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Millions of collisions at 300 km/h - Ouch!

Rotor blades are made of strong materials that look indestructible, yet they get damaged by water and that costs a lot of money. Harald van der Mijle Meijer (TNO), and Julie Teuwen (TU Delft), put on their rainboots to research how rain impacts the wind turbines’ rotor blades. The goal? To get more insight into how the damage of blades evolves and even better: how to prevent it.

Damaged blades produce less electricity

Harald van der Mijle Meijer
Harald van der Mijle Meijer
In the current offshore wind farms, blade coatings get damaged due to the combination of wind and rain. The leading edge is especially affected since this is the foremost edge of the rotor blade and therefore the part which first meets the oncoming weather. “The high tip speed, strong wind, and rainy conditions cause erosion. The edges of the rotor blades will start to degrade, ultimately causing high maintenance costs and a loss of performance,” says Harald van der Mijle Meijer, senior consultant Wind Energy Technologies at TNO.

Damaged blade
Damaged blade

Formation of damage

Sure, you might think it’s just water, and hardly impactful. But it is. Harald: "The rain droplets hit the rotor blades at a relative speed of over 300 km per hour and they almost completely pass over their energy to the blade. Again and again. That hurts.” The damages caused by this are costly and time-consuming to repair. And they also decrease the energy yield. Reason enough to find solutions to prevent the current situation for wind farm owners.

Julie Teuwen
Julie Teuwen
The WINDCORE project aims to find out more about the formation of damage in the coating of turbine blades. How can we prevent it? The solution might seem simple: just add an extra coating. But, as Julie Teuwen, assistant professor in aerospace structures and materials at the TU Delft explains, “with that coating, you can still get erosion after a while. So it’s still a big problem, and still worth looking into.”

We can prevent damage

With the analysis of lots of data in TNO models the project found some sort of optimum, in which the tip speed is reduced during heavy weather conditions to prolong the lifetime of the blade, and still produce an acceptable amount of electricity.

Harald van der Mijle Meijer and Julie Teuwen
Harald van der Mijle Meijer and Julie Teuwen

Missing data

However, the project was not without challenges. Harald elaborates: “At the beginning, we found out that the data on precipitation was lacking. We needed to get data on the distribution and the size of the droplets, however, that information hadn’t yet been measured. We used the disdrometer data of KNMI site in De Bilt and extrapolated this to offshore conditions.

The rain-making machine

Julie demonstrating the rain making machine
Julie Teuwen demontrating the rain-making machine
Another challenge was that the machine we intended to use for the laboratory tests was broken. So TU Delft created and developed in a short period a completely new machine. This new machine can best be described as a “rain-making machine”, which does exactly what its name suggests.
A big pump provides so much pressure that water is pushed through tubes at a speed of usually 130-170 metres per second. A disk quickly rotates to “cut” the water, so that the water stream is turned into water droplets. These droplets are shot at a rotor blade sample. This way, the harsh weather conditions of rain and wind that rotor blades are subject to are recreated, so that erosion can be purposely caused - and so that it can be studied.

Rain-making machine
Rain-making machine

According to Julie, the “idea behind the rain machine is that we can test different coating materials in a laboratory setting, so that we can control all the relevant parameters. Then, we can get a better understanding of the mechanisms behind erosion. With that understanding, we can develop new coating materials or provide pointers to the coating manufacturers.”

Test samples
Test samples

Other than the coating material, the complex mechanisms of erosion are also being tested. How is damage initiated? How does damage progress? What are the performance losses? Answers to all these questions - and more - will help identify the incubation time. More importantly, it will help to find out how the incubation time can be prolonged as much as possible. Knowledge of this will help to potentially prolong the lifetime of rotor blades, and lower maintenance costs and efforts.

Test samples
Test samples

Combining knowledge of different disciplines

Of course, such a challenging and multidisciplinary project cannot be brought to a successful end without several partners. The open data from the KNMI was instrumental to understand the rain and wind conditions at several locations in the Netherlands. Industry partners, such as RWE and Shell – as operators of wind farms the main “problem owners” - are also involved to help solve the problem. And RWE ensured that relevant coating systems were tested in the project. Wind turbine manufacturer Suzlon has shared the designs of their blades and made the panels which were then coated and tested in the project. In a follow-up project, even more parties will be included, such as meteorological partners, operators, and research institutes.

Harald enthusiastically highlights that “we are proud of what we added to the research community on this topic”. As a result, the insights will not stay secret. Reports and articles have already been published to share what has been found so far.

RWE expertise in operation wind farms

Shell expertise in operation wind farms

Suzlon expertise in turbine blades

TNO expertise in wind turbine control, aerodynamics and O&M modeling

TU Delft expertise in materials and erosion behaviour

Damaged blade
Damaged blade

A maintenance-free blade

Finally, Julie shares Harald’s belief that “it is possible to end up with a maintenance-free blade”. Such a blade would not only significantly lower maintenance efforts and costs but also suspend the possible energy loss. This could easily save for the owner of an offshore wind farm of 770 MW approximately a million euros each year, if not more. This decrease in costs is so significant.

Find out more about the WINDCORE project

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