ABSTRACT
Plants, the most successful energy systems on earth, have been using solar energy for millions of years to convert carbon dioxide and water into oxygen and carbohydrates, that are subsequently converted into oil and coal. While fuel is usually considered as the main source of energy, it would be useless without the presence of oxygen; accordingly, oxygen (in air) can be considered as an important energy reservoir. Solar energy is also at the origin of most of the renewable energies: hydroelectric, wind mills, solar cells, biomass, etc. Only nuclear plants can deliver large amount of energy not originating from sun.
