Energy

Renewable and non-renewable

Some sources are renewable, i.e. they provide energy, which is constantly regenerated by means of chemical transformations (e.g. biomass) or physical transformations (e.g. water power, solar, wind power, etc.). In particular the sun, the wind, the water cycle, the tides, the heat of the Earth are non-exhaustible sources, which are always available and will never end. Biomass, instead, can re-generate within times that are similar to man’s life. With reference to wood, for example, it is always possible to have some combustible available, even though sometimes it is necessary to consume a small quantity of it and reforest in those areas where trees have been cut down. 
Non-renewable sources, instead, are characterised by long regeneration times, so long (millions of years) that after they have been exploited they are considered depleted. They are those energy sources that took millions of years to form, like fossil fuels (oil, coal, natural gas) or when our planet was formed, such as uranium. These sources, although there is still plenty of them, are limited and represent a sort of energy warehouse on the Earth.  At present, only 13% of the energy consumed in the world derives from renewable sources. All the rest derives from non-renewable sources, mainly from fossil fuels (71%) and 6% from nuclear plants (International Energy Agency (IEA) – Key World Energy Statistics 2011).