Combined cycle power plants

 What is a combined cycle power plant?

The combined cycle power plant is one where electricity is generated through the joint use of two turbines:

  • A gas turbogroup

  • A steam turbogroup

That is, for the transformation of the energy of the fuel into electricity, two cycles are superimposed:

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

  • The Rankine cycle (steam turbine): where the consumption of heat is related to the production of work or the creation of energy from water vapor.

central térmica de ciclo combinado de fuerteventura

Advantages of the Combined Cycle

The main characteristics of combined cycle power plants are:

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

  • High efficiency. The combined cycle provides greater efficiency for a wider range of powers.

  • Its emissions are lower than in conventional thermal power plants.

  • Low investment cost per MW installed.

  • Short construction periods.

  • Less surface area per MW installed if we compare it with conventional thermoelectric plants (which reduces the visual impact).

  • Low consumption of cooling water.

  • Energy savings in the form of fuel

Fundamental parts of a combined cycle plant

To understand the operation of a combined cycle power plant, you must first know the parts that make it up:

  • Gas turbine. Which consists of:

    • Compressor, whose function is to inject pressurized air for gas combustion and cooling of hot areas.

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

    • Gas turbine, where the expansion of gases coming from the combustion chamber occurs. It consists of three or four expansion stages and the temperature of the gases at the inlet is around 1,400ºC leaving the turbine at temperatures above 600ºC. 

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

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

It is very common for the gas turbine and the steam turbine to be coupled to the same shaft in such a way that they drive the same electrical generator.

Operation of a combined cycle plant

First, the air is compressed at high pressure in the compressor, passing to the combustion chamber where it is mixed with the fuel.

Next, the combustion gases pass through the gas turbine where they expand and their heat energy is transformed into mechanical energy, transmitting it to the shaft.

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

At the exit of the turbine, the steam condenses (returning to water) and returns to the boiler to start a new steam production cycle.

Currently the trend is to couple the gas turbine and the steam turbine to the same shaft, so that they jointly drive the same electric generator.

Environmental impacts of combined cycle plants

The use of natural gas for the generation of electricity through combined cycle technology is within the environmental policy of a large number of countries, since it offers a large number of advantages compared to other electricity production technologies.

Specifically, CO2 emissions in relation to the kWh produced are less than half the emissions of a conventional coal-fired power plant.

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