A website about wind farm construction: not only turbine erection but also balance of plant – access roads, crane pads, turbine foundations, power collection network, substation, meteorological mast and the economics behind it.
As wind turbine loads and foundations size keep increasing year after year sharpening the geotechnical calculations and modelling correctly the interaction between soil and foundation is becoming a priority.
The cost of foundations can represent a significant percentage of the investment in a new wind farm - even more in 2021, when steel and concrete are becoming every day more expensive.
An important topic that is becoming the focus of detailed studies is the soil bearing capacity degradation under cyclic loads.
This subject has been incorporated in the new version of the standard IEC61400-6 on Wind energy generation systems in Part 6: Tower and foundation design requirements.
Now, under certain conditions, a certain amount of "gap" below the foundation may be allowed.
“Gap” means that under certain situations the ground below part of the foundation might become uncompressed – as if the foundation was partially lifted, creating a "gap" (i.e., a separation between the structure and the soil).
This is something that previously was not allowed (unless the foundation was on rock).
The reason is that if the soil goes through several cycles of compression and decompression its bearing capacity might deteriorate. Basically the bearing capacity becomes lower and lower, putting at risk the stability of the structure.
This is a relevant change, as the IEC standard is one of the most important document (if not the most important) used in wind turbines foundation design.
The key idea behind the change is that if the soil below the turbine is not susceptible to the phenomenon of degradation under cyclic loads a certain amount of gap can be allowed.
Removing this “no gap” requirement means that a significant reduction in the diameter of the foundation can be achieved.
This happens because otherwise the foundation would have been bigger only to keep the soil below it always compressed.
The “no gap” requirement used to be one of the dimensioning constraints in wind turbines foundation design when the soil was good.
The key to allow some gap in the foundation design (and as a result, a smaller foundation and savings in concrete and steel) is to be able to justify that the soil characteristics will not will not degrade under cyclic loads.
This involves dynamic tests, which are time consuming, expensive, difficult to implement on site, unusual for most geotechnical companies and difficult to post process and interpret.
In some cases, even with a robust testing campaign, additional finite elements models have to be created to validate the design.
Will we see smaller foundations after this change in the IEC? We will need to wait several months to answer this question.
The constant search for higher rated power, taller towers and longer blades has pushed wind turbine manufacturers in an arms race to secure a position in an extremely competitive market.
Today in the onshore market there are machines with rated power close to 6MW, hub heights in a range of 150-165m and blades longer than 80 m. Several projects are currently under development considering these massive sizes.
Lately I have had the opportunity to analyse in depth a new solution which is emerging as an alternative to the traditional lattice boom cranes: the tower cranes.
I have analysed two scenarios, one with the standard lattice boom crane and an alternative scenario with the tower crane.
Standard scenario: lattice booms crane
Lattice boom cranes such as the Liebherr LG1750 have been the standard solution for the installation of the latest generation of turbines, with a tower height in a range up to 140+m.
This type of crane can be moved fully assembled between positions under certain assumptions (such as a very low road longitudinal slope and minimum road width of more than 6m).
If the crane has to be dismantled a substantial area for the boom assembly and disassembly process will be needed (in red in the image below).
Other characteristics that can have an impact on the project are:
A mountainous landscape: in this case the boom assembly area would be even more essential. This will have a substantial impact of civil works cost.
Very high installation rates (such as 3 or 4 turbines per week). The limited stock of lattice boom cranes suitable for this hub heights worldwide create a risk: either you book the cranes two years in advance (giving up the possibility of changing the schedule) or you wait - with the risk of losing the crane availability slot.
Alternative scenario: tower cranes
The idea of using tower cranes for wind turbines installation is not new in the onshore sector.
Said that, as far as I was aware of, the use of this typology of cranes has been negligible in the last few years.
Common sense tells me that the experience turned out not to be very positive (otherwise I presume that the concept would have been replicated, while that does not seem to be the case).
More recently, new models from Krøll Cranes have been used in wind farms at the opposite side of the planet, in Thailand and Australia.
Big players like ALE are suggesting that this new concept is reliable.
One of the main references is the Theparak wind farm project in Thailand, where 60 V136-3.0 MW where installed using this crane.
Here we have a list of some of the projects installed in Thailand with tower cranes:
The main pros of the tower crane are:
Road width required: only 4.5 m (even as little as 3.5 m according some sources).
Cranes boom is only 70 m long.
Advanced crane bases allow savings in the critical path.
Lower minimum lifting radius compared to lattice boom areas.
Installation rates about 2 hours per component. An installation rate of 1 WTG every 4 days has been reached in the Thai projects.
Operational up to wind speeds of 15m/s.
On the other hand, cons would be:
Lack of experience of the operators with these new set of cranes and low offer worldwide.
Uncertainty on the actual installation rates due to insufficient track record.
Real installation costs are still unknown.
Depending on the characteristics of the project, Kroll cranes has available these models:
How would an hardstand layout adapted to both the tower crane and the new generation of turbines look like?
It seems that a tower crane could work using a standard hardstand without the boom area:
Some 3d models where created on real WTG locations to assess the actual impact on cost of this new configuration. Quantity reductions in topsoil stripping, excavation and fill material may lead to a cost reduction around 5000€ per hardstands (being conservative).
A nice image with a 3D model of the hardstand analysed is included below:
Even if potential savings in the civil works seem to be easy to achieve, a real total project cost reduction can be confirmed only considering the actual installation costs, which are not so clear at this moment.
Are tower cranes going to be a more mainstream solution in the future?
Only time will tell.
I have received this email from Jasper from Lighthouse projects. I think it can be useful to other readers.
Regarding the tower cranes in compare to Crawler crane I would like to share with you some experience.
I have worked 2 years ago at windpark Krammer which consists out of 34 Enercon E115 turbines on a dyk with no space for storage components or installation of the cranes.Therefore we have used 2 Liebherr EC1000 Tower cranes in the project to build the wind turbines.
My experience is that with the limited space on site it is easier to install the crane. Another benefit is that crane capacity because the crane can lift up to 100t we could pre assemble the rotor and generator up front and lift the generator in one lift which is efficient. In addition what we saw is that you can lift longer you can lift up to 12 m/s or more.
A disadvantage of this type of crane is that it must be extended at some point. Mast sections must then be placed in between so that the hook of the crane becomes higher.
Since some years ago almost all wind turbine manufacturers (“OEM” - I hate acronyms) have modified their tower to foundation interface.
The previous technical solution to connect tower and foundation was based on an embedded steel section (like a “ring” inside the foundation).
It did not work properly and the issues caused by this element might be easily subject of several articles, about the problems caused by the ring and on the solutions developed to fix those problems (i.e. retrofitting and repairs works necessary to ensure the necessary lifetime of the turbine foundations).
In the last years (I would say since around 2010) the embedded ring has been replaced with a pre-stressed anchor cage, as shown in the following picture:
The design methodology for this element is usually based on simplified hypothesis:
The first assumption is that the tensile/compression strength on each element is calculated assuming a uniform load distribution, usually using formulas such as:
T = 4Md / (n*D) + N/n
T = Maximum tension force on the more loaded bolt
Md = Bending moment from the tower
n = Number of anchors
D = Average diameter of the anchor cage
N = Axial force
This is usually known as the “Petersen approach”.
Petersen is a German engineer who wrote a book about steel structure design appropriately titled “Stahlbau” (“steel construction” in German) where this calculation method is presented.
The second assumption is that the tensile force is distributed between concrete and steel if there is no decompression.
If decompression happens (something that will always happen under ultimate limit state factored loads) all the tensile force will be taken by the steel.
The only problem with this approach is that the first assumption is only true in case there is no decompression.
This approach leads to conservative results, as it does not account for the force re-distribution due to the stiffness change when decompression occurs.
However, it is very easy to obtain the maximum tension the more loaded bolt or to calculate the needed number bolts or their dimensions for a given tension.
When decompression occurs the stiffness in the “compressed” side and in the “tensioned” side stops having the same value, as the concrete stops providing its stiffness (this is the magic of pre-stressing, before de-compression the concrete is somehow taking some tension, a thing that concrete rarely does).
In the compressed side we will have an area of concrete under compression and bolts in tension (due to the prestressing), that take the compression by de-tensioning.
In the tensioned side we will only have the bolts in tension.
Similarly to a hyper static structure the stiffer side (in this case the one under compression) is able to take more load.
This works like a beam supported by springs:
In the picture on the left all the springs have the same stiffness. This would be the current design model, as in the formula shown above.
In the picture on the right the tension springs (right side of the beam) have only half the stiffness.
This is just to show how stiffness affects the force distribution, in a real anchor cage the loss of stiffness when decompression occurs might be over 80% as the concrete area contribution it is much bigger than the bolts area (total stiffness would be Es*As+Ec*Ac, being Es and As the area and elastic modulus of steel, and Ec, Ac the ones from concrete).
The “softer springs” on the right take less load, that is redistributed to the more rigid area on the left.
Please note that this would not happen in an isostatic structure (with only two supports)
As the neutral axis moves, the redistribution of forces changes. This lead to a non-linear calculation.
To perform this analysis we can implement a model with a homogenized concrete-steel section, and with variable parameters depending of the location of the neutral axis. Using this type model, we would be able to obtain the maximum stress on concrete and the tensile force on the pre-stressing element.
This way the anchors size may be adjusted, and we will get a more accurate value for the concrete compression which is slightly underestimated with the current models.
I am not going more deep into this boring details about calculations but I think that it is interesting to know that there is still room for optimization in anchor cage design.
The IEC (acronym of International Electrotechnical Commission) has just released a new design code. More precisely it is a new section of an existing code, the IEC61400.
The IEC is an international organization that prepares and publishes international standards for all electrical, electronic and related technologies, including energy production and distribution devices.
The IEC 61400 is a set of design requirements developed specifically for wind turbines – to be sure that they are appropriately engineered against damage from different type of hazards within the planned lifetime (currently, 20 to 30 years). If you are familiar with the wind business you will probably know that this is one of the key international standards.
The IEC 61400 has several sections.
Section 1 deals with the wind turbine loads (more precisely, “design requirements”) in most of the world. A relevant exception would be Germany and some of neighbouring countries, where DIBT is used.
The new section released is the IEC 61400-6:2020 Tower and foundation design requirements.
If you are a wind turbine foundation designer, you are already aware that there is not really and internationally accepted design reference for wind turbines: there are some national references (such as the French CFMS Recommendation, or the Chinese FD 003-2007), some guidelines from certification bodies (such as the DNV guidelines), and recommendations from associations (AWEA for example has a recommendation for foundation design, but not a specific code for wind turbine foundation).
If we assume a similar applicability of this code as the one from the IEC61400:1 my opinion is that this is going to be one the more relevant technical reference (if not the most important) in the market for the next few years.
I am not going to enter deep into the technical detail of this standards, but there are a few points I would like highlight:
The new standard specify that foundation gapping does not need to be the limiting factors for foundations in all the cases. This opens the option to reduce the foundation size importantly when the soil is good enough.
Specifies the applicable codes for concrete design and provide guidance in how to perform some calculations (for instance cracking, dynamic shear modulus, etc…)
Has a set of very interesting annexes providing specifics about seismic calculation, strut and tie modelling, rock anchors, etc…
Specifies that there should not be decompression of the tower flange under the extreme (un-factored) loads.
Provides guidance about how to apply the sub pressure and perform the equilibrium verifications (this may modify some existing practises in some countries).
There are several interesting sections in this code, and many about towers and concrete towers that I have not yet analysed deeply but it seems that we might see some changes in the way we design at the moment.
It looks somehow unusual that this code has been issues by an Electrotechnical Commission – given the subject, it looks more like a code that should have been created by an institution of civil/structural engineers.
However I also believe that this type of reference and guidance was much needed in the sector, so I am happy that the IEC had taken the initiative of releasing such code.
If you either work for a WTG manufacturer or any electric utility the situation in which you are asked to do a complete analyse of a project very quickly (like in 2 or 3 days) is very common.
You are usually asked to do this task for several reasons:
Preparation of a first rough layout with preliminary quantities.
To compare different alternative WTG layouts.
For a technical due diligences.
Until not so many years this type of assessments was done using the specialized design software: MDT, Civil 3D, Istram-Ispol, Clip, etc.
These programs are more focused on a detailed design and the processes are not easy to adapt to a different scenario, where quick results are required.
In the last years Infraworks has emerged as a powerful alternative.
It is a planning and design platform from Autodesk that has experienced an impressive development. It offers a bunch of very useful tools that can be easily adapted to the BoP design of a wind farm.
Some of the principal qualities are:
It allows engineers to quickly create a preliminary design in a realistic environment, making possible to position the point of view anywere in the wind fare and immediately be able to visualize the existing conditions.
The program supports data from multiple sources: GIS (shapefiles, geodatabases, etc), CAD, raster, and all kind of BIM-data. These data is integrated into an interactive 3D model.
Cloud technology is integrated through BIM 360. Different users from the same team can work dynamically on the same projects in remote - i.e. from any point in the planet, a feature especially useful in these "work from home" days.
Multiple alternatives called “Proposals” can be generated for the same project.
The program allows to extract quantities, create shadow analysis, analize conditions with style maps using “style rules”, etc.
It is that it is very user-friendly: it takes only a few days to learn the basics and start doing your own projects.
One of its major advantages included in the last releases is that it is possible to import detailed designs from other software such as 3Ds Max, Civil 3D or Revit.
What I like the most is the dynamic flow of work implemented in the most recent versions between Infraworks and Civil 3D. In this sense, I like to think about Infraworks as a good complement to Civil 3D. We can either:
Start the design in Civil 3D and export it to Infraworks.
Create the predesign directly in Infraworks feeding the model with data available in all kind of source formats: shapefiles, raster, etc.
The synchronization between both programs is not bidirectional: any change done in the Civil 3D model is automatically included in the Infraworks model but not the other way around.
Going more deeply in what the program can offer, here are some comments specific for the wind farm BoP items:
The software offers a quick way to obtain quantities for big construction areas such as crane hardstands.
The option “Land Areas & Grading Behaviours” allows the user to easily calculate quantities for gradings and landfills.
One point to improve is that currently there is no way to export this data to any other format: the information must be extracted "manually" to an external BoQ.
It works in a similar way as explained before with the hardstands.
For example, we can quickly model the foundation bottom pit and calculate the excavation volumes.
We can do it to analyse a certain position or, taking advantage from GIS format files and other formats, do an overall analysis of all the positions.
Infraworks has two typologies of roads, which will be used depending on the needs of both the designer and the project:
Planning roads: These are lightweight roads that use spline geometry. You can add planning roads to a model or import roadway data as planning roads. This format does not allow to extract quantities from the model.
Component roads: these are configurable roads in cross section, vertical and horizontal geometry. They provide a precise control of geometry and grades. This is the type of road we are interested in when creating a BoP design in a wind farm because they have available features such as the grading tool and the mass balance quantification.
The interesting thing about this tool is that we can have a full control of all the parameters from a 3D environment and we can export to quantities to a .csv file format.
Additionally, the intersections between roads are automatically created and very easy to handle and edit.
Hydraulics analysis and drainage calculations are hard to deliver in few hours - at least if you want to do a good job.
However, Infraworks offers a useful and easy module to deal with this matter. Some of the main available services are:
Watershed analysis: it includes point watershed creation, watershed analysis along roads and calculation of flows for a calculated watershed according to different methods (rational, regression, etc).
Culvert design analysis with automated culvert placement, culvert analysis and culvert reporting
Roadway inlet and pipe analysis, including automated inlet and pipe placement and inlet and pipe design analysis.
As a drawback, it is worth point out that drainage tools are only available in combination with a BIM 360 account.
Furthermore, depending on the kind of service required, we will need to have "cloud credits" available for certain processes such as culvert analysis.
In any case, it is difficult to find in the market a tool that gives such a powerful and interactive tool for preliminary drainage calculations.
Creation of realistic videos
Useful realistic videos or tours around the model can be created in a very easy way. They could not only look really fancy in a presentation but also are useful to assess whether there is a major mistake in the predesign. You ave an example at the beguinning of this article: the creation of this video took no more than 15 minutes from when the model was ready.
There is also the option to create advanced customized cross section profiles.
This could be of great use to model other elements linear elements such medium voltage electrical cable trench or high voltage lines.
Beyond all the great functionalities described so far, we must not lose the perspective: as it is now, Infraworks is not intended for a full detailed design. As a user you frequently feel that the program lacks "advanced capabilities" that would be highly appreciated.
Autodesk is aware that the software has a great potential and the company is working constantly to improve the product. To have a good idea of the expected developments in future releases I recommend to visit the webpage “Infraworks Public Roadmap”:
These new developments are based on the feedback given by the users. Suggestions and proposed new features are posted in the “Infraworks Idea Forum”. I have personally posted some of them myself.
I also include the link to the Youtube official channel which I consider a good audio-visual reference for those who want to crack on with the software:
In short, Infraworks is already a tool to take into consideration, not only in preliminary designs but also as a support for final designs.
It will be interesting to see whether future developments will transform in a main reference for designers - not only for wind farm projection, but also for all kind of civil works modelling.
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.
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.
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.
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.