environmental and concers
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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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