Development And Application Of Tidal Stream Energy Extraction Models

Tidal stream energy has attracted attention from governments and researchers with its high predictability,relatively small environmental impact and relatively mature extraction technology,and thus have developed rapidly.Tidal stream energy development is a multiple scale problem,including foil-scale,turbine scale,array scale,and gulf/near-shore scale.Numerical models based on the concept of "shallow-water model+turbine effect"act as a main tool for large scale tidal stream energy studies.However,the representation of turbines and their arrays has always been the crux for this type of models.Existing turbine representation methods can be categorized into two main classes:Firstly,increasing bed resistance.Such methods cannot accurately reflect the form and scale of turbines,effect,and can only be used to qualitatively explain some macro problems.Secondly,adding a second order resistance to the space where the turbines located,referenced to the commonly-seen second order resistance relationship in the natural world,and calibrate the thrust coefficients with physical model experiments.However,experiments can only provide scenarios under limited ideal conditions,which cannot completely cover the complex situations that may occur in actual sea areas.In response to the above problems,this thesis aims to establish a cross-scale tidal stream energy extraction model that is capable of predicting macro-tidal wave progression and micro-energy extraction process simultaneously.The main objectives of this thesis includes the following aspects:(1)Analysing the feedbacks of a tidal steam turbine on the surrounding hydrodynamic environments during the process of energy extraction,based on physical experiments and numerical simulations.(2)Extending existing tidal stream energy conversion theories,allowing the consideration of free surfaces and shear flows encountered in actual sea areas,followed by an analysis of power coefficient and energy absorption efficiency under the above conditions.(3)Establishing a mathematical model that couples macro tidal wave progressions and micro turbine energy extractions processes by combining shallow water equations and blade element momentum analysis.The model is then used to study the impact of large-scale tidal stream energy extraction in a real site.Highlights of the above studies are as below:·It is found that the turbulence intensity in the near wake of a turbine has frequencies of rotor rotation,blade sweeping,and their multiplications.The far wake can be represented using self-similar functions and Gaussian distributions.It is also verified that the effect of azimuthal stress on the axial flow velocity and turbulence intensity is not significant·Using a three-dimensional stream function projection method,the two-dimensional uniform flow linear momentum actuator disc theory(LMADT)is extended to a three-dimensional shear flow version.Power coefficients and efficiencies under vertical shear flows and free surfaces are given·The coupling of multiple scale phenomena is realized.A Shallow Water Equation+ Blade Element Momentum model that meets the requirements of large-scale tidal stream energy extraction studies is established.