The steam-electric power station is a

power station A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the generation of electric power. Power stations are generally connected to an electrical grid. Many pow ...

in which the electric generator is steam driven. Water is heated, turns into steam and spins a

steam turbine A steam turbine is a machine that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Charles Parsons in 1884. Fabrication of a modern steam turbin ...

which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser. The greatest variation in the design of steam-electric power plants is due to the different fuel sources. Almost all

coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when dead ...

nuclear Nuclear may refer to: Physics Relating to the nucleus of the atom: *Nuclear engineering *Nuclear physics *Nuclear power *Nuclear reactor *Nuclear weapon *Nuclear medicine *Radiation therapy *Nuclear warfare Mathematics *Nuclear space *Nuclear ...

, geothermal, solar thermal electric power plants, waste incineration plants as well as many natural gas power plants are steam-electric.

Natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon di ...

is frequently combusted in gas turbines as well as

boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central h ...

s. The waste heat from a gas turbine can be used to raise steam, in a

combined cycle 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 turb ...

plant that improves overall efficiency. Worldwide, most electric power is produced by steam-electric power plants. The only widely used alternatives are

photovoltaic Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The photovoltaic effect is commercially u ...

s, direct mechanical power conversion as found in hydroelectric and wind turbine power as well as some more exotic applications like tidal power or wave power and finally some forms of geothermal power plants. Niche applications for methods like

betavoltaics A betavoltaic device (betavoltaic cell or betavoltaic battery) is a type of nuclear battery which generates electric current from beta particles (electrons) emitted from a radioactive source, using semiconductor junctions. A common source used is ...

or chemical power conversion (including

electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...

) are only of relevance in batteries and atomic batteries. Fuel cells are a proposed alternative for a future

hydrogen economy The hydrogen economy is using hydrogen to decarbonize economic sectors which are hard to electrify, essentially, the "hard-to-abate" sectors such as cement, steel, long-haul transport etc. In order to phase out fossil fuels and limit climate ch ...

History

Reciprocating steam engines have been used for mechanical power sources since the 18th Century, with notable improvements being made by

James Watt James Watt (; 30 January 1736 (19 January 1736 OS) – 25 August 1819) was a Scottish inventor, mechanical engineer, and chemist who improved on Thomas Newcomen's 1712 Newcomen steam engine with his Watt steam engine in 1776, which was fu ...

. The very first commercial central electrical generating stations in New York and London, in 1882, also used reciprocating steam engines. As generator sizes increased, eventually turbines took over due to higher efficiency and lower cost of construction. By the 1920s any central station larger than a few thousand kilowatts would use a turbine prime mover.

Efficiency

The efficiency of a conventional steam-electric power plant, defined as energy produced by the plant divided by the heating value of the fuel consumed by it, is typically 33 to 48%, limited as all heat engines are by the laws of

thermodynamic Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of ther ...

s (See:

Carnot cycle A Carnot cycle is an ideal thermodynamic cycle proposed by French physicist Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. By Carnot's theorem, it provides an upper limit on the efficiency of any classical thermodyna ...

). The rest of the energy must leave the plant in the form of heat. This

waste heat Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utility ( ...

can be removed by cooling water or in

cooling tower A cooling tower is a device that rejects waste heat to the atmosphere through the cooling of a coolant stream, usually a water stream to a lower temperature. Cooling towers may either use the evaporation of water to remove process heat and c ...

s. (

cogeneration Cogeneration or combined heat and power (CHP) is the use of a heat engine or power station to generate electricity and useful heat at the same time. Cogeneration is a more efficient use of fuel or heat, because otherwise- wasted heat from elec ...

uses the waste heat for

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 pipes for residential and commercial heating requirements such as space heating ...

). An important class of steam power plants is associated with

desalination Desalination is a process that takes away mineral components from saline water. More generally, desalination refers to the removal of salts and minerals from a target substance, as in soil desalination, which is an issue for agriculture. Saltw ...

facilities, which are typically found in desert countries with large supplies of

natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon di ...

. In these plants freshwater and electricity are equally important products. Since the efficiency of the plant is fundamentally limited by the ratio of the absolute temperatures of the steam at turbine input and output, efficiency improvements require use of higher temperature, and therefore higher pressure, steam. Historically, other working fluids such as mercury have been experimentally used in a

mercury vapour turbine A mercury vapour turbine is a form of heat engine that uses mercury as the working fluid of its thermal cycle. A mercury vapour turbine has been used in conjunction with a steam turbine for generating electricity. This example of combined cycle ...

power plant, since these can attain higher temperatures than water at lower working pressures. However, poor heat transfer properties and the obvious hazard of toxicity have ruled out mercury as a working fluid. Another option is using a supercritical fluid as a working fluid. Supercritical fluids behave similar to gases in some respects and similar to liquids in others.

Supercritical water Supercritical water oxidation (SCWO) is a process that occurs in water at temperatures and pressures above a mixture's thermodynamic critical point. Under these conditions water becomes a fluid with unique properties that can be used to advantag ...

or supercritical carbon dioxide can be heated to much higher temperatures than are achieved in conventional steam cycles thus allowing for higher thermal efficiency. However, these substances need to be kept at high pressures (above the

critical pressure In thermodynamics, a critical point (or critical state) is the end point of a phase equilibrium curve. The most prominent example is the liquid–vapor critical point, the end point of the pressure–temperature curve that designates conditions ...

) to maintain supercriticality and there are issues with corrosion.

Components Of Steam plant

Condenser

Steam-electric power plants use a

surface condenser A surface condenser is a water-cooled shell and tube heat exchanger installed to condense exhaust steam from a steam turbine in thermal power stations. These condensers are heat exchangers which convert steam from its gaseous to its liquid state ...

cooled by water circulating through tubes. The steam which was used to turn the

turbine A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced by a turbine can be used for generating e ...

is exhausted into the condenser and is condensed as it comes in contact with the tubes full of cool circulating water. The condensed steam, commonly referred to as

condensate Condensate may refer to: * The liquid phase produced by the condensation of steam or any other gas * The product of a chemical condensation reaction, other than water * Natural-gas condensate, in the natural gas industry * ''Condensate'' (album) ...

. is withdrawn from the bottom of the condenser. The adjacent image is a diagram of a typical surface condenser.Energy savings in steam systems
''Figure 3a, Layout of surface condenser'' (scroll to page 11 of 34 pdf pages) For best efficiency, the temperature in the condenser must be kept as low as practical in order to achieve the lowest possible pressure in the condensing steam. Since the condenser temperature can almost always be kept significantly below 100 °C where the

vapor pressure Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases ...

of water is much less than atmospheric pressure, the condenser generally works under vacuum. Thus leaks of non-condensable air into the closed loop must be prevented. Plants operating in hot climates may have to reduce output if their source of condenser cooling water becomes warmer; unfortunately this usually coincides with periods of high electrical demand for

air conditioning Air conditioning, often abbreviated as A/C or AC, is the process of removing heat from an enclosed space to achieve a more comfortable interior environment (sometimes referred to as 'comfort cooling') and in some cases also strictly controlling ...

. If a good source of cooling water is not available,

s may be used to reject waste heat to the atmosphere. A large river or lake can also be used as a ''heat sink'' for cooling the condensers; temperature rises in naturally occurring waters may have undesirable ecological effects, but may also incidentally improve yields of fish in some circumstances.

Feedwater heater

In the case of a conventional steam-electric power plant using a drum boiler, the surface condenser removes the

latent heat of vaporization 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. T ...

from the steam as it changes states from vapor to liquid. The condensate pump then pumps the condensate water through a

feedwater heater A feedwater heater is a power plant component used to pre-heat water delivered to a steam generating boiler. Preheating the feedwater reduces the irreversibilities involved in steam generation and therefore improves the thermodynamic efficiency of ...

, which raises the temperature of the water by using extraction steam from various stages of the turbine. Preheating the feedwater reduces the irreversibilities involved in steam generation and therefore improves the

thermodynamic efficiency In thermodynamics, the thermal efficiency (\eta_) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc. For a h ...

of the system.Fundamentals of Steam Power
by Kenneth Weston,

University of Tulsa The University of Tulsa (TU) is a private research university in Tulsa, Oklahoma. It has a historic affiliation with the Presbyterian Church and the campus architectural style is predominantly Collegiate Gothic. The school traces its origin t ...

This reduces plant operating costs and also helps to avoid thermal shock to the boiler metal when the feedwater is introduced back into the steam cycle.

Boiler

Once this water is inside the

or steam generator, the process of adding the

begins. The boiler transfers energy to the water by the chemical reaction of burning some type of fuel. The water enters the boiler through a section in the convection pass called the economizer. From the economizer, it passes to the steam drum, from where it goes down the downcomers to the lower inlet water wall headers. From the inlet headers, the water rises through the waterwalls. Some of it is turned into steam due to the heat being generated by the burners located on the front and rear waterwalls (typically). From the waterwalls, the water/steam mixture enters the steam drum and passes through a series of steam and water separators and then dryers inside the steam drum. The

steam separator A steam separator, sometimes referred to as a moisture separator or steam drier, is a device for separating water droplets from steam. The simplest type of steam separator is the steam dome on a steam locomotive. Stationary boilers and nuclear r ...

s and dryers remove water droplets from the steam; liquid water carried over into the turbine can produce destructive erosion of the turbine blades. and the cycle through the waterwalls is repeated. This process is known as

natural circulation Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convecti ...

. Geothermal plants need no boiler since they use naturally occurring steam sources. Heat exchangers may be used where the geothermal steam is very corrosive or contains excessive suspended solids. Nuclear plants also boil water to raise steam, either directly passing the working steam through the reactor or else using an intermediate heat exchanger.

Superheater

After the steam is conditioned by the drying equipment inside the drum, it is piped from the upper drum area into an elaborate set up of tubing in different areas of the boiler, the areas known as

superheater 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 ...

and reheater. The steam vapor picks up energy and is superheated above the saturation temperature. The superheated steam is then piped through the main steam lines to the valves of the high-pressure turbine.

References

External links

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