Some energy sources are renewable and some are not. Learn about coal and nuclear energy in this educator's resource and the accompanying e2 episode. Fossil fuels like coal, oil and natural gas cannot be replaced: they are a non-renewable source of energy. A renewable energy source is one that is so abundant that we will never exhaust the supply, or one that can easily be replaced. In order to reduce the emission of greenhouse gases, we need to find new energy sources that don't pollute, while also improving the ways in which we use old energy sources.