Optimizing the configuration of tidal turbines in storm surge barriers

The flow between bridge piers and through storm surge barriers and barrages is an untapped and promising source of water energy. This energy can be harvested with tidal or hydro turbines. In 2015, five turbines with a total capacity of 1.2 MW were retrofit in a flow opening of the Eastern Scheldt storm surge barrier (the Netherlands). These turbines form world's first commercial-scale tidal fence. However, there is still a major challenge to optimize the configuration of these turbines based on their energy yield and their possible environmental effects to the hinter-lying estuary.

This thesis presents a model tool to optimize the energy yield and impact on the environment of installing turbines in flood defences by altering the turbine placing. Mapping out the effects of turbines on the flow is the central question. To answer this question, this research consists of three parts: (1) measuring the field situation, (2) testing a turbine in the laboratory and (3) setting up an analytical model that is coupled to a regional flow model.

In the first part of this study (1), unique, high-resolution data of the flow through the Eastern Scheldt storm surge barrier and around the turbines were investigated. In particular, for the first time in the literature, commercial-scale turbines are used to determine the effect of tidal turbines on the water flow. The power output of the turbines is also quantified. The data is used to derive an analytical model of the flow around a turbine in a barrier. This model can calculate the power of tidal turbines and the resistance of the barrier and turbine for different forms of the installation and variable strength of the external flow.

In the second part of this study (2), these insights were refined in laboratory tests, in which the configuration of the turbine and barrier was varied. This method is more representative of real turbines because it has a larger scale factor (1:9) than is usual in the literature. The tests show that the generated power strongly depends on the position of the turbine relative to the barrier. The data also show that the combined resistance of a barrier and turbine is lower than the sum of the individual resistances. These outcomes are used to successfully validate the previously developed analytical model.

In the last part of this study (3), the developed analytical model was implemented in a larger-scale numerical flow model. In this larger-scale model, the small-scale flow around a barrier with turbines is linked in an efficient way to the large-scale water movement in a tidal basin. This makes it possible to optimize existing or new tidal power stations, both at the level of the entire barrier and at that of a single flow opening. The impact on the environment can therefore be determined with the model, even more accurately than was previously possible.

The research in this thesis shows that the effect of the turbines on the flow at a larger distance is smaller than previously thought. This offers the possibility, for example, to install more turbines and harvest more energy without exceeding the acceptable environmental impact (e.g. ecological effects). This study has contributed to confidence in the technical and economic feasibility of turbine installations that can be built in hydraulic engineering works in the Dutch Delta. The developed calculation tool is freely available to investigate energy yield and environmental effects of tidal energy projects worldwide.