Characteristics of Hydropower

(Transcript of the video commentary.)

Flowing water is a necessary condition for life on Earth, but it also holds immense power. It has shaped the face of our landscape for millions and millions of years. Finally, humans have managed to harness its potential.

They used water to turn the blades of their water wheels. The motion was transmitted to other devices, and the flowing water thus ground grain, pumped water into irrigation canals or powered saws and trip hammers. Waterwheel technology continued to improve in an effort to make the best use of hydro power.

Energy of water is inexhaustible as it is constantly renewed by the water cycle in nature. It is driven by the power of the sun and gravity. The sun evaporates water from the surface of rivers, lakes and oceans. The water vapour rises up and condenses into clouds that are carried away by the wind. The water they contain falls to the ground as rain, snow or hail. There, gravity takes hold and pulls the water to lower altitudes. As the rivulets join together, they form streams, rivers and eventually flow into the sea. The water is evaporated by the sun's rays and the whole cycle begins again. Water energy is clean and renewable.

At present it is mainly used to drive turbines. The flowing water hits the turbine blades and makes it spin. The kinetic energy of the water is converted into the rotational energy of the turbine. This is transferred by a common shaft to the rotation of an electric generator, which produces electricity. In 2022, hydropower generated 4,300 TWh of electricity, covering about 15 percent of global consumption. The largest contributors were hydroelectric plants in China, Brazil, the US and Canada. Worldwide installed capacity was 1,220 GW. But not all hydropower plants are the same, just as not all rivers and natural conditions are the same. Some rivers have a high head, others have a lot of water, whereas in some places, the river flow is relatively small but has water all year round, while elsewhere the river flow fluctuates according to the seasons. Different types of power stations are built to make the most of the site’s potential.

An accumulation hydroelectric power plant always has a reservoir of water created by artificial damming of the valley. The difference between the height of the reservoir and the river below it provides sufficient gradient and therefore kinetic energy to the water flowing to the turbine. In some cases, the pressure energy of the water discharged from the bottom of the reservoir is used. In addition to generating electricity, the reservoir is often used for drinking water storage, river level regulation during floods or droughts, and recreation. The reservoir can be created by different types of dams.

Concrete arch-shaped dams are thin shells firmly anchored in the steep slopes of surrounding deep canyons. The concave shape of that shell allows for the immense water pressure to be directed to the stone walls of the canyon.

Earth-filled dams oppose the water pressure by their weight. They can be made of earth, clay or stones, with a concrete core. The water side of the dam is built from solid materials in order to prevent an erosion of the core.

Gravity dams are usually built from heavy mixtures of concrete and rock and they retain water only by their weight. A lighter version of the gravity dam is a buttress dam, which has support pillars on the outside.

A power plant without a large reservoir is a so-called derivation plant. It provides the necessary head by diverting a portion of the river flow into a derivation channel that is much less steep than the original watercourse. After a few kilometres, the difference in levels is such that the water can be released to the turbine. This principle was already used in the Middle Ages when building a millrace. If there is sufficient water flow in the river for most of the year, it is possible to build a power plant directly on the river. This type, called run-of-the-river, needs only a low water rise and is therefore suitable for flat regions. Its performance depends strongly on the variation in flow due to the seasons. Different turbines are suitable for different types of power stations. The most commonly used are the Kaplan, Francis, Pelton and Banki-Michell turbines.

The Kaplan turbine whose shape resembles a ship propeller has adjustable runner blades as well as guide vanes. This allows it to maintain a constant power output even with fluctuating flow. Kaplan turbines are typically used in places with relatively high, variable rates of flow and lower hydraulic heads. The Francis turbine has fixed runner blades but adjustable guide vanes. In this way, the turbine is able to run at a constant revolution frequency and drive the electric generator regardless of changes in the rate of flow. It is suitable for heads between 20 and 700 meters. The world’s largest power plant, Three Gorges Dam, is equipped with Francis turbines with a diameter of 10 metres. The Pelton turbine uses the hydraulic pressure of a high water column, which is converted into kinetic energy of the jetting water. This is directed through a nozzle tangentially to the spoon-shaped buckets on the runner. The water jet is split in half by a ridge in the bucket which effectively turns the stream around, and since water is almost incompressible, it gives away almost all its energy. Pelton turbines are often used in power plants with a very high hydraulic head, typically in mountainous regions like Austria or Switzerland.

The inexpensive Banki-Michell turbine, also known as a cross-flow turbine, is suitable for small hydropower plants. It has a simple design and its performance does not change significantly with changes in flow. The turbine has the shape of a horizontal cylinder fitted with numerous blades, arranged radially and tangentially. The water directed by the nozzle hits the top of the turbine, is reflected into the turbine, and hits the blades again as it exits the bottom of the turbine. Because water flows day and night, in good weather and in storms, the hydroelectric plant is very flexible. It can supply baseload electricity, but it can also serve as peak-load thanks to its fast start.

A special type of hydropower plant is a pumped storage plant. Its primary purpose is not to generate electricity, but to store it. The pumped storage plant consists of an elevated reservoir and a low-lying watercourse or dam. The surplus electricity, e.g. from renewable sources, is used to pump water into the upper reservoir. The electrical energy is converted into potential energy of the water. During the time of electricity demand, water is released from the upper reservoir to the turbine. Its potential energy is converted into kinetic energy, which is converted into electrical energy. The pumped storage plant can quickly change its mode of operation from pumping to power generation within just a few minutes and in this way to dynamically respond to the actual load situation in the grid.

While some run-of-the-river hydropower plants affect the river only minimally, the construction of a massive dam represents a large environmental impact. A vast area is flooded, animals and plants lose their habitat, and people have to move out. The natural migration of fish is disrupted and methane releasing silt can be deposited at the bottom of the dam.

However, the construction of the reservoir also brings some positive aspects. In addition to generating electricity, it becomes a reservoir for drinking water. It can regulate the flow of the river during droughts or floods and thus protect the surrounding landscape. Its large body of water can be used for recreation and eventually create a new habitat where many aquatic and coastal animals and plants can thrive.

Because it is a clean, renewable and highly flexible source of energy, hydropower is very important. But hydroelectric power plants can't be built everywhere — not all countries have suitable locations, not all rivers flow through suitable terrain or have enough water. In some parts of the world, the hydropower potential for large-scale power stations is virtually exhausted and only small hydropower plants can be built on small watercourses. However, a number of sites remain unexploited. It is estimated that, if the entire global hydropower potential were used, hydropower plants could generate up to 16 PWh per year.