Zoom on hydroelectricity

hydroelectricity, coal, electricity consumption, European Union Hydroelectricity is the third most used electricity source across the world after coal (30%) and gas (17%). In 2009, worldwide hydroelectricity production was an estimated 3,000 TWh, that is to say nearly a fifth of global production. Some countries, like Norway, Iceland and Switzerland get at least half of their electricity from hydropower. However, this large-scale electricity generation system is sometimes overshadowed by other renewable sources, such as wind and solar power. If you have ever wondered how a dam works, this overview will give you an insight into the main assets of a mature technology.

A wide range of power plants using hydropower

Hydroelectricity is mostly generated through dams. They are built on waterways with a relatively high water flow. The dam’s role is to capture water and create a reservoir lake. Opening the dam’s gates allows the reservoir’s water to flow down into penstocks, i.e. long metal pipes driving the water to the power plant located underneath. Forcing water into these narrow pipes make it run at a higher flow rate so it can drive a turbine, which generates electricity. The plant’s power output will thus depend on the height of fall and the water flow rate it provides. Once this cycle is completed, water generally goes back to its original stream, whose flow rate is regulated. There are four major kinds of hydropower plants:

‘Impoundment’ hydropower:

  • ‘Hydro-peaking’ plants: their mid-sized reservoirs are suited for occasional use to cover peaks in demand.
  • ‘High head’ hydroelectric stations are built on high-gradient streams with a height of fall of several hundreds of metres. They are also meant to cover peaks in demand. Thanks to the stream’s high gradient, they can be put into operation very rapidly.
  • ‘Medium head’ stations are mostly built in medium-altitude regions, with a height of fall of a few tens of metres. Like the latter types, they are not meant to run continuously.

Run-of-river hydropower: these ‘low-head’ stations are built on major streams with a very high flow rate. These dams don’t hold back the natural flow of water and thus produce electricity in real time.

Pumped-storage hydropower stations pump water back upstream during off-peak periods in order to store electricity for future peaks in demand (see our article on bulk energy storage).

Tidal power plants are a still little known but attractive intermittent electricity source. Using tidal movements to drive turbines, tidal power’s greatest advantage lies in its steady and predictable electricity production.

A strategic advantage…

When properly managed, water can be an unlimited energy source, while neither emitting directly greenhouse gases nor producing waste. Storing water in reservoir lakes above power stations allows to store electricity on a high scale, which can be made available very quickly through a high-voltage grid. Besides, although they are expensive to build, hydroelectricity stations have a long lifetime and low operational and maintenance costs. These advantages are the reason why hydropower is considered one of the energy sources that cause least harm to the environment.

… for countries with an adequate topography

Hydropower has an intrinsic weakness, though, as it requires the combination of topographical and hydrological factors only few countries enjoy. Moreover, these states must be able to afford extremely costly initial investments. In Europe, the only country that still enjoys a high hydroelectric potential is Norway. Besides, dams are colossal construction projects, which often require population transfers and sometimes cause violent social conflicts (as is the case for the Belo Monte dam in Brazil, which will eventually displace 25,000 people).

In strictly environmental terms, although a hydropower plant pollutes little once it’s built, its construction inevitably upsets surrounding ecosystems. The creation of artificial reservoir lakes floods natural areas which then decompose and can emit CO2. These lakes can also cause the drying up of aquifers downstream of the power station. Besides, dams act as barriers for fish, which can’t go up the rivers anymore.

The European Unions is currently at the heart of these contradictions. A 2000 directive aims at safeguarding aquatic resources, for instance by tripling the number of waterways on which the construction of hydroelectric stations is prohibited. Parallel to that, the “super hydro project” should unlock nearly 500 million euros to fund the construction of new power plants, thus increasing the production of hydroelectricity by 7 TWh. European legislation has not yet really solved the dilemma between preserving ecosystems and producing burden-free energy.

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