tidal kites
March 2, 2011 11:55 PM   Subscribe

Green electricity from the artificial, tethered ray (the fish, not the beam). Video here.
posted by megob (21 comments total) 5 users marked this as a favorite
Hopefully, seaweed and other marine organisms know to steer clear of the kite's path, in the meantime.
posted by Blazecock Pileon at 11:59 PM on March 2, 2011

How does the tethering help? The usual (green) energy sources involve a fixed collector; a wind turbine fixed against the ground, spun by the wind, or a waterwheel fixed around an axle, set into motion by a stream. How does using a flexible tether help? The article in the first link suggests that some kind of "swooping" is more efficient, but I don't understand why.
posted by twoleftfeet at 12:10 AM on March 3, 2011

It's Evan Longoria, isn't it?
posted by dirigibleman at 12:27 AM on March 3, 2011 [1 favorite]

the artificial, tethered ray

the famous Ray
posted by flapjax at midnite at 12:51 AM on March 3, 2011

The article in the first link suggests that some kind of "swooping" is more efficient, but I don't understand why.

It's a water kite. Imagine that water flowing inland is a steady, directional wind with slight local variations. Like a toy kite in air, the wing follows the direction of strongest wind. This way the turbine is always in the strongest possible wind stream. When the tide goes back out the kite just swings around to swoop and swing in the opposite general direction.

This is also nice because they don't have to build a large rigid structure to hold a turbine. Those tethers are certainly much cheaper than some kind of ocean-bottom towers. The only concern I can see is submarine traffic (which can be rerouted around the kite farms) and sea life. If they had oxidizing steel parts I can see sharks trying to eat them and baleen whales could accidentally suck one in while filter feeding. No idea what the risk of that is, really.
posted by clarknova at 1:12 AM on March 3, 2011 [1 favorite]

It looks like a good idea. I remember watching a TED talk with some guy who was showing how kites are better for generating power from the wind - this is a nice extrapolation on that. And hopefully they'll be nothing like famous Ray's pizza, which is akin to nothing that is good or positive or useful or worthwhile.
posted by From Bklyn at 2:52 AM on March 3, 2011

As a boy who grew up on an island off the coast of Maine, I love this idea. The sun might not come out, and the wind might not blow, but the tide just keeps on coming and going.
posted by lobstah at 3:46 AM on March 3, 2011

This is also immeasurably better than early plans for tidal power which involved replacing beaches with massive concrete turbine facilities.
posted by clarknova at 5:42 AM on March 3, 2011

So, I'm guessing it "swims" near the bottom of the ocean, in those areas that are 50-300 meters deep? So boats could still travel over it?

I really like this idea, and I'm guessing life in the ocean itself is much more treacherous than the off chance a fish runs into this thing. Not to say they shouldn't consider that issue, but I really feel like people who say "What about the birds/fish?" when it comes to wind/tidal energy are missing the big picture. Yes, it'd be terrible if renewable energy killed an endangered species, but consider the alternative. Global warming will kill many other fragile species if we continue burning fossil fuels the way we do. We need to do something to alleviate the effects of fossil fuel pollution and the economic fallout of running scarce on those fuels. We're simply using those resources faster than they recover, and I don't care what school of economics you practice or what you're political stripes are. That's not sustainable.

Fighting against this kind of thing is simply falling into the hands of the energy companies, who would love to maintain their holds on the market even as prices rise and the environment suffers.

And just to be clear, I'm also pro-nuclear so long as it's run safely and properly. Meaning more regulation and oversight than the nuclear lobby would like, although I think that's only fair if they're also asking for huge government contracts to build the things in the first place.
posted by mccarty.tim at 6:04 AM on March 3, 2011

With free-stream turbines, power is all about swept area. I appreciate the simple design of these little kites, but a tiny little turbine travelling in lazy figure of 8s does not cover a lot of water. I've seen a half dozen tidal turbine companies go out of business over the last 10 years because no one wanted to spend millions to deply a 10kW toy (the government of Ireland's investment notwithstanding). Go big or go home!
posted by Popular Ethics at 6:47 AM on March 3, 2011

swept area
That's exactly the point, I think. I wondered before how to estimate the amount of energy to be generated by these kites. In fact, the swept area seems to be minuscule, taking into consideration that the turbine itself is only the small rotor at the end of the torpedo-like "body". One will need a really huge swarm of these things to make the whole thing it work, I'm afraid ...
posted by megob at 7:04 AM on March 3, 2011

Maybe I misunderstood, but I assumed these were prototypes and the eventually installed would be some order of magnitude bigger.

Here's the TED talk, there are some significant differences.
Saul Griffith
posted by From Bklyn at 7:12 AM on March 3, 2011

IMO, too many moving parts. Solar cells generally come with 25 year warranties. This thing has a fin that needs to toggle back and forth in salt water about ten times a minute, or about 5 million times a year. Then there is all the stuff in the water you need to worry about with a tethered line, like seaweed and the occasional sea creature. Maybe this would work in some niche markets, but it is not going to redefine the energy market or anything.
posted by cman at 7:31 AM on March 3, 2011

taking into consideration that the turbine itself is only the small rotor at the end of the torpedo-like "body".

I may be wrong, but I think that's the innovation of this design - the flexible tether isn't just so that the kite will swing around as the tide changes, it's because the kite is computer controller to guide itself in figure 8 patterns. The "swept area" of the flow is not just the tiny area of the turbine - the wing of the kite translates the water flow into movement which is apparently up to 10 times faster than the flow of water passing it (eg in a 1m/s flow the kite would be moving at 10m/s). The tiny turbine attached to the wing is in this 10x faster flow - so consequently has much more energy being funnelled through it.

So the "swept area" of the turbine vis-a-vis the original flow is a lot more complex than the simple area of the turbine, and presumably is some fraction of the area of the kite wing behind it, which is much much larger.
posted by silence at 8:28 AM on March 3, 2011

So the "swept area" of the turbine vis-a-vis the original flow is a lot more complex than the simple area of the turbine, and presumably is some fraction of the area of the kite wing behind it, which is much much larger.

Right. It's even more advantageous because you can count (some fraction) of the entire figure-of-8 outline. But even so, the kite would have to be pretty large to trace out the swept area of the "Seagen" turbine (two 50ft diameter rotors), and that's just their prototype. The Minesto prototype will be "1/10" scale of their planned "150-800kW" turbines, which probably means a paltry 15 kW. At that low power (even scaled up), they would have to be *very* cheap to be competitive, and nothing offshore is cheap.
posted by Popular Ethics at 8:46 AM on March 3, 2011

Working out the math:
The Seagen swept area is 50'^2 * pi/4 * 2 or about 4000 sq. ft
Assuming you can count 100% of the swept area,
Assuming the kite has a 20' wing span (a big kite, but maybe doable)
An equivalent power (1.2MW) kite would have to travel 200', or a figure of eight somewhat less than 100' on a side. Double that with a 10' kite. That's not too huge actually.

posted by Popular Ethics at 8:57 AM on March 3, 2011

Apparently they plan to ultimately use a 12m (40ft) wingspan kite. I guess the wingspan is comparable to the Seagen rotors, but it still seems huge to me.
posted by Popular Ethics at 9:31 AM on March 3, 2011

i think the advantage over the Seagen rotor is that this system will be able to optimise itself for the current flow speed by changing the angle of attack of the kite. Huge rotors (like windmills) don't tend to work well at either very slow flow speeds (simply not enough to turn the rotor) or very high flow speeds (they have to feather the vanes for safety).
posted by silence at 9:53 AM on March 3, 2011

There is one more thing I ask myself. What happens between tides? There has to be a moment when the water is more or less motionless. Won't the kite sink to the ground, before alining itself to the direction of the new tide? And who will start it again? Or is it supposed to be lighter than water, and will go upwards, awaiting the counter-tide? Maybe it is intended like this. Didn't understand this from the info on their site.
posted by megob at 9:58 AM on March 3, 2011

silence: You have the same problem with kites, only moreso. Too high a speed and you risk snapping the tether. It's not really a problem with tidal generators though. The water speed is predictable and reliable. Seagen can claim at least 60% capacity factor, which is twice what the best performing wind turbines produce.

megob: They mention somewhere on the site that the kites are neutrally buoyant, so between tides they just float there. They might find they have to actively control the kite's buoyancy though to account for varying sea temperatures. That would add to its complexity.
posted by Popular Ethics at 10:54 AM on March 3, 2011

... also not getting tangled in the tether as the tide ebbs and reverses would be difficult.
posted by Popular Ethics at 11:49 AM on March 3, 2011

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