In an electric grid, the electricity production has to be in line with the demand at every moment. Run-of-river hydropower plants, nuclear power plants and new renewable energy sources such as photovoltaic and wind are not fully flexible to react to demand. Therefore, storage and pumped-storage hydropower plants are operated to balance the production and consumption of electricity. Due to lower runoff in winter, the electricity production from hydropower is relatively low while the electricity demand is higher than in summer. Therefore, artificial storage lakes (called reservoirs) mainly in the Alps are filled with water during spring and summer runoff in order to generate more electricity in winter. Such seasonal storage reduces the need for electricity imports in winter.
The Swiss Energy Strategy 2050 foresees a stepwise withdrawal from nuclear power and a considerable increase of electricity generation from new renewable energies, mainly photovoltaics. When the nuclear power plants will be phased out in the coming decades, the Swiss electricity production in winter will be significantly lower than today, also because production from photovoltaics is lower than in summer. Due to these future shifts in the Swiss electricity production, the need for large-scale seasonal storage of electricity increases.
Extending existing reservoirs to increase seasonal electricity storage
One way to respond to this increasing demand is to extend existing reservoirs of storage hydropower plants by heightening their dams. This was done in the last decades for some reservoirs in the Swiss Alps, e.g. Mauvoisin (VS), Luzzone (TI) and Vieux Emosson (VS). By heightening these dams by 6 %, 8 % and 39 % of their previous heights (measured from the deepest point of the foundation to the dam crest), the reservoir volumes increased by 17 %, 23 % and 93 %, respectively. Such considerable reservoir volume increases result from the fact that the valleys are wider at higher elevations above the riverbed.
A systematic potential study over Switzerland
To assess the energy potential of such reservoir extensions, researchers at ETH Zurich systematically investigated 38 existing reservoirs and corresponding dams in the Swiss Alps with a net storage volume of at least 20 Mio. m3 today. For the heightening of these dams by 5 %, 10 % or 20 %, they conceptually studied the required adaptations and rated them based on a set of eight criteria (Fuchs et al. 2019). They considered the additional reservoir areas, the dams with their auxiliary structures (e.g. spillways) and the corresponding hydropower plants. Then they sorted the heightening options by their rating and classified them into “well suited”, “moderately suited” and “not to be further investigated”. The researchers summarized the “well-suited” heightening options in one scenario, while they additionally considered the “moderately suited” options in a second scenario.