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Forum Index : Windmills : Anemometer Turbulence?
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| pntrbl Newbie  Joined: 12/07/2008 Location: United StatesPosts: 32 |
New guy. 1st post but I've been lurking and learning ..... Since anemometers resemble the Lenz type of turbine are they less likely to have turbulence issues? There's one up at the local fire station that I can access data from online, but it's horribly sited. About 30 feet up with a 2 story building, numerous pine trees, and even some radio towers between it and the prevailing wind. I don't know if I should believe it or not, but I'm definitely willing to listen to all opinions ..... Thanx. SP |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5117 |
Hi SP Your right, it wouldn't be accurate with all those wind barriers around. The only way to get a accuraqte measurement is to mount the anemometer several meters above everything, but thats not easy to do. Things like lattice radio towers are OK, but next to a tree is bad, or even worse a big building. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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robbo Regular Member  Joined: 25/03/2007 Location: AustraliaPosts: 71 |
Mine is situated directly in line with the mill, (Same heights, same winds etc). Even when the mill has something blocking the prevailing winds, then the anenometers is getting the same amount of wind. Its the only way to get accurate measurements, of where your mill will be. Good luck. "the Earth was not given to us, by our fathers, rather, it is lent to us by our children". |
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| Dinges Senior Member  Joined: 04/01/2008 Location: AlbaniaPosts: 510 |
Hm. If that anemometer is so badly sited I'd take its measurements with a grain of salt. 'Indicator' of windstrength at best, definitely not a measurement. Here's some quick and simple rules to follow when installing an anemometer: http://www.weathershack.com/education/anemometer.html Peter. |
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oztules Guru  Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Hmm I see no point in having a anemometer where the mill isn't. I think Robbo has the right idea. There seems to be little value in having a measuring device not measuring exactly what your are trying to harness. I would go one further, use an axial directional anemometer bolted directly on the mill. Yes, just like on the big machines (although they use radial for wind window limit reasons, not mppt) Now you will see what it sees, not what you wished it saw. Mine will be a modified psu fan with the iron core removed, and the hall sensor left inside. It will be mounted on the windmill behind the blades. Yes there will be some wind shading, but I'm interested in what the mill is seeing at any moment in time. With this we can do something positive. (mppt) It doesn't matter if it is not quantified in any known measuring system. We only need to measure change, and feed that back into the system. With wind measured any other way, you can only look and procrastinate and wonder what to do with it. I have seen no positive use for wind information measured by normal means. (other than site development.... even then it needs to be where the mill will be ... warts and all). They are just figures of history.... and cannot be used in real time. So I have no use for them. The wind that I get is the wind that I get, knowing what it is in M/sec won't help me develop more power. So for me, a properly positioned and mounted anemometer (as per weather bureau )is as useful as a sack full of wet mice..... ...........oztules Village idiot...or... just another hack out of his depth |
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Gill Senior Member Joined: 11/11/2006 Location: AustraliaPosts: 669 |
G'day SP, From the replies I'm sure you've got the idea of how important wind measurement can be to the windmiller. However for the beginner, the ideal is often a long way off with much experimentation, observation and learning. Until the ideal comes within reach the wise would use the best resources available to him. If the choice is between the fire stations anemometer and say your wet piece of string hanging from the cloths line, then go for the anemometer. Be aware of it's limitations and assess any data accordingly. After all the ultimate, perfect anemometer for the windmiller is unachievable and all existing ones are a compromise. You are correct concluding a rotating cup type is less affected by turbulence than say a prop type. For accurate readings the prop type must track the wind. Wind tracking is typically done by using wind pressure on a tail to turn it. The wind that has turned the tail has therefor not passed through the prop at the same angle as it should have. The more turbulence the greater the error potential. You may note, whilst this error is in true wind speed, theoretically, it may accurately reflect the wind speed the generator prop sees if it's pointing direction is fixed to the wind gen's pointing direction. Finer points such as these depend on what you are doing with the information gathered. For general wind and weather monitoring these points are irrelevant and you don't say what your wind information is for. Still, I hope that's helped. was working fine... til the smoke got out. Cheers Gill _Cairns, FNQ |
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| pntrbl Newbie Joined: 12/07/2008 Location: United StatesPosts: 32 |
Well thanx for the discussion guys. All good information to me. Really, I'm still in "feasabilty" mode at this time. I think we've got a decent amount of wind around here, the vegetation's all flagged to the north because of it, and I've seen the tumbleweeds doin' 35 ...... But I'd better get my own anemometer up to know. Is there a way to quantify turbulence? I'm thinking I'd like to know how high I need to be for clean air. I've read you should be 30 ft above anything for 300, but that's not gonna be practical for me due to city limits. On the other hand if I find the air get's noticeably less turbulent at some height that'd be a good thing to know. SP p.s. to Oztules; the mppt reference went right over my head, I haven't a clue, and why the anemometer behind your rotor? |
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| oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
pntrbl "p.s. to Oztules; the mppt reference went right over my head, I haven't a clue, and why the anemometer behind your rotor?" MPPT= maximum power point tracking. This is many things to many people, but the bones of it is that we need to match the alternator (linear voltage output compared to rpm but square function of power to rpm) to the power in the wind (cubic function of speed of the wind). The alternator becomes a dynamic driver for a static battery load. As the wind power increases... some thing has to give eventually. If we were to graph their power curves, they dont match up well at all, particularly if we wind the stator for good low wind performance. In this case if we get decent wind that should produce a lot of power, we will either stall the blades (sometimes we sort of design for this to have a placid low wind performer) or develop too much power in a stator who's resistance is too high (because we wound for low speed cut in), and so may suffer burn out if the furling is not good enough to stop this happening.... and all scenarios in between. On the way to trying to burn out the stator, we must be developing heat in the stator and so by definition we are not being terribly efficient. As the speed increases this relationship gets worse, until we dissapate more power in the stator as heat than power into the batteries. This is not what we were trying to achieve. So, Mppt, is a black box which matches the blades performance to the power going into the batteries to maximise the latter while keeping the former performing at it's best possible tip speed ratio for the current wind. So it (somehow) must match the alternator to the battery bank, while keeping the blades on song.... nice act if you can pull it off. To my knowledge (limited), no one seems to have perfected this black box yet. There are many different approaches to making it work. The theory works well in practice for solar cells.... to match the sun to the cell to the battery bank, it is easier to design, and has few variables to deal with, but the wind version seems more elusive. Even the big boys seem to have hit a wall on this one, and so it is there to be solved. (I will be using the anemometer set up as described above by me as part of the equation.) The non-purists figure any means of matching the load better (than none) is a step towards mppt. This can range from resistance in the line to help keep out of stall, capacitors across the line to load up more at lower speeds, or capacitors seriesed to give a smoother startup curve (capacitive reactance changes as a function of frequency), where they present a lower load at lower frequencies and a act like a piece or wire at higher frequencies..... bit like a rpm dependent anti-resistor (at cutin, resistor drops almost no voltage,.. at full load it will drop considerable voltage, the capacitor will do the opposite.). There is star/delta switching, pwm chopping, multiple / staggered coil stators, and plenty of others. All directed towards solving the inequatable relationship between wind speed and battery charge current. You can match the load better by winding the alternator such that it cuts in quite late. This will have the effect of matching the squared graph line of the gen power to the cubic wind power line on their straighter sections. This makes them a close approximation of each other. (if you look at the X=YxY graph to a X=YxYxY line, they both straighten out a lot as they move away from their origin. If you were to cut these lines out and move them around the page a bit, you would find that by changing their origins you can get a match of sorts for the straighter sections. All that remains is to build a small booster to get around the late start up problem we had created (fairly easily done). Variable pitch blades, particularly on grid tied induction turbines can achieve a power increase by matching the torque to the fixed-ish shaft rpm, and can be loosely brought into the mix. Now, I intend to use the measurement of available power in the wind as a reference for my attempt at mppt. I don't much care what is in the way of the anemometer, just so long as it sees exactly what the mill sees... including direction -off- the wind, and solidity as speed increases. No point measuring a 40 mph wind, when the mill is pointing 30 degrees off the wind at the time. The calculations are meaningless. So behind the blades will do, or if I can see a way to mount one in front of the blades, then maybe there as well. Behind is where the big boys do it, but they're blades are moving very slowly in the center, mine will be also, probably 120-220 rpm. It will also make a fair difference as to how the blades are mounted... on arms or direct to rotor From initial tests, the interference is not insurmountable. An offset can be calculated (if necessary) for that from the current rpm. What I need is a solid known variable for my try at mppt to work. Many have failed before me, so I won't feel alone when this doesn't work. And when it doesn't, I will match the load via the booster method with higher cut-in, which I think is the best method by a mile at the moment. ........oztules Village idiot...or... just another hack out of his depth |
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| Dinges Senior Member Joined: 04/01/2008 Location: AlbaniaPosts: 510 |
I see little point of measuring the disturbed airspeed (near the ground, with turbulence, ec.) and not the 'real' airspeed (which is a function of height too !). There's little point in mounting a windturbine on a too low tower in disturbed, turbulent wind, though I'm aware that a lot of people have to make compromises. Doesn't subtract from the point though that it makes very little sense to invest a lot of time and effort building a windgenerating system and then install it in a bad site. Bit like mounting PV panels in the shade. One point I do agree with is that it's probably best (unless you're the met office) to install the anemometer close (upwind) to the windturbine. Even on a perfect site, the wind at a height of 10 m (internat'l meteorological standard) vs. 30 m (I don't know how tall your tower will be; perhaps 30+ m ?  ) makes a large difference in measured windspeed. There's little use having the windturbine at 30 m and the anemometer at 5 m, unless you only want to use the anemometer readings as an indicator of the wind the turbine sees.
So, the most important thing: know *what* you want to measure: the wind the turbine will see, or the 'real' wind (at an arbitrary 10 m height, as is the standard for the met office). Different horses for different courses. |
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| pntrbl Newbie Joined: 12/07/2008 Location: United StatesPosts: 32 |
Wow. MPPT. I've read that 4 times oztules and I think I have a glimmer! I picked up on velocity cubing in Piggott's "Windpower Workshop" and I already knew the swept area of a rotor increases on the square. Charging current increasing on the square was news tho and yeah, I can see where that's a basic conflict. Even if you don't burn out the stator making a lot of heat's no help ..... Thank you for your time and now I'm gonna search the forum to see what I can learn about low windspeed boosting. SP |
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| pntrbl Newbie Joined: 12/07/2008 Location: United StatesPosts: 32 |
The city has a 35 ft height limit on me. What's that? A little over 11 meters? I'm pretty sure I can sneak an anemometer up to 40-50 ft without no one noticing. If the wind is usable up there I'll build a mill at 35 and once everyone's gotten used to seeing it on the skyline we''ll sneak her up there where the good stuff is! SP |
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| oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Pntrbl, Yes the voltage rises linearly. Double the rpm, we double the volts.... unloaded coil Volts = EMF I will use E=EMF rather then measured volts. This will become useful when we load the windings to calculate the power out under load. The measured volts will be dictated by the battery voltage and diode loss. The EMF will be what is actually generated by the action of the magnetic field on the copper coils. ie a 12v battery will drag the alternator measured volts down to 13 volts or so (plus diode drop), but the EMF will be the rpm/ rpm per volt. With some ohms law where E=IxR and Watts = IxE then we replace the I in the watts equation with E/R and we find W=ExE/R . Resistance (R) stays constant (ignoring heat) so we measure our watts as a squared function of EMF in the alternator. If you want to read about matching the load from a very smart fellow, try this: http://www.fieldlines.com/story/2006/3/17/185646/194 You won't do better than listening to what he says. .........oztules Village idiot...or... just another hack out of his depth |
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| pntrbl Newbie Joined: 12/07/2008 Location: United StatesPosts: 32 |
Thank you for that link OZ. Most illuminating..... And now I have another message board to monitor! SP |
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