Unused Hydropower Segments
Hydropower with very low head differences
Water wheels (water wheel) are efficient converters, but are limited in their power output. In addition, their costs also increase with reducing head difference so that currently hydropower sites with less than 2 m head difference are considered economically marginal.
|Breast wheel||Poncelet wheel|
With the development of turbines in the 19th century serious hydropower exploitation started.
Thousands of weirs were built into our rivers in order to reduce the gradient (to prevent erosion), to extract water either for irrigation or for water wheels and for navigation. It is estimated that the power available accounts for more than 500 MW in Germany, and 500 – 1000 MW in England; other countries probably have similar power available. Today, the hydropower at all these places is unused since there is no cost-effective and ecologically acceptable energy converter. Standard technologies such as Kaplan Turbines require very large diameters and extensive in- and outflow structures, so that their costs rise with reducing head difference where high flow volumes must be processed. Also, the effect of the turbine on fish passing through is considered negative due to propeller strike and very low pressures in the suction tube.
Large rivers contain a significant potential as kinetic energy or energy of the moving water. This energy was utilised e.g. by ship mills. Today, the problem of generating energy from a free stream has importance in the area of tidal flows and for decentralised electricity / energy generation in large rivers.
section of a floating mill -
The main problem lies in the comparatively low energy density of flowing water ( 1m3 of water with a flow velocity of 1.5 m/s or 3 kn. only contains 1.69 kW of energy), and the low efficiencies of the energy converters. A large number of energy converters has been suggested, but so far no commercialy viable solution has evolved.
Small pressure differences in pipelines
In many pipeline systems pressure reduction is required at certain point. Usually, the energy is dissipated. The utilisation of pressure differences could be of particular advantage in remote locations, where no power supply exists, or in pipelines with high volume flows. Historcially, piston-type machines were suggested and apparently also used for this purpose.
These machines have the disadvantages that they produce intermittent power, and that during the necessary valve opening and closing pressure surges can develop.
FP7-Energy-2007-1-RTD project number: 212423