Economic and technological developments are linked with shifts in sources of energy. The trend is towards the adoption of higher energy content sources, as the shift from coal (solid) to oil (liquid) and natural gas (gas) illustrates. This shift can be simplified into five major phases, including one speculative about the future:
- Before the industrial revolution (18th century), energy use relied only on muscular and biomass sources. Most of the work was provided by manual labor and animals, while the biomass (mainly firewood) was used for heating and cooking energy needs. Other sources of energy, such as windmills and watermills, were present but their overall contribution was marginal and for very specific purposes (e.g. milling flour).
- By the mid-19th century, the industrial revolution brought a significant shift in energy sources with the usage of coal, mainly for steam engines, but increasingly for power plants. The use of thermal energy to generate mechanical energy was the core driver of this transformation. It mainly took place in areas in proximity to coal fields.
- As the 20th century began, the major reliance was on coal, but a gradual shift towards higher energy content sources like oil began. This second major shift saw the introduction of internal combustion engines and oil-powered ships.
- In the late 20th century, the preeminence of petroleum products as the main energy provider reached a high level of dependence on the world economy. A massive distribution system for petroleum was established, including pipelines, storage tanks, and liquid bulk carriers. As the level of technical expertise increased, more efficient sources of fossil fuels were tapped, such as natural gas, and an entirely new form of energy, nuclear fission, became available. Renewable energy sources, such as hydroelectric, wind, and solar, started to be tapped but remained marginal sources.
- The 21st century will be characterized by major shifts in energy sources with gradual obsolescence of fossil fuels (decarbonization), like coal and oil, for more efficient fossil fuels such as natural gas. Advances in biotechnologies underline the growing potential of biomass-derived fuels while wind and solar energy will also account for a notable share of energy sources. Nuclear energy, particularly if nuclear fusion becomes commercially possible, may also play a significant role, but this remains speculative. A new transition is likely to be the usage of hydrogen, mainly for fuel cells powering vehicles, small energy generators, and portable devices.