Friday, 23 November 2012

Seven key principles for converting wave energy

  1.  Provide a point of reaction. Power is captured when wave induced motion is damped. The force resisting the motion results in an equal force in the opposite direction. This has to be opposed by something; no sky hooks allowed.
  2. The DoF where power is extracted must have a restoring force. There must be a way to restore the change in position due to wave induced motion. Gravity is the simplest method; buoyancy does the job for heave, pitch and roll motion. If power is captured from surge, sway or yaw motions, a restoring force must be supplied by mechanical springs, or by coupling to a motion that has buoyancy. 
  3. Radiated waves capture waves. When the power take off (PTO) system is run in reverse (motoring), the waves generated indicate the potential for capturing ocean waves. Good performance results from radiation of waves that have the same direction of travel, the same amplitude, and the opposite phase, as the waves you want to capture. These are the waves that the incoming wave splits into once it reaches the device: the waves reflected and/or transmitted by the wave energy converter.
  4. Get the level of damping just right. Like baby bear’s porridge.
  5. Match natural period to wave period. Radiating a wave which is out of phase with the waves you want to capture is important. At resonance the phase takes care of itself. The options are to design for a natural period that equals the design wave peiod, to have a range of operating modes with different natural periods, or to use the PTO force to trick the system into behaviour with the desired natural period.
  6. Size matters. The smaller the device, the narrower the bandwidth of the capture curve, and the lower the natural period. These disadvantages trade off against cost, so clearly size is an important cost of energy consideration.
  7. Manage losses. Reduce losses (PTO, overheads) in low energy seas; shed loads and power in high energy seas.

Points 1&2 might appear painfully obvious but please don't skip over them. It is important to identify which degree of freedoms (DoF) are being damped by the PTO system, and, for each DoF, to identify what is being pushed against (point of reaction) and what pulls it back again (the restoring force). Ignoring points 1&2 will confound design efforts, and could lead to mistakes such as the point of reaction or restoring force being provided by moorings that were specified for station keeping.

Point 3 deals with the mechanism behind wave energy absorption. Some may disagree that this is indeed the mechanism, and I will address these concerns at a later time.

Points 4-6 are what are needed to enforce the rather esoteric amplitude and phase requirements described by point 3. Baby bear's porridge is neither too hot nor too cold, and likewise the PTO force is highest between the two extremes of no damping, and damping so high there is no motion.

Point 7 has aspects of don't waste all the energy you've just captured about it. It also covers design choices such as capacity factor, design sea state, and survivability.

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