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.
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.
The main differences between combined cycle steam turbines and conventional steam turbines are:
- 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.
New Procedure for Combined Cycles Power Plants
"Procedure for increase the output and the thermal
efficiency of the combined cycles power plants"
Publication Nr. EP 1 808 588, Date: 18.07.2007
Inventor: Youssef Mustafa (CH)
Applicant: Thermal PowerTec Ltd, Zürich (CH)
The research and the development of the gas turbine and steam turbine cycles led to
a new procedure for the reduction of exhaust steam and exhaust gas losses below the 40% of
the supplied heat flow.
Combined Cycles Power Plant with Vacuum GT Expansion
The procedure concerns an improvement for combined cycles power plants (Fig. 27a. 27b & 27c).
The gas in the gas turbine are expanded on a vacuum pressure,
the gas are cooled by the HRSG, which are arranged in a vacuum container, and compressed by
a gas compressor on the atmospheric pressure. The gas is further-cooled in the HRSG on the
exhaust gas temperature.
In the gas turbine the supplied heat flow is increased by reducing the pressure of the 2.
sequential combustion and the exhaust gas temperature is reduced by the gas expansion into
the vacuum range. These make possible in the steam generator the use of a Single-pressure
cycle with low exhaust gas temperature, in order to achieve minimum exhaust steam and
exhaust gas losses. Thus the thermal efficiency of the combined cycles plant can be
increased by more than 0.02 (i.e. hth net >0.60) and the power
output by more than 18%.
In the gas turbine with vacuum expansion and sequential combustions the pressure of the LP combustion
is reduced. The LP cooling air of the gas turbine taken from the air compressor by the corresbond
pressure has a lower temperature and can be used, without cooling down. So only one gas turbine air
cooler (HP air cooler) is necessary.
Repowering of Existing CCPPs:
CCPPs can repowered purely in order to benefit from the increase of efficiency
and power output even though they are far from the end of their design life. Repowering an
existing CCPP is possible and can be achieved by supplement of a vacuum gas turbine stage
and a gas compressor and by changing a part of the HRSG.
Expected profits in a 300 MW CCPP:
a) A power increase of 16% respectively 48 MW is expected. This can be evaluated with
approx. 30 mil $. These will exceed the construction costs to a large extent.
b) The increase of the thermal efficiency of 2 per cent points results for a power increase
of approx. 10 MW without fuel cost. This can be evaluated with approx. 1.8 mil $/year.
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
particularly in the high temperature of the turbine blades. In order to bear the usual
temperatures of 1000 to 1200°C, 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 on different pressure levels
from the compressor and used for the cooling of combustion chamber and turbine blades.
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
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.
GT-Air Cooler (Air/Water-Steam Heat Exchanger):
The air/water-steam cooler works as a steam generator. The cooling water 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 steam cycle.
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
For more information on Air/Water-Steam GT-Cooler see:
"AIR COOLER FOR POWER STATION PLANT AND USE OF SUCH AN AIR COOLER"
Inventor: YOUSSEF MUSTAFA (CH)
Applicant: ALSTOM TECHNOLOGY LTD (CH)
EP 1590603: GT 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%
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