How much wind is enough wind?

[Oly]

Hi all.  We've got an off-grid cabin that is currently solar powered, small system, 150W of panels. The entire cabin is wired 12V with a few outlets wired for AC that can run off a generator or inverter.  We use very little electricity, mostly just some CFL, with most appliances being propane-powered.

In the future we'd like to spend more time there and would like to have more available power. My plan has been to install wind power, as expanding the solar system would be quite expensive. Been lurking here for some time collecting ideas.  I'd probably go with a 10' diameter or so Piggot design, keeping it as simple as possible as this will be my first build.

My question is, how much power can I expect to generate with this size/type of generator?  I know this depends, among other things, on the wind at my location. The state of Oregon has an "Anemometer loan program" (http://me.oregonstate.edu/alp/) run by one of the universities in conjunction with a grid-power supplier. If the site is a good candidate, they'd install an anemometer and all the associated data-collection equipment and monitor the site for a year, for free.  We'll, I applied for the program but was rejected due to insufficient wind.

Their response after a paper evaluation was:

"Estimates obtained from the regional wind map model suggest the annual average winds in the area to be

        10m      6.0 mph (2.7 m/s)

        30m      7.8 mph (3.5 m/s)

        50m      8.7 mph (3.9 m/s)

From a meteorological perspective, these estimates seem reasonable. The proposed site is sheltered somewhat to the west and to the south.

Data from a number of historical sites in the general area are available and can be used to verify the model estimates.  A sensor was present for a limited time on a nearby ridge and showed a Mean of 8.4 mph.  This sensor was only in place during a summer period but was at a much higher elevation (3000 ft. compared to 2000 ft. for our site).  To the northeast there was another site which showed a mean of 7.0 mph.  This site was in place longer but was on a 20 foot tower.

Considering the site and wind model information we would expect the site to have an annual average wind speed of between 7.0 and 9.0 mph at the 30 meter level. Recommendation:    This does not appear to be a good candidate for the Oregon Anemometer Loan Program.  The site has relatively low winds."

Well, I'm not going to take this news and walk away. I know the main ambition of many folks on this site is to collect energy from the wind at low wind speeds. So, how much wind is enough wind? I reckon it'll be a trade-off between blade diameter and tower height. I'd like to keep the tower as low as possible for cost reasons and to keep it as out-of-site as possible for the neighbors. I've read many folks here write about 30-foot heights for their machines. That is what I was planning on,  but if I'm not going to get any usable power at that height with a 10' machine I need to reconsider the plans.  The response above said I could expect average wind speed of 6mph at that height.

Where is the better bang-for-buck here, going larger diameter or going higher up, and are there calculations or models I can use to figure my bang-for-buck? I haven't looked much into towers yet; is there a general rule of thumb for what towers cost per foot? I'd prefer to construct my own tower rather than buy one.

Thanks for your input, (and please tell me I've got enough wind to make this worthwhile!)

Oly

[Flux]

If you really are down to 6mph average then that is very low.

At 6 mph you will have difficulty extracting any power from a 10 ft machine.

Assuming that you have a Rayleigh distribution of wind, using Michael klemens good turbine figures you may hope for about 30kWh/month.

Even that is optimistic and I suspect nearer 20.Raising tower height will help, depending on the local terrain. At 60ft I think you might make the 30kWh/month or a bit more.

If the figure turns out to be nearer 7mph then things look a lot better.

To go out and buy a commercial turbine would be folly but if you can do it yourself for a low cost then go for it.

A lot will depend on the seasonal distribution of the wind, at some times it may not be viable, at other times it might do quite well. Average wind speed is only a first indicator.

Flux

[wdyasq]

"Well, I'm not going to take this news and walk away. I know the main ambition of many folks on this site is to collect energy from the wind at low wind speeds. So, how much wind is enough wind?"

I live in an area with 12mph average winds. I consider this speed 'marginal', at best. At 6mph winds, one would have 1/4 of that energy. The parasitic things will be near the same, just the energy to turn the blades. The diodes eat a bit of energy also.

If I were ebtting, I would bet on the side of the "Anemometer loan program" folks and take money home more than 9 times out of 10. There are a lot of incentitives for states to have 'green power' and they get to add 'independant producers' and the capasity in the guesstimates. I'd bet someone is selling the 'energy credits' one gets once an installation they assist in is installed. But, that is just my honest, observed trust in goobernment agencies.

Ron

[Countryboy]

You want your tower to be a minimum of 30 feet higher than anything surrounding it for 300-400 feet in all directions, otherwise you will get slower turbulent air.

The cost of a tower depends on the type of tower.  A freestanding tower will be more costly than a guyed pipe tower.

In Home Power magazine #37 Oct/Nov 93, and #39 Feb/March 94, Mick Sagrillo wrote the articles Tower Economics 101 and 103.  It would be worth your time to find and read these articles.  As Mick says, the 3 most common mistakes in windpower installations is too short a tower, too short a tower, and too short a tower.

Usually, the best bang for the buck is going higher up.  However, if all you have is poor winds, it may be better for you to build a generator with oversized blades, running a higher TSR.  Simply set the furl point to a lower windspeed, so you don't burn the alternator up if you get unexpected higher winds.  If you are not going to live at the cabin so you can keep an eye on the turbine constantly, I'd recommend setting a low furl point regardless.  (unless you plan to shut the turbine down when you are away, and are there to watch it when it is running.)

There are maps for America showing wind zones.  IIRC, a 10 foot turbine in wind zone 2 'should' generate about 100 KwH a month.

Keep in mind that average wind speeds are exactly that, averages.  1 hour of dead calm, and 1 hour of 14 mph winds is an average of 7 mph.  Yet, you will be able to extract more power in that one hour of 14 mph winds than you would be able to get out of 2 hours of 7 mph winds.

[Countryboy]

However, if all you have is poor winds, it may be better for you to build a generator with oversized blades, running a higher TSR.

Better meaning less financial cost.  You will always get better power the higher you go.

[wpowokal]

http://www.homepower.com/files/towereconomicsseries.pdf

Allan down under

[Shadow]

I find it hard to believe you only gain 1 mph by going from 33 feet to 100 feet and only 1 more mph going to 162 feet! So according to that it wont do you any good to go with a higher tower. So you might look at what Harrie just built and go BIG and slow.

[jimjjnn]

I agree with you Shadow. Something don't sound right here.

[DanB]

I think you made a typo there...

at 6 mph youd have about 1/8 the energy than you would at 12.

[Oly]

Thanks for the reference to the Klemens site.  I've taken his table and worked up the scenarios I may be able to do. Could you check my calcs? With a 20 foot diameter machine at 8mph winds I'd get 340 kWh per month?

[Oly]

Thanks everyone for your comments and links.  Looks like, to make it worthwhile, I'll need to go larger AND taller.  Much more so. I was hoping to avoid this for a first build.  Harrie's 20 foot low-wind setup looks like the direction to go, but being a newb I think I'll start with a 10 footer and see what I get at 30 meters up (8mph). If it looks worthwhile I can then go bigger. And if it doesn't look worthwhile, I'll have had fun trying!  I hope you guys are right about the wind speed estimates at higher elevations at my site seeming unrealistically low compared to the 10 meter estimate.

If anyone else has any real-world low-wind success stories I'd like to hear them. How little wind do you have, and how have you modified your turbine design to maximize output at low speeds?

[Ungrounded Lightning Rod]

Be REALLY CAREFUL with big mills, even in a low wind area.

They still have to survive the occasional storm.  So they, and their mounts and furling systems, have to be as strong, properly constructed, and correctly adjusted as mills for a high-wind area.

[nothing to lose]

As Countryboy said,

"Keep in mind that average wind speeds are exactly that, averages.  1 hour of dead calm, and 1 hour of 14 mph winds is an average of 7 mph.  Yet, you will be able to extract more power in that one hour of 14 mph winds than you would be able to get out of 2 hours of 7 mph winds."

Same thing I say.

Myself I think most of the charts and studies of wind speed averages are very flawed. Perhaps looking more towards large scale commercail systems to power a city than for home built private use types the average would be more important.

Studies showing the actual hours of true windspeeds are the only acurate way to measure and I don't know anyplace to find those. Say Xhours 0-5mph winds, Xhours 5-10mph, Xhours 10-15mph, etc... per month would be far more meaningfull than saying 7mph monthly average.

20MPH for 4 hours is 80, divide by 24hrs, average daily winds 3.33 mph!

Well certainly no-one will make much power at 3.33mph right. But wait, just how much usable power can you make at 20mph for 4 hours though???

So if your average is only 7mph X 24hrs thats a total of 168mph and what really counts is how long you get how much at once, not the average!

 Do you have steady 7mph winds always, probably not. So for every hour you are below 7mph you must spend some time above 7mph to average.

 So say 168/12mph=14 hrs or 168/15Mph=11.2hrs or 168/40mph=4.2hrs.

As you can see,depending how the average is acumulated you may have some pretty decent winds there for making power. More than half a day maybe at 12mph, or 40mph for 4 hours, either way it's decent power and that's what counts.

I get several hours of good strong winds here a day, various hours of dead calm and gentle breezes. I bet my average would say forget it, but it's those good hours that make the real difference, not the average!