|Fig. 1: This illustrates where parts of the wing set of a crosswind kite power device is crosswinding during conversion of the wind's kinetic energy.  (Source: Wikimedia Commons)|
Today, wind power is one of the fastest-growing power sources in the world. So far, the bulk of this growth has been in the form of conventional wind turbines, which are rigid systems with rather limited maneuverability. Although modern devices can be "downwind tracked," this is only possible around the turbine's vertical axis, which implies that only a rather small part of the available wind energy can be harvested. To increase efficiency, wind turbines must grow in size, and the bigger the device, the bigger the rotor blades needed to increase the yield. The machines must get bigger for another reason as well: at higher attitudes they encounter stronger and steadier wind regimes.  High altitude winds represent an untapped energy source that is even larger than the world's energy needs. Consequently, the towers that support the blades have grown from 50 meters in the 1990s to about 80 meters in the 2000s and about 100 meters today. Some devices operate at heights up to 150 meters, but structural constraints and technical limits hamper going much higher. In the actual wind towers, the outermost 30% of the blade surface contributes for about 80% of the generated power. Thus, the tower and the inner part of the blades do not directly contribute to energy generation. Yet, the structure of a wind tower determines most of its cost and imposes a limit to the elevation that can be reached. But there is another way to harness this untouched power of the winds in the troposphere: the use of kites, also known as crosswind kite energy. 
Instead of trying to harvest wind energy near the Earth's surface, where the winds are volatile and the wind speed is low, researchers seek to go where the wind is blowing constantly and the wind speed is much higher - several hundred or even several thousands of meters above ground. This is because wind speed is essential to the amount of energy a wind turbine can convert to electricity.  A kite is essentially a light and controllable aerodynamic flying device that flies in a cross wind and receives wind energy; in a kite power system this energy is somehow transported via a cable apparatus to a nearby ground station. Researchers have investigated different ways to harness this energy. A more frequently investigated system operates much like what happens when one allows a conventional kite to climb while pulling a line wound around a drum at the same time. The climbing kite creates a high traction force. As the drum rotates to pay out the line, it is coupled to a generator to produce electricity.  Upon reaching the maximum tether length, the kite's angle to the wind is reduced, so that the entire wing rotates and aligns with the apparent wind. The kite is then pulled back to its original position using the generator module as a winch and the next pump cycle is started. Depowering significantly reduces the traction force during reel-in by up to 80 percent. Thus, the energy consumed during retraction is only a fraction of the energy generated during unwinding. Fig. 1 shows this process. 
As you may know, conventional wind turbines have some detrimental effects to the environment, even though it is a clean, renewable energy source. One of the disadvantages is that wind farms create a lot of noise, which is why wind farms are usually built far away from residential areas.  Along with noise pollution, wind turbines also provide a sort of visual pollution, which is why most people would not enjoy a wind turbine sitting in their backyard. Also, the winds near the surface of the Earth are unpredictable, so wind farms will have to be placed in places where winds are consistent and abundant, such as coastal regions.  Taking this into account, the biggest concern of all is that a kite-based system would be impractical, because it could never replace a conventional power plant. Also, safely suspending and maintaining kite systems hundreds of meters off the ground in storms and abnormally high winds can pose as a large problem. Transferring the harvested power all the way back to the generator on the Earth's surface efficiently is also no easy task. But a lot of energy can be produced by kites while saving a lot of money. Kite power systems can be considered as modular, flexible, small-scale power production plants. They could therefore be applied to single family houses because their material is very light and affordable.  Even though energy kite systems are not on the market yet, they can be the breakthrough that replaces conventional energy production from harmful fossil fuels, decreasing the carbon dioxide in the atmosphere and possible hindering the detrimental effects of global warming. Overall, I believe energy kites are a safer, more efficient energy source choice, and will benefit our planet tremendously if successfully constructed and released to the market.
© Tai Thomas. 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.
 M. Canale, L. Fagiano, and M. Milanese, "Power Kites for Wind Energy Generation," IEEE 4384641, IEEE Control Systems 27, No. 6, 26 (December 2007).
 R. E. Wilson and P. B. S. Lissaman, "Applied Aerodynamics of Wind Power Machines," Oregon State University, July 1974.
 U. Ahrens, M. Diehl, and R. Schmel, eds., Airborne Wind Energy (Springer, 2013).
 B. W. Roberts, "Windmill Kite," United States Patent US6781254, 24 Aug 04.