What are secondary energy sources?

Secondary energy sources are also referred to as energy carriers, because they move energy in a useable form from one place to another. There are two well-known energy carriers:

People make electricity and hydrogen from the conversion of other sources of energy, such as coal, nuclear, or solar energy. These are called primary sources.

For many energy needs, it is much easier to use electricity or hydrogen than it is to use the primary energy sources themselves.