|02-15-09, 07:01 PM||#1|
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About small wind power, Part 1
Part 1: Reality Check
There's nothing quite as sexy in the homemade electricity world as a modest sized wind turbine. It moves. You can see it making power for you. Visions of cutting the grid umbilical or finding fat power-company checks in the mailbox can lead to fainting spells. But hold up -- before you plunge into the confusing, delicious, noisy, and sometimes hilarious world of small windpower, there are a few home truths you ought to consider.
First, we're going to set vertical axis wind turbines (VAWTs) off to one side for this discussion. They have their place -- grinding grain, lifting water, powering remote sensing devices -- but they do not and never can generate meaningful amounts of electrical power. Physics precludes it, and no amount of wishful thinking ever won an argument with Physics.
What's left are the small HAWTs, and a sorry bunch they are. What is "small wind"? Depends on who is talking, probably, but let's stick to turbines bigger than seven feet diameter but smaller than twenty. Larger birds require major, major investments and infrastructure; anything smaller than seven feet is a science fair project. Size matters: you can reliably gauge a wind turbine's output by its diameter, as output (both instantaneous and over time) is almost perfectly proportional to swept area. The obvious corollary: a slightly larger wind turbine may produce very much more power. An 11ft turbine sweeps twice as much area as an 8 ft turbine, tho each blade is only 1.5ft longer. They should be rated 1000W and 2000W respectively, tho more on those numbers in a moment.
A similar phenomenon applies to wind speeds in your location: power available in the wind is the cube of wind speed; very small increments of wind speed can make a huge difference in output. The air 100 ft above the ground may be twice the wind speed at head level and carry eight times as much energy. That's why we put wind turbines on tall poles and why we don't put them on rooftops. It's also why most locations are poor for wind power: in a rare instance of scalability, almost any size turbine is not worth having if your mean wind speed is below 11 mph. Winds below that simply lack energy, and no turbine of any design will produce meaningful juice. Very few places have average wind speeds above 11 mph.
Okay, so you have 12 mph winds, an eight foot turbine makes 1000W, multiply that times 24 hours ... ka-ching! Twenty-four kWh per day!
Well, no. That 1000W faceplate rating is about as honest as those 6hp ShopVacs that run on wall outlets. Both are 'peak' ratings and represent the amperage produced just before melting. Most wind turbines pluck their marketing numbers at "rated wind speed", often around 28 mph, which is where most of them furl or fly to bits. The wind may only achieve 28 mph 1% of the time. Most of its life, your turby will be mucking around at outputs roughly one fifth to one quarter of its peak power. That's a better number for projecting with. Given 12mph wind speeds and that 8ft turbine, you'll probably average 5-6 kWh per day into the grid, a little less into batteries.
About RPMs: an alternator produces lots more juice at higher RPMs than at low. That's one reason VAWTs don't work, and it's why very small HAWTs often spin at up to 1000 RPMs. There's a parameter in wind turbines called Tip Speed Ratio, or TSR. It's a function of blade design and simply measures how fast the fastest part of the blade moves relative to wind speed. Higher TSRs are theoretically more efficient, but in truth drag and turbulence quickly cancel out the gains. High TSR turbines tend to be noisy in normal operation and are hard on blades, bearings, and coils. Many companies rely on high TSRs to compensate for undersized alternators, because copper and magnets are expensive. Consensus recommends 5 to 8 is a good range of TSRs -- blade tips moving five to eight times true wind speed.
All turbines -- wind, water, or steam -- are miserable with engineering binds. Wind turbines are especially bothered by these tradeoffs, given the variable nature of their medium and the realities of location. The binds become acute as wind turbines get smaller. You can have a wind turbine that turns in light winds; it will be poor in strong ones. If the alternator gets up to voltage quickly, it will saturate quickly. If it spins fast, it is noisy; if it spins slow, it makes no power.
Your best hope (and there is hope, honest) is to match a wind turbine to your wind resource and electrical needs, put it on a fairly tall tower, and let it do its thing. They are in no wise perfect, but when the planets align they will deliver goodly chunks of power -- day and night, clouds or sun, winter and summer.
Part 2: Selection, siting, and costs