![]() High temperatures require expensive alloys made from nickel or cobalt, rather than inexpensive steel. High pressure steam requires strong, bulky components. In a thermal power station, water is the working medium. The efficiency of a heat engine, the fraction of input heat energy that can be converted to useful work, is limited by the temperature difference between the heat entering the engine and the exhaust heat leaving the engine. Working principle of a combined cycle power plant (Legend: 1-Electric generators, 2-Steam turbine, 3-Condenser, 4-Pump, 5-Boiler/heat exchanger, 6-Gas turbine) In addition the capital costs of combined cycle power is relatively low, at around $1000/kW, making it one of the cheapest types of generation to install. They gave costs of between 78 and 100 €/MWh for CCGT plants powered by natural gas. 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. Over time the secondary steam cycle will warm up, improving fuel efficiency and providing further power. This avoids the need for separate expensive peaker plants, or lets a ship maneuver. The gas turbine cycle can often start very quickly, which gives immediate power. Multiple stage turbine or steam cycles can also be used, but CCGT plants have advantages for both electricity generation and marine power. Stationary CCGTs burn natural gas or synthesis gas from coal. In contrast, a single cycle steam power plant is limited to efficiencies from 35 to 42%. These achieve a best-of-class real (see below) thermal efficiency of around 64% in base-load operation. This is a combined cycle gas turbine (CCGT) plant. The turbine's hot exhaust powers a steam power plant (operating by the Rankine cycle). In stationary and marine power plants, a widely used combined cycle has a large gas turbine (operating by the Brayton cycle). The approach is called " combined heat and power" (CHP). Vacuum-insulated piping can let this utility reach as far as 90 km. ![]() ![]() However, in cold climates it is common to sell hot power plant water for hot water and space heating. Very low temperature bottoming cycles have been too costly due to the very large sizes of equipment needed to handle the large mass flows and small temperature differences. Historically successful combined cycles have used mercury vapour turbines, magnetohydrodynamic generators and molten carbonate fuel cells, with steam plants for the low temperature "bottoming" cycle. Heat engines can only use part of the energy from their fuel, so in a non-combined cycle heat engine, the remaining heat (i.e., hot exhaust gas) from combustion is wasted. This is more than 84% of the theoretical efficiency of a Carnot cycle. That is, from an overall efficiency of the system of say 34% for a simple cycle, to as much as 64% net for the turbine alone in specified conditions for a combined cycle. Usually the heat passes through a heat exchanger so that the two engines can use different working fluids.īy generating power from multiple streams of work, the overall efficiency can be increased by 50–60%. The principle is that after completing its cycle in the first engine, the working fluid (the exhaust) is still hot enough that a second subsequent heat engine can extract energy from the heat in the exhaust. Combining two or more thermodynamic cycles improves overall efficiency, which reduces fuel costs. The same principle is also used for marine propulsion, where it is called a combined gas and steam (COGAS) plant. On land, when used to make electricity the most common type is called a combined cycle gas turbine ( CCGT) plant. Gateway Generating Station, a 530-megawatt combined cycle natural gas-fired power station in Contra Costa County, California.Ī 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.
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