Rigid kite (airborne wind energy)

A rigid kite for airborne wind energy (AWE), also called a rigid-wing or fixed-wing airborne wind energy system, is a tethered, aircraft-like wing that is flown autonomously in fast crosswind loops or figure-of-eight patterns to harvest wind energy at altitudes that are generally inaccessible to conventional tower-mounted wind turbines at fraction of total material usage. Unlike soft-wing (fabric) kites, rigid kites are built from stiff materials such as fibre-reinforced composites. The rigid construction gives them substantially higher lift-to-drag ratios than soft wings, allows precise aerodynamic control using conventional control surfaces, and makes it possible to mount wind turbines directly on the wing. The theoretical basis for crosswind power generation was established by Miles L. Loyd, who showed in 1980 that a tethered wing flying crosswind can generate far more power than a wing held stationary in the wind: because the wing's airspeed greatly exceeds the ambient wind speed, the aerodynamic force it produces is comparatively very large. In Loyd's idealized analysis the available power grows with the cube of the wind speed and with the square of the wing's lift-to-drag ratio, which strongly favours aerodynamically efficient rigid wings. Rigid kites are divided into two main categories according to where electrical power is generated. In fly-generation (fly-gen) systems, onboard turbines generate electricity as the kite moves along its high-speed crosswind trajectory, and the power is transmitted to the ground through a conductive tether; because power is extracted from the aerodynamic drag of the turbines, these are also called drag-mode systems. In ground-generation (ground-gen) systems, the kite's lift creates a high tension in the tether, which is reeled out from a ground-based drum to drive a generator; because power is extracted from the lift force, these are also called lift-mode systems. Ground-gen systems operate in a cyclic "pumping" mode, alternating between a reel-out (traction) phase and a reel-in (retraction) phase. Beyond grid-scale electricity generation, rigid-kite platforms are being explored for adjacent applications. Their ability to loiter persistently at several hundred metres altitude with a permanent tether link makes them candidates for surveillance and security infrastructure, a market segment announced by Kitemill in 2026. The low material usage, transportability and small ground footprint of rigid-kite systems also make them attractive for power delivery in off-grid and remote locations, such as island microgrids where conventional wind farm logistics are unfeasible. In addition, a tethered wing carrying onboard sensors can serve as an atmospheric measurement platform, continuously profiling wind, temperature and other meteorological quantities at altitudes between those covered by ground masts and research aircraft.

History

The idea of using kites to extract energy from the wind predates modern airborne wind energy, but the quantitative foundation of the field is usually traced to the 1980 paper "Crosswind kite power" by Miles L. Loyd, from Lawrence Livermore National Laboratory. Writing in the aftermath of the 1970s energy crises, Loyd derived the now-standard expressions for the power available to a tethered wing flying crosswind, in both lift and drag mode, and estimated that a single wing of airliner size could generate several megawatts. The work received limited attention at the time, and the concept lay largely dormant for two decades before being revisited as advances in composite materials, low-cost inertial and satellite navigation sensors, embedded computing and autonomous flight control made practical systems conceivable. A growing academic community consolidated around the topic in the early 2010s, and the field's first comprehensive monograph was published in 2013. Sustained development of rigid-wing hardware accelerated in the 2000s. Makani Power, founded in 2006 and acquired by Google in 2013, became the most prominent rigid-wing fly-gen developer. Its Wing 7 demonstrator achieved autonomous crosswind flight with onboard power generation, and the programme culminated in the M600 prototype, a rigid wing of roughly 26 m span carrying eight onboard turbines and rated at 600 kW. In 2019, Makani flew the M600 from a floating spar buoy off the coast of Norway, the first offshore demonstration of a utility-scale airborne wind energy system. Makani ceased operations in February 2020; its technical report, flight data and simulation code were subsequently released to the public. In Europe, development concentrated on ground-generation systems. Ampyx Power, founded in 2008 in The Hague, developed rigid-wing gliders that launch and land horizontally using a catapult and onboard propellers; after a series of small demonstrators that proved autonomous pumping-cycle operation, the company built the 12 m span, 150 kW-class AP3. Ampyx Power was declared bankrupt in May 2022 after failing to secure further investment, and its concept has since 2023 been continued by Mozaero, a company formed by former Ampyx engineers. The Norwegian company Kitemill, also founded in 2008, pursued fixed-wing ground-gen systems with a vertical takeoff and landing (VTOL) subsystem; its KM1 prototype completed several hundred test flights, and in 2023 the company unveiled the 16 m span, 100 kW KM2 as its first commercial model. Kitekraft, a 2019 spin-off from the Technical University of Munich, develops a fly-gen system based on a rigid boxplane airframe. Coordinated research efforts, including the European AWESCO and AWETRAIN doctoral training networks, IEA Wind Task 48 and the industry association Airborne Wind Europe, have since brought together the academic and industrial groups working on the technology.

Working principle

Crosswind flight

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