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Roof-top Wind Farms

- The Symphonic Sounds of a Lifestyle

continued from page 2

by Chris Devaney

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 Musician’s Union is having a cow!  There’s 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-X’s are howling, working hard, almost non-stop.  The short-bladed gennys spin furiously now, I can’t 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 it’s 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, it’s dramatic.  As the sleek body spins out, the rotor rapidly loses energy and the blades decelerate.  It’s 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 doesn’t 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-X’s 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.  She’s running for no other reason than that she smells the wind, and because that’s what she’s 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 70’s, 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-X’s are pumping out a little over 300 watts each when steady. That’s 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 don’t 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: “how much power do you get from those things?”

To put things in perspective, we’re not talking about huge amounts of power here.  The Air-X’s, 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 doesn’t 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

We’re on fire now! There’s 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-X’s 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 genny’s at this speed.  It’s like the brain telling the body, “Eat any more of that double chocolate fudge cake and I’ll make sure you regret it!”  I don’t always listen, neither do the Air-X’s.

In principle, the Air-X’s 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 today’s electronic feedback circuitry is, it can’t 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 can’t tell if it’s excitement or anxiety - they seem sometimes to be one and the same.

At the upper end, the Air-X’s 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-X’s 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 “Heeeere’s 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 you’ll get goose-bumps.  All over!  It’s 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-X’s 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.

Meanwhile, the race gets hot.   The big green Mallard is like a buffalo in a storm.  It points its head rock steady into the wind and does what it is programmed to do.  The buffalo survive, the Mallard generates electricity.  The massive blades gulp the howling air and do so quite easily.  A slight struggle arises as the wide sheet-metal tail-vane momentarily fights to stay in the wind.  It wins the battle and the rotor spins until the wind gives up.  The Mallard is very comfortable at this speed and wouldn’t mind seeing it on a more constant basis.

Output: Power output is as good as it gets.  Twin Air-X’s are outputting 400 watts each, occasionally stretching their limits to 550 watts in brief gusts.  The ammeters are pegged at 30 amps, real world amperage might be around 45 amps.  Voltage is pretty well controlled at 14.7 volts or slightly less.

The Mallard, the shaggy monarch on the southwest roof, is starting to shine.  Still well below the Air-X output, it’s generating 15-20 amps at the high end of this wind band at about 14 volts.  Although a microprocessor controls the voltage output of the Air-X’s, the Mallard will continuously feed electrons to the battery as long as its voltage is above battery voltage.  It cares not one bit about what is going on inside the Air-X.

Could be, I begin to think, this beast needs some control.

The Air-X units have an integral charge controller inside the fish-like body.  It senses the voltage on the batteries and limits its output to avoid overcharging them.  It is user adjustable within a narrow range.  The Mallard is an uncontrolled device.  If the voltage generated in the unit is higher than the battery voltage, it charges the batteries whether they like it or not.  Overcharging batteries can cause damage and produce hydrogen gas in the process.

One type of external charge controller often used to regulate uncontrolled sources is a diversion load controller or dump load controller.  It is placed on the battery bank and when the batteries are full, it diverts the excess charge to an electronic bottomless pit like a hot water heater.  When the battery voltage falls again to some percentage below full charge, the diversion load shuts off and battery charging resumes.  I have one of these, I just haven’t installed it yet.  

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