Combined cycle advantages and working principle

A combined cycle plant uses the hot exhaust of a gas turbine to generate steam for a steam turbine, so the same fuel produces power in two cycles.

Advantages

• High overall efficiency, 50-60%, against about 35-40% for steam and 30-35% for simple gas turbine plants.
• Lower fuel cost and lower CO2 per kWh.
• Short construction time; gas turbine part can run first (phased construction).
• Low capital cost per kW and less space than a coal plant.
• Quick start-up; good for intermediate and peak loads.
• Less cooling water (only the steam part, about one-third of output, needs a condenser).
• Lower emissions (natural gas fuel).

Working principle

      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 |<---+
                         +-----------+

1. Air is compressed in the compressor (1-2) and fuel is burnt in the combustion chamber (2-3).
2. Hot gas expands in the gas turbine (3-4) and drives generator G1.
3. The exhaust gas (about 500-600 C) passes through the HRSG where it heats feed water, evaporates it and superheats the steam; gas leaves through the stack at about 100-150 C (4-5).
4. Steam expands in the steam turbine (a-b) and drives generator G2; it is condensed (b-c) and pumped back to the HRSG (c-d).

The gas cycle adds heat at a very high temperature (about 1200-1400 C) and the steam cycle rejects heat at a low temperature (about 35-45 C). Using the gas cycle's waste heat as the steam cycle's input gives

$$\eta_{cc} = \eta_{GT} + \eta_{ST} - \eta_{GT}\eta_{ST}$$

For example, $\eta_{GT} = 0.35$ and $\eta_{ST} = 0.30$ give $\eta_{cc} = 0.35 + 0.30 - 0.105 = 0.545$, i.e. about 54.5%.