Run-of-river hydropower – in simple terms

Hydropower supplies around 60 percent of the energy produced in Switzerland. This is good, because electricity from hydropower is renewable, indigenous, reliable and climate-friendly (CO2-free). Around half of the hydroelectric power comes from power stations on rivers and streams - i.e. run-of-river power stations. They are divided into different categories depending on their size and type of production. You can find out here what they are - and how hydroelectric power plants work.

Hydroelectric power is one of the renewable energies. The flowing water (kinetic energy) is conducted through a turbine (mechanical energy), which drives a generator, where it is then converted into electricity (electrical energy).

How exactly run-of-river power plants work can also be read and looked up here in our dossier Energy Knowledge by means of graphics and videos (Hydropower A-Z).

Run-of-river power plants are built on rivers and use the energy of water flowing down a gradient. In most cases, this is only a few metres, but since several hundred tonnes of water can flow down per second, run-of-river power stations can have a large capacity of several megawatts (MW).

Electricity for 70,000 households

One example: Axpo's Eglisau-Glattfelden power plant has a maximum drop height of eleven meters. At a flow rate of 500 m³/s, this means that around 500 tonnes of water per second (a cubic metre of water has a mass of just under a tonne) flow through the seven new Kaplan turbines, which together have a capacity of 43.4 MW and produce an average of 310 GWh of electricity per year. This is enough to supply almost 70,000 households with an average electricity consumption of 4500 kWh.

Due to the even flow velocity of rivers, the output is also constant throughout, i.e. hydroelectric power plants supply reliable band energy around the clock 24/7, although this is subject to seasonal fluctuations. As a general rule, the effective annual production of all hydroelectric power plants fluctuates plus/minus 20 percent depending on precipitation, runoff and (storage) management.

In Switzerland, the rivers Aare, Reuss and Rhine in particular are used intensively to produce hydroelectricity. On the High Rhine between Schaffhausen and Birsfelden (BL) alone, eleven run-of-river power plants with an installed capacity of around 830 MW produce an average of around 5 TWh of electricity per year. A total of 4132 MW of capacity is available in run-of-river power plants in Switzerland to produce around 17,800 GWh of electricity annually.

Large and small, low and high pressure

Run-of-river power plants are divided into large hydropower (more than 10 MW capacity) and small hydropower (see also Energy Knowledge/Hydropower A-Z).

For this purpose, power plants are also differentiated according to the gradient that they can exploit. This is because the greater the gradient, the greater the pressure with which the water drives the turbines. This is why there are low-pressure (gradient up to 15 metres), medium-pressure (up to 400 metres) and high-pressure power stations.

Low-pressure power plants dominate the run-of-river power generation. The Axpo Eglisau-Glattfelden power plant, for example, with its 11-metre head, belongs in this category. And also the Rheinau AG power station, in which Axpo has a 50 percent share. The useful gradient there is around 10.5 metres, the turbines can process a water volume of 400m3/s, have a capacity of 37 MW and produce an average of 244 GWh of electricity.

However, there are also high-pressure run-of-river power plants in Axpo's portfolio, such as the Fätschbach power plant. It is located in southern Glarus in the municipality of Linthal. It uses the water from the Fätschbach stream in the Urnerboden region, whereby the available water is continuously used to produce electricity with practically no significant intermediate storage. The water is collected in a 10,000 m3 compensating reservoir, where it flows through a 3.2-kilometre-long pressure tunnel and a 1.1-kilometre-long pressure pipe over a usable gradient of around 578 metres to two turbines with a capacity of 15 MW. This will produce around 80 GWh of electricity, three quarters of which is generated in the summer half-year.

Axpo: Switzerland's largest producer of hydropower

By the way, did you know that Axpo is Switzerland's largest producer of hydroelectric power? The annual production of all the power plants (run-of-river, storage and pumped storage plants) that Axpo and its subsidiaries own or co-own is around 7.7 TWh of energy.

More information about hydroelectric power can also be found in this video: