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Roof-top Wind Farms by Chris Devaney

continued from page two

20-25 MPH

The roof is alive now.  This is my favorite part of the spectrum. There is hardly a break for the orchestra.  The Musicianís Union is having a cow!  Thereís always at least one genny spinning, more often, all three; each competing for center stage.  Strong gusts are frequent and accelerate the rotors vigorously.  This is power!

The Air-Xís are howling, working hard, almost non-stop.  The short-bladed gennys spin furiously now, I canít even see the blades.  They seem to be most comfortable at this speed and they perform better than I expect.  The bearing whine has peaked out, but itís hard to say for sure, as it is overpowered by the rising voice of the wind-howl.  They are a little less likely to lose the wind at this speed but when they do, itís dramatic.  As the sleek body spins out, the rotor rapidly loses energy and the blades decelerate.  Itís eerie, it seems like the Air-X wind-howl shouts itself to an instantaneous whisper...sound manifests where there is no sound! 

My best guess is that the human brain, in order to compensate for the abrupt stimulus change, invents a sound to deal with the void.  In any event, the void doesnít last long for as soon as the unit spins out, its own momentum carries it around in a full circle smacking it right back into the wind. 

The rotor instantly responds and accelerates wildly and the music continues.  Watching, listening, feeling the Air-Xís action, I imagine a sleek, high-spirited Arabian running at a full gallop . . . head down, mane plastered back, firing across a meadow with her large black eyes ablaze with fury.  Sheís running for no other reason than that she smells the wind, and because thatís what sheís bred to do.

Meanwhile, on the southwest side, the big green Mallard gets a five-foot armful of air and responds to the challenge.  The previous sewing machine hum breaks off and transforms seamlessly into a growl.  Now the air pushing through the blades sounds like the fan on a big American-made, early 70ís, V-8 Cadillac with 10 times more air flow passing through.  Smooth, powerful, surprisingly responsive, it feels like he has power to throw away.  

Output: Power output is fine in this regime. The Air-Xís are pumping out a little over 300 watts each when steady. Thatís feeding in 25 amps or so per unit.

The Mallard, the big green Cadillac, still lags behind at around 15 amps at best. Odd, because it just sounds so powerful! 

I hear many different opinions from folks about the viability of a small wind farm.  In discussing it, sometimes I have to deal with irrational fears like helicoptering the roof off or the fear that it will attract bats (I donít know where that one came from).  Other concerns are more realistic: like lightning strikes or potential EMF interference, etc.  Most of the time the discussion boils down to: ďhow much power do you get from those things?Ē

To put things in perspective, weíre not talking about huge amounts of power here.  The Air-Xís, under the right (but rare) conditions, will over-power and produce a maximum of 550 watts each, the Mallard is rated at 800 watts and may actually put out more.  Combined, the power output is way shy of 2 kw.  But, the wind doesnít blow constantly and when it does it is a variable resource.

A single wind generator or a small wind farm is a great way to get started in the energy production arena.  Depending on the setup, this could easily power a single room with lights, a modest amount of computer time, TV and a few other goodies in turn.  A single room powered only from alternative energy sources while the rest of the house is still grid tied not only works to reduce the monthly bill, but also begins the mental and lifestyle adjustment toward conservation that is crucial to the successful deployment of a non-fossil fuel or nuclear based society.

25-35 MPH

Weíre on fire now! Thereís a big whopping difference in the energy available in the wind from 25 to 35 mph.

The wind itself is howling now and the Air-Xís howl right back at it not giving an inch except when they get thrown out of the stream.  A lot is going on inside the gennyís at this speed.  Itís like the brain telling the body, ďEat any more of that double chocolate fudge cake and Iíll make sure you regret it!Ē  I donít always listen, neither do the Air-Xís.

In principle, the Air-Xís max out at around 28 mph and their brains, the micro-controllers, attempt to slow the rotor down.  Output power, as a consequence gets throttled back.  But the blades are furious.  As fast as todayís electronic feedback circuitry is, it canít always keep up with the wind, and with a good strong gust, the blades often accelerate way past the pre-programmed shut-down speed. I n addition, the body has difficulty in maintaining a stable heading in the fierce winds and it oscillates slightly about its vertical axis as it gets buffeted by the turbulent slipstream on the downside of the foil.  You can see it happen as it wiggles and shudders, occasionally letting go and spinning right out of the wind with a howling sigh.  In this wind regime, I start to get tense.  I canít tell if itís excitement or anxiety - they seem sometimes to be one and the same.

At the upper end, the Air-Xís are wild.  I feel the dawn of a strange uneasiness about to unfold.  Now, they are running like a thoroughbred, ears pinned back, nostrils distended as if snorting the smoke from an unseen fire.  The Air-Xís are fighting to hold their space in the wind.  The dual howls combine to produce a soundtrack that could easily accompany the climax of a Hollywood thriller-chiller.  Intensity builds and builds, the excitement is almost too much to handle, and with one last furious gust of air... just as Jack Nicholson breaks the window and is about to say ďHeeeereís Johnny!", just as the east side Air-X aligns it self back in the wind... just as...no, no, a thousand times no...FLUTTER!

Egad! The first time you hear a carbon-fiber composite blade go into a flutter state youíll get goose-bumps.  All over!  Itís like a door being unexpectedly slammed right in your face.  It only occurs for a brief few seconds and usually in a sequence of two or three blasts separated by a second or so.

When we were kids, we used to shove balloons into the rear spokes of our bicycle and peddle like Satan himself was chasing us.  If you can recall that auditory delight and multiply it a hundred, you have some idea what flutter sounds like.

The sudden onset, the quick shift from a fast moving aria to the auditory chaos of flutter, makes this a most objectionable sound.  You can bet this will wake you up.  It may even resurrect the recent dead.  For the brief second or two that it lives, it imparts a sense of self-destruction.  But not to fear: Flutter was actually a designed-in safety feature of early Air-Xís to limit the rotational speed of the unit.  It does a marvelous job at hauling down the speed of the rotor, but at the cost of a highly unpleasant auditory experience.

 
 

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