In a combined cycle power plant (CCPP), or combined cycle gas turbine (CCGT) plant, a gas turbine generator generates electricity and the waste heat is used to make steam to generate additional electricity via a steam turbine; this last step enhances the efficiency of electricity generation. The combined-cycle unit combines the Rankine (steam turbine) and Brayton (gas turbine) thermodynamic cycles by using heat recovery boilers to capture the energy in the gas turbine exhaust gases for steam production to supply a steam turbine as shown in the figure "Combined-Cycle Cogeneration Unit". Process steam can be also provided for industrial purposes. The thermal efficiency of a combined cycle power plant is the net power output of the plant divided by the heating value of the fuel. If the plant produces only electricity, efficiencies of up to 60% can be achieved. In the case of combined heat and power generation, the Energy Utilisation Factor (overall efficiency) can increase to 85%.
These plants are generally require high capital expenditure but lower operation and maintenance cost.
Simple Cycle Power Plants (Open Cycle)
A Simple Cycle Combustion Turbine (SCCT) is a type of gas turbine most frequently used in the power industry. It differs from a combined cycle machine in that it has no provision for waste heat recovery.
The modern power gas turbine is a high-technology package that is comprised of a compressor, combustor, power turbine, and generator, as shown in the figure "Simple-Cycle Gas Turbine".
In a gas turbine, large volumes of air are compressed to high pressure in a multistage compressor for distribution to one or more combustion gases from the combustion chambers power an axial turbine that drives the compressor and the generator before exhausting to atmosphere. In this way, the combustion gases in a gas turbine power the turbine directly, rather than requiring heat transfer to a water/steam cycle to power a steam turbine, as in the steam plant. The latest gas turbine designs use turbine inlet temperatures of 1,500C (2,730F) and compression ratios as high as 30:1 (for aeroderivatives) giving thermal efficiencies of 35 percent or more for a simple-cycle gas turbine.
SCCTs require smaller capital investment than either coal or nuclear power plants and can be scaled to generate small or large amounts of power. Also, the actual construction process can take as little as several weeks to a few months, compared to years for base load power plants.
Reciprocating Engines
Small electricity generators are often powered by reciprocating engines burning diesel, biogas or natural gas. Biogas is often combusted where it is produced, such as a landfill or wastewater treatment plant, with a reciprocating engine or a microturbine, which is a small gas turbine. The reciprocating engine is widely available today. It is the most commonly used technology for distributed generation. The technology is mature, and reciprocating engines are manufactured inexpensively in large quantities. IC engine generators for distributed power applications, commonly called gensets, are found universally in sizes from less than 5 kW to over 7 MW.
In a combined cycle power plant (CCPP), or combined cycle gas turbine (CCGT) plant, a gas turbine generator generates electricity and the waste heat is used to make steam to generate additional electricity via a steam turbine; this last step enhances the efficiency of electricity generation. The combined-cycle unit combines the Rankine (steam turbine) and Brayton (gas turbine) thermodynamic cycles by using heat recovery boilers to capture the energy in the gas turbine exhaust gases for steam production to supply a steam turbine as shown in the figure "Combined-Cycle Cogeneration Unit". Process steam can be also provided for industrial purposes. The thermal efficiency of a combined cycle power plant is the net power output of the plant divided by the heating value of the fuel. If the plant produces only electricity, efficiencies of up to 60% can be achieved. In the case of combined heat and power generation, the Energy Utilisation Factor (overall efficiency) can increase to 85%. 
A Simple Cycle Combustion Turbine (SCCT) is a type of gas turbine most frequently used in the power industry. It differs from a combined cycle machine in that it has no provision for waste heat recovery.
Small electricity generators are often powered by reciprocating engines burning diesel, biogas or natural gas. Biogas is often combusted where it is produced, such as a landfill or wastewater treatment plant, with a reciprocating engine or a microturbine, which is a small gas turbine. The reciprocating engine is widely available today. It is the most commonly used technology for distributed generation. The technology is mature, and reciprocating engines are manufactured inexpensively in large quantities. IC engine generators for distributed power applications, commonly called gensets, are found universally in sizes from less than 5 kW to over 7 MW.