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Forum Index : Windmills : Low windspeed choices
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Glipski Newbie  Joined: 26/08/2008 Location: United StatesPosts: 10 |
Sorry if it's posted in a blatantly obvious place somewhere on the site, but searching didn't turn up the answer for me. My question is this: I'd love to build either an axial or F&P (my brother is an authorized service tech) mill, but my worry is that I seem to have rather low wind speeds in my area. The average speed is in the 6mph range. However, I own 15 acres and my land is the highest point around, so constructing a tall tower wouldn't be a problem. I've lived on this property for a few months now and I've yet to see a day without wind, just not sure of the speed. I'm in the process of building an anemometer to start logging the actual windspeeds, so I'll be armed with actual data instead of my visual assumptions based on treetops. I've seen F&Ps here, axial flux, 3 bladed and 6 bladed mills...what is the general consensus for low-speed applications? Thanks, and sorry if this is a silly question. |
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GWatPE Senior Member  Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Hi glipski, low windspeed means low wind energy. You could put in an iron cored machine as an ornament. I would maybe opt for an axial flux unit with a boost cct. At least most of the energy captured could be converted to electricity. You would still need a large capture area on the rotor to get useful power. 6mph gives about 2.5m/s, or 5.4W/m^2 of rotor area. allowing for blade efficiency etc, would lower this to approx 2W/m^2. This would mean a huge area. Any mill would probably be an ornament. I would probably put in some solar. Gordon. become more energy aware |
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| Glipski Newbie Joined: 26/08/2008 Location: United StatesPosts: 10 |
How large is "large" for capture area? I have lots of room to build any size mill I'd need. The wind maps for the area state 6mph average windspeed, however it's always really windy in my general area. Especially up high where the treetops are. What would a "good" windspeed be for utilizing a mill? |
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Haxby Guru  Joined: 07/07/2008 Location: AustraliaPosts: 423 |
GWatPE gave you the calculation. You will get 2 watts per metre squared of wind swept area. So a mill with a diameter of 5 metres would give you 40 watts at your 6mph wind speed. Not much at all. But you should definitely build the anemometer... If you fix the anemometer to a tethered weather balloon, you will be able to see what the wind speed is above the trees. It just might surprise you how much wind is up there. |
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| GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
The average windspeed is an indication only. I have a windmull and the average windspeed where I live I measured with a weather station at 4m/s. I would rate my location as marginal. If I had to buy a commercial unit it wouldn't be viable. There would be periods of wind that would be viable at your location. There is often bias with met bureau readings, unless you are exactly at the measuring site. The best indicator is flagging of trees where you live. A good location has the trees bent away from where the wind comes. This is a little subjective if the winds are from all directions where you live. do some measurements at your site. If the weather station has averaged windspeed, then rule of thumb gives wind peaks at 2x the 10min average. Be careful when interpreting the weather station data. It is easy to over estimate wind eneregy if peak windspeeds are recorded. Gordon. PS remember wind energy = 0.5 x air density x wind capture area x wind velocity^3 where air density is approx 1.2kg/m^3 wind velocity is in m/s wind capture area is in m^2 blade efficiency is approx 0.2-0.3 generator efficiency will depend on type. 0.4-0.95 become more energy aware |
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| Glipski Newbie Joined: 26/08/2008 Location: United StatesPosts: 10 |
Thanks guys. I'll measure the wind for awhile and then buy the solar panels :-) |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
I have no doubt that you could design a windmill suitable for a low wind area. It would have a big turbine to catch enough energy but it would also have to be strong enough to survive on the few days of the year when the wind is stronger than usual. It would be interesting to see others comments but I think a suitable mill would have many blades. Put up the anemometer then make the decision!  |
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| Glipski Newbie Joined: 26/08/2008 Location: United StatesPosts: 10 |
How does one go about determining the number of blades for a mill? Why is 3 the "norm" and not 2 or 4 or maybe 7? Is it because of drag? How about a VAWT instead? If it had lots of torque but not alot of speed, could I add in a small gearbox to increase output speed into an axial flux mounted in a sort of motor driven generator type of setup? |
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philb Regular Member  Joined: 05/07/2008 Location: United StatesPosts: 96 |
This link should answer the first part of you questions. Hope this helps. 
"Marcellus Jacobs is the man who almost singlehandedly invented the first practical wind-powered electrical generating system. He's the man who originated nearly all the noteworthy advances in the field from 1930 to 1956. And he's the man who dominated this specialized mini-industry until the day he decided to move on to other interests." http://www.motherearthnews.com/Renewable-Energy/1973-11-01/T he-Plowboy-Interview.aspx philb |
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| Glipski Newbie Joined: 26/08/2008 Location: United StatesPosts: 10 |
Great reading, thanks for the link! |
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