Modelling the effects of pumped-storage on physical conditions in Lake Suldalsvatn

In a context of climate change, use of renewable energies should highly increase in the coming decades. While wind power and solar energy are intermittent energy sources, a need for energy storage to maintain balance in electricity production is required. In this perspective, Norway intends to adapt and develop its hydropower plants in order to become a "battery" for Europe and provide electricity in peaking periods. The increase in production will rely on the expansion of the current capacity, and in the future existing reservoirs and hydropower plants will be used to pump water from the downstream reservoirs and store it in the upstream ones when electricity demand is low. Water will be released to generate electricity in high demand periods. Lake Suldalsvatn, located on the Western coast of Norway, is part of Ulla-førre, the largest hydropower plants system in Northern Europe. The water comes from an area of about 2000 m2 and includes 16 lakes of different sizes. Lake Suldalsvatn is the lowermost reservoir of the regulated area before the water flows into Hylsfjord through Suldalslågen River. Today, it receives turbinated waters from Kvilldal and Suldal hydropower plants, and supplies water to Hylen power plant. In the future, new operational regimes could include alternation of pumping phases and production phases through installation of a new pump. Lake Sandsavatn located at a higher elevation could be used as upstream reservoir to store water pumped from Lake Suldastvan. The purpose of this study is to examine physical changes in the daily, seasonal and yearly fluctuations in Lake Suldalsvatn under pumped storage regime. The paper assesses consequences of more rapid and more frequent water level changes (short-term variations) as well as modification of reservoir filling time scale (long-term variations) on circulation patterns, seasonal stratification in temperature and oxygen and ice formation. The 3D-hydrodynamic model GEMSS was used to calculate flow velocity, water level fluctuations, water temperature, ice cover and oxygen concentration for different pumped storage scenarios. The paper presents the results from the study and outlines possible operational strategies seeking win-win situations for both increase in electricity production and survival of local eco-systems.