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Most scientists agree that emissions of pollutants and greenhouse gases from fossil fuel-based

electricity generation account for a significant portion of world greenhouse gas emissions; in the

United States, electricity generation accounts for nearly 40% of emissions, the largest of any source.

Transportation emissions are close behind, contributing about one-third of U.S. production ofcarbon

dioxide.[16] In the United States, fossil fuel combustion for electric power generation is responsible

for 65% of all emissions ofsulfur dioxide,the main component of acid rain.[17] Electricity generation

is the fourth highest combined source ofNOx,carbon monoxide,andparticulate matter in the

US.[18] In July 2011, the UK parliament tabled a motion that "levels of (carbon) emissions from

nuclear power were approximately three times lower perkilowatt hour than those of solar, four

times lower than clean coal and 36 times lower than conventional coal".[19] Though Solar PV

generation is positioned as environmentally friendly, fabrication of PV cells utilizes substantial

amounts of water in addition to toxic chemicals such as phosphorus and arsenic. These are often

overlooked when promoting PV. Because of strict environmental regulations in the United States, for

example, PV fabrication is often performed in countries with lower standards,such as China,which

produces approximately half the world's PV panels.The selection of electricity production modes andtheir economic viability varies in accordance with demand and region. Hydroelectric plants, nuclear

power plants, thermal power plants and renewable sources have their own pros and cons, and

selection is based upon the local power requirement and the fluctuations in demand. All power grids

have varying loads on them but the daily minimum is the base load, supplied by plants which run

continuously. Nuclear, coal, oil and gas plants can supply base load, with the low-carbon option

being nuclear. Thermal energy is economical in areas of high industrial density, as the high demand

cannot be met by renewable sources. The effect of pollution is also minimized as industries are

usually located away from residential areas. These plants can also withstand variation in load and

consumption by adding more units or temporarily decreasing the production of some units. Nuclear

power plants can produce a huge amount of power from a single unit. However, recent disasters in

Japan have raised concerns over the safety of nuclear power, and the capital cost of nuclear plants is

very high. Hydroelectric power plants are located in areas where the potential energy from flowing

water can be harnessed for moving turbines and the generation of power. It is not an economically

viable source of production where the load varies too much during the annual production cycle and

the ability to stop the flow of water is limited. Renewable sources other than hydroelectricity (solar

power, wind energy, tidal power, etc.) due to advancements in technology, and with mass

production, their cost of production has come down and the energy is now in many cases cost-

comparative with fossil fuels. Many governments around the world provide subsidies to offset the

higher cost of any new power production, and to make the installation of renewable energy systems

economically feasible. However, their use is frequently limited by their intermittent nature. If natural

gas prices are below $3 per millionBritish thermal units,generating electricity from natural gas is

cheaper than generating power by burning coalTurbines

Large dams such asThree Gorges Damin China can provide large amounts ofhydroelectric power; it

has a 22.5GWcapability.

All turbines are driven by a fluid acting as an intermediate energy carrier. Many of the heat engines

just mentioned areturbines.Other types of turbines can be driven by wind or falling water. Sources

include:

Steam - Water is boiled by-
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Nuclear fission The burning offossil fuels (coal, natural gas, or petroleum). In hot gas (gas turbine),

turbines are driven directly by gases produced by thecombustion of natural gas or

oil.Combined cycle gas turbine plants are driven by both steam and natural gas.

They generate power by burning natural gas in agas turbine and use residual heat togenerate additional electricity from steam. These plants offer efficiencies of up to

60%.

Renewables. The steam is generated by: Biomass Solar thermal energy (the sun as the heat source):solar parabolic

troughs andsolar power towers concentrate sunlight to heat a heat transfer

fluid, which is then used to produce steam.

Geothermal power.Either steam under pressure emerges from the groundand drives a turbine or hot water evaporates a low boiling liquid to create

vapour to drive a turbine.

Ocean thermal energy conversion (OTEC): uses the small difference betweencooler deep and warmer surface ocean waters to run a heat engine (usually

a turbine).

Other renewable sources:Large dams such asHoover Dam can provide large amounts ofhydroelectricpower; it has

2.07GW capability.

Water (hydroelectric) - Turbine blades are acted upon by flowing water, producedbyhydroelectric dams ortidal forces.

Wind - Mostwind turbines generate electricity from naturally occurring wind.Solarupdraft towers use wind that is artificially produced inside the chimney by heating it

with sunlight, and are more properly seen as forms of solar thermal energy.

Reciocating engines[edit]

Small electricity generators are often powered byreciprocating engines burningdiesel,biogas or

natural gas.Diesel engines are often used for back up generation, usually at low voltages. However

most large power grids also usediesel generators,originally provided as emergency back up for a

specific facility such as a hospital, to feed power into the grid during certain circumstances. Biogas is

often combusted where it is produced, such as a landfill or wastewater treatment plant, with a

reciprocating engine or a microturbine, which is a small gas turbine.

A coal-fired power plant in Laughlin, Nevada U.S.A. Owners of this plant ceased operations after

declining to invest in pollution control equipment to comply with pollution regulations.[8]

Photovoltaic panels[edit]
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Unlike the solar heat concentrators mentioned above,photovoltaic panels convert sunlight directly

to electricity. Although sunlight is free and abundant, solar electricity is still usually more expensive

to produce than large-scale mechanically generated power due to the cost of the panels. Low-

efficiency silicon solar cells have been decreasing in cost and multijunction cells with close to 30%

conversion efficiency are now commercially available. Over 40% efficiency has been demonstrated in

experimental systems.[9] Until recently, photovoltaics were most commonly used in remote sites

where there is no access to a commercial power grid, or as a supplemental electricity source for

individual homes and businesses. Recent advances in manufacturing efficiency and photovoltaic

technology, combined with subsidies driven by environmental concerns, have dramatically

accelerated the deployment of solar panels. Installed capacity is growing by 40% per year led by

increases in Germany, Japan, California and New Jersey.

Other generation methods[edit]

Wind-powered turbinesusually provide electrical generation in conjunction with other methods of

producing power.

Various other technologies have been studied and developed for power generation. Solid-state

generation (without moving parts) is of particular interest in portable applications. This area is

largely dominated bythermoelectric (TE) devices, thoughthermionic (TI)

andthermophotovoltaic (TPV) systems have been developed as well. Typically, TE devices are used

at lower temperatures than TI and TPV systems.Piezoelectric devices are used for power generation

from mechanical strain, particularly inpower harvesting.Betavoltaics are another type of solid-state

power generator which produces electricity from radioactive decay. Fluid-

basedmagnetohydrodynamic (MHD) power generation has been studied as a method for extracting

electrical power fromnuclear reactors and also from more conventional fuel combustion

systems.Osmotic power finally is another possibility at places where salt and fresh water merges

(e.g. deltas, ...)Electrochemical electricity generation is also important in portable and mobile

applications. Currently, most electrochemical power comes from closed electrochemical cells

("batteries"),[10] which are arguably utilized more asstorage systems than generation systems; but

open electrochemical systems, known asfuel cells,have been undergoing a great deal of research

and development in the last few years. Fuel cells can be used to extract power either from natural

fuels or from synthesized fuels (mainly electrolytichydrogen)and so can be viewed as either

generation systems or storage systems depending on their use.

There are seven fundamental methods of directly transforming other forms of energy into electrical

energy:

Static electricity,from the physical separation and transport of charge(examples:triboelectric effect andlightning)

Electromagnetic induction,where anelectricalgenerator,dynamo oralternator transformskinetic energy (energy of motion) into

electricity. This is the most used form for generating electricity and is based onFaraday's

law.It can be experimented by simply rotating a magnet within closed loops of a conducting

material (e.g. copper wire)
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Electrochemistry,the direct transformation ofchemical energy into electricity, as inabattery,fuel cell ornerve impulse

Photoelectric effect,the transformation of light into electrical energy, as insolar cells Thermoelectric effect,the direct conversion of temperature differences to electricity, as

inthermocouples,thermopiles,andthermionic converters.

Piezoelectric effect,from the mechanical strain of electricallyanisotropic molecules orcrystals. Researchers at the US Department of Energy's Lawrence Berkeley National

Laboratory (Berkeley Lab) have developed apiezoelectric generator sufficient to operate

aliquid crystal display using thin films of M13 bacteriophage.[7]

Nuclear transformation, the creation and acceleration of charged particles(examples:betavoltaics oralpha particle emission)

Static electricity was the first form discovered and investigated, and theelectrostatic generator isstill used even in modern devices such as theVan de Graaff generator andMHD generators.Charge

carriers are separated and physically transported to a position of increased electric potential. Almost

all commercial electrical generation is done using electromagnetic induction, in whichmechanical

energy forces anelectrical generator to rotate. There are many different methods of developing the

mechanical energy, includingheat engines,hydro, wind and tidal power. The direct conversion

ofnuclear potential energy to electricity bybeta decay is used only on a small scale. In a full-size

nuclear power plant, the heat of a nuclear reaction is used to run a heat engine. This drives a

generator, which converts mechanical energy into electricity by magnetic induction. Most electric

generation is driven byheat engines.The combustion offossil fuels supplies most of the heat to

these engines, with a significant fraction fromnuclear fission and some fromrenewable sources.The

modernsteam turbine (invented bySir Charles Parsons in 1884) currently generates about 80% of

theelectric power in the world using a variety of heat sources.
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