How does the blockage ratio influence the performance of a tidal stream turbine array?

The blockage ratio significantly influences the performance of a tidal stream turbine array by affecting the flow velocity and turbulence intensity experienced by the turbines. The blockage ratio is defined as the ratio of the swept area of the turbine(s) to the cross-sectional area of the channel or flow path. A high blockage ratio means that the turbines occupy a significant portion of the flow area, while a low blockage ratio means that the turbines occupy a relatively small portion of the flow area. A high blockage ratio can lead to an increase in flow velocity around the turbines, as the water is forced to flow through a smaller area. This increased velocity can increase the power output of the turbines, but it can also increase turbulence intensity. Increased turbulence can lead to increased drag on the turbine blades, reducing their efficiency and increasing the risk of cavitation. It can also increase the fatigue loading on the turbine structure, reducing its lifespan. Furthermore, a high blockage ratio can create a backwater effect, which is a rise in water level upstream of the turbine array. This backwater effect can reduce the available head for the turbines and can also affect sediment transport patterns. A low blockage ratio, on the other hand, can reduce the flow velocity around the turbines, decreasing their power output. However, it can also reduce turbulence intensity and the backwater effect. Therefore, there is an optimal blockage ratio that balances the competing effects of increased flow velocity and increased turbulence. The optimal blockage ratio depends on several factors, including the channel geometry, the turbine design, and the spacing between the turbines. For example, in a narrow channel with strong tidal currents, a lower blockage ratio may be preferable to minimize turbulence and the backwater effect. Careful hydrodynamic modeling and optimization are essential for determining the optimal blockage ratio for a tidal stream turbine array.