Combined Cycles Power Plants
Gas Turbine Cooling
The increasing of the hot gas temperature on the inlet of the gas turbine increases the efficiency of the gas turbine and thus of the CCPP. The problem of the gas turbine is the cooling of combustion chamber and particularly in the high temperature of the turbine blades. In order to bear the usual temperatures of 1000 to 1200C, apart from the use of developed materials the turbine components are cooled additionally from the inside, by compressed and cooled cooling air.

Open Loop Air Cooling

For this open cooling a part of the compressed air is removed from the compressor and used for the cooling of combustion chamber and turbine blades. In the case of a gas turbine with sequential firing the compressed air is removed on different pressure levels from the compressor. The extracted cooling air must be cooled down, before it used as cooling air in the turbine. After cooling of turbine components the cooling air is mixed in the turbine with the main gas flow.
Several concepts are available for matching the cooling requirements to the CCPP.
Open cooling concepts can be used for cooling the compressed cooling air:
- Water injection (quench cooler)
- Steam injection
- Water-Steam cooling in heat exchangers

For avoiding the loss of demineralised make-up water and the loss of evaporating heat energy in the exhaust gas, the use of water-steam-cycle for the air cooling in CCPP is the most economical solution (Fig. 060C).

GT-Air Cooler (Air/Water-Steam Heat Exchanger):

The air/water-steam cooler (30) works as a steam generator. The cooling water (32) is supplied from the feed water of the steam turbine cycle. In the gas turbine cooler the water is evaporated, superheated and return to the HP superheater (21).

The air/water-steam cooler has limitations of operation range. For a given bundle geometry the limitations depends on ambient temperature and inlet temperature/pressure of cooling water.

For more information on Air/Water-Steam GT-Cooler (30) see:

 Patent WO 2004/072544 A1: GT Air Cooler

Closed-loop steam cooling

This steam cooling system permits the higher firing temperatures required for increased efficiency. Gas turbine cooling steam is supplied from the steam turbine cycle. The steam cools the gas turbine and return to the steam cycle. The closed loop steam cooling is in the development.
GE Power Systems developed the closed loop steam cooling (H System). The GE's H System permits the higher firing temperatures and designed with the capability to achieve 60% thermal efficiency.
The gas turbine cooling system is integrated with the steam cycle. The supply of cooling steam is from HP steam turbine exhaust. The steam is delivered to the gas turbine stationary parts through casing connections and to the rotor through a conventional gland connection. The cooling steam is returned to the steam cycle at the reheat line.

If the gas turbine steam cooling (H System) of GE Power Systems and the gas turbine vacuum expansion of TPT used in a combined cycle plant, then a net thermal efficiency of over 62% is expected.