Airborne wind energy (AWE) is the direct use or generation of wind energy by the use of aerodynamic or aerostatic lift devices. AWE technology is able to harvest high altitude winds, in contrast to wind turbines, which use a rotor mounted on a tower.
The term high-altitude wind power (HAWP) has been used to refer to AWE systems.[1] However, semantically HAWP might also include wind energy conversion systems that are somehow positioned at a large height from the ground or sea surface.
Various mechanisms are proposed for capturing the kinetic energy of winds such as kites, kytoons, aerostats, gliders, gliders with turbines for regenerative soaring,[2] sailplanes with turbines, or other airfoils, including multiple-point building- or terrain-enabled holdings.[3] Once the mechanical energy is derived from the wind's kinetic energy, then many options are available for using that mechanical energy: direct traction,[4] [5] conversion to electricity aloft or at ground station, conversion to laser or microwave for power beaming to other aircraft or ground receivers. Energy generated by a high-altitude system may be used aloft or sent to the ground surface by conducting cables, mechanical force through a tether, rotation of endless line loop, movement of changed chemicals, flow of high-pressure gases, flow of low-pressure gases, or laser or microwave power beams.
Winds at higher altitudes become steadier, more persistent, and of higher velocity. Because power available in wind increases as the cube of velocity (the velocity-cubed law),[6] [7] assuming other parameters remaining the same, doubling a wind's velocity gives 23=8 times the power; tripling the velocity gives 33=27 times the available power. With steadier and more predictable winds, high-altitude wind has an advantage over wind near the ground. Being able to locate HAWP to effective altitudes and using the vertical dimension of airspace for wind farming brings further advantage using high-altitude winds for generating energy.
High-altitude wind generators can be adjusted in height and position to maximize energy return, which is impractical with fixed tower-mounted wind generators.
In each range of altitudes there are altitude-specific concerns being addressed by researchers and developers. As altitude increases, tethers increase in length, the temperature of the air changes, and vulnerability to atmospheric lightning changes. With increasing altitude, exposure to liabilities increase, costs increase, turbulence exposure changes, likelihood of having the system fly in more than one directional strata of winds increases, and the costs of operation changes. HAWP systems that are flown must climb through all intermediate altitudes up to final working altitudes—being at first a low- and then a high- altitude device.
An atlas of the high-altitude wind power resource has been prepared for all points on Earth.[8] A similar atlas of global assessment[9] was developed at Joby Energy.
Energy can be captured from the wind by kites,[10] kytoons, tethered gliders,[11] tethered sailplanes, aerostats (spherical as well as shaped kytoons), bladed turbines, airfoils, airfoil matrices, drogues, variable drogues, spiral airfoils, Darrieus turbines, Magnus-effect VAWT blimps, multiple-rotor complexes, fabric Jalbert-parafoil kites, uni-blade turbines, flipwings, tethers, bridles, string loops, wafting blades, undulating forms, and piezoelectric materials,[12] and more.[13]
When a scheme's purpose is to propel ships and boats,[14] [15] the objects tether-placed in the wind will tend to have most of the captured energy be in useful tension in the main tether. The aloft working bodies will be operated to maintain useful tension even while the ship is moving. This is the method for powerkiting sports. This sector of HAWP is the most installed method. Folklore suggests that Benjamin Franklin used the traction method of HAWP. George Pocock was a leader in tugging vehicles by traction.[16]
See also: Kite control systems. HAWP aircraft need to be controlled. Solutions in built systems have control mechanisms variously situated. Some systems are passive, or active, or a mix. When a kite steering unit (KSU) is lofted, the KSU may be robotic and self-contained; a KSU may be operated from the ground via radio-control by a live human operator or by smart computer programs. Some systems have built sensors in the aircraft body that report parameters like position, relative position to other parts. Kite control units (KCU) have involved more than steering; tether reeling speeds and directions can be adjusted in response to tether tensions and needs of the system during a power-generating phase or return-non-power-generating phase. Kite control parts vary widely.[17] [18]
See also: Crosswind kite power. The mechanical energy of the device may be converted to heat, sound, electricity, light, tension, pushes, pulls, laser, microwave, chemical changes, or compression of gases. Traction is a big direct use of the mechanical energy as in tugging cargo ships and kiteboarders. There are several methods of getting the mechanical energy from the wind's kinetic energy. Lighter-than-air (LTA) moored aerostats are employed as lifters of turbines. Heavier-than-air (HTA) tethered airfoils are being used as lifters or turbines themselves. Combinations of LTA and HTA devices in one system are being built and flown to capture HAWP. Even a family of free-flight airborne devices are represented in the literature that capture the kinetic energy of high-altitude winds (beginning with a description in 1967 by Richard Miller in his book Without Visible Means of Support) and a contemporary patent application by Dale C. Kramer, soaring sailplane competitor, inventor.
A research on airborne wind turbine technology innovations reveals that the “Kite type AWTs” technique, the most common type, has high scope of growth in the future; it has contributed for about 44% of the total airborne wind energy during 2008–2012. The kite type AWTs extract energy through wind turbines suspended at high altitudes using kites such as multi-tethered kite, kite and dual purpose circular fan, rotary wing kites etc.[19]
Electricity generation is just one of the options for capturing mechanical energy; however, this option dominates the focus of professionals aiming to supply large amounts of energy to commerce and utilities. A long array of secondary options include tugging water turbines, pumping of water, or compressing air or hydrogen. The position of the electric generator is a distinguishing feature among systems. Flying the generator aloft is done in a variety of ways. Keeping the generator at the mooring region is another large design option. The option in one system of a generator aloft and at the ground station has been used where a small generator operates electronic devices aloft while the ground generator is the big worker to make electricity for significant loads.
The “Carousel” configuration several kites fly at a constant height and higher altitudes, pulling in rotation a generator that moves on a wide circular rail. For a large Carousel system, the power obtained can be calculated as of the order of GW, exposing a law that see the power attainable as a function of the diameter raised to the fifth power, while the increment of cost of the generator is linear.[20]
One method of keeping working HAWP systems aloft is to use buoyant aerostats whether or not the electric generator is lifted or left on the ground. The aerostats are usually, but not always, shaped to achieve a kiting lifting effect. Recharging leaked lifting gas receives various solutions.In case of productive winds the aerostats are typically blown down by the aerodynamic drag applied on the wide and unavoidable Reynolds surface excluding them de facto from the HAWP category.
Conceptually, two adjacent mountains (natural or terrain-enabled) or artificial buildings or towers (urban or artificial) could have a wind turbine suspended between them by use of cables. When HAWP is cabled between two mountain tops across a valley,[3] the HAWP device is not airborne, but borne up by the cable system. No such systems are known to be in use, though patents teach these methods. When non-cabled bridges are the foundation for holding wind turbines high above the ground,[28] then these are grouped with conventional towered turbines and are outside the intent of HAWP where the tethering an airborne system is foundational.
Lightning, aircraft traffic, emergency procedures, system inspections, visibility marking of system parts and its tethers, electrical safety, runaway-wing procedures, over-powering controls, appropriate mooring, and more form the safety environment for HAWP systems.
There have been several periods of high interest in HAWP before the contemporary activity. The first period had a high focus on pulling carriages over the lands and capturing atmospheric electricity and lightning for human use.[29] The second period was in the 1970s and 1980s when research and investment flourished; a drop in oil price resulted in no significant installations of HAWP. Return on investment (ROI) has been the key parameter; that ROI remains in focus in the current development activity while in the background is the renewable and sustainable energy movement supporting wind power of any kind; but HAWP must compete on ROI with conventional towered solutions. A test center at Lista, Norway provides independent verification of research.[30]
Early centuries of kiting demonstrated that the kite is a rotary engine that rotates its tether part about its mooring point and causes hands and arms to move because of the energy captured from higher winds into the mechanical device. The tension in the lofted devices performs the work of lifting and pulling body parts and things. Airborne wind energy (AWE) for HAWP was birthed thousands of years ago; naming what happened and developing the implied potentials of tethered aircraft for doing special works is what is occurring in AWE HAWP. What is "low" for some workers is "high" for others.
Autorotation is the basis of a large sector of AWE technology. High altitude wind power research and development centers frequently are dependent on blade autorotation: SkyMill Energy, Joby Energy, Sky Windpower, BaseLoad Energy, Magenn Power, and Makani Power are making and testing airborne wind energy conversion systems (AWECS) that employ autorotation of blades to drive the shafts of generators to make electricity at altitude and send the electricity to earth via conductive tethers.[40]
