Energy exists in various forms, including mechanical, thermal,
chemical, electrical, radiant, and atomic
and are all interconvertible. Forms of energy come from sources qualified
as renewable and non-renewable, which include chemical
reactions (mainly combustion), nuclear reactions (fission
the effect of gravity (mainly hydraulic) and direct (photovoltaic)
and indirect (photosynthesis and wind) solar energy conversion.
The concept of renewability is based upon the scale of human events
and if the source can be replaced during that period. Fossil fuels
are the most common source of non-renewable energy since oil or
natural gas reserves would take millions of years to replenish
themselves through anaerobic decomposition. Inversely,
wood is a renewable biomass energy source as long as adequate conditions
are kept for reserves to be replenished. Rates of exploitation / deforestation
in a number of areas are so high that biomass may be considered as a
non-renewable source in those circumstances.