Printable 3D concrete wind turbine towers

Yes, you're reading right – you can print your concrete tower.

I have discussed in many previous articles how I see some evidence that we are reaching the maximum size for steel towers, mainly because of transportation issues.

For higher towers concrete towers could help solving the problem, as they can be transported in pieces and assembled on site.

Among the different technologies available for concrete I have just discovered this exiting evolution: a Danish company specialized in 3D concrete printing, COBOD, partnered with GE Renewable and LafargeHolcim to develop a large printable tower.

COBOD already printed some years ago a full scale building, a small but beautiful 50 m2 office with curved walls.

For this interesting evolution they already made a prototype about 10 meters tall.

The concrete is extruded by the machine in a sort of ribbon, and the internal and external sides of the tower are reinforced by a “wavy” central section.

Currently the solution that they are targeting is a hybrid tower (that is, with the top section made of steel) with an on-site printed base.

Dancing in the wind

I have discussed in other post the phenomenal growth of the dimensions of wind turbines in the last 2 decades. Bigger rotors, taller towers and more MW has been the industry trend year after year.

There is some evidence that we are reaching the limit – blades of more than 50m length pose significant logistic challenges, while steel tower more than 100 meters tall can be subject to strong vibrations and dangerous oscillations under certain circumstances.

Such vibrations can be induced by several external sources such as an unbalanced rotor, an earthquake or the wind itself.

They are dangerous because they can damage the turbine due to fatigue loading (the weakening of materials due to cyclical loads). Some type of foundation can also partially lose stiffness – for instance monopile foundations.

Additionally, these vibrations can also trigger resonance phenomenons in the tower – you can follow this link to see of how “soft soft” and “stiff” tower are designed based on the blade passing frequency.

You can see a good full scale example of this problem in the video above and read here more about wind turbine vibrations.

There are several technical solutions currently being studied to dampen the tower reducing the vibrations.

Among the most interesting concept that I have seen I would mention tuned mass dampers – basically an auxiliary mass connected to the structure with spring and dashpots (viscous friction dampers), friction plates or similar energy dissipating elements.

These dumpers are called “tuned” because they have been designed keeping in mind the natural oscillation frequencies of the structure they have to protect. The two main parameters are the spring constant and the damping ratio: by varying them it is possible to damp harmonic vibrations.

I do not know if tuned mass dampers that can work with the first fundamental frequency of  industrial size wind turbines (below 1 Hz) are currently available – however I have found quite a lot of  studies on the topic.

A similar technological solution is the tuned liquid column damper. In this case a liquid inside an U shaped tank. By varying the geometry of the tank and the depth of the liquid different damping frequencies can be achieved.

The main benefits of this solution are the geometrical flexibility (you have to put the dumper somewhere inside the tower or the nacelle – I can assure you that the space there is very reduces) and low cost.

Another variant is the pendulum damper. In this solution, the length of the pendulum is calculated to match the fundamental frequency of the WTG.

Mass Damper (a) and Pendulum Damper (b)
Copyright O. Altay, C. Butenweg, S. Klinkel, F. Taddei
Vibration Mitigation of Wind Turbine Towers by Tuned Liquid Column Dampers
Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014

Where have all the wind turbine gone? Foldable towers

Perima foldable wind turbine tower - folded. Copyright Pantano et al., Springer

In previous post some years ago I have described two alternative solution for the wind turbine tower that should help solving the problem of the huge cranes that are currently needed for the erection of the wind turbines components.

One is the self-lifting precast tower developed by Esteyco, a Spanish engineering company that developed several interesting technical solution.

The other is the Nabrawind solution – again, a Spanish company that developed a self-erecting tower. They also have another interesting product, modular blades that can be assembled.

Some days ago, I have discovered another technical solution that share some similarities with these two concept but with an interesting twist: a group of Italian engineers has developed a “retractable” tower, basically a telescopic mechanism that can be folded bringing the blades down to the ground without using cranes or other equipment.

Theoretically it could be operated from a remote location, even if I guess that some kind of supervision during the operation is advisable.

Why should you want to make your wind turbine disappear?

The authors mention several reasons, for instance minimization of the visual impact (you can make your WTG almost invisible during the day and having it work at night).

I can also think at other uses – minimization of bird impact (folding the tower during the migration period) or increased safety during extreme wind (for instance during the monsoon season in south east Asia).

The idea is not only a concept –a working prototype has already been built in southern Italy.

Perima foldable wind turbine tower - erected. Copyright Pantano et al., Springer

It is a small wind turbine (55 kW), at least for what is today the standard in utility scale projects (3 to 5 MW). Additionally it has only 2 blades, which I think can help when you retract the tower.

However the hub height is 30 meters, quite a reasonable figure.

It is interesting to observe that this technical solution needs a deep foundation, basically with a depth equivalent to the hub height.

It is mentioned the possibility to modify the concept to use the foundation hole as a well to extract water. Quite an interesting side benefit I would say.

The authors are not sharing the cost of the tower and the ancillary elements, although I suspect they could be several time the cost of the standard, non-retractile  tubular steel tower.

Finally, it would be interesting to know the applicability of this solution to WTGs in the MW class.

The authors mention a dimensioning bending moment of around 300 kNm. Such value is two orders of magnitude lower of the values that are common in industrial size turbines, so it is not immediately evident that the idea can be scaled without major modification.

An additional problem would be the length of the foundation pit.  Reaching depths of 50 meters and below, although not impossible, introduce new issues – for instance the need of very specialized drilling equipment.

Perima foldable wind turbine tower - technical details. Copyright Pantano et al., Springer

Second life: the destiny of turbine foundations after decommissioning

I have discussed in another article the challenges associated with the disposal of the blades when a wind farm is decommissioned or repowered.

But what happen to the foundations?

The destiny of a foundation will depends on local regulations, on the environmental requirements that are normally given with building permits and on the wish of the owner of the wind farm and of the land.

As a general rule, foundations are at least partially dismantled. The first centimetres (20, 50 or even one meter) are removed and the rest of the foundation is left in place and buried below a layer of organic soil.

Sometime the entire foundation is removed. This is a complex activity, and blasting or at least many hours of hydraulic hammer are needed.

The third option is to bury the foundation below a small hillock.

In case a repowering is planned there is also a fourth and more interesting possibility: giving a second life to the old foundation integrating it in the bigger, new foundation. There is a group of company that is studying this possibility under the name “FEDRE” (Fondations d’Eoliennes Durables et Repowering – French for “Long lasting wind turbine foundations”).

The concept that is being developed is how to reuse part of the existing footing for the new foundation - adaptation the existing one on the short term and working with reusable foundations designed ad hoc on the long term.

In case the foundation has to be dismantled some difficulties may be experienced.

They are very “dense” in steel (on average a foundation can easily have more than 100 Kg of steel for each cubic meter of concrete). Due to the concentration of rebars in some areas of the foundation (above all, in the centre) it can be more difficult and time consuming to separate the steel from the concrete.

Usually steel is separated from concrete and melted again. In some countries the reinforcement bar are even used “as they are” without being melted and reformed (i.e. they are straightened and used again in another structure).

The presence of steel makes more complicate grinding the foundation in smaller elements to use it again as a construction material, for instance to build roads (in the nucleus of the embankment) or for earthworks as a filling material.

It is worth mentioning that some with turbine have concrete tower or hybrid (concrete + steel) solution. They could be equally difficult to recycle.

Repowering and decommissioning will probably gain momentum in the near future.

2018 and 2019 saw only a few hundreds of MW decommissioned (unsurprisingly mostly in Germany, where the installed capacity is huge). However the numbers should increase steeply in the next years when more and more wind farms will end their 20 years of supported tariff.

Unless they are able to close some king of PPA (power purchase agreement with a counterpart willing to buy the electricity at a certain price) it could prove to be not economically viable to sell the power at spot prices.

 

Good vibrations: wind induced resonance and turbines oscillations

Have you ever wondered why sometimes the wind turbines (and other similar tall structure) sometime vibrate?

Under some conditions the wind blowing on the tower create vortices.

These vortices appears regularly on both sides of the tower, creating low pressure zones first on one side and then in the other.

This beautiful sequence of vortices is called von Karman vortex street. von Karman has been a pioneer of aeronautics

For these reason the tower will start moving perpendicularly to the wind, first toward one side and after toward the opposite.

The tower has a very low structural damping – when the oscillation start its reduction is very slow because the steel tower has a limited capacity to absorb the kinetic energy.

It also has a very low natural frequency (the frequency at which the tower will tend to vibrate when subjected to external forces).

The vortices created by the wind will appear at a frequency that depends on the speed of the wind and the diameter of the tower.

The formula to calculate this frequency is very simple:

f = St · U / D

where

f is the vortex shedding frequency

St is a value called Strouhal number (in our case it is around 0,2)

U is the wind speed

D is the diameter of the tower

At a certain wind speed the vortices will appear and shed at a frequency equal to the natural frequency and the tower will resonate. The wind speed that start the resonance is called the critical speed.

If the wind blow at the critical speed for enough time the amplitude of the vibration will increase and you will see the tower oscillating.

This is a simplistic summary of a very complex phenomenon. It is however a limiting factor for the use of higher steel tower. In additional to the risk of catastrophic failure the wind induce vibrations will also generate additional fatigue loads, shortening the life of the tower.

It is also interesting to observe that a structure has more than a natural frequency.

Vibrations in the lowest natural frequency (first mode) will have this shape:

First wind turbine tower vibration mode

However, sometime a turbine can vibrate in what is called “the second mode” the second lowest natural frequency):

Second wind turbine tower vibration mode

Following this link you can see a real world example of how is a tower vibrating in the second mode.

What can we do to avoid these dangerous vibrations?

Some solutions are structural - basically aimed at increasing the damping of the tower or changing the way the mass is distributed.

It is not easy to change the mass distribution in a wind turbine tower (basically its an inverted pendulum).

Nevertheless it is possible (and it is becoming increasingly common) to install dampers in the wind turbines, either only temporarily during the installation or as a permanent feature.

Other solutions are aerodynamic - the idea is to change the shape of the tower adding elements that disrupt (or "spoil") the vortices. Conceptually they are similar to the spoilers used in cars or planes, in the sense that they are intended to mitigate an unwanted aerodynamic effect.

An example are helical strakes, sometimes called "coils" "ropes". This is a concept developed by Christopher Scruton and other scientist such as William Weaver in the fifties and sixities and used often in chimney and similar structures.

However sometimes they do not work as you can see in the video below.

In general, the effectiveness of this solution is driven by two parameters:

  1. Diameter of the strake (usually defined as a ratio of the diameter of the tower)
  2. Pitch (the distance along the cylinder axis that is needed to complete one full turn of the strake)

Definition of pitch and height

From experimental tests in wind tunnels it has been found that the optimum height of the strake is approximately 10% of the diameter of the tower (that is, around 40 cm for a standard steel tower with a diameter of around 4 meters).

For the pitch the results of the test shown an optimum value in the region between 5 and 15 diameters (that is, one full turn of the rope should be done between 20 and 60 meters).

Obviously the smaller the pitch, the more the strake is parallel to the tower.

There are however other secondary factors that influence the efficiency of this solution, such as the area of the cylinder covered by protrusions (usually called “strake coverage”) and the pattern of the ropes (usually three or four independent ropes are used to create the helix).

One of the biggest problems of the strakes is that they greatly increase the drag coefficient (the resistance opposed by the wind turbine tower to the flow of the wind).

The implication is that the loads at the bottom of the tower will increase as well, so that a bigger foundation could be needed.

For this reason strakes are normally used as a temporary solution only during the installation – above all in the most critical part of the procedure, when the tower is installed but there is still no nacelle on top.

Removable strakes are wrapped around the tower, either before the lift while the tower segment is on the ground or after installation.

Bonus: poetry & technology (from a LinkedIn post of Hervé Grillot)

A soft spot for BoP: the soft-spot foundation

The soft spot in the middle of the foundation. Credits: CTE-wind.com

The soft spot foundation is another interesting technical solution that appeared in the marked some years ago.

Basically, it is similar to a standard shallow foundation with a certain amount of a foamy, soft material just in the middle below the anchor cage. I understand that different alternative materials can be used, being a standard solution Expanded Polystyrene (EPS, the type of material used to protect big electronic items during shipping).

The result is a change in the distribution of the forces that makes the foundation behave like a circular, ring-like structure.

As a consequence there is an increase of the lever arm of the stabilizing moment, and so ultimately a smaller foundation can be built.

The benefit are the savings in concrete and steel - some percentage point, that in big farms can mean a lot of money.

I know that it seems a bit strange - and possibly even counterintuitive - that just by putting some soft material in the middle of your foundation you will achieve savings in the quantities of material needed.

Me too I was surprised when I first discovered the solution. However, it looks like it really works.

The solution has been using in dozens of project worldwide. Additionally, I have seen that it has been certified by DNV-GL.

You have to consider that to work this foundation need to transmit an increased pressure to the soil below (because due to the soft spot that does not transmit pressure there is less area left).

Therefore it is usable only in situations where the bearing capacity is good - for instance in rocky areas.

I have seen this foundation for the first time on a project developed by CTE, a French engineering company specialized in wind energy.

It also look like they have some kind of copyright on the solution (or possibly only on the name).

However, lately I have noticed that it has been offered also by other engineering firms.

RUTE precast modular wind turbine foundation

Some days ago I have been contacted by Doug Krause, founder of RUTE - a green start up proposing an interesting solution for the wind turbine foundation.

Taking inspiration from the technology used in bridge construction RUTE is proposing a system of post-tensioned beams connected to a central hub. Each beam has an anchor system connecting it to the soil, and the foundation is delivered to the wind farms in around 20 elements.

Among the benefit the fact that the system is modular, less prone to quality problems (it is manufactured in specialized plants) and, at least in principle, reusable after 20 to 30 years for a new foundation: the lifetime of the components is over 40 years.

Decommissioning is also probably easier with such structure, at least compared with a standard shallow foundation.

Installation times can be cut as well – as it is delivered hardened it is ready for installation in few days from the start of the works.

I had a look at the technical specifications and I have seen that the bottom of the excavation is at the same depth of the standard solution, so no savings here. I have also noticed that in some situation soil substitution could be needed.

I have seen in their website that there is already a full-scale prototype built, so it’s much more than a concept. It has been installed at the Palmers Creek Wind Farm (Minnesota) on a 2.5 MW GE turbine with a 90 meter hub height.

Addenda (10 June 2019): I've received an email from the founder of the company. I post it here for the benefit of all readers.

Thank you Francesco for noticing RUTE.
That's a picture of our rock anchor, bulb T girder, model TG. The one we built in Minnesota is a box girder style, BX Foundation. It behaves just like an inverted T, spread foundation.

RUTE's biggest value to the BOP contractor is time. So most of the foundation works can happen off the project books and schedule. So a project can close finance and be erecting towers the same month. We'll hope to prove that claim this year.

Apart from the main BOP driver, the facility owner can run a pro forma out 30 years, or 40 years, the normal term of the land lease. And in those cases a foundation with bridge design, like ours, lasts well past 40 years. That's just a function of the post-tensioning which keeps the concrete in permanent compression. So there's an order of magnitude less fatigue damage than conventional reinforced concrete.

I can share some pictures from inside the foundation. You can walk around inside it and inspect.

Best Regards,
Doug, RUTE

Wood towers for wind turbines

I always believed that wood towers for wind turbines were a solution possible only in small, domestic WTGs (somewhere around 10 kW to maybe maximum 100 kW). There are several example available, for instance this product of InnoVentum.

Well, I was wrong: I see that some years ago (2012) a Vensys 77 1,5 MW turbine has been installed on a 100 meters tower. That is quite a number: a 77 meters rotor is considered small for today standard, however it fully qualify as a “utility scale” solution.

This full scale prototype followed a 25 meters test tower built by the same companies some years before.

Developed by 2 German companies (TimberTower & TiComTec) it has been built near Hannover. The foundation is standard (concrete) and the connection between the tower and the foundation is made trough 4 meters long steel rods.

With a somehow unusual octagonal cross section the tower diameter is comparable to a standard concrete or steel solution. I see however that other geometries are possible (hexagonal or dodecagonal).

The life span of this solution looks similar to the steel alternative (20 years). Unfortunately I haven’t been able to find information regarding the cost. For the sake of clarity it is not 100% wood – few steel elements are used inside the tower.

 

Long term instrumented monitoring of wind turbine foundations cracks evolution

Fiber Bragg grating sensors (Image copyright fbgs.com)

Some weeks ago I have discovered that, as I am currently enrolled as a university student (getting “slowly but steady” a second degree in Economics) I have full access to the Elsevier database.

This is an enormous amount of information, including all the best scientific papers and technical articles published by industry journals.

I am using this possibility to learn more and stay updated on several niche topics that I found interesting – from recycling of wind turbine to bird strikes to foundations pathologies.

Browsing the database, I recently stumbled upon an interesting article published by Jack McAlorum et al. from the University of Strathclyde (Glasgow).

The paper is called “Deterioration of cracks in onshore wind turbine foundation”.

The authors instrumented an octagonal slab foundation (sometimes called “star foundation” or “wall foundation”) to monitor the evolution of existing cracks.

I already wrote a couple of posts on foundation cracks: they can be due to a variety of root causes, such as:

  • Design mistakes
  • Errors in the composition of concrete mix
  • Extreme temperatures
  • Errors in the execution of the wind turbine (for instance, concrete poured in different batches creating construction joints)
  • Failures due to the use of an embedded can (this is a frequent failure reason for older wind turbines)

The paper does not specify the reason for the cracks. However, as typical, the most severe cracks were in the side of the wind turbine facing the wind (as the concrete is in tension there).

What it is interesting is the fact that the behaviour of the foundation has been monitored for a very long period (over 9 months) and under standard operating conditions. This is very unusual: while other key component of the turbine like the gearbox are constantly monitored and the data is collected trying to detect problems and predict failures, I have never heard of such monitoring for the foundation.

Additionally it is interesting the type of sensor used: instead of standard accelerometers or strain gauges the researchers used a strain sensor based on fibre-optic called “fibre Bragg gratings” (FBGs).

Basically it is a sort section of optical fiber treated in a way that some specific wavelengths are reflected and some are transmitted. They can be used as a strain sensor because when they are deformed the transmitted and reflected wavelengths shift, allowing a calculation of deformation.

Cracks can evolve with 3 different displacement type:

  1. Opening (the crack becomes wider)
  2. Sliding (one face of the crack slides on top of the other)
  3. Tearing

Through the monitoring period no significant evolution of cracks was observed. Basically, the wind turbine owner was lucky: cracks did not deteriorate and no intervention was needed.

Unfortunately, the cost associated with the monitoring are not shared, so it is difficult to make a business case (cost of immediately repairing the cracks with grouts or epoxy resins vs. cost of monitoring to see if the intervention is needed).

I also see that this solution only allow monitoring visible cracks. This is a strong limitation, as several failures originate in a non-visible area of the foundation.

Said that the idea is certainly interesting and useful, above all considering that some turbine are kept in operation for a very long time, even exceeding the design life of the foundation (usually 20 years).

 

Readers' questions: may I use an anchor cage somewhere else?

I’ve just received this question from a reader. As I believe it’s an interesting topic I’ve decided to answer with a post instead of an email or in the comment section.

“I'm custom broker and I have to classify under HS Code an anchor cage. I have consulted to Classification Office of Argentine Customs Service and they ask me if the anchor cages are designed to be used exclusively in the construction of wind generators, or if they could be used in other constructions, for instance an antenna tower.
I would appreciate if you could help me on this matter.
Kind regards.”

The answer is no – they can’t  be used somewhere else.

There are several applications for foundation cages: power transmission pylons, light poles, mobile phone antennas and other type of towers.

However, anchor cage are dimensioned to fit a specific type of tower. For instance, different wind turbine models have different anchor cages (both the number of bolts and their diameter might vary). You can’t take a generic anchor cage and put it below another tower: the number of bolts, diameter of the tower and size of the bolt would not match.

Some years ago there has been a famous mistake in a wind farm in Brazil – the wrong anchor cages have been shipped (and embedded in the concrete of the foundations). The mismatch between tower bottom and anchor cage was millimetric, so the installation crew tried to install the towers for hours before discovering the mistake. It was the anchor cage for a different model of tower for that specific wind turbine model.

It’s interesting to note that at least a wind turbine manufacturer offer a range of anchor cages with different bolt lengths compatible with a specific wind turbine model. This allow for a greater customization of the foundation and savings in material.