Energy sources: fossil fuels, nuclear, and renewables

The primary energy sources used to generate electricity fall into three broad categories. Fossil fuels (coal, natural gas, and oil) are burned to release stored chemical energy as heat. Nuclear energy comes from splitting uranium atoms (fission) to release heat. Renewables (solar, wind, hydropower, geothermal, and biomass) draw on naturally replenishing energy flows rather than a finite stored fuel. Nearly all electricity generation converts one of these primary sources into electricity as a secondary energy carrier.