CoolIP index                 Most recent edit: Wednesday October 24, 2012

Low Flying-Angle Mitigation

Low Flying-Angle Mitigation

Low flying-angle is a discouraging reality of most kite-based AWE. Flying higher requires ever more excessive tether-scope, weight, drag, & negative lift.

KiteLab's meshed AWE array concepts mitigate this problem. Only the upwind margin of such an array need use low angled tethers. All subsequent cells of the elevated mesh can enjoy short vertical tethers or even tilt tethers windward for lift & reduced drag. The wider downwind a meshed array is proportioned, the more low flying-angle penalties are avoided.

Notes:

Vertical tethers are ideal for membrane wingmills to passively accept wind from all quarters. Surface work-cells can be turret-less.

A meshed array can have its entire margin consist of low angled tethers, independent of wind direction. Such an array need not rotate if its lifters all swivel on leaders.

A thin upwind "whiskey-line" can tilt a single element AWECS vertical or upwind. A thick conductor cable located downwind is rendered far more supportable.

Putting wings all along tethers also fights low flying angle & maximizes airspace by higher frontal solidity.

Meshes, lattices, arches, trains & other array configurations maximize footprint & airspace. An upcoming report summarizes the topic.

CoolIP                       ~Dave Santos             21Oct2010        M2368

Sept 1, 2012: (parent post: AWES6886)

This note elaborates on earlier ideas:

Turbine rotors for AWE face severe operational and scaling challenges. With increasing size, dangerous kinetic and gravity loads soon overcome the advantages rotors enjoy. The risk of rotors fouling with tethers is present at all scales, except for small caged rotors, which tolerate the added weight of the cage, by their inverse cubic mass advantage.

We are aware of conventional HAWT economy-of-scale and have often pondered large turbines integrated into 2D AWES arrays as ungainly flocks. What if a tight mesh of small caged rotors were made on a vast scale? Could this strange method of tapping a large projected wind area actually beat a monolithic HAWT? One thing is for sure, a sufficiently large fabric of small turbines will outpower the largest possible HAWT, so this may be just like the inherently small crystalline silicon solar cell is ganged.

What is the optimal size of such a caged rotor unit? Probably not much bigger than a meter across. Go too small and low Re effects start to dominate. Small rotors turn at high RPM, have superior power-to-weight, and thermal dissipation. As small units, they could be made in fast automated high production.

Note the multi-small-rotor concept is thriving, the 16-rotor e-volo is just one instance, Joby Aviation's 8-rotor scheme was another, but these concepts are limited by large rigid support structures. A megascale tensile fabric of tiny whirligigs is even more radical.

========Bob adds on September 1, 2012:
At a guess, the trend to giantism in conventional windmills is due to the economics of tower size.  Dave S. shows that subdividing the turbine improves the weight/area ratio, and the shaft speed.  Smaller generators are easier to cool.  With robotic manufacture, material can be divided among many units with economies of frequency overbalancing economies of scale.  Using many elements usually gives a gradual failure mode, improving overall reliability.  The additional tether weight for a flying generator limits the altitude, and the extra weight aloft increases risks, but greatly simplifies the ground rigging.
               ~ Bob Stuart

===Harry counters some on September 1, 2012 
One sad aspect of small turbine rotors . . . lower conversion efficiency than larger diameter rotors. Both sizes operate in wind of almost identical density . . . . . that condition alone gives the efficiency edge to the larger rotors. To some degree, Selsam's multi-rotor SuperTurbine
concept is able to compensate for the lower conversion efficiency of a smaller rotor.

There is the unresolved challenge of seeking innovative methods by which to keep larger rotors airborne and to transmit power to ground level.
             ~Harry


Comment and development of this topic will be occurring here.       
All, send notes, drawings, and photographs!

Terms and aspects:   

  • Re :: Reynolds number  
  • whirligigs       wiki    |    images   |   Whirligigs are also known as pinwheels, buzzers, comic weathervanes, gee-haws, spinners, whirlygigs; whirlijig; whirlyjig; whirlybird; or plain whirly.
  • e-volo  eVoloIMAGEs
  • e-volo - a German prototype electric multicopter with 16 rotors, the first electric multicopter in the world to achieve manned flight. The large number of low-cost motors make it economical, quiet, and provide redundancy.

Related links:

Commentary is welcome:

  • ... the tree seems to a grounded towered cousin to this discussion.      ~JoeF
  • quivering aspen trees   ... video
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