Potential increase in energy storage capacity in the Swiss Alps to support the massive development of renewables: Case study of the Grande Dixence hydropower scheme

Only 42 % of the mean annual production of large hydropower plants in the Swiss Alps is stored in their reservoirs. Once the reservoirs are full at the end of the summer semester, plants are operated as run-on-the-river in order to avoid spilling. A significant part of the hydropower resources are thus sold at discount prices and cannot be transferred to the winter semester. It is precisely in the winter semester that Switzerland imports large volumes of electricity from its neighbours. In order to support the nuclear power phase-out planned in the Swiss Energy Strategy 2050, hydropower must not only maintain its 2012 production level but mainly step-up its role in the grid and improve its remuneration. Following this strategy, the present production of the nuclear plants will be mainly replaced by renewables and the total electricity storage capacity must therefore be increased by 10 to 30%. Hydropower schemes face two challenges: to reduce the lack of electricity production in the winter semester and to lay a suitable foundation for the increased storage demand of intermittent renewables such as wind and solar. In this work, we propose a methodology to evaluate the potential increase in energy storage capacity of a portfolio of large reservoirs in Alpine regions, considering future climate scenarios and future reservoir operation scenarios corresponding to different economic conditions (e.g. more/less PV and wind production) and reservoir management time scales (seasonal, multiannual). In addition to exploring the performance of existing reservoirs in different future conditions, we assess the possibility to link reservoirs (mutualising the use of storage) and/or to raise dams (increasing storage and annual runoff regulation). In fact, several large dams provide the structural capacity to be heightened by at least 10% of their height. Additionally, there are strategic locations that could be used to create new large reservoirs. This methodology is applied to a test site located on the left bank of the Rhône River in Switzerland. Our method uses both recent and future hydrology, as well as operational data to determine the optimal reservoir live storage capacity for a hydropower scheme. In addition to intra-annual energy and water transfer, we also consider the transfer in between years to account for adverse climate conditions. In order to apply this methodology to the Grande Dixence hydropower scheme, we show two opportunities to increase the seasonal storage capacity: heightening of the main dam and the creation of a new upstream reservoir. Our study shows that the largest Swiss reservoir with 400 hm3 can provide more seasonal storage in all climate scenarios, however a full heightening of 10% may not be optimal regarding the different intra-annual operation modes investigated. However, should the operator wish to guarantee production in extreme dry years the reservoir heightening would also provide the possibility of establishing a pluriannual reserve or extra room for pumped storage.