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| Fig. 1: An example of a Fly-Gen airborne wind energy device, where electricity generated on an aircraft is transmitted to the power grid via a tether. (Source: Wikimedia Commons) |
There has been a significant push to shift our energy sources from fossil and nuclear fuels to renewable technologies. A relatively new generation of energy systems aims to harness atmospheric winds that cant be reached by traditional wind turbines. Atmospheric winds tend to be faster and more consistent than surface winds, which poses an interesting opportunity to harvest energy at higher rates with atmospheric winds than with surface winds. [1] In fact, a recent global climate model suggests that high-altitude wind power devices could extract more than 1,800 TW, which is approximately 100 times greater than the present global power demand (~18 TW). [2] By contrast, surface winds can only provide 400 TW. [2]
All wind energy devices work on the principle of using aerodynamic forces to displace a generator to produce mechanical work. Airborne wind energy devices use a tether to extend the device into the atmosphere. This principle inherently has a consequence of extracting kinetic energy from atmospheric winds, which could potentially have climatic impacts. At energy extraction rates to meet the global power demand, however, uniformly distributed airborne wind energy devices are unlikely to have substantial affects on the Earths climate. [2]
Airborne wind energy devices can be broadly categorized into Ground-Gen and Fly-Gen systems. [3] Ground-Gen systems generate electricity on the ground via tension on the aircraft tether, while Fly-Gen systems generate electricity directly on the aircraft.
Ground-Gen systems can be further categorized into fixed and moving ground stations. In fixed ground station systems, the aircraft tether is winched on motor-generators. As the aircraft flies and applies tension on the tether, the tether is unreeled which generates electricity. When the end of the tether is reached, the winch consumes a smaller amount of electricity to pull the aircraft back. For moving ground station systems, the tether is kept at a constant length while the ground station tether attachment point is attached to a rotating or linear generator. By controlling the direction of tension applied on the tether, the generator moves to generate electricity.
Fly-Gen systems employ wind turbines onboard the aircraft to generate electricity. These aircraft can take many forms, such as winged aircraft, quadcopters, or even balloons. [3] While winged Fly-Gen systems stay in the air using lift forces generated from their wings similar to most Ground-Gen systems, quadcopter Fly-Gen systems depend on thrust from rotors and balloon Fly-Gen systems (such as the system shown in Fig. 1) use lighter-than-air buoyancy. No matter the method of staying aloft, Fly-Gen systems employ turbines to generate electricity onboard the aircraft and transmit the electricity down a specialized tether.
One area of interest with airborne wind energy devices is the implications of scaling size. Since airborne wind energy devices dont occupy land space, there are very few limitations to their theoretical size. One study applies a dynamic model to crosswind kites of varying size and finds that a power output of 45 MW is feasible for a kite with a 2,000 m2 wing area and a 1,200 m tether in only 10 m/s of wind. [4] While no existing airborne wind energy device has approached this amount of power output, Makani Power Inc. is currently planning to produce an offshore Fly-Gen system with a 65 m wingspan that can generate 5 MW. [3]
Airborne wind energy devices pose significant opportunities for increasing our reliance on renewable energies. There is enough wind energy to satisfy global power demands and deployed airborne wind energy devices do not impose significant environmental or societal pressures. In order to commercialize more airborne wind energy devices, more research and development needs to be done to make the multitude of Ground-Gen and Fly-Gen concepts a reality.
© Eric Chang. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] C. Archer and K. Caldeira, "Global Assessment of High-Altitude Wind Power," Energies 2, 307 (2009).
[2] K. Marvel, B. Kravitz, and K. Caldeira, "Geophysical Limits to Global Wind Power," Nat. Clim. Change 3, 118 (2013).
[3] A. Cherubini et al., "Airborne Wind Energy Systems: A Review of the Technologies," Renew. Sustain. Energy Rev. 51, 1461 (2015).
[4] M. Loyd, "Crosswind Kite Power," J. Energy 4, 106 (1980).
