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Roof-top Wind Farms

- The Symphonic Sounds of a Lifestyle

continued from page 1

by Chris Devaney

10-15 MPH

10 to 15 mph wind is typical here where I live and it’s a rare day when I don’t get any wind in this regime.

Now, the Air-X’s harmonize pretty well, taking turns dominating the wind song as they increase and decrease in speed quite regularly.  Hardly ever do they spin continuously in this band.  But when they do, it’s pleasant.  The sound is now above a whisper, more like moving wind through the tops of trees in the summer.  Even at different speeds, hence, different pitch, they’re not discordant.  It’s an ear-pleasing treat; a low-key tune that lulls and fades, rises again, then lulls and fades once more and so on, well into the night.  It makes it easy to sleep, easy to forget that they are even there.

Note:   In this regime when both Air-X’s cycle up and down in a variable wind it produces an interesting whooing ghost-like sound. It could frighten a young’un at first.

Filling in the bottom end is the Mallard.  It’s awake now and has taken over the bass cello seat in the orchestra.  Still, there’s too little wind to keep it performing with any regularity.  Nevertheless, when it plays, the Mallard “Ace Bass”, adds a peculiar depth to the wind-song lullaby.

Output: The LEDs on the Air-X’s are lit up as they spin. At the upper end of this band, the Air-X’s are putting out about 60-70 watts of power. I’ll see the ammeters on the gauges bounce around the 5-ampere tick mark. Not a lot of power, but who is going to throw it away?

Ace Bass, the Mallard 800, is having problems moving his needle. I think he’s still getting dressed.

“Bulls-eye” Bradley came by one day and informed me that “Them things’ll, likely as not, helio-co-pop-ter your roof into the middle of next week some day.”

The Air-X’s are mounted to a mast secured to a gable wall and extend 6 feet above the peak of the roof— one on each side of the roof (east and west). The Mallard is mounted the same way on the southwest corner on the lean-to addition and is shielded by the gambrel roof on the north side. It also is 6 feet above the roof. None are attached directly to the roof. The standard roof-mount kit includes rubber vibration isolators that install between the mast and the structure.  It’s a good idea to check the isolators each year as they are prone to crack and deteriorate. I do this, and I’ve never had my roof or any part of it helicopter itself into another place and time.

15-20 MPH

The song gets interesting here.  This is a powerful breeze.  The deep, bearing-rumble from the Air-X’s are completely gone and replaced by a whine.  The air sound is now a low howl as the foils slice through the wind.  With eight times more power available in the wind at 15 mph than at startup, the units are much more active.  Their aerodynamic bodies and short fish-like tails make them dance comically in and out of the wind.  As a result, there’s a dramatic rise and fall in the dominating wind howl and an occasional solitary bearing thump if one yaws out of the slipstream.  One will be spinning hard and I can hear the air madly rushing through, while its sister on the other end of the house, loses its wind, spins out and decelerates rapidly almost to silence.

Meanwhile, the powerful sounding Mallard gets some wind, and he gears up smoothly.  Initially it sounds like a large fan pushing air around.  But quickly, the fan sound is replaced with what sounds to me like a well-tuned sewing machine.  A big one.  One on steroids...a 16 cylinder Bernina affixed to the top of a ’71 Porsche 917 Short-Tail rocketing out of turn 8 and about to accelerate home.  No howling, less prone to rapid acceleration and deceleration or falling out of the wind, the big green sewing machine is getting ready to breathe some fire!

When all three are spinning it makes an interesting, continuously varying, wind-song with hardly any repeat measures in the composition.  It’s moved beyond a lullaby, beyond a subdued symphony, now its more like a fast-paced jazz ensemble, free-wheeling and making it’s own rules.  The not-so-faint bass sounds from the Mallard fill out, maybe even temper the more active, driving sounds from the Air-X’s.

Output: Power output is developing quite well now. The Air-X’s are producing much more power than the Mallard. I see about 12 amps being fed to the batteries from each Air-X and of course, much more from the frequent sudden gusts.

The Mallard lags noticeably in the power curve. I’ll see about 4-7 amps from the big green sewing machine.

Roof-mounting has some drawbacks performance-wise.  Friction causes air to move slower around an object.  Wind shear results in a wind velocity profile where the velocity increases according to a 1/7th power law from the ground upward.  What this means is air close to the ground (or the roof) moves much slower than 30 feet above it.  The net result is that there’s less energy captured by a roof-mounted generator because out of necessity, it is much closer to the effective ground (the roof) than a tower-mounted unit would be.  Add to that thermal effects that create a strong eddy effect in the air confronting (and immediately behind) a spinning rotor.  As moving air picks up heat from a hot roof the smooth flowing streamlines break down into chaotic eddies that can adversely affect the directional stability of the wind generator.  This is one reason the Air-X’s spin themselves out of the wind for no apparent reason.  At the onset of turbulence the gennys will give the visual impression they are “hunting” for wind.

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