Clear Science!

We talked about fossil fuels and said that the energy we get from them comes from their chemical bonds. The largest source of energy in the U.S. (and the world) is fossil fuels, a lot of that going toward electricity generation.

We get the energy from fossil fuels through combustion, which is the scientific term for burning. Combustion requires two things: a fuel and an oxidant. In our example the fuel is the fossil fuel octane. The oxidant is oxygen, which comes from the air. Our cars have a fuel tank, but if we were on the moon we would need a fuel tank and an oxidant tank. On Earth we’re spoiled because there’s an oxidant everywhere. (Other substances like chlorine and fluorine are also oxidants.)

Octane and oxygen have more energy in their chemical bonds than the products of combustion, which are carbon dioxide and water. That energy has to go somewhere, so it is released as heat. This is an exothermic reaction. In reality, combustion is rarely perfect, so ash and light and other things are formed, too.

This is why we like fossil fuels: you dig them up out of the ground and they have energy in them ready for the taking. That’s so easy! And easy = inexpensive.

Fossil fuels are a huge fraction of where we get energy. Some of these are used at power plants to make electricity (mainly coal and natural gas) or to power something like a car directly (mainly petroleum). Where does the energy come from in fossil fuels, and why do we use them so much?

Petroleum is a mixture of organic compounds, which are molecules based on carbon. Take octane as an example, shown above. Oct means eight, and octane is a molecule with 8 carbons. (Sometimes people get tired of drawing all the hydrogens in organic molecules, so they draw them as simple stick figures, with corners signifying carbons and the hydrogens implied but not drawn.) Coal (a solid) is mostly longer molecules, and natural gas is shorter molecules.

The energy in fossil fuels is stored in the bonds between the atoms. If you break these bonds, energy comes out. So we break them, and this is how the internal combustion engines in our cars run, as well as the external combustion engines in our power plants.

We are going to talk about energy for a while. Humanity uses energy a few ways. Sometimes we make electricity first and then power things with that. Other times we burn fuel directly to power an engine or a heater. But let’s step back and ask: where does energy come from in the first place?

Shown above are several primary generation methods for making electricity. What’s actually happening is that we are taking energy from somewhere else and converting it to a more useful form.

Fossil fuels like coal or oil have energy stored in the chemical bonds inside them. When we use fossil fuels, we are claiming that energy. Nuclear generation is similar, but it’s from the atomic bonds within the nuclei of the fuel materials.

If a material is high in a gravitational field it also has energy, and that is what we’re claiming with hydroelectric generation. Water up high is being lowered, and we get the energy liberated by lowering it. Wind generation claims the kinetic energy from blowing wind. Solar generation claims the radiation energy emitted by the sun.