Published data imply an average thermal efficiency of about 0.34 for U.S. electricity generating plants. With clever use of thermodynamics and technology, modern gas and steam turbines can be coupled, to effect dramatic efficiency increases. These combined-cycle power plants now reach thermal efficiencies in excess of 0.60. It is shown how the laws of thermodynamics make this possible.
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The U.S. EIA calculates the primary energy equivalent for solar thermal, solar photovoltaic, hydroelectric and wind-generated electricity using the typical fossil-fueled plants heat rate of 10300 MJ/kWh (electricity) to approximate the quantity of fossil fuels replaced by these sources. Note that EIA reports primary energy in BTU; I’ve converted to joules, with 1 BTU = 1055 J and 1 Quadrillion BTU = 1.055 EJ.
15.
Although the focus here is on power plants burning fossil fuels, these are by no means the only, or necessarily preferred, methods for large scale electricity generation. The long-term future is likely to bring a diverse mix of technologies, including renewable solar-thermal, photovoltaic, wind energy, geothermal electricity, fuel cells running on hydrogen, and nuclear power. In the near-to-medium term, however, it is likely that combined-cycle systems will have the largest impact.
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See Ref. 10, Secs. 8.6 and 8.7.
21.
Here, does not account for the imperfect efficiency of the heat exchanger that links the low-temperature end of the gas turbine with the high-temperature end of the steam turbine, or of the efficiency of the electrical generators.
22.
See p. 418 of Bejan’s book, Ref. 10, where the derivation of this equation is the objective of a homework problem.
24.
If the “heat recovery steam generator” (HRSG) in Fig. 4 has an efficiency of 0.85, and the gas turbine efficiency is 0.40, this implies a HRSG loss of of the input energy. Additionally, if the electrical generators bring a loss of, say, 0.02 of the input energy, then using these numbers as a rough guide, a gross efficiency of about 0.67 for the combined gas and steam turbines is needed to achieve a net efficiency that approaches 0.60.
© 2012 American Association of Physics Teachers.
2012
American Association of Physics Teachers
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