What makes an energy source renewable?

Answer 1

A useful definition of a renewable resource is one "that can be replenished naturally with the passage of time." Typical examples of renewable sources of energy include solar (thermal & photovoltaic), wind (turbines), water (dams, tidal, wave), geothermal, and biomass.

There are a number of caveats that should be mentioned here however, such as the fact that at long enough time scales all sources of energy are ultimately finite. For example, the hydrogen that fuels the Sun's conversion of energy into light which yields photosynthetic biomass and that drives Earth's atmospheric wind pressure systems along with its hydrological cycle is consumed irreversibly. Geothermal energy arises from the natural radioactive decay of unstable elements deep in the Earth's interior and such radioactive decay is also finite in nature.

In addition technologies needed to harness these renewable forms of energy require material inputs which are not themselves "renewable". For example, both multi-junction photovoltaic solar cells and wind turbine-generator magnets require rare-earth elements.

Given these qualifications there is no reason to exclude nuclear fission from renewable energy mix. After all both thorium and uranium are a lot more common than rare-earth elements such as indium, neodymium, or praseodymium. Furthermore it is estimated that the oceans of the world contain dissolved within it some 4.5 billion tons of uranium; and its rate of consumption in fast-spectrum breeder reactors would be constantly renewed by additional deposition from rivers and streams.

Answer 2

An energy source is considered renewable if it can be naturally replenished on a human timescale, such as solar, wind, hydro, geothermal, and biomass. These sources are sustainable and emit minimal greenhouse gases when generating electricity.