Hello, we’re here at The Ocean Cleanup’s headquarters in Rotterdam.
I’m here with our technology manager, Arjen.
We’re now well into our test campaign with our new system in the Great Pacific Garbage Patch.
So, with this video, we will try to enlighten you on some of the more technical details
of the challenges we face with getting this technology to work.
So, Arjen, could you start with explaining what the technical challenges were
with the first campaign last year?
What we saw is that Wilson concentrated plastics when it was in the Great Pacific Garbage Patch,
so plastics were attracted by Wilson but not always on the inside of the system, so to speak.
And the same happened on the inside, so plastics were gathered on the inside of the U-shape.
Where they were supposed to go but then after a little while they disappeared again.
When we saw that happening offshore, we did some experiments with GPS drifters,
so that’s basically a floating GPS sensor that is floating with the plastics and measures its own position.
So we could see how the plastics would behave, but then by a measurement.
So, in other words, sometimes the plastic is moving slower than the system.
Sometimes it’s moving faster.
Yeah, exactly.
So, that’s something that needs to be addressed in the system design in order to
keep the plastics inside.
Right.
So, what caused this effect.
Why did we see the plastic leaving the system?
Yeah, that’s what we started investigating, of course.
And, what we, what the design of Wilson was based on is that there is a speed difference
between the plastics and Wilson.
So, in order to capture the plastics, there needs to be a speed difference.
So, Wilson should overtake the plastics, so to speak, and then the plastics move inside.
But we found that that speed difference is not consistent.
It’s there.
It was not as large as we wanted it to be.
So, in other words, Wilson moved a bit slower than we expected it, but also,
sometimes the relative velocity of the plastics versus Wilson was in the other direction.
So, speeding up and slowing down.
Is that actually, conceptually equal or do we see pros and cons for both?
There are some differences.
For instance, the wave direction.
When you speed up the system, like this, then the system is moving in that direction and
the wind is coming from that direction.
That means that also the wind waves, so that’s like the steeper, shorter waves.
The shorts things that you see here.
Yeah, the little ripples and the really steep waves, which have the most impact on the system,
but also on the plastics.
They’re coming from this direction, which is good, because those are the waves that
could potentially flow over the floater.
If that happens in this situation, it’s not an issue.
Then it’s just water going over.
Sure.
If you slow it down, then the wind is coming from the other direction and the wind waves
are coming from the other direction, and then if the waves are going over the pipe,
they could take the plastics with them.
And that we don’t want.
So that would be, for instance, a benefit of going faster instead of going slower.
So, the overtopping could be a bigger problem, if you slow down rather than speed up.
Yeah, exactly.
Are there also advantages to slowing down?
Yeah, in the speed up situation we would need to create wind-catching area, a sail or, well,
anything that catches wind, in order to generate that speed.
And that is technically quite difficult because you need to engineer something that’s floating
on top of the waves, or maybe even above it.
And in that wave zone, there’s all kinds of dynamics, waves are hitting it,
it needs to be very robust and very strong to survive that location in the ocean.
If you would slow it down, you can deploy something in the water.
A parachute, as we do within this test now, but that you can submerge a little bit so
it’s outside of that wave zone, so it’s a much more steady environment, so to speak.
And, it’s easier to engineer something to survive there.
So, what approach did we take to try and sort of get quantitative data on this?
How many buoys do we need to create enough sail area?
How big does the parachute have to be?
How do we go about doing that?
Well, we started with doing simulations and numerical analysis on this.
There’s different forces acting on the system and on the plastics, and we need to very clearly
understand which those forces are, in order to use them or to know what we are designing for.
So on the one hand you have the cleanup system that’s affected by wind, waves, and current,
and you have the plastics that’s also affected by wind, waves, and currents
– but in a different way.
Okay.
So, there is, if you have the wind here in orange in this drawing, you see that close
to the water surface, there is a lower wind velocity than high up in the sky,
because the water causes friction and that slows the wind-down, so to speak.
So, acting on this, on the floater, is like the bottom of that wind profile.
So, the lowest wind velocity and the little bit higher wind velocity,
so it’s like an average of that, that’s pushing the floater forward.
Then the current on the other side, shows the same thing.
The current is mainly wind-driven in this area, so there’s not a lot of tidal current,
meaning that wind is, well, blowing the water away, so to speak.
So, at the surface, there’s the highest current speed, and then going deeper into
the water, that reduces due to friction.
So the screen hanging below the floater sees the average of these different current velocities.
The plastics are generally much smaller particles, so they have less depth into the water,
so they see the high parts of that current.
And that means that only due to currents, the plastics are moving faster
than the system would do.
So, in other words, if you wouldn’t add a parachute, or you wouldn’t add buoys,
the plastic actually goes faster than the system on average?
Due to current, yes.
But, then there’s also the wind that’s making, that makes that
the system is going a little bit faster than the plastics because
it’s well, the higher bits here and the low bits over there.
So, well, who wins?
Depends on the conditions: the wind speed, the current speed at the moment, directions,
so that will be different every time.
So, we can’t rely on that.
So, how do waves propel the system and the plastic?
On the plastics what we see is mainly a mechanism that’s called Stokes Drift, and that means
that if you have a wave in the ocean and then the water particles are moving in a circle
– that’s what a wave is.
But it’s not a perfect circle, due to wave kinematics.
So it is, well as in this picture, it’s going in a circle, but every, well
every round, so to speak, is moving a little bit to the side.
That’s what’s called Stokes Drift.
And any particle that is, or object, that is moving quite like a water particle,
like a piece of plastic because it’s relatively small and light, is moving with that water particle.
That’s also every circle moving a little with it.
So the Stokes drift is impacting the plastics and pushing it forward, so to speak.
The cleanup system is relatively large and heavy, so it’s not exactly following
the water particles, so a water particle here is moving, but the system will remain
more or less where it is, not exactly.
But there is a mechanism that’s called second-order drift that’s basically, well there’s different
effects, but it’s, for instance, a pressure difference on one side and the other, and
that’s also pushing the system forward.
But as they are different mechanisms, they have different effects in different wave conditions.
So, we cannot rely on a consistent speed difference because of that.
Sure.
So, there’s three factors: wind, waves, and currents – they all impact both the
system and the plastics, but in a different way.
In different conditions.
And that changes in different conditions.
Which makes it complicated Yeah
So, how do we go about trying to put any numbers behind this and quantify this?
Well, what you see is, if we go for speeding up, then we mainly exploit the wind
and the waves pushing the system forward, so we try to maximize those forces, basically.
For the slow down option, it’s more using the current or the drag, actually, of the system.
So, we try to maximize the drag to slow it down as much as we can.
So what we try to do is understand those forces and those mechanisms as good as we can
to design a system that uses those forces and is not so much effected by the other forces
that we try not to use in that specific configuration.
The plastic behavior and the forces on the plastic is given
because, well, the plastic is out there as it is and it’s impacted as it is, but we do try to understand it
as good as we can.
And, for instance, that Stokes drift on the plastics, that seems relatively straight forward
when we started looking into it, but it became quite complicated,
because we don’t exactly know how well such a plastic particle is following
- a plastic object - is following the water particle.
So, if it would perfectly follow the water, then there is a theoretical formulation
for Stokes drift.
The blue plusses over here, that you see, that’s the effect of Stokes drift on the plastics.
Theoretically.
Because we only had theoretical value for that.
And what we want to have a consistent speed difference is to have those plusses either
always on the bottom, or always below the system, or always above.
Sure.
And if that’s the case, then it’s consistent.
So, what you see here, the green line for Wilson, you see sometimes it’s above
and sometimes it’s below.
So that explains the observations there.
For the other options you see that it is a bit less.
You see in very low wind speeds, some outliers here, but apart from that extreme
fourteen fender version, it’s all below, so that would work theoretically.
And for the other ones it’s a bit closer, obviously, because they are less impacted
by the wind.
So, what you see is that, well there seems to be a pretty clear relation between the
speed of the system and the wind speed here with the speed up versions,
the relationship between the Stokes drift and the wind conditions is less clear.
Yeah, and that’s probably because the Stokes drift is mainly caused by those really small wind ripples.
Because we have very small plastic particles in there, we see that small ripples already
cause Stokes drift and they are basically always there.
If there’s a really low wind speed, then those ripples are there.
You can see that in a pond, if there’s a little bit of wind, you see the ripples on the water.
If the wind speed increases, then the ripples remain there but some higher and longer waves are added to it,
so to speak, but they don’t cause so much of that Stokes drift.
So that’s more or less, not exactly, it’s going a little bit up,
but it’s more or less constant over wind speed.
So, looking at this plot, it appears to be that if you look at the six-fender configuration,
the system already appears to be almost always above the plastic speed.
So looking at that, the six fender configuration would already look very good?
But this of course is still a simplification.
Yeah it is, so that’s one thing.
There is inaccuracies in here, there’s some assumptions but, also, there’s other effects at play.
So this shows the effect of wind on the cleanup system and Stokes drift on the plastics.
But there’s the other forces as well that we just spoke about.
And, for instance, there is an effect that for this test we will use those fenders
to generate the wind force.
That’s something we use for this test, because we need something to generate wind area.
For a next version cleanup system, we might most likely design something else.
But those fenders, they are moving in the waves and they are radiating some waves as well.
They’re creating extra Stokes drift?
Yeah.
They are adding to that.
And then what happens is that this here is the same as the previous plot
but then, as a density plot.
So you see here that yellow area in the plot is like where most of the plastics is.
And the spread?
Yeah, and the spread.
So, that’s still on the right side of that six fender line, the pink one here, but, if
you add that Stokes drift, due to the fenders, you see that it’s changing the picture,
and there you see that the most of the drifts, or the speed difference is, well, equal.
Which is not what we want.
So, those different factors at play can change the picture again.
So here we’re looking at the speed up and with the current understanding,
theoretical understanding, of course it’s unperfect,
we make assumptions, but it appears that
speed up should, in most cases, be faster than the range of plastics that we can expect.
But what’s the story for the slow down, and how does this relate to the slowing down concept?
Well, the slower the plastics move through the water, the more we have to slow down
the system to be able to capture the plastics and you can see that in another type of plot here,
where here you see the angle at which the plastics are encountering the system.
We want that to be between plus and minus 90 degrees, because that means
it’s coming from the open end.
So, zero degrees is.
So exactly 90 degrees would just be like this?
Yeah, nothing happens.
Technically 89 degrees would be just enough to capture plastic.
Yeah, exactly.
But if it’s 90 or more than it’s coming from the wrong direction.
Go like this?
Yeah, and that in itself is not so much of an issue because then you don’t capture
but you also don’t retain because the plastics inside are moving out again.
So that’s the situation we had with Wilson.
And you see here that density plot again, where you see most is indeed nicely between
that plus and minus 90 degrees, but some is still out here and that means that in some
conditions, the plastics are moving out.
If that happens every couple of days or so, then still you lose all the plastics in the end
and every couple of days you need to start over with collecting.
So that’s something that we need to squeeze between the lines, so to speak,
by designing the system in a good way.
So, it’s fair to say that looking at these results, we see especially in low wind speeds
that we see these critical conditions, where perhaps the plastic is not being retained
– is that fair to say?
Yeah, if you look at the speed difference, indeed, as you see here that this happens
mainly in the wind speeds below 10 meters per second, which is very low wind speeds.
So that’s critical for capturing the plastics and for retaining plastics as well.
In the high wind speed conditions, that could be critical cases for retaining if overtopping
becomes a factor, but those are things that we all now want to test out
in the Pacific Garbage Patch to exactly see what happens out there.
So, speed up or slow down – where’s your money on it right now?
My money’s still on speed up, because there’s like a theoretical limit to slow down.
Speed up, you could, theoretically speed up as much as you like and always create that
consistent speed difference.
So, thank you, Arjen.
And, we’re very excited to see what’s going to happen in coming weeks.
Of course, we’re going to keep you posted of what’s going to happen here on social media.
And, naturally, our aim is to prove the technology, to get to this first system that works,
that catches plastic and retains it consistently,
so that we can then start scaling to a whole fleet to cleanup the Great Pacific Garbage Patch.
But, I think, either way, we’re going to learn a lot, and which will, of course,
help us improve this knowledge that we see right here.
So, if you have any more questions, feel free to comment below.
If there’s anything unclear, just ask.
And, for now, thank you for watching and see you next time.