Introducing W2W power flow diagrams

 

Wave to wire (W2W) power flow diagrams are interesting because they: 

• Provide a level playing field for comparing wave energy converters (WECs).
• Describe the physics of the power capture mechanism.
• Make it clear what subset of the process is being modelled, which is useful for model design and interpretation of results. This gives insights into important work such as the US Wave Energy Prize and WECCCOMP.
• Highlight challenges specific to wave energy, which can inform design.

In Control of Cost of Energy

That's me off for a few days to Ireland. The Maynooth University Wave Energy Workshop is being held on 20 Jan. The workshop webpage includes a remote sign-in link for anyone who isn't able to attend in person. As Maynooth specialises in control, we can expect that to be the focus of many of the talks. I suspect my poster will be the least mathematical by far! Nevertheless, this is a perfect opportunity for me to evangelise my latest pet topic, that of broadening the function of control. Rather than limiting control to a means of improving power capture, we can consider it a powerful tool for reducing cost of energy.

Here is my conference poster.    

Why not use wavemakers?

"If you are trying to make a good wavemaker, why not use wavemakers?"

If we take the statement 'a good wave absorber is a good wave maker' to its logical conclusion, then why not consider the wavemakers in our wave tanks? Most tanks for simulating deep water waves have a series of bottom-hinged flaps, mounted on the edge of the tank.

Falnes's diagram: just a bunch of old maths?

What does Falnes's well-known diagram depicting the principle 'to absorb a wave you must make a wave' (Fig 1) actually show? Is this achievable or just a mathematical slight of hand, a bit of physics-sudoku with no relevance to the real world?

The radiation paradox

Ya cannae get aroond conservation o' energy, ya reekie oxtered sassenachs yae. (Translation from the Scots: You can't get around conservation of energy).

Conservation of energy from a far field perspective

Consider an arbitrary boundary around a big bit of sea. In order for a wave energy converter (WEC) to absorb energy from within that boundary, the waves leaving the boundary must have less energy than the waves entering the boundary. The only mechanism to capture useful energy while ensuring less energy in the waves leaving the boundary, is to create waves that destructively interfere with waves that would leave the system in the absence of energy capture. To create waves while absorbing energy involves the radiation of waves by damped (by a power take off system that is) wave excited motion.

Doctor doctor! I see waves before my eyes!

Q: If I can see waves being radiated, that's good ... right?

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?

Does Falnes's wave maker diagram apply to all WECs ?

Fig 1: Falnes's wave absorber = wave maker diagram

Falnes's assertion that 'a good wave absorber must be a good wavemaker', and his diagram depicting the amplitude and phase conditions for optimum absorption have been around since the early days of wave energy theory. I'd always thought that the wave absorber = wave maker diagram (Fig 1) was general, and applied to all wave energy converters. Would you be surprised if I told you that it applied to point absorbers only? (by point absorber I mean a body that is at least an order of magnitude smaller than the wavelength of the incident waves)

Wrong turns on the way through the 96% capture maze

The photo below (Fig 1) was used to show that 96% of the energy in the wave coming from the right had been absorbed by the duck. In a previous post I showed how we can draw this conclusion from the photo alone. Like navigating through a maze, my first attempts at solving this problem took me up several dead ends. It is worth discussing these, as I'm sure I'm not the only one to have made these mistakes?

Can a photo demonstrate percentage of wave power absorbed?

Photo of Duck in action, by Jamie Taylor, the University of Edinburgh

This conundrum puzzled me for a while, so I'd like to share it with you. The question is whether it is possible to determine what percentage of wave power was being absorbed by the Duck in the time lapse photo.

 

About this photo 

This photo has an interesting history behind it: it was presented as proof to the British public (and funding bodies) that the Edinburgh Duck was capable of absorbing 96% of the power in a wave. Yet the story told by this photo is not as simple as it first seems.