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Old 02-21-09, 06:49 PM   #1
Bob McGovern
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Default A brief wind power tutorial

Urps -- not sure how that happened. Thanks to this site's History feature, here's a recovered version.

***

I've found discussions surrounding wind energy usually benefit if people have a good sense of the physics and forces involved. The science is both straightforward and subtle. If this is old hat, forgive; if it is new, hopefully a few crudely-executed sketches can give a feel for what is happening where air meets machine, and why some designs are inherently more effective than others. No math, but it helps if you are a sailor.

First, good electricity-generating turbines are not pushed by the wind so much as they are pulled. Making electricity needs medium-high RPMs at the alternator, and blades that are pushed can never go faster than the true wind speed -- usually much slower. These are called "drag turbines". Good turbines rely on aerodynamic lift to turn them: the forces involved are greater, and the blades can spin many times faster than the wind, just as a catamaran can sail faster than true wind speed.

WHOA -- sound like a perpetual motion machine? The secret is a phenomenon called "apparent wind", which is the vector sum of the true wind and the blade (or boat) speed. It looks like this:



A blade with tip-speed ratio (TSR) of 5 will spin 50mph in a 10mph wind, creating its own apparent wind of ~51 mph. The apparent direction of that wind moves closer to the blade's rotational plane the faster it spins; likewise, the blade tip sees a faster and shallower apparent wind than the blade root.

As the apparent wind contacts the leading edge, it splits. Some air goes along the convex back, some goes along the concave front. The streams adhere to the blade surface and adjacent air by boundary layer effect.



The concave side air slows down, generating a net positive air pressure; the air flowing over the convex back of the blade speeds up, creating a net (and stronger) negative air pressure. Both sets of pressures act perpendicular to the surface of the blade. The forces act in all different directions and intensities, but the net force is a flexing and forward one. The flex force is canceled by the blade's stiffness; what's left is a forward vector of surprising strength:



There's a fairly narrow window of apparent wind direction -- or attack angle -- in which a lifting airfoil will operate efficiently. The flow along its sides must be smooth, and it must remain attached to the blade surface. If it detaches, the result is lost lift, terrible drag, and turbulence -- with attendant noise and vibration. These conditions are known as stalling (attack angle too steep) and luffing (attack angle too shallow.)



The red squiggles are turbulence, eddies that form when flow detaches.

Wind turbine designers have developed numerous strategies to keep the apparent wind angle correct, which is especially hard given that different parts of the blade are moving different speeds, and to keep lift strong and constant along the full length of the foil. These strategies can be grouped in pairs: Taper and draft, twist and pitch. Near the hub of a propeller-style-turbine, the apparent wind is slower and closer in direction to the true wind; the blade root has a wide chord (breadth) and a fairly deep draft, and it faces more toward the true wind. The blade tip is traveling very fast: too much breadth or 'cup' would create crippling drag, so the blade there tends to be narrower and flatter. The tip also experiences an apparent wind nearly in line with its rotational plane, so it needs very little twist:



Finally, the best HAWTs (horizontal-axis wind turbines) are able to rotate the entire blade to vary TSR, optimize lift, and keep the alternator spinning at its prime electricity-making RPM. They use taper, draft, twist and pitch all at once. Inexpensive turbines like my Bergey XL1 may use straight blades, extruded as if from a pasta machine. They lose remarkably little in terms of efficiency in their sweet spot. But in light or very strong winds, they luff or stall (or some of each!), lose lift, and make ungodly noise.

You may see curved blades appearing on some machines: as with most such innovations, it is worth asking why, and why commercial turbines rely on a straight leading edge. Bending the tip back may keep flow attached and reduce noise somewhat, but the lifting forces pull outward rather than forward, tip spillage and drag is increased, and you are losing torque at the blade ends, right where you most want it.

Finally, it's worth a brief look at why the world's best engineers design HAWTs rather than VAWTs (vertical axis wind turbines). The very worst VAWTs are pure drag machines: paddles pushed by the wind. Their maximum apparent wind is the true wind MINUS their own rotational speed, so they can never go fast and the wind's force is gutted. And two-thirds of the time, a given blade is either contributing nothing or plowing headlong INTO the wind. These generally fall under the heading of Savonius Rotors. Their TSRs are always less than 1.0, and their efficiencies are gruesome.



Better, though not much, is a class of semi-lifting VAWTs classified as Darrius, Modified Darrius, or Gyro Rotors. These include the famous 'eggbeaters' and more modern designs. Almost half the time, their blades are on or near a "beam reach," moving perpendicular to the true wind, which sailors know is the fastest, most-efficient point of sail. But the back blade is operating in the wind shadow of the front one, its draft must somehow be inverted, the downwind blade is dragging some, and the upwind blade is a sea anchor. Darrius blades are pulling maybe 50% of the time. They aspire to TSRs of 2.5; they do spin faster than the wind, but not much.

Finally, look afresh at the humble HAWT, or propeller-style machine. Its blades are purely lifting foils, though they suffer the disadvantage of reduced speed near the hub. They have TSRs of 5-8, usually. All three blades experience the same apparent wind speed and direction all the time. All three are traveling at 90 degrees to the true wind (that ideal beam reach), pulling in the same direction all the time. A HAWT experiences great lift over a 100% power stroke. That's why GE, Mitsubishi, and Vestas build them that way.


Last edited by Bob McGovern; 02-22-09 at 12:09 PM..
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Old 02-22-09, 02:33 AM   #2
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Quote:
Originally Posted by Bob McGovern View Post
I've found the various discussions surrounding wind energy usually benefit if people have a good grasp of the physics and for
Bob, it seams your message got burned by an old retired nuclear plant

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Old 02-22-09, 02:45 PM   #3
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Wow, another great post. Thanks Bob!

Is this part of the previous series?
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Old 02-23-09, 10:36 AM   #4
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Geez, and I thought you geeked out in the other threads...woah.

Thanks a lot for writing this up man. This was awesome. Stuff like this makes me wish I lived outside the city on a bigger plot of land so that I could set up something like this.

Is there even a point to doing it if you live in a residential area with a 50x110 foot yard? Or am I just going to irritate the hell out of the neighbors with the noise? Cause I'm guessing the amount of energy it's going to produce isn't going to be very much.
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Old 02-23-09, 11:37 AM   #5
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Quote:
Originally Posted by Higgy View Post
Is there even a point to doing it if you live in a residential area with a 50x110 foot yard? Or am I just going to irritate the hell out of the neighbors with the noise? Cause I'm guessing the amount of energy it's going to produce isn't going to be very much.
Looking at your wind resource, it might make sense. Fall and winter are pretty decent, especially near the lakes. Depends on ground clutter.

As for neighbors.... Phew. That's always the tough bit. Stuff is easy; people are complicated. If all your neighbors experience are the impacts (visual, noise, disruption) but not any benefits, they are gonna oppose it. Best strategy is to pull in as many people as possible. Make the wind turbine in some way theirs, as well. Could be a larger machine set off to one side, with everyone investing some money and benefitting by share. But more important is to get people emotionally invested. Come round with drawings, noise data, photos. Carry a tall stick around the yard with them and say "Here? What if we moved it over here so it doesn't block your window?"

On install day, you can ask them over, buy some cases of good beer and rope in some local musicians to play for tips and booze while the tower goes up (Winnepeg music is kewl!) A proper old time barn-raising. Wind power is unavoidably obvious. If people can look at a turbine and say "Some small part of that is me," they will come on board and stay on board.

Short answer: if you have the wind, the space, and the goodwill to put up a 70ft tower, go for it. If not, better not.

PS: It's not geeky if it involves backhoes and concrete.
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Old 02-23-09, 11:40 AM   #6
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Very good description Bob. Thank you.
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Old 05-09-09, 08:14 PM   #7
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Impressive work Bob.

i am working on a new style of wind turbine, i always think outside of the box, so be kind,
i think this one will be more effecient than the vawt and it will be 60% effecient as hawt. I Call It The "FAWT" Silo. "fixed axis wind turbine Silo" it will hide the prop from passers by. check it out at my site. Home

best regards
Sam
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Old 05-10-09, 10:05 AM   #8
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The only flaw I found in your writing was "All three blades experience the same apparent wind speed and direction all the time." this might appear to be true but it's not, because when a blade is at the top if it's rotation it's higher then the blade below it at that is at the bottom of it's rotation, I don't have the chart in front of me, but if my memory is correct a 12 foot blade in a 12mph average wind speed on a 120 foot tower is going to have closer to 11mph wind speed on the tip of the lowest blade and that 1mph diffidence at those speeds is going to translate to nearly half the energy hitting that lower blade, because the energy in wind is cubed as speed increases.
As for the city lot question, most city lots a tower has to be the hight of the tower from the property line unless your neighbors give you and ok, so why not put it on a short tower? turbulence, I've seen the airometer spin backwards at my parents house because of the turbulence next to the buildings as it's only 30 feet up, this is a little 3 cup wind speed indicator, so for it to spin backwards the wind has to swirl around it perfectly, granted this has only happened a few times that I have seen, but it's because of the buildings, for this reason the most basic rule is no less then 30 feet higher then anything within 200 feet and ideally 50 feet higher then anything within 200 feet, as any ground clutter causes drag, there are charts out there comparing wind speed in a field with knee high grass, to a golf course mowed lawn, at 100 feet up! that is why it's always best to go with the tallest tower you can get and why roof top mounts are a bad bad bad idea, that and you are attaching a spinning moving wind catching device to your roof.


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