The phrase "fuel source" refers to something that provides energy to power or run a certain system or machine. It can be anything that produces energy, like gasoline for cars or coal for generating electricity. Full definition
Global investments in new electricity capacity from renewable sources have exceeded those in fossil fuel sources for the past seven years, and the gap is growing. (algore.com)
Here's the reality: jobs in dirty energy are on the decline as fuel sources become more scarce and less expensive options become available. (climaterealityproject.org)
They are also primary fuel sources for brown electricity generation, and so brown electricity prices rise and fall with the costs of these raw materials. (zerocarbonista.com)