19-09-2012, 01:35 PM
GAS-FIRED COMBINED-CYCLE POWER PLANTS
Gas_Fired_Combined_Cycle_Power_PlantEN.pdf (Size: 464.4 KB / Downloads: 78)
AT A GLANCE
A gas-fired combined-cycle power plant, also known as a Combined Cycle Gas Turbine
Power Plant, combines the strengths of two thermal processes in ideal fashion:
electricity production using a gas turbine together with a steam turbine. The acronym
normally used to describe this system is CCGT.
Around two thirds of the electrical power generated is produced by the gas turbine.
In a similar way as an aeroplane engine, a mixture of compressed air and fuel is combusted.
The hot gases that this process creates drive the turbine and, with it, the generator
that is coupled to it.
The rest of the electrical power generated, roughly a third, is produced by the steam
turbine using the hot exhaust gases leaving the gas turbine. In the heat recovery steam
generator (HRSG) the exhaust gases transfer their heat to the circulating water: the
pressurised water vaporises, causing the temperature in the system to rise. The steam
drives the steam turbine and, with it, the generator that is coupled to it.
Gas-fired combined-cycle power plants are technologically advanced and used
throughout the world. Compared with other types of power plant, they are highly efficient:
the state-of-the-art EGL plants in Italy achieve a percentage in the region of 56,
meaning that the energy supplied in the form of fuel is converted into electricity as
efficiently as technically possible.
Investment costs are comparatively low because the main components are largely
standardised. The extremely powerful gas turbine allows for a compact power plant,
which minimises construction time to around two and a half years.
The gas turbine: the heart of the power plant
The gas turbine is the first stage in the process of producing
electricity. The gas turbine compressor draws in air
from the environment via a filter (1. on the model). This
air is compressed in the compressor (2. on the model),
which means that it is elevated to a higher pressure, and
then directed into the combustion chamber. Fuel is fed
into this chamber in the form of natural gas, and combustion
takes place. This process produces hot gases that
are allowed to “relax” in the turbine, which means that
they are brought to virtually ambient pressure. The gas
spreads out and expands. The energy that this releases
is converted into a mechanical rotation just like a toy
balloon when air is escaping from it. Here too the pressure
compensation (expansion) brings about the motion,
the action of the balloon “darting off”. The mechanical
rotation powers the compressor and the generator. The
generator converts this energy into electricity.
The high-voltage system
The alternating current generated in the plant cannot be
stored. Power plants therefore need an electrical system
that reliably conveys the electricity produced to consumers.
The power plant’s own transformers convert the
electricity produced so that it can be fed directly into the
high-voltage network. In the event of a disruption to the
high-voltage network, the plant automatically begins to
run down in a safe mode. As soon as the electricity grid
is available again, and the corresponding demand exists,
the power plant automatically connects to the grid again
and supplies the required output.
The issue of efficiency
A plant’s availability is the key to its cost efficiency: to
what extent is the power plant ready for operation (after
scheduled and unscheduled downtimes)? Gas-fired combined-
cycle power plants can achieve very high values upwards
of 95 percent if maintained well. This means that
they are connected to the grid roughly 345 days of the
year on average.
Compared with other types of power plant, gas-fired
combined-cycle power plants boast a high level of efficiency.
They convert the energy introduced in the form of
natural gas into electricity with the least amount of loss
that is technically feasible. It is not possible to achieve
zero loss given that friction and heat transfer always
cause some energy to be lost. The gas turbine manages
a level of efficiency of around 35 percent. The water/
steam cycle coupled to it significantly raises the efficiency
for the entire plant by using the heat energy emitted
by the exhaust gases from the gas turbine to produce
steam and, therefore, to generate electricity.
Cost matters
The investment costs for a gas-fired combined-cycle
power plant are lower than those for coal-fired power
plants and other conventional thermal plants, standing at
around 0.6 million euros per megawatt of installed capacity,
whereas for coal-fired power plants, for example,
twice that amount can be expected. The reasons for this
are the largely standardised main components of this type
of power plant and the short construction time of just two
and a half years or so. Depending on the design of the
plant, it is even possible to go into operation with just the
gas turbine first and to complete the water/steam cycle
in parallel.