Study On Performance Of Wave Energy Conversion System With Double Energy Harvesting Principles

With the unceasing depletion of fossil energy resources and the increasing of pressure for environmental protection worldwide,more and more maritime countries cast their eyes over the exploitation and utilization of marine renewable energies.Among marine renewable energies,wave energy has gained wide attention because of its large energy flux density and long annual utilization time.The problem of efficient utilization of wave energy has not been resolved well so far,which severely prevent the industrial development of wave energy utilization.Therefore,it is very necessary and significant to propose a novel efficient wave energy converter(WEC)and simulation techniques that accurately reflect the physical characteristics of the WEC.In this thesis,a novel WEC which has proprietary intellectual property rights is proposed.The device is mainly composed of two hinged floats,a hanging pendulum and a set of power take-off(PTO)system.Taking full advantage of the technological merits of raft-type and pendulum-type WECs,the device can capture both wave kinetic energy and wave potential energy efficiently.Both numerical simulations and model experiments are adopted in this paper to investigate the performance of the WEC which is comprised of two kinds of energy harvesting system principle.A dynamical model of the device motion in small wave conditions in frequency domain based on potential flow theory is established and validated.The effect of six PTO system arrangement forms on the optimal capture width ratio of the device under the condition of regular waves is compared.The effects of PTO system physical property,float geometrical features,pendulum geometrical and physical properties and wave frequency on the device performance is investigated,and it is revealed that the performance of the WEC has a substantial improvement compared to that of raft-type and pendulum-type WECs.A comparative analysis of the effect law of cross sectional shapes of the two floats which largely affect device performance on the device performance from the economic perspective is conducted.Based on the above-mentioned model in frequency domain,a dynamical model in time domain with the consideration of the effect of viscous drag force on structure motions is presented.The influence mechanism of PTO system,drag force coefficient,wave parameters,mooring system and current on the device performance is analyzed and the effect law of viscosity on the WEC of double energy harvesting principles is clarified.Based on turbulent flow equations,multi-degree of freedom nonlinear structure motion equations,PTO system equation and mooring system equation,a fully coupled mathematical model of fluid-structure-PTO-mooring system is presented.Then,the influence rule of PTO system,coefficient of kinetic viscosity and wave current condition on the device performance is analyzed.On the basis of numerical simulations,a physical model of the WEC is established based on the Froude number similarity criterion,and model experimental research is carried out.The influence law of PTO system setup and pendulum length on the relative rotations between each structure and the energy capture efficiency of the device in different wave and current conditions is measured and analyzed.Then the experimental results are made comparisons with the numerical simulation ones based on various mathematical models,which validates the correctness of the fully coupled model based on viscous fluid theory and clarifies the veracity and applicability of various mathematical models in different wave conditions.In general,the simulation results of the fully coupled model based on viscous fluid theory are the most accurate,followed by ones obtained by the dynamical model with the consideration of viscous drag force.