Clean Power

Published on January 29th, 2016 | by Joshua S Hill

133

Mammoth 50 MW Wind Turbine Blades Could Revolutionize Offshore Wind In US

January 29th, 2016 by  

A new design for mammoth wind turbine blades longer than two football fields could deliver 50 MW offshore wind turbines.

The research for the new wind turbine blades designs has been conducted by the Sandia National Laboratories, a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corp., for the US Department of Energy’s National Nuclear Security Administration. According to Sandia, it was challenged to design a low-cost offshore 50 MW turbine with wind turbine blades of more than 650 feet, or 200 meters in length.

That’s two and a half times longer than any existing wind turbine blade.

“Exascale turbines take advantage of economies of scale,” said Todd Griffith, lead blade designer on the project and technical lead for Sandia’s Offshore Wind Energy Program. Sandia has been working on wind turbine designs for a while now — including 13 MW systems using 100 meter blades, which are the basis for Sandia’s Segmented Ultralight Morphing Rotor (SUMR) designs.

Sandia-1

Sandia’s 100-meter blade is the basis for the Segmented Ultralight Morphing Rotor (SUMR), a new low-cost offshore 50-MW wind turbine. At dangerous wind speeds, the blades are stowed and aligned with the wind direction, reducing the risk of damage. At lower wind speeds, the blades spread out more to maximize energy production. (Illustration courtesy of TrevorJohnston.com/Popular Science)

50 MW wind turbines are a long way off, but according to Sandia, “studies show that load alignment can dramatically reduce peak stresses and fatigue on the rotor blades.” This would not only reduce blade costs, but eventually lead to the mythical 50 MW wind turbines.

And these developments are vital to the offshore wind industry in the US.

“The US has great offshore wind energy potential, but offshore installations are expensive, so larger turbines are needed to capture that energy at an affordable cost,” Griffith said.

“Conventional upwind blades are expensive to manufacture, deploy and maintain beyond 10-15 MW. They must be stiff, to avoid fatigue and eliminate the risk of tower strikes in strong gusts. Those stiff blades are heavy, and their mass, which is directly related to cost, becomes even more problematic at the extreme scale due to gravity loads and other changes.”

That’s where Sandia’s segmented designs come in (as seen above). Not only do they provide more cost-effective manufacturing and installation — as the blades can be manufactured in segments rather than as a massive long blade, which extends to transportation and installation as well — but, inspired by how palm trees move in storms, the blades would be positioned downwind, and the segmented sections would bend in the wind while retaining segment stiffness.

“At dangerous wind speeds, the blades are stowed and aligned with the wind direction, reducing the risk of damage,” explained Griffith. “At lower wind speeds, the blades spread out more to maximize energy production.”


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I'm a Christian, a nerd, a geek, and I believe that we're pretty quickly directing planet-Earth into hell in a handbasket! I also write for Fantasy Book Review (.co.uk), and can be found writing articles for a variety of other sites. Check me out at about.me for more.



  • Karl the brewer

    Does anybody know if the total surface area of the wind farm changes pro rata with the turbine diameter / height? I.e You can put up less turbines for more power but run out of space for them more quickly.

  • Najeeb Ullah

    wow.. that is going to be a true game changer

  • Matt

    I thought they were going to rotate the turbine. Face it into the wind to fully deploy, down wind to fold. But the partial fold in as wind increases means no always down wind. Wonder how they are handling the stress of tower shadow. I guess there are at least computer models to back this up.

  • I wonder if just rotating each blade at the hub to feather it out of the wind would be more feasible and just as effective?

  • Jenny Sommer

    I wish they would look beyond wind turbines. Any advantage in material use or even price/cost from now on will be marginal compared to prospects for high altitude wind/Kite powered GW scale plants.

    • Jens Stubbe

      Jenny unsubsidized wind power was per 2014 sold on average in USA for $0.035/kWh and the cost is going down fast (lower if you calculate the cost for the entire design lifetime and much lower if you focus at the cheapest projects). You once wrote that the Kite powered wind system target $0.02/kWh. This is what the wind industry target per 2025 but will probably reach the target much sooner because it is a significant reduction in the current cost trend towards cheaper electricity from wind turbines. Your belief that there are only marginal improvements underway belies a lot of research activities going on right now and several projects that are in test phase now.

      Getting to utility scale operation before you have significant track record will be impossible and bankability too will remain doubtful. Best guess is that although the mathematics add up the project will not fly.

      • Jenny Sommer

        You don’t have to undercut the cheapest US generation cost to be competitive in most any parts of the world.

        Unsubsidized wind in Europe is still more expensive.
        The target for KiteGen is ~6€/MWh for large carousel type installations.
        There are more benefits like fast deployment even at deep offshore locations. High CF at sites with bad wind resources in lower layers. Easy to ship to very remote locations.

        An EROEI boost to over 350 alone should be worth more research.

        We will see. Last posts by KiteGen say they are past development and testing.

        • Jens Stubbe

          Jenny there is room for enough wind turbines to power the European grid two times over and the Danish wind cost on an unsubsidized basis is pretty close to the US level and 50% higher for offshore. And interesting Danish offshore is now $6/mWh and therefore cheaper than Kitegen target.

          The declared target in the industry is to close the gap between onshore and offshore.

          China has decided to gamble on their own substandard turbines but could switch to modern wind power and half their cost over night.

          Recently EU has decided to undermine the Danish energy policy by basically declaring that we will face EU courts if we continue the successful policies. This is a serious blow to green technology development and especially wind power because EU is the play ground for lobbyist with quite different agendas and also generally mismanaged and without talent.

          The EU countries has for five decades had less growth than the world and less growth than any comparable countries.

          • Jenny Sommer

            Danish offshore wind is $0.6ct/kWh?
            I don’t doubt that there is enough room for turbines to power Europe and I am the last one to say we should not deploy as many as fast as possible now.
            I am saying it would be nice to need 50times less material/concrete, less transportation (trucks, ships, roads), higher CF at a given site, no towers, smaller fundaments, got 4 times the turbines per space.
            There are just a lot of advantages.
            The disadvantage is clearly the shared airspace with aviation.
            Maybe China or the middle East are the right places where huge, remote wind farms could supply synfuels.
            It’s also still unclear how ships should be powered without oil.

          • Bob_Wallace

            ​”I am saying it would be nice to need 50times less material/concrete, less transportation (trucks, ships, roads), higher CF at a given site, no towers, smaller fundaments, got 4 times the turbines per space.”

            I’m sure a lot of venture capitalists have looked at the numbers and I’m not seeing any buying in.

          • Jenny Sommer

            Was waiting for you to say it again.
            So it might turns out to not be interesting to VCs for numbers.
            Yet it is sad that nonfinancial benefits are ignored by anybody apart from KiteGen themselves which are still not closing shop.

          • Bob_Wallace

            And I’ve been watching you bring kites up over and over.

            I’m not sure what “nonfinancial benefits” are possibly being ignored. What I do know is that this idea has received a lot of attention over time but there’s no sign of any interest from investors.

            There are several things that work, but don’t work well enough. Unless the financials make sense a workable idea is simply going to fall by the wayside.

      • eveee

        Yup. Higher towers and higher capacity factors are a coming. Once they are here, Katie bar the door. Wind is really going to smack some high knee.
        http://cleantechnica.com/2015/08/04/wind-could-replace-coal-as-us-primary-generation-source-new-nrel-data-suggests/
        We are just now about to see the first generation of 135m towers on the East Coast of all places. If they were in the Midwest, they would be fabulous.

  • globi

    “Conventional upwind blades are expensive to manufacture, deploy and
    maintain beyond 10-15 MW. They must be stiff, to avoid fatigue and
    eliminate the risk of tower strikes in strong gusts. Those stiff blades
    are heavy, and their mass, which is directly related to cost, becomes
    even more problematic at the extreme scale due to gravity loads and
    other changes.”

    Actually, windturbine blades are not particularly stiff. At the tip they are actually quite elastic, such that one could it employ it at a playground as a seesaw.

    The reason why the axle has a slight incline is actually because of the fact that the blade bends quite significantly at high wind conditions (otherwise the blade tip would hit the tower).
    http://www.nordseeone.com/images/windpark/2014_6M_HD_CrossSection.jpg

    • eveee

      Thats right.Upwind turbines are designed to avoid hitting the tower. It also lowers the vertical gravity based torque on the main hub bearing a little bit. With the blades are swept forward a bit, once the hub is tilted back, the upper blade is nearly vertical, but the lower one is pointed out aways in front of the turbine.

  • Benjamin Nead

    Interesting to see now folding blades on wind turbines. This is something found on high performance and contest Free Flight rubber-powered model airplanes since
    the late 1930s.

    • globi

      And on birds since the dawn of time.

      • Benjamin Nead

        Your point? I’m comparing the similarities of a small man-made mechanical object that few people, apparently, are aware of to a much larger and more recently invented one profiled here. If we’re talking about an analogy in the natural world then, yes, I’m sure they exists. But dinosaurs were around long before chickens. 🙂

  • Foersom

    When upscaling wind turbines the blades becomes longer. Even when the wind turbine only makes few rotations per minute, the longer blades causes speed of the wingtip to be very high. Currently for 8 MW turbines with 164 m rotor diameter, the wing tip speed is up to ~300 km/h. That leads to wear and tear for the wingtip. I doubt we are going to see HAVT wind turbines at >20 MW.

    • eveee

      The wingtip does not vary with rotor size. The rpm of larger rotors decreases while the tip speed remains the same.
      https://en.wikipedia.org/wiki/Tip-speed_ratio

      • Ronald Brakels

        So if the tips are moving at 300 kilometers an hour a 200m blade will make 4 revolutions a minute. That means the “thwop” from passing the plylon downwind might last around a second and a half. The blades are so long that if you were at the base the thwop from the top of the near the hub would reach you about two fifths of a second after the thwop from the tip, making it longer lasting but gentler.

        Of course, thwop is less of a problem out at sea than on land since there are fewer neighbours around.

        • eveee

          But the poor fishes and seagulls. They don’t want thwop.
          Thwop is less of a problem if the blade tips are not moving at Mach 1. Thats what make the load noise of a whip crack. Or a wise crack.
          The aeroacoustic calculations have left me … dizzy. Toooo many variables. Need a simulator. Do you know there are people actually writing programs about stuff like that?

          • Matt

            I know GE is has wind turbine blade simulation software. Had a friend who had a consulting job working on it.

          • eveee

            Yeppur. This stuff is more than you can handle on a napkin.

          • just_jim

            “We’re going to need a bigger napkin.”

          • Ronald Brakels

            Well I imagine that what with the continuous thunder of collasping vacuum in the wake of the supersonic blade tips, no one would ever complain about thwop again.

            There are people who write programs about this stuff? How lazy! In my day it was all hands on knowlege. We’d tie a person to the end of a blade and rotate them until either the blades or the ropes holding them failed. That’s how you can real knowlege you can feel in your guts and that’s why there are arrest warrants out for me in the Netherlands.

          • eveee

            Arrest warrant? Why? Ferris wheels and amusement parks do the same things. … to children. And they fill them full of cotton candy, soda pop, and other bad stuff. Then if the kids get off, they give them back to their parents. Who get to experience all that in a different way.

            Yes. the blade tip speed is only a problem if its very high. Its really weird sitting under a gigantic turbine rotating. It takes so long for the blade to get directly overhead, but then it goes past quickly. It looks slow. But its really moving fast. 100s of mph. If you think about it, its a ridiculously difficult mechanical design. But the MEs wind pioneers have done a magnificent job. Here is a bonus. The Tvind 1MW upwind three bladed community built wind turbine is still in operation. A marvel.
            https://c1cleantechnicacom-wpengine.netdna-ssl.com/files/2015/05/tvindkraft.jpg
            http://cleantechnica.com/2015/05/21/oldest-operating-wind-turbine-world-turning-40/

          • Ronald Brakels

            Something about how if they change their mind after they are roped to the windmill blade then you have to stop otherwise it becomes kidnapping. They are so strict in that country.

            I don’t know what they were worried about. We tilted the windmill so he wouldn’t scrape the ground.

          • eveee

            Oh, the tethered wind turbine concept.

          • Ronald Brakels

            If we’d charged them for admission like a ferris wheel does we probably would have been fine.

          • Jenny Sommer

            But can your Turbine do that…

            https://youtu.be/hjS_OpY7xYI

            or that

            https://youtu.be/uR_1hNrtL8U

            I guess there is a hard limit how far you can throw one with a turbine.

          • ROBwithaB

            Baah.., Bunch of Danish Hippies!

      • globi

        Which means that at some point you’d need more than 3 blades in order to still cover the same area in a given time.

        Also, if one quadruples the power of a wind turbine by doubling the length of the blades, one needs to half the rpm in order to keep the tip speed steady. Thus, the torque at the nacelle/gearbox will increase eightfold.

        I doubt that one 50 MW wind turbine would be cheaper than four 12.5 MW turbines. Especially if one needs additional folding contraptions and the blades are constantly exposed to tower-wind-shadow (possing fatigue issues).

        • Jens Stubbe

          More than three blades are unlikely but we may see a number of different approaches to make more power out of slower blade speed in the center.

          You are absolutely right in assuming that one 50MW wind turbine will be considerably more expensive than four 12.5MW but that does not compute into lower cost of electricity for multiple wind turbines.

          • eveee

            Are you a wind turbine designer? You seem to know it pretty well. These are things I find only the most dedicated wind guys know.

          • Jens Stubbe

            No but I live in Roskilde and until last month my lab and office was at Risoe, which is the global center of wind turbine development. Also my cousin invented the Dominant Danish design and a good friend was an engineer on the famous Tvind wind turbine.

          • eveee

            Yes. I am not a designer, either, but I studied wind energy for a long time in and out of school. I specialized in wind energy. Read everything I could get my hands on. Back when I graduated, I was rejected for a job with one of the few large turbine manufacturers of the time. They couldn’t understand why an electrical engineer would be needed for wind energy. LOL. But I did wind up learning all this stuff by spending much time with mechanical and aero engineers. Nowadays, there is plenty of electrical engineering involved what with power electronics, generators, and distribution systems. The business is huge now, much to my joy and satisfaction.

          • globi

            Four 12.5 MW instead of one 50 MW wind turbine does not compute into higher cost of electricity either.

          • Jens Stubbe

            You are right in assuming that there is no telling whether four 12,5MW will be the better or worse choice than 50MW. However the North Sea is the worlds most trafficked ocean and important for oil, gas and fishing grounds as well. There is enough accessible wind resources there to power the electrical grid in Europe two times over. As 50MW turbines will be spaced further apart there will be more room for the other activities that has to continue (except of cause oil and gas :-)).

            Currently three are no 12,5MW wind turbines and certainly no 50MW turbines but the technological development is fast and could provide materials and design solutions such as the discussed concept that could pave the way.

            The incremental process adopted by the industry is to develop a new design and then refine it over the years where the performance including the rated power increase.

            The lead-time for new designs is increasing, which suggests that the size growth perhaps is decreasing.

          • Bob_Wallace

            The industry keeps making turbines larger and larger. That would seem to suggest that larger means cheaper electricity.

        • Steven F

          The 3 blade configuration used on wind turbines is the most efficient configuration. Adding more blades will actually reduce the amount of power captured by the turbine and increase loads on the tower.

          • eveee

            Right. I dug up a paper on it. Good stuff. Aerodynamics in wind turbine design. It helps to understand wing or sail theory, too. Makes it easier to relate tip speed ratio, angle of attack, Betz law, etc.
            http://large.stanford.edu/courses/2007/ph210/hall2/

          • globi

            The 3 blade configuration is the most cost effective and not necessarily the most efficient configuration (since wind energy is free, the costs of a wind turbine is much more important than its efficiency – contrary to a gas turbine).

            More blades can increase efficiency, because they can reduce drag.
            Why?
            Because the more blades you have, the lower the speed of your blade.
            Keep in mind: The number of blades increase the drag linearly but the drag is a function of speed squared.

            https://upload.wikimedia.org/math/9/9/a/99a6015b6a230860c9b1517b238e5de9.png
            https://en.wikipedia.org/wiki/Drag_%28physics%29

          • eveee

            The studies also say that lower tip speed ratios can also lead to lower efficiency. Higher number of blades leads to lower tip speed ratio.

          • globi

            With the same number of blades a lower tip speed ratio reduces efficiency.
            But the optimal tip speed ratio is inversely proportional to the number of blades. (The 3 bladed has an optimal TSR of about 7, the 2 bladed about 10 and the old multibladed windpump about 1).

          • eveee

            I think the graphic in the paper is best. A picture is worth a thousand words? It really gets complicated to explain after a while. The graphic shows a bunch of TSR curves for each blade number.
            Did you view that MacLean youtube video on aerodynamics. The guy has some very interesting things to say.

          • globi

            I’ll watch the youtube video tonight.

            In your paper there’s a graph regarding number of blades and aerodynamic efficiency (figure 1): http://www.nrel.gov/docs/fy00osti/28410.pdf

          • eveee

            Yes. Thats the one. Its really a set of curves, one for each blade number or rotor type. By the way, thanks for all your comments. Its great to converse with someone who knows this area.

          • globi

            Ok, I’ve watched it. It’s an informative lecture.

            I think your main point would be that the wake of a large wingstructure is substantial. Undoubtedly wake turbulence is a significant hazard in avation. https://en.wikipedia.org/wiki/Wake_turbulence

            I would argue though that this is not really issue with the blades of a wind turbine, because the wind is pushing the wake of each blade downwind, before the next blade will hit it. Besides, the higher the number of blades the lower the speed of each blade.
            http://www.noaanews.noaa.gov/stories2011/images/vattenfall-image_300.jpg

            However, after seeing the picture above, I would even argue that multbladed windturbines would actually be beneficial to windfarms because the slower moving blades would produce less turbulent wake.
            Nevertheless I don’t we’ll see more than 3 blades, because it would probably be too costly (and cheaper to increase separation).

            https://www.youtube.com/watch?v=Hl2mp90Qp1Q

          • eveee

            Thanks for the graphics. I was trying to pry open this whole business of blades rotating within the swept area. It always bothered me that the blades never cover the complete area, so some flow travels past the blades. Seems like that is an inevitability. The difference in efficiency as blade number increases is small. At first blush, it wouldn’t seem that amount of efficiency difference would account for the flow not encountered directly by the blade. (But there has been discussion historically that some of the flow goes around the turbine, because the wind slows down in from of it.) That should happen no matter what scale of rotor. TSR after all, implies that the wind moves a distance as the tip moves. In all likelihood, it seems that the greater number of blades must have a lower TSR to avoid the adjacent next blade encountering some of the wash from the previous one.
            The mind opener from the youtube video is that there are effects far from the blade (wing) surface. Its not just wake. He is actually saying their are effects upwind and on top and bottom of the blade, not just downwind. I think.
            Particularly, I am interested in the wingtip vortex mistake and the concept that there are effects far from the wing (blade). The rotating blade makes the entire theoretical effort much more complicated than a blade. If we don’t get the blade concepts right, we have no chance of understanding something more complicated like a turbine. The rotating wake adds a twist to the discussion. 🙂 I don’t believe I have ever seen a text or paper adequately explain all this. Particularly given McLeans comments on wingtip vortex, I am skeptical of handwaving explanations.
            Ideas?

          • globi

            In all likelihood, it seems that the greater number of blades must have a lower TSR to avoid the adjacent next blade encountering some of the wash from the previous one.

            It’s the other way round. For instance, 2 blade wind turbines have a higher velocity/TSR than 3 bladed turbines not because the blade has a different shape, but because the blade simply needs to move faster in order to cover more of the area in a given time (if it moves slower it misses more area and if it moves faster it creates more drag).
            A less bladed turbine simply needs to move faster in order to cover the same area in a given time. This is not disputed.

            For any given wing structure the size of the vortices are proportional to its speed. Since more blades reduce TSR they also reduce vortices.

            Now, since wind energy is free, there’s no benefit in increasing turbine efficiency by a couple of %, if one needs to pay 30% more on material.

          • eveee

            Yes. I get what you are saying. The comment about wash simply refers to how soon the blade gets to where the previous blade was, not wingtip vortex. There is a wash along the whole length of the blade, too, not just the tip.
            Adding further to our discussion and my hopefully expanding understanding of aerodynamics, there is a youtube video that explains what is really happening in wing lift and with streamlines and pressures around objects. Its a really nice video.
            Fundamentally, a fluid can only have pressure differentials when the flow curls or bends. Thats really what the principle is all about. Atmospheric lows are created by rotation, for example. Examining the Betz theory and looking at the cone of low pressure following the wind turbine, it becomes immediately apparent that the downwind flow has rotation imparted by the rotating blade. That rotation goes together with a lower pressure behind the turbine. The difference in pressure and the flow of mass are what the Betz calculation are about. Your picture of a wind turbine creating clouds downwind shows their is a lower pressure in their wake. Low pressure=clouds or rather vaporization. The air near the water is loaded with moisture. The drop in pressure causes it to precipitate.
            Here is that nice video about pressure and rotation. If the streamlines are curved, the particles are undergoing acceleration. A pressure difference goes with that. 3:15 to 3:26.
            https://www.youtube.com/watch?v=XWdNEGr53Gw

          • eveee

            I have been trying to find references for that. Found anything?

        • eveee

          I don’t think you need to increase the number of blades to cover the area. Its a bit paradoxical, but it doesn’t work that way. In fact, adding a lot of blades reduces efficiency. The most efficient design is the single bladed turbine. Don’t laugh. Its been analyzed.
          If you double rotor diameter, you double radius. If you double radius, you increase force by the square of 2 or 4. The power will increase by cubed or 8. The torque increases the same as force, so by 4.
          I would like to see more details of the study. Just like everything else in technology, it has been said many times before that things were at a limit. It was said at many points in wind turbine development over the years. I feel there must be some practical limit, but I don’t know where it is.

          • Steven F

            From a strictly aerodynamic standpoint one blade is less efficient than two which is less efficient than 3. However from a performance cost point of view one and two blade designs are attractive due to large cost reductions by reducing the number of blades. Unfortunately test on actual turbines shows that vibration gets increasingly worse as blades are eliminated.

            In two blade designs vibration induced by the interaction of the blade with the tower wake causes increased stresses that can counter act the potential cost reduction. Most early turbine designs were two blade designs. Including the 1938 1MW Putnam Turbine.

            The one blade design also suffers from balance vibration and high tip velocity .The tip velocity is getting close to the speed of sound at peak efficiency and getting the counter weight set exactly right proved to be very difficult. Overall the one blade design had more vibration issues that two blade design.

            So overall by the mid 80’s no one was interested one and two blade designs. Practical experience with them showed the cost benefit was not as good as it first appeared.

          • eveee

            Awesome. Another guy that knows about the Putnam design. You must also know about the Gedser one. And Tvind. And Mod-3.

          • globi

            Thanks.
            No offense to your link, but this book is much more elaborate in this regard: http://www.amazon.com/Wind-Energy-Explained-Theory-Application/dp/0470015004

            And also shows that the power coefficient is rising faster with more blades (as a function of the tip speed ratio).

            More blades have the advantage that the tip speed ratio is smaller. Keep in mind: Reducing the blade speed by 50% reduces the drag by 75%.

            I’m not advocating more blades. All I’m saying is: If the blade length is tripled and the tip speed ratio unchanged, then some of the wind passing through the rotor area will be missed.

            Example: With a maximum tip speed of 84 m/s, it would take one blade 5 seconds to cover 120°. If the wind has a speed of 12 m/s, it will travel 60 m (so a significant portion of kinetic energy of the air molecules will pass the rotor area with interfering with any blade).

            http://1.bp.blogspot.com/-DCRyt-_WTF0/UvN3tekhV5I/AAAAAAAADv8/RbbfmiRjTmg/s1600/PowerCurve3MWV16480MW.png

          • eveee

            Excuse my lapses. The Stanford reference was remedial. I should have given this NREL reference to you and the Stanford one to Brooks. I was exhausted. A nice deep paper like this is much more fitting to your level of understanding.
            http://www.nrel.gov/docs/fy00osti/28410.pdf

            There seems to be two opposite effects acting regarding number of blades. The aero efficiency goes up slowly as the number of blades increases, but the effect is small. On the other side, drag increases as the number of blades increase.

            See the Fig 1 Aero efficiency versus tip speed ratio family of curves, showing aero efficiency increases as number of blades increase.

            http://www.nrel.gov/docs/fy00osti/28410.pdf

            I saw a really nice discussion about aerodynamics from a retired engineer where he showed how even aero people often get the aero theory wrong.

            https://www.youtube.com/watch?v=QKCK4lJLQHU

            He was doing flow around an object, like a wing for one. One of the main points he made was that the air was disturbed in a wide area around any object, not just in the vicinity of the object. He even had something to say about wake turbulence or wind tip vortices that showed conventional wisdom was wrong.

            On that basis, I reserve judgement on your comment that,

            ” a significant portion of kinetic energy of the air molecules will pass the rotor area without interfering with any blade. And if there’s no interaction there’s also no energy extraction).”

            It don’t disagree. I just don’t know. Watch the video and come to your own conclusions.

            On the subject of multiple blades, the theory says multiple blades are higher efficiency with diminishing returns as the number of blades is increased. There is a counteracting effect due to drag on the increased number of blades, but I have not found anything satisfactorily substantive on that.

          • globi

            I think one can see the answer in this equation:
            https://upload.wikimedia.org/math/9/9/a/99a6015b6a230860c9b1517b238e5de9.png

            Double the number of blades = A is doubled, TSR/v is halved.

            Since v is squared drag would actually go down with more blades.

            And this is approximately confirmed with data from the 1, 2 and 3 bladed turbine.
            There’s no data with more blades because nobody ever built a modern multi-bladed wind turbine, because it wouldn’t be cost effective (much more material for very little gain).

          • globi

            And again the torque increase by a factor of 8 and not just by a factor of 4.

            1. Doubling the rotor diameter, quadruples the rotor area and with it power (Power is simply proportional to the rotor area).
            2. Power is torque multiplied by rpm.
            3. Doubling the rotor diameter reduces the rpm by 50% (if the tip speed ratio stays constant).
            4. 4*Power = 8*Torque*(1/2)*rpm

          • eveee

            I see. You are saying torque increases because omega reduces for longer blades. And thats because the tip speed remains constant. I get it. Thanks. More specifically, rotational torque at the hub. My analysis really applies to Force along the axis of the hub. If the tower height doesn’t increase, its correct for tower torque, not rotational torque.

          • globi

            I feel there must be some practical limit, but I don’t know where it is.

            Wind energy can meanwhile already be produces for less than 5 cents/kWh. IMHO, whether this is done with 40’000 or ‘just’ 10’000 wind turbines has little relevance.

          • eveee

            Its true that lower cost is the goal. Larger turbines have lower energy costs. Wind turbine designers wouldn’t even consider larger turbines if it wasn’t advantageous. I thought the graphic showed that pretty clearly. The costs are still falling. Higher towers are also a major cost reducer. The enabler for all that is lowering transportation costs to the site.

        • eveee

          Here is a nice discussion about the number of blades by Andy Hall.
          Thats better than me trying to explain it. Enjoy.
          http://large.stanford.edu/courses/2007/ph210/hall2/

          • Brooks Bridges

            I thought I read some years ago that a single blade wind turbine was very efficient. Just a counter weight on other end.

            Anyone else?

          • eveee

            Well, if you mean less materials, probably. Technically, a higher number of blades is more aerodynamically efficient. Here is a paper that summarizes some of the findings.
            http://www.nrel.gov/docs/fy00osti/28410.pdf

          • Brooks Bridges

            Thanks! Very comprehensive paper. Didn’t realize I was so ignorant of the enormous number of variables in a design.

            Can’t believe I dreamed the one blade design but hard to justify anyone ever building one.

          • eveee

            Yes. I hesitated to even reply definitively. There are so many variables, that any answer can be misleading or wrong if it doesn’t have some context.

          • globi

            see my response below.

    • Jens Stubbe

      Nonsense. There are huge benefits in making wind turbines bigger and that trend will continue as component become stronger and lighter and cheaper.

      • eveee

        Yes. What happens is that as simpler construction, system design, and materials reach a limit, better construction, system design, and materials step in to relieve the limit. Blades started out wooden, aluminum, and all kinds of other stuff. Then they became fiberglass composite, single piece. Generators have changed a lot, too. Even towers have undergone an evolution. In the beginning designs were fixed speed. This quickly evolved to variable speed with feathering. Design of large turbines has changed since the early Mod-3 two bladed design and the seminal Tvind upwind three bladed rotor. NASA ERDA quickly learned (in Sandusky Ohio as I recall) that downwind rotors and lattice towers were not optimum.

  • JamesWimberley

    The hub height does not increase proportionately with rotor diameter, as the sea clearance is fixed. Take this as 30m. Going from 50m blades to 100m ones (2x) only raises the hub from 80m to 130m (1.6x). The tower cost is proportional to the hub height, plus weight. The economics of going big are therefore attractive at sea. On land, you run into transport problems for the largest parts. Onshore sizes have stagnated at around 3 MW on an 80-100m tower for a while. There have been a few 8MW ones in Belgium on 120m hubs, possibly near large canals, but there is no boom in supersize.

    • I recall reading that blimps were being explored for heavy lifting applications, but haven’t heard anything recently. They may show promise as a way to deliver components to remote wind farms.

    • Ulenspiegel

      No, the 7,5 MW turbines are land locked. The bottleneck for transport in trucks and even more trains is the generator of these direct drive turbines. The 7.5 MW is too large, the new ones are around 5 MW.

      Why get larger when the 5 MW turbine gives better economy?

      BTW: The larges towers are around 150m.

    • Jens Stubbe

      Vestas is going into test phase with an around 3,5MW with around 150 meter hub height this year and it is for onshore. The not so precise specifications is probably part of the ongoing attempts to bewilder the competition. The Vestas 8MW was announced as a 6MW at first and will probably soon see an upgrade similar to the 6MW from Siemens that has now been upgraded to 7MW.

      I think it is fair to assume that higher hub heights are a part of the future for wind power and mainly so because they can increase the capacity factor, which makes designing grids that are dominated by wind power more trivial.

  • Karl the brewer

    Is that 200m per blade?? So the diameter is going to be ~ 400m or quarter of a mile!!!

    • Matt

      Yes 200m per blade!

      • Karl the brewer

        So that means 400m metres for the diameter and lets say 30m for clearance. So from the sea to tip of blade is going to be 430m. Empire State is 440m! https://uploads.disquscdn.com/images/eb07d46e09b2c977b763f2d52db1b2eb11c013fdcfe874b19a4ea9d55fae07ff.jpg

        • ROBwithaB

          Well, not quite, if there are three blades.
          If there were only two, it would be an accurate calculation.
          With three, the blades are angled at 120 deg to each other, so the height will be less.
          I’ll let someone else swoop in with the correct answer….

          But still scarily big.

          • Jenny Sommer

            Hub high will still be the same and while you can’t measure 400m tip to tip, clearance will still be ~430m from the surface… And if you stop one blade top vertically you have a total high that you can measure from base/surface to tip.
            Sea to tip 430m like he said.

          • ROBwithaB

            Indeed. I was being pedantically silly.

          • Damon Wright

            How many Godzillas is that in height?

            I can just see the movie possibilities for these turbines as they bravely defend Japan and the civilized world from undersea horrors bent on destroying our most prized cities.

  • eveee

    The downwind blades can be more flexible, and lighter. That helps with tower stress and weight. There is going to be a huge lateral force on a turbine with that much power.

    • ROBwithaB

      But… surely the lateral stress on the tower is more a function of the wind loading than the weight of the blades?

      • eveee

        Thats right. Lateral thrust is wind loading on the blades mostly. Vertical loads are easier to handle than horizontal ones, for the most part. But rotor/generator weight increases tower cost, too.

  • Alaa

    I don’t know why they keep on going about wind turbines. In contrast with solar and storage the wind turbines lose because they having moving parts. It seems to me that the only advantage they have is that they create jobs for people to regularly fix them!

    • eveee

      Perhaps the staggering size of the global wind energy market has something to do with it.
      http://www.evwind.es/wp-content/uploads/2014/06/Wind_Power-Capacity-672×372.jpg
      Its expected to double by 2020.

      • Foersom

        World wide installed wind is well beyond 400 GW at end 2015.

        • eveee

          Yep. Thanks. Its just hard to find graphics including 2015 this early.

    • sault

      It’s easier to build wind turbines off shore and in uneven / hilly terrain on land. Plus, there’s plenty of wind in areas that don’t have good solar resources like the UK and other similar regions. We just need to build what makes sense given the local resources / conditions.

    • Ronald Brakels

      In many areas wind is the cheapest new generating capacity available. So the advantage is it is the cheapest option. Definitely much cheaper than fossil fuels when any reasonable accounting of greenhouse gas emissions and health costs are made.

    • Kevin McKinney

      And unless and until storage hits very large levels, it’s nice to have nocturnal generation capability.

      • Bob_Wallace

        I expect that any technology that is not tightly correlated with wind or solar and can produce for less than 8 cents or so is going to get used simply because it would decrease variability and lower the need for storage and dispatchable generation

        • eveee

          Bingo. Think wave, tidal, anything. There are some papers on this that show the time constants are different in response to storms between wave and wind. Thats a big plus.

    • Bristolboy

      As others have said, wind is cheaper than solar, especially out of the tropics. In places like the UK (especially anything north of Birmingham), Ireland, Sweden, Finland etc solar is much more expensive than wind. Furthermore, in Europe the highest energy demand is in winter when the solar resource is less, but wind resource peaks.

    • Jens Stubbe

      Interesting point you are making there. Wind power unsubsidized is half of solar these days and the capacity factor is double up, so wind is way ahead of solar and will be for the next decades too even if the cost trend for both wind and solar follow the trajectories there are on respectively.

      You could easily power USA without storage using only wind power but that would be impossible with solar.

      As for the moving parts the most efficient utility scale solar projects all have moveable parts and in the future they will shift to double axis trackers for better capacity factors and overall economy.

      • eveee

        You might be interested in the Finnish paper about renewables and a super grid in Asia. They say they use 60% wind to power the grid. Doesn’t surprise me. The steppes have tons of wind. And it happens when its needed in winter.
        Solar, wind, geo, biomass, hydro, wave, … lots of possible mixes when the grid is large and some of the renewable sources are flexible, but low variability. Read the source paper, if you like.
        http://cleantechnica.com/2016/01/04/finland-shoots-russia-nuclear-energy-option-super-grid-option/

    • Steven F

      The best solar panels will convert about 30% of the captured light into electricity. Solar thermal is not much better because the steam turbine can only convert 40% of the captured thermal energy into electricity. In comparison a wind turbine can easily convert 80% of the captured energy to electricity. This conversion efficiency advantage of wind is one reason why wind is the cheapest source of electricity.

    • Brooks Bridges

      and gain because they produce power at night.

    • I used to favor solar over wind. Once you start reading how quickly wind is being deployed and the capacity of the wind farm vs the solar farm the benefits become more apparent.

      BTW solar isn’t maintenance free, grot has to be cleaned off from the surface of the panels on a regular schedule.

    • Rikaishi Rikashi

      Wind and solar are complementary since wind speeds tend to pick up at night-time, and turbines generally produce power 24/7 while solar produces during the peak consumption times.

      Having both minimizes the storage or load-balancing you need for a fully renewable grid.

  • Dan Kegel

    I wonder if part of the idea would be useful for very small turbines. The elevator pitch might be “drones are now cheap enough to be toys; why can’t small wind turbines be cheap?”

  • Jamset

    What would be the distance from the tip to the water?

    50MW turbines probably mean you need less of them in the bay, making it easier for boats.

    • JamesWimberley

      The rotors are clear of anything but large ships. A navigator who can’t avoid hitting a turbine base should not be in charge of a boat. The limitations on fishing are a conservation plus.

      • Martin

        Like the captain of the Exxon Valdes?

  • Hans

    So do they only exist as a computer model? Or is there at least a scaled prototype?

    • Hans

      Additional: what does design mean in this case. Is it just some drawings, or have at least some simulations been carried out?

  • Doug Cutler

    Collapsing blades are very innovative, I love that sort of thing. Still I’m wondering about possible storm conditions including a high degree of surface turbulence where super long blades would still be at risk. Perhaps someone with a better knowledge of ocean surface meteorology could enlighten us.

    • sault

      The tower would probably be high enough to keep the blades above the turbulent layer, especially when they fold back and stay around hub height.

      I’m more concerned about turbulence off the tower during normal operation. Maybe this is why the blades aren’t completely vertical when the turbine is generating to give the airflow some distance to normalize after being disrupted by the tower. Wouldn’t want any weird harmonics hitting the blades every time they pass downwind of the tower.

      • eveee

        Yup. Tower shadow is the main reason downwind blades have been avoided so long. Two bladed rotors have been shunned for dynamic balance. Those two set up harmonics in the entire structure which increases fatigue and lowers life.
        I am skeptical about downwind blades. Perhaps the articulating design can lower some of the stress of tower shadow. From Theoretical and Applied Mechanics, a hub reduces torque stress compared to a pinned junction. NASA DOE used this as the basis of their two bladed design.
        https://upload.wikimedia.org/wikipedia/commons/b/b5/Mod-2_Wind_Turbine_Cluster3.jpg
        It teetered at the hub.

        • sault

          Is it possible to make a tear-drop shaped tower that rotates at the base to keep the whole structure facing the wind? Maybe bringing the wind tracking and turning equipment out of the nacelle and bringing it much lower on the tower (below blade height) could lighten the materials needed to build the tower and make it easier to service the equipment itself.

          • eveee

            If the complexity can be added and it works, yes. I must say, structural engineers are quite conservative. It would be lovely to see some stress portraits on structures. Except for vertical gravity loads, most of the problematic loads are dynamic, not static. My guess is, mech eng will opt for a simple tower. The rotor on the other hand, has had constant fiddling over the years. This articulated rotor thing has been bandied about for many years. Think about the centrifugal forces that operate when a rotating mass is brought closer to the hub. It will try to speed up the rotor. There are other dynamic twisting forces. The deep articulation into the horizontal, IMO, is not a constant operation, but only used to stow the blades in severe weather. It does, however, dramatically lower the torque stress on the blades. Holding a tremendously long blade horizontal is going to put stress at those joints. Maybe something less drastic would work as well or better. Whats wrong with furling the blades along their axis? Its worked so far.

          • Bob_Wallace

            Rotate the nacelle so that the folding blades are downwind only in extreme wind conditions?

            BTW, someone talked about wind turbulence in severe storms. Having been on a sailboat during two really nasty blows my experience is that winds are very directionally steady.

          • eveee

            Yes. Thats the idea. For battening down in storms, to save the turbine. Even feathered (the flat part of the blade along the wind direction) blades still have some resistance and there is a large torque at the blade root. If the blades are bent back and on a fulcrum or pin, torque is eliminated. This is a TAM (mechanics) discussion. Downwind doesn’t need stiff blades to resist being pushed into the tower like in upwind designs when its rotating. Its going to be tricky getting the right amount of flexibility and damping on the blades of a downwind rotor to spare it fatigue failure.

            That same kind of storm design could be used on an upwind rotor as well. The nacelle would be rotated around the opposite of the way its normally used in operation, just for storm survival. Its only there for that reason. IMO, the blades would never be folded like that in operation. There has been design attempts at blades with a more limited amount of flexibility allowing them to bend back.

          • I had the exact same thought myself.

            The moment of inertia pressures on the bearing in the lower part of the tower would be ABSOLUTELY HUGE.

          • Burnerjack

            Maybe a simpler(?) answer is to put the hub on a boom away from the mast giving a ‘normalizing’ distance. Using the power genset to counterweight the assembly. Just a thought.

        • JamesWimberley

          Tower shadow declines as rotor diameter increases. A 2x diameter rotor surely does not need a 2x diameter tower, and even if it did the proportion of the swept disk that is occluded by the tower in the bottom half still goes down.

          • eveee

            The tower shadow effect does not refer to energy lost by the shadow. Rather, it refers to the sudden drop in wind speed as the blade passes behind the tower. This suddenly reduces the load on the blade once per cycle and causes it to wobble increasing dynamic stress and increasing fatigue.

  • Martin

    It would be interesting to know how the folding part works.

    • sault

      Probably the blades are carbon fiber sections with hinges between them. You’d need a lot of hinges to get the streamlining they modeled in the simulation. Just a guess, though.

    • JamesWimberley

      If they follow through with the biological inspiration, the hinges would have to be some very strong, very elastic material like a palm frond. With powered hinges, there is too much to go wrong.

      • Martin

        Thank you. 🙂

      • Carl Raymond S

        I can see a way that it doesn’t need power at the joint to re-extend. Gravity provides the force if the blade is extended while at bottom of the sweep. Of course, that means it needs more energy to rise, but the wind helps lift it, and it already has momentum. The blades would need to be extended little by little over 100 or so rotations.

      • Ronald Brakels

        “Too much to go wrong?” If only you had been part of the group with the job of deciding whether or not to go ahead with Olkiluoto.

      • eveee

        Yes, the hinge is a major problem. It has to carry static and dynamic loads in multiple directions. And do it for over 20 years out in the open. With little maintenance.

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