Limmern Pumped-storage Power Plant
Background on Hydropower
Hydropower from A to Z
Danger of flooding
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 stations
Run-of-river power stations use river currents to generate electricity. Usually, these are low-head power stations and go into operation when water volumes are high.
The water (head water) is routed to the turbines. The kinetic energy drives the turbines that transform the energy into electricity by means of generators. The electricity is then fed into the power grid. The water used to generate electricity is released back into the river (return flow).
Run-of-river power stations produce base load energy, and, in contrast to storage or pumped storage plants, cannot adapt the power volume to demand. The generated power volume is dependent on water flow and the flow velocity of the river.
In addition to power generation, run-of-river power plants often contribute to flood protection. In order to ensure the unhindered passage of fish and boats, fish ladders and weirs are installed in the river.
Small-scale hydropower plants
As a rule, the term small-scale hydropower plant refers to run-of-river power plants with a capacity of less than 10 MW.
The parallel metal rods of a rake collect debris. As a rule, they are installed to protect the hydropower plant downstream and, when placed ideally, they can also be used to guide fish into safe bypasses.
Storage power station
A storage power station is a hydropower plant that stores water in a reservoir and uses it as needed to generate electricity. The powerhouse with turbines and generators is located at the foot of the dam. The storage power plant uses the elevation difference between the reservoir located at a higher altitude and the powerhouse located at a lower altitude.
To generate electricity, water is routed from the reservoir to the turbines through penstocks. The created kinetic energy drives the turbines that transform the energy into electricity by means of generators. The electricity is then fed into the power grid. The water used to generate power is usually released into a river.
Storage power plants are normally not in continuous operation. Their purpose is to store water that occurs at different times owing to weather conditions. Storage power plants are put into operation when electricity consumption peaks due to daily or seasonal fluctuations. Storage power plants are an important supplier of flexible peak energy.
Pumped storage plant
(1) To generate electricity, water is routed from the reservoir to the turbines through penstocks. The created kinetic energy drives the turbines that transform the energy into electricity by means of generators. The electricity is then fed into the power grid. The residual water is released into the lower storage pond.
(2) In contrast to pure storage power plants, pumped storage plants not only generate peak energy. They can also transform power surpluses produced during off-peak periods into valuable peak energy. To do so, water from the lower storage pond is pumped up to the reservoir located at a higher altitude and used later to generate power. In pump operation the generator works as a motor. The generator is powered by electricity from the grid.
A pump turbine simultaneously carries out the functions of a turbine and a pump. Depending on the mode of operation, the pump turbine changes its rotation direction.
Pumped storage is a proven method to balance out supply and demand in a power grid in an eco-friendly, efficient way. Pumped storage plants play an important role in the security of supply and stabilisation of the power grid.
Residual water regulation
Flowing waters must be able to fulfil their natural functions even when water is extracted. The residual water regulation defines the minimum water volume that must be maintained in river- and streambeds at all times.
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.
Untenstehend finden Sie weiterführende Informationen
Brochures and Factsheets
Overview of brochures and factsheets
- Brochure Hydropower (in German), 887 KB
- Factsheet Hydraulic power plant Beznau (in German), 675 KB
- Factsheet power plant Löntsch (in German), 1 MB
- Factsheet power plant Rüchlig (in German), 1 MB
- Factsheet power plant Sarganserland - Gigerwald (in German), 1 MB
- Factsheet power plant Sarganserland - Mapragg-Sarelli (in German), 1 MB
- Publications pumped storage plant Limmern
- Leaflet flood, 983 KB
Ensuring that fish have access to watercourses is an obligation in Switzerland since the introduction of the revised Water Protection Act in 2011. According to the revised Water Protection Act, structural measures to restore unhindered fish migration near hydropower plants are reimbursed by Swissgrid, the national grid company, through grid utilisation fees.
Power plants: Every plant is a unique case
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.
Elevator at the Eglisau-Glattfelden plant: On the German side, the fish take the elevator.
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.
Behavioural studies: How a fish ticks
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 station.
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, descent systems were realised at the Axpo small hydropower plants Rüchlig and Stroppel – to date the only such ladders in Switzerland.
Fish descent: Research underway
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.
Fish descent: Small hydropower plants
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.
Monitoring fish descent 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.