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.

Modifications to wind farm access roads: a step-by-step guide

One of the problems that occurs frequently to engineers working on wind farms is how to modify existing access roads to allow the transit of special vehicles.

I was contacted by several readers of the blog who had this issue - the last one was Egil from Norway who led me to write this article.

In general, the problem is usually a curve that is too tight, a change of slope that is too fast (i.e., a road crest in the vertical alignment where the change of slope is too sudden and the truck “hits” underneath) or a combination of the two.

I am describing in this post the procedure I have followed in the past. If you have followed different steps please drop me a line.

The example is taken from a project where the blade truck was hitting below (there is usually very little space under the trailer – as little as 20 centimeters).

The first step is to send a topographer to create a cartography as detailed as possible of the area.

Both GPS and “traditional” topography will be fine while I would avoid LIDAR because you would have too many points to work with. It is sufficient to work with a simplified model.

You will usually receive the results in AutoCAD DXF or a similar format, ready to use.

The next step is to create a tridimensional model of the existing road. I usually work with AutoCAD Civil 3D – however there are several similar software in the market.

Then I use AutoTurn to calculate the path of the truck. I really like this software as it gives me a very reliable simulation of the wheel path and the swept path area.

AutoTurn simulation of the the path followed by the truck. The problem area is on the bottom.

The following step is to use Civil 3D (or your favourite software) to calculate longitudinal & cross sections (representing the ground "below the truck" and "as seen from the wheels")

This help me deciding if and were changes to the existing roads are needed. You can see in the image below where the trailer is "touching" the road. In this example I was already aware of the problem as I was contacted by the colleagues on site.

A section showing the blade trailer (hatched rectangle) and the elevation of the existing road below. As you can see the truck is hitting the ground below.

The next step is to modify the geometry of the road "manually".

This is done changing the elevation of the points in the topography one by one in the area of the problem recalculating the longitudinal profile and the transversal sections until I reached a shape that looked OK in terms of torsion, slope and free space below the truck.

The elevation of the problem area before and after (points marked in red)

You can see in the image above marked in red the points that I have changed. At two points the ground has been lowered, at one point it has been raised.

When you are satisfied with the solution you can export all relevant information (digital elevation model, cross section, longitudinal profiles, etc.) and send it to the topographer on site so that the changes can be implemented.

In the case of bends, if only minor modifications are needed, the same procedure can be followed changing points as needed.

Blade mover – a flexible dual transport system

The two elements of the blade mover

One of the many problems posed by the huge wind turbine blades currently in the market is how to move them quickly and safely.

In some situations (like in the factories, or when blades are temporarily stored before installation) space can be extremely limited and there is the risk of damages. Additionally manipulating and moving the blades can be a time consuming activity.

To solve this problem a Danish company (SH Group) has developed an interesting solution, currently used by a blades manufacturer.

The “Blade Mover” uses two elements, one at the root of the blade (a trailer pulled by a vehicle) and the other at the tip, where a self-propelled vehicle with a diesel engine is driven by a technician using a remote control.

The vehicle at the tip is extremely manoeuvrable, allowing full control on the steering wheels – you can basically orientate the wheels in every direction you may need.

The tip element - an independent vehicle

The system has been develop to carry blades with a weight up to 80 Ton.

I also see from the pictures and the YouTube videos that it use custom frames. The frames are connected to the blade mover using a flexible sliding mechanism with safety pins.

The tip frame. All components can be modified to fit a specific blade

One additional benefit of this system is that it can work on uneven terrains absorbing height differences with a hydraulic mechanism.

The blade mover in action. The technician is using a remote control system.

 

BoP vs. BoS - similarities and differences

Lately I have had the pleasure to spend a lot of time with my friend Alessandro.

Alessandro is an engineer specialized in the design and construction of photovoltaic plants – basically, a "solar energy" version of mine.

We spent some time discussing similarities and differences in the BoP (“Balance of Plant”) of wind farms and the BoS (“Balance of System”) of photovoltaic plants.

As you are reading this blog you will probably know that BoP and BoS basically mean “everything but the wind turbines (or the panels, in the case of BoS)”

Both can have quite an impact on the economics of the project. For wind farm is usually in the 20% to 30% range of the CAPEX while for solar plants it is typically much more – even above 50% of the investment total.

I have made a quick number with some projects currently under development in Southern Italy and I see that for medium size projects (10 to 20 MW) the cost of the modules is only 40%.

This could sound counterintuitive but is a consequence of the unstoppable reduction of the price of the solar modules. At the current rate the price is decreasing 75% every 10 years and this trend does not seem to change.

As a consequence, the BoS becomes every year more relevant (because it is not decreasing at the same rate, so its relative weight keep increasing).

Let's take a look at similarities and differences between BoP and BoS.

In both cases you will need internal roads and probably a substation to connect to the grid (unless the project is very small – for projects of few MW sometimes it is possible to connect directly to the grid in medium voltage).

Additionally, sometime the panels have a shallow foundation (“ballast”) that reminds somehow the shallow foundation of wind turbines on a much smaller scale.

Furthermore the engineering works to be done (geotechnical survey, topography, electrical and civil design, etc.) are very similar.

And this is more or less where the similarities end.

The differences are much more remarkable. For instance, a substantial amount of the BoS budget comes from the support structure of the panels, inverters and trackers.

Inverters are the elements that convert the electricity produced by the solar modules for DC to AC

Trackers are used to rotate the panels in order to have them always in the best position to maximize energy production. They are optional, but they are used frequently because they are generally a cost effective technology.

It is also very unlikely that you will see a solar plant on a steep terrain (with a strong inclination), while this situation is frequent in wind farms (many of them are placed on mountain ridges).

This happen because there is a limit to the height difference that can be absorbed changing the length of the elements that sustain the panels. Additionally, excessive height differences can make the work of the trackers more burdensome with an increased risk of failures.

For these reasons the usual maximum slope in a solar plant is usually around 5% or 6% - and therefore earthworks are limited and less expensive (at least compared to some projects that I have seen on top of mountains where a lot of blasting was needed).

For the foundations I mentioned before the shallow “ballasted” solution. This is basically a block of concrete holding the modules in place.

However the use of piles is usually more cost effective. Several alternatives are available depending on the geotechnical characteristics of the soil (helical piles where cohesion is low, driven piles when the soil is more dense, etc.) and in addition to the monetary advantage they are also usually faster to install and easier to decommission at the end of the life of the project.

Nabrawind Transition Foundation

Some days ago I have been contacted by Miguel, Sales And Marketing Manager at Nabrawind.

Nabrawind is a Spanish company working at several interesting breakthrough concepts – including a modular blade that I will try to describe in another article in the future, a self erecting tower and the innovative “Transition Foundation”.

Miguel asked me if I was interested in receiving material about the Transition Foundation solution they have developed.
I was obviously very happy to accept his offer and share with you what I have learned.

This alternative foundation use a 20 meters tall transition element in steel and cast iron in the lower section, at the bottom of the tower.

A detail of the transition element

The transition element is connected to the ground through three “feet” that allow different technical alternatives for the foundation: the standard solution (“shallow” or “gravitational”) plus two cheaper options – piled foundations (for standard soils) and rock anchors (when bedrock is very near to the surface).

The three alternatives solutions for the Nabrawind foundation - shallow, piled and with rock anchors

For a 4MW wind turbine the piles are expected to have a diameter of 1.5 meters with a depth in the 15 to 20 meters range.

A 15 meters pile would need approximately 80 m3 of concrete and 21 Ton of steel in total (that is, for the three piles).

This figure indicate very substantial potential savings in the amount of concrete and important reductions in the quantity of steel as well.

The piles are connected to the transition element via anchor cages (obviously smaller than the normal anchor cage used with standard solution).

In addition to the savings in the quantities the other main benefit of the solution is the speed. You will need only one or two days to drill the hole for the pile, and the installation of the reinforcement bars and concrete pouring is very quick as well (both operation should last between 2 and 4 hours in total).

The anchor cage variant promise to be even faster, needing only three concrete blocks (one for each “foot”) to level the surface and distribute the loads and 6 post-tensioned rock anchors with a length in the 15 meters range.

The Transition Foundation is more than a concept – the first foundations using this solution have been built in a wind farm in Morocco for a 3.6MW wind turbine on a 144m tower.

The have 24 meters long piles with a 1.2 meters diameter, for a total of only 81 m3 of concrete and 25 Ton of steel – a remarkable result.

How the foundation looks like (notice the 3 elements)

In the last picture you can see a detail of the completed works for the foundation.

How the completed foundation look like

Lift me up: the braced foundation

The braced foundation is a partially precast foundation that lift the wind turbine some additional meters above the ground.

Developed and patented by Esteyco (a Spanish engineering firm) is a technical solution validated, certified and used in several wind farms worldwide.

This solution increase the hub height up to 5 meters, which usually results in a significant increase of the annual energy production.

The “braces” are elements of precast concrete – basically double beams with a rectangular section transmitting the loads from the tower and stiffening the foundation.

They are on top of a cast in situ circular concrete slab that transmit the loads to the ground. This slab has a circular edge beam below, whose function is to absorb bending moments and contribute to the overall stiffness.

In the middle there is a central ring, while the tower rest on a smaller upper slab.

The main benefit of this solution is the increase of energy production – 5 meters of additional hub height can bring an annual increase in the 1% to 2% range depending on local wind condition.

Although this could look like a small number, compounded over 25 to 30 years it can really make a difference for the economics of the investment.

You first question could be something like “why not to use an higher tower”?

Generally, towers are designed, manufactured and sold with specific heights. Each wind turbine manufacturer has a portfolio that include only some heights (e.g. 90m, 100m, 110m, etc.).

Therefore you could find yourself in a position where the project could theoretically use a different hub height not offered by the wind turbine manufacturer.

Although every now and then project specific tower are designed and built this is not the standard and it has several implication in terms of time, cost, etc. Therefore it could be better to go for an off the shelf solution that gives you those additional few meters that your project need.

According to Esteyco this solution is also quicker to execute, at least in big wind farms. I do not have real world feedback to comment on this, although my impression is that the number of precast or partially precast foundation solutions used in the market is increasing.

This solution as a certain versatility because it can be used with different soil condition, including difficult geotechnical situation that needs piles.

It also use less material due to its geometry. I do not have actual figures to comment on the final cost, however my impression is that the real benefit will come from the additional production and that the saving in materials will be offset by the increased manufacturing complexity.

This solution has already a certain track record. I see that it has been used in Italy, Mexico, India, China and Saudi Arabia (in Dumat Al Jandal, a wind farm that I tendered 8 or 9 years ago – this gives you an idea of how long it may takes for a project to materialise).

It has also been certified by DNV-GL and TUV, undoubtedly a strong plus.

All the pictures are stolen from the presentation that Esteyco has given at India Windergy 2017.

Medium voltage power cables in wind farms: an introduction

This post is an extension of the previous short article I wrote some years ago on the characteristics of wind farms medium voltage system.

I wrote it with the help of my friend and colleague Kamran, who spent more than an hour answering my questions on the subject. Thank you Kamran!

The medium voltage network is one of the elements that compose a wind farm project, the other being foundations, earthworks, substation and high voltage line.

Some elements could be missing: I have seen several projects without substation, for instance in France where  small wind farms were connected to the grid directly at medium voltage level. However, you will never see a project without at least several hundred meters of medium voltage cables.

Wind turbines generally produce energy with a voltage around 600V – 700V. Subsequently the voltage is raised by a transformer that can be located in the nacelle, at the base of the tower or less frequently externally in a small box near the tower.

The objective is to minimize the electrical losses, and several voltage level are theoretically possible - I have seen projects with MV levels varying from 12kV to 33kV and higher.

The objective to achieve working at the design of the medium voltage system is obviously finding the sweet spot that optimize Capex (what you pay for cables and transformers cost) and Opex (mainly the electrical losses that you will have in the cables), selecting a rated voltage compliant with local regulations and cable types that are commonly used in the country where the wind farm is located.

Cables are rated by their effective cross sectional area in mm2 – the greater the section, the greater the amount of current they can transport.

Standard sections frequently used in wind farms are 70, 95, 120, 150, 185, 240, 300, 400, 500 and 630 mm. Greater sections are commercially available but already the 400 to 630mm sections are hard to use in construction due to their weight and bending radius.

The bending radius is usually expressed as a function of the diameter. For instance, “10x D” would mean that the minimum bending radius is 10 times the diameter of the cable. This parameter is significant because you will probably need some narrow bends in your cable, for instance at the bottom of the foundation if the transformer is inside the turbine. Large binding radius can make the work at the construction site very hard.

The cables are made of several layers with different functions – many technical alternatives and constructive techniques are available in the market but in general you will find (from the centre to the most external layer):

  • A conductor core made of copper or aluminium
  • An insulation layer, usually made of cross linked Polyethylene (XLPE)
  • A metallic screen to stop the electric field
  • An external sheath, protecting the cables from corrosion, humidity and mechanical stress. In some projects this most external layer is selected to have special properties such as for instance enhanced resistance to fire or protection from aggressive chemicals or even termites (I have seen this last feature in Australia)

Different medium voltage cable layers. Copyright image Yuzh cable

Cables will be delivered to the wind farm in cable drums made of wood.

The standard design strategy is trying to minimize the number of cable drums because making the joints between different sections of cables is an expensive and highly specialized task.

There are however limits to the size of drums – basically both its weight and dimension must allow safe transport and manipulation.

The amount of meters of cable that can be transported on a drums depend on the cable type and diameter – for wind farms you will usually receive some hundreds of meters in each drum.

Single core vs. three core cables

There are two main typologies of MV cables commercially available, single core and three core.

In single core cables each comes with his own screen while in three core cables the three phases share a common metallic screen. If you select the single core technology you will need to use three different cables, one for each phase.

Aluminium vs. Copper cables

The material used for the conductor of the cables for wind farms is always almost always aluminium.

Theoretically, copper cables are available and copper has several desirable characteristics - for instance it is a more efficient electrical current conductor and requires a smaller cross section to carry the same amount of power as an aluminium conductor.

However, with the current relative prices of copper and aluminium, copper cables are simply too expensive so they are never used for the reticulation of wind farms – you will probably see them inside the substations, where distances are shorter.

The cost of raw materials such as aluminium represent a relevant percentage of the final cost of the cable. For this reason I tend to see the MV cables almost as a commodity.

Overhead vs. Buried cables

In the majority of countries, the cables are directly buried in a sand bed in the bottom of the trenches (or in very rare cases, inside a duct).

Every now and then, I see a project with an overhead medium voltage line, for instance in India or South Africa. However, they tend to be more the exception then the rule.

 

Crane hardstands for installation of wind turbines: a handbook

Crane pads engineering and construction handbook. Copyright STOWA

Due to the pandemic this year we have been forced to go on vacation by car – the majority of flights from Germany to southern Italy have been cancelled, so we are making a 2000+ Km car trip with our three kids.

While my wife was driving I searched something to read and I have found something unexpected and extremely interesting: a 121 pages handbook only on wind farm crane pads!

The owner of the document is a bit unusual – STOWA, the Dutch knowledge centre of the regional water managers. However it makes sense if you consider that many projects in the Netherlands are on reclaimed lands and water authorities and municipalities are involved in the permitting process.

Behind the document there is a supervisory committee composed by the very best wind companies in the country, such as Fugro, ABT, H4A, Tencate and many others.

They will give you a very detailed view on the topic: after an introductory section describing the standard wind turbines and cranes now in the market the handbook describe in detail the process, from the geotechnical investigations to the design to the execution, operation and maintenance.

I have worked personally with some of the authors and I can guarantee you that they are very experienced professionals, so this is an extremely valuable document.

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.).

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.