Flying Electric Generators

Discussion in 'Science' started by HereWeGoAgain, Apr 1, 2019.

  1. The Rhetoric of Life

    The Rhetoric of Life Banned

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    Like a box kite with a generator eh?
    Some sort of tethered glider?

    I say harness the jet stream dagnabit!
    [​IMG]
     
  2. roorooroo

    roorooroo Well-Known Member Past Donor

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    Here is another study, funded by the U.S. government. Granted, the study is old, but interesting (skip over the explanation of the computer modeling). Lots of details and things to consider concerning the construction and use and hazards of an FEG. The study/computer modeling suggested that the maximum realistic Power output would be 2MW. Can't necessarily believe an old study, but since land-based turbines can do 2MW, why go through the trouble of flying a 2 MW generator 4 miles up?

    Miscellaneous items: In post #175, under the diagram of the vectors: The tension would be 1.414 x drag, not 2 x drag.

    Anecdotally, (ignore this anecdote if desired) I have an acquaintance who has worked in the electrical department at the South Texas Nuclear Project for 25 years - had the opportunity to talk to him about generators. He said that large generators are built at less than 30KV. However, he remembered that a European company in the 1990s had designed higher voltage generators using modern high density plastics. He wasn't sure on the specifics, but thought a 48KV unit was installed somewhere in Europe, and a 90KV unit was installed in Japan. He hadn't heard any more about these higher voltage generators so he assumed they ended up as failures. All the installations he knew about since then have used less than 30KV generators to his knowledge.
     
  3. roorooroo

    roorooroo Well-Known Member Past Donor

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  4. roorooroo

    roorooroo Well-Known Member Past Donor

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    Here is a video of Sky Windpower doing a test of a small prototype:



    What puzzles me is this: When the craft is flying up off the trailer, it is in "lift mode". Electrical power from below is being fed up the conductors into the craft making the generators act like motors. The blades are spinning a certain direction which we will refer to as "lift direction" (whether clockwise or counterclockwise is difficult to tell, but the actual direction doesn't matter).

    Once the craft is in the air, the truck starts driving forward to simulate a wind blowing on the craft. The craft tilts back a bit, with its leading edge going up slightly and it's rearward edge going down slightly. The claim is that the craft is now in generation mode and is producing electrical power. This implies that all power from below has been removed and the generators stop functioning as motors and start functioning as generators and are sending power down the conductors.

    But there is a problem I think. The way this craft is designed, wouldn't the blades have to turn in the opposite direction of "lift direction" to produce power? I don't see any pitch controls on the blades. So think of an electric box fan - when it is being powered, it is running a particular direction and blowing air out the front. But if it is turned off, and wind is blowing through it from the front to the back, it rotates in the opposite direction.

    In the video, there is no evidence that the blades ever stopped and began turning in the opposite direction.

    Am I missing something?
     
  5. roorooroo

    roorooroo Well-Known Member Past Donor

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    I guess this thread is dead.

    Here is another video demonstrating Sky Windpower's concept:



    Again, the way Sky Windpower is showing this, it seems to me that those blades are going to have to completely stop and then start turning in the other direction to change from lift mode to generator mode.

    Even if the blades have pitch control, there will be a certain amount of time where no lift is coming from the blades while transitioning from motor mode to generator mode. And in most of the literature, it says the blades don't have pitch control, only collective control for simplicity and ruggedness. So something isn't right here.
     
  6. yguy

    yguy Well-Known Member

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    I can't see the videos, but I don't get your logic either. You analogize the turbine to a box fan that reverses rotation if air is pushed through it backwards, but the wind will not reverse direction relative to the turbine. Moreover, I'd be surprised if the generator wouldn't work just as well rotating in either direction.
    Murderer. :smile:
     
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  7. roorooroo

    roorooroo Well-Known Member Past Donor

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    :angel:

    Thank you for responding. Sorry that you can't see the videos. Can you google "youtube sky windpower videos"? I hope you can look at them and respond. There is a "real" video of a small Sky Windpower prototype being flown, but there is no evidence presented that the craft is actually giving an electrical output. There is also an animated video of a "jet stream" craft.

    Let me attempt to explain further the issue I see with blade direction. Forgive me for going back to the basics.

    You are correct that the generator produces electricity in either direction. That isn't the issue I see.

    If you can't see the videos, the Sky Windpower unit looks exactly like a four rotor drone. Each rotor is connected to its own motor/generator. It takes off and flies up to altitude using electrical input into the generators which causes them to spin like electric motors. Two of the four rotors rotate one direction, and two rotate in the opposite direction. This makes complete sense because it negates the torque on the drone, regardless of whether it is lift mode or generating mode.

    Now, let's consider just one of the rotor and generator/motor units. Let's say that it is turning clockwise to provide lift. It is pulling air from above and directing it downward. The leading edge of each blade is tilted up from the trailing edge. Again, the air movement is from above the blade down to below the blade, thereby providing lift.

    Once at altitude and in the jet stream wind, the craft "front" tilts up and the rear tilts down. (I am using "front" as that edge of the craft which the jet stream wind hits first.) The input power is cut and the motor changes to a generator. The jet stream wind is now making the rotors spin instead of the motor making the rotors spin.

    But here is the problem: At this point, the air movement of the jet stream wind is from the bottom of the blade to the top of the blade. The air movement through the rotor is opposite of what it was in lift mode. And this necessarily makes the rotors turn in the opposite direction of the lift direction. This means the rotors will have to come to a stop and then began rotating in the opposite direction. And therefore the craft will have no lift/not enough lift for the length of time it takes for the rotors to stop and then build back up to speed in the opposite direction to maintain lift.

    If you have a fan handy, use it to visualize what is happening. Hold the fan horizontal and imagine the direction the motor has to turn it to provide lift. Then tilt the fan up at an angle and imagine now that the wind is blowing through and causing the blade to spin. Isn't it going in the opposite direction? Look at the images below so you see what I mean.

    [​IMG]

    [​IMG]
    Again, the generator isn't the issue. The issue is going from lift mode to gen mode. Electrical power is removed from motor. Motor/rotor mass has to have time to completely stop. Wind begins spinning the rotors in the opposite direction. It takes time for the rotors to get back to speed to cause lift. All during this time, there is minimal to zero lift. The craft will lose altitude and may become unstable.

    Your thoughts? Do you see what I am saying?
     
  8. Derideo_Te

    Derideo_Te Well-Known Member

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    I take your point about the first video but the resolution is too poor to make out the blades clearly.

    In the second video there is nothing to indicate that the rotors are providing any lift at all and it might have become airborne from the wind producing lift on the central wing but since no forces were indicated it tells us nothing.
     
  9. roorooroo

    roorooroo Well-Known Member Past Donor

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    Thank you for responding. For the first video, do you agree that, as the video is presented, those blades would have to reverse rotation when going from lift mode to generation mode? If the blades must stop and then reverse, there is no lift and the craft would fall until it again created lift. Look at what I am showing in the diagrams in post #182. Grab a fan and see what is happening with the blades.

    Take the fan, turn it on, and note the direction that the blade is spinning as it blows air out the front. Now turn the fan off and place it in the wind so the air enters the front and note which direction the blades are spinning. This is analogous to what is happening in the videos.

    I agree that it is hard to tell what the blades are doing, but the physics is that if the blades have to stop and reverse, there is no lift for that amount of time, and the craft should fall for that amount of time. I can't see how that craft would have produced electricity because I didn't see any change in lift as it went from lift mode to generation mode. If I am wrong, I sure wish someone would explain it to me.

    If you do the fan experiment, you will notice that the blades spin one way under power, and the opposite way when being turned by the wind. The craft in the first video would have to do the same thing.

    In the second video, I am certain that the implication is that the blades are being used for lift, then switching over to generation mode once in the jet stream. And as presented, those blades would have to stop and reverse. The craft would fall for the amount of time it takes to stop the blades and for the wind to get them back to speed.
     
  10. Derideo_Te

    Derideo_Te Well-Known Member

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    I agree with your reasoning and I am trying to imagine a scenario where the blades do not need to stop.

    If the blades are on a pivot at the hub that would have them hanging vertically when not in motion and have stops that would prevent them from swinging in either direction more than 45 degrees then when the motor starts and turns clockwise the blades are now deflected back and provide lift. Once airborne the motor stops so the drag flips the rotor blades the other way and wind passing through them generates power.

    Not sure if that is completely feasible because I am still trying to figure this out but a system like that would not need the rotor to reverse direction because the blades switch over automatically. The ability to spot that blade switch over and the amount of drop involved could be too slight for us to identify given the less than optimal resolution in that video.
     
  11. roorooroo

    roorooroo Well-Known Member Past Donor

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    I agree with you that the issue could be solved mechanically. However, one of the issues with large rotating wing FEGs is the massive forces on the mechanical components. Much of the literature says that the rotors/blades would use "collective" control only, not "cyclical" or pitch controls. The cyclic or pitch controls would be subjected to massive forces and would quickly fail.

    Considering a very small FEG like in the video, perhaps the small amount of mass in the blades and generator could be stopped quickly, reversed, and brought up to speed quickly. But when we are talking about massive rotors and generators weighing hundreds of tons, the inertia would prevent a quick reversal.
     
  12. Gelecski7238

    Gelecski7238 Well-Known Member Past Donor

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    Does the lift have to be push instead of pull? There would be no need for transition reversal using pull like an ordinary aircraft.
     
  13. roorooroo

    roorooroo Well-Known Member Past Donor

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    Not sure I understand what you mean. Could you elaborate please?

    What I see with a rotor providing lift is that it is pulling air from above and pushing it down all at the same time.
     
  14. WillReadmore

    WillReadmore Well-Known Member

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    During the lift phase, the blades will be working to push air down, using power from the ground - like a helicopter on an electric cord.

    During power production, the blades will need to be rotating in the other direction - with the wind. (Or maybe the blades could reverse pitch).

    During power production, that vid shows part of the wind energy going to power production and part going to keeping the vehicle aloft - supposedly by using some of the wind resistance against the blades.
     
  15. Robert

    Robert Well-Known Member Past Donor

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    I have not yet seen what you speak of but I do understand propellers which are what Rotors are.

    Both are designed to spin in air and during the spinning phase, the pressure above the rotor for helicopters produces lift. It is not the force of the air moving down so much as the lack of pressure above the rotor. So that is why it is called lift. Propellers produce it so it creates a lift to move forward. Airplane wings provide lift.
     
  16. roorooroo

    roorooroo Well-Known Member Past Donor

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    Okay, I did quite a bit of reading concerning helicopter autorotation, which is essentially what is happening with this craft when it is in generation mode. Helicopter autorotation is the principle that a copter can use to land if its engine quits.

    Due to the "further from perpendicular, closer to parallel" direction of the wind over the blades, it may very well be that the blades turn in the same direction as when in lift mode. Complicated stuff, it depends on the fact that the outer tips of the blades are rotating faster than the blades near the center. Also has something to do with the twisting of the blades due to different forces from tip to hub, and also with the cyclic angle of the rotor hub.

    Lots of stuff on the net about autorotation, much of it confusing, and some of it conflicting. Here is a straight forward site that seems understandable.

    http://www.pprune.org/forums/showthread.php?s=&threadid=104671

    Now, the power that can be derived from the wind when it is almost parallel to the blades is very low. This is why a helicopter cannot maintain altitude when autorotating. The helicopter has to be moving forward at a certain speed for the blades to provide lift, and again, it only provides enough lift to keep the copter from falling like a stone - it is possible, but not guaranteed, to manage a controlled landing under autorotation.


    Further reading reveals that the FEG as seen in the video, when in generator mode, is operating as a gyrocopter as in the following link.

    https://en.wikipedia.org/wiki/Autogyro

    The gyrocopter has a motor that turns a propeller to provide forward motion, with a free-spinning, non-driven blade up top. Concerning the comparison, instead of the FEG having a motor which moves the FEG forward in the air, the FEG is held stationary by the tether and the wind blows past it with relative motion.

    Now, what needs to be realized is that to achieve maximum power from a blade in the wind, the wind needs to be perpendicular to the blade. The blade will spin in the "natural" direction that makes sense - as I was thinking in previous posts. But when the wind is blowing almost parallel to the blades, as in a gyrocopter, an autorotating helicopter, or an FEG as in the video, the blade actually spins in the opposite direction of "natural" and the energy produced by the free-spinning blade is a very small fraction of the available energy in the wind. I assume it varies as a function of the sine.

    At this point, I am quite skeptical of a heavier than air FEG, especially considering that this idea has been in consideration since 1979, possibly longer, and the start-up companies have been trying for at least a decade, some longer, and have not created a viable device to this point. I wonder how much money Bill Gates kicked in and how many tax-payer funded grants have gone into the endeavor. I wonder if that is the angle?
     
  17. cerberus

    cerberus Well-Known Member Past Donor

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    This is where string theory comes in?
     
  18. Derideo_Te

    Derideo_Te Well-Known Member

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    I agree with you that it would not work at that scale. What might work would be two sets of rotors with clutches. At one end you have the lift rotor and the other end the generator rotor. Disengaging one clutch and engaging the other would not involve the same stresses although there might still be a slight drop as the lift came off. The motor itself could work in the same direction it would just switch from using electricity to generating it.
     
  19. tecoyah

    tecoyah Well-Known Member Past Donor

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    I say we all just begin building these in our neighborhood.

    Or for a bit more
     
  20. Derideo_Te

    Derideo_Te Well-Known Member

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    The first video concept could be used on the huge upstate water reservoirs that feed NYC. The downhill flow of that piped water could be diverted through those turbines.
     
  21. tecoyah

    tecoyah Well-Known Member Past Donor

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    There is also this.
    Already in use and expanding.
     
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  22. tecoyah

    tecoyah Well-Known Member Past Donor

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    Some good ideas.

    The flying one is at the end.
     
    Last edited: Apr 20, 2019
  23. WillReadmore

    WillReadmore Well-Known Member

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    Once a helicopter is unpowered, any wind motion parallel to the ground is irrelevant to the task of managing rate of descent. So it can't maintain altitude because helicopters aren't designed to ride updrafts and it has no source of power. I think the general autorotation strategy is to store energy in blade rotation speed and forward speed until near the ground, and then to use that energy to reduce descent speed.

    For an FEG one has to add the tether so one can make use of the wind. All the wind power used for lift and total power generation will be supplied by the tether. So, when calculating the force on the tether the total power generation capacity and the lift requirement have to be present in the equation. The estimates of massive power generation capacity is directly translated as one of the several components of required tether strength.

    Another factor is airspace. At 4 miles high, each of these vehicles could be found anywhere within a substantial diameter determined by tether angle. So, moving this idea to scale would require dotting the countryside with significant circles of aircraft exclusion zones. There is no requirement for general aviation aircraft to have navigation equipment capable of helping pilots to find these circles. Even in remote areas, farmers, ranchers and others use minimally capable aircraft in the business of managing the land. Plus, commercial flights make extensive use of airspace below 20K feet.

    Dotting this nation with 20k foot long cables is something that this pilot would most definitely oppose - even if it were possible, which I doubt.
     
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  24. Gelecski7238

    Gelecski7238 Well-Known Member Past Donor

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    You have shown the four rotors pulling the craft upwards in a photograph. Why do you then turn the illustration of a single rotor upside down? If you didn't invert it, you wouldn't need to reverse the rotation during transition.

    However, if the long tether won't be perpendicular to the ground, the rotors will cut it if the transition tilt is too much. If that's the case, then the rotor blades will have to pivot like they do on a helicopter and not rely on tilt.
     
    Last edited: Apr 20, 2019
  25. roorooroo

    roorooroo Well-Known Member Past Donor

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    Oh crap. :-x I see what you are saying, don't know how I had such a brain fart here. Give me a few minutes to post new drawings....
     

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