About Francesco Miceli

Hello!My name is Francesco and I'm a civil engineer specialized in EPC (that is, "turnkey") wind farms projects.I'm currently based in Hamburg, Germany and I'm developing several interesting project all around the world - southern Europe, LATAM and various other countries.If you want to contact me please don't leave a comment in the blog (I don't check them very often) - you can use the contact form.You can write me in English, Spanish and Italian.To find a (somewhat concise) description of my non-wind business activities you can visit my webpage - www.francescomiceli.comIf you want to know more about my work, here you can download my CV - www.windfarmbop.com/CV_Francesco_Miceli.pdfHope you like the blog!Francesco

The gentle art of EPC wind farms: design responsibility

In the ideal FIDIC world there is little space for the discussion about design responsibility.

Either the Employer (FIDIC Red) or the Contractor (FIDIC yellow & silver) is going to do all (or almost all) the design.

Unsurprisingly, real life is more complicate than this. You will have an hard time trying to get reasonable offers from subcontractors without providing at least a preliminary design: subcontractors are usually bidding on many project at the same time, and their engineering department is often quite busy with running projects.

There are basically several different scenarios:

Employer provide the design, contractor build the wind farm. This happen often. In this situation, you will have a tight control over what it’s build. However, if something goes wrong it can be a problem to prove that the problem has been the construction and not the design – that is, you are left with an interface risk.

Employer provide the preliminary design, contractor provide the constructive project. This second option is very common as well. The gentle art here is to force the subcontractor to take full responsibility on the existing design.

Design “started” by employer and completed by the subcontractor. The main difference with the previous point (preliminary design for tender) is that something more detailed than a preliminary design but less detailed than a ready for construction project. Same story, you will usually want the subcontractor to warrant the existing documentation to avoid disputes.

Both design and construction provided by the contractor. This would be the “pure” EPC. In the wind energy business is not as frequent as you might think.

In general, the employer will try to retain some control on the design process and at the same time unload the risk and responsibility on the subcontractor. The gentle art consist in incorporate in the contract provision for design review.

Additionally, the employer will need to drive the subcontractor in the right direction using the proper mix of technical specification, quality requirements, industry standard and a properly drafted scope of work.

Last but not least, sometime the employer has a “main employer” or commitments with other parties (e.g. lenders) – all this obligation that can (and usually will) impact the design must be passed down as back to back as possible.

Gearbox in wind turbines

Why do you need a gearbox in a wind turbine?

The short answer is that you don’t need one – if you are using a direct drive WTG. But even if the solution without gearbox is used by several manufacturers (e.g. the Goldwind 2.5 PMDD, Enercon models, etc.) the majority of makers decided to include this technology.

Purpose of the gearbox is to increase the rpm (revolutions per minute). The blades rotate very, VERY slowly. It is also important to mention that the longest the blade, the lower is the tip speed of the blade: you do not want to increase it to avoid generating noise and to lower the loads on the blade itself.

In order to reach the correct rotational speed and generate power at the frequency needed by the grid you will need to use a gearbox between the main shaft (connected to the blades) and the secondary, “high speed” shaft linked to the rotor in the generator. The conversion ratio depend on the WTG model, but can be around 1:100.

The gearbox must survive over 20 years with very high, cyclical loads. Torque can be extreme during emergency shutdown, and is usually high during start ups. The failure of a gear box is a very big problem, as you will have a long production downtime and you will need a crane to disassembly the broken component and install the new one.

Additionally, gearboxes should be as silent as possible, have low vibrations and dissipate quickly the heat produced by the internal mechanisms. Therefore lubrication systems and vibration absorber mechanism are crucial in their design.

Gearboxes are usually built using planetary gearing system, and are equipped with several  auxiliary system. For instance, it is possible to analyse the density of particles dissolved in the lubricant oil and the way the gearbox vibrate to detect problems and predict possible failures.

 

Wind farm earthing and optical fiber cables

In this post of many, many years ago I explained how wind farm trenches are usually built.

In addition to the medium voltage cables, in the trenches usually there are also 2 other type of cables:

  • Earthing cables
  • Optical fiber cables

The earthing cables are usually made of copper and they are used to dissipate fault currents, coming usually from lightning or short circuits.

Typically the earthing cables connect all the wind turbines with the substation. In the turbine side, they are usually connected with an earthing bar inside the tower.

Additionally, there is also a second earthing system inside the foundation connecting the earthing bar with the steel rebars inside the concrete.

This system usually grant low earthing resistance (<10 Ω) in the majority of cases. In specific situations (for instance, wind turbines in rock with high resistivity) it can be necessary to use additional measure to lower the resistance, for instance using several auxiliary copper rings around the foundation.

The optical cables bring all the information recorded in the wind turbine and the met mast to the SCADA system. Usually there is a software installed in a specialized server in a separated room of the wind farm substation.

From there, the information reach the stakeholders via an internet connection (usually there are remote control centers).

The fiber optic cable usually have from 4 to 16 fibres and, due to the distance, they are usually single mode. The topology of the optic fiber cables connection depend on the redundancy asked to the system. If you really want to avoid the risk of losing control of the turbine in case the optical fiber is damaged somehow, then the correct solution would be a redundant ring topology where a WTG is connected to the SCADA system following 2 different paths.

 

Wind derivatives: is hedging the risk the next step for wind energy?

Weather derivatives are not a new product. The first contract were traded over the counter in the ‘90s, with the Chicago Mercantile Exchange (CME) introducing from 1999 a broad set of products like futures and options that are widely traded today.

They are a financial product that can help hedging the risk associated with the inherent variability of weather.

They are not like an insurance. With an insurance, you know that one or more events (for instance, a hurricane damaging your wind farm) will trigger a payment if certain contractual conditions are met.

Derivatives are more “continuatives”. Simplifying a lot, you can get money if a certain index is above (or below) a certain threshold in a given time frame.

For instance, a common contract traded in the CME is linked to the (cumulated) difference between the actual temperature and 18⁰C. Basically, if the weather is warmer than usual you will have a payoff: this will lower the business risk of companies whose activity is linked to cold weather (for instance, selling products to household heating).

In the previous example the index underlying the derivative is the temperature. In the case of wind energy, derivatives can be built around 2 different concept – wind speed (as measured at the met mast or in a meteorological station) or wind power (that is, hedging the actual production of the wind farm).

A second categorization would be the typology of contract built around the chosen index. At least theoretically, all the standard structures are possible – e.g. futures, options, floors and other types of cash settlements.

I’m writing this post because I’ve noticed that, in countries where the wind energy has a high penetrations, wind derivatives are not a mental experiment – they are already a reality: for instance in Spain there is a specific market for them, and this second product cover the highly developed German market.

My first impression is that wind conditions are very local – therefore it can be hard to find an off the shelf financial product considering a wind index that match the conditions of the area where the wind farm is operation. Possibly these products are more useful for an utility (trying to hedge the risk on a nationwide level) than for a small energy producer.

Medium voltage power cables in wind farms

I already discussed in another post how the wind farm cable trenches are usually built.

However, for a more comprehensive explanation, some additional words on the medium voltage cables are needed.

The power produced by the wind turbine is usually evacuated to the substation using a medium voltage (MV) cables connection.

This cables are usually buried. This solution is slightly more expensive but it offer much more protection to the cables than an overhead line (that is, a line where the cables are hanging from poles). I do however see every now and then projects with overhead MV systems.

The reason for evacuating the power generated by the wind turbines with a medium voltage system is purely economic. A low voltage solution would have very high power losses (“Joule losses”) in the cables – basically the resistance of the conductor would create too much heat.

A higher voltage will decrease the current flowing in the cables and the related losses.

However, high voltage equipment is very expensive. Therefore the medium voltage solution is a reasonable compromise (the optimum balance) between the losses in the cables and the cost of the equipment.

It’s important to highlight that the MV level used in the wind farm can be discretionary – that is, you can work at 20 kV, 33 kV, 34.5 kV, etc.

In situations where there is no specific requirement from the owner of the wind farm (or from the owner of the grid) the smartest choice is usually to select  the MV level used in the country where the wind farm will be built.

Other significant project constraints that must be checked are the allowable current (how many amperes are transmitted by the cable – to be sure that the capacity of the cable is not exceeded) and the voltage drop between the 2 sides of the circuit (usually it should be less than 1.5%).

Last but not least you want to minimize the aforementioned power drop (increasing the diameter of cables, for ins A rule of thumb is that the losses should be less than 2%, but some wind farms have more aggressive requirements.

How to measure wind resource

Met mast, weather vane and anemometer

I have to  start with a disclaimer – I’m not a specialist in wind resource analysis.

However, through the year I’ve seen several time this process so I think I can summarize it with a reasonable level of accuracy (and obviously if you spot a mistake, please let me know).

The wind resource assessment is done for several reason: to define the most suitable wind turbine given the local meteorology, to define the layout of the wind farm and, above all, to calculate the expected energy production of the wind farm – which is obviously a key input in the calculation of the profitability of the project.

To calculate the wind resource you will need to measure several variables:

  • Wind speed
  • Wind direction
  • Wind shear
  • Wind turbulence
  • Air density
  • Roughness of the area

This variables are usually measured installing one or more meteorological mast (“met mast”) in the area where the wind farm is planned  - obviously with the exception of the roughness, which is assessed by a specialist keeping into account the topography and the vegetation of the area.

This activity is called “site measurement campaign”.

The met mast is a tower made of steel (or, more unusually, in concrete) where the measuring equipment is installed. Ideally the met mast should have the same high of the wind turbines that are going to be installed in the area – however, to save money sometimes shorter masts are used.

The equipment installed on the met mast include usually the following:

  • Anemometers (usually there are several anemometers at different heights)
  • Weather vane (to record the direction of the wind)
  • Barometer
  • Thermometer

All the information collected is safely stored in an element called “data logger”. Auxiliary elements in a met mast are solar panels, a protective lightning rod on top, anti-vandalism fence and obviously the foundation.

Ideally, at least 1 year of data should be recorded. However longer measurements (2 to 5 years) have less uncertainties and capture better the seasonal and intraday variability of the wind in the area.

After the site measurement campaign the wind resource assessment start.

The first step is to “clean” all records before processing, removing errors that can occur due to malfunctioning of the instruments.

After, several key parameters are defined:

  • Mean speed
  • Wind rose
  • Wind speed distribution
  • Wind shear
  • Wind turbulence
  • Air density, pressure, temperature

The last step is to use this parameters to estimate electrical power production. There are quite a lot of commercial software in the industry, being some of the most widely used WAsP, WindPRO and OpenWind .

This software will try to optimize the wind farm layout to maximize energy production considering certain limitations – for instance, they will leave a distance of at least 6 rotors from one wind turbine to the other in the direction parallel to the wind.

Finally, when the layout is defined, the software will combine the power curveof the WTG with the wind speed distribution of the site to have the power output.

 

Wind sector management – how to put more wind turbines in the same area

Wind sector management - image curtesy of wasp.dk

Wind sector management - image curtesy of wasp.dk

In many project my colleagues from the wind and site department (the people who calculate the best wind turbine model and the optimal layout in a wind farm) are forced to put quite a lot of wind turbines in a reduced space.

Each of these wind turbines generate a “wake effect” – basically, they create turbulence in the wind.

These turbulences can affect other turbines nearby, increasing loads. This is not good, because higher loads usually means more problems due to component failures.

Wind sector management it’s a solution to this problem – basically, when the wind is blowing from a certain direction some turbines are automatically shut down.

There are basically 2 alternatives: you can shut down the turbine upstream (the one creating the turbulence) or the one downstream (the one suffering the increased loads).

Stopping one or more wind turbines will obviously result in a loss of production. However, the guys in wind and site often found that, even considering these losses, the global output of the wind farm is higher in a densely packed wind farm with wind sector management then in a configuration without it.

In the market there are also more advanced solutions that, instead of stopping completely the wind turbines, change only some parameters of the WTGs. For instance the optimization algorithm could decide to change the speed of the rotor or the pitch of the blade.

Wind sector management is one of the curtailment that a wind farm can have. Other typical restrictions are linked to environmental issues (noise, shadow flickering, birds or bats) or to requirements coming from the grid.

Dynamic loads on wind turbines

I discussed in another post the relevance of resonance analysis in the design of steel tower for wind turbines and the different technical solutions currently in the market.

But what are the loads that could induce resonance in the wind turbine?

There are several different type of dynamic loads:

Unbalanced rotor. This basically means that the blades have not the same weight. This problem can happen for several reasons: accumulation of snow, ice or simply variance in the production of the blade itself in the factory. The consequence is that the centre of gravity will move with the rotation of the blades inducing a centrifugal force in the system.

Tower shadow effect. It is also known as dam effect – basically, the tower affect the speed of the wind nearby. This will affect the blade passing in front of the tower, unbalancing the rotor.

Wind shear. The wind will usually have a different speed increasing from the bottom of the rotor to the top.

Errors in the configuration of the turbine. In this category I include any type of asymmetry generated by design mistakes, assembly errors, software errors, etc. For instance, a difference in the pitch of the blades could create a dynamic load.

What is wind turbine certification?

Wind turbine type certificate: certification process steps

Wind turbine type certification is the accreditation, done by a reputable third party (“Certification Body”), that a manufacturer is selling a wind turbine that meet relevant standards and codes.

TUV, DNV-GL, Bureau Veritas (among others) are examples of Certification Body.

The scope of certification, according to the industry standard IEC 61400-22, can be:

  • Prototype certification: the evaluation of a new wind turbine design
  • Type certification: the evaluation of a wind turbine design and serial manufacturing process

Additionally, there are 2 other type of certifications are available:

  • Component certification: this is usually done for the most critical main components (e.g. the gearbox, transformer, etc.)
  • Project certification: the expected behavior of a group of WTGs on a specific project site. It would include the assessment of country specific laws and regulations, foundations, electrical network, etc.

In general type certification has several benefits, such as better credibility of a new WTG model and easier access to financing and to new markets. It makes clear that it’s possible to manufacture, install and maintain wind turbines of a certain model.

Therefore the type certification process is usually the most important - even if it's often achieved starting with prototype certification in a previous phase.

Type certification goes through several steps, some mandatory and some optional.

Mandatory steps are:

  1. Design basis evaluation. This step check if standards, assumptions, methodologies, etc. used in the design are in line with IEC 61400-22.
  2. Design evaluation. In this step the certification body verify that the design has been made following the design basis of the previous step.
  3. Manufacturing evaluation. Here a quality system evaluation and a manufacturing inspection are performed.
  4. Type testing. This is a set of laboratory and field tests to blades, gearbox, loads and power performance.
  5. Final Evaluation. In this step the findings of the evaluation are provided.

 The optional steps are the evaluation of the foundation design and foundation manufacturing plan and the measurement of type characteristics.

Wind farms: top 5 of myths & urban legends

A curious consequence of writing a blog about wind farms is that every now and then I’m contacted by someone hating wind energy.

In the majority of the cases, the writer also ask for help to stop or boycott a wind farm somewhere in the word, apparently ignoring the fact that wind turbines help me to bring food on the table every night.

To add insult to injury, in the emails they often ask me support to confirm some pseudoscientific “facts” about wind turbines.

I’ve collected some of them in this post – other are coming from anti-wind energy propaganda websites that I read on the subway when I’m bored to discover new, unexpected effect of windmills.

Some of them are similar to the urban legend of the albino alligator in the sewers of New York.

Feel free to contribute!

5. Wind farms causes bush fires. Well, it’s true that (very, very infrequently) a wind turbine burns somewhere. But to say that they are burning woods and bushes it’s a bit too much.

4. The foundations are poisoning the water. Unless you pump 1 million cubic meters of bentonite to do a piled foundation nothing will happen to the water – a foundation is basically only a big stone.

3. The turbine is scaring the fishes – I’m not fishing as much as before. This is courtesy of a fisherman in Juchitan de Zaragoza (Oaxaca, Mexico). I doubt that a fish 10 meters underwater can really hear a WTG.

2. You need more energy to build a wind farm than the energy produced by the turbines. I’ve seen several (serious, scientific) papers on the subject. The energetic payback is usually only a few months.

And the winner is...

1. Wind farms change the weather locally. I love this one. The theory is that, slowing the wind, wind farms modify somehow the climate in the area – some urban legends say that they warm it, some others that they make it more foggy. Well, at least here in norther Germany where I’m living they are not making it any warmer.