Bachelor’s Thesis 2026 – Gregory Sauthier
In an electricity system incorporating an increasing share of variable renewable generation, the flexibility of hydroelectric power stations plays a vital role in maintaining grid balance. Optimising existing facilities is therefore a particularly promising way of increasing this flexibility without the need to build new infrastructure.
This Bachelor’s thesis, carried out in collaboration with the Forces Motrices Valaisannes (FMV), focuses on the Mörel hydroelectric power station and examines various options for improving its operation and its contribution to system services. The study focuses in particular on primary and tertiary control, whilst taking into account the hydraulic and mechanical constraints associated with more flexible operation
Characterisation of the power station’s operation
An initial part of the work involved analysing several years’ worth of operational data in order to characterise the actual operation of the scheme. As an inconsistency in the inflow flow sensor was identified, the hourly flow rates for 2023 to 2025 were reconstructed using the power-flow relationships obtained from on-site measurements. In particular, this analysis confirmed a minimum operating flow corresponding to approximately 25 % of the maximum flow and provided a better characterisation of the availability of the water resource throughout the year.
The power station’s ability to contribute to primary control was then assessed by comparing three complementary approaches: 1D hydraulic simulations, scale model tests and measurements taken directly at the power station as part of the project SmallFLEX Goms. The results suggest a primary adjustment capacity of 3 MW per unit, provided there is a sufficient water supply in the Eggen reservoir.
Part-load operation also raises the issue of cavitation in Francis turbines. Pressure and noise measurements in the suction pipes have made it possible to identify the most sensitive operating conditions. The analysis shows, in particular, that the rise in the water level downstream is an effective means of reducing the risk of cavitation without modifying the turbine: raising the downstream water level by one metre would increase the pressure at the runner outlet by approximately 10 kPa.
Making the most of the compensating orifices for tertiary control
Another option being considered is to increase the power station’s tertiary control capacity. The proposed solution is based on the reuse of existing pressure-equalising orifices acting as turbine bypasses. Three systems for their controlled opening were designed and dimensioned: hydromechanical, magnetic and electromechanical.
Hydromechanical operation appears to be the most promising solution: it retains the original safety function of the relief orifice, requires no external power supply and allows the valve to close by gravity. This solution has also been compared with the construction of a new bypass fitted with a relief valve.
Significant economic potential
Finally, an economic model based on actual production data for 2023–2025 and the system services markets made it possible to assess the value of this additional flexibility.
Raising the tertiary control ceiling from 30 to 60 % of maximum power could increase annual revenue by 1.4 to 2.5 1Q-3Q, whilst the introduction of the primary setting would bring 0.6 to 1.4 1Q–3Q additional. The combination of the two services is expected to lead to an estimated increase in revenue of 2.4 to 3.2 % per year.
This study thus demonstrates how a better understanding of the hydraulic behaviour of an existing structure, combined with targeted modifications to its equipment, can enable to increase its flexibility and value to the electricity grid whilst making the most of existing infrastructure.
