Gas and Steam Turbine Power Plants
New Procedure for Increase of Efficiency & Power
developed by TPT
Combined Cycles Power Plant CCPP
Today the CCPP is the most used power plants. CCPPs use a combination of two thermodynamic
cycles: the gas turbine cycle (Brayton cycle) operating in a high-temperature and
the steam turbine cycle (Rankine cycle) in a low-temperature range by using steam
production in a heat recovery steam generator (HRSG). The combined cycle concept exploit the
high-temperature potential of modern gas turbines and the low-temperature (cold end) of the
The combined cycle power plant offers high thermal efficiency, low emissions,
low installed cost, flexibility in fuel selection and low operation and maintenance cost.
CCPPs are suitable for daily cycling operation due to short start-up times and for continuous
base load operation. Part load efficiencies are also high due to the control of the gas turbine
inlet mass flow using inlet adjustable vanes.
Figure 045a shows a standard HRSG with a triple water-steam pressure.
CCPP can be cooled by a cooling tower, a direct-cooling system or air-cooled condensers
ensuring a wide range of applications. Where water is scarce, CCPP are advantageous because
the cooling requirement is low due to the fact that the main coling requirement applies only
to the steam process (33% of the supplied heat flow or 57% of total output).
The fuel flexibility of CCPP in limited to gases and some oils. The fuels that can be fired
are those which are widely available in most parts of the world.
Main differences between combined cycle steam turbines and conventional steam turbines; in combined
cycle steam turbin:
- fewer or even no steam extractions for the feed water heating
- shorter start-up times
- lower live-steam pressures, 100 to 160 bar (160 to 300 bar by STPP)
Today the net thermal efficiency of the combined cycles power plants lie between 0.56 and 0.58.
The losses by the exhaust gases and condensation of the exhaust steam are still relative
high and lie between 42% and 44% of the supplied heat flow.
The typical gas and steam temperature profiles in a HRSG consisting of a superheater, evaporator,
and economizer operating at a single pressure. Because the gas temperature entering the HRSG is low
(480 - 565 °C), the steam generation will also be lower than in conventional steam generators for
the same gas flow. The economizer duty in the HRSG will also be low, leading to a high exit gas
temperature. Also the effect of steam pressure is significant - the higher the steam pressure, the
higher the exit gas temperature from the evaporator and the lower the steam generation rate, leading
to a smaller duty in the economizer and a higher exit gas temperature. This is the reason for
considering multiple-pressure units.
Important tasks take place in the CCPP, which are especially handled by TPT and are at
research and development.
The following aspects can be handled by TPT:
- Optimization of thermodynamic design of GT air coolers
- Equalization of the water flow distribution in the parallel evaporator tubes
- Flow stability in the parallel evaporator tubes
- Behavior and performance of GT air coolers at different loads
- Failure analysis & reliability/availability for improvement of existing heat exchangers