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. In most cases, these energy sources have technical challenges to overcome before they are economically competitive with conventional methods of electricity generation. Approaches to overcoming these challenges are a field of active research, and are described on the relevant generation method pages.
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 structures 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. The environmental and economic benefits of non-cellulosic ethanol have been heavily critiqued by many, including Brad Ewing of Environmental Economics & Sustainable Development[1] and Lester R. Brown of Earth Policy Institute[2]
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.