What are the two types of hydropower

Different types of hydropower plants

Distinguishing criteria

After Mode of operation A distinction is made between run-of-river and storage water power plants. In run-of-river power plants, the available water energy is used continuously, in storage power plants (dams) as required to generate electricity.

You can also use the hydropower plants after the Height of fall differentiate: In the range up to about 25m one speaks of low pressure power plants, up to 100 m of medium pressure power plants and over 100 m of high pressure power plants. In the low-pressure range, apart from the Pelton turbine, all turbine designs are used in very different installation variants. In the medium pressure range, vertical Kaplan turbines or - depending on the water flow and head - Francis turbines are mainly used. Finally, the high-pressure power plants are a domain of the Francis and Pelton turbines, the latter being used all the more likely the higher the head with a relatively small amount of water.

Run-of-river power plants

In Germany, the vast majority of hydropower generation comes from run-of-river power plants. With these, it is usually not possible to control the water flow depending on the electricity demand. As a rule, they are therefore operated continuously around the clock and feed their electricity into the public supply network to cover the base load.

In run-of-river power plants, due to the low gradient, mostly Kaplan turbines used.

In the past, some power plants had the option of accumulating an additional amount of water when there was less electricity demand, which could then be delivered to the turbines when there was a peak demand, thus providing an electricity reserve. Due to the associated surge and sink operation (water level fluctuations) in the rivers, this is currently not permitted under the currently applicable legal regulations in Germany.

Small hydropower plants

The usual power limit to differentiate between “large” and “small” systems is 500 kW. Often it is about earlier mill systems that had a water wheel at a weir or through diversion into a mill ditch and today the majority are equipped with turbines (mostly Kaplan or Francis turbines). All of the old mill locations can often be traced back in history many hundreds of years, as the small hydropower was the nucleus of mechanized work (mills, hammer and stamp mills) and at the beginning of industrialization the only available energy source. Many place and field names (e.g. Mühlgasse, Mühlsteig) still indicate the long tradition and wealth of ideas of our ancestors.

For this reason, old weirs and mill ditches are an integral part of our grown cultural landscape and cannot be imagined without it. Mill ditches enlarge the runoff profile and protect against flooding, they enlarge the water body and thereby form valuable biotopes and thus habitats for animals and plants. These valuable biotopes can be preserved through further use of the old mill locations. At the same time, the respective operator is given a livelihood, which at the same time obliges him to preserve the valuable cultural space for future generations.

In the case of major changes to old locations, it should be considered whether the energy gain justifies the intervention in the landscape. When decommissioning old facilities, it is also important to ensure that valuable cultural areas or biotopes are irretrievably lost as a result.

Storage water power plants

In the case of storage water power plants, for example, the water is stored in a dam that dams a stream or river in a high lake and fed from there to the turbines of the lower-level power plant via pressure pipes or pressure tunnels. An expansion tank (“water lock”) is built in front of the pressure pipe downpipe. When the turbines are switched off, the surge tank absorbs the amount of water pushed in by the reservoir and thus dampens the rise in pressure in the pipe and turbine. The water locks are designed as shafts standing vertically on the pressure pipe and can reach great heights. In storage power plants, depending on the height of fall Francis turbines or Pelton turbines their areas of application.

Storage water power plants are usually not intended for continuous operation, as otherwise their storage basins would soon be empty. Rather, its purpose is to store the water that arises differently in weeks, months and seasonal changes and to postpone it if there is an increased demand for electricity. They are therefore also called “top-performance power plants”.

The storage water power plants often serve other purposes at the same time, such as flood protection, drinking water storage, irrigation purposes or the needs of shipping.

Pump storage power plants

In pumped storage power plants, the high-lying storage basin is usually not filled by a natural, continuous inflow. Where there are such natural tributaries, they usually only have a complementary function. Rather, all or most of the water comes from a deeper basin and is pumped up with electrical energy.

At first glance, this may seem absurd, as the amount of energy required for pumping up must inevitably be greater than the electrical energy that can be generated afterwards with the water that is pumped up. Technically and economically, this two-fold energy conversion from electrical current to potential energy and back still makes sense: It makes it possible to use the underutilized capacities of the base load supply for pumping up the water in times of low electricity demand. When demand peaks then occur, the turbines are switched on and convert the potential energy of the pumped water back into electricity. The thing is also financially worthwhile, as in this way, for example, cheap nighttime electricity can be converted into expensive daytime electricity.

In practice, pumped storage power plants achieve an efficiency of around 75% (i.e., around 1.3 kWh must be used to generate 1 kWh), so that a quarter of the energy used is lost.

Tidal power plants

Already in a document from the 11th century a tidal mill is mentioned, which in the port of Dover used the high or ebb current to drive its grinder. Such mills were particularly effective in the estuary funnels of the rivers, where the current is strongest.

Today, the energy of the tides can also be used to generate electricity. A prerequisite, however, is a sufficient tidal range, such as that achieved in the Rance estuary near St. Malo in France. The difference between the highest and lowest water level is about 12 to 13 meters here. In the 1960s, the estuary was therefore closed off with an artificial dam against the sea and the considerable gradient between high and low tide was used for a hydroelectric power plant that is able to generate around 600 million kWh of electricity annually.

However, tidal power plants have the disadvantage that their maximum capacity shifts daily by around 50 minutes with the rhythm of the tides. The natural prerequisites for this are only available in a few places on earth.

Wave power plants

Smaller performances can also be achieved by using the waves. However, only on favorable coasts such as England, Norway, France or Denmark.

In one process, the waves are directed into a concrete chamber. The sudden rise in water compresses the air in the chamber, and the resulting compressed air drives a turbine. The negative pressure when the shaft sloshes back is also used to drive the turbine.

Another method uses the ups and downs of the waves to make a piston designed as a floating body do the work. There are a number of other variants, for example in the form of a collecting funnel that carries the waves up several meters and then directs them to a turbine.