Abstract
Realistic evaluation of tidal-stream power extraction effects on local hydrodynamics requires the inclusion of the turbine’s operating conditions (TOC). An alternative approach for simulating the turbine’s array energy capture at a regional scale, momentum sink-TOC, is used to assess the impact of power extraction. The method computes a non-constant thrust force calculated based on the turbine’s operating conditions, and it uses the wake induction factor and blockage ratio to characterise the performance of a turbine. Additionally, the momentum sink-TOC relates the changes produced by power extraction, on the velocity and sea surface within the turbine’s near-field extension, to the turbine’s thrust force. The method was implemented in two hydrodynamic models that solved gradually varying flows (GVF) and rapidly varying flows (RVF). The local hydrodynamic effects produced by tidal-stream power extraction for varying the turbine’s operating conditions was investigated in (i) the thrust and power coefficient calculation, (ii) flow rate reduction, and (iii) tidal currents’ velocity and elevation profiles. Finally, for a turbine array that operates at optimal conditions, the potential energy resource was assessed. The maximisation of power extraction for electrical generation requires the use of an optimum turbine wake induction factor and an adequate blockage ratio, so that the power loss due to turbine wake mixing is reduced. On the other hand, the situations where limiting values of these parameters are used should be avoided as they lead to negligible power available. In terms of hydrodynamical models, an RVF solver provided a more accurate evaluation of the turbine’s operating conditions effect on local hydrodynamics. Particularly satisfactory results were obtained for a partial-fence. In the case of a fence configuration, the GVF solver was found to be a computationally economical tool to pre-assess the resource; however, caution should be taken as the solver did not accurately approximate the velocity decrease produced by energy extraction.
Highlights
Strong tidal currents are produced by local geographical constrictions such as narrow straits, channels, off headlands, and between islands and landmasses or basins
As the rapidly varying flows (RVF) solver approximates more accurately the flow rate reduction than the gradually varying flows (GVF) solver, the higher blockage ratio reported by the GVF (B = 0.7) suggest that the alternating direction implicit (ADI) model underestimates the flow rate decrease caused by energy extraction
Coastal tidal-streams can be approximated as two-dimensional flows with low Froude numbers, whose potential energy resource can be assessed with depth-averaged shallow water equations
Summary
Strong tidal currents are produced by local geographical constrictions such as narrow straits, channels, off headlands, and between islands and landmasses or basins. Three main approaches have been implemented to simulate energy extraction from turbine arrays They are based on: (i) the drag effect of turbines (bed roughness), (ii) the axial component of the thrust force produced by turbines on the flow (momentum sink), and (iii) the numerical implementation of LMAD-OCH. This last approach relates the turbine operating conditions (defined by the blockage ratio and turbine-wake induction factor) to the momentum extracted by the turbine. Assessment of the tidal-stream potential energy resource considering optimal conditions of turbine performance
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call