Chapter 8 Lyes KADEM [Thermodynamics II] 2007

Combined Gas-Steam power cycles

The objective of this cycle is to combine a Brayton cycle with a Rankine cycle. This combination improves significantly the thermal efficiency of the power plant.The idea behind a combined gas-steam power cycle is to use the exhaust high temperature gases from the gas turbine to heat the steam within the boiler of the steam turbine.

Fig.8.24. Combined gas (Brayton)- steam (Rankine) cylce.

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Chapter 8 Lyes KADEM [Thermodynamics II] 2007

T-s diagram:

Fig.8.25. T-s diagram of a Combined gas (Brayton)- steam (Rankine) cycle.

Numerical example:

- Simple Brayton efficiency: 36.8%- Simple Rankine efficiency: 35%- Combined Brayton and Rankine cycles: 56.4%

Fig.8.26. Difference between gas turbine (left) and steam turbine (right) blades.

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Chapter 8 Lyes KADEM [Thermodynamics II] 2007

Other practical application:

BMW TurbosteamerAlthough combined cycles have traditionally only been used in large power plants. BMW has a proposal to deploy such a system in automobiles by using exhaust heat to drive a steam turbine. The idea is that 80% of the heat energy from the exhaust gases is used to heat a fluid and form steam that is then conducted directly into an expansion until linked to the crankshaft of the engine. This results in 15% increase in the thermal efficiency and additional 14 horsepower.BMW is designing the system to be small enough to fit on any of its current production vehicles and hopes to have the system production ready in ten years.

Fig.8.27. Schematic representation of BMW’s turbosteamer.

Combined Gas-Steam cycle with CyclePad

Fig.8.28. Combined gas-steam cycle with CyclePad.

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Chapter 8 Lyes KADEM [Thermodynamics II] 2007

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Example

A simple steam power plant operates between pressures of 10 kPa and 4 MPa with a maximum temperature of 400ºC. The power output from the steam turbine is 100 MW. A gas turbine provides the energy to the boiler; it accepts air at 100 kPa and 25ºC, has a pressure ratio of 5, and a maximum temperature of 850ºC. The exhaust gases exit the boiler at 350 K. Determine the thermal efficiency of the combined Brayton-Rankine cycle.