Water-based Energy Production

09/10/2010

River currents represent a considerable source of energy. Electricity can be produced by using them to drive turbines. This form of energy access is commonly used throughout the world and has provided a significant share of energy production for several decades.

Harnessing the Energy in River Currents

Hydroelectric power plants use water's kinetic energy (generated by its movement), which has been known to humankind for centuries- it is this energy that drives the watermills beside rivers.

Modern hydraulic energy is harnessed in plants where electricity is produced. Artificial waterfalls or dams are used to channel the flow of water and facilitate the use of its kinetic energy. Water used to produce hydroelectric power can be stored for use during peak demand periods. This source is also highly efficient- 90% of water energy is converted into electricity. It also does not emit any greenhouse gases and is renewable. These advantages explain hydraulic power plants' popularity.  In 2008 they provided some 16% of the world's electricity.

A hydroelectric power plant has 3 main components:

   • A dam: its purpose is to create a large waterfall to drive turbines and to store enough water to supply the plant at all times (during low-water periods). Aside from the dam itself, there are pipes to transport the water to the plant, particularly in mountainous areas.

   • A diversion channel: this takes the water from its natural environment (river or lake) to supply the dam reservoir. This may be an open channel, an underground conduit or a pipeline.

   • A plant: this houses the turbines that are powered by the waterfall and drive the electricity generator (usually an alternator). One of the largest dams in the world is Itaipu in Brazil, where 62,200 m3 of water passes through the turbines per second, 125 times the rate of the Seine River in Paris!

Apart from producing energy, a dam can also be used to regulate a river's water levels. In addition, it provides a reservoir of water for farm irrigation and is also sometimes used for leisure activities (beaches and water sports).

Before building a hydroelectric dam, three conditions must be met:

   • Topographical: gorges or narrow river sections are generally the best places for building dams. Similarly, a large, flat valley is ideal for holding water.

   • Geological: the rocks on which the dam is built must be stable and non-porous for both efficiency and safety reasons.

   • Hydrological: there must be sufficient rainfall in the catchment area (all upstream water) supplying the dam basin to fill it and compensate for water loss through evaporation from the reservoir.

There are two main types:

   • Gravity dams: these are entirely built on extremely solid ground because it supports the retained water's full force. Gravity dams are made of concrete, earth or rock backfill.

   •    Arch dams: These are shaped like a convex arch, and are supported by their rocky side walls. The walls must be strong and are regularly inspected. This type of dam is used in narrow valleys or canyons, such as are found in mountainous areas.

Dams Require Constant Monitoring

A hydroelectric dam's design must take the many risks into account and minimize thesm as much as possible. Surveys need to be conducted on:

   • The dam's ability to withstand flooding. All dams today have draining channels to stop them from giving way in the event of severe flooding.

   •  The dam's ability to withstand earthquakes and, more generally, the stability of the ground around the reservoir basin.

Moreover, dams must be constantly monitored for water seepage in the body of the dam or below it, buckling in the dam, etc.

Significant Environmental and Human Impact

The construction of a dam often has significant environmental, economic and human repercussions.

Filling a dam requires displacing many people and often involves flooding large areas of farmland.

For example, the construction of the enormous Aswan Dam on the Nile in Egypt (retention capacity of 160 billion m³) led to a significant lowering of silt content in the water downstream. The river delta, which previously extended as far as the sea, began to recede. Farmers then had to use more fertilizer to maintain crop yields due to the lack of silt.

The impact on wildlife and flora is also considerable. Stagnant water behind the dam tends to be under-oxygenated, which damages the quality of the area's ecosystems. Over-oxygenated water with micro air bubbles resulting from sudden discharges of water is also harmful to the area's ecosystems.

Another factor is the fact that dams retain more than just water. They also retain sediment eroded by the river water that supplies the reservoir basin. Therefore, they tend to silt up sooner or later. The Sanmenxia Dam on the Huang Ho in China lost 41% of its storage capacity due to the mud deposits in the first 4 years after it was commissioned. This phenomenon can also be seen on dams on the Rhone River and the Camargue coastal area in France.

In summary, dams are a clean, renewable source of energy. In some parts of the world, hydroelectric production has significant potential that is thus far under-used. With its huge river basins such as the Congo and the Nile and the Great Lakes region, Africa has great potential for dam works. However, implementing this type of solution would require precisely measuring its impact on the environment and local populations.