hydrodynamic effects of tidal turbine arraysA tidal current turbine converts kinetic energy from a tidal current into electricity. A horizontal axis tidal turbine works in a similar way as a horizontal axis wind turbine. Kinetic energy of the flow is transformed in shaft mechanical energy through the blades of the turbine. This shaft mechanical energy is further transformed into electricity. In order to be as efficient as possible the turbines are dimensioned as big as possible. However due to negative effects of surface waves and the steeper part of the boundary layer at the seabed, a clearance of 8 m is reserved at the top and a clearance of 25% of the depth is reserved at the bottom. In a water depth of 30 m this would result in a tidal turbine with a diameter of 15 m.
For economic reasons tidal current turbines are expected to be applied in array formations, similar to wind farms. A tidal farm differs from a wind farms in the fact that the boundaries (free surface and sea bed) are relatively close. One can imagine that a turbine in the wake of another turbine can experiences negative effects due to the wake. By knowing the characteristics of the wake behind the horizontal axis tidal current turbine the farm design can be optimized to produce as much power as possible. Research on horizontal axis tidal current turbines is mainly focused on optimizing the design of the turbines, scale model tests and CFD tools are used to assess the hydrodynamic characteristics of the turbines. The optimization of the tidal turbine farm is not the only point of interest, the impact on the environment is important as well. How does the farm influence the general flow, sediment transport, water quality etc. are all important environmental aspects. This can be an opportunity for Delf3D- FLOW, as Delft3D-FLOW is used to solve (far-field) flow problems in large water bodies with a free surface. The objective of the project is to develop a (far-field) modeling approach, based on the Delft3D open source hydrodynamic software. The focus is on the large scale hydrodynamic effects of the tidal current turbines. The goal is to come up with a practical tool for the development of a tidal farm, which implies the interest in the far field. The main criteria which are of interest: - Is the wake (turbulence) modeled accurately enough?
- Is the interaction of multiple wakes modeled accurately?
- Is the resistance represented accurately?
- And finally the energy production of the turbine should be modeled sufficiently as well.
These criteria raise the important question how accurate is accurate enough? Numerical models used as practical tools for wind farms show accuracies of 5 to 15% , this will also be the desired accuracy for this thesis. Another important issue concerns the definition of large scale. Because of practical reasons horizontal axis tidal current turbines will be placed at least 4 rotor diameters away from each other, which means there influence should be modeled accurately enough starting from this distance. Studies have shown that this region can be classified as the far wake region and this study focuses on modeling the far wake region as accurate as possible. |