Materials Science in Airborne Wind Energy (AWE) Part 1

AWE systems (AWES) materials run the gamut from “energy drones” with composite airframes, complex power electronics, and advanced avionics, to “rag & string only”, pure engineering polymer, like UHMWPE or nylon as examples. Obviously the high-complexity platforms entail less than 100% structural material with recycling challenges for a truly sustainable circular economy.

High-complexity advocates hope to achieve key operational advantages, like longer service life and superior control. They seek high performance at max L/D. CAPEX is high. Maintenance resembles conventional aviation, with high OPEX. Every sort of material finds a place in these designs, including toxic unrecyclable elements, with diverse materials science applicable. Composite AWES airframes and conventional HAWT blades are built very similarly, with common O&M and lifecycle properties. Most of energy drone type jobs are inside a factory. Repairs and regular inspections are slow and complex. Control is demanding, at high speed, with scant margin for error. Crashes are generally dangerous, with a total loss of the platform.

The Rag & String AWES camp seeks highest power-to-mass, using the highest strength-to-mass material kept at its working-load. Any non-structural material aloft is parasitic to raw performance. The idea is “a truck not a sports-car”; max-power by max-area, rather than highest L/D. Generators and controls are kept on the ground. This AWES archetype has converged on single-skin (SS) fabric wings derived from NASA’s Apollo program, notably the NASA Power Wing (NPW) and SS derivatives. The scaling path is simply a fractal load-path network, from the fabric weave with rip-stop threads, to as many stages of load path networks needed to reach extreme-scale wing dimensions. Crashes are generally benign, and the wings hop right back up.

Cheap polymer fabric reaches pay-back very fast, in a few weeks, while advanced polymer lifetime is indefinite, if over-tensioning and abrasion is avoided. Some (careful) kite pros use the same lines for years. UV life is no longer a major issue when fabric is properly specified for UV-protecting pigmentation (carbon black, iron oxide, titanium white) and/or sealer (like sunblock lotion). Construction of fabric wings is sewn or taped. In principle, simple sails made from polymer roll-stock polymer could be made at many meters a second, in one super-factory, to power the world in a year or two of production. Field repairs are fast and cheap. Its mostly outdoor jobs, “sailing in the sky”, based on supervisory auto-piloting rather than full control autonomy.

This video is great because it shows SpaceX using SS NPW-derivative wings to return large launch components from the edge of space to a precise landing. Andre Bandarra recreates a SpaceX SS wing at subscale, with a bit of advice from Ozone, a top fabric wing maker for extreme sports. It shows how one can make a good AWES platform with nothing but a sewing machine and some polymer fabric and line. One can also buy cheap COTS TRL9 polymer SS kites, to instantly be working at the frontier of AWE, the next wind energy revolution.

The emerging engineering science of Metamaterials creates novel properties from the arrangement of parts in a periodic lattice, rather than the choice of substrate material (polymer, metal, ceramic, etc.). Mechanical Energy Metamaterials are now well known, from microscopic to bench-top scale, and provide direct analogs for kite energy metamaterials, as regular periodic Airborne Wind Energy (AWE) “kite networks”, as a many-unit scaling strategy, consistent with Network Theory. Repeating units of lines and kites can be arranged many ways, including traditional kite trains and arches. This exciting new RE concept space considers kite networks to harvest and aggregate wind energy at extreme-scale.

Mechanical Energy Metamaterials have boundaries of surfaces and edges. The engineer can choose to convey energy within the bulk of the material, or along surfaces and edges. Energy metamaterials have converged on basic crystal geometric orders, particularly hexagonal and tetrahedral cell and prism lattices. These have long been classified mathematically, and particular lattice models (ie. Kagome Herbertsmithite) offer unique formal properties. The mathematical physics become more complex and exotic as spin-liquid statistics, knot & braid theory, topological order, and various other paradigms are applied. There is no shortage of applicable science to creating an extreme-scale “kite matter” metamaterial with dynamic similarity to its smaller cousins.

Kites meet every formal criteria for metamaterial identification, like periodic order and auxetic and negative-index properties. For a theoretic kite metamaterial, wind energy is harvested in the bulk, from any direction, takes the form of spontaneously resonant lattice-waves conveyed to surfaces and edges, and ultimately to Power Take-Off (PTO) points at the surface, for coherent power-pumping action. Already, toy kite lattices self-oscillate with coherent internal waves that can be felt at the surface. In fact, traditional kite lattices are covered with aero and mass dampers to tame them. A kite energy metamaterial is simply a matter of optimizing well known kite principles for power output.

As AWE theorists have analyzed energy kite network metamaterial challenges, they have hit on the “SpiderMill” paradigm [Ockels AWEC 2011], a radially symmetric plant that accepts wind equally from any direction by simply tilting rather than rotating, rather like a spiderweb in plan. The unit kites have come to be called “Kixels”, which may also individually tilt away from prevailing wind to generate lift, without needing to rotate. A SpiderMill may consist of a layered stack all the way to very high altitude. Its an open question how best to rig and fly a SpiderMill, and what bulk harmonic modes to tap. This is the current “wind metamaterial” frontier, aimed at GW-rated unit-plants.

Generic SpiderMill Concept: