Combined Cycle Power Plant
A combined cycle power plant uses two stages of turbines. A gas turbine converts chemical energy of natural gas, oil or other combustible gasses/liquids to produce a rotational motion and heat. The exhaust gas ist used to produce steam in specialized heat exchangers. The steam drives a steam turbine which produces about 70 percent of the total power. Both turbines are coupled by a clutch or a gear to drive the generator.
Modern combined cycle power plants operated with a mixture of natural gas and oil have efficiencies about 50--60 percent. Compared to a typical modern coal power plant with its 45 percent the efficience of the gas operated power plants is much higher.
Natural gas and oil have a further advantage compared to coal: The specific carbon dioxide emissions are much lower. Specific emissions are calculated as grams carbon dioxide per kilowatt hour of electricity.
The natural gas reserves and ressources are limited. This reduces the potential of this type of power plants. An alternative way to get the high efficiency but to use coal is a power plant, where a combustible gas is produced by coal gasification. This process might be equipped with a carbon dioxide sequestration facility which keeps this green house gas away from the atmosphere and stores it underground.