Sources of Energy
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 or fusion), 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.