Grades 9–12 · Systems
How can one power plant make electricity from the same fuel in two stages?
In a gas-turbine power plant, compressed air mixes with fuel and burns. The hot gases expand through a turbine, turning machinery that drives a generator and also helps power the compressor.
In the diagramHot exhaust powers a second engine cyclegas turbineheat recoveryexhauststeam turbinegenerator 1generator 2cooling + exhaust lossesThe second cycle uses remaining thermal energy.
The gases leaving that turbine still carry substantial thermal energy. A combined-cycle plant sends them through a heat-recovery steam generator, where energy passes to a separate water-and-steam circuit rather than going directly out with the exhaust.
The resulting steam expands through another turbine connected to electrical generation. Afterward it is cooled and condensed so the water can circulate again. Two different engine cycles therefore use successive parts of the energy released by the original fuel.
The second stage increases the fraction converted to electricity, but it cannot use every remaining bit. Exhaust and cooling still carry energy away, and pumps and other equipment consume power. Performance depends on operating conditions, equipment design and how efficiently both stages work together.
About this explanation
Starting knowledge: Understand a heat engine, an electric generator and heat transfer between separate fluid streams.
Sources
- U.S. Department of Energy — How Gas Turbine Power Plants Work