Monday 3 December 2012

How to design a device that absorbs no wave power

  1. Do not provide a point of reaction for the PTO. e.g the PTO opposes surge motion between two floating vertical plates, one behind the other; ensure one of these plates is light and porous so it offers little resistance to the PTO force; ensure they are close together to minimise excitation phase difference.

  2. Avoid restoring forces in the degree of freedom (DoF) where power is extracted. There is no buoyancy restoration in surge, sway or yaw, so extract power from these DoF, ensuring that extra spring is not inadvertently applied. Moorings provide spring in these DoF, so just let the device drift where it wants.

  3. Avoid useful wave radiation. Ensure that when the power take off (PTO) system is run in reverse (motoring), the waves generated are as small as possible. Better still, generate large waves that travel in the opposite direction, or have a similar phase, to the waves you're trying to cancel out. The next three points describe pragmatic methods for achieving this goal. 
  4. Get the level of damping wrong. Light as a feather or heavy as a brick.

  5. Mismatch natural period and wave period. Design the device so that the natural (resonant) period is as far as possible from the energy period of the waves you want to capture. 

  6. Make it really really small. This will make the bandwidth of the capture curve really narrow; combined with a mismatch between natural period and wave period this will ensure very little energy is captured indeed.

  7. Pump up those losses. Induce turbulent flow by lining exterior surfaces with mesh beaches similar to those used in wave tanks. Use a leaky transmission system. Design the bit just above the water-line so that small motions induce slap and slam loads. Install the biggest generator possible to ensure you stay at  the low end of the generator efficiency curve during normal operation.

Of course, it may be difficult to apply of all these principles simultaneously. For example, to have a really really small device with a really really big PTO you may require dimensionally transcendental engineering, as used in Dr. Who’s police box.

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