Brayton cycle
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The Brayton cycle is a cyclic process generally associated with the gas turbine. It is unique among power cycles in being an open system.
Model
A Brayton engine consists of three components:
- A compressor
- A burner (or combustion chamber)
- A turbine
Ambient air is drawn into the compressor, where it is pressurized—a theoretically isentropic process. The compressed air then runs through a combustion chamber, where fuel is burned, heating that air—a constant-pressure process, since the chamber is open to flow in and out. The heated, pressurized air then gives up its energy, expanding through a turbine (or series of turbines)—another theoretically isentropic process. Some of the work extracted by the turbine is used to drive the compressor.
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Since neither the compression nor the expansion can be truly isentropic, losses through the compressor and the turbine represent sources of inescapable working inefficiencies.
In general, increasing the compression ratio is the most direct way to increase the overall power output of a Brayton system.
Applications
The efficiency of a Brayton engine can be improved in the following manners:
- Reheat, wherein the working fluid—in most cases air—expands through a series of turbines, then is passed through a second combustion chamber before expanding to ambient pressure through a final set of turbines. This has the advantage of increasing the power output possible for a given compression ratio without exceeding any metallurgical constraints.
- Intercooling, wherein the working fluid passes through a first stage of compressors, then a cooler, then a second stage of compressors before entering the combustion chamber. While this requires an increase in the fuel consumption of the combustion chamber, this allows for a reduction in the specific heat of the fluid entering the second stage of compressors, with an attendant decrease in the amount of work needed for the compression stage overall.
- Regeneration, wherein the still-warm post-turbine fluid is passed through a heat exchanger to pre-heat the fluid just entering the combustion chamber. This allows for lower fuel consumption and less power lost to waste heat.
- A Brayton engine also forms half of the combined cycle system, which combines with a rankine engine to further increase overall efficiency.
- Cogeneration systems make use of the waste heat from Brayton engines, typically for hot water production or heating.