The Nant de Drance power plant is a pumped storage power plant with a capacity of 900 MW in a subterranean cavern located between the two reservoirs of Emosson and Vieux Emosson in Valais, Switzerland. The plant became operational on 1st July 2022 and comprises six sets of radial type reversible pump-turbine units of 150 MW capacity each. In pump and turbine operation, the variable speed enables adaptation to the head variations while increasing the global plant efficiency. Thus, variable speed allows a wider operating range. In pumping mode, the power consumption can be adapted to the requirements of the electrical grid.
For operational reasons it was decided to perform efficiency tests with the installed acoustic transit time (ATT) flow meters. For this, it was decided to perform in-depth computational fluid dynamics (CFD) simulations and a detailed analysis of the measurement uncertainty. The challenges for the simulations were, on the one hand, the swirl in turbine operation and, on the other hand, the distorted flow field in pumping operation due to the flow contraction in the guard gate. Another factor that challenges the flow measurement is that the conduits are not round. The bottom has been flattened to allow vehicles. Accordingly, neither standard elevations for the crossed path nor standard weights can be used.
Based on a series of different simulations and measured cases the integration uncertainty could be estimated. Due to the pronounced swirl in turbine operation, the integration uncertainty was estimated to be +–0.75%, while the estimation is +–0.2% for all other cases. Some sensitivity tests performed have confirmed that the integration method with OWISS (Optimized Weighted Integration of Simulated Sections) weights is a robust procedure.
Link to the involved subject area: CFD Simulations
