We hate to call certain businesses ‘fraudulent,’ but certain math and measurement errors have been coming up in exactly the same way for years now on web pages, and in national magazine advertisements, with over half a dozen culprits online. We frankly find it hard to believe that anyone purporting to be a ‘legitimate’ supplier to homebrew wind turbine enthusiasts would enter into the business without having the slightest idea of how Ohm’s Law, Alternators, and Blades actually work. But there they are…and they would love to part you with your money very quickly because it is possible that YOU (and THEM) have not done even the most basic homework on how wind turbines work!

DISCLAIMER: We don’t even TRY to compete with these folks. Our focus is on wind power education, teaching seminars, selling books about wind power, plans, etc. Yes, you can buy a wind turbine ‘kit’ from us, but the waiting list is long, and we turn down many requests. We are too busy teaching to build many turbines at all.

And, we will not call them “fraud” — instead, “uneducated and ignorant sellers who want your money.”

Here’s an example of a company that has no clue about what they are really selling:

This ‘wind turbine permanent magnet alternator’ manufacturer has no idea whatsoever about how to measure wind turbine output in watts. They take open circuit voltage (a mostly useless measurement in battery-charging systems, except for determining cut-in speed) and multiply it by amperage measured into a battery bank. This is meaningless data! The battery bank ‘clamps’ the system voltage to its own level, so to actually measure watts (volts x amps) you can only use the voltage your battery bank is at. Let’s call it 13 volts for a 12 volt battery bank to cut these people a volt of slack (increased instantaneous power output due to increased system voltage during charging).

These data points were taken from an actual website of a ‘wind turbine PMA supplier’ who shall remain unnamed. There are many others just like them. Note what happens when you take their data (freely available from their website!) and PROPERLY calculate Watts output, versus claimed:

Actual watts vs. claimed, anonymous internet seller

This is basically absurdly BAD alternator performance! This thing is not suited whatsoever for any sort of wind turbine use.

Here’s how the RPM curve should look for a decent wind turbine alternator. 2 of ours are on the chart. It’s basic, but is actually measured from ours (the 2 different versions in our book). Hugh Piggott’s alternators perform similarly, as do most commercial wind turbines. Won’t be exactly right with all turbines on this curve, but they all should follow it pretty closely (in the 9-10 foot diameter range for rotors):

What’s wrong with the very first diagram, of watts vs. RPM? What’s not right there?

Well…. other than they are not measuring watts correctly, and that there is no curve for power IN from the shaft…..Why would anyone care about alternator performance above 500 RPM with a 10-foot diameter wind turbine? You are getting into an amazingly high Tip Speed Ratio to do that. How could you possibly achieve that high of a TSR in a 10-foot machine? Why would you even want to try? Even with a 3 or 4 foot diameter rotor, this is still an absurdly high RPM. It looks good on the website but is really meaningless information for the homebrew wind turbine builder. Name a single commercial wind turbine that commonly spins at 1500 RPM?

Also, The graph that we measured above is NOT intended to imply more power coming out than going in.  Remember, the “power in” line on this graph is from a theoretical ‘perfect’ turbine that works right at Betz — no such thing exists. The graph simply shows a real-world alternator attempting to take more power out of the wind than is available, thus stalling the blades. This is adjusted for with changing the resistance of the line (Homebrew Wind Power, page 264).

The end result of it all is what a wind turbine power curve should look like — Wind MPH vs. Watts output. The RPM really depends on your swept area, rotor diameter, etc. In fact RPM vs watts data (above) is only useful in the alternator matching stage of your wind turbine design…..your blades themselves, their length, pitch, etc. will give you this final data set, which is what you are after in the first place. This one is from our 2 different 10 foot diameter turbine designs, and tracks very closely with commercial turbines of similar size:

THE BOTTOM LINE:

Don’t let anyone hoodwink you into buying their product. Do your OWN math, do your OWN homework, and see what the scientific and engineering literature says. Don’t believe US, the authors of this book, either, we make math errors too. But at least we know how to measure freakin’ Watts of output!….and feel free to correct us if we have made errors here!

DAN FINK

DAN BARTMANN

We get bombarded here with questions about short towers:

• I built your turbine, but they won’t let me put it up on a tall tower, is it OK to put it up on a 20 foot tower in the garden behind the house?
• So and So’s website said that their wind turbine design works better than yours right on the ground!
• Such and Such wind turbine design “eats up turbulence for lunch,” it says so on their website and since it’s on the internet it must be true! And some of the text in their website blinks!
• “They” (or “everyone”) say that VAWTs (vertical axis wind turbines) work better on short towers and in turbulence than HAWTs (HAWTs are what we build in our book).
• “They” say that if I buy their turbine, I can put it on my roof because the wind speed increases going over the rooftop, just like an airfoil. Why doesn’t your turbine do this?
• There’s a very reputable big company that sells a grid tie wind turbine on a 30 foot fiberglass tilt up tower, and it’s only a 30 foot tower. They obviously know what they are doing, so I’m going to buy one and line my bird cage with pages from your book. Thanks for wasting my $39.95. (OK, we made that one up, but we are sure we’ll be getting emails like that soon).

It’s all Hokum! Hogwash! People are making this “data” up for good reason–most folks, when considering a wind turbine, do not want to or are not allowed to build tall towers. And that really puts a big crimp in wind turbine sales. There’s no guarantee from the manufacturer of energy output per month with any wind turbine, it depends on your wind regime. And once you sign on the dotted line, all you can do is scratch your head a year later and wonder why your turbine is putting out only a tenth the energy per month that the website and friendly sales rep said it would. Frankly, the authors spend as much time convincing folks that wind power is NOT right for them as we do helping folks with wind turbine questions.

The only published, scientific test results out there completely disprove all of this nonsense about rooftop turbines and short towers. Look at it this way — the earth’s surface causes “friction” to the wind. This has been extensively field tested and computer modeled for decades now. Friction slows the wind down. And the power available to you in the wind goes up by a factor of 8 when you double the wind speed!

Here are a few of those articles, from reputable wind power scientists:

Hugh Piggott’s measurement of urban wind speed over a roof top at Wind Speed Measurement in the City.

Comments by Hugh Piggott “Rooftop wind turbines are a load of nonsense”

Rooftop Turbines: Rooftop Mounting and Building Integration of Wind Turbines
By Paul Gipe

The case (or not) for Roof Mounted Wind Turbines by Nick Martin

Andy Mahoney’s Rooftop Mounting Failure at http://navitron.org.uk/forum/index.php?topic=942.0

Some test data from the authors:

Co-author Dan F and Otherpower.com Stator Specialist Rich C fly the exact same little wind turbine, the 7-footer that’s described in detail in Homebrew Wind Power. We both really enjoy this little machine.

We had no way to easily make this a scientific test…but we DID measure kilowatt-hours per month from identical turbines at sites only 1/2 mile away, during both high-wind and low-wind months of the year.

• RIch’s turbine was on a 50 foot tower, with hub height about 10 feet above the nearest tree within 300 feet. However, the hillside stretches up above his turbine, so there was turbulence caused by that. It was a ’so-so’ location by normal standards (which say your turbine must be 30 feet above anything within 300 feet in any direction) but far less obstructed than DanF’s, below.
• Dan F’s turbine was on a lattice tower, 40 feet in the air, right next to his house. The turbine is above the house roof peak by about 10 feet. There are numerous trees in the area taller than this within 100 feet. DanF broke all the tall tower rules he talked about in Homebrew Wind Power because he didn’t listen to his own advice wanted to prove a point about the importance of tall towers.

During the same period of 3 months, Rich’s identical 7-foot wind turbine made over 10 times the amount of energy per month (in kilowatt-hours, the only really important measurement) because of Rich’s fairly OK tower site, compared to DanF’s bad tower site.

Ten times the energy……

What were you THINKING, DanF?

(authors note — DanF was thinking that it would be cool to have an easily accessible tower top where he could test different small turbines and swap them out with little effort. But that’s not much use when the tower is not tall enough)

Well, at this time we have no errata whatsoever for the book!

Thanks to astute readers we are finding a few errata! Nothing critical.

Our Stator Master Rich C found a typo in the Glossary, but that’s about it so far.

If you are reading along and find a typo, please post here and let us know. We’ll fix it for the next printing. Dan F is keeping a master copy of the book with all typos and such noted for correction.

Click the comments balloon above for the list of errata as it gets longer, and to post your own.

Editors note: This really is coming in handy, thanks everyone for pointing out these errors!

OK, well, there is one “sleeper” in there. It was left in to make sure our editors were paying attention — It’s a photo of a common electrical part from the hardware store, but the part shown is not what is described in the caption. A free autographed copy of the book to the first reader that spots it and posts here.

We attribute the lack of major corrections and typos to the incredible skill and attention to detail incredible luck of the authors and copy editors. Thanks, everyone!

DAN F