Hydroelectric power: how it works and the most important hydroelectric projects around the world

In many countries, hydroelectric power is by far and away the leading source of renewable energy. Take Italy, for example: hydroelectric power plants produce 17.6% of all the energy produced within Italy, compared to 8.9% produced through photovoltaics. But how does a hydroelectric power plant work, and how is electricity produced from the movement of water?


Hydroelectric energy: how does it work?

Hydroelectric power is produced in hydroelectric power stations. Typically, it starts with the damming of a water course, which can take place at quite a distance from the hydroelectric plant itself. This may involve a dam over a river, or a dyke. In either case, the damming structure stops the flow of water, creating a reservoir where the water collects. The result may be the creation of an artificial lake or the filling of a large cistern. This water reserve is connected to charge basins by means of channels, diversion tunnels and so on. A controlled flow of water is then channelled from these basins towards the hydroelectric turbines, where the process of transforming mechanical energy into electrical energy begins. More specifically, the water is transferred from the artificial reservoir to the turbines, passing through penstocks which are specially designed to increase the hydraulic power with which the flow of water meets the turbine to the maximum level possible.

The blades of the turbines are designed specifically to exploit the full energy of the water, and as they rotate they turn a central shaft attached to an alternator, that is, a rotating electrical generator that can convert mechanical energy into electrical energy. At this point all that remains is to pass the electricity obtained in this way through a transformer; the task of the transformer is to increase the voltage and lower the current intensity, so that the electrical power can then be transmitted over the power lines of the grid connected to the plant. Once it arrives at its destination, the electrical power is again passed through a transformer to lower the voltage and increase the current intensity, so that it can be used to supply electricity to industrial or domestic devices.

It should be pointed out that once the water has passed the turbines, it has completed its task and it is then conveyed through drainage channels downstream of the dam, where it is reintroduced into the water course from which it originated.


Different types of hydroelectric power plants

Now that we know how hydroelectric power is produced, it may be interesting to consider the different types of power plant; these consist of conventional power stations, run-of-river power stations and pumped storage power stations.

With conventional power plants, everything starts from a reservoir located upstream, which may be an artificial lake created by a dam, or a natural lake. The water from this is channelled through penstocks towards the hydroelectric turbines as we saw earlier.

In the case of run-of-river power stations there is no actual reservoir. Quite simply, the power plant exploits the natural flow of the water course, without creating forced channels for the water to ‘fall’  through. The turbines are located at natural ‘jumps’ in the water course, that is, where it passes from one level to another. These power stations are heavily dependent on the flow of the water course itself and do not allow actual control of the water flows, unlike reservoir power stations.

Finally there are the pumped storage power stations. These have two reservoirs, one upstream and one downstream. The flow is continuous, as is the energy production. At times of lower energy demand, the surplus energy is used to pump the water from the downstream reservoir to the upstream one, so that there is an additional energy reserve to cover peak demand periods.


Hydroelectric power in Italy

There are about 4,300 hydroelectric power stations in Italy which produce 46 TWh a year: the sector employs over 15,000 personnel. Thanks to investments made in the past in this sector and to its natural mountainous terrain, Italy is the fourth biggest hydroelectric power producer in Europe, after Norway, Sweden and France.

The largest hydroelectric power plant in Italy is at Entracque, in the province of Cuneo. This power station started operation in 1982, and uses 3 different reservoirs: the Chiotas dam reservoir, situated at an altitude of 2,000 metres; the Rovina lake, at 1,500 metres; and the Piastra Dam reservoir, at 1,000 metres. All of these are connected by long underground pipes, reaching a power generation capacity of 1,065 Gigawatts, 5% of total capacity in Italy and sufficient to meet the needs of the whole of Turin province.


The largest hydroelectric power stations in the world

There are estimated to be about 62.500 power plants for producing electricity globally. In contrast to what many people believe, the plants that generate the most power are neither coal-fired power stations nor nuclear power stations. 9 of the 10 largest power stations are actually hydroelectric. The largest power station, with a capacity of 22.5 Gigawatts is the Three Gorges Dam in China, built in 2006 on the river Yangtze; the second largest, with an installed capacity of 14 Gigawatts, is the Itaipu Dam, on the river Parana, between Brazil and Paraguay. It should be noted that the South-American plant is capable of generating more electricity in practice, achieving 89.5 billion kilowatt hours compared to the 86 generated by the Three Gorges Dam.


Advantages and disadvantages of hydroelectric power

One of the great advantages of hydroelectric power is the possibility to produce huge quantities of electricity with a net reduction in harmful emissions, and with no limits: as long as there is water, it will be possible to continue to produce energy in this way. Moreover, it should be pointed out that hydroelectric power stations allow energy to be produced economically, with the most significant costs being incurred only during the first phase, that is, in the construction of the dam.

The damming of the water itself provides a further advantage: good control over the flow of water, thus limiting the danger of flooding, for example.

As we know, hydroelectric power has disadvantages, too. Long and exceptional periods of drought can reduce electricity production significantly; the construction of dams and the consequent creation of large artificial reservoirs result in serious disfigurement of the environment, leading to the elimination of entire habitats, as well as significant deforestation. Added to the environmental impact are the potential effects on the local community. These may include the destruction of whole towns and villages, as well as disasters linked to errors of calculation in the construction or management of the dams, from the Vajont Dam tragedy onwards.