Monthly Archives: December 2017

Characteristics of wind turbine blades

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Maintenance of a wind turbine blade

The most elegant element of the wind turbine is, at least for me, the blade.

Blades are currently reaching incredible lengths (onshore we are almost at 70 meters, offshore they can be even bigger) and, as I discussed in this post, can be made of several materials.

The cheap solution is fibreglass, more heavy, while the technological advanced, lighter (and more expensive) solution is carbon fibre. They are submitted to several loads of different origin – not only aerodynamics but also inertial, gravitational and other loads induced for instance by ice.

The main design drivers are aerodynamics, aeroelasticity (the correct damping of the blade) and fatigue behaviour.

But there are other technical requirements, and more technology hidden in the blade:

They must resist lightning. For this reason they often incorporate metallic elements to conduct the electricity to the tower and from there down to the ground. Lightning strikes are a relatively frequent – so frequent that there is a specific norm on the topic (IEC 61400-24). They usually hit the nacelle or the blades. The surprising part is that many lightning striking the turbines are upwards – that is, they go from the turbine to the sky. The metal conductor, usually in copper or steel, can be embedded in the surface of the blade or can be inside it.

They must resist ice. Some models include a mechanism (usually fan heaters or resistors) to warm them and avoid the accumulation of snow and ice, pernicious for stability, production and potentially even dangerous for people working in the area. There are also microwaves solution, that have a low energy consumption, and “defensive” (or preventive) solutions, such as hydrophobic foils. Basically, the ice will not stick to the blade.

They must resist erosion. 20 years of UV, sandstorms can seriously damage the surface of the blade, impacting production. Several solutions have been developed, such as special paints and epoxy or acrylic materials.

They must resist strong winds. During the life of the turbine, the blade can (and it probably will) be exposed to extreme winds.

They must be silent. A relevant percentage of the wind turbine noise is generated by the blade  - usually around the tip. In several countries it’s compulsory to reduce the noise level under a certain dB threshold.

One of the most intriguing characteristics of the blades (at least for me) is the fact that  they are “twisted”.

Conceptually, wind turbines blades works like the wings of a plane.

But on the wings of a plane, the speed is the same from the root to the tip, while on the blade increase from the root (where the blade is moving relatively slowly) to the tip (where speed is maximum).

Therefore, in order to have the correct angle of attack and keep constant the mechanical torque in each section of the blade, the angle of attack decrease from root to tip.

 

The gentle art of EPC wind farms: design responsibility

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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.