February 2019 - Wind farms constructionWind farms construction

Lately I’ve been spending some time trying to learn something more about quality. Although I see that there is no consensus on the business effectiveness of some of these technique I’ve decided to take a certification (ASQ Six Sigma Green Belt) to have a first-hand experience.

One of the first concept I’ve learnt is the difference between defect and defective. This is the standard definition in the wind industry:

“Defect” is defined as a non-conformity, a failure to comply with the Technical Specifications, a flaw in design, manufacturing, workmanship or damage.

“Defective” is defined as a part that has one or more defects and fail due to it.

The key concept here is that in principle it is possible to have one or more defect on a component without having safety or operational problems.

Wind turbines warranties (and presumably other similar equipment) usually are based in the concept of defective – that is, of failure of the component to operate correctly.

The logic is that a failure is usually a black or white concept: either the gearbox is working or it is broken.

However, seen from the perspective of the customer, this definition is not reassuring: a component could have a defect that, even if it’s not preventing the turbine to work, is making it underperforming, unreliable, deteriorating quicker than usual, etc.

Basically the concern of the customer is that the turbine seller will simply “try to keep the turbine alive” until the defect warranty expire (usually after 2 years). Afterwards, it will become a problem of the customer.

For this reason the clause with the definition of defect and defective is usually extensively negotiated. Some possible wording, from the least to the most favourable to the customer, are:

Defective is a part that fails due to a defect
Defective is a part that has a defect that could reasonably cause adverse effects on production or safety
Defective is a part that doesn’t match the Technical Specifications
Defective is a part that contain a defect

Obviously the last one is very onerous for the company who has to mantain the turbine, while the second can offer a reasonable level of protection to both parties.

I’ve received a question regarding material selection for wind turbines blades. The reader asked why there is a predominance in the use of composite materials for the blades instead of wood, steel and aluminium and other materials used in the first glorious, pioneering years of wind energy.

Please note that I’m by no mean an expert so the only intention of this post is to give a very general introduction to the subject. This is a very broad topic involving different engineering branches.

In general the 2 design drivers are weight and stiffness.

A blade should be as light as possible for a variety of reasons:

To lower gravity induced fatigue loads
To be easily transported and installed
To have a better performance

However, it should also be stiff (that is, rigid) for several other reasons:

To withstand loads (both wind loads and gravity loads). Wind loads are function of wind speed and length of the blade, and increase from the root to the tip of the blade. Gravity loads are function of the material density.
To prevent collision between the blade and the tower under extreme wind
To prevent instability (local or global buckling) maintaining its shape

For these reasons blade designers try to minimize the mass for assigned stiffness levels – it is to find a balance between aerodynamic and structural requirements.

So we want less weight (that is lower density) and more stiffness.

Stiffness is expressed by the Young’s modulus of the material – basically the relationship between force and deformation. In general blades are very flexible, stronger in the flapwise direction and weaker in the edgewise direction.

And here is the reason for the use of composite materials. For a given Young Modulus, the material with the lower density is the composite (resin plus glass fiber).

You can see graphically this relationship in a type of graphic called “Ashby Plot” (I attach a version stolen online from a document of the University of Cagliari.

Ashby plot for a wind turbine blade

Wind farms construction

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.

Monthly Archives: February 2019

Wind turbines defective parts warranty

Why wind turbine blades are made of composite materials?