Combined Cycle Power Plant Principle

Combined cycle power plant is a cycle that utilizes exhaust gases from gas turbine (power plant) to heat water in the boiler, in this case is called HRSG (Heat Recovery Steam Generator) and the steam that is produced by HRSG is used to drive steam turbine.

Exhaust gases from gas turbines coming out at pressure and temperature above 500 oC. Exhaust gas cannot be utilized as working fluid if it has low pressure and high temperature (high enthalpy). Regenerator can be used to utilize this exhaust gas by heating gas out of compressor before entering combustion chamber.

Several constraints of regenerator usage as follow:

  1. Regenerator resulting in pressure drop between compressor outlet and combustion chamber inlet which causes the increase in compressor work due to certain turbine inlet pressure. Compressor outlet pressure should be higher.
  2. Regenerator cause rise in back pressure turbine that cause drop in turbine work.
  3. Regenerator difficult to serve high flow rate.

To avoid the matters above, utilization exhaust gases from gas turbine is with HRSG boiler. This can be clearly understood, where the exhaust gases from gas turbine are still contain relatively high energy, which can be utilized as energy source for steam cycle. Therefore, the two cycles can complement each other thermodynamically. Thus it can be combined into one combined cycle power plant consisting of gas turbine and steam turbine that drive each generator separately.

Figure 1: Combined Cycle Power Plant

Combined cycle power plant as shown in Figure 1 above, in addition to producing high efficiency and greater power output, combined cycle is flexible, easily ignited with no full load, suitable for base load operation and turbine has cycle and has high efficiency in wide load area. The disadvantage is associated to is complexity because basically of installation of two combined technologies in complex power plant.

By using combination of recycled gas, two main advantages can be obtained; it can add power and save fuel costs. The addition of electric power without increasing fuel also means it will increase thermal efficiency of system and can be raised from about 24% to 42%. The magnitude of this efficiency improvement depends on temperature of cooling water which is used in the plant and the size of flue gas temperature in power plant. Colder cooling water temperature so will make higher temperature of exhaust gas, furthermore efficiency will also greater.

Another reason election of combined cycle power plant is fast construction time so that when surge in electricity demand will be met within short time, combined cycle power plant can be built gradually. The first stage was built gas generator power plant to meet surging demand, while HRSG and steam generator power plant is built and operated later when the electricity demand has increased. Combined cycle power plant can be operated as generator for peak load and for base load.

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