- From the beginning of history up to the industrial revolution
(18th century), mankind's sources of energy relied only on
muscular and biomass sources. Most work was provided by manual
labor and animals, while the biomass (mainly wood) provided
for heating and cooking energy needs. Other sources of energy,
such as windmills and watermills were present but their overall
contribution was marginal.
- By the mid 19th century, the industrial revolution brought
a major shift in energy sources with the usage of coal, mainly
for steam engines, but increasingly for power plants.
- 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 inaugurated the era
of the internal combustion engine and of oil-powered ships.
- In the late 20th century, the emphasis on petroleum products
as the main provider of energy has reached the point where
the world economy highly depends on the internal combustion
engine and supporting industries. As its level of technical
expertise increased, mankind was able to tap on more efficient
sources of fossil fuels, mainly natural gas, and energy released
by matter itself (nuclear fission).
- The 21st century will be characterized by major shifts in
energy sources with a gradual obsolescence of polluting fossil
fuels, like coal and oil, for more efficient fossil fuels
such as natural gas. Advances in biotechnologies let anticipate
the growing usage of biofuels. Nuclear energy, if nuclear
fusion becomes commercially possible, may also play a significant
role. A very important change in energy sources is likely
to be the usage of hydrogen, mainly for fuel cells powering
vehicles, small energy generators and numerous portable devices.