Mini hydro
Hydropower is elecrical energy from a waterbody generated by exploiting the potential energy that it produces with a jump or a path downhill. Hydraulic systemns are feasible wherever there is a steady and sufficient flow of water, subject to what is referred to as the minimum vital flow necessary to protect the ecosystem.
The installation of a hydroelectric plant is preceded by a design phase, relating to: geomorphological characteristics of the site, assessment of water resource and its potential, appropriate choice of turbines and generators, and studies on economic conditions and environmental impact.
Hydropower technology is currently recording new interest, particularly as regards small-scale hydroelectric. Mini and micro hydro plants, although of limited power, have considerable advantages from both a technical and economical point of view.
A first rough diffrence is between systems that use a fall of water through a drop and those who exploit the speed of the currents. In the first case the plant power depends on both the jump and water flow rate. In the second case the power is determined by the waterbody speed and the active surface of the turbine.
Despite what it is commonly thought, a hydroelectric plant does not always require high waterfalls and large waterbodies. Besides this kind of power stations, there are also "flowing water" plants, which exploit the water flow, instead of the power arising from the fall.
In both cases, the operating system starts with a barrier along the course of a river, creating a small basin (intake). The weir is always interrupted by the so called "fish scale": a slide, or a real scale, to allow the passage of fishes along the river and not to alter the river ecosystem.
While excess water overflows in the lower course, the basin water flows through a grid along the bank, which blocks any transported material. The grid is connected to a pipeline that carries water to the load tank: a room where it is collected and maintained at uniform levels of pressure and flow. Passing from a grid with a rake for a further filtering (the bar screen), water enter in the pipelines and then in the load tank of each turbine, making them turn.
The turning turbines rotates a generator connected to their axis, where electrical energy is generated (then directly placed on transmission lines). After passing through the turbines, the water returns to the river downstream, without any environmental impact or emission.
Turbine types
Turbine types depend mainly on flow rate and on the waterfall height.
- Pelton wheel action turbine, waterfalls between 50-1300m.
- Turgo action turbine, waterfalls between 15 and 300m.
- Cross-Flow Turbine (Banki-Michell or Ossberger), waterfalls between 5-200m: it works in two stages, the first with a small degree of reaction, the second full-action.
- Kaplan jet turbine, waterfalls between 2-20m.
- Bulb turbine derived from the previous one, used for high flow and very low waterfalls that use the tidal flow. .
- Francis jet turbine, waterfalls between 10-350m.
- Peace turbine, designed to work with water currents, thus without using the potential energy of a waterfall: it is a kinetic turbine, a new type at its early stage of development.
