Witt Energy

Optimising Bouy Design for small 6 d.o.f. Wave Energy Converter 

WITT Energy have a granted patent for a unique mechanical transmission system that takes whatever input to torsion transfer, hence the name WITT for the device (which alternatively stands for `What Is That Thing?’)  That is the device, converts vibrations from the full 6 degrees of freedom into rotation of a shaft in a single direction. Such motion can then generate electrical power. Many use cases are currently under investigation that produce small amounts of energy (c. 5Watts) for powering offshore sensors and devices by employing the WITT as a wave energy converter (WEC).

In order to extract significant power from a WITT WEC, it is a requirement  that the forcing motion should be beyond the resonant natural frequency of the device. This creates a dichotomy, because natural sea states have most of their power spectra at frequencies that are too low for the compact WITT devices that would fit in a buoyant sealed unit.  The challenge in this Study Group is to how to design a bouy and tethering system so that WITT experiences oscillation inputs at sufficiently high frequencies.

An increase in natural frequency of a floating vessel can be achieved by changing the relationship between its centre of gravity and centre of buoyancy. Alternatively, one could change the mooring system from a single mooring point to a two or three point bridle where the attachments are higher up the vessel.  Or one could vary elasticity of the various moorings so the vessel oscillates in a non-linear way for example by having peripheral floats which would displace water as the vessel oscillates (effectively acting as individual spar buoys that undergo simple harmonic motion as they are forced below the surface and resurface). Alternative designs have proposed tethering a group of vessels using catinaries that can lead to higher frequency differential motion between each vessel.

As this WITT is based on a spherical pendulum, it is agnostic to the direction of oscillation, so the bouy could rock in any direction. The target natural frequency would ideally be higher than 1 Hz, but lower frequencies should  not be dismissed, as work has been carried out using counterbalances, although they sacrifice energy.