Crosswind kite power

Crosswind kite power is a class of airborne wind-energy conversion systems (AWECS) characterized by a kite system, flying crosswind. These systems can be used as high-altitude wind power (HAWP) devices. A flexible wing or a rigid wing can be used in the kite system. A tethered wing, flying crosswind at many times wind speed, harvests wind power from an area, many times exceeding the wing’s own area. Crosswind kite power has all the advantages of AWECS over conventional wind turbines – access to more powerful and stable wind resource, high capacity factor, capability for deployment on and offshore at comparable costs and no need for a tower. Additionally, the crosswind kite has excellent aerodynamic efficiency, usually compared with the outer parts of conventional wind turbine blades. However, a conventional rotating blade carried aloft in a kite system has the blade cutting crosswind and is a form of crosswind kite power.

Crosswind kite power schemes taught in patent that has teachings that are now in public domain for use by the kite energy systems community and all others.

Charles McCutchen, Peter R. Payne taught crosswind kite power in their patent .

Myles Loyd introduced crosswind kite power systems in his seminal work "Crosswind Kite Power" in 1980.

Types

Scheme of the main types of airborne crosswind power systems

The three main types of crosswind kite power systems are distinguished by the method of the power removal from the wing: an onboard generator, a tether lift and separate motion transfer.

Tether pulling

In the systems of this type, an electrical generator, pump, or tasking line is installed on the ground. There are two subtypes, with or without a secondary vehicle.

US8066225 teaches crosswind kite power including kite farming multiplicities of tethered wings of any sort.

In the subtype without a secondary vehicle,"Yo-Yo" method, the tether slowly unwinds off a drum on the ground, due to the windward pull of the kite system's wing, while the wing travels crosswind, that is, left-right of the wind's ambient direction, along various paths, e.g., a figure-8 flight path, or optimized lemniscate paths, or circular paths (small or large radius). The turning drum rotates the rotor of the generator or pump through, perhaps, a high-ratio gearbox. Periodically, the wing is depowered, and the tether is reeled in, or, using the crosswind for a constant pull, the tether is re-connected to a different section of the drum while the wing is traveling in a "downwind" cycle. In some systems two tethers are used instead of one. An example of a system like this is KiteGen. In another system, three tethers are used to crank a load.

In another subtype, a secondary vehicle is used. Such a vehicle can be a carousel, a car, railed cart, wheeled land vehicle, or even a ship on the water. The electrical generator is installed on the vehicle. The rotor of the electrical generator is brought in motion by the carousel, the axle of the car, or the screw of the ship, correspondingly.

Onboard generator

In the systems of this type, one or more propellers and electrical generators are installed on the wing. The relative airflow rotates the propellers, which transfer the power to the generators. Produced electrical energy is transmitted to the ground through an electrical cable laid along the tether or integrated with the tether.

Separate motion transfer

In this type, an electrical generator or pump or tasking line is installed on the ground. Transfer may be by separate lines or by utilizing the actual tether set in working loops or fan-belting manner; or the tether set in bi-tether or tri-tether may work ground cranks.; the placement of the ground devices maybe be behind, under, to the side, or upwind or downwind of the tethered wing set. The moving tethers, loops, belts, or chains drive loads on the ground for performing tasks, perhaps make electricity, pump water, move materials, grind or cut. One method has a separate belt that extends at approximately the speed of the wing. Because of the high speed of that belt, a gearbox may not be required for specific purpose.

Patents giving instruction on crosswind kite power

• US8066225 Multi-tether cross-wind kite power. Benjamin Tigner filed in Jan 19, 2009, but has a priority date of Jan. 31, 2008. He teaches crosswind kite farming to make electricity.
• US 3987987 Self-erecting windmill by Charles McCutchen and Peter R. Payne. They filed in January 28, 1975. Their work is now in the public domain.

Theory

In all types of the crosswind kite power system, the useful power can be approximately described by the Loyd’s formula:

$P={2 \over 27}\rho _{a}AC_{L}({C_{L} \over C_{D}})^{2}V^{3}$

where P is power; C_L and C_D are coefficients of lift and drag, respectively; ρ_a is the air density at the altitude of the wing; A is the wing area and V is the wind speed . This formula disregards tether drag, wing and tether weights, change of the air density with altitude and angle of the wing motion vector to the plane, perpendicular to the wind. A more precise formula is:

$P={2 \over 27}\rho _{a}AC_{L}G^{2}V^{3}$

where G is the effective gliding ratio, taking into account the tether drag.

Example: a system with a rigid wing, having dimensions 50m x 2m and G=15 in the 12 m/s wind will provide 40MW of electric power.

Challenges

By June 2013 there are hundreds of deployed crosswind kite power systems. One traction leader Skysails used for ship propulsion in a manner similar to traditional sails. Crosswind kite power systems for electrical energy generation are now being sold to retail customers by Pacific Sky Power which has crosswinding in the moving blades. Some of the challenges that crosswind kite power systems must overcome to achieve mainstream acceptance are: regulatory permissions, including use of airspace and land; safety considerations; reliable operation in varying conditions (day, night, summer, winter, fog, high wind, low wind, etc.); third-party assessment and certification; lifecycle cost modeling .

History and prospects

Crosswind kite power was brought again into focus when Loyd carefully described it in 1980. It was not possible to create an economical automatic control system to control the wing, though passive control of crosswinding kite systems has been ancient. This has changed in the passing years, when computational and sensory resources became not only affordable, but cheap. In the same time, significant progress was made in the materials and wing construction techniques. New types of flexible kites with good L/D ratio have been invented. Synthetic materials, suitable for the wing and tether, became affordable. Among those materials are Dyneema, carbon fiber, Dacron, and rip-stop nylon. A large number of people became engaged in the sport of kitesurfing, kiteboarding, kite buggying, snowkiting, and power kiting. The field continues to undergo rapid development in the airborne wind energy community. Most of the progress has been achieved in the last 10 years. Multiple companies and academic teams work on crosswind kite power. In May 2013 Google acquired a Californian company developing systems with onboard generators . Over fifty methods of crosswind kite power are being explored; no winner has been proven as most promising.

Gallery

• Crosswind kite power system with tether lift, drawing.
  Crosswind kite power system with tether lift, drawing.
• 
  Crosswind kite power station with separate motion transfer with two wings offshore, artist's impression.

See also

• Airborne wind turbine
• High-altitude wind power
• Unconventional wind turbines