Investigation of kinetic river turbines in open channels

Providing 16% of the global electric energy supply hydropower holds a substantial contribution to the renewable energy sources. But only 35% of the estimated worldwide economic potential on hydropower is currently used for power production – most of it by conventional damming methods where rivers are retained by to gain potential energy which may be used by classical turbines. Damming of rivers is a major intervention into nature as ecological connectivity and natural sediment transport are disturbed. Therefore alternative facilities for energy extraction in ecological sensible or remote areas have to be analyzed, among them hydrokinetic turbines which are driven by the free stream velocity of rivers.

The hydrokinetic tubines investigated at the institute are horizontal-axis lift type turbines. The maximum efficiency of such a machine is theoretically limited by the deceleration of the flow. According to Betz, the maximum achievable power is calculated as:

In order to obtain a high power with the smallest possible rotor, the impeller is connected to a diffuser which increases the effective projected area of the machine. Using CFD-based development of hydrokinetic turbines, in a first step the diffuser is designed and optimized. Therefore, the runner is only considered indirectly through a model. The results from the simulation of the optimum diffuser geometry are used as boundary conditions for the design of runner blades and guide vanes. Finally, a unsteady simulation of the entire system - including support elements - must be performed in order to verify functionality and determine the turbine characteristics.

Many potential sites for the direct use of the river flow velocity provide space for turbine parks. Therefore, advantages and disadvantages of different turbine arrangements are investigated at the institute. Figure 2 shows the streamlines for a parallel arrangement of an infinite number of turbines with the distance a. The graph shows that this distance has a significant influence on turbine performance. For small distances, the performance can be increased by up to 15%. It should be noted that the increase in power mainly results from the blockage of the flow cross-section and the increase the upstream water level.

There are several approaches investigating the interaction between kinetic turbines and the river. For a first approach 2D shallow water solvers (e.g. tidalFoam) can be used. Here the flow quantities are depth-averaged and the flow depth is determined by the shallow water equations.

In order to be able to make a detailed determination of the water level in the immediate vicinity of a kinetic turbine, a two-phase simulation is necessary, using for example the "Volume of Fluid" method. Figure 3 shows the water level for the flow around a kinetic turbine. In the upstream, the water is slightly increased by the obstacle “turbine”. Right above and in downstream of the turbine a local wave pattern occurs induced by the weak flow of the turbine.