Frequency Domain And Time Domain Analysis On Mechanism Of A Point Absorber Wave Energy Convertor

In recent decades, the increasing demand for energy and the need for a dramaticdescent of the CO2-emission have stimulated the scientific investigation and technicaldevelopment concerning the electrical power harvesting from ocean waves. A numberof wave energy converters (WECs) have been proposed all around the world; theseincludes oscillating water column WEC, overtoping WEC and oscillating body WEC.This thesis focus on a point absorbor WEC, which takes advantage of the heavemotion of a buoy to drive a linear generator directly or to drive a hydraulic power takeoff (PTO) system indirectly. And it develops frequency domain and time domainmethods to investigate the work principle and wave energy capturing characteristicsof a heaving point absorbor WEC.This thesis firstly studies the direct drive point absorbor WEC in the frequencydomain. Based on the linear wave theory, by assuming that the power take off force(PTO) is modeled to be linear and restricting the consideration of heave motion,motion equation of a direct drive point absorber WEC in the frequency domain isestablished, and its frequency response function (FRF) is obtained. The expression ofpower capture width in regular waves and random waves are derived, the influencesof buoy geometry, FRF, PTO damping coefficient and wave climate on power capturewidth are investigated. It is found that the smaller difference between the naturalfrequency and peak frequency of the wave spectrum, the larger amplitude of FRF, andthe greater PTO damping coefficient can lead to a relatively larger power capturewidth.Since it’s impossible to incorporate the effects of nonlinear characteristics ofPTO and control on power capture performance of a point absorbor WEC infrequency domain, this thesis conducts research on point absorbor WEC in timedomain as well. State space approximation of the convolution term in time-domainequation is introduced to accelerate and simplify the numerical modeling. Thismethod is adopted to simulate a direct drive point absorbor WEC with linear PTO, the accuracy of this method is validated by comparing the time-domain simulating resultswith the frequency-domain calculating results.In the following, this thesis investigates the influence of nonlinear characteristcsof PTO on power capture performance of a direct drive point absorbor WEC. Statespace approximation method is employed. It’s reasonably assumed that the pistondamping force is propotional to its velocity when the velocity is small, and is constantwhen the velocity is large enough. It’s shown that WEC has the optimal efficiencywhen deployed in suitable wave climate, and stronger wave climate might not lead toan increasing of its wave energy capture performance, and instead result in thedamage of its structure or mooring system.This thesis makes an initial disscution concerning point absorbor WEC withhydraulic PTO. Only the heave motion of the buoy is taken into account, rationalsimplification is adopted for the hydraulic system and state space approximationmethod is employed. The time-domain mathematical model of a point absorbor WECwith hydraulic PTO is established. Numerical simulations in regular waves andrandom waves are carried out. Further study on control of WEC could be performedon the basis of the current research.