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Energy Sources

Where does energy come from? Primary energy sources can be generally grouped into two categories: fuels and flows. Fuels are dense stores of energy that are consumed with use (such as fossil fuels and nuclear fuels). Flows are natural processes that have energy in their motion (such wind and solar energy). All of the energy for fuels and flows on earth initially came from the sun.




In contrast to fossil fuels, formed and stored over millions of years, renewable fuels are ones that humans can make or harvest from current solar energy cycles.


Coal, oil, and natural gas are fossil fuels, which means that they are hydrocarbon compounds formed from dead plants and animals over millions of years compressed in the earth. 

It takes a lot of energy to hold protons and neutrons together in a nucleus, so nuclear fuels are much more energy dense than a chemical fuel like coal or oil.


Page last updated: November 20, 2022​


One category of energy sources that humans use are referred to as flows. These flows have energy associated with motion and come from natural processes.

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