Design of a Throttled Surge Tank for Refurbishment by Increase of Installed Capacity at a High-Head Power Plant

The Swiss confederation aims to phase out nuclear power production with its Energy Strategy 2050 program by increasing the renewable energy contribution to its overall energy generation. Hydroelectricity, which is the most important form of renewable energy in Switzerland, supplying almost 60% of the electricity in 2015, should increase its production capacity to achieve this goal. The case study presented in this paper focuses on the replacement of the third turbine in the Gondo high-head power plant with a turbine with a higher discharge capacity. The results of one-dimensional (1D) numerical simulations shown that throttling the surge tank is an efficient measure to adapt the existing hydraulic system for the increased discharge. Physical-scale modeling was performed to validate the design of the grid throttle placed at the bottom of the lower chamber of the existing surge tank. The grid throttle geometry and its head losses are compared with two existing similar throttles in Switzerland. Finally, prototype tests of the temporal evolution of water levels in the surge tank using the throttle coefficients obtained experimentally showed good agreement. Hybrid modeling using a combination of 1D numerical models, threedimensional (3D) physical models, and prototype tests are highly recommended for checking the transient performance of the waterway after a refurbishment of turbines with increased design discharge. Furthermore, placing a throttle at the bottom of an existing surge tank is often an effective and economical solution in the case of small increases in installed capacity.