Wind Turbine Towers
The tower is perhaps one of the most important parts of a wind turbine. Air is fluid like water and any obstruction to the wind creates turbulance (like the wake behind a boat). Turbulence caused by nearby objects robs power, and causes much more stress on the mill..
Ideally a tower should be 30' above anything within a 300' radius in order to keep the turbine up in clean, non-turbulant wind. Often the best time/money spent on a system is on a nice tall tower.
In practice, we cannot always follow guidelines, we have to work with available resources (time and money). If there are few obstructions on the ground, the ground is flat - and an effective tower is quite easy.
Free standing towers
Require significant foundations, they have no guy wires. They are either lattice towers, or made from large diameter pipe or tubing. Their main advantage is their appearance, and the very small footprint required. Most very large (utility scale) wind turbines are on free standing towers, but they don't seem very commonly used in smaller system because of their cost.
Common on medium sized installations. Usually erected with a jin pole or a crane - the latter being also required to install the wind turbine.
Both types mentioned above require climbing & heavy equipment to install so therefore we shall concentrate on the Tilt-Up tower.
Tilt-up towers can be made from wooden utility poles, steel lattice radio towers, or sections of steel pipe coupled together. They are simple to make and can be tipped up with a winch, or a truck. All work with tools and heavy wind generators can be done while safely standing on solid ground.
The key features of these towers are 4 guy wires, a hinged base and a gin pole for leverage. To raise a tower, the 2 side guy wires are attached and tightened while the tower is laying on the ground. This prevents lateral movement during the process. The guy wire opposite the winch side is cut to the correct length and attached to the earth anchor to prevent the tower from falling over if it passes plumb.
A winch or vehicle is attached to the remaining free side over a gin pole for leverage, and the tower is slowly pulled up. Using the turnbuckles, all guy wires are tightened. To lower it, simply reverse the process.
As a rule the jin pole to be at least 1/3 as long as the tower is high. The longer the better as it is easier to raise the tower. If its too short, the forces really add up, and it goes up and down too quickly
Pictured above is a drawing of a simple pipe tower which is easily scaled up or down. There are other ways of doing things but it's typical and very workable for smaller wind turbines.
This tower has 2 sets of guy wires. The side guy wires should be inline with the pivot as they must support the tower while its being raised and lowered. The top set of guy wires should be attached as near the top of the tower as possible such that the blades of the wind turbine cannot hit it. So for a 8ft machine, the top guy wires are about 4ft from the tower top.
There also needs to be a heavy cable (3/8" thick) that runs from the tower top to the end of the jin pole. This cable takes all the force when raising, or lowering the tower.
The top guy wires take almost all the force from the wind turbine, so they should be heavier than the bottom ones. 1/4" thick steel cable is OK for a 10' wind turbine.
The second set of guy wires should be about where the two pipes join - half way up. They serve mostly to keep the tower rigid, they can be lighter , 3/16" cable being fine.
On a smaller installation these sizes can be obviously be reduced but remember, the stronger it is.. the longer it will last. Stainless steel cable, although expensive, can be worth the extra cost, particularly incoastal regions and where regular relocation is required.
Wooden poles can be very effective and economical. Some people have simply used a tall tree from the forest, bored a 1 inch hole near the bottom for the hinge, built a metal collar on which to mount the wind machine. The height averages only 20-30 feet though. A straight, treated utility pole would make for a much more permanent tower. These are fairly easy to find in 20-30 foot lengths, and may be found (with difficulty) in more useful 50-60 foot sizes too.
A firm favourite. Great success can be had using 2 inch galvanized steel water pipe for towers. The maximum recommended height for a 2 inch water pipe tower is 42 feet, which is 2 pieces of pipe. Attach another set of guy wires at the coupling between the two pipes. Also, it is good idea to make couplers between each section that extend into the pipe on both sides for added strength. Smaller towers can be made from standard builders scaffolding pipe, the aluminium type being light and portable. These can be joined with the standard external coupler.
These towers are very strong and lightweight, but expensive if purchased new. Used lattice towers can occasionally be found through ham radio enthusiasts. Though generally not designed for tilt-up use, the base can be modified to be hinged. They can be made from cheap electrical conduit etc., although the construction is complex
5 - 6ft diameter wind turbines.
It can be made from two sections of steel or aluminium scaffold. Depending on the wind turbines fitting it may be sleeved or a stub may be needed for the wind turbine to slip onto.
8 - 10ft diameter wind turbines.
This can use 2.5" dia Sched 40 steel pipe which comes in 21' sections, so the tower itself is made from two lengths of pipe. The top section (2.5" dia) slips down about 1 foot or so into the bottom section (3" dia). Pipe is measured by its inner diameter.
As most wind turbines use 12" of 2.5" pipe for the yaw bearing, so at the top a stub of 2" pipe about 13" long is needed for the wind turbine to slip onto. The jin pole is also from 2.5" pipe. Pipe is measured by its inner diameter.
There is a gusset (2" pipe) in between the tower and the jin pole, it's length is not all that important - it is there to reinforce the point at which the jin pole is welded to the pivot. 6-8' long would be fine.
A wind generator foundation must be very strong, especially the hinge. If the tower, foundation, or hinge are not strong enough, it will probably fail on erection and if not, come crashing down in a strong wind. The only time when there is a great deal of sideways stress on the base though, is during raising and lowering.
Pouring a concrete foundation that extends down below frost line is highly recommended. 1/4 inch steel hinge brackets can be embedded directly in the concrete. 1/2 inch steel plate would be better.
Alternately a steel baseplate can be made complete with hinge and mounting holes. this can be staked down for temporary (mobile) installations, or bolted on to concrete or rock for permanant installations.
See the diagrams above for details.
Your earth anchors for the guy wires must also be very strong--they have to resist the lateral thrust that is put on the windmill. As your propellor rpm increases, the blades act more and more like a solid sheet of plywood sitting up in the wind.
There are a number of different earth anchors that can be used; a rugged mountain installation in the rocks might require a couple of different kinds. Generally, anchors are designed to utilize the weight and shear strength of the surrounding soil to help hold them in place.
The radius of your guy wires should be between 50 and 75 percent of the tower height. It is very important to lay out your 4 anchor foundations perfectly perpendicular to each other around the tower foundation--otherwise, the 2 side guys will not stay tight while raising and lowering the tower. Leveling is a an issue too. If the guy wire pads are at different elevations, you will run into the same problems with loose and tight guy wires.
Use the highest quality turnbuckles that you can find. Try to find turnbuckles that have removable bolts at each end to hold the guy wire in. Use the thimbles that come with your cable clamps to avoid fraying the cable. Avoid turnbuckles that have hook ends--if you can't find them with bolts or solid loops, be SURE to using baling wire on each hook so nothing pulls loose during raising or lowering.
When fastening your guy wires with Crosby U-shaped cable clamps , make sure to use enough clamps on each connection (3 are recommended), orient them correctly, and tighten them VERY hard! The clamps should all be oriented the same way, with the forged saddle on the live end of the cable. Remember, don't ever put the saddle of the clamp on the dead end of the cable!
This is the all-around strongest solution. The hole for the concrete should be bell-shaped, so it is wider at the bottom than at the top--this allows the soil over the anchor to help hold it in place (see diagram below). A metal loop for the guy wire can be embedded directly in the concrete, preferably at close to a 90° angle from the attachment point near the tower top. Your footing should extend down below frost line. If you set an auger with a plate into concrete, the end should point toward the tower...In other words it should be parallel to the guy wire.
If your soil has good shear strength (i.e. it is not loose, dry and crumbly) you can purchase earth augers to anchor your guy wires. These metal rods have an angled plate at the end that allows you to screw it into the ground. Again, they should be angled into the ground so that they are parallel with the guy wire.
Anchored guy wires directly into large rock outcroppings using a homemade 'piton' made of rebar. Drill a 3/4 inch hole in the rock with a hammer drill, again angled away from the windmill. Pound in a #6 rebar and weld a loop for the guy wire on the end. Epoxy in the hole might also help. Our first version used #4 rebar and seems very strong, but a larger diameter would be better.
Metal T posts can make fast and easy earth anchors for 20 to 30 foot towers if you can pound them in deep enough. If not, dig out around the post and pour in concrete for a very strong anchor on a 30 foot metal pipe tower. With fence posts, they should be angled into the ground at 90 degrees to the guy wire.
Here is a way to anchor into solid rock:
Drill a hole (usually 6" deep), and epoxy re-bar into it. Each guy wire is attached to a turn buckle, and inside the loop we have a 'thimble' - which keeps the turnbuckle from wearing into the steel cable. The guy wire is fastened with 3 cable clamps.
Concrete makes for a nice guy wire anchor. Another possibility for guy wire anchors is a 'dead man'. Take a log (or something large and heavy) - wrap a chain around it, and bury it. If it's large enough it'll be fine.
Simply pound 4' stakes made from steel scaffold pipe into the ground. Lots of pounding, but they seem quite solid and a 10' machine works fine with it. This may not be such a good option if the soil was loose, or wet. Where it's dry/rocky and the soil is dense/clay like just a few inches down and it works fine.
Windmill towers are subject to all sorts of vibration. The propellor, changing wind direction, generator noise, and effects of the wind directly on the tower can cause all sorts of harmonic vibration, some audible and some not. Be sure to Loc-tite® all bolts on the tower. If harmonics are causing problems (such as a swaying metal pipe tower), the problem can sometimes be fixed by changing where the guy wires attach to the tower, or by adding extra guy wires farther down the tower.