Wind site assessment

The wind site assessment (or "wind and site" assessment) is one of the most important steps in the development of a wind farm.

Basically is a in depth analysis of the site conditions of the area where a wind farm could be built.

Purpose of this assessment is calculate energy production and suitability of a specific WTG model to the local conditions.

Such study is usually performed by different stakeholders – external consultancies (possibly on behalf of financial institutions), wind turbines manufacturers and even developers (if they are big enough to have a wind & site department in house).

The inputs for the site assessment are:

  • Wind data
  • Topography & Roughness conditions
  • Other environmental conditions

Wind data includes usually raw data from one or more met masts. Measurement period should be sufficiently long, ideally several years. Key data are the wind rose (from where the wind is blowing), the distribution of the wind speeds (it follows a Weibull distribution) and the normal and extreme wind speeds.

Topography & Roughness conditions have an impact on turbulence, flow inclination and local speed up effects that can be key in the selection of the correct wind turbine.

Other environmental conditions include parameters such as temperature (both very high and very low temperature will need a special “package” and usually leads to decreased energy production), air density (will change the loads, and if very high or low could lead to a derating) and seismic actions (will the tower withstand earthquakes?).

All this data is cross checked against standard wind classes. These classes have been defined by the IEC, an international committee of experts, and are often used to categorize a wind turbine model. For instance, wind turbine A could be certified for wind class I (strong wind) while wind turbine II could be certified for wind class III (weak wind).

It’s important to highlight that usually there is uncertainty on one or more parameters. Therefore different assumptions are made by the wind & site engineers. The interesting part of the story is that, depending on where you are working, you will be interested in “twisting a bit” the numbers in a different direction.

For instance, an external consultant will usually be more conservative when analysing energy production (as he doesn’t want to be blamed if the actual production is lower).

Conversely, a WTG manufacturer could possibly give you higher number when calculating energy production. This is good for 2 reasons: because more production means more money for the prospective customer, and because considering higher loads put the engineer on the safe side when assessing expected life of the key components of the wind turbine.

Land lease and site access: 5 usual mistakes

You would be surprised to discover the amount of problems that are generated by missing, uncomplete or wrongly defined land lease and site access agreement.

Land lease contracts must be negotiated by the project company (that is, the developer of the wind farm) with the landowners.

It’s extremely rare to have the whole wind farm built on the land of a single owner. Usually, wind farms are built in agricultural areas – therefore these contacts must be negotiated with several counterparts.

The most usual problem connected with such contracts are:

  • Incomplete land acquisition. I’ve frequently seen layout changes at the very last minute because the project company couldn’t close one (or more) leasing agreement. The consequence is that roads, crane pads and/or wind turbines have to be repositioned.
  • Wrongly defined land occupation. A classic situation – it could happen for instance that the developer has a contract granting few meter of width to build a road. The problem is that to build a 5 meters wide road, you will need much more space to move with construction equipment, to store materials, to have space for embankment, etc.
  • Right of way wrongly defined. This is a classic as well – developers are sometime not aware of the amount of space needed to move the blades. This might force you to touch wall, trees or other properties not included in the agreement.
  • Social conflicts due to different payment terms. In some situations, a conflict may occur if neighbors are getting paid different amount of money for the same land lease agreement. Correct strategy here is to offer a fixed sum to all.
  • “Aerial” rights. I’m not sure about the terminology. However, I’ve worked on a project where, when the WTG was orientated in a certain way, the blades were rotating over a land plot without a land lease. Guess what? Yes, we had to move the turbine.

Split EPC contracts

As you probably will be aware of if you are reading this blog, an EPC is a typology of contract where a company agree to develop the engineering, procurement and construction of a facility (in this blog, a wind farm) for a fixed, “lump sum” amount.

The key advantage of this type of contract is the existence of a single point of responsibility.

This improves in some situation the bankability of the project, as it puts the investors in a simpler position - if somethings go wrong, they have only one counterpart to deal with and there is no room for common discussions like “it has been built correctly but the engineering is wrong” or “I was on schedule, but this other subcontractor is late”.

However, sometimes the EPC contract is split. This word can be used with different meanings.

If it is referred to the contract between the wind farm developer and the main EPC contractor, it usually means that 2 different contracts are created, one for the onshore construction and another for the offshore supply. This is normally done for taxes purposes.

The second meaning refers to the fact that 2 different contracts are created – one for the supply of the wind turbines and another for the balance of plant. In this case, there are 3 parties involved: the developer of the wind farm, the wind turbines supplier and the construction company.

With this setup, a third agreement is needed to deliver a single point of responsibility despite the split. This third agreement will “wrap-around” the other 2 contracts defining coordination, interfaces and guarantees. Obviously, the lender will try to keep the other 2 parties jointly liable as much as possible.

The third meaning arise from the main contractor perspective. In this case, splitting a contract means dividing the task between 2 or more subcontractors (usually one for the civil works and another for the electrical works).

For instance, the main EPC contractor (for example, the wind turbines manufacturer) could be interested in closing 2 other EPCs – one for the civil works and another for the electrical works. Usually, splitting contracts will reduce cost and increase the risk and complexity of the project.

Technical due diligence of a wind farm

It is a hard task to compress in a blog post the reasons behind the technical due diligence of a wind farm and the several points that must be evaluated.
In a nutshell, in the clear majority of the wind farms developments are built using borrowed money.
The equity (cash at risk) is put by the developer, while the debt (money given against some form of security) is provided by a financial institution, or more commonly by a pool of institutions.
There are obviously exceptions to this rule, that is wind farms developed only with cash coming from the books of the company investing in the project. Nevertheless, these are exception and what is common is to have most the budget (up to 70%) provided by a financial institution such as a bank.
The lenders will be obviously interested in being sure that the financial model behind the project is solid.
Therefore, they will ask for a due diligence to identify, quantify and (if possible) mitigate technical risks.
In general, the lender will check what he considers appropriate.
Normally 3 macro categories are checked:

Financial due diligence, including for instance

  • Hypothesis
  • Budgets
  • Financial models

Legal due diligence, including items such as

  • Land lease
  • PPA
  • Contracts (e.g. TSA) & subcontracts

Technical due diligence

There is obviously an overlap between the various categories – for instance, some items are not purely “technical” or “legal”.
The technical due diligence should investigate in detail several key points.
A short, non-exhaustive list would include at least the following items:

  • Site suitability (wind resources, turbulence, data solidity)
  • Choice of WTG model (track record and match with the wind resource, power curve, certification, etc.)
  • Archeological y environmental constrains (impact on flora and fauna, such as birds and bats)
  • Access to the area (road survey and works outside the wind farm)
  • Geotechnical survey (ground risk)
  • Noise study (a big problem in inhabited areas)
  • Shadow flickering & visual impact
  • Grid connection
  • Electrical losses (are they calculated correctly?)
  • Projects for the BoP (foundations, MV, substation, etc.)
  • Congruence of the time schedule of the project
  • Interface between subcontractors
  • Allocation of risk

WTG tower – concrete or steel?

One of the key decision in a wind farm is the type of tower that will be used to reach the desired hub high.

In the infancy of the wind industry, lattice towers where used – you can still see them in very old wind farm, for instance in southern Spain.

However, this technology was not really a good fit when the hub high reached 50+ meters. The following step has been to switch to tubular steel tower with a circular section, which has been (and still is) the standard technical solution.

In parallel, the concrete tower solution has been developing. This can be either hybrid (the lower part of the tower is made of concrete and the upper section of still) or a full concrete solution (except for a small element on top of the tower that act as a sort of adapter between the last section and the nacelle.

The components of the tower can be either precast in an existing factory or cast in situ in a factory specifically built for the project, usually in the wind farm area. Obviously, this second alternative make sense in big wind farms, with dozens of wind turbines.

Regarding the assembly process, there are different technical solutions in the market. However, in general each tower section is composed by several elements (usually from 2 to 6) that must be assembled together with vertical joints to compose a complete tower section.

After, the different tower sections are assembled together and united with horizontal joints.

The joints are usually filled with grout, and a system of cables run through the tower usually all the way down to the foundation.

The foundation of  a concrete tower is usually smaller and different from the standard shallow foundations used for steel towers.

Is a concrete tower a good choice?

As often, the answer depends on many factors.

From an economical point of view, to simplify the problem, concrete towers are usually competitive when the wind turbine is high (100 m. and above).

From the technical perspective, there are several advantages of concrete over steel:

  • No restriction in the geometric design
  • Greater stiffness (good for resonance) and damping
  • Greater maximum hub height possible
  • Smaller foundation due to increased weight

Ineffable concepts: bankability of a wind farm project

Wind energy, as probably all niche sector, is full of acronyms and hard to define terms.

One of them is the “bankability” of an EPC project.

In a nutshell, it express the idea that the lenders are fine with the development and are ready to put on the table a relevant percentage of the money (easily up to 70% or more), normally with a consortium of financial institutions.

In this post we will focus on the “Capex” part of the problem: we will ignore all analysis related to the expected cash inflow (PPA, wind data, financial models, etc.) and Opex (basically, Operation & Maintenance of the turbines and substation for 20 years).

In a EPC project the money will normally flow from the banks through the developer to one or more subcontractors.

The banks will check several different features of the contracts between the developer and the subcontractor, being  the most important point a fixed completion date and price. To reach this target, they will try to minimize the ability of the subcontractors to claim extensions of time and additional costs.

If unable to reach completion on time, the subcontractor(s) will have to pay delay liquidated damages (DLDs). These DLDs are normally expressed as “dollars per day of delay”. The amount is obviously project specific, but is usually several thousand USD per day and I’ve seen project with over 50K USD/day. Obviously banks likes high DLDs.

Another type of liability is the performance liquidated damages (“PLDs”) that the contractor will have to pay if it fails to meet the performance guarantees. These are usually linked to power curve and an availability guarantee for the whole wind farm, but can also (and do often) include other concepts more directly related to the civil and electrical works of the BoP.

Banks also like large caps on liability – being “uncapped” the best scenario for the lenders (which never happen in the real word).

Connected to these concepts there is the need for the lenders (and the project sponsor) to be able to get money from non performing subcontractors. Therefore, some kind of security (often in  the form of a Parent Company Guarantee) is requested to the main EPC contractor.

Bank also appreciate proven technologies, and like to pay for wind turbine with extensive track record.

In general, a project  may become “bankable” even in a situation where the bank is not 100% satisfied if the sponsor (the wind farm developer, or the shareholders behind) are ready to put a bigger percentage of money, lowering the risk profile for the bank.

Federal Aviation Administration (FAA) Compliance for Temporary Wind Turbine Lighting

This is my first “guest post” in the blog.

I’ve been contacted by Mr Stewart Erwin who asked me to incorporate his article. I think it’s interesting (even if it’s focused only in the US market) and on topic.

It was originally published on LinkedIn.

Mr Erwin works for Carmanah – feel free to contact him for more info.

 There are new changes for wind turbine construction this year. In December, the FAA announced new guidelines for temporary obstruction lighting to increase safety for pilots and flights. To comply, the FAA now requires a FAA L-810 steady-burning red light that can maintain autonomy for 7 days at 32.5 candela on all turbines once they reach a height of 200 ft (61m).  In addition, the FAA reminded the industry that submitting a Notice to Airman (NOTAM) is not accepted to justify not lighting the turbine (FAA AC 70/7460-1L).

If power is not available for temporary lights, the FAA recommends the use a self-contained, solar-powered, LED steady burning red light that meets the photometric requirements (L-810) instead.  Choosing the correct light to meet compliance can sometimes be confusing.  The guidelines are very specific and many solar lighting manufacturers will only have one light that can meet these specifications. It is important to understand the FAA compliance in full, in order to select an appropriate solar product.

Submitting a Notice to Airmen (NOTAM) to justify not lighting the turbine during construction, is prohibited.


L-810 compliant solar lights must also meet the FAA guidelines for candela and autonomy. Some solar lights on the market will have a candela of 32.5 and state they can stay lit or last for 7 days. However, staying lit/lasting for 7 days is different than having autonomy for 7 days. Autonomy refers to how long the light will last if all solar charging is removed – this ensures that if a solar light encounters 5-days of overcast, on days 6 and 7 it will still shine at 32.5 candela. The goal is for light output to remain consistent if it encounters days when the system will store little to no power (FAA EB 76).

Let’s take a look at candela. To meet FAA standards, L-810 lights must have a minimum intensity of 32.5 candela (cd), and that the minimum vertical beam spread must be 10 degrees and the center of the vertical beam spread between +4 and +20 degrees (FAA AC 150/5345-43G). Temporary lights must also sustain autonomy for 7-days at 32.5 cd.

Let’s recap. To comply with 2016 FAA standards during wind turbine construction, your company must:

1. Light wind turbines once they reach 200ft during construction. (submitting a Notice to Airmen (NOTAM) to justify not lighting the turbine is prohibited)

2. Use a FAA L-810 compliant lights with a minimum intensity of 32.5 candela (cd)

3. Ensure temporary solar lighting systems have 7 days of autonomy at 32.5 cd

Trenchers in a wind farm: do they make sense?

In a nutshell, the answer is yes – at least if you are able to find one nearby at a reasonable price.

In general, you will have several alternatives to dig your cable trenches:

  • Backhoe
  • Backhoe with hydraulic breaker hammer
  • Explosives
  • Trencher

The smarter choice will depend on the hardness of the rock.

In general, the use of explosive for trenches in wind farms is extremely rare – it is normally limited to foundation excavation.

Backhoe is always an easy solution, above all in situation where the trenches follow a very irregular path with a lot of change of direction.

However, it’s production is lower when you have compact, very hard rock (for instance, basalts and granites).

On top of that you can have a border of the trench which is not very “clean”, as it could be difficult to open a truly rectangular section.

All in all, it will usually be a better choice in small wind farms on soils or soft,, fractured rock.

The biggest problem with trenchers is that it’s not always easy (or cheap) to find one for your project. They are very performant with long distances, as they can easily open from 1 to 3 meters per minutes in soils (e.g. silts and clays).

Obviously they are somehow slower in rocks. In a very soft one (e.g. gypsum) you should work at a pace of around 1 meter per minute (from 40 to 80 m/h), going all the way down to few meters per hour while you go up in  the Mohs scale.

As an order of magnitude, in limestone you would get from 15 to 40 meters per hours, in sandstone from 10 to 30 m/h and in gneiss less than 10 m/h.

Cost drivers in Electrical Balance of Plant

Due to my education as a Civil Engineer there I already wrote a substantial number of posts regarding cost of the civil BoP.

However I do not want to neglect the electrical side, which as you might already know is usually accountable for approximately 50% of the total cost  of the balance of plant of a wind farm.

I went through the cost of several projects I’ve worked at in the last 6 or 7 year together with a very good friend that I’ve left in Madrid to see if it was possible to find a recurring pattern in the numbers.

Unfortunately, the Electrical Works costs are much more fragmented than the Civil Works, where few “usual suspect” such as concrete, steel and earthworks dominate the scene and are the key cost drivers.

If you are working in the wind business you will be probably thinking  that the most expensive items will be the main transformer.

This is not always the case: in project where we had to quote a long overhead line, it absorbed up to 40% of the electrical budget, quite an impressive figure. Even shorter overhead lines could easily end in the 10% to 20% range, that in a multimillion project  is obviously a big number.

The second item competing with the transformer in the Top 3 is the medium voltage cabling system.

Obviously is extremely difficult to give a number because it will depend on the layout of the wind farm (will it be a row of WTGs or a “cloud” of scattered positions?). Nevertheless, numbers in the 3 to 4 million USD are not unusual even for medium size wind farms.

Then you have the transformer, the last of the Top 3 items. This is the easiest item to quote, usually somewhere around 1 million USD.

Last but not least we have “the rest”. This include everything from the switchgears to the high voltage equipment to the capacitor banks, substation facility and other fancy equipment in the substations.

The impact of all this item can be huge, from 30% all the way up to 70%. Obviously, with such fragmentation it becomes clear that from the cost structure point of view Civil Works and Electrical Works are totally different.