Physicist Alex Bolonkin calculates that modern artificial fiber power transmission is 2682 times more efficient by weight, & more than 540 times by cross-section, than bare copper. KiteLab has found mechanical power transmission far superior to electrical conductors by many direct experiments. Aluminum is hardly better, given its low fatigue life. This sorry picture does not even count flygen weight. Some hope higher voltages than Alex's 1000V assumption can work, but there are terrible operational & design trade-offs. It increasingly seems flygen ventures lack competative viability, especially for high-altitudes.

This from the NASA annotated AWE bibliography-

Bolonkin, A. (2004). Utilization of Wind Energy at High Altitude.  Providence : AIAA. http://arxiv.org/ftp/physics/papers/0701/0701114.pdf

• The use of artificial fibers instead of copper
  • For 20 MW and voltage 1000 V the cross sections are Cu: 20,000 AF: 37 mm^2
  • Cable length of 25 km, weight of Cu: 8930 tons, AF: 3.33 tons
  • AF is also much cheaper

Companies like Joby Energy & Makani Power insist that the best way to harvest upper wind is jumbo aerobatic kiteplanes operating at high speed. Many challenges to this view continue to emerge. JoeF has spotted a new concern, Wake Turbulence Hazard, which i thought about & find significant.

We have two good clues to reason from. There is vast experience with Wake Turbulence Hazard at at airports, which was a mysterious killer until it was understood. Training & operational procedures were put in place & the issue receded. The basic precaution is to increase spacing of aircraft when awareness of the hazard exists; generally  when an aircraft is following too close behind another on take-off & approaches at low altitudes in the absence of crosswind (I will always remember long ago taking off with my father in a Cessna light plane only a few thousand feet behind a giant C-5A Galaxy & enjoying the opportunity to probe the massive wake).

Another fine clue is Prof John Dabiri's finding that wind farms consisting of High Re HAWTs suffer disproportionally from wake interference compared the wind farms of Lower Re VAWTs whose weaker wakes dissipate far more quickly. His research was able to show that one can crowd far more capacity of VAWTs into the same space & actually reverse the longstanding conventional wisdom that HAWTs are always intrinsicly superior.

What this suggests for hot kiteplanes is that performance & safety margins will be considerably degraded by Wake Turbulence from neighbors & increased spacing &/or staggered altitudes will be required to reduce the effect. On the other hand, slower AWE kites should enjoy the same advantage Dabiri found for densely packed Low Re turbines & be able to harvest considerably more power from a given parcel of airspace.

Reference-

Dabiri JO (2010) “Order-of-magnitude enhancement of wind farm power density via counter-rotating vertical-axis wind turbine arrays,” submitted. [arXiv preprint]

It has been previously reported to the Forum that the KiteLab's Tri-Tether concept not only is a good way to transmit power from a looping kite to the ground, but can in principle be "back-driven" to keep a kite in the sky without wind. Today a successful demonstration occurred at the opening of the 10th Annual Windless Kite Festival, in Long Beach, WA (USA), in the Long Beach Elementary School gym. The fliers were competitor Michael Dirk, judge Don Ostey, festival founder Scott Durran & "yours truly". Retired Boeing engineer & Kite Master Dick Durran materially assisted & served as the senior expert witness.

Three kiters stood apart in a triangle pattern, each with a kiteline attached to a shared toy kite (Winnie-the-Pooh). Each kiter tugged on the line in turn (three phase power) & easily sustained the kite circling aloft. This suggests that the elusive high-value goal of persistent flight is practical without tow-vehicles, lifting gas, aerial-refueling, conductive cords, & motors aloft. Had the experiment been outdoors, transition to & from wind-powered flight could have been shown. By this sort of method, groundgen AWE has a clear capability to maintain flight thru calm. Many other applications, like communications, are enabled. The inventor requests a ban on military development of this technology.

Technology Readiness Level (TRL) is a metric used by NASA & other federal agencies to support decision making. It informs the focus & prioritization of governmental R&D.

We can state with certainty that Low-Complexity AWE methods, like piloted/passive-control & COTS components like the ground-based generator, UHMWPE tether, & parafoil kite are, taken separately, at the highest TRL9 level. Flygens, conductive tethers, & hot kiteplanes are well down the TRL scale (~3-6). The tiniest Low-Complexity COTS-based AWECS can be reasonably rated near TRL8, but all utility-scale systems are at about TRL3.

KiteLab Group proposes NASA treat high-TRL solutions as especially worthy of early support to set the performance baseline against which low-TRL AWE progress is measured.