Potential assessment study

for a historic hydropower plant

On behalf of a large German energy supplier, we are currently working on a refurbishment study for a historic hydropower plant that has been in operation since 1956 and is equipped with 2 vertical Kaplan turbines.

Technical characteristics of the system

With a runner diameter of D = 3.70 m and a nominal head of around H = 8.5 m, each turbine's nominal output is 6.6 MW. According to the operator's information, this power plant is more or less still in its original condition and will undergo extensive refurbishment measures in the coming year. The turbine casing, in particular, urgently needs to be replaced, as it has been severely affected by cavitation damage.

3D CAD modelling to prepare the CFD study

Based on a 3D surface scan of the entire turbine geometry, a closed 3D CAD model was developed in the first step in preparation for a comprehensive CFD study.

Numerical flow simulation of the actual state

The next step was the numerical flow simulation of the actual state to obtain a sound database for subsequent design optimisation. On the one hand, this showed that the known nominal output of the turbine could be confirmed exactly using the CFD simulation. On the other hand, detailed analyses of the simulation results also revealed deficiencies in the existing turbine hydraulics.

The following illustrations show the pressure distribution on the suction and pressure sides of the relatively long runner blades at maximum discharge. The visualisations illustrate unfavourable blade inflow and cavitation zones on the suction side, which occur in the area of the pronounced damage to the turbine casing.

Potential for improvement in the intake manifold flow

The draft tube flow of the original turbine configuration also shows room for improvement. Regarding the turbine's full-load operation, the following figures show velocity distributions at different sectional planes in the draft tube. The pronounced flow separation in the draft tube cone, which opens strongly on one side, has a particularly detrimental effect on turbine performance.

Economic benefits of the planned measures

The optimisations currently underway show that both the turbine efficiency and the cavitation behaviour can be significantly improved by optimised runner blades, an extended turbine casing and a weaker opening draft tube cone. The economic efficiency calculation carried out in parallel with the CFD study shows that the annual work of the turbines can be increased by several percentage points as a result.