In combined cycle power plants the heat rejected by the higher temperature cycle is recovered and used by the lower temperature cycle to produce additional power and gives high efficiency. Some of the combined cycles are:gas-steam, steam-organic fluids and gas-organic fluids
The higher temperature cycle is known as topping cycle whereas the lowering temperature cycle is known as bottoming cycle. The topping cycle can be Diesel, Brighten, and Rankine cycles but the bottom cycle is basically Rankine cycle.
The heat in the exhaust gases of the simple gas turbine plant can be used to generate steam in the waste heat boiler and the steam generated in the boiler is used to drive the steam turbine for generating electrical power.
Combined Cycle Plants Advantages:
Some the advantages of Combined cycle plants compared with conventional reheat cycle (steam turbine) power plants are given below:
- The efficiency of the combined cycle plant is better or higher than the turbine cycle or steam cycle plant. The efficiency of combined cycle power plant will be of the order of about 45 to 50%.
- The capital cost of combined cycle power plant with supplementary firing is slightly higher than the simple gas turbine plant and much below those of the classical steam turbine power plant of the same capacity. This is one of the major advantage of combined cycle power plant.
- The combined cycle power plant is more suitable for rapid start and shutdown than the steam power plant. Therefore these plants accepts load variations quickly and helps in maintaining the stability in the electrical grid.
- The cooling water requirement of the combined cycle plant is much lower than the normal steam turbine power plant having same capacity output. In these type of plants almost 2/3rd of the power is generated by the gas turbine plant which requires very little or no cooling water. Conventional cooling water system (once through) or recycle system is adopted. In case of lack of sufficient water at the site, dry cooling system can also be provided for cooling. Less water requirement for combine cycle plants is also major advantage while selecting the site of the plant.
- The combined system offers self-sustained feature. For example if the plant is shut down due to any fault or maintenance, no start up power supply can be required to start the unit. Gas turbine offers to start the plant and supply power to plant auxiliary loads. Therefore combined cycle plants has advantage of self sustaining feature.
- Simple gas turbine units can be converted to combined cycle operational power plants using gasified coal. The two phase development requires short installation time for peaking power plus the future capability for efficient operation for a base load generation. Only the burners on the gas turbine need to be replaced to accomplish the fuel conversion.
- Combined cycle plants have the advantage of high ratio of power output to the area occupied. Therefore for designing a combined cycle plant space requirement is not a concern
- It has the advantage of low operational personnel.
- Maintenance duration (down time) for combined cycle power plant is significantly low. This improves the plant availability feature
- When the natural gasified coal is used as the fuel, combined cycle greatly reduces CO2 emissions. Also the amount of heat rejected into the surrounding atmosphere (environment) is less than for completing system, because of the higher overall efficiency. When the coal gasification is employed, up to 98% of the sulphur in the coal is recovered. Thus SO2 emissions are quite lesser than the conventional coal fired power plant.
- The amount of solid waste from coal gasification combined cycle is less compared to conventional steam turbine plant.
- The superior efficiency of the combine cycle power plants over conventional reheat cycle plants not only conserves the fossil fuels but also drastically reduces the emissions and waste per unit of electricity generated.