycle plants add a steam turbine to capture it.

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