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 y