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A combined cycle power plant is an assembly of
heat engine In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do work (physics), mechanical work. It does this by bringing a working substance from a higher state temperature to ...

heat engine
s that work in tandem from the same source of heat, converting it into
mechanical energy In physical sciences Physical science is a branch of natural science that studies abiotic component, non-living systems, in contrast to life science. It in turn has many branches, each referred to as a "physical science", together called the " ...
. On land, when used to make electricity the most common type is called a combined cycle gas turbine (CCGT) plant. The same principle is also used for marine propulsion, where it is called a
combined gas and steam Image:COGES diagram.svg, left, A turbo-electric COGAS power-plant Combined gas and steam (COGAS) is the name given to Ship transport, marine compound powerplants comprising gas turbine, gas and steam turbines, the latter being driven by steam gener ...
(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 engine, the
working fluid For fluid power Fluid power is the use of fluids under pressure to generate, control, and transmit power. Fluid power is subdivided into hydraulics using a liquid such as mineral oil or water Water is an Inorganic compound, inorgan ...
(the exhaust) is still hot enough that a second subsequent heat engine can extract energy from the heat in the exhaust. Usually the heat passes through a
heat exchanger A heat exchanger is a system used to transfer heat between two or more fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids are a Phase (m ...

heat exchanger
so that the two engines can use different working fluids. By generating power from multiple streams of work, the overall efficiency of the system can be increased by 50–60%. That is, from an overall efficiency of say 34% (for a simple cycle), to as much as 64% (for a combined cycle). This is more than 84% of the theoretical efficiency of a
Carnot cycle
Carnot 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.


Historical cycles

Historically successful combined cycles have used
mercury vapour turbineA mercury vapour turbine is a form of heat engine that uses mercury (element), mercury as the working fluid of its thermal cycle. A mercury vapour turbine has been used in conjunction with a steam turbine for Electrical generator, generating electri ...
s,
magnetohydrodynamic generator A magnetohydrodynamic generator (MHD generator) is a magnetohydrodynamic converter __NOTOC__ A magnetohydrodynamic converter (MHD converter) is an Electromagnetism, electromagnetic machine with no moving parts involving magnetohydrodynamics, the stu ...
s and
molten carbonate fuel cell Molten-carbonate fuel cells (MCFCs) are high-temperature fuel cell A fuel cell is an electrochemical cell An electrochemical cell is a device capable of either generating electrical energy Electrical energy is energy derived from electri ...
s, with steam plants for the low temperature "bottoming" cycle. 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. However, in cold climates it is common to sell hot power plant water for hot water and space heating. Vacuum-insulated piping can let this utility reach as far as 90 km. The approach is called "combined heat and power" (CHP). In stationary and marine power plants, a widely used combined cycle has a large
gas turbine A gas turbine, also called a combustion turbine, is a type of continuous Continuity or continuous may refer to: Mathematics * Continuity (mathematics), the opposing concept to discreteness; common examples include ** Continuous probability di ...
(operating by the
Brayton cycle The Brayton cycle is a thermodynamic cycle named after George Brayton that describes the workings of a constant-pressure heat engine. The original Brayton engines used a piston compressor and piston expander, but more modern gas turbine engines ...

Brayton cycle
). The turbine's hot exhaust powers a
steam power plant A thermal power station is a power station in which heat energy is converted to electricity. Typically, water is heated into steam, which is used to drive an electrical generator. After it passes through the turbine the steam is condensed in a s ...
(operating by the
Rankine cycle The Rankine cycle is an idealized thermodynamic cycle A thermodynamic cycle consists of a linked sequence of es that involve and into and out of the system, while varying pressure, temperature, and other within the system, and that eventually ...
). This is a ''combined cycle gas turbine'' (CCGT) plant. These achieve a best-of-class real (see below)
thermal efficiency In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quan ...
of around 64% in base-load operation. In contrast, a single cycle steam power plant is limited to efficiencies from 35 to 42%. Many new power plants utilize CCGTs. Stationary CCGTs burn
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon gas mixture consisting of methane and commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carbon dioxid ...

natural gas
or
synthesis gas Syngas, or synthesis gas, is a fuel gas Fuel gas is any one of a number of fuels that under ordinary conditions are gaseous. Many fuel gases are composed of hydrocarbons (such as methane or propane), hydrogen, carbon monoxide, or mixtures the ...
from
coal Coal is a combustible , Germany ) , image_map = , map_caption = , map_width = 250px , capital = Berlin , coordinates = , largest_city = capital , languages_type = Official language , languages = German language, German , ...

coal
. Ships burn
fuel oil Fuel oil (also known as heavy oil, marine fuel, bunker, furnace oil, or gasoil) is a fraction A fraction (from Latin ', "broken") represents a part of a whole or, more generally, any number of equal parts. When spoken in everyday English, a f ...
. 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 plant Peaking power plants, also known as peaker plants, and occasionally just "peakers", are power plants that generally run only when there is a high demand, known as peak demand, for electricity. Because they supply power only occasionally, the power s ...
s, 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 The Fraunhofer Institute for Solar Energy Systems ISE (or Fraunhofer ISE) is an institute of the Fraunhofer-Gesellschaft. Located in Freiburg, Germany Freiburg im Breisgau (; abbreviated as Freiburg i. Br. or Freiburg i. B.), commonly referred ...
assessed the
levelised cost of energy The levelized cost of energy (LCOE), or levelized cost of electricity, is a measure of the average net present cost of electricity generation for a generating plant over its lifetime. It is used for investment planning and to compare different me ...
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 costs of combined cycle power is relatively low, at around $1000/kW, making it one of the cheapest types of generation to install.


Basic combined cycle

The
thermodynamic cycle A thermodynamic cycle consists of a linked sequence of thermodynamic process Classical thermodynamics considers three main kinds of thermodynamic process: (1) changes in a system, (2) cycles in a system, and (3) flow processes. (1) A change in ...
of the basic combined cycle consists of two power plant cycles. One is the Joule or
Brayton cycle The Brayton cycle is a thermodynamic cycle named after George Brayton that describes the workings of a constant-pressure heat engine. The original Brayton engines used a piston compressor and piston expander, but more modern gas turbine engines ...

Brayton cycle
which is a
gas turbine A gas turbine, also called a combustion turbine, is a type of continuous Continuity or continuous may refer to: Mathematics * Continuity (mathematics), the opposing concept to discreteness; common examples include ** Continuous probability di ...
cycle and the other is
Rankine cycle The Rankine cycle is an idealized thermodynamic cycle A thermodynamic cycle consists of a linked sequence of es that involve and into and out of the system, while varying pressure, temperature, and other within the system, and that eventually ...
which is a
steam turbine A steam turbine is a machine A machine is any physical system with ordered structural and functional properties. It may represent human-made or naturally occurring device molecular machine that uses Power (physics), power to apply Force, f ...
cycle. The cycle 1-2-3-4-1 which is the
gas turbine power plant A gas turbine, also called a combustion turbine, is a type of Internal combustion engine#Continuous combustion, continuous and internal combustion engine. The main elements common to all gas turbine engines are: * an upstream rotating gas compres ...

gas turbine power plant
cycle is the topping cycle. It depicts the heat and work transfer process taking place in high temperature region. The cycle a-b-c-d-e-f-a which is the Rankine steam cycle takes place at a low temperature and is known as the bottoming cycle. Transfer of heat energy from high temperature
exhaust gas Exhaust gas or flue gas Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler Poland Poland ( pl, Polska ), officially the Republic ...
to water and steam takes place by a
waste heat recovery A waste heat recovery unit (WHRU) is an energy recovery heat exchanger that transfers heat from process outputs at high temperature to another part of the process for some purpose, usually increased efficiency. The WHRU is a tool involved in cogene ...
boiler in the bottoming cycle. During the constant pressure process 4-1 the exhaust gases in the
gas turbine A gas turbine, also called a combustion turbine, is a type of continuous Continuity or continuous may refer to: Mathematics * Continuity (mathematics), the opposing concept to discreteness; common examples include ** Continuous probability di ...
reject heat. The feed water, wet and super heated steam absorb some of this heat in the process a-b, b-c and c-d.


Steam generators

The steam power plant gets its input heat from the high temperature exhaust gases from
gas turbine A gas turbine, also called a combustion turbine, is a type of continuous Continuity or continuous may refer to: Mathematics * Continuity (mathematics), the opposing concept to discreteness; common examples include ** Continuous probability di ...
power plant. The steam generated thus can be used to drive
steam turbine A steam turbine is a machine A machine is any physical system with ordered structural and functional properties. It may represent human-made or naturally occurring device molecular machine that uses Power (physics), power to apply Force, f ...
. The Waste Heat Recovery Boiler (WHRB) has 3 sections: Economiser, evaporator and superheater.


Cheng cycle

The
Cheng cycle The Cheng cycle is a thermodynamic cycle which uses a combination of two working fluids, one gas and one liquid. It can therefore be considered a combination of the Brayton cycle and the Rankine cycle. It was named for Dr. Dah Yu Cheng. The compa ...
is a simplified form of combined cycle where the steam turbine is eliminated by injecting steam directly into the combustion turbine. This has been used since the mid 1970s and allows recovery of waste heat with less total complexity, but at the loss of the additional power and redundancy of a true combined cycle system. It has no additional steam turbine or generator, and therefore it cannot use it as a backup or supplementary power. It is named after American Professor D. Y. Cheng who patented the design in 1976.


Design principles

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. In a
thermal power station A thermal power station is a power station in which heat energy is converted to electricity. Typically, fuel is used to boil water in a large pressure vessel to produce high-pressure steam, which drives a steam turbine connected to an electric ...

thermal power station
, water is the working medium. High pressure steam requires strong, bulky components. High temperatures require expensive alloys made from
nickel Nickel is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

nickel
or
cobalt Cobalt is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical element ...

cobalt
, rather than inexpensive
steel Steel is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appe ...

steel
. These alloys limit practical steam temperatures to 655 °C while the lower temperature of a steam plant is fixed by the temperature of the cooling water. With these limits, a steam plant has a fixed upper efficiency of 35–42%. An open circuit gas turbine cycle has a
compressor A compressor is a mechanical device that increases the pressure Pressure (symbol: ''p'' or ''P'') is the force In physics, a force is an influence that can change the motion (physics), motion of an Physical object, object. A force ...
, a
combustor A combustor is a component or area of a gas turbine A gas turbine, also called a combustion turbine, is a type of continuous Continuity or continuous may refer to: Mathematics * Continuity (mathematics), the opposing concept to discreteness ...
and a
turbine A turbine ( or ) (from the Greek , ''tyrbē'', or Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', ...

turbine
. For gas turbines the amount of metal that must withstand the high temperatures and pressures is small, and lower quantities of expensive materials can be used. In this type of cycle, the input temperature to the turbine (the firing temperature), is relatively high (900 to 1,400 °C). The output temperature of the
flue gas Flue gas is the gas Gas is one of the four fundamental states of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior th ...
is also high (450 to 650 °C). This is therefore high enough to provide heat for a second cycle which uses steam as the working fluid (a
Rankine cycle The Rankine cycle is an idealized thermodynamic cycle A thermodynamic cycle consists of a linked sequence of es that involve and into and out of the system, while varying pressure, temperature, and other within the system, and that eventually ...
). In a combined cycle power plant, the heat of the gas turbine's exhaust is used to generate steam by passing it through a
heat recovery steam generator A heat recovery steam generator (''HRSG'') is an energy recovery heat exchanger that recovers heat from a hot gas stream, such as a combustion turbine or other waste gas stream. It produces steam that can be used in a process (cogeneration) or use ...
(HRSG) with a
live steam Live steam is steam Steam is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for ...
temperature between 420 and 580 °C. The condenser of the Rankine cycle is usually cooled by water from a lake, river, sea or
cooling tower A cooling tower is a heat rejection device that rejects waste heat Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing Work (thermodynamics), work. All such processes give off some w ...

cooling tower
s. This temperature can be as low as 15 °C.


Typical size

Plant size is important in the cost of the plant. The larger plant sizes benefit from
economies of scale In microeconomics Microeconomics is a branch of mainstream economics Mainstream economics is the body of knowledge, theories, and models of economics, as taught by universities worldwide, that are generally accepted by economists as a basis ...

economies of scale
(lower initial cost per kilowatt) and improved efficiency. For large-scale power generation, a typical set would be a 270 MW primary gas turbine coupled to a 130 MW secondary steam turbine, giving a total output of 400 MW. A typical power station might consist of between 1 and 6 such sets. Gas turbines for large-scale power generation are manufactured by at least four separate groups – General Electric, Siemens, Mitsubishi-Hitachi, and Ansaldo Energia. These groups are also developing, testing and/or marketing gas turbine sizes in excess of 300 MW (for 60 Hz applications) and 400 MW (for 50 Hz applications). Combined cycle units are made up of one or more such gas turbines, each with a waste heat steam generator arranged to supply steam to a single or multiple steam turbines, thus forming a combined cycle block or unit. Combined cycle block sizes offered by three major manufacturers (Alstom, General Electric and Siemens) can range anywhere from 50 MW to well over 1300 MW with costs approaching $670/kW.


Unfired boiler

The heat recovery boiler is item 5 in the COGAS figure shown above. Hot gas turbine exhaust enters the
super heater A superheater is a device used to convert saturated steam or wet steam into superheated steam or dry steam. Superheated steam is used in steam turbines for electricity generation, steam engines, and in processes such as steam reforming. There are ...
, then passes through the
evaporator An evaporator is a device used in a process to turn the liquid form of a chemical substance, such as water, into its gaseous form - vapor. In this process, the liquid is evaporated, or vaporized. Uses One example of an evaporator is a radiator ...

evaporator
and finally through the economiser section as it flows out from the
boiler A boiler is a closed vessel in which fluid In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order ...

boiler
. Feed water comes in through the economizer and then exits after having attained saturation temperature in the water or steam circuit. Finally it flows through the evaporator and super heater. If the temperature of the gases entering the heat recovery boiler is higher, then the temperature of the exiting gases is also high.


Dual pressure boiler

In order to remove the maximum amount of heat from the gasses exiting the high temperature cycle, a dual pressure boiler is often employed. It has two
water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known li ...

water
/
steam Steam is water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fl ...

steam
drums. The low-pressure drum is connected to the low-pressure economizer or evaporator. The low-pressure steam is generated in the low temperature zone of the turbine exhaust gasses. The low-pressure steam is supplied to the low-temperature turbine. A super heater can be provided in the low-pressure circuit. Some part of the feed water from the low-pressure zone is transferred to the high-pressure economizer by a booster
pump A pump is a device that moves fluids (liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers to a fluid flow, flow in which the ...

pump
. This economizer heats up the water to its
saturation temperature The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding envir ...
. This saturated water goes through the high-temperature zone of the
boiler A boiler is a closed vessel in which fluid In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order ...

boiler
and is supplied to the high-pressure
turbine A turbine ( or ) (from the Greek , ''tyrbē'', or Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', ...

turbine
.


Supplementary firing

The
HRSG A heat recovery steam generator (''HRSG'') is an energy recovery heat exchanger that recovers heat from a hot gas stream, such as a combustion turbine or other waste gas stream. It produces steam that can be used in a process (cogeneration) or use ...
can be designed to burn supplementary fuel after the gas turbine. Supplementary burners are also called ''duct burners''. Duct burning is possible because the turbine exhaust gas (flue gas) still contains some
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
. Temperature limits at the gas turbine inlet force the turbine to use excess air, above the optimal
stoichiometric Stoichiometry is the calculation of reactants and products in chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical A chemical s ...
ratio to burn the fuel. Often in gas turbine designs part of the compressed air flow bypasses the burner in order to cool the turbine blades. The turbine exhaust is already hot, so a regenerative air preheater is not required as in a conventional steam plant. However, a fresh air fan blowing directly into the duct permits a duct-burning steam plant to operate even when the gas turbine cannot. Without supplementary firing, the ''thermal efficiency'' of a combined cycle power plant is higher. But more flexible plant operations make a marine CCGT safer by permitting a ship to operate with equipment failures. A flexible stationary plant can ''make more money.'' Duct burning raises the flue temperature, which increases the quantity or temperature of the steam (e.g. to 84 bar, 525 degree Celsius). This improves the efficiency of the steam cycle. Supplementary firing lets the plant respond to fluctuations of electrical load, because duct burners can have very good efficiency with partial loads. It can enable higher steam production to compensate for the failure of another unit. Also, coal can be burned in the steam generator as an economical supplementary fuel. Supplementary firing can raise exhaust temperatures from 600 °C (GT exhaust) to 800 or even 1000 °C. Supplemental firing does not raise the efficiency of most combined cycles. For single boilers it can raise the efficiency if fired to 700–750 °C; for multiple boilers however, the flexibility of the plant should be the major attraction. "Maximum supplementary firing" is the condition when the maximum fuel is fired with the oxygen available in the gas turbine exhaust.


Fuel for combined cycle power plants

Combined cycle plants are usually powered by
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon gas mixture consisting of methane and commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carbon dioxid ...

natural gas
, although
fuel oil Fuel oil (also known as heavy oil, marine fuel, bunker, furnace oil, or gasoil) is a fraction A fraction (from Latin ', "broken") represents a part of a whole or, more generally, any number of equal parts. When spoken in everyday English, a f ...
,
synthesis gas Syngas, or synthesis gas, is a fuel gas Fuel gas is any one of a number of fuels that under ordinary conditions are gaseous. Many fuel gases are composed of hydrocarbons (such as methane or propane), hydrogen, carbon monoxide, or mixtures the ...
or other fuels can be used. The supplementary fuel may be natural gas, fuel oil, or coal.
Biofuel Biofuel is fuel that is produced through contemporary processes from biomass, rather than by the very slow geological processes involved in the formation of fossil fuels, such as oil. Since biomass technically can be used as a fuel directly (e. ...

Biofuel
s can also be used. Integrated
solar Solar may refer to: Astronomy * Of or relating to the Sun. ** A solar telescope 175px, The Swedish 1-m Solar Telescope at Roque de los Muchachos Observatory, La Palma in the Canary Islands. A solar telescope is a special purpose telescope used ...

solar
combined cycle power stations combine the energy harvested from solar radiation with another fuel to cut fuel costs and environmental impact (See: ISCC section). Many next generation nuclear power plants can use the higher temperature range of a Brayton top cycle, as well as the increase in thermal efficiency offered by a Rankine bottoming cycle. Where the extension of a gas pipeline is impractical or cannot be economically justified, electricity needs in remote areas can be met with small-scale combined cycle plants using renewable fuels. Instead of natural gas, these gasify and burn agricultural and forestry waste, which is often readily available in rural areas.


Managing low-grade fuels in turbines

Gas turbines burn mainly natural gas and light oil. Crude oil, residual, and some distillates contain corrosive components and as such require fuel treatment equipment. In addition, ash deposits from these fuels result in gas turbine deratings of up to 15%. They may still be economically attractive fuels however, particularly in combined-cycle plants. Sodium and potassium are removed from residual, crude and heavy distillates by a water washing procedure. A simpler and less expensive purification system will do the same job for light crude and light distillates. A magnesium additive system may also be needed to reduce the corrosive effects if vanadium is present. Fuels requiring such treatment must have a separate fuel-treatment plant and a system of accurate fuel monitoring to assure reliable, low-maintenance operation of gas turbines.


Configuration

Combined-cycle systems can have single-shaft or multi-shaft configurations. Also, there are several configurations of steam systems. The most fuel-efficient power generation cycles use an unfired heat recovery steam generator (HRSG) with modular pre-engineered components. These unfired steam cycles are also the lowest in initial cost, and they are often part of a single shaft system that is installed as a unit. Supplementary-fired and multishaft combined-cycle systems are usually selected for specific fuels, applications or situations. For example, cogeneration combined-cycle systems sometimes need more heat, or higher temperatures, and electricity is a lower priority. Multishaft systems with supplementary firing can provide a wider range of temperatures or heat to electric power. Systems burning low quality fuels such as brown coal or peat might use relatively expensive closed-cycle helium turbines as the topping cycle to avoid even more expensive fuel processing and gasification that would be needed by a conventional gas turbine. A typical single-shaft system has one gas turbine, one steam turbine, one generator and one heat recovery steam generator (HRSG). The gas turbine and steam turbine are both coupled in tandem to a single electrical generator on a single shaft. This arrangement is simpler to operate, smaller, with a lower startup cost. Single-shaft arrangements can have less flexibility and reliability than multi-shaft systems. With some expense, there are ways to add operational flexibility: Most often, the operator desires to operate the gas turbine as a peaking plant. In these plants, the steam turbine's shaft can be disconnected with a synchro-self-shifting (SSS) clutch, for start up or for simple cycle operation of the gas turbine. Another less common set of options enable more heat or standalone operation of the steam turbine to increase reliability: Duct burning, perhaps with a fresh air blower in the duct and a clutch on the gas turbine side of the shaft. A multi-shaft system usually has only one steam system for up to three gas turbines. Having only one large steam turbine and heat sink has economies of scale and can have lower cost operations and maintenance. A larger steam turbine can also use higher pressures, for a more efficient steam cycle. However, a multi-shaft system is about 5% higher in initial cost. The overall plant size and the associated number of gas turbines required can also determine which type of plant is more economical. A collection of single shaft combined cycle power plants can be more costly to operate and maintain, because there are more pieces of equipment. However, it can save interest costs by letting a business add plant capacity as it is needed. Multiple-pressure reheat steam cycles are applied to combined-cycle systems with gas turbines with exhaust gas temperatures near 600 °C. Single- and multiple-pressure non-reheat steam cycles are applied to combined-cycle systems with gas turbines that have exhaust gas temperatures of 540 °C or less. Selection of the steam cycle for a specific application is determined by an economic evaluation that considers a plant's installed cost, fuel cost and quality, duty cycle, and the costs of interest, business risks, and operations and maintenance.


Efficiency

By combining both gas and steam cycles, high input temperatures and low output temperatures can be achieved. The efficiency of the cycles add, because they are powered by the same fuel source. So, a combined cycle plant has a thermodynamic cycle that operates between the gas-turbine's high firing temperature and the
waste heat Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing Work (thermodynamics), work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste he ...
temperature from the condensers of the steam cycle. This large range means that the
Carnot efficiency Carnot's theorem, developed in 1824 by Nicolas Léonard Sadi Carnot ''Sous-lieutenant'' Nicolas Léonard Sadi Carnot (; 1 June 1796 – 24 August 1832) was a French mechanical engineer Mechanical may refer to: Machine * Mechanical system, ...

Carnot efficiency
of the cycle is high. The actual efficiency, while lower than the Carnot efficiency, is still higher than that of either plant on its own. The electric efficiency of a combined cycle power station, if calculated as electric energy produced as a percentage of the
lower heating value The heating value (or energy value or calorific value) of a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance, a material with a definite chemical compos ...
of the fuel consumed, can be over 60% when operating new, i.e. unaged, and at continuous output which are ideal conditions. As with single cycle thermal units, combined cycle units may also deliver low temperature heat energy for industrial processes,
district heating District heating (also known as heat networks or teleheating) is a system for distributing heat generated in a centralized location through a system of insulated pipe Insulated pipes (called also preinsulated pipes or bonded pipe ) are widely ...

district heating
and other uses. This is called
cogeneration Cogeneration or combined heat and power (CHP) is the use of a heat engine In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, ...

cogeneration
and such power plants are often referred to as a combined heat and power (CHP) plant. In general, combined cycle efficiencies in service are over 50% on a
lower heating value The heating value (or energy value or calorific value) of a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance, a material with a definite chemical compos ...
and Gross Output basis. Most combined cycle units, especially the larger units, have peak, steady-state efficiencies on the LHV basis of 55 to 59%.


Fuel heating value

Heat engine efficiency can be based on the fuel
Higher Heating Value The heating value (or energy value or calorific value) of a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance, a material with a definite chemical compos ...
(HHV), including
latent heat of vaporisation The enthalpy of vaporization (symbol ), also known as the (latent) heat of vaporization or heat of evaporation, is the amount of energy (enthalpy) that must be added to a liquid substance to transform a quantity of that substance into a gas. Th ...
that would be recuperated in condensing boilers, or the
Lower Heating Value The heating value (or energy value or calorific value) of a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance, a material with a definite chemical compos ...
(LHV), excluding it. The HHV of
methane Methane (, ) is a chemical compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes a ...
is , compared to a LHV: a % increase.


Boosting efficiency

Efficiency of the turbine is increased when combustion can run hotter, so the working fluid expands more. Therefore efficiency is limited by whether the first stage of turbine blades can survive higher temperatures. Cooling and materials research are continuing. A common technique, adopted from aircraft, is to pressurise hot-stage turbine blades with coolant. This is also bled-off in proprietary ways to improve the aerodynamic efficiencies of the turbine blades. Different vendors have experimented with different coolants. Air is common but steam is increasingly used. Some vendors might now utilize single-crystal turbine blades in the hot section, a technique already common in military aircraft engines. The efficiency of CCGT and GT can also be boosted by pre-cooling combustion air. This increases its density, also increasing the expansion ratio of the turbine. This is practised in hot climates and also has the effect of increasing power output. This is achieved by evaporative cooling of water using a moist matrix placed in the turbine's inlet, or by using
Ice storage air conditioning Ice storage air conditioning is the process of using ice for thermal energy storage Thermal energy storage (TES) is achieved with widely different technologies. Depending on the specific technology, it allows excess thermal energy to be store ...
. The latter has the advantage of greater improvements due to the lower temperatures available. Furthermore, ice storage can be used as a means of load control or load shifting since ice can be made during periods of low power demand and, potentially in the future the anticipated high availability of other resources such as renewables during certain periods. Combustion technology is a proprietary but very active area of research, because fuels, gasification and carburation all affect fuel efficiency. A typical focus is to combine aerodynamic and chemical computer simulations to find combustor designs that assure complete fuel burn up, yet minimize both pollution and dilution of the hot exhaust gases. Some combustors inject other materials, such air or steam, to reduce pollution by reducing the formation of nitrates and ozone. Another active area of research is the steam generator for the Rankine cycle. Typical plants already use a two-stage steam turbine, reheating the steam between the two stages. When the heat-exchangers' thermal conductivity can be improved, efficiency improves. As in nuclear reactors, tubes might be made thinner (e.g. from stronger or more corrosion-resistant steel). Another approach might use silicon carbide sandwiches, which do not corrode. There is also some development of modified Rankine cycles. Two promising areas are ammonia/water mixtures, and turbines that utilize supercritical carbon dioxide. Modern CCGT plants also need software that is precisely tuned to every choice of fuel, equipment, temperature, humidity and pressure. When a plant is improved, the software becomes a moving target. CCGT software is also expensive to test, because actual time is limited on the multimillion-dollar prototypes of new CCGT plants. Testing usually simulates unusual fuels and conditions, but validates the simulations with selected data points measured on actual equipment.


Competition

There is active competition to reach higher efficiencies. Research aimed at turbine inlet temperature has led to even more efficient combined cycles. Nearly 60% LHV efficiency (54% HHV efficiency) was reached in the , using a GE H-technology gas turbine with a NEM 3 pressure reheat boiler, using steam from the heat recovery steam generator (HRSG) to cool the turbine blades. In May 2011
Siemens AG Siemens AG ( ) is a German multinational Multinational may refer to: * Multinational corporation, a corporate organization operating in multiple countries * Multinational force, a military body from multiple countries * Multinational state, ...
announced they had achieved a 60.75% efficiency with a 578 megawatt SGT5-8000H gas turbine at the Irsching Power Station. The
Chubu Electric , abbreviated as Chuden in Japanese, is a Japanese electric utilities provider for the middle Chūbu region of the Honshu island of Japan. It provides electricity at 60 Hz, though an area of Nagano Prefecture uses 50 Hz. Chubu Electric ...
’s
Nishi-ku, Nagoya is one of the 16 wards in the city of Nagoya } is the largest city in the Chūbu region of Japan. It is Japan's List of Japanese cities by population, fourth-largest incorporated city and the third most populous urban area. Located on the ...
power plant 405 MW 7HA is expected to have 62% gross combined cycle efficiency. On April 28, 2016, the plant run by
Électricité de France Électricité de France S.A. (literally ''Electricity of France''), commonly known as EDF, is a French multinational corporation, multinational electric utility company, largely owned by the French state. Headquartered in Paris, with €71.2 bil ...
in
Bouchain Bouchain ( vls, Boesem) is a commune A commune is an intentional community of people sharing living spaces, interests, values, beliefs, and often property Property (''latin: Res Privata'') in the Abstract and concrete, abstract is what ...
was certified by Guinness World Records as the worlds most efficient combined cycle power plant at 62.22%. It uses a
General Electric General Electric Company (GE) is an American Multinational corporation, multinational Conglomerate (company), conglomerate incorporated in New York State and headquartered in Boston. Until 2021, the company operated through GE Aviation, aviat ...
9HA, that claimed 41.5% simple cycle efficiency and 61.4% in combined cycle mode, with a gas turbine output of 397 MW to 470 MW and a combined output of 592 MW to 701 MW. Its firing temperature is between , its
overall pressure ratio In aeronautical engineering Aerospace engineering is the primary field of engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicle ...
is 21.8 to 1. In December 2016,
Mitsubishi The is a group of autonomous Japan Japan ( ja, 日本, or , and formally ) is an in . It is situated in the northwest , and is bordered on the west by the , while extending from the in the north toward the and in the south. Japan ...

Mitsubishi
claimed a LHV efficiency of greater than 63% for some members of its J Series turbines. In December 2017, GE claimed 64% in its latest 826 MW HA plant, up from 63.7%. They said this was due to advances in
additive manufacturing 3D printing, or additive manufacturing, is the construction of a three-dimensional object from a CAD Computer-aided design (CAD) is the use of computers (or ) to aid in the creation, modification, analysis, or optimization of a design. T ...
and combustion. Their press release said that they planned to achieve 65% by the early 2020s.


Natural gas integrated power and syngas (hydrogen) generation cycle

A  
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon gas mixture consisting of methane and commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carbon dioxid ...

natural gas
integrated power &
syngas Syngas, or synthesis gas, is a fuel gas Fuel gas is any one of a number of fuels that under ordinary conditions are gaseous. Many fuel gases are composed of hydrocarbons (such as methane or propane), hydrogen, carbon monoxide, or mixtures the ...
(
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
) generation cycle  utilizes semi-closed (sometimes called closed)  
gas turbine A gas turbine, also called a combustion turbine, is a type of continuous Continuity or continuous may refer to: Mathematics * Continuity (mathematics), the opposing concept to discreteness; common examples include ** Continuous probability di ...
cycles where fuel is combusted with pure
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
in a presence of the working fluid of the cycle which is a mix of combustion products and H2O (steam). The integrated cycle implies that, before combustion,
methane Methane (, ) is a chemical compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes a ...
(primer natural gas component) is mixed with working fluid and converted into
syngas Syngas, or synthesis gas, is a fuel gas Fuel gas is any one of a number of fuels that under ordinary conditions are gaseous. Many fuel gases are composed of hydrocarbons (such as methane or propane), hydrogen, carbon monoxide, or mixtures the ...
(mix of H2 and CO) in a catalytic (without an indirect heat supply) reactor by using sensible heat of the hot working fluid leaving, in the simplest case, the
gas turbine A gas turbine, also called a combustion turbine, is a type of continuous Continuity or continuous may refer to: Mathematics * Continuity (mathematics), the opposing concept to discreteness; common examples include ** Continuous probability di ...
outlet. The largest part of produced
syngas Syngas, or synthesis gas, is a fuel gas Fuel gas is any one of a number of fuels that under ordinary conditions are gaseous. Many fuel gases are composed of hydrocarbons (such as methane or propane), hydrogen, carbon monoxide, or mixtures the ...
(about 75%) is directed into the combustion chamber of the gas-turbine cycle to generate power, but another part of syngas (about 25%) is withdrawn from the power generation cycle as
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
,
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom. It is the simplest molecule of the oxocarbon family. In ...

carbon monoxide
, or their blend to produce chemicals,
fertilizer A fertilizer (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American E ...

fertilizer
s,
synthetic fuel Synthetic fuel or synfuel is a liquid fuel Liquid fuels are combustible or energy-generating molecules that can be harnessed to create mechanical energy In physical sciences Physical science is a branch of natural science that studies abio ...
s, etc. The thermodynamic benefit owing to this modification is substantiated by
exergy In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these qua ...
analysis. There are numerous technological options to separate syngas from working fluid and withdraw it from the cycle (e.g., condensing vapors and removing liquids, taking out gases and vapors by
membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecule A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A m ...
and
pressure swing adsorption Pressure swing adsorption (PSA) is a technique used to separate some gas species from a mixture of gases (typically air) under pressure according to the species' molecular characteristics and affinity for an adsorbent of multilayer adsorption ...
separation,
amine gas treating Amine gas treating, also known as amine scrubbing, gas sweetening and acid gas removal, refers to a group of processes that use aqueous solutions of various Amine#Aliphatic amines, alkylamines (commonly referred to simply as amines) to remove hydrog ...
, and glycol dehydration). All the environmental advantages of semi-closed gas turbine cycles as to an absence of and the release of non-diluted (in ) in the flue gas stay the same.  An effect of integration becomes apparent with the following clarification. Assigning the efficiency of syngas production in the integrated cycle a value equal to a regular syngas production efficiency through steam-methane reforming (some part of methane is combusted to drive endothermic reforming), the net-power generation efficiency (with accounting for the consumed electricity required to separate air) can reach levels higher than 60% at a maximum temperature in the cycle (at the gas turbine inlet) of about 1300 °C. The natural gas integrated cycle with adiabatic catalytic reactor was firstly proposed at Chemistry Department of Moscow State Lomonosov University (Russia) in Prof. M. Safonov (late) group by M. Safonov,  M. Granovskii, and S. Pozharskii in 1993.


Integrated gasification combined cycle (IGCC)

An
integrated gasification combined cycle An integrated gasification combined cycle (IGCC) is a technology using a high pressure gasifier to turn coal Coal is a combustible black or brownish-black sedimentary rock, formed as stratum, rock strata called coal seams. Coal is mostly car ...
, or IGCC, is a power plant using synthesis gas (
syngas Syngas, or synthesis gas, is a fuel gas Fuel gas is any one of a number of fuels that under ordinary conditions are gaseous. Many fuel gases are composed of hydrocarbons (such as methane or propane), hydrogen, carbon monoxide, or mixtures the ...
). Syngas can be produced from a number of sources, including coal and biomass. The system uses gas and steam turbines, the steam turbine operating from the heat left over from the gas turbine. This process can raise electricity generation efficiency to around 50%.


Integrated solar combined cycle (ISCC)

An Integrated Solar Combined Cycle (ISCC) is a hybrid technology in which a
solar thermal Solar thermal energy (STE) is a form of energy and a technology Technology ("science of craft", from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officiall ...

solar thermal
field is integrated within a combined cycle plant. In ISCC plants, solar energy is used as an auxiliary heat supply, supporting the steam cycle, which results in increased generation capacity or a reduction of fossil fuel use. Thermodynamic benefits are that daily steam turbine startup losses are eliminated. Major factors limiting the load output of a combined cycle power plant are the allowed pressure and temperature transients of the steam turbine and the heat recovery steam generator waiting times to establish required steam chemistry conditions and warm-up times for the balance of plant and the main piping system. Those limitations also influence the fast start-up capability of the gas turbine by requiring waiting times. And waiting gas turbines consume gas. The solar component, if the plant is started after sunshine, or before, if there is heat storage, allows the preheat of the steam to the required conditions. That is, the plant is started faster and with less consumption of gas before achieving operating conditions. Economic benefits are that the solar components costs are 25% to 75% those of a
Solar Energy Generating Systems Solar Energy Generating Systems (SEGS) is a concentrated solar power plant in California, United States. With the combined capacity from three separate locations at 354 megawatt (MW), it was once the world's List of solar thermal power stations, ...
plant of the same collector surface. The first such system to come online was the
Archimede combined cycle power plant Archimede combined cycle power plant (also known as Centrale a ciclo combinato Archimede, once Centrale Enel Priolo Gargallo) is an integrated solar combined cycle (ISCC) power generation plant at Priolo Gargallo Priolo Gargallo (Sicilian: ''Pri ...
,
Italy Italy ( it, Italia ), officially the Italian Republic ( it, Repubblica Italiana, links=no ), is a country consisting of a peninsula delimited by the Alps The Alps ; german: Alpen ; it, Alpi ; rm, Alps; sl, Alpe ) are the highest ...

Italy
in 2010, followed by Martin Next Generation Solar Energy Center in
Florida Florida is a U.S. state, state located in the Southeastern United States, Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia (U.S. state), Geor ...

Florida
, and in 2011 by the Kuraymat ISCC Power Plant in
Egypt Egypt ( ar, مِصر, Miṣr), officially the Arab Republic of Egypt, is a transcontinental country This is a list of countries located on more than one continent A continent is one of several large landmasses. Generally identi ...

Egypt
, Yazd power plant in
Iran Iran ( fa, ایران ), also called Persia, and officially the Islamic Republic of Iran, is a country in Western Asia Western Asia, West Asia, or Southwest Asia, is the westernmost subregion A subregion is a part of a larger regio ...

Iran
,
Hassi R'mel Hassi R'Mel ( ar, حاسي الرمل, Berber language: Ḥasi Eṛṛmel) is a town in Algeria located near the Hassi R'Mel gas field, 18th largest gas field worldwide. Hassi R'Mel is located in Laghouat Province northwest of Ghardaïa. It is the ...
in
Algeria ) , image_map = Algeria (centered orthographic projection).svg , map_caption = , image_map2 = , capital = Algiers , coordinates = , largest_city = capital , religion = , official_languages = , languages_type = Oth ...

Algeria
, Ain Beni Mathar in
Morocco ) , image_map = Morocco (orthographic projection, WS claimed).svg , map_caption = Location of Morocco in northwest Africa.Dark green: Undisputed territory of Morocco.Lighter green: Western Sahara, a United Nations lis ...

Morocco
. In Australia CS Energy’s Kogan Creek and Macquarie Generation’s
Liddell Power Station Liddell Power Station is a coal-powered thermal power station A thermal power station is a power station in which heat energy is converted to electricity. Typically, fuel is used to boil water in a large pressure vessel to produce high-pre ...

Liddell Power Station
started construction of a solar Fresnel boost section (44 MW and 9 MW), but the projects never became active.


Bottoming cycles

In most successful combined cycles, the bottoming cycle for power is a conventional steam
Rankine cycle The Rankine cycle is an idealized thermodynamic cycle A thermodynamic cycle consists of a linked sequence of es that involve and into and out of the system, while varying pressure, temperature, and other within the system, and that eventually ...
. It is already common in cold climates (such as
Finland Finland ( fi, Suomi ; sv, Finland ), officially the Republic of Finland (; ), is a Nordic country in Northern Europe. It shares land borders with Sweden to the west, Russia to the east, Norway to the north, and is defined by the Gulf of B ...

Finland
) to drive community heating systems from a steam power plant's condenser heat. Such
cogeneration Cogeneration or combined heat and power (CHP) is the use of a heat engine In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, ...

cogeneration
systems can yield theoretical efficiencies above 95%. Bottoming cycles producing electricity from the steam condenser's heat exhaust are theoretically possible, but conventional turbines are uneconomically large. The small temperature differences between condensing steam and outside air or water require very large movements of mass to drive the turbines. Although not reduced to practice, a vortex of air can concentrate the mass flows for a bottoming cycle. Theoretical studies of the
Vortex engine The concept of a vortex engine or atmospheric vortex engine (AVE), independently proposed by Norman Louat and Louis M. Michaud, aims to replace large physical chimneys with a vortex , revealed by colored smoke In fluid dynamics, a vortex (plur ...
show that if built at scale it is an economical bottoming cycle for a large steam Rankine cycle power plant.


See also

* Allam power cycle *
Cheng cycle The Cheng cycle is a thermodynamic cycle which uses a combination of two working fluids, one gas and one liquid. It can therefore be considered a combination of the Brayton cycle and the Rankine cycle. It was named for Dr. Dah Yu Cheng. The compa ...
*
Combined gas and steam Image:COGES diagram.svg, left, A turbo-electric COGAS power-plant Combined gas and steam (COGAS) is the name given to Ship transport, marine compound powerplants comprising gas turbine, gas and steam turbines, the latter being driven by steam gener ...
*
Cost of electricity by source Different methods of electricity generation Electricity generation is the process of generating electric power Electric power is the rate, per unit time, at which electrical energy Electrical energy is energy derived as a result of movem ...

Cost of electricity by source
*
Heat recovery steam generator A heat recovery steam generator (''HRSG'') is an energy recovery heat exchanger that recovers heat from a hot gas stream, such as a combustion turbine or other waste gas stream. It produces steam that can be used in a process (cogeneration) or use ...
*
Hydrogen-cooled turbo generator A hydrogen-cooled turbo generator is a turbo generator A turbo generator set or turbine generator set is the compound of a steam turbine A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do Wor ...
*
Integrated gasification combined cycle An integrated gasification combined cycle (IGCC) is a technology using a high pressure gasifier to turn coal Coal is a combustible black or brownish-black sedimentary rock, formed as stratum, rock strata called coal seams. Coal is mostly car ...


References


Further reading

*Steam & Gas Turbines And Power Plant Engineering ISBN C039000000001, R Yadav., Sanjay., Rajay, Central Publishing House, Allahabad *Applied Thermodynamics , R Yadav., Sanjay., Rajay, Central Publishing House, Allahabad. * *


External links

{{Authority control Thermodynamic cycles Mechanical engineering Power station technology Energy conversion Articles containing video clips