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Scaling Laws for AWES Design

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  • dave santos
    To correctly envision the largest possible unit scales for AWES, all applicable scaling considerations must be accounted for. Many scaling issues are
    Message 1 of 1 , Dec 24, 2012
      To correctly envision the largest possible unit scales for AWES, all applicable scaling considerations must be accounted for. Many scaling issues are operational; others are fundamental to the basic "Theory of Operation" of the AWES.

      Online educational pages linked below, called Physics Documents, by John Denker, nicely introduce engineering scaling considerations. Denker invokes a key heuristic- "Always Look for the Scaling Law." Loyd himself overlooked critical AWES scaling laws in Crosswind Kite Power*, and many AWE scientists and engineers are making badly mistaken assumptions by failing to apply important scaling laws.

      AWES's top scaling law came from Galileo, that mass (co-linear with volume) grows at the cube of dimension, while critical cross-sectional strength, only grows at the square. This supports the aircraft designer's axiom of "minimal mass aloft" and enables predictions about scaling up soft-kites filled with air v. rigid wings. Until we find a standard formal name, lets call this our "Volumetric Mass Scaing Law". We can also keep in mind a corollary rule, that excess mass aloft in an AWES is parasitic of net power available for other work..

      Here's a misc. scaling law, from Denker, of interest in the question of what advantage the largest practical kite has over smaller kites-

      "6-21 The energy in the pressurized gas inside a bubble scales like volume, i.e. like diameter cubed. "

      This means that the bigger ball of pressure the kite can create (and opposed volume of vacuum) the potential energy scales at the cube of diameter. This is an "economy-of-scale" that best applies to soft kites like Mothras, that by their quasi-2D character, postpone the basic volume-mass structural scaling law limit. 

      This volume-energy law effectively offsets the well-known rigid-wing advantage of high L/D, and formally supports the old KiteLab conjecture that rigid and soft wings have a roughly comparable power potential (by mass) at small to medium scales, but that purely tensile soft wings naturally scale well beyond rigid. 

      Note also a favorable thermal buoyancy factor to large soft kites, which Peter Lynn Sr. has observed with his megakites. This extra lift can be a combination of solar gain and ram-air heat-of-compression (and low-pressure cooling above the kite wing).

      The best validated AWES megascale architecture seems to be soft overall, at or even beyond the single km scale, perhaps with arrays of embedded or hosted rigid structures optimally below their scaling limit dimension (~100m). 

      Denker's page-


      * For example, Loyd- Table 1: "Strength-to-Weight" ratio is misleadingly presumed constant across a large kite scale. The paper remains a classic despite inevitable small errors and major omissions (like noting zero downwind VL crosswind modes). Loyd makes many correct statements about scaling up wind power, especially the advantage of the largest effective unit scale, with cited references.

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