Speaker 1Rain 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 Nikolai Frost-Jensen-Hohansen. Nikolai describes how leading-edge coatings are tested and how to get faster, more realistic results. Welcome to Uptime Spotlight, shining light on wind energy's brightest innovators. This is the progress powering tomorrow.
Speaker 2I'm here at the Danish Technical University RISO campus. I'm here with Nikolai Frost-Jensen, and you're a development engineer here, working in the Rain Erosion Test Facility, right?
Speaker 3Yes. 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 bleeds together in the field of rain erosion.
Speaker 2Yeah, 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 yet. Yeah. Yeah. Now, I understand that you have already done an episode with Alan here before about the Rain Erosion Testing Machine, but for people who haven't seen that, maybe we could just like really quickly explain. Can you explain what Rain Erosion is?
Speaker 3Our 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 here in sometimes the rainy Danish environment, but also some of the more severe weather in Australia, there, of course, the droplets are impinging on the leading edge of the blade. And over time, that builds up fractures and damages, 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're 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, as close as we can, the same processes on the real turbine.
Speaker 2So leading edge erosion wears away the surface, causes two problems. One, aerodynamics are affected, so you get less power and two, it'll eventually eat through the laminate in the blade, right? And so then you've got to get into a structural structural problem. Right. So that's 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. You would ideally you would have a blade that didn't ever erode over the entire lifetime. Or at least, you know, 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 work. If you want to just reproduce what happens in the field, you know, tip speed on a wind turbine blade is going to be what like 900 meters a second, maybe. Yeah, so that's like 360 kilometers an hour, which is, I guess, like a racing car or something. But it's yeah, 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. 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.
Speaker 3Yeah, it depends a little bit on the so if you had to say, so 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, you know, near 35 years of rain. And then again, also depending on what is that rain climate you are trying to simulate, because, you know, different places in the world get much, much different kind of rain.
Speaker 2Yeah, yeah. So 1600 hours is like two months, a little bit more than two months, just flat out, flat out. And then you got to get an intern in here or something supervising at nighttime. I guess you don't just leave it running
Speaker 3and then for for the lower speeds we do at times run overnight with that that varies. And then besides that, of course, it runs more or less automatically. And then for a set amount of time, the tester runs so that if it's a long test, it might be one hour. Then it's it turns out the rain field that spins down the rotor and then we're imaging all three blades. And from those images, we're looking for any onset of of damage or what we term incubation. But that's that's really small local loss of materials, which you run it for
Speaker 2an hour, stop it, wipe off the lens of the camera, take a few photos, start it back up again. And then every hour you're taking photos or
Speaker 3depends on the test. And luckily, that part happens automatically. So it dries the the the camera dries the blades and then it takes the pictures and it spins back up the faster we go. The more often we take pictures because of the time onto onset, sometimes we're we're taking a picture every five minutes just to be able to capture the real onset, especially when we were about 160 meters a second. It's not uncommon that there is damage after the first five minutes. Oh, OK. And then so so we ideally we want at least five images before the onset. So our uncertainty doesn't get too large because that's also one of the differences between, you know, let's say normal fatigue testing and this view. This has a lot of parallels with fatigue testing. And, you know, we have this repeated load over time. We draw SN or VN curve as we term them. But the difference is that when you go to a fatigue test, you impose a predefined load or displacement on a single specimen and then you run it until it breaks. So you always know exactly when it breaks and you are imposed at that particular load you have imposed. But as I said before, our specimen is relatively long. So that means. At the tip, we might be 160 meters a second at the route. We might be 100 and then we are running it until then we're running it for a known amount of time and then we stop and observe it. So all we actually know is it failed somewhere in the time we didn't look at. And what we know then exactly at which position, which then corresponds to velocity. So it's that record then of in an ideal world, you will get damage starting at the tip, growing towards the roots. But what we then actually more often. See is that it's generally happening towards the tip, but it can be sort of randomly within the length of the of the specimen, especially in modern coatings on average much better, but also more unpredictable. If we were putting you in an old gel coat, we could get a very nice curve. It would start at the tip and generally get more rough and then progress down towards the root. Some of these newer elastomeric systems are an order of magnitude better, but then it might just randomly fail at them. The middle and that's really where the tricky part of the material science needs to come in.
Speaker 2Yeah, I would say definitely that that matches with my experience in the field in Australia. It's not, you don't just see a little bit of erosion one year and then the next little bit more. And it's just like growing kind of evenly and predictably across the whole wind farm. It's more patchy, like it might be all in one year, you know, 90% of the wind farm has suddenly got heaps of damage and then maybe nothing will happen for a few years after that. Do you think that it's related to imperfections in the coating? The coating material of the substrate itself or what's your thoughts?
Speaker 3Typically, we've done various studies also with, I have a PhD student that then looked at, took samples that have been a radioreson test and put them in the scanning tomography machines. And then often we do find that we can correlate it back to that as maybe it was an air void. Leon in our section, famous for all of his modeling work, also correlates a lot with any defects or stuff being really stress risers. The next part is then also, you know, some materials. Can just be more sensitive to having those kind of defects on top of it as well. And I think, of course, there's lots of complexity within the topic, but one, if there's one trend I've seen happening from, let's say, older style gel coats and top coats and whatever, up to many of the LEPs we see now is that we've gone in the direction of being much more near pure resins. These elastomeric coatings are almost pure. Elastomer with very little filler. And you say older systems paints, if you ever look at a cross section and an electron microscope or anything else on a paint system, you see this is not as much paint as it's like gravel held together with a little bit of polymer. So another way to look at it is it is 95% defects and 5% defect free. So you can say these systems, you know, they will fail when they're subjected, they will have damage growing in a tons of places. But it becomes very predictable and improving. But the other way that we've seen now, we are having these materials that are, you know, less than 5% fillers and a lot of the fillers can be one of those defect drivers. So now we have materials that are mostly defect-free, but, you know, we struggle at these like 40 centimeter lengths to get something that's defect-free. So then it also follows that, you know, you're covering 40 meters at the tip sometimes with LEPs, you're not going to get that defect-free. And it mimics a lot of other things within sort of fatigue in general.
Speaker 2Regular charts that we use in comparing fatigue testing, which is how materials behave when they've got a small load, too small to break the material. And then you apply it enough times and fatigue failure would mean that it would break eventually, even though the individual load was too small to ever break it once. When you're looking at that kind of failure, you characterize materials by looking at the strain, right? Stress or strain. It depends on what you're doing. So, yeah, on one axis, you've got S, whether that's strain or stress, and then you just count the number of cycles that it has until it breaks and you get a nice curve for a material. Now, with rain erosion, you're saying that on one axis, you have the speed.
Speaker 3Typically the velocity instead.
Speaker 2Yeah, the velocity of the blade section relative to the rain, I guess. And then on the other axis, you're counting how much water it has ever seen. Right? Yeah. Yeah.
Speaker 3Okay. You can say there's two schools to solve fatigue problems. Either you, you know, in metals, add in tons of dislocations and stop any crack growth because it grows into something else, or you go like the Rolex way and you make your hairspring in your watch out of a single crystal so there are no defects. All the atoms are sitting exactly where they are. And I think for the big challenge for wind turbines is that we cannot make anything defect-free.
Speaker 2Not at a cost that anyone.
Speaker 3Not at a cost that makes sense.
Speaker 2Is going to choose over coal or gas power, whatever it is.
Speaker 3No. And that's sort of one of the big realizations, sort of the big shift that we're trying to push in the science and the industry is to start focusing in on figuring out how to make damage-tolerant materials. So as I mentioned, we do a lot of sectionist mechanics of damage, but it comes from a background in fracture mechanics. You say that the normal design criteria has been VN curves, but that is, you know, a purely empirical design observation. We just say at this point it failed. We have no real knowledge of why it failed and what makes it has nothing to do with the material properties as such. So we are hoping to go into a bit more the part of developing materials that will stop cracks themselves. And there are various sort of strategies you can use. And also to define where this material will naturally arrest itself and that might not always be the ones that are those that are now the best performing erosion coatings. We might find there's that other avenue and hopefully something that will allow us to finally do some lab scale tests that are not the rain erosion test.
Speaker 2Yeah, you've got, say you want your wind turbine blade surface to last you for 35 years, you know how much rain it's going to see in that time, say the tip speed is 90 meters a second. You just can spin it around. You can spin it around in this machine at 90 meters per second and for as long as it takes, just keep on putting water on it until it's seen its full lifetime worth of water. So it's sort of accelerated because your blade doesn't have to experience any non-rainy periods. So you kind of save some time that way. So if you're going to take 60 actual days or so two months or maybe it's six months if you just want to work business hours, that's obviously extremely expensive test, right? So now you want to accelerate it. And how can you do that? You can either spin the blades faster, right, which is something that you do. Can you tell me about like what speeds you're running tests to try and compress that time frame?
Speaker 3So if you look to the current standard within the recommended practice, we tend to test at 160 meters a second tip speed and then also down at 100 meters a second, so being close to the reels. And the big thing really there is that. We were talking that for many composite, there might not be a plateau, but some of this rain erosion, they might actually have because they follow a power curve with an extreme, sometimes like quite an extreme exponent. So we talk with negative exponents of about sometimes up to three or four, which means like just this tiny jump in velocity can be orders of magnitude in lifetime. So, you know, for example, an example and a test that might take one hour. Might take one or two days at 160 meters a second test if we trusted the VN curve and extrapolated it down to 100 meters a second, that could take, you know, you were talking these 30 days or two months, worst case in some of them. So it is so sensitive on that curve where you end up landing.
Speaker 2Okay. So you're saying when you speed it up, it doesn't like you run it twice as fast, say, to keep the math simple. Yeah. It's not just
Speaker 3twice as, you know, it's to a high exponent. So a test that takes one day at 160, if you drop to 130, it took a week. Okay. You know, that we're going 30 meters a second down and it can, and it's some of these really counterintuitive things. Humans are just not good at understanding power curves. Yeah. We continue to see them all the time on these log plots. And it just looks like straight lines. But it's so counterintuitive what the consequence of a power curve really is. And it's, you know, increasing the tip speed of the turbine by one or two meters a second can all of a sudden be like you are having no problems with rain or ocean. You're maybe repairing every five, 10 years doing some spot repairs to like all of a sudden it's end of the world and you're repairing every year just because, you know, it had had a benefit in the drivetrain or the plate design or whatever to just inch of that tip speed.
Speaker 2It's a, it's a big, to be fair, it is a big, a big, it's a big, it's a big benefit. Yeah, yeah. And I, I just have to, to fight
Speaker 3on my side of the fence and just say there's a consequence.
Speaker 2It's been really interesting for me working, changing from, you know, I was working in the design and development. Now I work in operations and maintenance and I've totally flipped which issues I think are important or not in, not important based on that new perspective. But that should actually be intuitive to all of us because we've all seen pictures, or I don't know, maybe not every viewer has, but we, most of us have seen pictures. Of a blade that has erosion on it, right? It's at the, it's at the tip. You don't see erosion like all the way down and obviously the tips are moving faster, but it's only, you know, the last bit. And if you keep on moving inboard, there's a point where it just suddenly stops. And there's no huge difference in, you know, from like this area with erosion to this area without erosion, it's basically the same speed. It's a very small difference, but one might be, you know, eroded all the way through the laminate and then a few centimeters away, there's just like absolutely nothing wrong with it. Right. It's just a tiny little pitting. So I guess we should, we should have known that intuitively, even if we don't understand it.
Speaker 3But again, you're seeing it on the paper and like, what does one meter, meter a second is, you know, three and a half kilometers an hour. It's like crawling, like it's a very slow walking pace, faster, and that's enough to, to, to, to all of a sudden lose you half or more of your lifetime.
Speaker 2Yeah. Okay. But so I guess that that's a risk then, because you can accelerate the speed. I mean, you're changing the behavior of the material, so what, how much can you accelerate it? Can you go at 200, 300 meters per second? Would you, you know, get a really fast test or are you starting to measure something that is interesting in the lab, but doesn't correlate to reality?
Speaker 3That is even, even I think most agree that when we go to 160 meters a second, we're probably over accelerating it and, and, and potentially tricking into some failure modes that are both not realistic. Yeah. I mean, it's, it's, it's, it's a, it's, it's, you know, we're designing, the risk is you end up designing materials that are good for the test, but not good for, for real life. Yeah. One of the big things that complicates it within this, because this is contrary to, to normal fatigue loading, we, we can deal in, in linear mechanics, you know, Hook's Law and, and, and, and, and Young, Young's Modulus are, are valid to use. But when we go into, to, to impact loadings, then all of a sudden things are happening on the microsecond scale or sub-microsecond scale, and then materials are no longer necessarily behaving linearly. We are having to account for viscoelasticity and, and all of these things that then compounds difficulty on top. And to how that then correlates back again, that, that's, you can, there can be velocities where you trip over a, like the material is starting to behave very differently at, at, at, at a higher speed. And then it can also be, we see sometimes for polymers, if you are, you said there are two main ways of trying to solve rain erosion. Either you're, you're the anvil or the trampoline. Either you want to be so hard that a droplet hits and it, it shatters and, and blows apart. So you can say leading edge on airplanes, they use a lot of metallic solutions because you can get away with it there and your, the, the, the yield strength of a, of a metal becomes high enough that you can just shatter a droplet. Where the other option is that we typically can't go that in, in many options on, on blades, then we, many of the LEPs are soft, like we try to, instead of shatter the droplet, disperse the energy. By deforming. So having a lot of soft elastomers that then bounce in, but that can then be, there are some polymers that, that, you know, are borderline being hard enough to work and that the risk can be, if you go too high in velocity, you trip over that. critical threshold where it's just not strong enough anymore. And then you're sort of not, then you're giving a big advances to a soft system where at a lower velocity, it might have the opposite relation. -
Speaker 2Yeah, I think that's really interesting 'cause it's like, of course everyone realizes if you accelerate a test, it's not gonna be exactly the same as it behaves in the field, but you would probably expect or at least hope that it would at least rank materials correctly to the way if you test 10 materials in here and then number ranking one, two, three, four, five would match the number ranking of how they would last in the field. But it sounds like that's not necessarily the case. And especially if you're comparing hard and soft materials. -
Speaker 3They can be very different.
Speaker 2Okay, so if that kind of acceleration just going faster and faster isn't the solution or at least isn't the only solution, what's your other option for accelerating testing? -
Speaker 3So the option that the direction we've been going is based on some studies with ideas back where we looked into the effect of different drop sizes. But one of the sort of unintended consequences of that study is that we ran the tester with much more rain sometimes than normally. And then when we went back through all the data and normalized it, we found that if we, even though we, one of the tests had twice the amount of rain flow into the field, when we normalized it down, we could get all the curves to collapse back on themselves again. So that also showed, okay, we're maybe not at the limit of adding, even though I'm saying this is like torrential downpour, that might not be too much rain. So our tester, we have a second rain field that allows us to run on that rain field alone, twice the amount of water, and we can also potentially run both rain fields together to triple. And that's so fast, it seems to linearly scale. So if you double the rain flow, you have the exposure time. And if you put, we haven't tested it fully yet, but hopefully when we triple the rain flow, we go to one third and then all of a sudden, you know, one and a half or two months becomes a couple of weeks, which is much, much more realistic to go. And exactly where that outer boundary exists, that's a good question. And that we have to figure out. -
Speaker 2Yeah, okay, so you've been adding in more little needles to get more water falling out of the machine at once. -
Speaker 3For the audience, the normal tester has 600 needles sitting around this radial manifold and a radial rain field. We have then a second set of manifolds with 1,200 needles allowing us to flow twice the amount of water. -
Speaker 2But what did you say the rate was, the maximum rate? -
Speaker 3So if you actually go through the system and then if you imagine you're sitting one, if you look at one point on the blade and you are counting the impacts, you do see that we have an impact rate of between 0.1 to 0.3 Hertz. So it's, you know, it's somewhere between three and 10 seconds and an impact on the exact same position when you account for the size of the droplet. -
Speaker 2Okay, and so that would be a problem if it had just been hit and before it can kind of return to its steady state. - Yeah, that's some of the concerns.
Speaker 3And of course there is the chance that, you know, a droplet would, it is a random process. So you could have two sort of more simultaneous droplets, but for two droplets to interact, they really need to hit within microseconds of each other. Like most of the energy and impact events have dissipated over a few microseconds. So then it's fairly rare that those events will happen. -
Speaker 2Okay, but the actual rainfall rate, the, you know, rain intensity here didn't sound that high to me when I was comparing it to like what I look at at the sites that I work with in Australia. You said, what did you say, 35? -
Speaker 3So if you measure, if you put a rain gauge in the center of the rain field for the normal, it's about 35 millimeters an hour. -
Speaker 2And how would that like change? Like if you, so maybe that's a down, that counts as a downpour in Denmark, but I know that, you know, like 100, 150 millimeters an hour is not out of the question for some of the sites that I work with in Australia. So what would you, like, are they going to have accelerated damages, these sites, because they are just seeing too many rain droplets one after the other, or like what would, what would you think would be the difference? -
Speaker 3Don't necessarily think so. So then how is the rain connected to the wind? Is there then becomes the big question because a lot of downpours that are near or onshore, we've found, or then it might be raining, but if it's not windy as well, the turbine is not at the rated speed and it's not going to hit the same amount of water. So that connection between is it windy and is it rainy at the same time? I think that that is the first sort of next step in asking where it goes. There's a lot of other complications you can add on, on effects of drop sizes, but from an experimental standpoint, the main thing we've found is the, the amount of water you hit. And that is sort of mainly driven by drain intensity, forward velocity of the droplet and how fast the turbine is spinning. -
Speaker 2All right, so it sounds like you're going to keep on adding more and more droplets until you reach some sort of limit, practical limit or until it doesn't make a difference anymore. -
Speaker 3Yeah, of course we're limited in the end by how much our water system can, can, can push and we'll- -
Speaker 2I guess you can only turn up the flow rate in an individual needle so much before it just becomes a steady stream. -
Speaker 3And we might actually go to a steady stream at some point. That's also, we're talking about what the size of the droplet is and it's again, much dependent. Right now, there's the test that runs now. It's about 2.4 millimeter diameter droplets. And for the majority of rain, that is actually quite large. So another option is that we just force it out as a spray, some very small droplets, which at least in Denmark is more reminiscent of the majority of the rain we get, but that's still gives damage. Because what in many cases matters is how much water is hanging around in the air. -
Speaker 2Okay, okay, interesting. But I did want to just talk about some of the, 'cause it's not, rain isn't the only thing that can dictate how, how resistant a blade is to being eroded, right? So you were telling me before that you're looking at doing some different temperature tests. Maybe there's some other things that you're looking at. Can you tell me about any, anything else besides rainfall rate that you're looking at? -
Speaker 3Yeah, so for example, we do have an upcoming project starting here in the next year as the CTP, in a CTP collaboration called Protect, where we'll be looking into various metrological drivers. So one of the things that we'll be testing, both high and low temperature. So one of the things we can also do is go, reheat the entire chamber and water up to 30 degrees. It becomes a little bit closer to some tropical rain, but also going down to eight degrees, change the average size of the droplets, and maybe also have mixed. So we could have one set of rain, and then we could have one set of rain, and then we could have one set of rain, you
PodcastTranscript AI