Combined Cycle Power Plant Explained

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What is a combined cycle power plant?

A combined cycle power plant (CCPP) is a type of power plant that combines a gas turbine and a steam turbine to generate electricity. The gas turbine is used to generate electricity directly, and the exhaust gases from the gas turbine are used to generate steam in a heat recovery steam generator (HRSG). The produced steam is then used to drive a steam turbine, which also generates electricity.

Combined cycle power plants utilise both the Rankine thermodynamic cycle, and Brayton thermodynamic cycle, hence, they are 'combined cycle' power plants. The Rankine cycle describes the thermodynamic properties of a heat engine that uses a fluid as the working fluid (typically steam), whilst the Brayton cycle describes the thermodynamic cycle of a heat engine that uses a gas as the working fluid.

Combined Cycle Power Plant Schematic

Combined cycle power plants are more efficient than traditional power plants that use only gas turbines or steam turbines; this is because combined cycle power plants utilise the heat from a gas turbine's exhaust gases to generate steam, consequently reducing wasted energy.

Combined Cycle Efficiency (Brayton and Rankine Cycle in Series)

Good to know - a 'power plant' and 'power station' are the same thing, only the wording is different.

 

What are the main parts of a combined cycle power plant?

The main parts of a CCPP are:

  • Gas Turbine
  • HRSG
  • Steam Turbine

 

How Combined Cycle Power Plants Work

The following is a simplified overview of how a combined cycle power plant works:

  1. Air is drawn into the gas turbine compressor.

  1. The air is compressed and then mixed with fuel in the combustor section of the gas turbine.

  1. The fuel-air mixture is ignited, and the hot exhaust gases are directed to the turbine section of the gas turbine.

  2. The turbine section rotates, driving the gas turbine generator and producing electricity.

Gas Turbine (Combustion Turbine)

  1. The exhaust gases from the gas turbine are directed to a HRSG.

  2. As the exhaust gases pass through the HRSG, their heat passes to the HRSG pipes, which are full of water. The water is heated and turns to steam.

  3. The produced steam is directed to a steam turbine.

  4. The steam turbine rotates, driving a generator, thus producing electricity.

  5. Exhaust steam from the steam turbine is condensed in a condenser and returned to the HRSG.

 

Note that both the gas turbine and steam turbine are used to generate electricity. It is however the gas turbine that produces electricity and the heat required to make steam, which is then used to drive the steam turbine.