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Ocean Wave Energy Exploitation in Deep Seas
W. Finnegan 1 & J. Goggins 1,2
1 College of Engineering and Informatics, NUIG.
2 Ryan Institute for Environmental, Marine and Energy Research, NUIG.
w.finnegan1@nuigalway.ie jamie.goggins@nuigalway.ie
Abstract
Ocean wave energy is one of the world’s most powerful
forms of energy and, as of yet, is a relatively untapped
natural resource. The main goal of this research is to
challenge some of the opinions of current developers
[1, 2] by exploring the advantages and efficiencies of
relatively small Wave Energy Converters (WEC’s),
which operate in the heave motion. This is to be
achieved by optimising their structural shape and
developing reliable methods for carrying out Structural
Health Monitoring of the devices.
1. Introduction
Since the publication of Stephen Salter’s
groundbreaking paper on his Wave Energy Duck [1],
thousands of patents have been issued for Wave Energy
Converters (WEC), incorporating a variety of methods.
However, as of yet, no ‘winning’ device design has
being established. This leaves plenty of scope for
Ireland to become a world leader in the development
and design of the necessary technologies. Furthermore,
Ireland is located next to a vast ocean, maximising the
available wave energy.
The energy in ocean waves is the most dense of the
renewable energies and is available up to 90% of the
time, compared to wind energy which is available 20-
30%.
2. Project Objectives
• Develop an analytical approach for calculating the
wave forces on a vertical cylinder in deep water.
• Use computational fluid dynamics (CFD) to
explore the wave forces on complex geometries
for axisymmetric vertically oscillating wave
energy converters (WEC’s).
• Develop a method of matching the frequency of a
WEC to the to the peak frequency of the wave
spectrum of a given location by optimising its
geometry.
• Perform physical experiments in order to verify
the results of the study.
• Explore methods for the structural health
monitoring of offshore devices.
3. Analytical solution for calculating the
wave force on a floating cylinder
The derivation uses the wave-water problem [3] and
eigenfunction expansions to obtain the velocity
potentials. A boundary value problem was set up to
derive the interior and exterior velocity potentials. The
velocity potential is then integrated over the structural
31
surface to calculate the excitation forces. The analytical
solution is arrived at by taking an asymptotic
approximation for a low frequency incident wave [4].
4. Computational Fluid Dynamics
A variety of complex geometries will be examined
using the fluid-structure interaction aspect of CFD, as
the frequency of the incoming linear deep water wave
changes. In order to insure the properties of the incident
wave are accurate, a study for creating an accurate and
optimum model was carried out. The next stage was to
explore the 2-dimensional interaction between the
incident wave and a floating rectangular prism.
5. Conclusion
An analytical solution for determining the excitation
forces on a floating vertical cylinder has been
developed. A method for generating realistic waves in
CFD has been developed and the CFD fluid-structure
interaction is being explored.
For every sea or ocean region around the world the
energy and properties of the waves is unique. Therefore,
it is necessary to design a WEC depending on its
expected location. The outcome of this project will
enable a designer to optimise the geometry of the
device by matching its natural frequency to the peak
frequency of the wave spectrum of the location. This
will aid to increase the frequency range of a vertically
oscillating axisymmetric WEC when mechanically
tuning the device to match the frequency of the incident
wave, in order to force it to resonate. Furthermore,
designers will have user-friendly methods to carry out
reliable SHM of devices.
6. Acknowledgments
The first author acknowledges the information
provided by the Marine Institute and the financial
support provided by the College of Engineering and
Informatics Postgraduate Scholarship.
References
[1] Salter S.H., 1974. Wave power. Nature. 249(5459): p.
720-724.
[2] French M.J., 2006. On the difficulty of inventing an
economical sea wave energy converter: a personal view.
Engineering for the Maritime Environment. 220.
[3] Linton C.M. and McIver P., 2001, Handbook of
mathematical techniques for wave/structure interactions.
Boca Raton, Fla.: Chapman & Hall/CRC. 304 p.
[4] Finnegan W., Meere M., Goggins J. The Wave Excitation
forces on a Floating Vertical Cylinder in Water of Infinite
Depth. World Renewable Energy Congress 2011 – Sweden 8-
11 May 2011, Linkoping, Sweden. Paper No. 475