Wave Energy Prize results

The US wave energy prize results are through. Four teams completed the challenge of at least doubling the ratio of energy capture to design loads for a baseline design. Here are my views on how they achieved this.

AquaHarmonics

Prize: $1,5 m
Device: Surface piercing point absorber with a flexible tether to ground; power capture from heave, surge and sway.

Why did they win?  By their own admission, what won it for them was the control. I couldn't find any more information on the control other than its description as 'latching/declutching' on the team page.

Latching means putting on the brakes when the buoy slows to a halt at the top or bottom of its cycle, and holding it in that position for just the right amount of time to induce a quasi-resonance. The challenge is knowing how long to keep it latched before letting go. This can be calculated exactly for a regular wave, involves some guesswork in a wave tank when you know what the spectrum is, and is much less effective in real waves when you don't have real-time knowledge of the wave spectrum. The type of control is not new: Budal and Falnes were looking into this almost 40 years ago. However, this team designed the control from scratch, and were focused on practical implementation from the start, so I wouldn't be surprised if they have done something interesting in the implementation.

Latching is one way of inducing quasi-resonance, and collectively these methods are known as 'reactive control'. Reactive control is well suited to small buoys that extract power from heave motion: 'point absorbers'. The reasons for this are:

1. It's a cheap way of increasing the power to mass ratio: the heave natural period of small buoys is lower than the periods of the waves you want to capture. Increasing the resonant period with a heavier buoy is expensive; it is much cheaper to induce quasi-resonance with control.
2. It's a cheap and failsafe way of increasing the power to extreme load ratio: without control the buoy doesn't interact well with big waves. As you can turn down, or turn off, the control, the design loads will be lower.

The team page also mentions no limit in operational stroke, and hence no end-stops. This will no doubt have been a significant contribution to their success, as it both increases the power that can be captured and reduces the peak loads. The only details about the implementation is mention of a chord winding and unwinding on a (rotational) generator. Rotation doesn't need stroke limits, but translation does. So my guess is that the translational stroke limit (i.e. the length of the string attaching it to ground) was designed not to be reached in these tests. This is no guarantee that end-stops won't be reached in more challenging waves. However, one of the advantages of reactive control is that the uncontrolled device has smaller motions.  You can choose to capture less power if the end of the tether might be reached.

I would also suggest that a sense of humour contributed to their success. Their profile picture (see above) looks like they have just been abducted by aliens for medical experiments. The video at the top of this post has an amusing sequence showing their control kicking in.

Calwave power technologies

Prize: $500 000
Device: A 'carpet' of subsurface plates, connected with hinges, with power captured from the relative motion at these hinges. The hinges in the direction of wave travel capture power from heave and pitch motions (which are coupled). It is not clear whether adjacent strips of carpet have hinges that also allow power capture from roll motion relative to adjacent strips. Large areas of carpet are supported by floating or fixed towers.

Commentary: I think this device has a lot of potential. The mid-water operation has several advantages:

• loads are spread out over time and space; 
• problems with the air/water interface are avoided, such as corrosion, flotsam or ice collision, wave aeration and slamming loads; 
• keeping the working surface away from the bottom minimises risk of boulder collision and benthic damage;
• a plant big enough to power a city would have minimal visual impact; boats could potentially sail over it. 

Make no mistake, subsurface operation presents serious challenges too: installation and maintenance will be tough. To be cost effective, the design should be built around mechanisms for raising and lowering the working surface cheaply, with limited use of specialist vessels. The drawings available on the web at the moment (see diagram above from the Berkley website) suggest a supporting structure that is more massive than the working surface. My impression is that this is over-engineered to an extent that is appropriate for a prototype device, and that subsequent reductions of the supporting structure could be a significant source of cost of energy reduction in the future.

Waveswing America

Prize: $250 000
Device: Sub-surface point absorbers connected to the seabed via a hinge. Power is captured from the relative translation of the buoy and the hinged support structure, which is nominally heave. The device has a pitch motion; it is unclear whether power is captured from this mode.

Commentary: It is interesting to note that this concept shares some of the advantages of the concepts that won first and second prize: it is submerged, and it is a point absorber with reactive control. Details about the type of reactive control were not given on the wave energy prize website. 'Reactive control' is used to refer both to latching, and to a type of control where some of the energy captured from waves is stored and used to drive the motion for a small part of each oscillation cycle. There is nothing on the team page which gives a definite positive identification of either of these. The video at the bottom of the team page shows smooth motion, which suggests latching was not used. The video also shows that the tank tests used air flowing through an orifice as the damping. So the full cycle of energy storage was not modelled.

This is a concept developed by Inverness-based developer AWS Ocean Energy. The original AWS concept developed by Teamwork technologies in Holland only allowed motion in heave. The addition of a hinge is an improvement on the original concept because it lowers the bending moment due to pitch oscillation and pitch offset. This increases the ratio of power capture to the loads you need to resist.

Oscilla Power

Prize: Just missed out.
Device: The technology they are showcasing is a novel power take off mechanism. Their device is a two body point absorber. The lower body is a moored sub-surface reaction plate, and this is connected by corner tethers to a surface float. Power is captured from the relative motion in heave, surge, sway, pitch and roll.

Commentary: Despite missing out on prize money, this concept is well worth a mention. The power take off is really innovative. It is a solid state material that converts material strain to electricity with a high efficiency. This is interesting as the material used in the power take off could potentially double up as structural strength. One of the challenges of this technology is the small motions required at the business end. So the movement of the prime mover has to be geared down. The problem space is quite different from other wave energy converters where motions are typically geared up.

The wave energy converter concept also has several interesting features. It captures from several modes of motion, which is good for the ratio of power capture to mass. The design has considered cheap installation and retrieval of the device, as well as a storm-protection mode. Furthermore, this device may be suited to reactive control. There is no mention of control methods, and this suggests an area for further improvements.


Disclaimer:
The views expressed here are my personal opinion. I am not aware of any conflicts of interest that would influence my opinions. I previously worked for AWS in Inverness, but not on their present concept. I have collaborated academically with Tim Mundon from Oscilla Power when we were students. My PhD was on reactive control so I am naturally quite excited that this contributed to the success of two of the teams.