What Makes Composite Materials So Ideally Suited To The Construction Of Wind Farm Blades?
Building a wind turbine is tricky business. These huge marvels of technology are at the cutting edge of materials technology. As the demand for green energy grows, the industry becomes more and more competitive.
While there are many challenges that relate to all aspects of a wind turbine's design, undoubtedly the most difficult components to get right are the blades. How many blades will produce the optimal amount of energy? How long do they need to be? How light can they be? Are they going to be strong enough?
These are just some of the questions that thousands of engineers and researchers are tackling across the world each and every day. As you might expect, the answer to many of their problems seems to be found in the form of composite materials.
Today we are going to look at the properties that wind turbine blades need to have for what maximum efficiency and reliability, and explain why composite materials are the only tools for the job.
Let’s get started.
What Properties Does A Wind Turbine Blade Need And Why?
When it comes to operating efficiency, bigger is better with wind turbine blades. They need to be as long as possible to extract the maximum amount energy out of the wind. Sounds pretty straight forward right?
Well as you probably already know, when you start building things on the scale of wind turbine blades the properties of the materials used in construction become your primary concern. Blade mass scales at the cube of the of the turbines radius, which means as they get longer – they get much heavier.
When blades reach the lengths that we are used to today, this creates buckling issues where the blade can easily be crushed from under its own weight when it reaches a certain size. Obviously, this is not an ideal thing to happen to a blade that is quickly spinning around in the air (often within a few hundred meters of populated areas).
The second issue that engineers are concerned about is stiffness. Again the sheer size of the blades and the rotational forces that are placed upon them can make things go a little bit wobbly. This means that materials that are not strong enough (and not stiff enough) can flex significantly while in use and there is a real risk of a blade striking the tower. This again is obviously a less than ideal situation.
Wind turbines are designed to allow for a little bit of wobble in the blades, it's an inevitable consequence of the limitations of even our most advanced materials. However, it goes without saying you want the blade to flex as little as possible.
Alongside these two main concerns are several “secondary” issues that need to be taken into account. These are things like longevity, ease of maintenance, damage resistance, and of course economic viability.
Early Building Materials
So to recap, what we are looking for in a material for a wind turbine blade is this:
- Something that can be made long and slim (while remaining stiff)
- Something that is light enough for it not to be crushed under its own weight
It’s seemingly obvious today that the perfect material for the job is going to be some kind of modern composite material. But that didn’t stop early engineers and researchers from trying other materials first.
The first wind turbine experiments were made with wooden blades. It’s easy to see why wood was a potential candidate. It’s a low-cost, low-density material that can be surprisingly resilient to fatigue. However, the natural variations that arise from wood being a natural material (alongside its ability to absorb water) quickly ruled the material out as a primary option.
This being said, small wood/epoxy composite blades (less than 10m) that were made in the 80’s are still in operation today in a handful of places.
Steel was also experimented with in the 80s, most notably in Germany. The combination of steel spars covered with a fibreglass skin was almost immediately noted to be better than previous wooden constructions. However, it was not without its own set of issues that arose from its higher density (causing problems with inertial and gravitational forces).
Aluminium was another candidate that was considered. And it solved many of the issues that were uncovered with steel (as it had a comparatively low density). However, the nature of the rapidly changing loads that are inherent to wind turbine blades made the material unsuitable.
It didn’t take long for engineers to figure out that composite materials were the way to go with blade design. To this day fibreglass is the most commonly used material in blade construction.
It is light, strong, durable, easy to maintain, easy to fabricate and potentially most importantly – it’s cheap. As if by magic fibreglass solves all of the problems that were encountered during the evolution of wood, steel, and aluminium blades in one fell swoop.
Additional benefits were also to be found that were potentially unexpected. For example, fibreglass does not conduct electricity anywhere near as much as metal based blades. Making it perfect for inevitable lightning strikes that could otherwise cause issues.
In terms of mass production, the world has not really moved on from fibreglass blades - what worked well then still works well now. Nevertheless, that does not mean that progress is not being made.
Modern composite materials like carbon fibre and Kevlar have been proposed and experimented with. They are undoubtedly better suited for the job and would allow engineers to create longer, thinner, lighter and stronger blades with relative ease. However, the mass production capability required to make them economically viable is lacking, to say the least.
A few companies (Vestas, Gamesa, DeWind) have decided to make the leap to carbon fibre and there are several carbon fibre blades in operation today. However, they are certainly in the minority and fibreglass is still the most common choice. Experiments are being conducted utilizing carbon fibre and fibreglass combinations (which are yielding some excellent results). But again they are still almost at the experimental level in terms of usage.
That said with the amount of research and investment that has been pumped into renewable technology in the last decade, we expect big changes in the near future. It’s easy to think that the world will move away from pure fibreglass blades reasonably soon. It’s almost inevitable that we end up using more advanced composites (and combinations) as technology advances and/or material costs come down.