OK so rather than “single-factor analysis” you are looking at two (2) factors…
(I’m not sure this design solves either of them)
I’m not “blaming” Makani for anything except their own results.
I merely use their well-intended effort as an example of being steered in a wrong direction by “visionary” thinking, where the vision is seen as so grand that everyone forgets to examine the details, but just go forward with what turn out to be unrealistic expectations based on some incompletely-analyzed “vision”.
The example given equates to 52 m under the system with a velocity of 26 m/s at the edge so all the water has a velocity change of 0.5 m/s per metre of depth resulting in a low resistance.The difference using a cushion of air is that the depth of the cushion could be 1/50 of the water depth for the same resistance.
Design thoughts:
The direction of the apparent wind onto the aerofoils is from outside the circle on the windward side and from within the circle on the leeward side. To automatically cope from zero to extreme wind conditions the aerofoils would need to be secured to the ring. As a general rule the greater the TSR the greater the potential but with very tall structures even with the added support provided by stays big chords are required for structural integrity. As the TSR ratio increases the friction does increase, so designing the system to peak with a TSR of 3 results in a harvesting capability of 375 MW in 11 m/s winds with a very high coefficient of performance. Dragging the mass stores energy proportional to the square of the velocity, so small changes in velocity can equate to a massive amount of energy. To enable the aerofoils to harvest extra energy from stronger winds, they have to be stronger, which increases their mass. But because the aerofoils are supported via the ring on water their mass is not an issue.
With any VAWT system with really big aerofoils those aerofoils must be able to independently rotate 360 degrees to feather ( regardless of their position on the support ring ) for safety in extreme weather. This led to the concept of controlling the pitch of the aerofoils.
Float the mass on a cushion of air and seal the air in to minimise the cost in energy of leakage. There are also friction losses on the seal.
Float the system on water (that is reasonably deep) so that the top of the water rotates with the system & the speed of water rotation decreases with depth and distance from the side of a system… This maximises the mass of rotating kinetic energy and the losses are found by Couette flow analysis. The shear stress = dynamic viscosity x shear rate so resisting torque is proportional to speed. Hence the ratio of power loss to energy stored kinetically remains constant.
So for the second choice there is no air cushion system.
In some way, could water drag of such a ring be compared to the air drag of a ring surrounding a wind turbine like on the video below?
I experimented a ring surrounding a wind rotor: the drag was significant but not huge. But water is 800 times denser than air.
Perhaps we could experiment with water drag by rotating (in the water) a buoy which would be heavy enough to have a draft comparable of that of the intended installation, keeping the proportions.
The air in the video is not rotating with the turbine, but constantly moving past it. The analogy of the video is more like trying to spin a circular ship on stationary water instead of creating a fluid flywheel.
To model the friction of a system with air:
A ring of a wheel supporting vertical axis aerofoils
A drum 2 metres bigger in diameter than the ring of the wheel
A conical base to the drum , the centre supporting the wheel hub.
The gap of 1 metre between the cone base and the ring ( under the ring )
To model the energy stored by the fluid is a lot more complex due to a density ratio of 800 to 1 and a depth ratio of 50 to 1, resulting in an apparent 40,000 to 1 ratio
The idea of cylinder walls that go straight down into the water containing a cushion of air would be interesting, but connecting the walls at the bottom to create a ring hull would have half the water friction. I believe, without any math or real analysis to back it up, that trying to create a whirlpool as energy storage would not work out. Just my off-the-cuff take on the whirlpool concept. Sounds pretty whacky actually. Then there may be environmental factors too - marine life, changing ocean currents, etc.
Let’s say you have (a channel of) perfectly still water, and you want to place a weight on it, but you want to have minimal contact area with the water. What you do I think is put the weight on top of a hollow tube, standing up, capped on the top. You increase the air pressure in the tube until it rises enough so that it barely touches the water, but you don’t raise it so much that the air rushes out from under the tube.
I think I’ll believe that a thing like that won’t experience much drag when you push against it.
I’ll also believe that it would be infinitely easier to build such a (circular) channel of still water on land than in the sea. And I’d be interested in a comparison of the drag of this compared to using train tracks.
OK I was thinking at first that a ring like that would have the least drag.
But then it occurred to me that the main drag would be " skin friction" with the water, proportional to the wetted area. Depending on submersion depth, if you bring the sides of the ring together at the bottom to form a closed hull, you would reduce that wetted area, reducing skin friction by up to almost 50%. Similarly, one could imagine a boat or ship with just sides and no bottom, or sides with a raised bottom and a hollow volume below, with air pumped into that volume below - who knows, but air leakage from waves, and the front (bow) and rear (stern) might be problematic. Of course there are existing schemes to introduce bubbles under ships to reduce friction…
I’m basically describing a kind of hovercraft, which would have a smaller wetted surface area than any closed hull. The idea was to reduce the wetted surface to the absolute minimum.
Since in @GuyM’s idea there are no bow and stern we don’t have to worry about them, or about waves if the channel is wide enough and has sides high enough to prevent waves from the wind.
How wide you want to make it becomes an engineering question. The narrower you make your “hovercraft”, the higher your air pressure needs to be to sit at the same depth, but also the narrower your channel can be, I imagine.
Engineering the Hovertrain - 1972
The Problem With Fast Trains: What Happened to Hovertrains?
So the spun water is going round & contained inside a massive cylindrical drum?
A virtual drum . The cables locating the hub forming the cone of the base . For simplicity the Side was considered to be the same distance out as the depth under the ring. In reality the shear rate can be lower on the sides.
How wide you want to make it becomes an engineering question.
The ultimate low friction method for a land system is to use a Sealed top with a central bearing.
Magnets fixed further out on to the rotating top and a side (diameter) which seals an thick air cushion by penetrating into the centre of a water canal that is wide and deep. Coils under the rotating magnets would be protected by a base support / brake to hold up the top (stationary) in the event maintenance, when the air pressure is lowered. There are losses on the wetted area but by making the canal wide and deep Couette flow analysis applies and shear rates are minimised. Note there are also losses under the rotating top. The air cushion needs to be thick except between the magnets and coils and slightly less between the top and the support / brake. The system may also need some rollers ( on pressurised jacks) inside the ID of the canal to to partially support the top to limit tilting.
It may not be practical to construct a single rotating top due to the span but it would be possible to construct a system with an inner and outer side and use 2 concentric canals.
This is an interesting conversation about an interesting topic, but without diagrams it is difficult to imagine what actual structure people intend to communicate, some posts more than others. The last few posts just make my eyes glaze over - can’t decipher them.
On point to note is there seems to be two opposing concepts being discussed:
Maximizing water drag, and dragging all the water you can along with the spinning ring, ostensibly creating “energy storage”.
Minimizing water drag by supporting the ring with air pressure in a restricted zone between an inner and outer wall sealed from air leakage by water immersion.
In point 1 it is hard to tell when someone is talking about open ocean or some container or what
True. Consider the cone of cables that locate the hub as a conical stationary base. The water speed is proportional to radius and depth so in the simplified example I gave you the shear rate is uniform under the whole system.
To Model it is possible to spin a disc on a body of water with a cone of cables to locate the centre of rotation. If bubbles of air are injected from the base you should be able to see how they rotate faster as they rise to the surface. If you have right equipment you could verify that the bubbles move at a speed proportional to radius and inversely proportional to depth.
Is there some diagram? I’m still confused when I read this. The cone of cables? I must have missed some key illustration, sorry.
OK nevermind I found the illustration at:
Looks like a bicycle wheel. Great, they can use a bike wheel to build a very small prototype.
The video in the PDF seems to expose some inexperienced engineering. The British accent implies they must know what they are talking about, but…
