Hydropower is the oldest artificial power source known to man; it was already employed in antiquity; the Roman architect Vitruvius describes a water wheel driven by a current. In the Middle Ages and during the industrial revolution hydropower was employed on a large scale, with 25 to 30000 water wheels reported in England alone in 1850. The main hydropower machine employed then was the water wheel; this meant that individual installations had – in today’s assessment – very small power outputs ranging from 1.5 to (occasionally) 50 kW. river_weir unused hydro power: a river weir With the development of turbines in the 19th century serious hydropower exploitation started, with the construction of large dams and river power stations. Today, most of the hydropower potential in Europe is developed; there are only three segments of hydropower which are still unused: (1) Hydropower with very low head differences between 0.5 and 2.5 m (2) The energy of currents (river or tidal currents) (3) Small pressure differences in pipelines (< 25 – 30 kPa) It is estimated that in Germany alone, hydropower potential of 500 MW with head differences below 2m exists, which cannot be exploited for the lack of a cost-effective and ecologically acceptabel converter technology.