We define “energy sources” as all natural phenomena, raw materials and physicochemical processes by which humans can extract usable energy. An initial and simple classification distinguishes energy sources according to the original state in which they are found. Primary energy sources are directly available in nature, thus not derived from the transformation of other forms of energy. Examples are fossil and mineral fuels (coal, oil, natural gas, uranium), and energy sources derived from virtually inexhaustible phenomena like the blowing wind, solar radiation, heat from the Earth and the dynamics of water. Secondary energy sources are, on the other hand, forms of energy derived or resulting from more or less-consuming energy conversion processes. Examples are fuels (petrol, diesel, etc.) and thermoelectricity.
Energy sources can be further divided into two additional categories: conventional and non-conventional (or alternative). Conventional sources have been used for a long time (ex. coal, oil, natural gas, nuclear and hydro-energy) and enjoy extensive popularity, with a strong place in the energy market. Non-conventional (or alternative) sources are being used more recently (ex. solar, wind, tidal energy or bioenergy) and is in a phase of expansion, as is their strength in the energy market.
Consumption of all modern fuels continues to grow over the coming 25 years in the World Energy Outlook central scenario, although growth in coal is cut to 0.2% per year on average. Oil demand rises steadily to 103.5 mb/d in 2040; gas consumption rises by nearly 50%, overtaking coal. But it is renewable energy that is the major growth story. In the power sector, 60% of all capacity additions to 2040 are from renewables.
Mtoe: Milion tonne of oil equivalent (toe) is a unit of energy defined as the amount of energy released by burning one tonne of crude oil.
The evolution of the mix of energy sources used in the world from the 19th century to the estimates expected for 2040. The First Industrial Revolution gave way to the rise of coal, which later declined after the 1920s and 30s in the face of the growing use of oil. In the second half of the 20th century, there was a considerable rise in the role of natural gas, later adding nuclear and renewable energies to the mix. Throughout this whole time, there has been a progressive reshaping of the traditional use of biomasses, thanks to the increase in access to modern and efficient energy sources.
Source: Smil, Energy Transitions(1800-1960)