Every couple of years someone will jump out of nowhere and claim the wind energy sector has it all wrong, the list is very long. Windtree, Makani and many many others that replace simple and sturdy as well as well understood mechanisms with highly variable and fragile ones, usually without having studied the corpses of those that came before and died on the hill of unnecessary complexity. The closest that ever came to realization was the Darrieus rotor project in Canada, that consumed a ton of resources and resulted in the Aeolian (Eole?), a huge (100m, I've seen it up close before they tore it down, most impressive) VAT that ended up running for a couple of hours before its lower main bearing gave up the ghost under the vibrations.
Wind turbines are hard, and the standard shape that we've settled on was not due to a lack of creative ideas but simply because it is by far the simplest and most robust shape that gets the job done.
Any company pitching some complex alternative should at a minimum mention in their deck the degree to which they have researched the failures in the field of predecessors and what will set them apart.
I love the idea, but given that it took decades to scale traditional wind turbines which are much simpler by design to powers in the range of MW, I can't see this happening.
These systems are insanely complex, you need tethers, winches, a drone that flies 24/7 and can safely and autonomously start and land in many weather conditions. It also looks like google gave up on Makani, which had airborne generators.
But on the other hand, this may be similarly complex/expensive as installing and maintaining a wind turbine on a tower that's 100m high is. These things tend to be pretty complicated and expensive as well. It's all about operational cost vs revenue in the end (and comparing that to other solutions). IMHO wind and solar are still making great leaps currently in cost effectiveness and efficiency. So you'd have to compete with/undercut that. Which is of course a challenge for just about any other form of energy generation currently (particularly anything nuclear of fossil fuel based).
The added advantage of this setup would be that you can land the things during a storm or for maintenance which potentially simplifies e.g. in the field repairs. Also, if they can be small enough that they can be mounted on a truck, deployment might be quicker or you might be able to re-deploy them to other areas. Currently building a new wind mill park is a large investment. Mass production of these things might enable more rapid roll out. So there are some upsides theoretically.
Safety might be a bigger issue. The cables would be a risk for planes and what happens if the cable snaps? Autonomous drones would be able to handle the latter probably (by crashing/landing in some safe spot). Also, lots of moving parts means those parts need more frequent maintenance/replacement. I imagine the stresses on the cable and mounts would be substantial (and proportional to the amount of power generated).
> The only press they ever received was from credulous buzztech sites that just wanted something flashy to grab eyeballs, so they weren’t used to anyone actually being engaged enough intellectually to point out the lack of clothes that they were wearing.
I love the way this is written.
I think I am missing something here, because I don't quite get how the wind does continual work on the device to generate power. I mean, at the start the device goes downwind unreeling a tether, so it is pulling on the tether and doing work, so far so good. Then it reaches the maximum extent and... starts more or less staying where it is, staying aloft? Okay. But it's not systematically moving, so no longer doing work. What am I missing?
I couldn't find a good explanation either, but my guess was different from some others I've seen here:
I think it doesn't generate continuously. I think the plane spends some energy to get up into the fast-moving air, while the tether pays out some of its length easily to let it climb. I think it only does this "once", or as infrequently as possible.
Once the plane is in the fast-moving air, the generators kick up their field current to extract energy from the tether being unreeled further, as the plane turns broadside to the wind to haul the tether out despite this drag. It's generating power as the tether pays out.
Then the tether runs out.
So the plane dives/glides back towards the base, while the generators run in reverse, reeling the slack back in. But it doesn't return so far as to get out of the good wind. So as soon as most of the tether is reeled back in, the plane turns to haul on it, the generators go back to generating, lather, rinse, repeat.
I think it only lands when the wind dies, or if it gets too strong to remain safely in the air.
This is one scheme that runs the generator on the ground. Makani and Ampyx both run generators in the air and transmit power down to the ground on the tether.
Imagine for a moment that you built a wind turbine generator with just one blade. Instead of two other blades, you just had a counterweight. Then replace the tower and hub with a tether, and drop the counterweight.
That's effectively what these technologies are doing.
In point of fact, all of the blades on a commercial wind turbine are shaped like wings, such that the reaction force of the air flowing over the wing acts to drive the wing in the direction of motion and adds a drag force away from the tether.
In the classical tower arrangement, you extract energy by opposing the rotation of the blade about the tower. In the energy kite arrangement, you extract energy by running the "propellors" in reverse as generators to oppose the forward thrust.
Single blade turbines approach the maximum possible under Betz' law regarding power extraction from a moving air mass but suffer from resonance and material fatigue to the point that it offsets all advantages. Twin blade rotors suffer from thump and material fatigue as well, three seems to be the optimum and is what almost all commercially available turbines use.
The single-blade model reduction is just a rhetorical device used to guide the reader on the principle of energy generation, not on viability.
As you point out, single-blade turbines have been done but they have their own suite of mechanical problems. A more practical way to save capital cost is to under-size the electrical system relative to the blade area. You may still only get 30% capacity factor of the blades and tower, but you could get 40% or better on the electrical system for a small win.
Thanks! That actually reminds me of something else I was wondering: why only three blades, leaving so much empty space for wind to pass through unimpeded? I understand adding a fourth or fifth would give you slightly less energy per blade, but wouldn't it give you more energy per windmill, for greater overall cost efficiency? Or is it the case that for some reason I'm not aware of, most of the cost is in the blades themselves, such that energy per blade is the most important factor?
The error in logic is the part where you see so much empty space for wind to pass through. The horizontal velocity of the wind is quite a bit lower than the circumferential velocity of the turbine blade. So only a relatively short distance passes before another blade comes along.
Basically, there is a relationship between number of blades, blade tip speed, and flow speed that is optimized. For the same flow speed, power, and area, a two-blade turbine has to spin faster to reach optimum and makes more drag. A four-blade turbine has to spin slower to reach optimum, but it is only barely closer to the Betz limit than the corresponding optimized three-blade turbine. The difference is small enough that its difficult to measure or simulate - some other inter-blade losses rise in importance as well.
It would give you more torque, so you will see this in windmills that require a lot of starting power such as water pumpers. For other purposes efficiency of extraction is more important than starting torque. Wind does not 'pass through unimpeded', the speed of the mill is carefully calibrated to slow down the maximum amount of air without causing it to pool behind the machine. That is precisely why there is a maximum amount of energy that can be extracted to begin with.
> It would give you more torque, so you will see this in windmills that require a lot of starting power such as water pumpers.
This is the classical rationale, but having worked on tons of fluid systems, I don't buy it. Its true that you will get a lower tip speed ratio, and therefore a lower RPM for the same power output.
But in practice, that just changes the gearing required for the fluid pump.
IMO, the real reason that fluid applications used four blades and more was that humanity didn't know any better. The theory behind wind turbine optimization wasn't fully developed until the early 1980's.
A windmill that isn't rotating yet is an entirely different device than one that is already turning. Once moving, indeed, the three bladed device with a gearing has the same torque as one that has many more blades. But when it isn't turning yet the device with more blades has far more torque than the one with only three blades and that is why it can get started at all.
Pumps are particularly unforgiving, most of the designs out there will have an uneven load distribution on a single rotation of the outgoing shaft to the pump and so will get stopped at the point where the required torque is at its maximum. Precisely because of the distinction made by the questioner: why does so much of the surface allow air to 'slip through'. Which it will in fact do when the mill is standing still or still moving very slowly.
So on a three bladed device the start-up torque will be a small fraction of what it is on a device with many blades, which prevents the three bladed device to start rotating effectively stalling the rotor until the amount of wind is high enough to overcome the initial resistance of the mechanism, which can be quite high. In low wind situations this will allow the multi-blade device to produce far more yield, in moderate wind situations it will produce somewhat less yield and in very high wind situations it may cause the three-blader to overspeed and destroy itself unless it has a very good furling mechanism (such as variable pitch). On a three blader the only parts active to generate this start-up torque are the three blade roots out to about 1/3rd of the rotor diameter.
This limitation applies to purely mechanical direct drives without any trickery such as start-up relays or clutches that only engage when the mill is spinning at a sufficiently high rate, in which case the rotor can stay unloaded long enough for it to get into a high enough rate of rotation that a torque convertor such as an electric motor or gearbox can be engaged. But that is a far more complex setup, one that waterpumpers that run for decades without maintenance can do without.
For electricity generating windmills the situation is a bit different in that most of these have an output voltage below which the energy the mill produces isn't used at all, no current will flow until the output voltage exceeds this lower limit. This effectively acts as a clutch that only engages when the mill is already in an efficient domain of operation. MPP tracking can help extract some power below that speed, something that may be useful in domains where there are long periods of low speed wind.
Just because there wasn't a good computational model for a long time doesn't mean there wasn't a wealth of practical knowledge about windmills empirically gained over three centuries. We already knew what worked and what didn't, we just didn't know why it worked, and we weren't able to scale up the smaller machines to the sizes we see today. The latter was also heavily affected by the development of stronger materials such as carbon fiber.
At a guess you have a highly developed theoretical knowledge about windmills, but not a very well developed practical knowledge, as in actually building some of these models to verify your theoretical knowledge. If you had you would not make such a simple error because you'd know from actual observation that this difference exists in practice and why it exists, rather than just to make a claim from what you know from your - in this case faulty - theoretical knowledge.
I don't know anymore than what was on their site, but my reading of it was that it works by moving the airframe (plane? glider?) around in the wind-stream. The tether would then be continuously moving back and forth (or in and out). I think this the meaning behind the figure-eight pattern they show.
If it's anything like some ocean wave generators, it might be able to generate power when moving in either direction, but I'm not sure about that.
The kite goes sideways relative to the wind, spooling out the tether, generating power, and speeding up. Just like a high-speed sailing boat it can reach several times the speed of the wind this way. Then when time comes the kite turns into the wind, letting the tether spool back in at no real energy cost (but not generating power during this period). The kite then turns again to repeat the process.
Note that while there are several different techniques, they all rely on the kite moving fast, this allows it to sweep a large area relative to its size, thus working around the fundamental limit that you can't extract more energy than some percentage of the directed kinetic energy contained in the air molecules moving through the swept area.
I wonder how closely these can be deployed? Safe operation might require large distances between base stations. If they can't beat conventional wind mills in power per area, that may be a deal breaker in many scenarios.
I wonder how close to humans they can be deployed. I'd guess that thing can fly quite a distance before it comes crashing down in case anything breaks.
I there a good argument it's not profitable, but this should be seen like going to the moon.
I personally don't can about 'alternative' energy, but a lot of people do, and getting tech that permanently runs hundreds of meters of even kilometres in the atmosphere will open up a lot of possibilities.
We need to stop thinking of the Earth in 2D and these projects help.
For once lets use the fact people want to invest in 'alternative' energy on something exciting.
There is are so many theoretical things you can do with platforms kilometres in the air.
Why do these companies seem to fixate on fixed wing aircraft? Why not use a big parachute style kite similar to what paragliders use? You could send control signals to collapse the wing when the end of the spool is reached, making it eay to reel in and/or direct it upward out of the powerband. Then let it expand back to full size to repeat. The cost, weight, and complexity would seemingly be much lower. I would argue the surface area of a parachute can be scaled up more easily than the wing area of a plane.
Less to go wrong -- foil kites can become tangled in their bridle lines due to fluctuating wind which lead to a stall. Tangled lines can prevent the safety release mechanism from depowering the kite: https://youtu.be/pU0mEWKf-_Q?t=15
I have a goofy idea: kite power to replace portable solar panels and generators rather than for the grid. Something small scale you can take in your car (backpack would be even cooler) to get a few watts to charge your phone. Would this ever work to, say, to the extent of having better power to weight than solar?
It looks like a lot of the cost and complexity is to allow takeoff and landing. Is anyone working on a system where the flying part never lands? You presumably have to supply some energy to keep it aloft on windless days, but that might be cheaper than landing gear.
You don't need an MVP to raise money. This is a polite way of saying that most of these are bordeline scams, they could work in theory and if the people behind them have done their homework they know full well they will never see them in large scale deployment but it is a nice way to get a bunch of cash to play around with and tinker. See also: flying cars, especially Moller.
http://www.wind-works.org/cms/typo3temp/pics/EoleCapChat045x...
Wind turbines are hard, and the standard shape that we've settled on was not due to a lack of creative ideas but simply because it is by far the simplest and most robust shape that gets the job done.
Any company pitching some complex alternative should at a minimum mention in their deck the degree to which they have researched the failures in the field of predecessors and what will set them apart.