The Energy Capture Analysis of a Cylindrical Wave Energy Converter

  • Mohamed Hariri. Nokob

Student thesis: Master's Thesis

Abstract

The field of ocean wave energy is very diverse in terms of design. It is the purpose of this work to model the energy conversion characteristics of a circular cylinder set to oscillate in gentle ocean waves. A semi-analytical eigenfunction expansion method is used to compute the hydrodynamic forces on the body. The fluid potentials and normal velocities are matched at the boundary separating the internal region beneath the floating cylinder (or above it if the cylinder is submerged) and the external region which includes the rest of the fluid. Three different configurations will be considered. The first is the case of a buoy that is constrained to oscillate in heave by a concentric pole fixed to the seafloor. The pole can be used to lower the buoy during a storm or can be the tower of an offshore wind turbine. Results in monochromatic waves shed light on the effect of the pole size on the hydrodynamic forces acting on the cylinder and on the power output of a linear power takeoff mechanism. The power output from irregular stochastic sea waves is also discussed. The results show that the effect of the pole is to increase the power output at high frequencies. Furthermore, the same power output can be achieved at some frequency for a given pole size by choosing the appropriate buoy size. In irregular waves, numerical results presented for the power indicate that the pole can be advantageous for certain geometries. An increased power per unit mass is also observed. The results can be used to design the buoy to achieve optimum performance. The second configuration to be discussed is the case of a cable-moored buoy set to oscillate in the vertical plane. Again a linear model is used to assess the power output from regular and irregular waves. Power is extracted from both the heave and pitch modes while particular attention is given to the effect of the pitch mode and to help answer the question of how beneficial it maybe to employ the added degrees of freedom. The results show that the share of pitch power is most notable at high frequencies when the heave power decreases sharply. The results also highlight the effects of some parameters such as the buoy radius, height and floatation level on the power output. The power per unit mass in irregular waves (directly related to cost) is found to admit a maximum at a fixed buoy height-to-radius ratio. The third configuration of interest is the case when the sea bottom-mounted cylinder is completely submerged with water. This is a simplified model of the so called Archimedes Wave Swing device used to extract wave energy. The work studies the effect of an added internal mass-spring system on the general performance of the device. The idea is to make the exterior cover of the device almost stationary to protect from wear while moving all the energy to the internal mass. The results show the difficulty of achieving that goal and therefore the ineffectiveness of the idea. Finally, the work tackles some of the numerical aspects of the problems at hand; mainly the speed of convergence of the Fourier series and how to make use of the Richardson Extrapolation method to speed convergence. A general comparison of the three configurations and conclusions are included.
Date of AwardDec 2011
Original languageAmerican English
SupervisorYoussef Shatilla (Supervisor)

Keywords

  • Ocean Wave Power

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