| 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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