Renewable Energy

Renewable Energy: an energy source that can be replaced at a rate either equal to that of human consumption or faster.

It is important to keep in mind that "renewable" is a relative term. It applies to human endeavors. Technically, energy does not "renew" itself. It just is. Going back to the first law of thermodynamics we see that energy can neither be created nor destroyed.
Renewable energy sources come from on-going natural processes on earth or in the solar system. When thinking about renewable energy we also tend to think of a low-pollution energy source. Therefore, due to the polluting nature of nuclear energy, it is not included.

If we try to get at what the originating sources of renewable energy are we will find that there are currently only two sources: the sun and gravity. Gravity technically only stores energy and it is the most difficult to understand when talking about it in terms of renewable energy. The work we get from gravity is due to "gravitational potential energy." More on that later.

Let's examine the sources of renewable energy:
Main Originating Sources:
Sun - gives us thermal and solar energy
Gravity - gravitational potential energy, gravitational binding energy

Sub-categories and sources:
Solar Power - sun
Wind Power - sun, gravity
Hydropower - gravity
Biomass - sun
Geothermal - radioactive decay, gravity
*Note: Geothermal is argued as not renewable by some.

As you can see, except for geothermal, the originating sources have a monopoly on renewable energy categories. The easier of the two to explain is energy supplied by the sun as gravity is still a bit of a mystery. For more information on Solar renewable energy visit The National Renewable Energy Laboratory HERE