Combined plant components and T-s diagram

A combined cycle power plant joins a gas turbine (Brayton) cycle as the topping cycle and a steam turbine (Rankine) cycle as the bottoming cycle. The hot gas turbine exhaust (about 500-600 C) raises steam in a heat recovery steam generator (HRSG).

Basic components

      air 1                fuel
      |                     |
      v                     v
   +------+     2     +-----------+   3   +------+
   | Comp |---------->| Combustion|------>|  GT  |===> G1
   +------+           |  chamber  |       +------+
      ||              +-----------+        ||  |
      ++=============== shaft =============++  | 4 hot exhaust
                                               v   (~500-600 C)
   stack <-- 5 --+-------------------------------+
                 |  HRSG (economiser,            |
                 |  evaporator, superheater)     |
                 +-------------------------------+
                     ^ d                  | a steam
                     |                    v
                 +------+             +------+
                 | Pump |             |  ST  |===> G2
                 +------+             +------+
                     ^ c                  | b
                     |   +-----------+    |
                     +---| Condenser |<---+
                         +-----------+

• Gas turbine unit: compressor (Comp), combustion chamber, gas turbine (GT), generator G1.
• HRSG / waste heat boiler: economiser, evaporator and superheater heated by GT exhaust (sometimes with supplementary firing).
• Steam unit: steam turbine (ST), generator G2, condenser, feed pump.

T-s diagram

  T
  ^              3
  |             /|
  |            / |   Gas cycle 1-2-3-4
  |           /  |   (Brayton, top)
  |          /   |
  |         2    |
  |         |    4
  |    a    |   /    4-5: exhaust heats
  |   /|    |  5     steam (d-a) in HRSG
  |  / |    | /      5-1: gas to stack
  | d  |    |/
  | |  |    1
  | c--b       Steam cycle c-d-a-b
  |            (Rankine, bottom)
  +--------------------------> s
  (gas and steam drawn per kg of
   their own fluid, schematic only)

The heat rejected by the gas cycle in the HRSG (4-5) is used as heat input to the steam cycle, so the overall efficiency (about 50-60%) is higher than either cycle alone.