Wave Energy Converter With Wave Sensor and Velocity Control
- Takashi Kawaguchi (Aqusys Corporation) | Kunio Nakano (Mitsui Zosen Steel Structures Engineering Co., Ltd.) | Shogo Miyajima (Akishima Laboratories (Mitsui Zosen) Inc.) | Taro Arikawa (Chuo University)
- Document ID
- International Society of Offshore and Polar Engineers
- The 28th International Ocean and Polar Engineering Conference, 10-15 June, Sapporo, Japan
- Publication Date
- Document Type
- Conference Paper
- 2018. International Society of Offshore and Polar Engineers
- absorbing wavemaker, resonance control, wave absorption and generation, point absorber
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- 9 since 2007
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This study is to demonstrate the bottom-mounted wave energy converter at Oarai port in Japan. This is a type of heaving point absorber which generates power by heaving movement of a float relative to the center axis. According to the point absorber theory, in regular waves of wave length λ, the maximum absorption width of a heaving point absorber is theoretically equal to the wave length divided by 2π. As the efficiency of the point absorber is λ/(2πD), assuming the float diameter is D, it can be larger than 100 %. The authors employed a resonance control with a wave sensor and succeeded in realizing this high efficiency.
For wave energy conversion, after the pioneering work of Masuda (1986) in the 1940s, the study in the North Sea by Salter (1974) was performed in the 1970s. At almost the same time as Salter, Budal and Falnes , Evans  and Newman  led the point absorber theory. One of the conclusions is that the upper limit of the wave energy absorbed by an axisymmetric float heaving in the waves of wave length λ is equivalent to the energy of the incident wave of width L=λ/2π, regardless of the size of the float. This width is named capture width. When comparing waves and wind, the wind energy is defined by the area of wind, whereas waves are defined by energy per width. In the case of wind, it is impossible to acquire energy exceeding the area of the wind, but the wave energy of the capture width L may be obtained regardless of the float width B. The capture width L and the capture width ratio η are defined as follows (Babarit and Hals (2011)),
Where Pabs is the absorbed power and Pw is the available wave power per unit crest length.
This efficiency is surprising. If the wave period T is 7s, λ/2π becomes 12m in deep sea waves. If the float width B is 5m, the capture width ratio η = L/B reaches 240%. This attractive effect is named the point absorber effect or the antenna effect. If the wave period T is 7s, λ/2π becomes 12m in deep sea waves. If the width of the float is 5m, λ/(2πΒ) reaches 240%. This attractive effect is named the point absorber effect or the antenna effect.
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