AWES were intended to harness high altitude wind energy (HAWE) which is both more strong and constant. So to reach this goal the tether(s) should be very long. However it is in the process of shortening, passing from the jet-stream zone initially envisaged to a zone below 600 m in altitude. Why not continue and end up with tether-wing proportions like those of kitesurfing (20-30 m tether length for 5-20 m² kite area)? This is because a second goal has now supplanted the first: it is a question of obtaining a product that is lighter and more mobile than a current HAWT of same power range.
These AWE systems are very successful. So one can deduce a AWES for electricity production with equivalent tether-wing proportions could be also more promising.
So a 10-20 kW AWES comprising a 20-40 m² kite for 30-70 m tether length could more easily be implemented to feed for example a house, above all if Low radius loop figure is used. Then the kite area and its power scale by the square of the tether length.
Using a single skin kite would also lead to a possible progress in power to weight ratio, efficiency, lower cut-in wind speed…
Annexed note: a low aspect ratio leads to a lesser efficiency but also to a possibly larger area compared to the diameter of the path (loop or figure-eight), so the tether length. Generally the aspect ratio of power kites, comprising SkySails’ is far lower than that of paragliders, their angle of attack being higher. The table 6-2 page 92 shows that a low aspect ratio of 2.45 leads to a glide number of 4.49.
I believe this will be the way forward. We could see Makani had a pretty short tether relative to the wing. Choosing the tether length is a very important design parameter. Many aspects go into that, for instance the minimum
possible size of the flying pattern, the elongation required for yoyo etc. But my feeling is that high altitude is just too difficult due to drag and mass constraints. That being said, a slightly higher altitude may be reached for AWE compared to a similarily sized HAWT.
A tiny wing, even with a high glider number (lift-to-drag ratio) would have a too low Power to space use ratio when the tether length increases a lot. So both the kite area and its power, are scaling by the square of the tether length.
As a result a 20 kW yo-yo kite in the end of 70 m tether length (comprising the cable for reel-in/out, the reel-out phase time being short, about 10 seconds) would roughly become a 8 MW kite in the end of 1400 m tether length (the reel-out phase being able to be 20 times longer). But it is not for now.
Let us begin with AWES in similar proportions and dimensions as kitesurfing equipment. This perhaps would lead to a market for individual houses (10 kW range).
For kitesurfing line lenght is chosen, I believe, due to a balance between the time it takes for the kite to travel across the wind window, access to higher altitude clean wind, jumping capabilities and price. I do think for AWE the ideal tether length is longer, but could be your requirement of kite area to land use may shift the balance towards shorter tether. Anyways, I think it has to be a calculated optimal length for that design and purpose
I listened to a talk from a Greenland expedition Vegard Ulvang and some other guys did using kites where they used 100 meter lines (4 x 25 meter line sets hocked together) on their kites to assess the winds higer up i belive. And he said one issue was that if the wind picks up or for some other reason you need to take the kite down, you have to be very alert because it takes a lot longer time to get the kite down compared to a kite with the standard 25 meter lines. So i guess responsiveness is a big reason for the shorter lines on windsurfing kites. Plus it will be a nightmare to find a long enough spot on the beach to lay out the lines if they where 100 meters…
This gives an idea of the extreme difficulty of finding a suitable site to deploy an AWES with a 1000 m long tether as intended usually. The wind area of the frontal airspace swept should be accordingly to compensate for the dimensions of the space used.
Reminds me of the joke about the guy looking for his keys under the lamp post cuz that’s where the lighting is good, or kids who only apply for jobs at retail stores, because mom never took them through industrial parks. There is a lot of wide open space in this world, but by definition, not many people are used to going there. No stores. No restaurants… Here in the Mojave desert you can drive for hours without passing much of anything besides endless sage-brush, lots of dirt and sand, maybe the occasional rock or hill…
Indeed an AWES with a long tether would be more implementable in a desert but a still longer electric cable would be required to connect the desert site to a nearby town if there is one. Given the cost of such a cable the AWES would have to be overpowered to compensate for it. It’s a safe bet that stores and restaurants would be built on such a desert site as soon as an operational MW range AWES is implemented, even a city.
Likewise, you don’t find many (to put it mildly) power plants in Antarctica. Mom does not say everything …
Well it might seem that way, as long as one is looking for excuses instead of a place to deploy a demo, but the desert is criss-crossed by huge, high-voltage powerlines, such as the ones a half-mile behind me that bring power from the Hoover Dam in Nevada, to the greater Los Angeles area. That is just one example. And this vast desert is also home to a surprising number of small towns, and neighborhoods “in-the-middle-of-nowhere” that you might never notice from the main roads until night falls, then you can see the lights. Failing that, a team could be charging batteries, compressing air, electrolyzing water, smelting aluminum, pumping water for growing crops - you know, all that stuff people talk about but will never do? But no, what people want in a huge, largely unpopulated world, is excuses, not a place to try your apparatus that doesn’t work very well. I’ve seen it for many years now. The last thing any “other-peoples’-money” supposed energy breakthrough team wants is to run out of excuses.
Doug, you raise some good points. Unfortunately, equipped desert sites are not readily available for some current (above all European) AWE companies. For example The Netherlands (where is TU Deft) is very populated. Shortening the tether is a simpler option, issues such as takeoff, landing, and flight control remaining similar.
An “apparatus that doesn’t work very well” is a valuable excuse. So we are also waiting for your solutions @dougselsam. What about power a small city in the Mojave desert with your short or long SuperTurbine ™?
Thanks Pierre: My point is that there are a lot of places to develop AWE. If you are near the ocean, do your testing from a boat. If you are near agriculture, get permission to fly over some farm fields. The vast open desert is just one example of open, flat places with wind. Antarctica sounds good too, but not for testing prototypes. The next factor is to test at appropriate scale for the level of development. If you don’t have an automated system yet, why test it at some huge scale? Be patient. If you have a system that works well, there would be a market.
I spend my days watering trees, developing my ranch properties right now. I did not realize the level of involvement required to manage these beautiful parcels. I think the difference is, While I am not making as much progress in AWE as I would prefer at this moment, I have not wasted any investors money. My investors were paid back with interest. I continue to develop additional relevant intellectual property in the offshore wind realm. I have not made false promises to power a small city, as you suggest might form a reasonable goal. To me, only an idiot would promise to power a small city before demonstrating the powering of maybe one house to start, then maybe a reasonable grid-feed project that would contribute power to a grid when the wind is blowing, and only another idiot would believe such a story and throw money at it. Well, some people have a lot of money to waste. Also, in most cases, if you put power to the grid it might be difficult to say exactly where your power goes, since it all mixes together on the grid. I do agree that initially developing AWE at a smaller scale makes more sense than trying to go too big, too fast. Talk of giant systems, before having small systems working well, seems like putting the cart ahead of the horse. To me it seems like an artifact of mass-insanity, but what do I know? I guess what I’m wondering is why such a basic concept needs to be explained to anyone.