Homebrew Wind Power
A Hands-on Guide to Harnessing the Wind
Sunday, December 7, 2008 18:29
This is a book for anyone who is interested in generating their own power from wind, or who wants to build a wind generator from scratch. The book starts off with the basics and explains what works and what does not work along with the theory and all the references you could want.
First the bad news: You need a tower, the tower may cost more than the generator, if you live in a city you may not be able to erect one, you can’t attach it to your roof, those fascinating looking vertical axis mills are poor performers, and you are going to have to do a lot of energy conservation if you plan on living off the output.
Now the good news: If you have ever dreamed of building one, this book provides the knowledge required to be successful. Even if you are an apartment dweller, it is a fascinating read. If this was a common book for school kids to write book reports on, there would be very few wind power scams. It might even inspire you to build a fully equipped workshop or to learn welding. This is the kind of book that needs to be donated to school libraries and science classrooms across the nation.
I was expecting to find that they used a commercial alternator — and was totally unprepared to see details on magnet placement, resin potting, coil winding, workshop safety, magnet handling safety — these people have been struggling with a myriad of trade-offs for years, building upon the designs and hard work of other pioneers; their current designs are indeed excellent pieces of low-tech engineering.
Finally, there is lots of humor here. Dog Haiku anyone? There are many excellent photos and diagrams, as well as some amusing ones. All in all, a worthy permanent reference for your bookshelf.
You can order a copy from OtherPower.
Homebrew Wind Power
A HANDS-ON GUIDE TO HARNESSING THE WIND
Dan Bartmann & Dan Fink
Foreword by Mick Sagrillo
ISBN: 978-0-9819201-0-8
Published by Buckville Publications LLC
John Bass says:
July 17th, 2009 at 12:12 am
The DanB “heavy duty” 48V stator from his web site says “This stator is wound with #15 gage wire. It has 9 coils and each coil has 105 turns in it. [...] The coils are wired in 3 phase star configuration [...] In our tests we believe this stator will be about 50% efficient at 1000 Watts. Sustained output above about 1000 Watts may overheat this stator”.
There are easy ways to get generator and rectification efficiency up above 97%, so that almost all of the harvested wind power available from the prop ends up in the batteries. Burning more than half of it in the generator and rectification is simply poor engineering. I can provide detailed plans for building a cheaper and more robust MPPT generator that is about 97% efficient at design load.
The DanB coil form, requires about 6″ per turn average, so 105 turns is about 53 feet with pigtails. Each phase with 3 coils, is then about 160 feet of #15 wire, with a nominal resistance of 0.003184 Ohms/ft, or 0.50944 Ohms/phase at room temperature.
The nature of the diode rectifier switching, combined with the precise battery voltage cut-in while charging batteries, blocks the multiple interphase current paths that would normally exist in 3-Phase AC generation and AC load systems. As such, the two phases with the highest voltage sum, will be the only two conducting current the majority of the time. Near the phase switching point, there will be two phases in parallel briefly. Because of this, the majority of the cycle will have only two phases conducting in series.
The simple equations are V = I * R, and P = V * I = I^2 * R
A fairly drained battery bank, will have a cut-in charge voltage +/- about a volt from the nominal 48V, and rise to around 56V when the bank nears fully charged. 1000W at 47V, implies sustained charge currents will start around I = P/V = 1000/47 = 21 amps for drained battery banks. The stator copper losses are then P = I^2 * R = (21^2)*(2*0.50944) = 450W copper heating losses, plus another P = V * I = 1.4V*21A = 30W losses in the rectifier.
With the copper buried in the vinyl ester resin stator casting, there is no way to cool the coils. The resin is a fairly good thermal insulator, even with ATH filler added. A 500W heater, buried in a resin insulator, is going to heat up way above 80C very quickly. Above 80C where the N42 and N50 magnets, just a tiny fraction of an inch on both sides of the stator, are permanently damaged and lose their field strength. By 80C, the resistance in the copper has increased about 23% or so, and the stator is now consuming (21^2)(2*0.68)= 600W in copper heating losses. The stator at this point is well into thermal runaway, and will destroy itself at rated power. The stator heating, also risks pushing the magnets well above their thermal limits as well. This is anything but responsible engineering … when many solutions exist to keep the product at safe operating limits by design.
Many resins, heated to several hundred degrees F, will burst into flames that will be fanned in this case by high winds and rotor windage. Before this design can be declared “safe”, the thermal runaway MUST be resolved by proper engineering, so that internal and surface temperatures in the generator do not exceed 170F at any time. There is high risk of structure and wild land fires using this product in high winds, when furling fails. Since the design violates various NEC and UL guidelines and standards, there is a high chance that even if a home owner convinced a code inspector to sign it off, that the insurance would deny damage claims since it is not a listed product. The broad litigation that will fall out from a loss of life, or major fire, will include anyone and everyone even remotely involved in the production, sale or promotion of this product.
I do not think giving this product such high marks is “responsible” advocacy for the wind power industry. This product has a high risk for loss of life, and major forest or wild land fires, if this design continues to be built and installed. The back lash from such a major loss, would be heavy regulation that is likely to kill the homebrew market by setting expensive regulatory and permitting standards that would damage the market for safe listed wind power products as well.
I tried to politely raise these issues last year in DanB’s forums, and found myself lynched by the DanB disciples, with TomW leading. I was kicked off the forums with DanB’s blessings. DanB has been warned better designs exist, and willfully has refused to listen to efficient designs that do not fail like this.
I do not need a wind generator in my back yard, but I will provide plans and guidance to anyone that would like to build a better, cheaper, efficient MPPT wind generator. A design that is much safer by sound engineering design principles, totally lacking in the DanB design. It’s interesting that designs that are 97% or better efficiency, also may need less copper and/or magnet material. So, selling poor 50% efficient designs, has the side effect of increasing magnet sales, which is where the profits are.
I will be releasing a broader analysis of the problems in the DanB generator design in a few weeks, that may catch some DanB advocates and partners in the cross fire because of their glowing reviews. We need “responsible” advocacy in this market, that includes looking well past what is accepted practice. Everybody has been doing it this way for a long time … and it’s just flat wrong.
John Bass
Senior Engineer