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Recursos educativos

Combined cycle thermal power stations

What is a combined cycle thermal power stations?

 The combined cycle thermal power stations are those where electricity is generated by means of the use in tandem of two turbines:

  • A gas turbine.

  • A steam turbine.

That is to say, for the transformation of the energy of the fuel into electricity two cycles work in tandem:

  • The Brayton cycle (gas turbine): taking air directly from the atmosphere and it is subjected to heating and compression to use it as mechanical or electric energy.

  • The Ranking cycle (steam turbine): where the consumption of heat is related to the production of effort or generation of energy from steam.

central térmica de ciclo combinado de fuerteventura

 

 

 

 

dvantages of the Combined Cycle

   The main characteristics of the combined cycle thermal power stations are the following:

  • Flexibility. The station can operate at full or partial load, up to a minimum of approximately 45% of the maximum power.

  • High efficiency. The combined cycle provides a higher level of efficiency over a wider range of powers.

  • Its emissions are lower than those of the conventional thermal power stations.

  • Low investment cost per MW installed.

  • Short building periods.

  • Less surface area per MW installed when compared to those of the conventional thermoelectric power stations (reducing the visual impact).

  • Low consumption of cooling water.

  • Energy saving in fuel.

3. Essential parts of a combined cycled power station

In order to understand the operation of a combined cycle thermal power station we must first learn about the parts making it up:

  • Gas turbine. Made up of a:
    • Compressor, whose function is to inject compressed air for the combustion of the gas and cooling of the warm areas.

    • Combustion chamber, where the natural gas is mixed (fuel) with the compressed air, producing the combustion.

    • Gas turbine, where the expansion of the gases coming from the combustion chamber takes place.

  • It has three or four expansion stages and the temperature of the gases at the input is at around 1,400ºC leaving the turbine at temperatures of more than 600ºC.

  • Recovery boiler. In this conventional boiler, the heat of the gases coming from the gas turbine is used in a waste-steam cycle.

  • Steam turbine. This turbine usually has three bodies and it is based on conventional technology.

 It is very usual for the gas turbine and the steam turbine to be coupled in tandem to the same shaft in such a way they operate a same electricity generator.

 

 

 

 

 

 

 

 

 

 

Operation of a combined cycle power station

In the first place, the air is compressed at high pressure in the compressor, going to the combustion chamber where it is mixed with the fuel.

Then the combustion gases go through the gas turbine where they expand and their calorific energy is transformed in mechanical energy, transmitting it to the shaft.

The gases coming from the gas turbine are taken to a heat recovery boiler to produce steam, and from this moment we have a conventional water-steam cycle.

 At the output of the turbine the steam is condensed (becoming water again) and returns to the boiler to start the new steam generation cycle again. Currently the tendency is to couple the gas turbine and the steam turbine in tandem to a same shaft in such a way they jointly operate a same electricity generator.

Environmental impacts of the combined cycles power stations

The use of natural gas for the generation of electricity by means of the combined cycle technology is included in the environmental policy of a large number of countries, as it provides a large number of advantages when compared to the alternative energy production technology.

Specifically, the CO 2 emissions with regards to the kWh produced are less than half of the emissions of a conventional coal power station.