Electricity generation

Electricity generation is the process of converting non-electrical energy to electricity. For electric utilities, it is the first process in the delivery of electricity to consumers. The other processes, electric power transmission and electricity distribution, are normally carried out by the electrical power industry. Electricity is most often generated at a power station by electromechanical generators, primarily driven by heat engines fueled by chemical combustion or nuclear fission but also by other means such as the kinetic energy of flowing water and wind. There are many other technologies that can be and are used to generate electricity such as solar photovoltaics.

History

Centralised power generation became possible when it was recognized that alternating current power lines can transport electricity at very low costs across great distances by taking advantage of the ability to raise and lower the voltage using power transformers.

Electricity has been generated at central stations since 1881. The first power plants were run on water power or coal, and today we rely mainly on coal, nuclear, natural gas, hydroelectric, and petroleum with a small amount from solar energy, tidal harnesses, wind generators, and geothermal sources.

Electricity demand

The demand for electricity is met in several ways. Large centralized generators have been the primary method thus far.

Distributed generation uses a larger number of smaller generators throughout the electricity network. Some use waste heat from industrial processes, others use fuels that would otherwise be wasted, such as landfill gas. Wind and solar generation tend to be distributed because of the low density of the natural energy they collect.

Methods of generating electricity

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

• Static Electricity, the physical separation and transport of charge (eg. triboelectric effect and lightning)
• Electromagnetic induction (as in an electrical generator, dynamo or alternator) transforms mechanical energy into electricity
• Electrochemistry (direct transformation of chemical energy into electricity, as in a battery, fuel cell or nerve impulse.)
• Photoelectric effect (transforming photon energy, as in solar cells)
• Thermoelectric effect (direct thermal energy differential to electric conversion, as in thermocouples and thermopiles)
• Piezoelectric effect (from the mechanical strain of electrically anisotropic molecules/crystals)
• Nuclear transformation, charged particle creation and acceleration (eg. Betavoltaics or alpha particle emission)

Static electricity was the first form discovered and investigated, and the electrostatic generator is still used even in modern devices such as the Van de Graaff generator or MHD generators. Electrons are physically (mechanically) separated and transported to increase their electric potential.

The vast majority of commercial electric generation is done via electromagnetic induction: mechanical energy is used to supply the energy to rotate an electrical generator. There are many different methods of developing the mechanical energy, such as heat engines, hydro, wind or tidal generators etc.)

Note that the direct conversion of nuclear energy to electricity (beta decay) is not how a nuclear power plant produces electricity; instead it uses the heat of a nuclear reaction to run a heat engine, and transforms the heat engine's resulting mechanical energy into electricity via magnetic induction (a generator).

Most electric generation is driven by heat engines. The combustion of fossil fuels supplies most of the heat to these engines, with a significant fraction from nuclear fission and some from renewable sources.

Turbines

All turbines are driven by a fluid acting as an intermediate energy carrier. A lot of all of the heat engines just mentioned are turbines. Other types of turbines can be driven by wind or falling water.

Sources includes:

• Steam - Water is boiled by:
• nuclear fission,
• the burning of fossil fuels (coal, natural gas, or petroleum). In hot gas (gas turbine), turbines are driven directly by gases produced by the combustion 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 a gas turbine and use residual heat to generate additional electricity from steam. These plants offer efficiencies of up to 60%.
• Renewables. The steam generated by:
• Biomass
• The sun as the heat source: solar parabolic troughs and solar 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 ground and drives a turbine or hot water evaporates a low boiling liquid to create vapour to drive a turbine.
• Other renewable sources:
• Water (hydroelectric) - Turbine blades are acted upon by flowing water, produced by hydroelectric dams or tidal forces.
• Wind - Most wind turbines generate electricity from naturally occurring wind. Solar updraft 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.

Reciprocating engines

Small electricity generators are often powered by reciprocating engines burning diesel, biogas or natural gas. Diesel engines are often used for back up generation, usually at low voltages. 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.

Photovoltaic panels

Main Photovoltaics 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 though, and multijunction cells with close to 30% conversion efficiency are now commercially available. Over 40% efficiency has been demonstrated in experimental systems. 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

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 by thermoelectric (TE) devices, though thermionic (TI) and thermophotovoltaic (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 in power harvesting. Betavoltaics are another type of solid-state power generator which produces electricity from radioactive decay. Fluid-based magnetohydrodynamic (MHD) power generation has been studied as a method for extracting electrical power from nuclear reactors and also from more conventional fuel combustion systems.

Electrochemical electricity generation is also important in portable and mobile applications. Currently, most electrochemical power comes from closed electrochemical cells ("batteries") , which are arguably utilized more as storage systems than generation systems, but open electrochemical systems, known as fuel 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 electrolytic hydrogen) and so can be viewed as either generation systems or storage systems depending on their use.

Producers

In 2005, USA continued to remain as the top producer of electricity with a global share of at least 25% followed by China, Japan and Russia.

Global warming

Emissions from electricity generation account for a signifcant portion of world greenhouse gas emissions; in the United States, electricity generation accounts for nearly 40 percent of emissions, the largest of any source. Transportation emissions are close behind, contributing about one-third of U.S. production of carbon dioxide .

See also

References

External links

• Power Technologies Energy Data Book
• Electricity Generation -- Guide to various methods of electricity generation, including both renewable and conventional sources.