Research On A Two-body Floating Wave Energy Converter

In recent decades, the increasing demand for energy and the call for environmental protection and CO2 emission reduction have simulated the development and utilization of clean energy. Ocean wave energy is a kind of clean energy and the conversion technology has achieved very big procress. A number of wave energy converters(WECs) have been proposed all around the world, including oscillating water column WEC, overtoping WEC and oscillating body WEC. A novel two-body floating WEC in demand of marine monitoring is put forward and researched by this thesis, which takes advantage of the relative motion of two oscillating bodies activated by ocean waves to drive a generator for electrical power output. The work principle and wave energy capturing characteristics of the two-body floating WEC are investigated based on the frequency domain, time domain methods and model experiments.Fristly, some basic theories related to this thesis are brirfly introduced, including the linear wave theory, the random wave theroy, the characteristics of motion of the floater subjected to waves, the hydrodynamic commercial software AQWA and principles of model experiment.Secondly, a novel two-body floating WEC applied to marine monitoring is proposed. The basic structure and working principle of the converter are introduced. The initial converter consists of a floating spheroid connected to a heavy submerged sphere by a tight rope. The power take off(PTO) system are mounted in the floater. Base on the linear wave theory, providing the PTO system is linear and the bodies are restricted to move in heave, then the heave motion equation of the WEC in frequency domain is established. The spring and damping coefficients of the power take-off device are optimized by means of resonance theory of undamped mass-spring system, and the numerical results indicate the proposed converter is reasonable to application. A two-body oscillating similar model is devised for experiments. The experimental results show that the power capture width ratio of the converter can be considerable under the action of regular waves.Thirdly, a constraint is firstly added to the system in order to solve the problem that the optimal damping coefficients obtained by numerical analysis may be too small. The phenomenon that when incident wave height becomes high the rope in initial scheme will be slack is not allowed in real application. The shape optimization process comparing the performances of the cylinder, elliptical cylinder, sphere, ellipsoid, cone as the floater according to the strain condition is completed and the sphere is found to be feasible. The optimal phase condition and damping condition for the maximum power output are found out after the analysis of the heave performance of the two-body system with two spheres, and its frequency response function(FRF) is obtained. The influences of PTO coefficients on FRF are investigated. The expression of power capture width in regular waves and random waves are derived. It is found that when the wave frequency or wave peak frequency is close to the natural frequency, the power output is remarkable.Since one floater generally moves in six degree of freedom and it’s impossible to incorporate the effects of nonlinear characteristics and control on power capture performance of the WEC in frequency domain, this thesis conducts research on the two-body floating WEC with two spheres by many motion modes in time domain as well. State space approximation of the convolution term in time-domain equation is introduced to accelerate and simplify the numerical modeling. The parameter identification is completed in frequency domain and identification steps are listed. The external forces and moments acting on the spheres due to the tight rope are deduced. Motions of the system and power absorbed by the PTO under regular and irregular waves are analyzed in detail. The effects of wave perhoid and wave height, and PTO coefficients on the power output are analyzed and the 12 natural perhoids of the whole system are decuded. By comparing the enengy output by heave motion only and by many motion modes in time domain, the conclusion that the proposed two-body floating WEC with two spheres mainly moves in heave is drawn. A two-body oscillating similar model with two spheres is devised and the converter is further studied by model experiments of which well performance has been got.At last, the thesis gives a conclusion to the contents and presents the prospect of this topic in future studies.