Building a wind turbine tower
Strictly speaking, a tower is nothing but a post that holds the wind turbine high enough to capture a stream of strong wind. Since the goal is so simple, there is no excuse in forgetting your primary responsibility around the wind turbine tower: Safety. I cannot stress this enough. A safe tower is a mandatory part of a wind turbine installation. Attempting to design, build, and raise a tall tower with no experience or proof of its strength is irresponsible and dangerous.
There are several ways to get a tower for your wind turbine.
- Buy an old tower
- Buy a new tower
- Build your own tower
Here is the first fork in the road: “Do It Yourself” or “Get it Engineered”. When building the generator and blades, there is a lot of latitude for creativity and invention. A tower is a totally different beast! If you decide to make it yourself, you will quickly find it takes more time, money, and effort than the windmill did!
Whatever you choose, unless the tower was specifically designed to support the wind turbine you have in hand, you probably must add parts or supports to make it work. If you salvage a truss-tower in good condition, you will probably be faced with making guy wire cables for it. Then there is the ground foundation, anchors for the guy wires around the base, the top stub that the windmill swivels upon… You get the idea.
I’m going to pass on a few pieces of information that should be helpful if you are going to modify a tower to mount a turbine. I cannot, however, tell you all about how to design and build a tower yourself. Not in good conscience. Either you have the technical background for it or you don’t. It hardly matters if you got the experience at school, work, or from the back of cereal box. If you don’t already know to tackle the problem, then you just aren’t going to learn it while surfing the internet.
Wind turbine loads
Knowing the loads a windmill imposes on a tower is so important that it almost goes without saying, however individuals who do not know how to calculate or measure these loads should not throw up their hands!
Picture your windmill as a machine that captures energy from the wind. That means that is slows the wind down, meaning there is a “drag” load (but it is usually referred to as thrust). It is also a big gyroscope. Its angular momentum causes all sorts of reactions bending this way and that, as it swivels left and right seeking the wind. Lastly, it is heavy enough that its weight can’t be ignored. Summary:
The weight of the windmill and blades is obviously an important load on the tower. Look at how their weight load the tower as you raise and lower it, too.
You can picture this as wind “drag”, but it’s not exactly that. The blades are actually producing “lift” – it’s just pointing backwards!
- GYROSCOPIC TORQUE
If you’ve ever tried the gyroscopic “bicycle wheel” experiment, you can remember how gyroscopes create forces at right angles to their motion. This happens in windmills, too.
When loads are applied to the tower, it will bend this way and that, and stresses build up in many of its parts. In the typical pole-and-guy-wire structure, the windmill applies all of its loads at the top of the tower, and the peak bending stresses occur right where the topmost guy wires are attached.
The tower’s peak stresses are transferred to the guy wires as tension loads. Since guy wires stand at an angle, they must take up even more tension to do the job. If the windmill applies 1000 pounds horizontally, the component of guy wire tension must respond with an equal 1000 pounds. The steeper the angle, though, the higher the tension in the guy wire to make up that horizontal load, or for the same size of windmill.
Tower column and base
While the guy wires react to windmill’s sideways loads, the cable tension also pulls down on the tower. Tension in all four guy wires is necessary at all times, otherwise they will go slack on the lee side. It all adds up to a lot of load on the tower’s foundation:
Weight of the tower + Weight of the windmill + Tension in the guys X number of guys
Since we all learned (and thoroughly enjoyed) trigonometry class in school, like I did, it’s a snap to calculate the tension load from any given side-ward load, and then adding the pre-tension in the cables, to work out the total weight that is going to be on the foundation. Now that the easy calculation is over, it is very wise to make sure that the tall column will not buckle under this compression load – especially if this tower is made from pipe. Trusses are much stiffer so overall buckling is less of a concern, but the individual leg elements can get into trouble if they are not supported by enough diagonals. That is why cheapo “TV Towers” are not suitable for wind turbines. They can take the bending, but not the crushing!
Raising the tower
This condition needs a lot of attention. It is not obvious, but the most dangerous time for a tower is when it is being raised or lowered. Human error and design errors show up in this stage, with disastrous results, unless you proceed carefully, slowly, and are checking it as you go. When you design a tower, too, paying attention to how it will be raised also pays off. By making the tower easy to raise & lower (unless it is permanent) you make maintenance and inspection much easier.
This was just a brief run-through. I personally think that towers should be engineered, not just “built”. You are the judge of your own capabilities. If you don’t know how to engineer a structure, you can get someone to do it for you, or buy a tower that already is designed for a windmill such as yours.