TYPES OF HYDROELECTRIC TURBINES
| The types of hydroelectric turbines are divided into 2 distinct groups: IMPULSE and REACTION. The runners of the first ones are not immersed in water, which is sprinkled to the tabs composing the runner itself making it move. On the contrary, in the reaction turbines the runner is completely immersed in water, which flows between the vanes moving them. |
PELTON
PELTON are impulse turbines, in which one or more nozzles (till six) and one or two runners turn the water pressure into mechanical energy. Indeed each nozzle, thanks to a needle valve allowing the regulation of the water flow, creates a jet which hits a spoon of a particular form fixed on the rotor of the turbine itself. Higher the output pressure from the nozzle (with proper quantities) is, more and more mechanical power the water gives back to the spoon impressing strength on the turbine rotor. Then the turbine, coupled in the right way, will give back the mechanical energy to the generator, which transforms it into electrical energy. PELTON turbines have to be protected from the load disconnection, which could bring the turbine to a destructive escape velocity, without closing the water flow valve too quickly to avoid water hammers. When the water goes out from the turbine, its speed is almost absent, so that the case containing the rotor and the nozzles do not have to get particular pressure and the construction can be light. PELTON turbines are used for water heads from 50 to 1300m. |
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TURGO
TURGO too is an impulse turbine. Differently from the PELTON one, TURGO turbine has blades of different form and disposition and it exploits a solution similar to the vapour turbines, where the jet hits not one but more blades. This allows TURGO turbine to use a bigger volume of water compared to PELTON, where each nozzle can interfere with the other one. Moreover, with the same tip speed, the smallest diameter allows to obtain a higher angular velocity, so that the turns multiplier in the coupling with the electrical generator is avoided.
This allows to decrease the costs and to increase the mechanical reliability of the system. TURGO turbines are used for water heads from 15 to 300m.
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BANKI
It is also called CROSS-FLOW and it is an impulse two-stage turbine. Water goes into the turbine thanks to a distribution system and it arrives at a first stage of the rotor functioning almost totally submerged. Subsequently, once abandoned the first stage, the water flow changes direction and reaches the second stage of the turbine, which is totally an impulse stage. The rotor of the turbine is formed by parallel disks, among which the blades are installed, characterised by bent tongues. The form of the rotor is similar to the one of a tangential ventilation system. The easy construction of the turbine allows to produce it in developing countries too. The efficiency of this turbine is less than 87%, keeping almost constant till low flow rates (>16%). BANKI turbines are used for water heads from 5 to 200m. |
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FRANCIS
It is a reaction turbine, in which water moves as in a pressure pipe. In fact, thanks to the adjustable nozzle blades, water is carried to the fixed blades rotor, to which it gives energy without coming into contact with the external air. In this type of turbine the water flow is almost always radial while the outlet is axial. FRANCIS turbines are used for water heads from 10 to 350m. |
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KAPLAN
They are reaction turbines and they are divided into two types: double (true KAPLAN) or single (semi-KAPLAN) regulation. In the first type both the rotor and the nozzle blades are adjustable. On the contrary, in the second type only the rotor blades are adjustable while the others are fixed. The possibility of adjusting the nozzle and the rotor blades allows to regulate the functioning of the system according to the flow rate and the water head. In case of constant flow rate the turbine can be also formed by fixed blades to reduce the costs. KAPLAN turbines are used for water heads from 2 to 20m. |
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BULB TURBINE
It is a reaction turbine and it is similar to the submerged part of a big outboard motor. It is specifically used to exploit the wave-motion of the tides or the submarine currents. The rotor of the BULB TURBINE derives from the one of the KAPLAN TURBINE. The generator and the multiplier are contained in a waterproof case immersed in the water. |
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ARCHIMEDEAN SCREW TURBINE
The main characteristics of this technology are the following ones:
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use of grids with big holes thanks to the capacity of the Archimedean screw to accept alluvial material and waste of big dimensions; |
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no combs and therefore no production of waste to be disposed of; |
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perfect compatibility with fishes; |
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low costs of the plant and its management. |
Archimedean screw turbines are used for water heads from 1 to 10m and water flow rates from 0,5 to 5,5 m³/sec.
The most important characteristic of these turbines is that they continue to function also with minimum water flow rates. Therefore, they are very proper for irregular water quantities.
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TURBINES FOR SEA CURRENTS
The hydroelectric turbines for the exploitation of the sea currents derive from the wind technology. In fact, they are composed by huge blades mechanically connected to the electrical generator.
They are complicated machines, used only in systems of big power.
These turbines function exploiting the sea currents caused by the tides and they can be installed only in some coastal areas.
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» Types of hidr. turbines |
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