Wind Turbine tower loading

[Rob Dolinski]

Hi all, I have a unique and interesting opportunity that I am pondering.

I have a rural property in southern MB with an 80’ free-standing antenna tower used by a local wireless Internet service provider (WISP). With Fibre To the home coming to our area in 2023 the WISP asked what I would like for them to do with the old tower once they are finished with it.

One option is for them to abandon the tower in place as it will cost more to remove the tower than it’s worth (crane & crew). Tower was built in 2016 so it’s pretty new.

With the prospect of a sturdy structure the first thought in my mind is wind turbine! It is an 80’ lattice structure with a concrete foundation 8’x8’ square and 8’ deep. Ya, a lot of concrete! The tower loading capacity at 80’ is 20 square feet but for every 8’ section of tower you remove the wind loading capacity increases by about 6 square feet so 72’ is good for 26 sq ft and 64’ is good for 32 sq ft.

My question is, how would you determine the loading of the wind turbine on the structure?

My first thought is worst case. If the turbine blades formed a solid circular disc, so long as the area of that disc was less than the tower capacity I’m ok. A disc with 2.5’ radius would have an area of 19.63’. If my assessment is correct I could safety install a wind turbine with 2.5’ (30”) blades.

I have had a anemometer/weather station at my place for the past 5 years so I calculated the annual average wind speed which is ~5m/s.

Should I consider a smaller turbine higher or larger at a lower elevation?

Smaller turbines need higher wind velocity to generate but the higher/smoother air might be more efficient.

A larger turbine that can operate with lower velocity but I would have to install it at a lower elevation 60’.

Looking for input. Thanks!

[Rob Beckers]

Rob, I've been digging through my notes from the days we still were deep into wind turbines (and at one time we working on tower design for a 6kW one). This is from my notes at that time: "use the Betz limit plus 30% for induced vorticity, and calculate the thrust needed to extract that amount of kinetic energy from the air", and that would make a somewhat worst-case since it assumes a turbine efficiency well beyond what any real-life one actually has. Alas, I don't have all the equations or an example handy to show how to calculate it. I'm sure I can recreate it, but don't have the time just now.

Using the thrust of a flat plate should work too, as it is even worse case than what we used above. It just results in an overly sturdy (and expensive) tower if the goal is to design something cost-effective as we were trying to do.

At the time the tower needed a safety factor of 2 to meet code, so it had to be able to meet twice the calculated load.

There is another consideration with wind turbine towers, that has to do with oscillations in the structure (first and second mode). Turbines, and towers, can and do self-destruct if they hit on a frequency that makes the tower + turbine oscillate, and oddly it is the stronger-stiffer towers that have more of a problem with this (because their oscillation frequency is higher, making it more likely to coincide with that of what the turbine produces, and they have less dampening). The frequency range is simply RPM/60 for the turbine, though how much energy is available at a frequency has to do with turbine thrust load (i.e. how much power it produces) for that frequency. The effects can be subtle; you may not see the tower oscillate but the vibration may be strong enough to make the bearing and other parts in the turbine wear out quickly.

Lastly, fatigue loading is an issue. The rapid change in thrust can cause structural fatigue, making a tower fail even though straight-up thrust calculations wouldn't have predicted this.

Because wind turbines are dynamic structures, very different from (for example) an antenna, their towers had to be designed quite a bit sturdier than just thrust load would suggest.

-RoB-

[Steven Fahey]

Hi Rob,
If you're still tuned in to this thread, there are ways to work it out yourself, but it takes a fair bit of reading up on structural design against wind and ice loads. This depends a lot on your technical background, experience with steel design, and attitude to doing math, etc.

Also, based on the simplest numbers you already have, a 32 square foot area gives you a wind turbine with a diameter of only about 6 feet. That's going to give you much power, although a 80' tower is a great place to put it.

There are ways to improve this greatly, but they aren't for novices.