Power in the Wind

 The wind is perhaps the most unpredictable and unwieldy source of power we  use.  The 'fuel' is the wind, and there is no controlling it, you need to take what's there and hopefully protect the system when things get out of hand.  Imagine a gas powered generator with no throttle control  - one moment it stops, the next moment its running full speed, and you need to figure out how to use the power that's coming out and figure out how to keep the engine from blowing up when the throttle gets pushed up a little too high!  All good wind turbines have some system to protect themselves in high winds.  I'll discuss that in more detail later. 

Do I really want a wind turbine?

 For people who live off the power grid wind and solar power can compliment one another nicely.  Often if the sun isn't shining then the wind is blowing.  Having a wind turbine in your off grid power system can allow you to have less solar and a smaller battery bank.  The cost of solar vs wind depends a great deal on your resources.  In some places (like Northern CO where we live) the sun shines over 300 days a year, so solar power makes a great deal of sense.  We're also fortunate to have a reasonable wind resource so the two compliment one another nicely.  In Some places the sun doesn't shine for weeks at a time and wind power might be the most cost effective solution.  We live 11 miles from the nearest utilities and for us wind power is very cost effective compared to our other options.

Can I get a wind turbine and unhook from the power grid?

Yes, you can - but it's not likely to be cheap or easy or cost effective if you allready have grid power.   Lot's of companies selling wind power systems and plans will tell you only what you want to hear.  Do your research - look for information provided from objective sources who have nothing to sell.  There are many good books and some good websites on the subject - it is wise to educate yourself before you spend your money. 

It almost always costs more to generate your own electricity than it does to buy it from the power grid.  Just the cost of batteries alone can often break down to a monthly cost greater than you'd have paid had you bought the same power from the grid.   These costs can vary, if you buy surplus/used equipment that can change your overall cost per KWH dramaticly in an off grid system.

 There does  tend to be an economy to scale with windpower though and with larger machines it can  become cost effective with a reasonably short payback period.  Before considering generating your own power to reduce your power bills - do everything you can to conserve.  Some estimate that one dollar spent towards conservation is better than 10 dollars spent on generation.

Wind Power Facts

 I cannot think of another 'industry' where I see more misleading information out there than with wind power.  Because accurate wind speed is difficult to measure and many customers are not well informed it becomes easy to make unrealistic claims about a machines performance and it seems unlikely that those claims will ever be contested.

Power in the wind(Watts) = 1/2 *rho*A * V^3

There is perhaps the most important formula to in wind power.  It tells you how much energy is in the wind (not how much you can extract) where:
rho = 1.23 (this is air density, 1 cubic meter of air weighs 1.23 kg at sea level)
A = swept area in square meters.  If your wind turbine has a blade diameter of 3 meters (about 10') then swept area is a bit over 7 square meters (or about 80 square feet).  Because of the formula for area, it's important to keep in mind that doubling blade diameter will cause the machine to sweep 4x the area.  Power available from a wind turbine is related to the *square* of the blade diameter.

V = velocity (wind speed, in meters per second).  Notice the V is to the 3rd power.  This means if you double wind speed you have 8 times the energy.  This is very important to think about.  If a machine is designed to produce usable power in a 10 mph wind then it has to deal with 8x that much energy in a 20mph wind, and 64 times that much energy in a 40 mph wind, and 512 times that much energy in a 80mph wind etc...  For the sake of the wind turbine and it's tower it must have some system by which it protects itself. 

So lets look at the formula above and consider a small 5' diameter (60") wind turbine in a 10mph wind.  This is a bit larger than the popular 'Air' series of wind turbines built by South West Wind power. 
 Diameter =5' = 1.524 m
Swept area = pi x r^2 = 1.8241 square meters
Wind speed = 10mph = 4.4704 meters/second
Power in the wind = 1/2 * 1.23 * 1.8241 * 4.4704^3 = 100.22 Watts.
And remember, if you double wind speed you have 8 times the power, so in a 20mph wind there would be over 800 Watts.!

So above you can see how much power exists in the wind in theory - and this is assuming we start with a certain wind speed and stop the wind.  Of course... we cannot stop the wind, then it would pile up around our wind turbine and the machine would quickly stop making power.  In 1919 Albert Betz theorized that at best we can only slow the wind by about 1/3 and he calculated that the most power you can take from the wind is 59.26% of the total power.
So the very best a wind turbine can do is 59% of what the formula above tells us.  This means that in theory, a 5' diameter wind turbine could produce 59 Watts in a 10mph wind.  In practice no wind turbine blade is perfect and a good blade on a small wind turbine might be around 30-35% efficient.  A good small 5' diameter wind turbine blade will likely produce about 30 Watts in a 10mph wind and this is mechanical energy into the shaft - it does not account for alternator/generator inefficiencies.  Most small wind turbines have good electrical efficiency in low winds and then it tends to drop off in higher winds.  Electrical efficiency of 80% or so should be expected in low winds and in higher winds (25 - 30mph) they are often down around 50% unless there are power electronics involved to improve the matching of the blades to the alternator/generator.

It's easy to use the above formula to see how much energy is available in the wind with any known wind speed.  Any claims of output greater than 59.2% of that are definitely breaking the laws of physics and any claims about a machine greater than 30% or so are probably unrealistic.  This is important to keep in mind because I frequently see people making impossible claims about wind turbines.

Swept area and rated output

 'Rated output' of a wind turbine is fairly meaningless compared to swept area.  I often see 'light duty' small wind turbine rated for 1000 watts or more in high winds and then I'll see another manufacturer with a nice big heavy slow machine thats rated for about the same output but in much lower wind.  Remember the power in the wind is related to the *cube* of the wind speed.  There is twice the power in a 25 mph wind as there is in a 20mph wind.  So the small 1000 watt wind turbine that claims to produce 1000 watts in a 30mph wind is not comparable to the larger 1000 watt wind turbine that produces 1000 watts in a 20mph wind. 

Wind speed (mph)  2      4       6       8      10      12      14      16      18      20      22      24      26  

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4'     12.56          .18     1.4    4.6    11     22     38      60       90      128    176    234    304    386

6'     28.26          .39     3.1    10     25     49     85      136     202    288    396    526    684    868

8'     50.24          .70     5.5    19     45     88     151    241     359    512    703    934    1215  1542

10'   78.5            1.1     8.6    29     70     137   237    377     564    800    1099  1460  1900  2410

12'   113             1.6     12     42     101   198   341    524     809    1152  1582  2102  2735  3469

14'   154             2.2     17     57     137   270   465    739     1103  1571  2156  2864  3727  4728

16'   201             2.8     22     74     179   352   607    965     1439  2050  2814  3739  4864  6171

18'   254             3.6     28     94     226   444   767    1219   1819  2591  3556  4724  6146  7798

20'   314             4.4     35     116   279   533   948    1507   2248  3203  4396  5840  7599  9640

When shopping for a wind turbine it's much more important to look at swept area than it is to look at the manufacturers rated output.

Towers

 We've gone over two very important things that I often think about.  The power in the wind is related to the *square* of the blade diameter (double diameter and you get 4x the power) and the *cube* of the wind speed.  You have a great deal of control over blade diameter - you can choose what size machine you buy.  What about the wind speed?  It's often suggested that the most economical choice is to put up lots of machines on short towers, or mount your wind turbine(s) to the roof.   Manufacturers say this stuff because it sells machines - people do not like the idea that a tower costs money and they love to hear that once they buy the wind turbine very little further cost/effort is required.  The wind is your fuel and wind turbines need good clean non-turbulent fuel and unfortunately you don't find that on the ground or on your roof.   It is usually suggested by most reputable installers and manufacturers that the most cost effective arrangement will be to get your wind turbine 30 feet above anything withing 300'.  If you have turbulant 10mph winds at 30' and 13 mph winds at 70' then it makes sense to go a bit higher, get out of the turbulance and get into a slightly higher windspeed where you might have twice the energy available!  This is not the sort of talk that sells machines, but it is reality.  In most good installations the cost of the tower is much greater than the cost of the machine itself and there is a good reason for that.

Can I build my own wind turbine?

 Sure you can!  Pictured above is our home brew 20' diameter wind turbine that powers our home and shop.  It's an axial flux machine built very much along the same lines as you'll see in Hugh Piggotts books.  I think it can be great fun and there are some good plans and parts available on ebay.  It does get a bit tricky I think to properly match a given blade set to a given generator/alternator so it makes sense to be prepared for a good bit of trial and error - or - get good plans.   I've seen  plans frequently on ebay for both battery charging machines and for direct grid tie systems.  Some plans are better than others.  Building a wind power system is not a cheap or easy project, you still need a tower, batteries etc so it makes sense to do your research as you would if you were buying a wind turbine.  I expect if I was just getting into that hobby I would buy a few sets of plans and read over them all before investing my time and money.

The Bottom Line

Windpower has it's place.  It should be quiet, reliable and easy to live with.  It is not always cheap or easy and if you're thinking of having a windpower system you really need to inform yourself about what's available, what's realistic etc.  There are many good books and resources available.  Check out the AWEA website.  Hugh Piggott and Paul Gipe have excellent books on the topics.  Look at the pages of manufacturers, but remember... they may just be trying to sell you something - they are not always the best source of information.  So...  I would say conserve energy, inform yourself, and have fun with it all!