Do we really need towers and foundations? Airborne wind farms

Airborne wind power with air generated energy. Image copyright Philip Bechtle et al. - "Airborne wind energy resource analysis". Article on "Renewable Energy" 141 (2019)

Airborne wind energy is a generic name that describe various technologies that have different levels of development.

They have in common the idea of using unmanned vehicles such as planes, kites, balloons or similar solutions to produce energy from the wind. These vehicles are generally “tethered” (that is, connected to the ground).

Their main promise is to give you “more energy with less material” – because they have a lower initial cost and they consume less material. They also have several other potential advantages. For instance, they could be deployed relatively quickly in areas where there is urgent need for electricity (for instance after a disaster).

Researches on this technology started in the seventies but accelerated greatly in the last 10 to 20 years, with dozen of company developing different ideas and registering hundreds of patents.

There is still no consensus on which is the best technical solution. Therefore there are significant differences between the technologies currently being developed.

A first distinction can be made between systems that produce energy in air inside the flying unit (“on-board generation”) and those that have a generator inside a ground station.

This ground station can be fixed or move (for instance on a loop track or an horizontal track). There are different possibility to produce energy in the ground station: for instance, the tether can unroll a cable around a drum, and the rotation of the drum can be used to produce electric energy.

This mechanism remember somehow a yo-yo – you can see how it looks like in the following picture.

The system work with a two-phase cycle: a "generation phase" where energy is produced and a "recovery phase" where the rope is rolled up again changing the flight configuration (and possibly, consuming some energy using an electrical motor to rewind the rope).

Airborne wind power with ground generated energy. Image copyright Philip Bechtle et al. - "Airborne wind energy resource analysis". Article on "Renewable Energy" 141 (2019)

Several others possible categorizations are possible, for instance depending on the wing type used by the system (rigid wings vs. flexible wings), their weight (lighter vs. heavier than air) or considering the take-off mechanism used (vertical vs. horizontal).

Rigid wings have a better aerodynamic efficiency. Additionally, they usually also have a longer durability. Soft wings, on the other hand, are lighter and more effective with ground generating systems (decreasing the flying mass increase the tension on the cable).

How high will they fly?

The objective is to fly higher that the current wind turbines, to find stronger and more consistent winds (as a general rule the turbulence of the wind decrease with height).

Although it would be nice to extract energy from jet streams at 8000 meters (the strong winds blowing in the upper atmosphere that can make your airplane fly faster) there is evidence that at such altitute the cable that connect the vehicle with the ground would dissipate a significant amount of energy due to aerodynamic drag.

This could imply operating the system at a much lower altitude – probably in the 200m to 2000m range, with an optimum that will be location specific.

Investigations are also focusing on how to solve the take-off problem: as the vehicles are not propelled they will have a very low speed during take-off, and this imply less controllability of the vehicle.

The inverse problem (landing) is equally relevant: it should be possible to stop the system quickly in case of an emergency, as it is possible with a wind turbine.

To solve these problems and to maximize energy production the companies working in this niche are using algorithms to control the flight.

Another problem that the engineers are trying to solve is how to find a balance between low cost and reliability: these machines have moving parts and are supposed to have a long useful life (at least compared to the current lifespan of a wind turbine, which range from 20 to 30 years). The cost of maintenance should not offset the low initial investment cost.

Making a prediction on “social acceptancy” is much more complex (and not only because I am biased and I believe that wind turbines are beautiful).

There is some possibility that people will think that kites and balloons are more attractive than standard wind turbine. Can you imagine a wind farm made from giant helium-filled balloons? I'm sure my children would love it.

A floating wind turbine. Image copyright Altaeros Energies

However, the path towards the implementation of these solutions still seems long and complex.

A few months ago (February 2020) Google closed Makani, one of the companies more advanced in the research for airborne turbines.

Makani was a start-up founded in 2006 and acquired by Google X in 2013.

Makani made several working prototypes and was considered in pole position to find a working solution, also because their parent company has very deep pocket. Therefore their closure was a shock for the industry.

We will probably still need towers and foundations - at least for a while.

Blades repair – how to fix it

I have discussed in other posts why wind turbines blades are prone to different type of damages and how to detect them.

But what happen when a problem is identified?

Changing the blade is usually the “last recourse” option: in addition to the cost of the blade itself there are the transportation costs plus the need to fine a main crane for the job (and it is not easy to find a free slot for a crane: due to their cost owners try to keep them busy 100% of the time).

Therefore efforts will be made to repair the damaged blade with a short downtime - ideally one or two days.

The technique used to fix the blade will depend on numerous parameters such as the entity of the damage (cosmetic, structural or affecting the efficiency of the blade), the region of the blades that suffered the damage (some areas such as the leading edge are more critical) are and the type of problem (cracks, debonding, impact damage, etc.).

Among the repair techniques currently used it is worth mentioning:

  • Filling and sealing (also called "dill & fill"). With this solution small superficial cracks, delamination and other similar non-structural problems can be repaired injecting the appropriate material (usually resin, or special fillers or gel). To do it injection holes going to the depth of the defect are created. Often the resin is pre-heated before being injected with manual guns or pneaumatic tools using compressed air. The material injected can have a curing time of several hours or even a day. Curing can be at ambient temperature or at a higher temperature depending on the chemical properties of the substances used. In case higher temperatures are needed heating blankets or similar tools such as ultraviolet lamps (UV) are used.


  • Coatings, tapes or shields. These solutions are especially designed for leading edge erosion repair and protection – a classic and frequent problem. Basically the idea is to use an additional layer to protect the leading edge.


  • Plug/patch and scarf repair for major damages. This solution involve removing the damaged region, leaving a straight, stepped or, if possible, tapered hole. Subsequently the patch is applied to close the hole. There are different alternatives for the patch: it can for instance be formed from a pre-impregnated composite fibre tape cut to shape, applied in layers using intermediate layers of adhesive or preformed to the correct shape and subsequently bonded.

Wind turbne blades repair patch. Image from Report on Repair Techniques for composite parts of Wind Turbine blades
(D. Lekou)

The adhesive used in blades repair is usually especially designed for this type of application. It has to be resistant to fatigue and cracks and have a short curing time (although for wind turbines located in extremely hot environments a slow curing adhesive can sometimes be a better choice). Two-component adhesive are frequent (basically the blade repair technician mix the two different components of the adhesive, activating the reaction).

Different repair works may be needed if the damage has be caused by lightning (a frequent occurrence) or if the problem is with the ancillary elements of the blade (like the vortex generators).

Wind turbine blade damage detection systems

Wind turbine blades damages. B.F. Sørensen, E. Jørgensen, C.P. Debel, H.M. Jensen, T.K. Jacobsen, K. Halling, et al., Improved design of large wind turbine blade of fibre composites based on studies of scale effects (phase 1). summary report.

Wind turbine blades are rarely subject to catastrophic failures, although you can find a bunch of videos on YouTube with blades flying away or falling into pieces.

They are however subject to several problems, such as cracks, debonding of the various layers, internal delamination, etc.

As an order of magnitude blade failures are accountable for approximately one fifth of the problems usually seen during the operational life of the wind farm.

Such problems have a direct impact on the profitability of the plant and can be extremely expensive to fix.

Some issues can be fixed leaving the blade attached to the rotor. However not all problems can be solved without dismantling the blade, and the fee associated with the cranes needed for the dismantling can be very high.

In the worst case scenario a blade substitution will be needed, and the transportation cost will add up.

A certain number of failure is attributable to design error (insufficient material strength under fatigue or extreme loads) or manufacturing problems.

However the overwhelming majority of issues seem to be attributable to extreme weather – basically strong winds, thunderstorm (including lightning strikes), ice, pollutants, etc.

The effect of ice accumulation is especially dangerous in case of asymmetrical accumulation, because it can create unbalanced loads in the rotor.

Pollutants, usually in the form of airborne particles (e.g. sand) can shorten the life of the blades accelerating the erosion of the superficial layer. A gel coat usually cover the outside of the blade to protect it from ultraviolet degradation, water, etc.

Therefore the erosion of the external layer may have two impacts: lower the efficiency of the blade (this will have a direct impact on energy production, and ultimately on money) and accelerating the degradation of the blade.

Some areas of the blades are more prone to damage: tip, root and joints accumulate the majority of reported damages. The distribution of failures has however a typical pattern: erosion is more frequent in the tip side (due to the higher local speed) while cracks are more frequent toward the root.

Blades crack and erosion regions. Zhang, H., 2016. Reducing Uncertainty in Wind Turbine Blade Health Inspection with Image Processing Techniques. Iowa State University, Ames, Iowa

A variety of methods has been developed to detect and monitor damage. There is a consensus on the fact that early detection of a problem mean usually lower maintenance costs.

The “perfect”, ideal method should be able to assess the full scale of the blade without contact, allowing for continuous remote monitoring.

Unfortunately such methods does not exist yet. Among the most promising techniques it is worth mentioning:

The use of strain sensors, either embedded in the blade or on the surface. It is the same type of fibre-optic called “fibre Bragg gratings” (FBGs) that I have described in another post. As each sensor measure only direct strain and shear strain only in one point of the blades many sensors will be needed to have the full picture. Another problem is that the sensor can give a "false positive" result if it fails due to creep, fatigue, etc.

Measuring the acoustic emissions of the blade is another extremely interesting method to find damages. Unfortunately it is a complex system that need many sensors and physical contact with the blade of many sensors – something that could be difficult to achieve in operational wind farms. Sensors operate on an extended frequency range (50 kHz to 1 MHz) and as in the case of the strain sensor are used not only for the blades but also for the other components (gearbox, bearings, etc.)

Ultrasound is an extremely common non-destructive technique, that basically analyse the ultrasound waves reflected by the different layers of materials and by discontinuities in the materials. The main problem of ultrasound detection is that usually physical surface contact is needed. On top of that, there is also usually time consuming (and it can be hard to process the signal). It can however be extremely useful for an in depth analysis of the situation of the blade.

Thermography is a technique that use infrared cameras to assess the temperature distribution. If there is a defect in the structure the flow of the heat will be disrupted, causing temperature gradients. Thermography comes in two types, passive (made from the ground using the “natural” temperature) and active (heating the surface).

The last method, and possibly one of the most promising giving the quick evolution of computer computational power, is Machine Vision. One or more cameras are used to capture images of the blades subsequently analysed with image processing algorithms.

There are several interesting studies on machine learning – basically using two set of image, one of blades with known failures and the other of blades without problems, to “train” the algorithm to detect issues.



Can a rotor be smart? Passive optimization systems

The spectacular growth of the dimension of the wind turbine has led to the introduction of several interesting technical solutions. Different type of towers (concrete, hybrid, lattice, self-erecting, etc.) and new technical solutions for the foundations appeared in the last decade, trying to have the lowest possible cost of energy.

The rotor of the turbine has followed the same trend. With the impressive size of the blades currently in the market (today we are around 70 meters, the width of a football field) it is not surprising to see a variety of new concepts already in the market or under development.

One of the key issues of very long blades is that it is difficult to optimize them: finding the “sweet spot” for a component exposed to a variety of wind flow characteristics during its operational life is not easy, above all if such characteristics are not uniform along the blade.

The engineers designing the blades are trying to achieve several goals, such as:

  • Increase the energy production (maximizing the aerodynamic efficiency and the power extracted from the wind)
  • Reduce the loads on the structure
  • Create a solution that can easily be transported on public roads and installed with cranes already in the market
  • Extend the life of the blades (we are moving to the 25 or even 30 years mark)

All these objectives should be reached with the lowest possible cost.

The term “smart rotor” refer to a variety of technical solutions whose purpose is to increase the production or reduce the loads.

They belong to two categories, passive systems (not controlled by a software or an operator) and active systems.

Among the passive systems the most interesting are:

Vortex generators. Believe it or not, you can buy the elements generating the vortex from 3M (the same company that invented the Post It all around my desk). You can use them to retrofit pitch-regulated turbines sticking them near the root of the blade, where the air flow is separated by the blade (that is, it is “stalled”).

They basically work reducing the separation of the flow, increasing the production 1 or 2 percentage points (they can be a lot of money).

A second passive technology is the bend-twist coupling.

As you will guess from the name it creates a link between the bend and the twist of the blade, with the object to reduce fatigue loads created by sudden inflow changes during turbulent wind conditions.  

The wind blowing on the blade is creating 2 forces – “lift” (the one pulling up the blade and making the rotor turn) and “drag” (the one bending the blade backwards). As a general rule the engineers try to minimize the drag and maximize the lift, achieving a high “lift to drag” ratio.

With the bend-twist coupling the loads are reduced because the blades “adapt” its shape changing its shape and the angle of attach when deflected.

This coupling can be achieved changing the geometry of the blade (geometric coupling) or by changing the direction of the fibre inside the composite material (resin + fiber) that constitute the blade.

This interesting technology is currently being investigating by several entities, including a heavy weight such as the Fraunhofer Institute for Wind Energy Systems.

Image Copyright Mark Capellaro, 2012 Sandia Wind Turbine Blades Workshop

I also described some months ago the serrations. They have a different scope (reduce the noise) but I believe they can be consider a type of passive optimization system.

Vortex bladeless wind turbines

I have always seen the wind induced vortexes as a problem – they create vibrations in the tower, that in some cases can start to resonate with the eigenfrequencies (the natural frequencies of the structure) and in the most extreme cases even collapse.

The existence of such vibrations is one of the reasons why it looks like that steel towers for wind turbine have reached their maximum height. At around 100 – 120 meters they start needing dampers and other anti-vortex solutions during installation and for the operational life.

What I was not aware of is that there is a Spanish start-up trying to develop a “bladeless turbine” which exploits this phenomenon to produce electricity.

I have some doubts on the idea of a “bladeless turbine” (I suspect that a wind turbine has, by definition, a rotating part). However the concept developed by the folks at Vortex is for sure very interesting.

The Vortex Tacoma (this is the name of the industrial version under development) is expected to have a height around 3 meters, a weight around 15 Kg and a rated power output of about100w.

Currently smaller scale prototypes are available and the target date for launch of the full scale production is end of 2020.

It looks like a big cylinder oscillating when the wind blow. I also see that they selected the same combination of materials as wind turbines blades (resins reinforced with carbon fiber and/or glass fiber), while for the bottom section anchored to the ground they have selected a carbon fiber reinforced polymer due to its resistance of cyclical loads.

If you wonder how does it generate energy it is with an alternator system with coils and magnets. The cool part is that, unlike wind turbines, you do not have gearboxes, shafts or any other rotating element. The benefit is not only less maintenance but also a noiseless operation.

An additional interesting characteristic of this technology is that many machines can be clustered together in a narrow space. Standard wind turbines have a distance of hundreds of meters from each other to avoid the wake effect (basically the turbulence in the wind caused by the turbine itself). The wake effect can have an impact not only in the energy production of the turbine but also on its lifespan, shortening it due to the demanding operational conditions.

On the other hand the bladeless solution thrive on turbulence so you can pack more Tacoma Vortex together in what would probably look like a forest of artificial trees.

Another very cool feature of this machine is its ability to change its rigidity to adapt it to the characteristics of the wind. Different environmental conditions will request a different setup from the vortex in terms of mass distribution and rigidity. According to the website of the developer the machine will be able to automatically “tune itself” in order to maximize the oscillations.

Wind turbine tower as a water battery: the Gaildorf Wind-Water Project

Did you ever think at the amount of empty, unused space in the bottom of a wind turbine? Any idea how to use it?

Well, the folks at Max Bögl (a German conglomerate active in several sectors) have decided that it could be a good idea to fill it with water (about 40.000 m3 per turbine, up to a height of 40 meters) and use it as a temporary energy storage, in what they call a “water battery”.

Basically, the idea is to use a pumping system to fill the bottom of the tower when energy consumption is low and production is high (for instance, during a windy night).

When needed, the water can be released opening a valve and, thanks to a network of pipes with a diameter of over 1 meter, it can be used to produce energy through three Francis turbine, with a total nominal power of around 16 MW.

The hydro electrical plant is relatively near, at a distance of around 3 Km and with a height difference of 200 meters.

The turbines installed are 4× 3,4 MW GE 137 on an hybrid Max Bögl tower. What is remarkable is the hub height, varying from 155 to a record 178 m. They claim this to be the highest onshore turbine tower currently in operation, and as far as I know with a tip height of 246.5 metres, this could easily be true.

The switching time between energy storage and energy production is not exceptionally fast (30 seconds) but is not outrageously long either.

Partially founded by the German Environmental ministry with over 7 mln. € the pilot project is currently being built in Gaildorf (southern Germany).

Among the benefits of this solution is noteworthy the high efficiency of conversion of the potential energy of the water into electricity using well-known, proven technologies.

The main issue that I see is that this system, to be implemented, need a hydro electrical plant nearby with his own “long term water storage basin”. Essentially the wind turbines are providing only an additional (and somehow limited) storage capacity. However, in order to be cost effective, this technology will also need a “standard” basin.

Wind turbine blades damage - an introduction

I have discussed in another post the different type of damages that a wind turbine blade can suffer and the various detection systems.

There is an extensive list of potential problems: cracks, delamination, debonding, erosion of the leading edge (the first area of the blade to “see the wind”) are some of the issues observed.

This variety of potential failures is partially explicable with the complexity of the blade and the materials used in its fabrication: usually blades are made with an external aerodynamic shell of a composite material, stiffened by a box beam.

A composite is a material fabricated mixing two or more components – in the case of the blade a “matrix” usually made with an epoxy or polyester resin with embedded “fibres” (frequently glass fibres or carbon fibres).

Internally, shear webs, a box beam or a similar solution is used to give stiffness to the blade.

Additionally other materials can be used – some kind of gel for the coating of the external surface, metallic conductors to protect the blade in case a lightning strike the tip (an event that is more frequent then what you might suppose), adhesive material for the joints and very light material for the core (for instance balsa or synthetic foams).

On top of the intrinsic complexity of the blade there are other factors that contribute to the variety of damages observed: during its life the blade is exposed to a wide range of situations that can originate a damage – from the factory (fabrication mistakes) to transport by ship, train or truck to manipulation before the erection to a very long (20+ years) operational life.

During the operational life harsh weather is a major to blade damages (e.g. rain, ice, lightning). Additionally moisture can contribute worsening the situation entering the cracks and acceleration the degradation of the materials.

Impact is another source of damages. There are quite a lot of study on wind turbines and bird impact – however other type of impacts are possible as well, for instance during transportation or storage (sometime blades are stored for months before erection).

Not all problems are easily detectable with a naked eye: for this reason a variety of tools has been developed to identify internal damages.

Among the solutions most frequently used:

  • Ultrasonic waves: they travel through composite materials and internal damages acts as discontinuities with a change in acoustic impedance that can be measured
  • Digital shearography: this is an interferometric technique that uses coherent laser illumination to measure surface deformation
  • Passive and active thermography: as in the case of ultrasounds internal damages create a change in the distribution of temperatures. “Passive” means that the environmental temperature of the blade is used while “active” means that the tool itself heat the blade.

What happen when an issue is detected?

Unfortunately there are not so many competent technicians able to repair a blade - and many of them work for the wind turbine manufacturers and keep travelling from wind farm to wind farm. There is often some kind of warranty involved, at least for the first years of the project.

Blade repair: not a work for me

Obviously the best solution would be to dismantle the blade and work on the ground.

Unfortunately sometime this is not possible - maybe because a crane is not available on a short notice or because the cost would be prohibitive.

Why wind turbine blades are made of composite materials?

I’ve received a question regarding material selection for wind turbines blades. The reader asked why there is a predominance in the use of composite materials for the blades instead of wood, steel and aluminium and other materials used in the first glorious, pioneering years of wind energy.

Please note that I’m by no mean an expert so the only intention of this post is to give a very general introduction to the subject. This is a very broad topic involving different engineering branches.

In general the 2 design drivers are weight and stiffness.

A blade should be as light as possible for a variety of reasons:

  • To lower gravity induced fatigue loads
  • To be easily transported and installed
  • To have a better performance

However, it should also be stiff (that is, rigid) for several other reasons:

  • To withstand loads (both wind loads and gravity loads). Wind loads are function of wind speed and length of the blade, and increase from the root to the tip of the blade. Gravity loads are function of the material density.
  • To prevent collision between the blade and the tower under extreme wind
  • To prevent instability (local or global buckling) maintaining its shape

For these reasons blade designers try to minimize the mass for assigned stiffness levels – it is to find a balance between aerodynamic and structural requirements.

So we want less weight (that is lower density) and more stiffness.

Stiffness is expressed by the Young’s modulus of the material – basically the relationship between force and deformation. In general blades are very flexible, stronger in the flapwise direction and weaker in the edgewise direction.

And here is the reason for the use of composite materials. For a given Young Modulus, the material with the lower density is the composite (resin plus glass fiber).

You can see graphically this relationship in a type of graphic called “Ashby Plot” (I attach a version stolen online from a document of the University of Cagliari.

Ashby plot for a wind turbine blade

Multirotor wind turbine: an update

Some time ago I wrote a post about the interesting concept of multirotor wind turbines, including the full scale prototype built by Vestas with 4 refurbished V29-225kW (that is, with a 29 meters rotor diameter).

It has been installed in a test site of the Technical University near Roskilde, in Denmark - I believe I’ve been there many years ago for the famous rock festival.

After running for approximately 3 years the prototype has been dismantled. The result of the test are still not public, but some information leaked.

For instance, an increase in annual energy production (AEP) of approximately 1.5% has been reported. It is due to an improved power curve, allowing the turbine to reach faster the nameplate capacity.

I’m not sure this result can be scaled to the current turbines in the market – however for a modern WTG a 1.5% increase is a lot of money.

Another counter intuitive fact is that the wake effect (the turbulence generated downstream by the WTG when the wind cross the blades) is minor in a multirotor. Don’t ask me why because I’m not an expert in fluid dynamics.

Additionally the load increase is not significant. That is good, because it has a direct impact on life expectancy of the turbine and on tower and foundation cost.

Moreover noise emission is not significantly higher. This point is especially relevant in Europe or other area with strong constraints in term of noise.

Serrations: how to reduce those noisy vortexes

Every now and then a new technical solution appears in the wind energy business and it’s slowly implemented in the new wind turbines.

A good example is the use of trailing edge serrations – not really a new idea (it has been around for several years) but a simple solution that it’s spreading and gaining acceptance in the industry.

Basically they are a method that help reducing the noise of the blade and they look like small triangles. You can see them in the picture at the beginning of the post, which I ironically stolen from an anti-wind energy website.

This solution is particularly beautiful because it can be retro fitted – meaning that it can be applied also to existing, working wind turbines.

They work reducing the turbulent boundary layer on the trailing edge of the blade, which is the source of a relevant amount of the noise. This is a complicate subject and I’m not an expert in acoustic, but in general what happen is that the turbulences and the vortices created by the  layer of air that separate from the edge of the blades are creating the majority of the noise that we hear.

It’s interesting to observe that the length of the triangles has an impact on the reduced frequencies (the longer the serrations, the bigger the reduction at the low frequencies). This help reducing the frequencies that are more annoying for humans.

Also, it is worth mentioning that the amount of dB reduction is function of the serrations flap angle.

Obviously they don’t do miracles, but a reduction of 1 or 2 dB(A) for their price is a good trade off.

It’s also worth to notice that they do not affect substantially the performance (that is, the production) of the wind turbine.