Researchers at ETH Zurich, led by Robert Boes, are developing solutions to optimize electricity production from hydropower plants to ensure hydro remains the backbone of Switzerland’s electricity supply.
“We must constantly work on optimizing [hydropower]. If we don’t, electricity production and storage at existing plants will slowly erode,” said Boes, who has headed the Laboratory of Hydraulics, Hydrology and Glaciology at ETH Zurich since 2009. Reservoirs have a natural tendency to shrink due to rubble and gravel, and sediment in the waterways inevitably leads to wear and tear on the turbines.
ETH Zurich researchers have developed solutions for efficient water management, calculated maintenance strategies for turbines and shown which locations have the potential for making the most effective and eco-friendly use of hydropower.
Better water management for run-of-river plants
There are 11 run-of-river power plants along the 36 km that the River Limmat flows from Lake Zurich to the River Aare. Lake Zurich resembles a large head reservoir through which water is drained into the Limmat. Authorities use the weir system at Platzspitz park to regulate the level of Lake Zurich and how much water flows into the river. Besides playing a role in flood protection, navigation and ecology, this water level is particularly relevant for electricity production.
Boes and his research team showed that optimized weir regulation at Platzspitz could allow around 2% more electricity to be generated in the Limmat power plants. This increase in efficiency would arise from a new management strategy that, first, permits lake water levels to be higher under today’s regulations and, second, uses weather models to better adjust water level regulation in Lake Zurich to expected precipitation and inflow volumes.
As a general rule, the more evenly water flows into run-of-river power plants, the more electricity they can produce. Especially in the case of small and medium levels of high water, the new regulations would make better use of the additional water present. “If the weather model predicts heavy rain, the smart weir system would release a little more water into the Limmat ahead of time. Then, when the predicted rain arrives, the lake would have more of a buffer and could continue to release water evenly despite the heavy rainfall,” Boes said. This would prevent the turbines from being overloaded by too much water.
Similar adaptations would be possible on other rivers on the Swiss Plateau downstream of Alpine lakes. Boes and his team calculated that electricity production from run-of-river plants could be increased by around 100 GWh per year if weir systems were managed more intelligently.
Protecting turbines against sediment
The fine silt that rivers carry acts like sandpaper, causing turbines to wear out over time and generate significantly less electricity. While many power plants feature sand traps, these often fail to remove enough of the tiny particles from the water.
To increase the sand traps’ effectiveness, protect the turbines and avoid production losses, Boes and his team investigated which types of trap are particularly effective. “Long traps with a gentle bottom gradient, which make the water flow as slowly as possible, work best. They let the particles settle more easily to the floor,” Boes said. These findings have been used to improve the sand trap at the Susasca hydropower plant in Graubünden. However, longer traps require more materials and take up more space, making them expensive. As a result, decisions on which structural adaptations make economic and technical sense will differ among power plants.
Boulder bypasses for reservoirs
Weather-related erosion causes stones, gravel and other sediments to enter reservoirs via their water intake and reduce storage volume. This sedimentation could reduce the storage capacity of Swiss reservoirs by around 7% by 2050. Small and medium-sized reservoirs use bypass tunnels as a structural measure against sedimentation by guiding stones, gravel and silt past the dam wall during floods. However, because floodwaters carry a great deal of sediment, the floor of the bypass tunnel is sometimes subject to pronounced wear.
Boes and his team have looked into this problem. For example, the researchers investigated which materials are best suited to lining the floor of such tunnels. They concluded that high-strength granite is best able to withstand the heavy wear and tear in particularly harsh conditions. Several bypass tunnels around the world have since been lined with granite.
Using the Solis reservoir in Graubünden as an example, the researchers were able to prove how effective bypass tunnels actually are. The tunnel has reduced annual sedimentation in Solis by over 80%. However, this requires adjustments to storage management: the power plant’s operators can further increase the tunnel’s effectiveness by bringing the water level in the reservoir down low enough, as this enables the inflowing river to transport particularly large quantities of rock and sediment and discharge them via the tunnel. These findings are also relevant for the operators of numerous other power plants.
More electricity through optimized turbine maintenance
Another way to deal with silt buildup in reservoirs is to channel fine sediments into downstream sections of the river via the headrace and turbines. “The problem with this is that it causes more turbine wear. But it can still be a worthwhile measure for Alpine reservoirs if alternative measures, such as bypass tunnels, would be too expensive or not feasible,” Boes says.
To better assess the feasibility of this approach to the problem of sedimentation, power plant operators need to know what damage the silt causes to the turbines and how much it reduces their efficiency. Boes and his team analyzed this problem in one hydropower plant in Valais and another in Graubünden. The researchers used their findings to develop a model that predicts when a turbine will lose output due to sediment wear and ought to be replaced. This enables plant operators to optimize the maintenance of their systems and ultimately produce more electricity.
The potential of Swiss hydropower
In addition, Boes and his team have been conducting research into the potential for expanding Swiss hydropower. For example, his research group investigated which areas of glacial retreat would be most suitable for new reservoirs and which existing dams might be raised to create more storage volume.
In 2020, the Swiss Federal Office of Energy used the results of these ETH studies on suitable sites as the basis for a roundtable discussion at which electricity companies, environmental protection organizations and cantons agreed on 15 hydropower plant expansion and new construction projects. Acting as facilitator, ETH Professor Emeritus Michael Ambühl also played a part in helping the parties reach a compromise. These projects were subsequently incorporated into a new Electricity Supply Act. Whether this legislation comes into force depends on the Swiss electorate, which will vote in June on the expansion of hydropower and other renewable energy sources.