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Research Papers: Energy Extraction From Natural Resources

Assessment of Wave Energy Extraction From Seas: Numerical Validation

[+] Author and Article Information
Anthony Lewis

Hydraulics and Maritime
Research Centre,
Youngline Industrial Estate,
Pouladuff Road,
Cork, Ireland

Contributed by the Advanced Energy Systems Division of ASME for publication in the Journal of Energy Resources Technology. Manuscript received January 13, 2012; final manuscript received April 21, 2012; published online August 7, 2012. Assoc. Editor: Srinivas Katipamula.

J. Energy Resour. Technol 134(4), 041701 (Aug 07, 2012) (8 pages) doi:10.1115/1.4007193 History: Received January 13, 2012; Revised April 21, 2012

In developing a wave energy converter (WEC), assessing and rating the device is a difficult, but important issue. Conventionally, a large scaled device (maybe large enough for accommodating a power takeoff (PTO) system) or prototype device is needed to be tested in wave tanks or in seas in different wave conditions so that a power matrix for the device can be defined using scaling or interpolation/extrapolation methods. Alternatively, a pure numerical simulation in time-domain may be used for assessing the power capture capacities of wave energy devices. For the former, it is convincing, but can be especially difficult in the early stages of development, when small scaled models are normally used; and for the latter, the pure numerical simulation may not be very reliable and convincing, especially when the dynamic problem is very complicated. In this paper, a method for assessing the captured wave power for a device from its power capture response is presented. In the proposed method, a measured or calculated linear power capture response of the device is combined with wave spectrum to compute the average captured power function. Once the average captured power function is obtained, the overall average captured power corresponding to the wave state can be easily calculated. If a linear power capture response is obtained from a model test, the power assessment based on this proposed method can be very convincing and reliable. To illustrate the application of the proposed method, an example of a fully linear dynamic system, including the linear hydrodynamics of the floating structure and a linear power takeoff, is considered. For such a system, the frequency-domain analysis can be employed to obtain the performance of the floating device under waves and the power takeoff system. The hydrodynamic performance of the wave energy converter is then used to define the power capture response and to calculate the average captured power functions in different sea states. Then, the captured power of the device in different sea states, i.e, the power matrix, can be calculated, and accordingly, the device can be assessed and rated. To validate the proposed method, a time-domain analysis is also performed for a cross-check. In the time-domain analysis, the hydrodynamic coefficients and responses are first assessed in frequency-domain, and then transformed into the relevant terms by means of impulse response functions for establishing the time-domain (TD) equation. By comparing the results from frequency-domain and time-domain analyses of irregular waves, it can be concluded that the proposed wave energy capture assessment method can be used in assessing or rating the device.

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References

Figures

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Fig. 1

Panels for the truncated circular cylinder

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Fig. 6

Heave RAO of the cylinder after PTO is on

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Fig. 7

Wave power capture curve

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Fig. 2

Added mass for heave motion of the cylinder in waves

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Fig. 3

Damping coefficient for heave motion of the cylinder in waves

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Fig. 4

Excitation force for heave motion

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Fig. 5

RAO of the heave motion (ζ3/A)

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Fig. 8

Wave power capture efficiency

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Fig. 9

Average captured power function in a sea state of Hs = 3.0 and T01 = 8.0 s (Bretschneider spectrum)

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Fig. 10

Radiation impulse response function

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Fig. 11

Diffraction impulse response function

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Fig. 12

Excitation forces for heave by different approaches

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Fig. 13

Heave motions by time-domain and frequency-domain analysis

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Fig. 14

Captured power time series

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