Turbine Orientation

Moving the lifter kite across the wind window (crosswind?) will result in two advantages. Firstly the tether tension increases which ensures good performance of the cable drive. Secondly it will cause the train of turbines to move across the wind window, effectively increasing the apparent wind speed. This will increase the power output of the turbines especially if they are oriented to face the ‘effective’ wind direction. (To keep on topic.)

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A s a preliminary remark for this topic an evaluation of the aeroelasticity effects of a multi-rotor structure compared to a single larger rotor of equivalent power would make it possible to sketch the most appropriate solution in crosswind flight mode. Account should also be taken of the pendulum effect with regard to the wear of the material, including the tethers, as well as the interaction of these effects. Then these issues could be better evaluated according to the turbine orientation.

Positive…
Yes, sweeping or looping a lifter increases line tension by increasing apparent wind.
Negative …
Aerobatics requires extra line length for safe working. This has drag and mass implications.
Positive…
The extra tension can be significantly beneficial for tension transmission (like yo-yo).
The higher apparent wind will be beneficial for high up line mounted inline turbines (Kitewinder Kiwee) or kite mounted turbines (like Makani)
Negative
Sweeping and looping from above will add cyclic loading to components and cause irregular output.

As an aside, I’m quite keen to use a longer line and looping (tight) lifter above larger Daisy types.

Check my numbers. If we change the orientation angle from 30 deg to 0 deg then the increase in output power will be: 1/cos cubed (30)=1.54. If we assume the average wind increases by 20% at altitude, then the power will increase by 1.2 cubed = 1.72. The power will therefore increase by about 1.54X1.72=2.66. With an increase in performance like this, perhaps we do not need cross-window action. The increased performance will more than compensate for the extra weight and complication of u-joints, rigid frames or pulleys.

A major problem with multiple coaxial turbines (Superturbine, Daisy etc.) is that the higher the elevation angle, the greater the cosine cubed losses. At 45 deg. The correction factor is 0.354 and for 60 deg. the factor is 0.125! It is therefore essential to find methods to reorient the turbines to face the wind if we want to capitalize on the higher winds at altitude. I suggest three methods to do this, all of which can be converted to cable drive to avoid airborne generators and conductive tethers. I would appreciate your comments on the three methods and perhaps you can suggest others.

I feel that it is much easier to increase the size of the lifter kite to provide more lift than to operate a turbine at a suboptimal angle. We require large amounts of lift to operate the cable drive. In order to generate significant amounts of energy we require a large lifter kite which must be automatically controlled, launched and landed. Increasing the size of the kite will not increase the cost much.

With multiturbine devices which we are discussing the only way to generate crosswind action is to traverse the wind window with the lifter kite. Since this kite will be controlled by a kite steering unit or auxiliary tethers, increasing the kite size will not appreciably affect the crosswind action. This crosswind action can be stopped in conditions of high wind to prevent damage to the turbines. If the wind is still too strong, and the maximum force is approached, then the kite system must be landed.

Moving the lifter kite across the wind window (crosswind?) will result in two advantages. Firstly the tether tension increases which ensures good performance of the cable drive. Secondly it will cause the train of turbines to move across the wind window, effectively increasing the apparent wind speed. This will increase the power output of the turbines especially if they are oriented to face the ‘effective’ wind direction. (To keep on topic.)

Check my numbers. If we change the orientation angle from 30 deg to 0 deg then the increase in output power will be: 1/cos cubed (30)=1.54. If we assume the average wind increases by 20% at altitude, then the power will increase by 1.2 cubed = 1.72. The power will therefore increase by about 1.54X1.72=2.66. With an increase in performance like this, perhaps we do not need cross-window action. The increased performance will more than compensate for the extra weight and complication of u-joints, rigid frames or pulleys.

Pierre,
I fail to see how the Kitewinder system can be adapted to multiple oriented turbines. Auxiliary cable drives would have to be rigidly connected together with some sort of frame. This is like the pulley system I propose.

But as the connection would be made by the angle pulleys rather by the hubs, some significant adjustments would be required. I think a preliminary work would be building the different variants (pulleys, bevel gears, universal joints, angle pulleys like for Kiwee, and so on) of multi-rotor facing the wind or the apparent wind.

An example of connection by the hubs is given below. Unfortunately the apparent wind would be different according to the side of the turbine.

propellers belt and rope-drive

The frame would be a bar like this, the turbine being correctly oriented:

And as the rope-drive transmission avoids the weight of the generator in flight while the system scales up, a single larger rotor would be simpler.

I like your idea of using bars to connect the oriented turbines. The problem is to connect the cable drive loops without interfering with the propellers. My idea, in the attached sketch involves a lot of pulleys. Any other ideas? The bars must have bends at the end to satisfy the orientation. Perhaps these bends can be adjustable to accommodate various tether angles.
DOUBLE PULLEYS.pdf (59.5 KB)

Hi @gordon_sp,

Your stepped axis with pulleys looks to be a workable solution. The question is balancing between the expected benefit by the orientation of the turbines, and the weight and complexity of the device.

According to the report below, SuperTurbine ™ looks to be efficient (see § 5):

I remake my sketch (see below) because the rope-drive is useless in the SuperTurbine ™ part. The rope-drive connects the lower rotor to the generator at the ground in Kitewinder patent way.

I think the Superturbine tests were done with 25 deg angle. This results in a 25.6% loss in power. For high altitude we have additional wind speed gains. The additional weight can be compensated by a larger lifter kite.

You are right about the rope-drive section. The question remains, how do you orient the turbines to be horizontal if they are connected to the bars? I can think of universal joints and sails (inefficient) or pulleys. Any other ideas?

Variants with universal joints, double universal joints, bevel gears has been discussed.

I think we have to keep it simple. All these variants, comprising pulley system, lead to additional weight and frictions. Certainly there are some (limited for a low angle) losses for SuperTurbine ™ but also some lift, and it is simpler and lighter.

And also an option for multi-rotor on a tether (instead of a bar) can be studied in yo-yo mode.

I think the focus on % loss is a little misguided. For any AWE, it is quite likely that zero elevation angle is plain impossible.

Conparing with a HAWT (regular windmill) yes you have losses, but you have benefits as well.

Cosine loss only means that you need more wing area to achieve the same power. This is bad, but other factors may be limiting the scale also, like tether strength, power rating of the ground eqipment etc.

And, for a superturbine type og windmill, if you point the rotors horizontally, you need to add an extra force to counter tether downwards pull.

Also remember higher elevation would mean more altitude and higher average windspeeds.

Im not sure cosine losses is the best place to start a design…

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Unfortunately the cosine cubed losses are more than the gain from increased wind speed. For example, increasing the tether angle will cause a cosine cubed loss of 46% whereas a 10% increase of wind speed will increase the power by 33%.

It is also possible to increase the altitude without modifying the angle of elevation by lengthening the tether.

But other issues would occur such like higher space use, and tether weight.

My example suggests increasing the tether angle from 30 deg to 45 deg. It would be even worse for greater tether angles.

Yo-yo mode requires crosswind action. Why not replace the turbines with arrays of kites which are lighter and cheaper. Kites will naturally turn to face the wind during crosswind action providing more power. This is similar to your Orthokitebunch system with a different power takeoff.

The parachute on chinese umbrella description.pdf (1.3 MB) does (did) not use crosswind action during yo-yo phases (The most basic airborne wind energy system).

Multi-rotor could make the same, enjoying a higher lifetime as the rotors are rigid, and also perhaps easier to implement. Indeed thin rotors are around the tether without transfer issue, only thrust. So the rotors are not connected and are adapted to different wind conditions according to altitudes.

It is true. The same is not easy for rotors excepted for facing rotors like for Makani or FlygenKite.

Like something you’d see sketched in the margin of a junior high school kid’s notebook

Indeed, because as you just wrote:

Yes Pierre, it definitely has the advantage of simplicity, and I’m probably one of the kids who sketched it in the margin of my notebook way back in the '60’s or '70’s. Also underwater versions. By the time I got to college all my margins were full of wind-energy-themed art, mostly just very vague themes.