We have been fairly closed with the WEC design that we are currently working on. However, we have submitted a patent application, and now feel more comfortable sharing some information - see our updated Technology page as well.
The Mocean WEC is a hinged raft. Wave forcing and the bodies’ dynamic responses leads to a motion about the hinge (called flex), which drives a power take-off mechanism that converts the kinetic energy into electricity. It has 7 degrees of freedom (DOF).
Historically, a hinged raft WEC consisted of two (or more) bodies of the same size and shape connected in a line by a hinge (or multiple hinges). The image on the right shows a simple conceptual hinged raft. The predominant force on the traditional hinged-raft WEC is the wave excitation force in flex, and so it absorbs the most power in waves that are approximately the same length as it. This means that rafts must be made long to extract energy from longer, more powerful waves. An 8 second wave in 40 m water depth is 100 m long.
However, the wave-body interaction can be more complex than just the waves inducing a bending moment about a hinge. There are inertial, damping, and spring forces; and there are wave excitation forces in other DOF. The hinged raft is a dynamic system with multiple degrees of freedom.
The resonant frequency of a simple, single DOF, mass spring system (see figure above) is the square-root of the ratio of the spring constant to the mass. However, in a system with multiple masses and springs, the resonant frequencies are found by solving a generalized eigenvalue equation for the mass and spring matrices (also in the figure above, and explained more here). There can be more than one resonant frequency, and in fact there can be as many resonant frequencies as there are DOF.
In a symmetric hinged raft WEC (i.e. the aft body is the mirror image of the forward body), many of the modes of motion are uncoupled from one another, which means they each act as single DOF, mass-spring system, and as such limited to the degree that the resonant response can be tuned.
The innovation of the Mocean WEC is in the design of the shapes of the bodies, which dramatically improves its dynamics and thus power absorption. The configurations are based around varying the ratio and position of the water-plane area to the submerged volume, where the water-plane area affects the hydrostatic restoring (spring) force and the volume affects the mass and added mass (inertial force). By changing these values one can induce coupling between the modes and tune the resonant response to improve performance in wavelengths that are significantly longer than the overall length of the machine.
Consequently, although the power take-off is solely in flex around the hinge, there is extensive cross-coupling with other degrees of freedom, and when excited by wave action the device responds not only in flex but substantially in heave and also pitch and surge. This results in greater cancellation of the incoming wave and a broader bandwidth response than a standard hinged raft.
The figures above show the profiles and performance (power/mass as a function of wavelength for unit amplitude waves) for three different geometries of hinged rafts. Each WEC is approximately 90 m long. The blue WEC is the standard hinged raft and its peak is at a wavelength of approximately 90 m. However, the red and yellow WECs have much boarder responses are are active in wavelengths significantly longer than their devices lengths.
Even armed with an approach for how to affect the resonant response of the hinged raft, it is still quite difficult to design for. There is a great deal to be learned and we are continuing to develop new high-performance geometric shapes. One technique that we use is non-linear optimization. The video below shows an optimization tool that we developed as part of our Wave Energy Scotland NWEC project. In it, we see hinged raft geometries being programmatically generated in Rhino3d and exported for evaluation with WAMIT.
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