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Fig 1: Duck radiating a wave |
We've established that wave energy converters need a radiated wave to cancel out the wave being absorbed.
Question is, if you are watching a device operating in a wave tank or a sea trial, and you can see a wave being radiated, does this prove that the device is doing a good job of power capture?
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Fig 2: Falnes's wave absorber = wave maker diagram |
Of course, Fig 2 is not intended to describe real life; it merely describes the requirements for the perfect absorption of a wave. In real life you'd expect imperfections.
What would ineffective wave absorption look like?
It's worth considering which conditions would result in ineffective wave absorption, and how this might look during operation (in the final step of the diagram).
Radiated wave too small
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Fig 3: Radiated wave is too small |
Radiated wave too large
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Fig 4: radiated wave is too large |
Fig 4 shows the case where the radiated wave has less than twice the amplitude of the incident wave; hence in Fig 4e we see a transmitted wave that is smaller than the incident wave. Conservation of energy tells us some power has been captured.
If instead the radiated wave was more than twice as big as the incident wave, we would observe in step (e) a transmitted wave that was larger than the incident wave. Conservation of energy would tell us that we would need to be supplying power to the system rather than capturing it.
Radiated wave out of phase
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Fig 5: radiated wave is out of phase |
At phase angles further away from those required, the interference would be constructive rather than destructive. This means we would observe a transmitted wave with a larger amplitude than the incident wave. Conservation of energy tells us this is possible only if we are putting energy into the system (driving motion).
In both cases the wave leaving the system would consist of some un-cancelled incident wave combined with wasted radiation.
Radiated wave in the wrong direction
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Fig 6: radiated waves in the wrong direction |
In a previous article I presented a variant of Falnes's diagram that applies to large devices, and discussed why Fig 2 refers to a small device. For large devices, it is necessary to radiate waves towards the incident waves, so waves radiated in the same direction as the incident wave are travelling in 'the wrong direction'. For small devices, it is necessary to radiate waves in the same direction as the incident waves, so waves radiated towards the incident waves (Fig 6c) are travelling in 'the wrong direction'.
When waves are radiated in the wrong direction, both un-cancelled incident waves and wasted radiated waves are leaving the system.
Doctor's advice
If you are watching a device operating in a wave tank or a sea trial and you can see waves radiating away, this is evidence that the device is not working efficiently. What you're observing is either wasted radiation; radiated waves that are not doing the job of cancelling out the waves you're trying to capture; or waves arising from the incident wave that have not been cancelled by a radiated wave. Both are undesirable, as the power they carry away from the system is power not being captured.
Image credits:
'Splishy splashy explored' by Rachel Carter http://www.flickr.com/photos/raggle/3163752268/
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Simple and useful...
ReplyDeletethanks!