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.
Historical cycles 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. 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.
In stationary and marine power plants, a widely used combined cycle has a large gas turbine (operating by the Brayton cycle). The turbine's hot exhaust powers a steam power plant (operating by the Rankine cycle). This is a combined-cycle gas turbine (CCGT) plant. These achieve a best-of-class real thermal efficiency (noted below) 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 or synthesis gas from coal. Ships burn fuel oil.
Basic combined cycle
The thermodynamic cycle of the basic combined cycle consists of two power-plant cycles. One is the Joule, or Brayton, cycle, which is a gas-turbine cycle, and the other is the Rankine cycle, which is a steam-turbine cycle. The cycle 1–2–3–4–1, which is the gas turbine power plant cycle, is the topping cycle. It depicts the heat- and work-transfer process taking place in the high-temperature region.
The cycle a–b–c–d–e–f–a, which is the Rankine steam cycle, takes place at a lower temperature and is known as the bottoming cycle. Transfer of heat energy from high-temperature exhaust gas to water and steam takes place in a waste-heat recovery boiler in the bottoming cycle. During the constant-pressure process 4–1, the exhaust gases from the gas turbine reject heat. The feed water and wet and superheated steam absorb some of this heat in the process a–b, b–c and c–d.
Steam generators
The steam power plant takes its input heat from the high-temperature exhaust gases from a gas-turbine power plant. The steam thus generated can be used to drive a steam turbine. The waste-heat recovery boiler (WHRB) has three sections: economiser, evaporator and superheater.
Cheng cycle The Cheng cycle 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 be used for 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, water is the working medium. High-pressure steam requires strong, bulky components. High temperatures require expensive alloys made from nickel or cobalt, rather than inexpensive 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 combustor and a turbine. For gas turbines the amount of metal that must withstand the high temperatures and pressures is small, so 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 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).
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 (HRSG) with a live steam temperature between 420 and 580°C. The condenser of the Rankine cycle is usually cooled by water from a lake, river, sea or cooling towers. This temperature can be as low as 15°C.
Typical size Plant size is an important factor in overall cost. The larger plant sizes benefit from economies of scale (lower initial cost per output power) 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, providing a total output of 400 MW. A typical power station might consist of between one and six 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 or marketing gas turbines 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 superheater, then passes through the evaporator and, finally, through the economiser section as it flows out from the 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 superheater. 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, boilers with two water/steam drums are often employed.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 superheater 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. This economizer heats the water to its saturation temperature. This saturated water goes through the high-temperature zone of the boiler and is supplied to the high-pressure turbine.
Supplementary firing The HRSG 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. Temperature limits at the gas turbine inlet force the turbine to use excess air, above the optimal stoichiometric 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.
Combined-cycle advanced Rankine subatmospheric reheating
Fuel for combined-cycle power plants Combined-cycle plants are usually powered by natural gas, although fuel oil, synthesis gas or other fuels can be used. The supplementary fuel may be natural gas, fuel oil, or coal. Biofuels can also be used. Integrated 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