How DTU’s Rain Test Facility Predicts Erosion

SPEAKER_1 (0:01)

Rain erosion is a massive problem for wind turbines around the world. And our understanding of rain erosion really starts with testing.

This week, Rosemary visits DTU's Rain Erosion Test Facility to speak with Nicolai Frost-Jensen Johansen. Nicolai describes how leading edge coatings are tested and how to get faster, more realistic results.

SPEAKER_2 (0:26)

Welcome to Uptime Spotlight, shining light on wind energy's brightest innovators. This is the progress powering tomorrow.

Rosemary Barnes (0:39)

I'm here at the Danish Technical University research campus. I'm here with Nicolai Frost-Jensen and you're a development engineer here, working in the rain erosion test facility, right?

SPEAKER_1 (0:50)

Yes.

Nicolai Frost-Jensen Johansen (0:51)

So rain erosion is my main topic I work with most days. Besides that, we're on the section of mechanics of damage, so we are spending a lot of time on fracture mechanics as well. But a lot of those speeds together in the field of rain erosion.

Rosemary Barnes (1:05)

Yeah, it's a complicated field with lots of different facets and still, I think it's developing fast, right? Like our understanding is definitely not complete. Yeah. Yeah. Now, I understand that you have already done an episode with Allen here before about the rain erosion testing machine. But for people who haven't seen that, we could just really quickly explain what rain erosion is.

Nicolai Frost-Jensen Johansen (1:27)

Our focus here is to try as well as we can in a lab setting to mimic the process of rain erosion on turbines. And to those that don't know even that level, it is, of course, as our turbines are rotating around, and here sometimes the rainy Danish environment, but also some of the more severe weather in Australia. Of course, the droplets are impinging on the leading edge of the blade. Over time, that builds up fractures and damage, the polymers or composites or whatever protective solution is on the edge, and it erodes or falls off and whatever it goes. And of course, the goal is not to try and test that in real life on a turbine, but to do it in the lab scale. So what has now become the industry standard is machines like this, a Whirling Arm Rain Erosion Tester, this one by R&D Test Systems. I think there is about 20 of this type of machine in the world now. As far as I remember, we are number 15

And the basic mechanism is that you might be able to see in the background, we have a big rotor where we have test sections in this length here, where that's mounted in the tester, and then we're spinning around underneath an artificial rain field. And as it spins, the blade is impacting the droplets, and thereby we're mimicking the same process, as close as we can to the same process as on the real turbines.

Rosemary Barnes (2:54)

So, leading edge erosion wears away the surface, causes two problems, one, aerodynamics are affected so you get less power, and two, it will eventually eat through the laminate in the blade, right? And so you got to get into a structural problem. Right. So that's one thing, obviously both of those are bad, and you want to prevent that. You don't want to be climbing up your wind turbine every year to replace this stuff. Ideally, you would have a blade that didn't ever erode over the entire lifetime, or at least you want to minimize the number of times you have to reapply. So that's why people who are manufacturing new leading edge protection products, they want to test it and see how it's going to go in the field. But if you just put it in the field, then I guess it's going to take you a full lifetime before you know if it will last a full lifetime, unless it fails earlier. So then they bring it here, and the goal of this is to accelerate that testing, right? So that you can figure out in a few days or weeks how it's going to last.

If you want to just reproduce what happens in the field, tip speed on a wind turbine blade is going to be what, like 900 metres a second maybe. So that's like 360 kilometres an hour, which is I guess like a racing car or something, but it's not abnormally fast.

So if you spin at that speed, then you're dumping water on it and you're trying to dump a lifetime's worth of water all in one go, and you're saying that that's 1600 hours worth of rain is about what a wind turbine would say in its lifetime.

Nicolai Frost-Jensen Johansen (4:29)

Yeah, it depends a little bit on the... So if you had to, sorry, if we're talking about the ways, there are different ways of accelerating and a normal tester, it would take about 1600 hour-ish to simulate those, near 35 years of rain. And then again, also depending on what is that rain climate you're trying to simulate because different place in the world gets much different kind of rain.