Before the average onshore wind turbine produces a single watt, more than 100 tons of structure, several million dollars in capital investment and a suitable site all have to come together perfectly. Bigger turbines produce more power, but in many places they need to compete for fewer acceptable sites. Put simply, tower-based turbines may be reaching the limit of the technology.

But what if the tower was done away with and the turbine was up high – really high. Average wind speed doubles every 30 meters in altitude. Power is based on the cube of the wind speed, so altitude is an attractive option. This is where many think the technology is headed: aerial wind turbines.

Some, like the Altaeros turbine, suspend a traditional turbine hub within a helium- filled doughnut-shaped structure. Others, like Google’s Makani airborne turbine, use a tethered airfoil to loft its multiple DC generators hundreds of meters high. Some even use kites to pull a cable through a generator pulley, much like the pull starter on a lawn mower. When the kite reaches its maximum extension, it furls and the tether is rewound.

Aerial wind turbines don’t need a big footprint or a heavy foundation. They can be flown nearly anywhere power is needed, although flying them over water somewhat reduces worries that a failure could damage structures on the ground. And, like Benjamin Franklin’s kite, lightning strikes are a concern and one not completely resolved. Power lines, airports and competition for airspace all conspire to limit their range, but there is still a lot of open space out there for airborne wind turbines.

The higher the turbine, the bigger the benefit. The wind gradient – the height above ground where surface friction stops having an effect - ranges from 457m over cities to 274m for open terrain. Then there is the wind profile power law, which says that wind speed increases logarithmically up to about 2,000m. Above this altitude winds stay relatively constant, at least until the jet stream takes over at the 10,000m mark. Either way, an aerial turbine at 600m encounters triple the wind power density of the tallest fixed-base turbine.

Dr. Fort Felker, center director of the National Wind Technology Center, part of the National Renewable Energy Laboratory, considers aerial turbines an intriguing opportunity with some notable advantages.

"A modest increase in height makes a strong difference in the wind resources available," he says. This enables a smaller machine to generate the same power as a larger ground-based turbine. He considers all the technologies promising, and says it is still too early to pick winners and losers. Like many in the industry, Felker is anxious for the players to "begin real trials in the market: get turbines on the grid and making power."

Most of the aerial turbines under development or in limited commercial release are designed to remain no more than 600m off the ground. Lower altitudes make them easier to control and reduce the weight of the tether. Since the tether is often used to transport power to the ground, it needs to carry metal conductors, and metal is heavy.

One startup, New Wave Energy, plans to eliminate tethers altogether, transmitting power to the ground through microwaves. Unlike other technologies, New Wave’s 400 m2 drones would hover at 15,000m and collect power through photovoltaic cells. At that height, winds are steady and air traffic nonexistent. Of all the aerial technologies, this one may be the farthest from commercial release.

The Buoyant Airborne Turbine (BAT) is Altaeros’ patented technology for suspending a bladed turbine within a 10.6-meters-in-diameter helium-filled doughnut-shaped structure (pictured on the front page). The turbine is designed to hover at 300m and survive 161 kph winds. Rudders maintain directionality, keeping the turbine facing into the wind. A pilot project underway in Alaska will test the company’s sweet spot – the ability to provide relatively low-cost power to remote villages. Generation costs are predicted to be around 18 cents per kilowatthour, about 1/4 of what diesel generation costs in remote regions. The BAT fits in two shipping containers and takes about one day to prepare and launch. Rated continuous output for the BAT is 30 kW.

Google’s Makani airborne turbine approaches the problem quite differently. Unlike the BAT, which needs helium to fly, the Makani hangs its four generators on an 8m wing and is designed to generate 30 kW in winds of 11.5 m/s. The tethered wing flies across the wind in vertical loops, controlled by an airborne flight computer. If the wind fails, the generators become motors, keeping the wing aloft until the wind picks up again. The generators are also used to launch and land the wing.

A similar approach is used by Sky Windpower, whose tethered device resembles a quadcopter. Like the Makani, its generators are back-fed to get the turbine to altitude, after which it remains aloft from the autogiro lift on the propellers. Several prototypes have been developed, but the company has yet to move to commercial release.

Perhaps the most unique aerial turbines are the tether-traction generators. Technologies include Ampyx Power’s glider, Kitenergy’s furling kite and Kite Gen’s adjustable airfoil unit. The Kite Gen airfoil pulls its ground station back and forth in a semicircle, powering alternators built into the base. Intelligent control surfaces on the kite guide it in a figure-8 pattern that is some 800m in diameter. Add enough kites and the company claims the system is capable of 100 MW output.

Both Kitenergy and Kite Gen are working on alternate technologies that drag the tether through a pulley system to power a ground-based alternator. When the tether reaches its maximum extension, Kite Gen’s version adjusts itself to a minimal drag configuration for rewinding at the ground station.
Kitenergy’s design uses two kites in a u-shaped configuration with each kite alternately pulling and furling the tether through the alternator assembly. The company's current version has been tested at an altitude of 800m, where it delivered 60 kW in controlled tests.

Ampyx Power’s tethered glider also works by pulling its tether through a ground-based alternator. This type of ground-based generation offers two advantages: the tether can be both lightweight and non-conductive, which reduces gravity-induced drag and the risk of lightning strikes. The Ampyx prototype Power Plane, with a 5.5m wingspan, has been flying since 2012 and is currently under redesign to ready it for commercial release.

Although wind is free, the technology needed to harness it is not. The Department of Energy's Advanced Research Projects Agency - Energy (ARPA-E) worked with the Makani project, but NREL's Felker is not aware of any joint commercial development projects involving his laboratory. What's more, the current fossil fuel boom is creating strong economic headwinds against commercial renewable energy development. Even so, work on developing cost-effective alternatives to traditional tower-based turbines is well under way.