Water and electricity
Water as a driving force
The power of water has been used since ancient times to obtain mechanical energy for different types of machines. The more common example is that of the water mills where water, properly channeled, made spin a special “wheel” which was connected (through the gears) to the grindstone. Several other factories have over time been operated by hydraulic energy: anvil, sawmills, mills, etc …
Only in the last century, man began to use the power of water to produce a more easily transportable type of energy and so usable from a great distance from the production area: electricity.
Before explaining how to use the power of water to get electricity it is good to briefly explain how to produce electricity. Electric current is achieved by rotating a magnet in the middle of a winding of metal wire. The magnetic field produced by the rotating magnet “induces” in the winding a magnetic field, and then electric current. The machines that do this are called “dynamo” or ” alternators” depending on whether they produce DC or AC. A dynamo used in everyday life is the one that powers the light of some bicycles. The magnet in this case is made rotating by the wheel and then actuated by the person who is pedaling. Of course, for the production of large amounts of electric current are built very large alternators and dynamos; inside these machines, there are no magnets (because in nature they are not so large) but of artificial magnets obtained by passing the electric current around the iron cores. The little current needed to create the electromagnet is produced by a smaller dynamo. The use of water for the production of electrical energy. After this brief explanation on how to get the electrical current is easy to see that in the process described the power of water is used to spin the dynamo or the alternator. This does not occur directly but through a “turbine”; a sort of mill “wheel” moved by the water and connected to the dynamo and to the alternator through a metallic shaft. The water to make the turbine rotate must have a certain speed; it is therefore necessary that the point where water is picked up is located in a higher place than the one of the turbine. All this can be achieved in various ways depending on the amount of water available, the hydraulic scheme of the stream used, the amount of energy to be produced. One method is to deviate with a small dam the water of a stream or a river, into a channel. The channel allows you to find water in the place where the electricity is produced. This place is called “hydroelectric power station” and it is a building in which there is a whole range of equipment including turbines and alternators. In particular the water, through the channel, is carried in a “loading tank” situated a little more upstream and at a certain height above the center; departing from this tank of large pipes ( “forced ducts”) in which the water falls at great speed making the turbines work and operating so the alternators within the plant. In case of torrential rivers it can happen that during periods of low there is not enough water to operate a hydroelectric plant. In these cases you can build dams and create artificial lakes in which you can store rain water to use it during the dry season. An artificial reservoir can be compared to a bathtub: on its bottom, generally in the vicinity of the dam, there is a drain to which are connected the forced ducts leading the water to the turbines of the power plant. If the tank is full and continues to get more water than the one used by the hydroelectric power station, this is gathered downstream of the dam by a surface-water drain which functions as the drain that prevents water going out of the bathtub when you forget the running tap. Sometimes the same water is used for more stations; these are arranged at a certain distance from each other and each station is located in a higher position than that of the next central. In this way the water after having operated the turbines of the first station can, traveling the difference in level that separates it from the next, reach the speed (and hence the energy) needed to make the turbines spin. Downstream of the second plant there can be a third station, situated in a lower position, and so on. Finally, water is returned to the river or the stream from which it was taken, or may be used for other purposes such as crop irrigation or service aqueducts.
Transportation and distribution of electricity
As already mentioned, the electricity generated in power plants can be easily transported and used in places, even distant: electricity we use at home may also have been produced hundreds of kilometers away from our countries. The energy transportation occurs through the high-tension wires. These are so called because the current that runs the length of them has a voltage of hundreds of thousands of volts. The electric current that we use in our houses instead has a voltage of 220 volts, while the current produced in the power stations has still a different voltage. There are some machines called “processors” that are able to change the voltage of the electric current. The electric current produced in power plants is transformed into high voltage current to be transported in electrical networks for long distances; then other transformers reduce the voltage to get current arrive at 220 volts in houses.
The dams are river levee works used to form the reservoirs (artificial lakes or basins) that have at least one of the following purposes:
- Electricity generation
- Drinking water supply
- Rolling flood
Dams are divided into three basic types based on how they resist the buoyancy of water reservoirs:
- Gravity, where the water pressure is opposed by the weight of the dam,
- Arc, which discharge the water pressure on the sides of the valley where the dam is set
- Arc-gravity (or with combined action), in which collaborate both the own weight of the dam and the arc effect.
Extra elements of a dam:
- the taking work, which includes the structures and equipment necessary to pick up the water from the tank for water distribution, irrigation, or production of electricity.
- the drain work:
– The spillway, through which the flow of the stream, with full tank, can cross the dam without damage to neither the artificial works nor the natural riverbed;
– The bottom drain, by means of which it is possible to rapidly empty the tank to allow the inspection of the normally submerged parts.
The old dam
The “old dam” is located along the gorge of Cellina few hundred meters downstream of the confluence of the stream Molassa.
It was the levee and taking work of the first industrial plants that have exploited the waters of the Cellina river for hydroelectric purposes.
It was built between 1900 and 1905 and used to divert some of the stream water in the intake channel that fed the four groups turbine-generator Francis – alternator to the horizontal axis of the Malnisio power station.
The dam is only 18 meters high; in fact it is not a dam itself but more a crossbar: levee work, placed orthogonally with respect to the stream used to capture only a part of the water without creating a reservoir upstream.
The flow channel was then less than the minimum flow rate of the stream in order to have always the water needed to operate the plant. In this way also, the water not diverted continued its travel along the channel, following its normal course in the riverbed of the stream.
The dam foundations are made of concrete material while the elevated part is made of squared stone; the slides on which the unused water falls and then continues along the natural riverbed, are covered with large square blocks in porphyry.
Above the dam a three meters large deck had been created and on which there was the road that connected Montereale with Andreis and Barcis.
Turbines convert kinetic energy into rotary motion. They are actuated by the water which falls from a certain level to a lower one, along forced ducts. The rotary movement of the “impeller” (as it is called that device, within the turbine, which is rotated by the water) is then transmitted to a dynamo or an alternator through a metallic shaft.
There are three basic types of turbines:
- The Pelton turbine, generally with horizontal axis, is particularly suited to deep differences in height. The impeller is constituted by a disc, in the periphery of which are mounted double spoon- shaped small shovels.
- The Francis turbine, with vertical or horizontal axis, is used for medium drops and medium flows. It is characterized by a spiral forced room: “snail”.
- The Kaplan turbine, generally with vertical axis, is used in hydroelectric plants of great power and represents the type of turbine with increased efficiency. The impeller has the form of a propeller with movable blades that can be oriented to better exploit changes in flow rate or in water speed.