Wind Generator Towers Mick Sagrillo copyright 1991 Mick Sagrillo Towers for wind generators come in a wide variety of shapes, sizes, heights, and prices. However, the often forgotten purpose of a wind generator tower is to get the wind generator way up there, and, most importantly, to keep it up there. "Keeping it up there" should, therefore, be the prime consideration in selecting a tower for a wind generator installation. Styles Towers for wind generators come in two basic styles: freestanding and guyed. A freestanding tower is just that; no wires or cables to help keep the tower in an upright position. They are self-supporting. These towers include the 3- or 4-legged lattice or truss-type of towers, and either metal or wooden poles. Guyed towers require the use of cables or guy wires to keep them standing. Most television and radio towers fall into this category. They can also be either lattice-type, or wooden or steel poles. Guyed towers are generally less expensive than freestanding towers, but, because of the guy wires, require considerably more space. 3 & 4 Leggers Most freestanding towers are of the lattice or truss style (figure 1). They are either 3- or 4-legged, with diagonal and/or horizontal braces holding the legs together. These braces are what give the tower its lattice or truss look. They also provide the strength and rigidity to keep the tower upright. Lattice or truss towers are tapered from top to bottom. Towers made with light gauge metal will be tapered more than heavy-duty towers. Light-duty towers will have a height to base ratio of about 4 or 5 to 1. This means the base will occupy, from leg to leg, one fifth of the distance of the height. An 80' tower would, therefore, have a span of from 16 to 20 feet between legs. Heavy-duty towers typically have a height to base ratio of about 9 or 10 to 1. An 80' heavy-duty tower would have a span of only about eight or nine feet between legs. The area that the tower base occupies only becomes important if space is a consideration. Towers that use angle iron for the legs will be 4-legged in order to get the diagonal and horizontal braces to bolt properly to the legs. The legs of the 3-legged towers are usually made of specially formed 120 degree angle iron, or round pipe or tubing. Lattice or truss towers always have some sort of ladder built into them so that you can climb to the top and service your wind generator. New heavy-duty towers sell for about $80 per foot, while used ones go for about $30 to $40 per foot. Used light-duty towers will sell for about $15 to $25 per foot while new ones sell for about double that price. Steel Poles and Tubes Metal structural steel tubes can also be used for wind generator towers. An example of this type of tower can be seen all along our interstate highways. Gas stations often use metal tubes to get their signs high in the air. These tubes are also frequently used by utilities for their high lines. They are very heavy duty, and usually taper from about one foot at the top to to three or four foot in diameter at the bottom. They usually incorporate some sort of removable steps for climbing to the top. There is no hard and fast rule for prices on these towers. Used ones are very hard to come by. New ones are usually sold by the foot while used ones sell by the pound, like scrap steel. $20 to $25 per foot is not an unreasonable price to pay for a used steel pole, as they sell for three to four times that new. Get a second opinion on any price quote. "Telephone" Poles Wooden poles, the creosoted kind used by utility and phone companies, can sometimes be used for wind generators. Unless you guy the pole, you should only put a small wind generator on one of these poles (more about this, and why, later). By small, I mean nothing larger than a generator with an eight or nine foot rotor. Larger rotor diameters will cause the wooden pole to sway. While this will usually have no effect on the pole itself, it can have a considerable impact on the wind generator and how it works. It can also be very unnerving! Wooden utility poles are sold by class, the class indicating its strength. Get the strongest that you can afford. Average price for these poles is in the $1,000 dollar range for a 70-footer, depending on the class and the utility you're dealing with. Their one big advantage is that they can almost always be obtained locally, thereby minimizing shipping costs. A disadvantage is that they can be very tough, and dirty, to climb. Unless the pole was in excellent condition and came with a guarantee of some sort, I would never consider using a used wooden pole for a wind generator. Any internal cracks could prove disastrous! Lattice/Guyed Towers Guyed towers of the lattice style use considerably lighter materials in conjunction with supporting, or guy, wires to get the job done. A 10 foot section of Rohn 45G tower, the most commonly used guyed tower for wind generators, weighs only 70 pounds. A 20 foot section of a freestanding Rohn SSV tower will weigh in at between 500 and 800 pounds! This is due to the tower's geometry. The secret to the guyed tower's strength is the guy wires (figure 2). Cables stretch from several points on the tower to three different equally-spaced directions away from the tower. The top guys keep the tower erect, while lower guy wires keep the tower rigid and prevent oscillation or wobble. Ideally, the guy wires should reach the ground at a distance from the tower base equal to 3/4's of the height. For example, an 80' tower would have the guy anchors spaced 60' from its base. This distance can safely be reduced to 1/2 the height of the tower, if necessary, without upgrading either the cables or the footings. While the base of a guyed tower is smaller than that of a freestanding tower, they none-the-less take up considerably more space due to the guy anchor locations. Lattice/guyed towers look like three-sided ladders. The three legs of the tower are parallel to each other, and in the case of the Rohn 45G, only 18" apart from base to top. Holding the legs together are evenly spaced horizontal and diagonal braces (figure 3). These braces make climbing this type of tower very simple. Used lattice-type guyed towers cost about $15/foot with all associated hardware: guy brackets, cables, turnbuckles, and anchors. New equipment runs two to three times used prices. Guying Poles and Tubes Metal tubes and wooden utility poles can also be installed with guy wires. By using guy wires, an otherwise light duty pole can be strengthened enough for use as a wind generator tower, within reason, of course. What we're trying to eliminate is excessive sway. If a tower will not support the static weight of a wind generator AND the weight of one or two people servicing the unit, then guy wires are not going to improve the situation. Tilt-up Towers An interesting variation of a guyed tower is a tilt-up tower (figure 4). Tilt-up towers are typically made of steel well casing, although any strong steel tube or wooden pole would work. The advantage of a tilt-up tower is that you don't have to climb it. You just tilt the tower along with the wind generator down to ground level. All service work can be performed safely on the ground. If you are deathly afraid of heights, then a tilt-up tower is just the ticket! Tilt-up towers have a built-in hinge at the base for tipping up and down. The raising and lowering is done with the help of a tractor, truck, or 4- wheel drive car. Fancy set-ups have their own built in winch to do the job of the vehicle. Tilt-ups have a shorter "tower", called a gin pole, attached at right angles to the tower that aids in raising and lowering. (Design and construction of a tilt-up tower will be covered in a future article.) They also have four sets of guy wires, rather than three sets like a conventional guyed tower does: one set is on either side of the tower to keep it from swinging from side to side while being raised and lowered; one set is used to pull the tower in an upright position and lower it; and the last set is opposite the front set and prevents the tower from tilting too far forward. While tilt-up towers are the most convenient to use, they do have a down side. Raising and lowering them can be a hair raising experience until you get used to it. If the tower, guys, and footings have been undersized, you'll find out during raising or lowering when the whole thing comes toppling down. Raising and lowering is rarely a one person job. There is just too much to keep an eye on. Also, there are some wind generators that don't work very well with tilt up towers. For example, generator that utilizes a gearbox is going to pose a problem at any oil changing time. (The ingenious person can usually find ways around these problems.) Loading on Towers The emphasis on a well-built and strong tower should be obvious. We don't want it to fall down or blow over. How that is accomplished may not be so obvious. Let's take a look at how a tower is designed and constructed, and why. Towers are designed to carry a certain amount of static weight, namely the wind generator and the associated bodies that dangle from the top to perform service work. This is the vertical. or downward, load on the tower, and is fairly easy to design for and build. If the legs won't support the weight involved, you just make them a little stouter. The wing generator and tower itself also present a certain amount of resistance to the wind, especially when the blades are spinning. This is known as horizontal or lateral thrust, and is not as easily designed for. The reason is that as the velocity of the wind increases, the power available in the wind, and subsequently the thrust, increases exponentially. (see "Wind Generator Tower Height" in Home Power #21.) When the wind speed doubles, that is, increases by a factor of two, the power increases by the cube of the velocity, or a factor of eight! Also, remember that the surface area that rotor presents to the wind is a function of ã x r2. While a 14' rotor is only twice the diameter of a 7' rotor, it has more than four times the surface area. Lateral thrust can get out of hand very quickly! Tower Physics This is lateral thrust is what causes most tower failures. What we have is an 80' (or whatever height you choose) lever arm! The wind is pushing on the wind generator rotor at the top of the tower. This is causing a bending action all the way down the tower. This bending action increases as we get farther away from the lateral thrust presented to the rotor on top of the tower. Remember, we have a lever arm. The longer the lever, the more we can move. In order to survive this lateral thrust, the tower is built heavier from top to bottom. Again, this is because the bending action increases as we get further away from the lateral thrust. The way we compensate for this is by using stronger materials for the legs as well as the braces as we go down the tower. The taller the tower, the heavier the bottom sections will be. Footings In addition, the wind is trying to topple the entire tower over through this lever arm action. Not only do we need a tower that gets progressively stronger as we go from top to bottom, but the attachments to the ground have to increase as the tower height increases. These attachments to the ground are generally known as footings. Footings act to anchor the tower in place and keep the wind from pushing the tower over. Each leg of a freestanding tower has its own footing. Footings are usually bell-shaped (figure 5). Guyed towers will have a footing under the tower itself, but individual guy anchors are usually imbedded in concrete pads (figure 6). Footings and pads are always set below the frost line. Footings and pads are designed to use the soil itself to help work against the lever action of the tower and keep themselves in the ground. If you were able to pull straight up on a footing or pad with enough force to dislodge it, it would not come straight out of the ground. Instead, you would pull a certain amount of soil out of the ground with the footing or pad (figure 5 and 6). The shape of the remaining hole would look like an inverted cone. By being designed this way, the amount of concrete needed, and therefore the cost, can be kept to a minimum, while maximizing strength. Vibrations In addition to lateral and vertical forces, a tower also has to withstand a variety of vibrations. These vibrations are set up in the tower due to the spinning of the rotor, the yawing on the wind generator, the electrical hum of the generator, and the interaction of the wind with the tower itself. These harmonic vibrations may become so severe as to be audible to the human ear. Also, the tower may begin to sway in the higher winds. This swaying can easily become an oscillation in a steady wind if it is uninterrupted by yawing. All towers have a natural frequency at which they vibrate or resonate. However, if not accounted for in the design of the tower, vibration can actually destroy a wind generator or tower and, especially, their welds. For this reason, the nuts and bolts of wind generators should be assembled with a thread locking compound (such as Loc-tite). An alternative is to use self locking nuts or "pal" nuts. One of my wind generators is mounted on top of a uniquely designed tower made of 3" thin-walled metal tubes. In about a 15 mph wind, the tower gives off an eerie low "moaning" sound when the generator yaws. It has put more than one visitor on edge on an otherwise quiet moonlit night. Rooftop Mounts Many people ask about mounting a small wind generator on a short tower on top of their house roof. My answer is always "don't"! Aside from the obvious problem of turbulence, the generator will cause the entire structure to resonate at some point. Rubber pads don't help at all. Smaller wind generators, which spin faster than larger units, are the worst offenders. Even if your house is structurally sound enough to hold the tower in place, the sound will drive you wild in short order. For the same reason, towers should not be attached to the walls of houses, either. If we're talking about a garden shed or garage, then maybe, but you may still end up dismantling everything. I know of one guy that built a small greenhouse out of fiberglass sheeting between the four legs of his wind generator tower. It was designed so that the four legs were the corners of the greenhouse. After two days of running the wind generator, the police came and told him he had to do something about the situation. By that time he was convinced anyway; he couldn't work in the yard without ear plugs. Final Caveats I am occasionally asked about putting a wind generator on the top of a tree. Trees don't make good towers. They are hard to climb safely. They're even harder to climb with wind generator parts and tools cluttering up your hands. They sway too much. Dead trees rot at the ground and fall over. Enough said! Finally, be wary of putting an oversized wind generator on an undersized tower. Many people learned this lesson the hard way in the mid- and late- 70s. For a time, the rage was to buy up old waterpumper towers and put Jacobs or Wincharger wind generators on them. A Jake with a 13 1/2' rotor has a swept area of 143 square feet presented to the wind (ã x r2 ). A Wincharger with a 12' rotor has a swept area of 113 square feet. Most waterpumpers came with an 8' wheel. That's only 50 square feet. Virtually all of these installations came crashing down. If you're going to err with a tower, err on the side of safety: overdo it. Who knows. Maybe someday you'll want to put up a larger wind system on your existing tower! Access Author" Mick Sagrillo, Lake Michigan Wind & Sun, E3971 Bluebird Rd., Forestville, WI 54213 ù 414-837-2267.