Hydraulics and Thermodynamics of Closed Surge Tanks for Hydropower Plants

This work has studied the hydraulic and thermodynamics of closed surge tanks for hydropower plants. Surge tanks are applied in hydropower plants to control and reduce the acceleration forces of the water during change of turbine flow. Surge tanks reduce the design pressure of the tunnel and pipes that convey water from the reservoir to the turbines, and enable automatic frequency and power control in the hydropower plant. Closed surge tanks are constructed as underground caverns filled with pressurized air located close to the turbines. The work comprises four different studies. In the first study (1), the benefits and challenges of two different surge tank types, the open and the closed surge tanks, were compared. The state-of-the-art design for these two solutions were described. In the second study (2), a hydraulic scale model of an existing hydropower plant with a closed surge tank was constructed in the scale 1:65 in the Hydraulic laboratory at NTNU. A new method for scaling of the absolute pressure in the closed surge tanks was developed and tested. The results of the hydraulic scale model was compared with field measurements from the prototype, and the accuracy was quantified. In the third study (3) the thermodynamics of the air in the closed surge tank was investigated. The impact of heat transfer from the air to the surrounding media was assessed, and different theoretical models for simulation of the thermodynamics where compared with field measurements from an existing hydropower plant. A new improved theoretical model for the thermodynamics was developed and programmed and tested in the freeware hydropower simulation program LVTrans. In the fourth study (4), the effect of installing a surge tank throttle on governor stability, power control and hydraulic transients was investigated. A new method for evaluation of the throttle effect has been developed, and a quantification of the effects for an example hydropower plant with a closed surge tank was conducted