Hydraulic turbines in mini hydroelectric plants
A hydraulic turbine is a power unit that allows the transformation of the potential energy of water into mechanical energy.
It consists of:
- a fixed-distributor component with the mechanical function of channelling and regulating the incoming flow to the impeller and the hydraulic function of converting the water’s potential energy into kinetic energy;
- a mobile-impeller component that is set in motion by water exiting the distributor with the task of delivering mechanical energy to the shaft on which it is mounted.
The choice of a suitable turbine is made by means of nomograms, into which project data is loaded (available drop and water flow), allowing the identification of the most suitable type of turbine and its size in terms of power.
This is an impulse hydraulic turbine which is suitable for facilities with a high drop, up to a few hundred metres. It is very similar to the turbines used in larger plants and has a horizontal or vertical axis. This turbine has many advantages:
- it operates at atmospheric pressure;
- it has a simple, robust build, is small and provides a high level of performance;
- it has a relatively low number of revolutions and can therefore be adapted to even very high water drops.
This impulse hydraulic turbine is similar to the Pelton turbine and is suitable for drops of between 30 and 300 metres. It is recommended in situations with considerable variations in inflows of turbid water.
Radial or cross flow microturbines
These are suitable for facilities with low and medium drops, from a few metres up to 100 and a capacity of between 20 and 1,000 l/s approximately. They are used in low-power plants. The advantages of these hydraulic turbines are similar to those of Pelton ones, except they have a lower level of performance. They are, however, easier to build and can also be adapted to lower drops.
This is a reaction hydraulic turbine for power with a lower limit of around 100 kW and this is why it is called a miniturbine. Advantages include a high speed of rotation suitable for small-medium drops, use in open chamber facilities with very small drops and the possibility of exploiting all of the available drop right up to the spillway. Disadvantages include its complex construction, issues surrounding sealing (due to the difference in pressure of the impeller between upstream and downstream), cavitation (due to the depression of the diffuser), friction and wear (due to high water speed against the blades).