13 January 2016

Kite power—latest in green technology?

Dominic Notter

Dominic Notter

Dominic Notter is a scientist formerly with the Swiss Federal Laboratories for Materials Science and Technology. His research interests are in environmental risk assessment and life cycle...


Geneva, Switzerland, is known for many things. It is the birthplace of the International Red Cross and the Geneva Conventions, and home to more than 270 nongovernmental organizations, and tens of thousands of intergovernmental meetings and diplomatic treaty negotiations.

It is also host to an enormous amount of steady, powerful winds that come off the adjacent lake—one of the largest in Europe—and the city is sometimes buffeted by other winds that descend from the nearby mountains, which are extensions of the Alps. These winds are so large, and their timing so predictable, that local sailors have names for them; for example, the cold, persistent, and vigorous northeasterly is called the “bise,” most likely a mocking reference to the French word “bisous,” meaning “kiss.” When the bise arrives, flagpoles on shore appear to nearly bend in half, and it can be thrilling (and terrifying) to be on the lake in a small sailboat.

It is no wonder that for two years in the early 2000’s, the team that won the America’s Cup sailing trophy came from this high, land-locked lake.

And the winds are not confined to just the immediate shoreline. From as far away as the Palais des Nations—site of the UN’s Geneva office—on the north end of town, one can watch hang-gliders, parachutists, and others leap from the cliffs of the nearby Salève mountain far to the south. (And in the winter, daredevils sometimes parachute elsewhere in Switzerland with skis attached to their feet.) They are able to pursue these sports because of the strong winds, which follow roughly predictable daily and seasonal rhythms, depending in part on whether the sun is heating up the mountainsides (early morning and evening) or the valleys (around noon), for example. These are just some of the reasons for the many strong winds in this high-elevation region; in short, there is an awful lot of wind to be found throughout the country.

Engineers have long sought to harness these winds. About a 90-minute drive to the east, close to the base of the Saint Bernard Pass, there is a 500-foot windturbine with rotors 300 feet in diameter, along with a smaller, futuristic “Darrieus rotor vertical axis wind turbine.”

And now, a new technology to harness the power of the wind may be coming along—or, rather, a new reimagining of an existing technology: the kite. A Swiss research consortium called SwissKitePower has been working on this form of renewable energy from wind power, as have engineers at KiteGen, on the other side of the Alps in Italy. The two teams have different approaches, but the general overall premise is roughly the same. The thinking is that at higher altitudes, wind power is stronger and more consistent—but more inaccessible. Large kites, however, could go up hundred of meters, or even kilometers, where they could readily collect the vast amounts of energy available at these heights. As the kite climbs, it would pull a line wound around a drum. As the drum spins to let out the line, it is coupled to a generator, which produces electricity. After the kite has reached a high enough altitude, the kite’s angle to the wind is reduced—much like a sailor on the lake luffs a sail—causing the kite to spill wind out of its wings and descend, during which the line is reeled back in with minimal tension and thus minimal expenditure of energy. Once descent is complete, the kite is allowed to climb once more, where it generates electricity once again.

Supporters say that this technology is not limited to high-altitude places like Switzerland or the Italian Alps. Instead, kite technology could theoretically be applied anywhere in the world. The potential is huge: a couple of hundred meters above ground there is nearly always a steady wind. But how far along is “kite power,” and how does it work? Could this green technology really be part of the effort to replace conventional power sources—and thereby help to cut down carbon emissions and global warming? Could it be scaled up to the size of today’s fossil-fueled power plants, and perhaps used to help fulfill some of the “individual action plans” agreed to by countries at the end of the Paris conference on climate last month?

Some engineering history

Humanity has used wind as a source of energy for thousands of years. According to written records from the Code of Hammurabi, the earliest windmills were built more than 4,000 years ago. In Europe, the first mentions of wind-powered mills date to the middle of the 9th century in England; and to at least 11th-century France. By means of windmills, wind energy was used to provide mechanical work.

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—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 more steady wind regimes. Consequently, the towers that support the blades have grown from 50 meters in the 1990’s to about 80 meters in the 2000’s and about 100 meters today. Some devices operate at heights up to 150 meters, but structural constraints and technical limits hamper going much higher.

But there’s another way to get way up to the place where the wind always blows. 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. (Wind speed is crucial for the amount of energy a wind turbine can convert to electricity: The energy content of the wind varies with the third power of the average wind speed. Consequently, for example, if the wind speed is doubled, its energy content is eight times as high.)

But how can the strong and steady wind at high attitudes be accessed? The basic principle is simple and convincing, although the devil is—as always—in the details, as seen in terms such as “Crosswind Kite Power” or “Airborne Wind Energy.” 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. The system may be operated in periodic pumping cycles, alternating between reel-out and reel-in of the tether. During reel-out, the kite is flying figure-eight maneuvers at high speed.

Currently there are several different systems under investigation. One system converts wind directly into electricity by small turbines attached to the kite. The electricity is transported from the kite to the ground station by an electric cable.

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.

Kite power systems are not available on the market yet, although the multinational electro-generating giant Alstom—manufacturer of France’s TGV high-speed trains—signed on as industrial partner. Research is going on for all major components, such as the light-weight materials used for the kites and the tether. Scientists are also looking to improve the aerodynamic efficiency of the kite, and to arrange for several kites to work in tandem (including the set of a single kite’s wing, a “train” arrangement for several kites at a time, and a “stack” configuration), the ground station, and so forth. A prerequisite for the successful commercialization of any kite power system is that it have automatic control, and synchronization of the drum/generator module for autonomous operation.

Such things call for a lot of collaboration from different fields. In the case of the Swiss program, a mobile testing program was developed by the University of Applied Sciences Northwestern Switzerland; EMPA—a research institution affiliated with the Swiss Federal Institute of Technology-Zurich (ETH Zurich)—created the new kites; and ETH-Zurich and the Swiss Federal Institute of Technology in Lausanne developed the control system. The research gets so complicated, with the need for so many players, because so much is going on: During operation both the kite and the winch must switch between different modes of operation—reel-out phase, upper transition phase, reel-in phase, and lower transition phase. These different operation cycles are accompanied by several boundary conditions, among them maximal force, maximal acceleration, minimal and maximal height, and maximal cable length. The kite controller has to provide input for the desired elevation angle of the kite and its orientation with respect to wind direction. The aim during the reel-out phase is to maximize power generation without exceeding the maximum in acceptable design forces.

Where things currently stand. (Or fly.)

Besides being “technically immature,” other factors hamper market penetration. As for any new technology, there are skeptics who forecast that the technology will not be socially accepted, citing the fears of interference with air traffic, shadows on the ground cast by the kites, noise from the ground station, or very low system design. The biggest concern of all is that a kite-based system would be impractical, because it could never replace a conventional power plant.

But a lot of energy can be harvested with a kite. Kite power systems can be considered as modular, flexible, small-scale power production plants. They could therefore be applied to single family houses, used for autonomous electricity production for events such as open air outdoor gatherings, and in the provision of electrical power for remote buildings such as alpine huts, settlements, and villages not yet connected to the power grid.

At the other side of the scale, it is possible to install huge power plants on the level of megawatts or even gigawatts. The Italian firm of KiteGen, located on the other side of the Saint Bernard pass, for example, developed a prototype with 27 megawatts of peak power in Turin. A configuration with 100 megawatts of peak capacity allows production of 500 gigawatt-hours per year, enough to supply 86,000 households. The no-fly safety zone over a nuclear power plant would be sufficient for one gigawatt of installed power. The potential is amazing!

And the price of the electricity that would be generated is quite reasonable; KiteGen estimates that a 100-megawatt plant would produce electricity at a cost of about 0.03 euros (about 3.2 US cents) per kilowatt-hour. That’s a fair price, considering that there are almost no direct emissions—which means no radioactive waste materials, no carbon dioxide, no particulate matter, no nitrous oxides, and no sulphur oxides.

In addition, even the problems that come along with conventional wind turbines—such as shade from the blades, accidents with birds, noise emissions, or impairment of the natural scenery—are negligible. The kites fly so high that they cast no shadow on the ground, there is no noise, and the kites are so far away that they are almost invisible. The kite power system has a much lower material intensity compared to conventional wind turbines, which need huge concrete foundations and tons of steel for the tower and the blades. In addition, there is no reliance on scarce metals or rare earth elements like neodymium for the magnet in a wind power plant. It can therefore be assumed that the environmental footprint is very much in favor to this technology.

If we keep all these facts in perspective, the only negative point is that the technology is not on the market yet.

Paraglider image courtesy Saleve paragliding school. Diagram courtesy SwissKitePower.