What is the Brayton cycle?
The Brayton cycle, also known as the Joule or Joule-Brayton cycle, is the ideal thermodynamic cycle for all gas turbine engines. Gas turbines are used for power generation and aviation. Air is the working fluid which turns the turbine and thus the shaft. The turbine’s shaft is connected to the compressor and either a generator (for power applications) or fan (aviation). The graphs on the right are P-v and T-s diagrams. This shows the relationship between the pressure and volume*, and temperature and entropy*.
The steps for the (ideal) Brayton cycle are:
- 1-2 (Isentropic) Compression: Fresh air is taken into the compressor. When compressed, it is sent to the combustion chamber. Ideally, this is done without any change in entropy. This is impossibly in reality, so it is actually adiabatic (no heat enters nor leaves the system).
- 2-3 (Isobaric) Combustion: The compressed air mixes with the fuel and is burned. This process is done under a constant pressure.
- 3-4 (Isentropic) Expansion: The power produced from the combustion, along with the exhaust gases, spin the turbine. The spinning of the turbine expands the exhaust, along with the increasing volume in the engine itself. This is really an adiabatic process.
- 4-1 (Isobaric) Heat rejection: The turbine produces power for the application using it. Some of this power is used to power the compressor as well. The exhaust gases are expelled, reducing the temperature and thus getting rid of heat.
No cycle is perfect. Every component has some deficiencies and there are always energy losses. Though gas turbine achieve up to 60% efficiencies (and growing), they typically do not do so without some alterations of the cycle. Technically, those engines use a combined Brayton-Rankine cycle, but improvements to the Brayton cycle itself include reheat, intercooling, and regeneration.
*The variables shown are actually the specific volume and specific entropy, or volume per unit mass and entropy per unit mass, respectively.
Improving the cycle: intercooling, reheat, regeneration
Intercooling is done by cooling the first stage compressor’s air to reduce its specific volume. This reduces the energy needed for the second stage compressor to put into it. I.e., it makes it easier to further compress the air. The lower temperature at the exit of the second stage compressor makes it easier for heat to transfer from the regenerator to the air. On its own, intercooling doesn’t make the engine much more efficient, but coupling it with regeneration makes it useful.
When the exhaust passes through the first stage turbine, it can be sent through a reheater. The oxygen content in the combustor is about four times the amount needed to completely burn the fuel, so there is excess oxygen that could be used. This oxygen-rich exhaust is sent to another combustor, the reheater, where some more fuel is added and burned to provide energy to turn another turbine. So the reheater uses the exhaust of the first stage turbine to burn more fuel and turn a second stage turbine.
The final outlet exhaust has a very high temperature. A regenerator takes this exhaust and uses it to heat up the compressor’s outlet air, thus reducing how much fuel is needed to heat up the air for combustion.
- Intercooling: Compressor air is cooled to make it easier for the next stage compressor to compress it. This reduces the energy needed to compress the air further.
- Reheat: Turbine’s oxygen-rich exhaust is used to combust more fuel. The reheated exhaust turns another turbine. This takes exhaust that would otherwise be wasted and uses it to produce more power.
- Regeneration: The high-temperature exhaust from the final turbine is sent through a heat exchanger to heat up the compressor’s outlet air. This reduces the energy and fuel needed to heat up the compressor air for combustion.