Hydraulic turbines, they transform energy into energy. Potential or kinetic energy of a fluid, in mechanics and then, if desired, electrical, if we need it so. These are mechanical devices with a rather high yield, estimated on average above 90%, and for this reason they are highly appreciated, even if their installation is not always very “agile”: it requires a lot of infrastructures.
The hydraulic turbines they generally consist of a fixed organ, the distributor, and from a mobile one: the impeller. The distributor gives the direction to the flow rate which then reaches the impeller, also adjusting it and transforming the pressure energy of the flow into kinetic energy. When this transformation occurs completely in the distributor, the hydraulic turbines are said to action, otherwise we can talk about hydraulic reaction turbines. Moving from the distributor to the impeller, where we witness the transformation of the potential and / or kinetic energy of the water - the one that arrives depending on the type - into mechanical energy yielded on the motor shaft.
The ancestor of electric turbines it is the water wheel widely used since the seventeenth century and based almost exclusively on the exploitation of potential energy. It could not provide great powers, so it did not immediately be appalled. It went better when we passed with the modern water turbines to the exploitation of kinetic energy.
Towards the nineteenth century, therefore, the Pelton-type hydraulic turbines, with the drawback, however, of rather expensive artificial basins due to the need for height differences of even hundreds of meters. Gradually, we have approached hydraulic turbines with less demands and also suitable for heights of a few meters (2 - 3), the Kaplan, which look like intubated propellers. Archived in the museums water wheels, or almost, today Pelton, Francis and Kaplan are the most used hydraulic turbines for the production of electricity.
The principle of operation in general is the exploitation of the fall of a fluid and is therefore linked to the height at which the upstream tank is placed, compared to the downstream tank, they are both part of the hydraulic turbines and are usually at atmospheric pressure.
Pelton hydraulic turbines
The hydraulic turbines of this type they are actionable and equipped with one or more nozzles that totally transform the water pressure into kinetic energy. We usually find them with jumps from 50 to 1300 meters. Each nozzle of these devices creates a jet with flow rate regulated by a needle valve, then there is a deflector plate that diverts the flow from the blades in order to avoid problems in the event of sudden load disconnection.
One drawback of the Pelton water turbines is the fact that they can process a limited volume of water, due to the risk of interference between the individual flows coming from two side-by-side nozzles. However, we find them both in vertical and horizontal axis, both in large plants and for micro-hydroelectric plants.
Turgo hydraulic turbines
It is about action hydraulic turbines also in this case but the jumps they can work on are very different: we are between 15 and 300 meters. Compared to the Pelton, this type provides blades with different shape and arrangement so that a jet hits several blades simultaneously. Thanks to this expedient, the hydraulic turbines Turgo they have no particular limitations on the volume of water.
As they are designed, they do not provide for the presence of a multiplier with a consequent reduction in costs and an increase in reliability. Widespread in Europe, the Turgo hydraulic turbines they are not very popular in Italy, at least not yet, in general they are recommended in situations with strong variations in flows or in murky waters.
Kaplan hydraulic turbines
Here are some hydraulic reaction turbines, unlike the previous ones, and also with axial flow, also perfect for low jumps, from 2 to 20 meters. The wheel blades are always adjustable, while those of the distributor can be fixed or adjustable, so there are two different types of these hydraulic turbines. The double adjustment one, the real Kaplan, is the one with all the adjustable blades while if only the wheel blades are adjustable we have a semi-Kaplan, or single adjustment.
The maximum power reached today by Kaplan water turbines is about 200,000 kW. From this "model" then derive the hydraulic turbines to be immersed in water in plants that exploit the motion of the tides: they are calledbulb and have the generator and multiplier suitably contained in a waterproof case.
Francis hydraulic turbines
We remain among the radial flow reaction hydraulic turbines, but the Francis have by force the distributor with adjustable blades and the impeller with fixed blades and are widely used for jumps between 10 and 350 meters. There is also the "Francis speedy" model, again with radial flow but axial exhaust. The peculiarity is that the water passes from the distributor to the impeller without coming into contact with the atmosphere at any time but are not water turbines with many advantages over the others, we find them almost only in pumping stations.
It is about hydraulic turbines which fully fall into the category "micro hydroelectric”Therefore of hydroelectric power plants with an electrical power not exceeding 100 kW, with limited environmental impact.
In this scale of action the most used are undoubtedly the Pelton, in addition to those called Banki, and are obviously ideal for limited water flow rates.
We find these micro plants both used with flowing water and within aqueducts, the conditions for their operation are two and essential. There must be a sufficient water jump, even a few meters, and a water flow rate, fairly constant and not seasonal, even just 0.5 liters per second are fine.
As for the concrete applications of hydroelectric microturbines, we look at both plants connected to the grid and isolated ones, with no preferences whatsoever. However, I cannot fail to mention that these specials are in the first context hydraulic turbines they can take advantage of the incentives dedicated to renewable sources as well as being an opportunity to make the exchange on the spot.
The microturbines have their advantages, therefore, and are also ideal for meeting the electrical needs of buildings without a connection to the network, if obviously there is sufficient water availability. Examples? Small isolated communities, farms, mountain huts and agritourisms.
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