Basin
Dam
Turbine
Penstock
Valves
Restitution Infrastructure
Transformer
Alternator
Combined cycle
Photovoltaic
Wind
Geothermal
Coal
 
The restitution infrastructures are mainly composed of a channel or tunnel either pressurized or in free surface, which return the flows used to the water stream through a proper artifact outlet.
The term “small hydro” is conventionally used for hydroelectric systems with a capacity up to 10 MW, which differ from plants with a higher capacity. In fact, while the latter require large barrages (dams) and extended artificial lakes for water accumulation, small hydro systems practically work like the old wind mills (obviously as a high-tech version), without almost any environmental impact.
On the contrary, they offer several environmental advantages. First of all they supply energy without emitting polluting substances, dust particles, heat and greenhouse gasses, thus helping to reduce local pollution and global warming.
From the point of view of energy, even if small hydro plants do have a limited individual capacity, they can be installed in arrays that include several systems, thus making a significant contribution to the national electricity production. In fact, this is an energy source that is considered essential to achieve European emissions reduction targets by increasing the amount of renewables that is deployed.