Open and closed Brayton cycles

The Brayton (Joule) cycle is the ideal cycle for gas turbine plants. It has four processes: isentropic compression (1–2), constant-pressure heat addition (2–3), isentropic expansion (3–4) and constant-pressure heat rejection (4–1). Its ideal efficiency is $\eta = 1 - \dfrac{1}{r_p^{(\gamma-1)/\gamma}}$.

Open Brayton cycle

 Air in        Fuel
  1 --> [C] -2-> [CC] -3-> [T] -4-> Exhaust
         \==================/ ==> Gen

• Atmospheric air is drawn in, compressed, mixed with fuel and burnt directly in the combustion chamber; products expand in the turbine and are exhausted to the atmosphere.
• Heat rejection 4–1 takes place in the atmosphere, so a fresh charge is used every cycle.
• Advantages: simple, light, compact, low cost, quick start; no cooler needed. Used in aircraft engines and most power-station gas turbines.
• Disadvantages: working fluid is combustion gas, so turbine blades suffer deposits, erosion and corrosion; only clean fuels (gas, light oil) can be used; output depends on atmospheric conditions; filters needed.

Closed Brayton cycle

       Heater (external heat)
  +--> [Heat exch.] -3-> [T] -4-+
  |2                            |
 [C] <-1- [Cooler] <------------+
  \=======shaft=======/ ==> Gen

• The same working fluid (air, helium, CO₂, nitrogen) circulates continuously. Heat is supplied externally in a heat exchanger (by burning any fuel, or from a nuclear reactor) and rejected in a pre-cooler before the compressor.
• Advantages: clean working fluid, so no blade fouling or erosion; any fuel (coal, nuclear) can be used; higher pressure level gives smaller components and better part-load efficiency (output controlled by changing pressure level); gases with better properties (helium) can be used.
• Disadvantages: needs large heat exchangers and cooler, so heavier, costlier and slower to start; requires cooling water; leakage of working fluid must be prevented.