How DTU’s Rain Test Facility Predicts Erosion

Rosemary visits DTU Wind & Energy Systems in Denmark, where Nicolai Frost-Jensen Johansen explains rain erosion testing, tip speed, and LEP defects. The Uptime Wind Energy Podcast is brought to you by Weather Guard Lightning Tech, creators of the StrikeTape Ultra LPS retrofit. Subscribe to Uptime’s Substack newsletter. And check out Rosemary’s "Engineering with Rosie" Youtube channel. Have a question we can answer on the show? Email us! Episode Transcript Announcer: Welcome to Uptime Spotlight, shining light on wind energy’s brightest innovators. This is the progress powering tomorrow.Rosemary Barnes: I’m here at the Technical University of Denmark, Risø campus, with Nicolai Frost-Jensen Johansen. You’re a development engineer here, working in the rain erosion test facility, right?Nicolai Frost-Jensen Johansen: Yes. Rain erosion is the main topic I work on most days. Besides that, we’re in the section for Mechanics of Damage, so we spend a lot of time on fracture mechanics as well. A lot of those meet together in the field of rain erosion.Rosemary Barnes: It’s a complicated field with lots of different facets, and I think it’s still developing fast. Our understanding is definitely not complete. I understand you’ve already done an episode with Allen about the rain erosion testing machine, but for people who haven’t seen that, maybe we could quickly explain what rain erosion is.Nicolai Frost-Jensen Johansen: Our focus here is to mimic, as well as we can in a lab setting, the process of rain erosion on turbines. For those who don’t know, as turbines rotate in a rainy environment, and in some of the more severe weather in Australia, the droplets impinge on the leading edge of the blade. Over time that builds up fractures and damages the polymers, composites, or whatever protective solution is on the edge, and it erodes or falls off.The goal is not to test on a real turbine, but to do it at lab scale. What has now become the industry standard is machines like this one, a whirling arm rain erosion tester from R&D Test Systems. I think there are about 20 of this type of machine in the world now. As you might be able to see in the background, we have a big rotor where the test specimens are mounted. We spin them underneath an artificial rain field, and as the blades spin they impact the droplets. That way we mimic, as closely as we can, the same process as on real turbines.Rosemary Barnes: So leading edge erosion wears away the surface and causes two problems. One, the aerodynamics are affected, so you get less power. And two, it will eventually eat through the laminate in the blade.Nicolai Frost-Jensen Johansen: Then you get into a structural repair.Rosemary Barnes: A structural problem, right. Both of those are bad, and you want to prevent them. You don’t want to be climbing up your wind turbine every year to replace this stuff. Ideally you would have a blade that never eroded over its entire lifetime, or at least you’d minimize the number of times you have to reapply protection.That’s why people manufacturing new leading edge protection products want to test them and see how they’ll do in the field. But if you just put a product in the field, it will take a full lifetime before you know if it lasts a full lifetime, unless it fails earlier. So they bring it here, and the goal is to accelerate that testing so you can figure out in a few days or weeks how it’s going to last.If you want to reproduce what happens in the field, tip speed on a wind turbine blade is maybe 90 to 100 meters per second. That’s about 360 kilometers an hour, like a racing car. So you spin at that speed and dump water on it, trying to deliver a lifetime’s worth of water in one go. And you’re saying 1,600 hours of rain is about what a wind turbine would see in its lifetime?Nicolai Frost-Jensen Johansen: It depends a little on the approach, because there are different ways of accelerating. In a normal tester it would take about 1,600 hours to simulate roughly 35 years of rain. It also depends on what rain climate you’re trying to simulate, because different places in the world have very different kinds of rain.Rosemary Barnes: So 1,600 hours is a little more than two months, running flat out. You’d have to get an intern in here supervising at nighttime. I guess you don’t just leave it running.Catching the Onset of DamageNicolai Frost-Jensen Johansen: For the lower speeds, we do at times run overnight. The machine runs more or less automatically for a set amount of time. If it’s a long test, that might be one hour. Then it turns off the rain field, spins down the rotor, and images all three blades. From those images we look for any onset of damage, what we term incubation. That’s a really small, local loss of material.Rosemary Barnes: So you run it for an hour, stop it, wipe off the camera lens, take a few photos, and start it back up again. Every hour you’re taking photos.Nicolai Frost-Jensen Johansen: It depends on the test, and luckily that part happens automatically. It dries out the blades, the camera takes the pictures, and it spins back up. The faster we go, the more often we take pictures. Sometimes we take a picture every five minutes just to capture the real onset. At about 160 meters per second, it’s not uncommon to see damage after the first five minutes.Rosemary Barnes: Okay.Nicolai Frost-Jensen Johansen: Ideally we want at least five images before the onset so our uncertainty doesn’t get too large. That’s one of the differences from normal fatigue testing. This has a lot of parallels with fatigue testing: we have a repeated load over time, and we draw curves from it. But in a fatigue test, you impose a predefined load or displacement on a single specimen and run it until it breaks, so you know exactly when it breaks and at what load.Our specimen is relatively long. At the tip we might be at 160 meters per second, and at the root we might be at 100. We run it for a set amount of time, stop, and observe it. So all we actually know is that it failed somewhere in the time we weren’t looking, and we know the position, which corresponds to a velocity.In an ideal world, damage starts at the tip and grows toward the root. What we more often see is that it generally happens toward the tip, but it can be somewhat random along the length of the specimen. That’s especially true of modern coatings, which are on average much better but also more unpredictable. With an old gel coat, we could get a very nice curve. Damage would start at the tip and progress toward the root. Some of the newer elastomeric systems do an order of magnitude better, but they might just randomly fail in the middle. That’s really where the tricky material science needs to come in.Rosemary Barnes: That definitely matches my experience in the field in Australia. You don’t see a little variation one year, a bit more the next, growing evenly and predictably across the whole wind farm. It’s more patchy. In one year, 90% of the wind farm might suddenly have heaps of damage, and then nothing happens for a few years after that. Do you think that’s related to imperfections in the coating material or the substrate itself?Nicolai Frost-Jensen Johansen: We’ve done various studies, and we’ve had PhDs look at examples in scanning tomography machines. Often we do find that we can correlate the damage back to defects. Leon in our section is famous for his modeling work, and a lot of it deals with defects acting as stress risers. Some materials can also be more sensitive to having those kinds of defects.There’s a lot of complexity within the topic, but if there’s one trend I’ve seen, from older gel coats and tough coats up to many of the LEPs we see now, it’s that we’ve moved toward near-pure resins. These elastomeric coatings are almost pure elastomer with very little filler. If you ever look at a cross-section of an older paint system under an electron microscope, it’s not so much paint as gravel held together with a little bit of polymer. Another way to look at it is that it’s 95% defects and 5% defect-free. Those systems will fail, and damage will grow, but it’s very predictable.Now we’re using materials with less than 5% fillers, and a lot of those fillers can be defect drivers. So we have materials that are mostly defect-free, but we struggle to get something defect-free even at a 40-centimeter specimen length. It follows that if you’re covering 40 meters at the tip of a blade with LEP, you’re not going to get it defect-free. That mirrors a lot of other things within fatigue in general.Fatigue, Damage Tolerance, and Tip SpeedRosemary Barnes: There are regular charts we use in fatigue testing. Fatigue is how materials behave under a load that’s too small to break the material in one go, but if you apply it enough times, it eventually breaks. You characterize materials by putting stress or strain on one axis…Nicolai Frost-Jensen Johansen: Stress or strain, depending on the case.Rosemary Barnes: …and then you count the number of cycles until it breaks, and you get a nice curve for the material. With rain erosion, you’re saying that on one axis you have the speed.Nicolai Frost-Jensen Johansen: Typically velocity instead, yes.Rosemary Barnes: The velocity of the blade section relative to the rain. And on the other axis, you’re counting how much water it has seen.Nicolai Frost-Jensen Johansen: You could say there are two schools of solving fatigue problems. In metals, you either add tons of dislocations so any crack growth is stopped because it grows into something else. Or you go the Rolex way and make your hairspring out of a single crystal, so there are no defects and all the atoms sit exactly where they should. I think the big challenge for wind is that we cannot make anything defect-free.Rosemary Barnes: Not at a cost.Nicolai Frost-Jensen Johansen: Not at a cost that makes sense.Rosemary Barnes: Not if you want people to choose wind over coal, gas, or whatever else....