This article is about a concept related to renewable energy, of which sustainable energy is a superset.

Sustainable energy sources are energy sources which are not expected to be depleted in a timeframe relevant to the human race, and which therefore contribute to the sustainability of all species. This concept is termed sustainability.

Sustainable energy sources are most often regarded as including all renewable sources, such as solar power, wind power, wave power, geothermal power, tidal power, and others.

Fission power and fusion power power meet the definition of sustainability, but there is controversy over whether or not they should be regarded as sustainable.

Renewable energy sources

Renewable energy sources are those whose stock is rapidly replenished by natural processes, and which aren't expected to be depleted within the lifetime of the human species.

The well-known renewable energy options can be classified by the natural process that provides their energy:

Direct solar energy:

• Solar cells use semiconductors to directly convert sunlight into electricity. Primary challenges with their use are low efficiency, energy-intensive manufacture, and power variability due to weather and nightfall.
• Solar thermal plants use concentrated sunlight as a heat source to power a heat engine which generates electricity. Primary challenges with their use are manufacture and maintenance of large mirror arrays and power variability due to weather and nightfall.
• Solar updraft tower plants use sunlight to heat a contained mass of air, setting up convection currents that cause air to exit through a chimney from which power is tapped. Primary challenges with their use are low efficiency, construction and maintenance of the large structures required, and power variability due to weather (a Solar updraft tower has enough heat capacity to function through night).

Indirect solar energy:

• Ocean thermal energy conversion uses the temperature difference between the warmer surface of the ocean and the cooler lower depths to drive a heat engine. The primary challenges with ocean thermal energy conversion's use are low efficiency and the construction and maintenance of large strutures in a sea environment.
• Wind power uses wind turbines to draw energy from large-scale motion of air. The primary challenges with wind power's use are the large areas required to produce useful amounts of electricity, and power variability due to weather.
• Hydroelectricity uses dams to draw energy from the flow of water from high-altitude areas to areas with lower altitudes. Primary challenges with hydroelectricity's use are the environmental damage caused by the construction of dams, and the scarcity of remaining sites for power generation.
• Wave power uses floats to extract mechanical energy from the motion of waves. Primary challenges with wave power's use are the large areas required to produce useful amounts of electricity, and disruption of coastal environments.
• Biofuel uses products of plants, animals, or bacteria to provide fuels that can be used in a manner similar to fossil fuels. The primary challenge with biofuel's use is the availability of suitable feedstock in sufficient quantity for large-scale adoption.

Radioactive decay within the Earth:

• Geothermal power uses the temperature difference between the earth's surface and its interior to drive a heat engine, generally at a location such as a hot spring where the heat has been transported most of the way to the surface by natural processes. The primary challenge with geothermal power's use is low power generation efficiency for most sites.

Rotation of the Earth:

• Tidal power uses dams to draw energy from the changes in water height due to tides produced by the gravitational influences of the moon and sun as Earth rotates. The primary challenges with tidal power's use are the large area required to produce useful amounts of electricity, and disruption of coastal environments.

Processes powered by solar energy will be renewed for as long as the sun remains on the main sequence (approximately 5 billion years). Processes powered by radioactive decay within the Earth will be renewed for time comparable to the half-life of uranium 238 (4.5 billion years) and thorium 232 (14 billion years). Processes powered by the Earth's rotation will last until the Earth becomes tidally locked to the Sun (though tidal acceleration would eject the moon from Earth orbit earlier). Both of these would take longer than the expected lifetime of the sun to occur.

Sustainable sources not considered renewable

Sustainable energy sources that aren't renewable are those whose stock is not replenished, but for which the presently available stocks are expected to last for as long as human civilization cares to use them.

These energy sources are derived from nuclear energy, as other forms of stored energy found on Earth do not have sufficient energy density to supply humanity indefinitely.

• Fission power uses the nuclear fission of heavy elements to release energy that drives a heat engine. Primary challenges with the use of fission power are the production of small quantities of highly-radioactive waste in the form of spent fuel, larger quantities of less-radioactive waste in the form of activated structural material, and (for use as a long-term power source) the need to perform intensive processing of highly-radioactive fuel bundles, both to reclaim unused fuel in spent fuel rods, and to reclaim plutonium 239 and uranium 233 that have been bred from uranium 238 and thorium 232, respectively.

• Fusion power uses the nuclear fusion of isotopes of hydrogen to release energy that drives a heat engine. Primary challenges with the use of fusion power are that the technology required to build a useful fusion power plant are still under development, and that substantial quantities of radioactive waste in the form of activated structural material is produced.

Fission power's long-term sustainability depends on the amount of uranium and thorium that is available to be mined. Estimates for fuel reserves vary widely, but if breeder reactors and fuel reprocessing are assumed, tend to be tens of thousands of years or longer (uranium is approximately as common in Earth's crust as tin or zinc (2 ppm), and thorium as common as lead (6 ppm)).

Fusion power's long-term sustainability depends on the amount of lithium that is available to be mined (for deuterium-tritium fusion), or the amount of deuterium available in seawater (for deuterium-deuterium fusion). Lithium is a reasonably common component of Earth's crust, being about 10 times as common as thorium (65 ppm). Deuterium (a hydrogen isotope) occurs wherever hydrogen is found (principally in water), at about 150 ppm. As it can be extracted easily from seawater, economically viable reserves of deuterium are for practical purposes unlimited.

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