Numerical Study on Design of Floating Body for Wave Energy Generation in Ocean

Abstract

The demand for renewable energy sources in the world are growing with the depletion of fossil fuel supply as well as the environmental concerns of global warming. Renewable energy sources are of different kinds and ocean wave energy sources are one among them. The earth is covered with two thirds of water and therefore this energy source is considered as a permanent source. It has a tremendous amount of untapped energy left in it. Especially, South Korea, as a coastal state, has a natural geographic advantage for the use of wave energy. Energy can be extracted from the oceans in the form of a Wave Energy Converter (WEC), and among the converters proposed, the Pelamis WEC is a notable one. Technical literature is limited for the floating type WECs and it was necessary to carry out related research to solve energy shortage problems. In order to design a large-scale floating system like the Pelamis, we need to study the motion of a floater, which is effected by wave characteristics, as well as to study the energy absorbing capabilities of the floater, thus, necessitating for an optimal design of the floating type WEC. Firstly, linear wave theory is used as the basis for two phase flows of air and water through six Degrees of Freedom (DOF) for a rigid body by Computational Fluid Dynamics (CFD) simulations. The boundary conditions for the inlet are assumed as the "flap type" and the "piston type" separately for the motion of a floating body. Numerical simulation of a single floating body and two floating bodies were carried out for the piston type. The change of angle between the two floating bodies was also investigated. The comparative results obtained for the flap type and piston type are consistent in same wave properties. According to the wavelength of waves generated by the result of evaluating the behavior of floating body, it is concluded that 0.3 m is the maximum amplitude of wavelength of 5 m, and 0.15 m is the minimum amplitude of wavelength of 1 m. 1.06 m is the maximum distance between the two floaters of wavelength of 6 m. Secondly, the numerical simulation of a floating body under different floater length, floater diameter and wave lengths were carried out separately, either fixed or varied, to compare the rotational energy absorption of the floater. The results obtained were crucial in showing the importance in variations of floater length, floater diameter and wave lengths. Three cases were considered. 1) In the first case, when the wave height increases and the floater diameter and length are kept fixed, the angular velocity increases. The average power absorption also increases proportionally. 2) In the second case, when the wave properties are kept fixed and floater length are kept varying, the average absorbed power increases initially, reaches a maximum value of 55.00 KW and then tends to decrease. 3) In the third case, when the wave properties and the floater length are kept fixed and the floater diameter is allowed to vary, the angular velocity decreases with an increase in diameter. As the diameter increases, the average absorbed power also increases. Thirdly, the tuning factor concept is employed, as it explains the relation between the maximum pitch angle and the length of a floating body and wavelength. The relation between tuning factor & pitch period for the generated waves is compared to analyze the effects of energy absorption. A wave period of 5.5 s and amplitude of 0.57 m from Marado island region was chosen. 12 cases (model A to L) of natural pitching period from 1.25 s to 2.8 s have been modeled. From the results obtained, it is concluded that model L has the maximum power absorption, namely 6 kW approximately. A maximum pitch angle of 1.91 degree was attained by Model F, and the maximum displacement of nearly 0.7 m was attained by Model L among models D, F and L. In the fourth place, the 'tuning factor' concept is extended to predict the floater and wave interactions to maximize the rotational power absorption of the floating body. Different variables of “wavelength”(35 m~100 m) was used to study the effects of energy absorption of a floating body. The results show that when the wave length is longer the energy absorption by the body is higher as they are proportional to each other. Also, there are variations in energy absorption due to the location of the floating body in a wavelength. Finally, the spring constant is adopted to control the motion of multi floating bodies and to calculate the total average power absorption. Three cases of different wavelengths, namely 20D, 30D and 40D has been modeled to analyze the total average power absorption. The average power absorption not only varies with the position of the floating body but also varies with wavelength. The results obtained shows a maximum total average power absorption of 9W approximately in 30D wavelength and a minimum total average power absorption of 4.3W approximately in 40D wavelength.