| On-farm wind power: Part 2 of a three-part series Farming the Wind By Dan Sullivan |
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When it comes to evaluating whether wind-generated electricity makes sense for you and to selecting the best equipment for your particular application and site, a host of choices and variables are worth considering. Will the savings be worth the up-front investment? (Generally speaking, there is at least a break-even point compared to conventional electricity over the estimated 20-year lifetime of smaller-scale wind machinery. As you go bigger, the savings over conventional power can be substantial, and don’t forget the huge savings to the environment.) Should I go with new or remanufactured equipment? (If a quality rebuild from a reputable company is available to suit your needs, you will likely save a bundle). Should I choose a system with two rotor blades or three? Ah, that’s a very good question. The spin on rotor blades
By Sagrillo’s own admission, the advice is somewhat counterintuitive. Blades can account for up to 40 percent of the cost of an entire wind system, so two blades over three can offer significant savings. And two blades spin faster under the same wind than three, meaning increased rotor speed and more electricity produced as coils of wire inside a generator create an electronic pulse each time they spin through a magnetic field. But like a great vessel on high seas, a two-bladed system is prone to the effects of yaw—in this case the generator pivoting on its bearings as it tracks the wind— “causing enormous strain on the generator, the blades, the welds and the fasteners,” explained Sagrillo. While three blades results in some level of inertia, “every time you add a blade it causes a bit of turbulence for the one behind it,” said Sagrillo, the stops and starts, or blade chatter, that a two-blades system experiences as it tries to maintain its plane of rotation make it too prone to damage from stress. Preventative maintenance
In his 17 years installing and inspecting wind machinery, Sagrillo said, the most common breakdowns are a result of flagrant neglect, such as failing to tighten a loose bolt. “Nobody is on the tower for four, five, or six years and guess what?—the turbine explodes. A bolt loosens, a nut falls off, another bolt loosens, and pretty soon the blade is in the tower. You’ve got two or three thousand dollars damage, and it all could have been prevented by tightening a bolt. The life of the system is directly related to the involvement of the owner. You should be checking welds, fasteners, nuts and bolts, looking for cracks…and if you find something wrong, take care of it.” Sagrillo also warned against going in for “maintenance free” gimmicks such as sealed bearings. “Sealed bearings are a sales pitch, he said. “From a maintenance perspective, there’s nothing advantageous to them.” Evaluating your wind resources Winter is the optimal time for wind power, Sagrillo said, because more molecules are present in the air. This, he said, is why you can expect to harness 13 percent more electricity from the wind in wintertime under the same wind speed and at the same location than at any other time of year. It’s also why wind power and solar power make such a great combination. “The sun picks up when there’s no wind around; they are remarkably complimentary, wind power and photovoltiacs.” (As well as temperature, Sagrillo said, altitude and humidity also affect the density of the air.)
It is not uncommon for two relatively close spots to differ in average wind speed by a few miles per hour or more, Sagrillo said, adding that even a small increase makes a monumental difference in a location’s capacity for generating electricity. This, he said, is due to the formula P=1/2dAV3 (where P=power, D=density of the air, A=area, and V=windspeed). Because wind speed is cubed, Sagrillo said, even in incremental increase in wind velocity becomes hugely significant as far as capacity to generate power. (An increase from 8 to 10 miles, he said, can result in twice the power generated.) And the one way to increase wind speed at any given site is to go higher. “As wind speed increases the amount of work that can be done increases as well,” Sagrillo said. “And as you increase away from the surface of the earth, wind speed increases dramatically. That’s why wind farms are in the 200-foot range, and they’re going to be approaching 300. To illustrate the cost-effectiveness of going higher, Sagrillo calculated the cost of putting two small-scale electricity-generating windmills on 30-foot towers (around $63,000) versus setting up the same model windmill on a 120-foot tower ($36,000) in order to generate roughly the same amount of power. “If you want to increase your output, increase your height,” Sagrillo said. “It’s always, always, always cheaper, and the reason is that the fuel isn’t down low; it’s up high.” How high can you go? A tower can be anywhere from half to four times the cost of the rest of your system, Sagrillo said. “What determines the tower height is the obstacles in your area.” “There are essentially two enemies to a wind generator. The first one is called ground drag, getting caught up in the earth’s zone of friction. You get into a laminar flow, just air over air, above the surface of the earth.”
Computer generated models help determine the wind resources at a specific site, Sagrillo said, and an investment in a professional wind audit is a sound idea before investing in a lot of expensive equipment and positioning it wrong. (Sagrillo himself teaches wind-assessor training for the Midwest Renewable Energy Association.) “Everybody’s got solar, it’s a democratic renewable, but wind is very site-specific. The problem with wind, you hear, is that wind is very unreliable as opposed to solar or hydro. But it’s not about the reliability, it’s about the tangibles. In reality, we don’t know on a day-to-day basis what the wind is going to do, but we do know on a seasonal basis.” This, he suggested, makes wind every bit as reliable as solar. [Editor’s note: A recent AP story posted on the Environmental News Network http://www.enn.com/news/2004-04-28/s_23255.asp tells of a new project in Norway where a combination of wind turbines, hydrogen generators, and fuel cells produce clean electricity year-round.] “The other enemy of a wind generator is something called turbulence.” [Turbulence is a swirling agitation of the air as the wind hits a physical barrier such as a building, tree, or hill]. “We actually use turbulence to prevent some wind-blown problems,” Sagrillo said, giving the example of planting trees for windbreaks to prevent erosion and snowdrift. To keep turbulence from interfering with rotor performance, Sagrillo said, “the rule of thumb is that all three blades have got to be a minimum of 30 feet higher than anything within 500 feet, or 30 feet above the tree line, whichever is higher. “And the thing you have to remember about trees is, they grow—towers don’t. So you’ll need to know the mature height of your trees in 20 to 30 years, the life of the wind system.” If the mature tree line is 60 feet and the blade length is 10 feet, that means a tower height of 100 feet minimum, Sagrillo said. “Below that you are going to have turbulence, and that turbulence is going to eat up your wind generator.” Sagrillo sketched a number of geographical scenarios to demonstrate how topography, prevailing winds, and accompanying turbulence might affect location. For a single hill rising from a plain he selected the peak as the optimum location and the base of the lee or downwind side (where turbulence would be greatest) as the worst. For a bluff facing prevailing winds, he suggested placing the tower at least 200 feet back from the ledge, just outside of the zone of turbulence. “It depends on the prevailing winds,” he said. “You want to know the prevailing winds and capitalize on them in fall, winter, and spring. You are looking for the most consistent winds.” (There are two exceptions to the fall, winter, and spring rule, Sagrillo said: the southeastern part of the United States and the far southwest, where wind can be a considerable resource in summer as well.) Tower of power “The three most common mistakes” when installing wind equipment, Sagrillo quipped, are “too short a tower, too short a tower, and too short a tower.”
Residential and small-farm-size towers come in three general types, he said, free-standing, guyed (supported with cables or guy wires), and “somewhere in between.” Free-standing towers typically have three legs (sometimes four), are purchased and assembled in 20-foot increments, and are supported by diagonal and horizontal latticework or trusses (and lots of concrete below). “This is the most expensive tower you can buy,” Sagrillo said. These towers taper up from the bottom; the lighter gauge construction material used, the more space the base of the tower will occupy. These towers also have ladders built into them so that the wind equipment can be accessed and serviced once it’s installed. New heavy duty towers, Sagrillo said, sell for about $80 a foot while used ones go for half that price or less. Used light duty towers cost about $15 to $25 per foot, he said, while new ones cost about twice as much. Guyed towers are considerably less expensive but also take up more space down below, Sagrillo said. They also weigh much less than a free-standing tower, he said, offering the example of a typical 10-foot section of a lattice-style guy tower weighing in at just 70 pounds. “The guy wires go out typically about 75 percent of the height,” Sagrillo said. (This compared to a typical light duty free-standing tower where the height-to-base ratio is 4 or 5 to 1 and a heavy duty free-standing tower where the base to height ratio is just 9 or 10 to 1. Guy wires attach to the pole at various incremental heights and go out to three equidistant points forming an equilateral triangle around the base of the tower. Guy towers also require the least amount of concrete, are easy to climb (if they are of the lattice type), and can be had (including all hardware) for about $15 a foot (new ones for two to three times that figure). The biggest drawback of guy towers besides space requirements (if that happens to be a factor), Sagrillo said, is that those available on the market today are only designed to handle up to about a 10kW system. The “somewhere in between” tower to which Sagrillo referred is a tilt-up tower—a good choice if you are somewhat acrophobic—typically costing somewhere in between the price of a free-standing and a guyed tower. “The advantage of a tilt-up tower is you don’t have to climb it,” he said. The trouble is, you can’t climb it, and breaking one down is no small task. “Not being able to climb it can be a problem if you just want to check something,” Sagrillo conceded. Tilt-up towers are supported by guy-wires going off in four different directions (forming a square around the base rather than a triangle) and are typically raised and brought down with the help of a gin pole (a braced lifting arm) and a vehicle, Sagrillo said. To wrap up his lecture on tower types, Sagrillo laid out the costs of a typical 120-foot guyed tower ($6,800, plus another $1,000 for concrete and rebar), a 120-foot tilt-up tower ($8,500 to $9,000, plus $3,000 in concrete and rebar) and a 120-foot free-standing tower (around $19,000, plus an additional $5,000 to $6,000 for concrete and steel). (These are tower-only estimates.) Although the wind is a force for generating electricity, it’s also a force to be reckoned with. “The wind sees the tower as a lever,” Sagrillo said, reminding us of the familiar and relevant equation that work equals force times distance. “The wind sees an opportunity to knock that tower over; it’s trying to knock it over, and it’s trying to pull that concrete out of the ground.” Coming up next: Lightning, stray voltage and earth currents; the downside of wind power; wind energy and the Farm Bill; incentives, regulations, programs and support. Dan Sullivan is senior editor at The New Farm. Mick Sagrillo
is owner of Sagrillo Power and light, a Wisconsin-based consulting
firm specializing in home-sized wind-turbine technology and educational
workshops. |
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