The main concept hydroelectric plants are based on is to transform the potential energy of resting mass of water and/or the kinetic energy of a water current into mechanic energy. Subsequently this energy will be converted into electric energy. Hydroelectric plants are subdivided into: big hydroelectric plants (or simply hydroelectric plants) and minor hydroelectric plants (or mini-hydroelectric plants). This subdivision depends on the power installed inside the plant and can take 10 MW as a reference value (actually in Italy minor hydroelectric refers to a maximum power of 3 MW). This subdivision is usually reflected on different types of plants: while large hydroelectric plants usually require wide surfaces to be submerged, with a significant environmental and social impact, small hydroelectric plants perfectly integrate with the local ecosystem (it directly exploits the river current).