Rankine Cycle
Prerequisites
Roughly 80% of the world's electricity is generated by spinning a turbine with steam. Coal, nuclear, gas (the steam portion of combined cycle), geothermal, concentrated solar: all use the same thermodynamic cycle. The Rankine cycle has been the workhorse of power generation since the late 19th century.
The Rankine cycle has four stages. A feedwater pump raises liquid water from condenser pressure (~0.05 bar) to boiler pressure (100-300 bar), requiring very little work because liquid is nearly incompressible. The boiler heats water into superheated steam at 540-600 degrees C. The steam expands through a turbine, producing shaft work as temperature and pressure drop. The condenser returns steam to liquid using cooling water, rejecting the waste heat the second law demands.
Compare across heat sources. The Rankine cycle does not care where the heat comes from. Coal: boiler burns pulverized coal. Nuclear: reactor heats pressurized water. CSP: mirrors concentrate sunlight. The turbine side is identical.
Trace the efficiency gains. Subcritical plants (below water's critical point at 374 degrees C, 221 bar): 33-37% efficiency. Supercritical: 38-42%. Ultra-supercritical (600+ degrees C, 250+ bar): 42-45%.
Why is the pump's energy consumption negligible compared to the compressor in a Brayton cycle?
Liquid vs. gas. Pumping nearly incompressible liquid water requires far less work than compressing a gas. The Rankine pump consumes less than 3% of turbine output, while the Brayton compressor takes 50-60%. This is a fundamental thermodynamic advantage of steam cycles.
The Rankine cycle explains why thermal plants need cooling water, why higher steam temperatures improve efficiency, and why nuclear runs at lower efficiency than modern coal (lower steam temperature due to material constraints).
An engineer designing a new supercritical steam plant notes the feedwater pump draws only 8 MW while the turbine produces 500 MW net. A comparable gas turbine (Brayton cycle) plant of the same net output needs over 600 MW just to run its compressor. What explains this large difference in parasitic power draw?
Pumping liquid water takes little work because its volume barely changes under pressure, while compressing a gas to the same pressure ratio requires shrinking its volume substantially, which costs far more work.
The answer is BLesson complete
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