Wind turbine towers – the bigger the better

Wind farm repowering with higher tower

It looks like the trend in the business is to go as high as possible.

The places with the best wind conditions have been already used in the first 20 something years of the wind industry.

Now it’s time to work with low wind sites – and probably, repower the older wind farms.

I consider tall a wind turbine tower of over 100 meters. They are not unusual and the majority of big OEM have solutions for low wind sites in their portfolio.

Both Nordex and Repower have tower of over 140 meters. Vestas has a 120 meter tall steel tower, Acciona has a 120 meter modular precast concrete solution and Enercon is ready to  market a 149 hybrid (concrete + steel) tower for the E-101.

Siemens / Gamesa is not behind – they just build a 153 meter tower in the Sarahnlom wind farm (central Thailand).

With the cost of energy free falling and an aggressive competition from the solar energy manufacturers and developers are squeezing every dollar from the projects.

The benefit of taller WTGs is not only an increase in energy production, but often also less turbulence. This usually means a longer lifetime for the wind turbine and lower cost for operation and maintenance (as the loads on the system are lower).

It’s difficult to say upfront which solution is better – for shorter tower is usually steel, for taller tower either concrete or hybrid.

However this is a general rule and it should be crosschecked against local market conditions.

Price of steel is more or less uniform (unless there are huge tariffs, like for instance in Brazil).

Price of concrete is much more dependent on local conditions – if it’s possible to buy aggregates nearby, if there are big cement plants, etc.

As a result, precast concrete towers are usually better in big wind farm (50+ wind turbines), where transportation cost for the steel solution are much higher.

An additional benefit of in situ concrete tower is that it will boost your local content.

 

Acronyms in wind farms construction

I’ve been informed by one of my most affectionate reader that some acronyms that I’m using in the blog are not immediately clear.

Therefore I’ve started a first list of the most used ones - special thanks to Janos for helping expanding the list:

AEPAnnual energy production.
BoPBalance of plant. All civil (roads, foundations, crane pads) and electrical works (cables, substation, etc.) in the wind farm.
CapExCapital Expenditures
CODCommercial operation date
COECost of energy
EISEnvironmental impact sudy
EPCEngineering procurement and construction. A type of contract (also known as "turnkey")
FIDICInternational Federation of Consulting Engineers  (Fédération Internationale Des Ingénieurs-Conseils)
HSHealth and safety
HVHigh voltage
IPPIndependent power producer
IRRInternal Return Rate
LDLiquidates damages
MLAMechanical Load Assessment
MVMedium voltage
MWMegawatt
O&MOperation and maintenance
OEMOriginal equipment manufacturer. Here, the company producing the wind turbine.
OMAOperation and Maintenance Agreement (sames as SMA)
OpExOperation expenditures
PCCPoint of common coupling
PPAPower purchase agreement
RoWRight of way. The legal right to use a certain route.
S&ISupply and installation
S/SSubstation
SCADASupervisory control and data acquisition
SMAService and Maintenance Agreement (same as OMA)
SoWScope of work
TSATurbine supply agreement. The contract between the wind turbine manufacturer and the wind farm developer.
W&SWind and site. Usually, either the assessment of the wind farm (W&S study) or the department doing it.
WFWind farm
WTGWind turbine generator

Chinese wind turbines - are they coming?

Well, apparently the answer to this question is "not yet".
I've been through an interesting article from Bloomberg - the  images of this post are taken from the same source.
As you can see, in addition to a new reshuffle in the top 3 (Vestas up again, together with GE), there is another important piece of information: Goldwind (and other Chinese companies such as Guodian United Power) are big but they are selling mainly in China.
European manufacturers have not been able to penetrate the Chinese market, but also the opposite is true.
Nevertheless, maybe the wind is changing: for instance, Envision has been recently awarded 4 wind farms in Argentina.

 

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