Hydropower: Renewable and climate-friendly

How it works

Hydropower is one of the renewable energies. In the past, hydropower was used for simple purposes by building a water wheel on a watercourse and using it to drive a mill or machine.

Today this is how it works: The kinetic energy of the flowing water is converted into the mechanical energy of the turbine, which is converted into electrical energy, or electricity for short, in a generator.

A distinction is made between different types of power plant depending on the type of construction. The most common are the so-called run-of-river power plants. They are built on rivers and use the energy of the flowing water. Usually this is a river section of only a few metres, but since hundreds of tons of water can flow down per second, run-of-river power plants dispose up to several megawatts of power. Due to the uniform flow velocity of rivers, output also remains constant, although it is subject to seasonal fluctuations. In the case of run-of-river power plants, a distinction is made between large-scale hydropower (more than 10 MW capacity) and small-scale hydropower (see also Hydropower A-Z). The former generates around 90% of Switzerland's total hydropower production.

Storage power plants have large reservoirs and use the gradient between the lake and the power plant. The water flows through large pipes or tunnels from the reservoir to the lower-lying power station (the differences in height can be up to 100 metres), where it drives turbines. It is then slowly fed back into a river via an intermediate reservoir.

Pumped storage plants have reservoirs at various altitudes. In such a system, the electricity can not only be stored and turbined, but also pumped up into the higher lake. Excess electricity from the grid is used for this purpose, for example when there is a great deal of sun and wind at the same time. Pumped storage power plants thus serve to store electrical energy and function like a battery that feeds its stored electricity back into the grid when demand is high (peak load).

Finally, Tidal power plants use the power of ebb and flow. Tidal power plants of this type are used, for example, in France, in Brittany near St Malo.

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Our hydropower

In Switzerland, around 60 percent of the electricity produced comes from hydropower. That's good, because this electricity production is domestic and therefore reliable, nearly CO2-free, storable and renewable. Switzerland is well ahead in Europe – only Norway, Austria, Iceland and Albania have a higher share of hydropower electricity production.

As Switzerland's largest producer of hydropower, Axpo makes a major contribution to this top position. Its hydroelectric power plant park (owned and co-owned) currently comprises around 60 plants. The Limmern pumped storage plant (LPSP) was also added to the portfolio. It not only stores and turbines water, but can also store excess energy from the grid and pump water from Lake Limmern to Lake Muttsee at 2500 metres above sea level. If necessary, this water can be re-turbined - the LPSP functions like a large battery.

Hydropower has a long tradition

For over 100 years, hydropower has been generating reliable energy in Switzerland, when needed at the push of a button. Axpo has also been using hydropower for a long time: Our Löntsch power station is a high-pressure storage power station with a reservoir in the Klöntal valley and headquarters in Netstal, Canton of Glarus. The plant was built between 1905 and 1908 by the former Motor AG, Baden, and taken over in 1914 by the then newly founded Nordostschweizerische Kraftwerke AG (Axpo since 2009).

Together with the Beznau hydraulic power station in the lower Aaretal in the Canton of Aargau, the Löntsch power station at the beginning of the 20th century formed the first significant interconnected operation between a high-pressure storage and a low-pressure run-of-river power station.

Hydropower from A-Z

Hydropower has many different aspects. Here is a quick overview of a few important keywords

Hydropower in keywords

Riverside woodlands, idyllic riverbank sections and gravel islands on the old Aare watercourse near the Wildegg-Brugg (Axpo), Rupperswil-Auenstein (Axpo and SBB) and Rüchlig (Axpo) power plants are popular recreational areas for hikers, sports enthusiasts, fishermen, and tranquillity seekers. However, the residual water in the old Aare watercourse between Brugg and Aarau also poses dangers, in particular when surge water occurs. This is the case, when the power plant turbines have to be shut down abruptly for technical reasons. The weirs open to the old Aare watercourse because the water flowing in from the Aare cannot be dammed. Within a short time, the residual water volume exceeds the legally prescribed threshold. Unexpected flooding can also occur owing to far away weather conditions.

Special alarm systems with flashing lights and loudspeakers have been installed at the Wildegg-Brugg and Rupperswil-Auenstein power plants to warn against sudden water level increases. In such a case, the system triggers a siren, which can be heard at great distances. The siren sounds in three 20-second intervals and is followed by an announcement "Attention, danger of flooding - please leave area!" At the Rüchlig power plant signs point out the dangers.
As is the case along streams and rivers in power plant areas, numerous such signs have been posted on the banks of the old Aare watercourse. They point out the danger of a sudden water level increase, which can also occur under good weather conditions. Picnic areas on the popular gravel islands can suddenly no longer be evacuated. In the worst case, flooding occurs.

Axpo has also published a flyer on the potential danger and the warning system. Flyers are available at community town halls in the region and also provided to various associations and institutions. 

Run-of-river power plants use the flow of a river to generate electricity. They usually have low heads and are used for large volumes of water.

The water (upstream) is directed to the turbines. The kinetic energy generated by the flow drives the turbines, which convert the energy into electricity via generators. The electricity is fed into the grid. The water used to generate electricity is fed back into the river (downstream).

Run-of-river power plants produce base load energy and, in contrast to storage or pumped storage plants, cannot adjust the quantity of electricity as required. The amount of electricity produced depends on the water flow and the flow velocity of the river.

In addition to generating electricity, run-of-river power plants often also serve as flood protection. Fisch ladders and locks are installed to enable the passage of fish and ships.

Small-scale hydropower plants are hydropower plants, usually run-of-river plants, with an output of less than 10 MW.

Video: How does a run-of-river power plant work? (Source: ARD, German only)

The rake’s parallel metal rods are used to catch floating debris. They are usually installed to protect the hydroelectric power stations behind them and can also be used to guide fish into safe bypasses when ideally arranged for fish descent. 

A storage power plant is a hydroelectric power plant that stores water in a reservoir and uses it for electricity production as required. The powerhouse with turbines and generator is located at the foot of the dam. The storage power plant uses this difference in height between the high reservoir and the lower engine house.

To generate electricity, water from the reservoir is fed to the turbines via pressure pipes. The resulting kinetic energy drives the turbines, which convert the energy into electricity via generators and feed it into the power grid. The water used to generate electricity iis drained (usually into a river).

As a rule, storage power plants are not in continuous operation. Instead, their task is to store water, which is produced in different ways as a result of the weather. Storage power plants go into operation when electricity consumption is at a peak due to daily or season fluctuation. This makes storage power plants important suppliers of flexible peak energy.  

Video: How does a storage power plant work? (Source: ARD, German only)

(1) To generate electricity, water from the reservoir is fed to the turbines via pressure pipes. The resulting kinetic energy drives the turbines, which convert the energy into electricity via generators and feed it into the power grid. After turbine operation, the water reaches the lower reservoir.

(2) In contrast to pure storage power plants, pumped storage plants can not only generate peak energy, but can also convert excess electricity, which occurs during so-called off-peak periods, into valuable peak energy. For this purpose, they pump water from the lower reservoir back up to the higher reservoir and reuse it for electricity production  at a later point in time. In this pumping mode, the generator works as a motor. It is supplied with electricity from the power grid.

In a pump turbine, the functions of the turbine and the pump are performed by the same machine. The pump turbine changes its direction of rotation depending on the operating mode.

Pumped storage is a proven method of balancing supply and demand in a power grid in an environmentally friendly and economical way. Pumped storage plants play an important role in ensuring security of supply and stabilising the electricity networks.  

Running waters must also be able to fulfil their natural function when water is removed. Residual water provisions specify the water volume that must be available at all times in river and steam beds when water has been withdrawn.

Residual water map of the FOEN

When hydropower plants generate electricity the water level increases and decreases on a daily basis. If demand and power prices are high or when grid stabilisation requires higher production, water is released. When demand is low the storage power plants feed in less power to the grid and retain water. These artificial high and low water levels can endanger water organisms in streams and rivers.

A surge tank is part of the drive water system of pump and storage power plants. Put simply, it is basically a vertical tank which serves to absorb the pressure surge in the piping of the hydropower plants when the valves (ball valves) close.

In the case of plants with underground water tunnels or pressure shafts, as is customary in Switzerland, the surge chamber is also built underground and usually in shaft form as an upwardly open riser in the tunnel system.

And this is how it works: There is a large amount of water in the pipelines, which drives a turbine with high kinetic energy. If the pipeline is closed by a slide valve, this amount of water is abruptly slowed down. A very high pressure briefly occurs at the gate valve, followed by a negative pressure, both of which can damage the systems.

The surge chamber dampens these pressure surges because it diverts the flowing water and thus delays and reduces the deceleration process. When the slide valve is closed, the water moves into the surge tank and swings up and down until it comes to rest. The high kinetic energy is converted into potential energy. 

Switzerland is a country without resources. This is commonly said. But that's not entirely true, because our country has water as a resource like almost no other country - that's why it's often referred to as the water tower of Europe.

The Swiss Alps are the source and continental watershed: the Rhine flows into the North Sea, the Rhone into the western Mediterranean, the Ticino (Po) into the Adriatic Sea and the Inn (Danube) into the Black Sea. More than 1,500 lakes, rivers and bodies of water as well as numerous glaciers can be used here as freshwater reservoirs or for the climate-friendly production of renewable energy, which accounts for around 60 percent of Switzerland's total production.

Most locations for the profitable use of hydropower have already been developed in Switzerland. The focus of the expansion of hydropower desired by the Confederation as part of the Energy Strategy 2050 is therefore on renewing and increasing the efficiency of existing plants. 

Switzerland currently has around 650 hydroelectric power plants with a capacity of at least 300 kilowatts and around 1000 small hydroelectric power plants. Hydropower production in Switzerland averages around 35,000 GWh per year (35 TWh). More than 90 percent of the hydropower plants are located in the main catchment area of the Rhine with its tributaries the Aare, Limmat and Reuss as well as on the Rhone. The largest producers in Switzerland are the cantons of Valais and Grisons, which account for almost 50 percent of Swiss hydropower.

Together with its partners, Axpo has around 60 hydropower plants in Switzerland with an installed capacity of around 4300 MW.

For the construction and operation of hydropower plants, energy producers such as Axpo require a concession from the canton and/or municipality from which the water used in the respective power plant originates. This concession allows the energy companies to use the water. As a rule, these concessions last between 40 and a maximum of 80 years. This period is intended to enable the operators to amortise the investments made over a long period of time. In return, the cantons and municipalities receive water fees from the energy producers, currently amounting to around CHF 550 million per year.

In Switzerland, energy producers such as Axpo pay fees for the use of water to the respective canton or municipality where the power plant is located. The water rate in Switzerland was increased in several steps to the current level of CHF 110 per kilowatt gross output. Currently, the cantons and municipalities receive around CHF 550 million in water fees per year. The water rate accounts for the largest share of fixed charges levied on Swiss electricity producers. According to a study by the Swiss Water Management Association there is no place where fees are even close to the level of Switzerland.

This discrimination against Swiss hydropower producers has a direct impact on their willingness to invest and thus on ensuring security of supply in Switzerland in the medium to long term. This is because the levies are not only high, but also form a fixed cost block. An open market with volatile prices, and, hence, fluctuating revenues is therefore confronted with a high block of fixed costs. If market prices are high, the fixed levies are acceptable. In the years with low revenues, however, losses result as in previous years. This leads to uncertainties in the strategic planning of producers because they have less financial resources at their disposal to invest in the maintenance and expansion of hydropower.

Fish bypasses/fish ladders

Many fish species use different habitats within their life cycle and are therefore dependent on migrations through water bodies. Despite existing hydroelectric power stations, fish can migrate up and down rivers and access different habitats through fish bypasses and ladders.

Since the Water Protection Act (GSchG) was revised in 2011, ensuring fish mobility in watercourses has been mandatory in Switzerland. Structural measures to restore the free migration of fish in hydroelectric power stations in accordance with the revised GSchG are reimbursed by the national grid company Swissgrid via the grid utilisation costs.

Owing to the different topography on the Swiss and German banks, two solutions with different migration systems were realised at Eglisau-Glattfelden. The facility has been under historic preservation since 1988, and structural changes to the plant are prohibited.

Fish ladders are not “off the shelf” products. Every power plant is a unique case based on its architecture and topography. Solutions must be tailored to the swimming behaviour of the fish and the respective plant situation according to state-of-the-art technology. Axpo, as the largest producer of hydropower in Switzerland, benefits from its long-standing experience with its plants. The company owns some 60 hydropower plants in Switzerland, including interests and procurement rights.

Fish lift at the Eglisau-Glattfelden power station: On the German side the fish take the lift

Sustainable solutions are found in the exchange with the responsible stakeholder groups. Planning and finding optimal solutions that bring environmental as well as technological and economic aspects in balance take place through close collaboration between Axpo and the monitoring groups. Monitoring groups include representatives from the respective canton, federal authorities and environmental organisations. Depending on the plant location, different construction methods must be taken into account, for example vertical slot passes, bypasses or fish elevator systems. A fish elevator is a good solution in cases where space conditions are tight like those on the German bank of the Eglisau-Glattfelden plant. Using very little space, the elevator system allows the fish to overcome the steep height differences. 

Before a technical solution can be defined, the fish migration patterns and behavioural biology must be known. Migration behaviour differs depending on the fish species. A fish ladder must accommodate the needs of some 30 species. Migrating fish that swim close to the ground, as well as those that swim close to the water surface, must be able to find the entry point. Fish always look for the strongest current, which lures them to the ladder entrances. Not all fish are strong swimmers. As a result, a ladder must be designed to accommodate weak swimmers such as the bullhead so that it can master the height difference between up- and downstream. In addition, larger fish, e.g. salmon, must have enough room in the ladder system.  

The new weir system with fish ladder at the Rüchlig power plant

Currently some 20 larger and smaller projects connected with fish migration in compliance with the new Water Protection Act are in the pipeline at Axpo.

Owing to the different topography on the Swiss and German banks, two solutions with different migration systems were realised at Eglisau-Glattfelden. A new fish ladder on the Swiss side and a fish elevator on the German side allow the fish to pass the plant more easily when migrating upstream.

In addition to an ascent ladder, decent bypass systems were realised at the Axpo small hydropower plants Rüchlig and Stroppel – to date unique in Switzerland.

While fish migration systems have become standard at Swiss hydropower plants with practical solutions such as fish ladders, systems for descent are still largely inexistent. Today, descent only functions by means of a weir or through the turbines. The current technology does not yet enable large run-of-river plants e.g. on the Aare, Reuss, Limmat or Rhine rivers to implement separate fish descent systems.

One problem is that the fish migrating downstream cannot find the entrance to the fish ladder because they follow the strongest current that leads directly into the turbines. In addition, there is not enough information about the migration patterns of many fish available as yet. Research to find solutions for fish descent is underway worldwide. Findings have been available since about 2008, particularly in the USA and Germany. 

As soon as a technology for fish descent systems has been developed, Axpo will implement projects for the restoration of fish migration systems in compliance with the revised Water Protection Act. Ultimately solutions must be found that benefit the fish and present minimal obstacles for power generation from domestic hydropower. Environmental as well as technical and economic aspects must be in balance to realise a sustainable solution.  

Depending on the plant geometry at small hydropower plants with a discharge capacity up to 80 m3/s, safe, state-of-the-art fish descent systems can be evaluated. To date there are only a few pilot systems in operation in Switzerland. As two of the only power plants in Switzerland, the Axpo Rüchlig (weir power plant) and Stroppel power stations have both fish ascent as well as descent systems. The fish are diverted from the turbines by means of a fine rake leading them to a bypass flap that allows passage for fish swimming close to the ground or close to the water surface. From there, they are safely routed downstream through a narrow canal.

The fine rake system is not suitable for larger power plant facilities. The rakes would require dimensions that would lead to large power generation losses, and would represent a great challenge in terms of rake cleaning, debris removal, operation, and maintenance.

The pond used to carry out success monitoring at the Stroppel power plant

The results of fish migration are assessed in an extensive success monitoring process. For a longer period of up to one year, the fish that use the ascent and descent ladders will be directed into ponds where they will be manually counted, recorded according to species and size, and checked for injuries. Detailed monitoring will provide new information on the functionality of the systems. 

It is hoped that in the upcoming years the research and monitoring findings at the pilot systems will provide new information concerning fish descent systems. Axpo is involved in a research project by the Aare-/Rheinwerke Association (VAR) with the objective of acquiring a better understanding of the complex behaviour of fish, and developing and testing structural measures for fish descent systems at large-scale hydropower plants. Among others, a pilot project, which will investigate additional or alternative structural and technical measures between 2017 and approx. 2020, is in planning at Axpo’s Wildegg-Brugg power plant.

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