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.
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.
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.
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.
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.
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.
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.
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.
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 .