Once More, with Feeling: Timber Towers

Modvion laminated wood tower. Image copyright Modvion

Approximately one year ago I wrote a post about a full scale prototype of a wind turbine tower made of wood.

It has been built in 2012, but after that the idea seemed to have stalled without progress: one of the companies involved in the construction, TimberTower GmbH, disappeared from the radar shutting down their website and I suspect they went out of business.

However, I see that someone else has taken up the challenge: Modvion, a Swedish start-up.

They could be more successful in moving from the prototyping phase to the industrialization for at least two reasons: they are coming from Sweden, a country with an extensive know how and network of companies active in wood construction, and they successfully went through a round of Venture Capitalists, Business Angels and European Union founding.

They target is to be ready to market in 2022 cross laminated timber towers in the 100 – 150 meters range. This means that they want to enter in the 4 to 5 MW segment, the current standard for utility scale WTGs. The prototype that they have just installed is 30 meters high.

Such tower could have several benefits – solve the current transportation problems (steel towers with diameters over 4 meters have huge transportation challenges due to bridges, cables, etc.), lowering the carbon footprint and possibly even be cost competitive against the current technologies (steel, concrete and hybrid).

I have no idea of the behavior of this solution from the resonance point of view although I suspect that the increased diameter at the base improve the situation. I also ignore how this tower would behave in case of fire: I have personally seen a fire very near to the wind turbines some years ago in Portugal.

Unfortunately their website does not share many technical details. I understand that it is a modular solution, with the total number of modules obviously depending on the tower height being a standard solution in the 30 - 40 modules range.

The tower section is circular, unlike the TimberTower solution that was octagonal.

Among the materials used for the tower I see glued laminated timber and laminated veneer lumber - basically a mix of wood and adhesives, with superior technical properties and more uniform characteristics as it is produced in a controlled environment.

The modules are joined together using double-treaded fasteners, preassembling on site 4 or more tower sections with a bottom diameter of 6+ meters (that is, more than a standard steel tower).

This concept is similar to some concrete modular tower solution with a key difference – the modules are assembled horizontally, so I guess that the need of big crane support is limited.

I also understand that the internals of the towers would be similar to the ones currently built (with elevator, ladder, space for transformer, etc.).

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.

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

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.

Peikko rock adaptor foundation

I've been asked by a reader of this website why there are no references to the various technical solutions available for wind turbines on rock.

The truth is that I’m not a specialist on this topic. However I’m learning, due to the fact that I’m currently working at several projects in northern Europe where it can be applicable.

To solve the problem I’ve decided to start with a video, that can be better than a 1000 words.

It’s an example of foundation on rock without anchor cage – one of several possible technical alternatives when the turbines are above shallow, unfractured rock.

Basically the tower rest on a steel “adapter” plate on top of a reinforced concrete block, and the turbine is fastened to the ground with dozens of post-tensioned anchors several methers long (figures above 9 or 10 meters are not unusual).

There are some very clear benefits with this solution if the geology is favourable: for instance less excavation, almost no blasting and lesser use of materials.

This video has been done by Peikko, a Finnish company specialized in steel elements. They have an interesting, unusual business model, as they do the engineering for the foundation and provide the steel but not the concrete or the manpower – therefore the foundation has to be built by another company.

Here a screenshot with the main elements of this solution:

One more video (possibly more detailed) on this technical solution here:

BoltShield® anchor bolts rust protector cap

Some weeks ago I’ve been contacted by a company developing an interesting product – a tailor made protector cap for anchor bolts.

I’ve notices that in some wind farms corrosion of the exposed side of the anchor bolt can be a problem. For instance, it’s not unusual to observe this phenomenon in areas with high salinity (e.g. Chile, or near the sea in the Netherlands).

If rusty, the bolt need to be cleaned before being tensioned. In theory this solution could improve the situation.

The solution, called Boltshield, is a metal cover cap available in several materials like aluminium and carbon coated steel (other similar products are made of plastic).

This cap should protect the upper part of the bolt, the nut and the washer from possible damages.

Additionally, coupled with paste or corrosion inhibitor, should prevent corrosion.

It’s a specific product line for the wind energy sector and apparently is already used in wind farms in  several countries (Italy, Finland, Scotland, Lithuania).

They claim that the market response is particularly interesting for the innovative screw-on system that allows an easy and safe screwing on the tie rod.

I didn’t had the opportunity to test this product (and obviously I’m not affiliated or compensated by them) so I can’t assure you that it delivers what promise. If you do have experience please drop me a line.

 Wind turbine foundations cracks – an update

I already discussed in another post a frequent problem of with turbines foundation, the appearance of cracks.

In general, my impression is that the new foundations with anchor cages are much more reliable that the previous technical solution (the “embedded ring” – the industry standard some years ago).

However, every now and then I still hear story of foundations that need some kind of  intervention due to mistakes during design and/or execution.

Unfortunately there is a lot of secrecy on this issues. Unlike other civil engineering products (e.g. roads, dams, etc.) problems with wind turbines foundations are generally hidden, probably due to the fact that they are mainly private investments and probably the companies experiencing an expensive problem prefer to have as little publicity as possible.

From several studies I’ve been able to found on the topic it seems that towers and foundations are accountable for less than 5% of WTG failures – being blades, gearbox and generator much more frequent sources of problems.

However,  while replacing a blade or a gearbox is “business as usual”, replacing a foundation is not  really an option – and any intervention will probably be quite expensive.

Problems in the foundations usually materialize as cracks in the concrete.

In many cases they are caused by the cyclical nature of foundation loads – with a lifespan of 20 to 25 years the foundation can be exposed to millions of loads cycles.

These cracks can be radial or circumferential, and appear both in the pedestal (the visible part of the foundation, where the tower connect to the foundation) and in the buried part of the foundation.

Usually these cracks tend to appear soon (1 or 2 years) and they doesn’t pose a danger to the stability of the wind turbine. However, water could infiltrate them damaging the reinforcement bars.

The position of cracks can be defined with ultrasonic devices.

These technology use the echo of sonic waves to create tridimensional images of the foundation. In practice a crack will appear as a discontinuity, reflecting the wave to  the receiver.

Should cracks on a foundation worry you?

It depends.

It’s important to note that not all cracks are created the same: shrinkage cracks or cracks in the grouting due to an excess of material are usually less critical than the appearance of voids (for instance below the load spreading plate or the bottom flange of the anchor cage).