(Transcript of the lesson commentary.)

History of water utilisation

Flowing water is a necessary condition for life on Earth, but it also holds immense power. People have used its energy since ancient times. The flowing water turned various kinds of water wheels. Their motion was transmitted by gears to other devices that ground grain, pumped water into irrigation canals, or powered saws or hammers. Water wheels were mostly wooden, later metal. A number of ways of bringing water to the wheel have been developed. This greatly affected their efficiency, which in the case of a well-designed overshot wheel can reach around 85 percent. Water wheels was gradually replaced by cheaper and more efficient turbines. 

Water cycle

The energy of water 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, renewable and inexhaustible. 

Hydroelectric power plant principle

Water energy is mainly used to drive turbines, which are more compact and efficient than water wheels. In a turbine, water energy is converted into rotational motion. There are different types of water energy that we can exploit. The best known is kinetic. The faster the water flow, the more energy it transfers to the turbine. The initial energy of the water could be potential energy — the water comes from a point high above the turbine, and as it passes through the penstock, its potential energy changes to kinetic.

Some types of turbines also use pressure energy. Water at the bottom of a high column, such as at the bottom of a reservoir, is under a large pressure. It then jets rapidly through a nozzle onto the turbine blade. The pressure energy is thus converted into kinetic energy, which is converted into rotational energy of the turbine. This motion is transferred by a common shaft to the rotation of an electrical generator, which generates electricity. The engine room with electric generators is located in a typical accumulation hydroelectric power plant under the dam, behind which there is a large water reservoir. 

Types of hydroelectric plants

Not all hydropower plants are the same, just as not all rivers and natural conditions are the same. Some rivers have a high head but low flow, others have a lot of water but without significant head. The flow of many rivers 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. 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 erosion of the core. 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. 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 the 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 a 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. 

Water turbines

To make the most of natural conditions, different types of turbines are used for different hydropower plants. The most commonly used are the Kaplan, Francis, Pelton and Banki-Michell turbines.

The Kaplan turbine is a reaction water turbine whose shape resemble a ship propeller. It 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 lower hydraulic heads and relatively high, variable rates of flow. They reach efficiencies of up to 90 percent.

Another type of reaction water turbine is the Francis turbine that 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. The turbine has a high efficiency exceeding 90 percent. It is suitable for heads between 20 and 700 meters. Due to its reversing capability, it is often installed in pumped storage power plants. 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. Power output can be regulated by narrowing or widening the nozzle. The efficiency varies between 80 and 95 percent depending on the size of the turbine. 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. 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 at the bottom of the turbine. It has a simple design and its performance does not change significantly with changes in flow. When used on streams with variable flow, it has an efficiency of around 85 percent. 

Advantages and disadvantages

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. 

Electricity production

Water flows day and night, in good weather and in storms. Hydroelectric plants are therefore very flexible. They can supply baseload electricity, but it can also serve as peak-load thanks to their fast start. In 2022, hydropower generated 4,300 TWh of electricity, covering around 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. In addition to generating electricity, hydropower plants also play an essential role in its storage. For this purpose, pumped storage plants are used.

They 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.

Although hydropower plants are very important sources of electricity, they cannot be built everywhere. Suitable locations with suitable watercourses are essential. It is estimated that, if the entire global hydropower potential were used, hydropower plants could generate up to 16 PWh per year. In some regions, the possibilities of building giant hydroelectric dams have already been exhausted, but there are still opportunities to build small hydropower plants on small watercourses.

The output limit under which a power plant may be considered as small depends on local legislation and usually is only a few MW. These small power plants are often driven by a cross-flow turbine (or Banki-Michell turbine); a cheap and simple design with the water passing through the blades of the cylindrical runner twice. Where higher yields are expected, Kaplan or Francis turbines may be installed. Throughout the year, the flow rates in small power plants vary significantly. Therefore, the power output fluctuates as well. If proper building and design solutions are applied, small hydroelectric power plants can be considered one of the most ecological and economical sources of electric energy.