would like 75W (12V) @ 11MPH, is that ok/possible?

[innergy]

i have an average of 11 MPH winds and would like around 75 to 100 Watt at that particular windspeed, is that possible? how? what's the math and things?

i would like to build a generator from scratch that may be able to produce at least 75 Watts at the windspeed of 11 MPH (or 4.5 M/s) with a blade diameter of less than 2 meter (is this even possible, should be my question also?) (something between 1 and 2 meters)

i have figured out that my average windspeed is 11 MPH year-round and that i will need no more power than around 100 Watt per hour, somewhat less is okay too (12 Volt at 6 to 8 Amps is what my powerconsumption will be, i have figured out also)

what kind of wire for coils would you recommend i need? and how many coils? what kind of magnets, size and how many? what should the cut-in be and the tsr? number of blades? size? etc? and if it's possible i would like to ask

thanks for your help,

Joel

P.s. i have ordered Hugh Piggott's windpower workshop book, but it's not in yet, and i'm not sure if i can extract this info from the book, so anyone experienced who can tell me i would sure appreciate that

[DanB]

An 8' diameter machine well built should do the job - you might get Hughs plans 'How to Build a Wind Turbine'.

[stephent]

Read the posts here...

http://www.fieldlines.com/story/2006/7/18/183548/011

use the wind calcs link there..

and do some data entering in the "Betz Limit" tab on the bottom of the sheet.

Looks do-able to me at 11mph wind and a 3 to 4 ft blade(s).

This is if you have something that will open the xls file on your computer anyway.

I use OpenOffice in Linux to do it.

[Flux]

Much depends on whether you want 75W at 11 mph or an average power of 75W from an average 11 mph wind. The cube law makes these things rather different.

With most machines on typical sites, one that will produce 75W at 11 mph will do much better for average power produced on a site with 11 mph average wind speed.

You will not get 75W at 11 mph with 2 M or less unless you do something rather better than average.

You may produce 75W average in a site with 11 mph average wind speed. I am inclined to agree with Dan and suspect you will need a bit over 2 M ( 8ft, 2.4M would be reasonable).

Unless you have actually measured the 11 mph average wind speed over several years you may be drastically in error if your figures are for an airport or wind maps.

Flux

[SamoaPower]

It's refreshing to see someone who is willing to start at the beginning. Too many jump right in and want to start winding coils and shaving wood before doing their homework. Site and needs assessment is the correct place to begin.

One point, that seems to be often overlooked, is that the annual average wind speed is NOT the speed for which you need to optimize your machine. The peak in the energy harvest curve, which is based on the usual Rayleigh time distribution curve, is the correct point.

This curve set is for my location. The yellow curve is the Rayleigh time distribution and the light blue is the energy distribution. The other curves are for my particular air rotor. If your machine is efficient at the energy peak, you will do well.

[jimovonz]

SamoaPower, If you were considering total system performance and were to factor in storage capacity, would the optimum design speed not be somewhat less? In any given system a large portion of the energy in the higher wind speeds is dumped to avoid overcharging the battery. This would tend to bring the optimum design speed back closer to the average...  

[SamoaPower]

Not if you make use of the dumped energy which only makes good sense to me.

[jimovonz]

But you do have to set some reasonable furl point...

[SamoaPower]

Sure, and I use a relatively low 25 mph to save wear and tear on the machine.

If you chop off the high end of the energy curve for whatever reason, unused dumping or furling, you will decrease the integral (area under the curve) but you won't change the speed at which the peak occurs. So, yes, you will lose on total energy harvest but not a major amount. I think I rather have the machine stay together.

[richhagen]

Hi Samoa, nice chart, we had this conversation before, but again, if your goal is total power produced, like in your grid tied system, fine, but if your goal is to use the generator with a battery storage system of limited size, not all power has equal value.  In effect, the goal is then to use the least fuel in the backup generator, which means that your optimization point is going to shift to the left, because in that case, dumped power has less value than power that can be stored in the batteries, and having more 'up time' becomes relatively more valueable.  Rich

[Ungrounded Lightning Rod]

But charging power is more valuable than dump load power.  (That's why you prioritize things that way.)

So you maximize your utility (value) by optimizing for a lower wind speed to get more (valuable) charging power at low winds (when you need it most, making it still MORE valuable), at the cost of losing a somewhat larger amount of (less valuable) dump load power to the earlier furling needed to protect the mill.

The situation is different if you're grid-tied and net metering.  With "infinite storage" all your power is equal and you want to maximize it.

[SamoaPower]

Rich and ULR,

You both make a good point.

About the only design element we have control over that tends to optimize for a given wind speed is the alternator. Designing the alternator to have better than usual efficiency at a selected wind speed doesn't necessarily mean that we have to give up much, if any, energy capture at lower speeds.

By designing for a higher cut-in speed, say 12 mph for an energy peak of 16 mph, we can get the alternator efficiency up to the 80% range at 16. Then, a la' Flux, we use a boost converter to recover the low end energy. Doing this also improves the efficiency near the furling point to 65% or so intstead of the usually accepted 50%. Should have fewer cooked stators.

 

[richhagen]

That is wonderful and all, and not meaning to start the electronics debate, but I don't happen to have a wonderful and reliable boost circuit handy for any of the small mills I have, and do realize that  the more stuff I add, the more likely something along the line from the turbine to the batteries will fail.  If I had a reliable boost converter, then it would make more sense to optimize for a higher wind because I could obtain a higher efficiency over a broader range, and at some point on the low end the amount of additional energy becomes trivial.  Rich