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    Cosmic Embryo #6: Swaying Trees To Produce Wind Energy
    By Richard (Dick) Go... | March 24th 2013 04:26 PM | 18 comments | Print | E-mail | Track Comments
    The earth has abundant energy sources, if only we would bother to tap them. Wind power has had its financial and ethical ups and downs, with wind farms using gargantuan, expensive wind towers decorating or despoiling the scenery,depending on your esthetics:
    Wind power resources on the eastern U.S.continental shelf are estimated to be over 400 GW [gigawatts], several times the electricity used by U.S. eastern coastal states…. The furthest advanced of a handful of proposed U.S. offshore wind developments is in Nantucket Sound,off the Southern coast of Cape Cod, Massachusetts. This proposal has engendered a widespread, well-organized, well-financed, and politically potent opposition.This movement’s strength, and the apparent contradiction of such opposition coming from a population thought of as politically liberal and environmentally concerned, have garnered national press coverage….  The project would provide clean energy but would intrude on a beautiful, unspoiled natural area. One can see the wind turbines as an ugly intrusion on the seascape or as a beautiful portent of clean energy for the 21st century; as a threat to tourism or an opportunity for year-round local jobs; as a menace to marine and avian species or as pollution reduction; as a government-financed boondoggle or a hedge against foreign oil dependence; as an example of market solutions to environmental problems or as greedy developers spoiling Cape Cod yet further. Because there is an oil-burning electric plant in the same area, the possibility of wind electricity displacing oil electricity makes the air pollution and energy security aspects of wind energy locally immediate and easy to visualize [1].

    As T. Boone Pickens said of his Pickens Plan to harness USA wind power: “I’m in the wind business…I’ve lost my ass in the business” [2]. Once we stop importing oil, with its price fluctuating due to world politics, wars, and market manipulation, the various renewable energy sources, such as wind, solar, geothermal, algal biofuel, etc., could start competing with one another on a level playing field, and the winners will sort out [3, 4].

    Around 2003 I bought a then newly invented Nightstar shaking flashlight [5, 6]. It works on the old fashioned principle that a magnet moving through a coil of wire will generate an electric current in that wire. This is the dual to the electromagnet: pass a current through a coiled wire and it will magnetize an iron rod inside it. Dynamic microphones and loudspeakers are another dual pair of technologies based on the same principle [7]. The Nightstar powers an LED, and a few seconds of shaking its cylindrical magnet up and down through itsinduction coil produces light. The housing is rugged, immersible, and waterproof. Of course, one never has to buy batteries. The motto of the company, embossed right on the flashlight, is: "Nature and technology can co-exist".

    I bought the flashlight as a birthday present for my wife, for use at our solar powered rough cabin in the woods of Manitoba, on a quarter section of wilderness we call Silver Bog. It is surrounded by 60 foot trembling aspen trees that sway in the frequent prairie winds. To make that flashlight work I had to shake its magnet up and down. And so I mused about harnessing the swaying of these trees, by roping a tree near the top and having the rope shake such a magnet up and down through an induction coil. I should have patented the idea, but recently was scooped [8]. As a theoretical biologist, I seem to have developed 10 thumbs, leaving the practical building of things to others. In retrospect, I've also wryly mused about harnessing the energy of exploding seeds [9], so I often don't take myself seriously.

    Howard Rees and Michael Faigen treat us to a magnificent vision of a world powered by swaying trees. We just tie trees together with ropes, pulleys and pull-retract generators (PRGs) to get up to 1 kilowatt of power per tree:

    The scale of an e-Tree installation can vary between those utilizing only a few trees (generating a few hundred watts) and those utilizing many thousands of trees (generating hundreds of KWs or even MWs)…. e-Trees installations promote the planting of trees (and hence, carbon sequestration) and produce electricity. e-Tree "forest rangers"… can be members of the indigenous population who shepherd the e-Tree farms by tending to the health of the trees (trimming, thinning and readjusting the PRG harnesses) and maintaining the equipment... [8].

    Before you rush outside to tie up your own trees and start wind farming, best to learn from the experience of Scott McGarry and Chris Knight, who hitched ropes, pulleys, computers and cameras to a tree and tried out the idea. Indeed, they estimate that one eucalyptus tree they tied up could theoretically yield 0.5 kilowatts = 500 watts [10]. But they were only able to extract 50 milliwatts of power in a good wind, and just harnessed it for wireless communications [11]. So what is going on here, that we can extract only 1 part in 10,000 of the mechanical energy a tree takes from the wind? These researchers surmise:

    This is due to a number of reasons: the tree dissipates wind power from all parts of the tree, whereas the experiments outlined considered just one point on the trunk. Harvestable energy is a fraction of the elastic energy within the tree, and losses due to internal damping and wind vortices cannot be harvested using the methods outlined in this article…. Of the proportion transferred to movement energy, just a small part of that will be available to a movement energy harvesting device for energy extraction. This is due to the fact that it is envisaged that such a device will only attach to one point or area of the tree, and not be able to take advantage of the movement of the many small leaves, twigs and branches widely distributed throughout the tree’s structure [10]. 

    So much promise, so little power delivered! Trees are obviously very clever at dissipating wind energy, else more would be blown over [12]. Wind pruning [13] and culling over millions of years has probably selected for these tactics, such as multimodal vibrations between the main trunk and branches [14], and wind stimulates root growth [15]. Those trembling aspen at Silver Ridge are not dancing their leaves for my amusement, but for a reason. So let me suggest an evolutionary approach to the problem: domestication, i.e., we designate the criteria for survival. For example, using bonsai methods [16], could we force trees to grow in shapes that transmit more wind force to our ropes?

    More subtly, can we grow trees with attached or embedded inductive coils, perhaps creating a symbiosis with bacteria that make permanent magnets [17], in such a way that they "dissipate" wind energy into electricity rather than into heat and air vortices? Here is a challenge for combining nanobiotechnology with some of the largest organisms on earth.

    Finally, physics based computer animations now sway us that we are looking at realistic swaying trees [18, 19, 20, 21] down to the level of splashing of raindrops on the leaves [22] and the reciprocal effects of the tree on the wind, and thereby on each other [23]. With such fine software, we could now turn from delightful animation to a deep understanding of the physics of trees, and resolve whether or not they could be harnessed for wind power.

    While we have a long way to go, it may no longer be true that “only God can make a tree” [24].

     

    (Thanks to Ille Gebeshuber and Miranda Stuart for preposting comments!)

    A conference on trees in wind has been held [12]. This depiction with kind permission of editor Bodo Ruck. 

       

    Experiment to measure wind energy from a tree. From [10] with kind permission of Multidisciplinary Digital Publishing Institute (MDPI AG).

    Tethered palm trees [25], with kind permission of Holly Gramazio.

    1.      Kempton, W.; Firestone, J.; Lilley, J.;Rouleau, T.; Whitaker, P. The offshore wind power debate: Views from Cape Cod. Coastal Management 2005, 33, 119-149.

    2.      Pickens, T.B. I've Lost My Ass In The WindPower Business. Available online: http://www.realclearpolitics.com/video/2012/04/11/t_boone_pickens_ive_lost_my_ass_in_the_wind_power_business.html

    3.      Gordon, R. Cosmic Embryo #2: QuittingImported Oil Cold Turkey. Available online: http://www.science20.com/cosmic_embryo/cosmic_embryo_2_quitting_imported_oil_cold_turkey (August 1, 2011),

    4.      Gordon, R.; Poulin, B.J. Quitting coldturkey: rapid oil independence for the USA. In The Science of Algal Fuels: Phycology, Geology, Biophotonics, Genomicsand Nanotechnology; Gordon, R.; Seckbach, J., Eds.; Springer: Dordrecht,2012; pp. 3-20.

    5.      EcoCentricNow Shake Flashlights. Available online: http://www.appliedinnotech.com/products/shake-flashlights/shake-flashlights.php

    6.      Vetorino, S.R.; Platt, J.V.; Springer,D.A. Renewable energy flashlight [UnitedStates Patent 5,975,714]; Applied Innovative Technologies: Aurora,Colorado, 1999.

    7.      Wikipedia Microphone. Available online: http://en.wikipedia.org/wiki/Microphone- Dynamic_microphone

    8.      Rees, H.D.; Faigen, M.J. Distributed System of Electrical GeneratorsUtilizing Wind Driven Natural Motion of Trees [United States Patent US7,936,079 B2]; 2011.

    9.      Beer, T.; Swaine, M.D. Theory of explosively dispersed seeds. NewPhytologist 1977, 78, 681-694.

    10.     McGarry, S.;Knight, C. The potential for harvesting energy from the movement of trees. Sensors 2011, 11, 9275-9299.

    11.     McGarry, S.;Knight, C. Development and successful application of a tree movement energyharvesting device, to power a wireless sensor node. Sensors 2012, 12, 12110-12125.

    12.     Ruck, B.;Kottmeier, C.; Mattheck, C.; Quine, C.; Wilhelm, G., Proceedings of the International Conference "Wind Effects on Trees",University of Karlsruhe, September 16-18, 2003; Institut für Hydromechanik,Universität Karlsruhe: Karlsruhe, 2003.

    13.     James, K.R.;Haritos, N.; Ades, P.K. Mechanical stability of trees under dynamic loads. American Journal of Botany 2006, 93, 1522-1530.

    14.     Spatz, H.-C.;Brochert, F.; Pfisterer, J. Multiple resonance damping or how do trees escape dangerously large oscillations? AmericanJournal of Botany 2007, 94, 1603-1611.

    15.     Coutand, C. Mechanosensing and thigmomorphogenesis, a physiological and biomechanical pointof view. Plant Science 2010, 179, 168-182.

    16.     Korner, C.;Menendezriedl, S.P.; John, P.C.L. Why are bonsai plants small? A consideration of cell size. Australian Journal of PlantPhysiology 1989, 16, 443-448.

    17.     Yoshino, T.;Maeda, Y.; Matsunag, T. Bioengineering of bacterial magnetic particles and their applications in biotechnology. Recentpatents on biotechnology 2010, 4, 214-225.

    18.     Diener, J.;Rodriguez, M.; Baboud, L.; Reveret, L. Wind projection basis for real-time animation of trees. Computer GraphicsForum 2009, 28, 533-540.

    19.     Habel, R.;Kusternig, A.; Wimmer, M. Physically guided animation of trees. Computer Graphics Forum 2009, 28, 523-532.

    20.     Song, S.-M.;Kang, Y.-M.; Lee, E.-J.; Ok, S.-Y. Tree animation based on hierarchical shape matching. In Multimedia and Signal Processing; Wang, F. L.; Lei, J. S.; Lau, R. W. H.; Zhang, J. X., Eds.;2012; Vol. 346, pp. 475-482.

    21.     Hu, S.; Chiba,N.; He, D. Realistic animation of interactive trees. Visual Comput. 2012, 28, 859-868.

    22.     Yang, M.; Jiang,L.; Li, X.; Liu, Y.; Liu, X.; Wu, E. Interactive coupling between a tree and raindrops. Computer Animation and VirtualWorlds 2012, 23, 267-277.

    23.     Akagi, Y.;Kitajima, K. Computer animation of swaying trees based on physical simulation. Computers&Graphics-UK 2006, 30, 529-539.

    24.     Kilmer, J. Trees and Other Poems; George H. Doran Company: New York, 1914.

    25.     Gramazio, H.Tethered palm trees. Available online: http://www.flickr.com/photos/several_bees/1425035/sizes/z/in/photostream/

    Comments

    Bonny Bonobo alias Brat
    Really interesting article Dick! I started googling this concept and found another interesting article called 'Artificial Trees to Harness Solar and Wind Energy'. The article explains that :-
    'Biomimicry — that is an emerging science — is going to be used to implement some of the nature’s processes in order to produce clean, environmental friendly energy. The tree, once installed, will replicate the functions of a solar-wind harvester.

    According to the company’s website its energy capturing powerhouse is going to be “Nanoleaf”. According to the Nanoleaves page, “A Nanoleaf is thin like a natural leaf, when outside forces, like the wind pushes the Nanoleaf back and forth, mechanical stresses appear in the petiole, twig and branches. When thousands of Nanoleaves flap back and forth due to wind, millions and millions of Pico watts are generated, the stronger the wind, the more energy is generated.”

    The Nanoleaves reflect back only a small portion of the sunlight and the remaining light is used to produce energy. “Due to the unique combination of photovoltaic and thermovoltaic in our Nanoleaves it converts this thermal radiation into electricity, even hours after the sun has set,” the page further states.

    They can even convert the infrared radiations into energy.

    The constant development in nanotechnology, the photovoltaic and themovoltaic materials will be easier and less costly to produce, bringing down the production and installation costs significantly.

    Of course the biggest benefit of such trees will be the availability of extremely nonpolluting electricity. But along with this they will also eliminate the need to create eyesores in the form of gigantic wind turbines and solar panels. These energy harvesting trees will look as natural as normal trees. This is like growing gardens and mini forests that are actually silent electricity generators.'

    The only problem is the 'nanoleaves page' isn't there! Must be nanoautumn or nanofall  :) 

    I also found some references to a Saraswati veena exotic wind harp called an Akasa, which is a veena that was tied up in the tops of trees for the strings to vibrate from the currents of wind which is sort of using the same concept of free wind powered energy creating free, unique and impromptu music.

    I suppose that one of our oldest weapons, the bow and arrow also utilises power contained in wood that has been stretched which then powerfully discharges the arrow with a little help from the archer?

    Most the skyscrapers that I have been into, can be felt to be swaying in the wind, maybe that wind power could also be trapped and used to power the buildings? The windows or window frames could be coated with a similar substances being used by the nanoleaves in the artificial trees above and use solar power to turn every window in the skyscraper into a power generating machine too? The possibilities are endless and yet seem quite feasible and a lot more practical, clean and recyclable than burning wood, coal and oil and polluting the environment and more importantly probably, the atmosphere, contributing to global warming.


    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    rholley
    Must be nanoautumn or nanofall
    Oder sogar auf Deutsch Nanoherbst?

    Try this poem (English translation provided)!




    Robert H. Olley / Quondam Physics Department / University of Reading / England
    rholley
    Which reminds me of this article by Patrick on the effects of tidal power:

    Pushing The Moon Away With Victorian Machinery

    Robert H. Olley / Quondam Physics Department / University of Reading / England
    Theoretical yield 500W; actual yield 50mW. Hmm, let me think. Ingredients: tree plus wind. Ooh, I know, let's take a tree some 10 m by 30 m and see how much kinetic energy flows through it in a 30 m/s wind. That's about 1000W. Well, we had better be realistic and halve it... That's 500W folks! Wow! 500W! Free and non-polluting! Green!
     
    Joy, joy, joy!!!
     

    Unfortunately:
    Trees are obviously very clever at dissipating wind energy, else more would be blown over [12]. Wind pruning [13] and culling over millions of years has probably selected for these tactics, such as multimodal vibrations between the main trunk and branches [14], and wind stimulates root growth [15]. 
    You can rely on it!

    In general, power transfer from the wind depends on creating a pressure differential while at the same time not stopping the air flow altogether. Thus there is an optimum overall drag coefficient which will result in something like 25% power transfer - at around half the unimpeded flow, half the dynamic pressure.  However trees are draggy objects and, even without evolutionary fine-tuning, most of the wind energy will be dissipated as useless turbulence. Genetically engineered leaves coupled to resonant branches might be a little less effective at "wasting" this energy but as we have not yet succeeded in producing rectangular eggs for easy packing, I won't be investing my money in home-grown windwills just yet. 

    Bonny Bonobo alias Brat
    but as we have not yet succeeded in producing rectangular eggs for easy packing, I won't be investing my money in home-grown windwills just yet.

    Could anyone seriously believe that a rectangular egg was easier to package? Wind trees are a great idea rectangular eggs aren't, ask any chicken or you could end up with egg on your face Derek :)
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    rholley
    Helen,

    I’m pretty sure you’ve Never Seen a Straight Banana!

    Here is a recent version by the Singing Chimney Sweeps



    I have heard that the original song (1926) was inspired by an advertising campaign by the chief banana marketing company, promising a prize to anyone who could find a rectilinear fructification of Musa × paradisiaca.

    Robert H. Olley / Quondam Physics Department / University of Reading / England
    Bonny Bonobo alias Brat
    Ha ha, very funny Robert, you are a mine of such interesting information!!!
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    I really don't know any chickens well enough to ask them such a personal question, Helen. However, I'm quite sure that if rectangular eggs were adaptive, chickens would evolve to match :)

    More seriously, the actual wind power for the scenario I suggested is nearer to 8 MW - my numbers were rubbish - so it is remarkable indeed that the experimenters only pulled 1/160,000 of the power out of the air flow. See also http://en.wikipedia.org/wiki/Betz'_law 
    Bonny Bonobo alias Brat
    Adaptive? I'd like to see you adapt or evolve to handle rectangular stools first :)

    More seriously, the actual wind power for the scenario I suggested is nearer to 8 MW - my numbers were rubbish - so it is remarkable indeed that the experimenters only pulled 1/160,000 of the power out of the air flow. See also http://en.wikipedia.org/wiki/Betz'_law
    So what does that mean in laywoman terms?
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    rholley
    In a science fiction story by Richard Cowper, the main protagonist found himself wondering if the rectangular fish finger had evolved as the optimum shape to escape through the mesh of a net.
     
     
    Robert H. Olley / Quondam Physics Department / University of Reading / England
    I'd like to see you adapt or evolve to handle rectangular stools first
    Then you are destined to experience a life of disappointment. Individual organisms, of which I am one, do not evolve. You have been watching too many of those tedious sci-fi films where "mutants" sprout extra eyes or wings.
    So what does that mean in laywoman terms?
    It means that trees are just about the worst possible starting point for utilising wind-power. Betz's law is what I said - the air must not be fully blocked by the power-collecting device but must not flow through unimpeded either. There is an optimum. Betz's law asserts the maximum possible efficiency to be ~59%. Well-engineered turbines might achieve 3/4 of this but are based on efficient blades that create lift forces (forces perpendicular to the air flow) which turn the generator shaft at the optimum rate for extracting the most energy. Natural objects rarely create useable lift, most of the forces experienced by objects in a wind are due to drag (forces in the same direction as the air flow) and this is inherently inefficient. According to Wikipedia, Betz's law can be beaten if the air flow is allowed to diverge, but that's obviously no use if you want to have trees working close together to use all the air flow. But whether you reckon on 20% or 80%, "random" draggy objects are unlikely to be useable at anything like that efficiency. And trees which have evolved to survive absorbing mechanical energy from the wind are going to have hardly anything available at all. My initial comment implied that the experimenters just dreamed of harnessing a substantial ppart of the total wind power passing though a tree, but having corrected the figures it would seem that they actually measured the available mechanical powr (presumably at the aerodynamic surfaces) to come up with their 500 or 1000W. That would be worth having if a generator could be cheap and simple but the pitiful amount they actually gathered suggests that the tree simply dissipates the energy - no surprise there. That's a seriously bad start to the engineering: you might just as well try to design a system of levers and gears all made out of porridge. 
       
    Gerhard Adam
    ...you might just as well try to design a system of levers and gears all made out of porridge.
    Now that would be a useful application of porridge.
    Mundus vult decipi
    rholley
    Now that would be a useful application of porridge.
    Are you trying to break up the Union?  Remarks like that are bad enough coming from a Sassenach!

     When Scotland started to obtain a degree of local government, it was referred to as Devolution.  Subsequent developments raise the question: Are the English making monkeys out of the Scots, or (as seems more likely) the Scots making monkeys out of the English?
     
     
    Robert H. Olley / Quondam Physics Department / University of Reading / England
    Gerhard Adam
    ..and why are we blaming the monkeys?  :)
    Mundus vult decipi
    Bonny Bonobo alias Brat
    Thanks for explaining so clearly Derek, I miss your physics lessons and blogs.

    So what about the 'Artificial Trees to Harness Solar and Wind Energy' article I found above that claims to be using biomimicry with its energy capturing powerhouse the “Nanoleaf”. According to the missing Nanoleaves page :-

     “A Nanoleaf is thin like a natural leaf, when outside forces, like the wind pushes the Nanoleaf back and forth, mechanical stresses appear in the petiole, twig and branches. When thousands of Nanoleaves flap back and forth due to wind, millions and millions of Pico watts are generated, the stronger the wind, the more energy is generated.”

    The Nanoleaves reflect back only a small portion of the sunlight and the remaining light is used to produce energy. “Due to the unique combination of photovoltaic and thermovoltaic in our Nanoleaves it converts this thermal radiation into electricity, even hours after the sun has set,” the page further states.

    They can even convert the infrared radiations into energy.The constant development in nanotechnology, the photovoltaic and themovoltaic materials will be easier and less costly to produce, bringing down the production and installation costs significantly.'

    Does this artificial tree with its nanoleaves sound at all feasible to you Derek, as a possible future energy source that is also not an eyesore?

    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    An artificial "tree" might work better than a natural one but what's the point?  I think a forest of metal imitation trees would be even more of an eyesore than a conventional wind farm - and I detest those things. But in any case what the nano brigade are talking about doesn't sound very feasible to me. 

    Existing wind turbines are reasonably efficient. The claim that millions and millions of Pico watts (sic) are generated is absurd - a picowatt is a million millionth of a watt so they are talking about millionths of a watt not the mhundreds of kilowatts produced by a decent turbine or the kilowatts hoped for in the scheme using natural trees. I'm not sure how much power is obtainable from infrared/thermovoltaic sources: the second law of thermodynamics severely limits the possible efficiency and it involves a lot of machinery to generate power: there's no obvious way nanotechnology could attain even a useable fraction of the thermodynamic limit. Of course photovoltaic cells are also quite feasible but there seems little point in having them flap around like leaves - a photovoltaic panel could generate many tens of kilowatts from the sunlight directly without the inefficiency of a thermovoltaic system, so why introduce all the problems of flexible connections, cells shading each other and so on just to gain a tiny fraction of a watt? You might just as well keep a glowworm in a jar next to the "tree" in order to augment the sunlight.

    Sometimes I think people are infatuated with the Tesco Effect: - "Every little helps" - which is patently silly when there still isn't a hope of achieving anything useful. 
     

    Bonny Bonobo alias Brat
    An artificial "tree" might work better than a natural one but what's the point?  I think a forest of metal imitation trees would be even more of an eyesore than a conventional wind farm - and I detest those things. But in any case what the nano brigade are talking about doesn't sound very feasible to me.
    The part I missed out from the original article this time around was the part that said 'Of course the biggest benefit of such trees will be the availability of extremely nonpolluting electricity. But along with this they will also eliminate the need to create eyesores in the form of gigantic wind turbines and solar panels. These energy harvesting trees will look as natural as normal trees. This is like growing gardens and mini forests that are actually silent electricity generators.'

    Maybe we could combine solar and wind power from artificial trees in our gardens with the massive artificial Xmas tree industry, kill 2 birds with one stone? That's a point, I wonder how birds would take to sitting in and building nests in power generating, artificial trees with vibrating, photosensitive nanoleaves?

    Anyway, from what you are saying this artificial tree wind and solar power nanotechnology is unlikely to take off in your opinion. I already have one of the wind up torches which don't use conventional batteries, just stored kinetic energy from the recent winding and it works beautifully. You would think we could all just as easily be able to extract power from the wind in our own gardens without having to look at the typically ugly and noisy modern windmills and solar panels. I personally really like the idea of windmills and solar powered energy that looks like natural trees, so I hope this technology does take off one day.
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    I'm not saying it can't be done at all, just that starting with the "tree" paradigm is the wrong place: "If it's Dublin you're wanting you shouldn't be starting from here." Unlike the Irishman, we do have a choice as to where we start and the obvious place is the efficient flying surfaces of a turbine or bird, not wasteful, draggy leaves.

    The problem of converting mechanical energy to useable power might conceivably be solved with minature "windmills" or in this case "flutterers" each one automatically tuning itself to the local airflow and coupling its mechanical power to a main shaft through variable nano-tech gear boxes. That's a complete self-adjusting power station inside every leaf.  Whether it would ever look like - and above all, sound like - a natural tree is another matter. Unfortunately, typical leaves are only a few inches long and are likely to result in a Reynold's number of a few thousand, so acheiving non-turbulent air-flow might be difficult. I think insects actually utilise the vortices and, contrary to popular myth, bumble bees do fly, but whether a flickering metal tree that sounds like a swarm of angry wasps is any improvement on a quiet, white windmill is another matter. 
    I wonder how birds would take to sitting in and building nests in artificial trees with vibrating, photosensitive nanoleaves?
    I'm sure we could genetically engineer them to enjoy our metal monsters - and make them lay rectangular eggs at the same time. Better still, replace the inefficient natural birds with nano-tech models.