Combined cycle power plants

A combined-cycle power plant is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy. On land, when used to make electricity the most common type is called a combined-cycle gas turbine (CCGT) plant, which is a kind of gas-fired power plant. The same principle is also used for marine propulsion, where it is called a combined gas and steam (COGAS) plant. Combining two or more thermodynamic cycles improves overall efficiency, which reduces fuel costs. The principle is that after completing its cycle in the first (usually gas turbine) engine, the working fluid (the exhaust) is still hot enough that a second subsequent heat engine can extract energy from the exhaust. Usually the heat passes through a heat exchanger so that the two engines can use different working fluids. By generating power from multiple streams of work, the overall efficiency can be increased by about 50–60%. That is, from an overall efficiency of say 43% for a simple cycle with the turbine alone running, to as much as 64% net with the full combined cycle running. Multiple-stage turbine or steam cycles can also be used, but CCGT plants have advantages for both electricity generation and marine power. The gas turbine cycle can often start very quickly, which gives immediate power. This avoids the need for separate expensive peaker plants, or lets a ship maneuver. Over time the secondary steam cycle will warm up, improving fuel efficiency and providing further power. In November 2013, the Fraunhofer Institute for Solar Energy Systems ISE assessed the levelised cost of energy for newly built power plants in the German electricity sector. They gave costs of between 78 and €100 /MWh for CCGT plants powered by natural gas. In addition the capital cost of combined-cycle power is relatively low, at around $1000/kW, making it one of the cheapest types of generation to install.