Start-ups are devising kites to turn wind power into a cheap source of power

March 21, 2011

Kites of all shapes and sizes will take to the breeze Sunday, filling the airspace around the Washington Monument. The capital’s annual kite festival will feature recreational fliers as well as competitions to determine the best homemade kites, the kite with the top aerial dance moves and the champion of Rokkaku-style kite-fighting.

Elsewhere, a different kind of kite contest is already in progress, and start-ups around the world are racing to win. The goal: to create a self-piloting kite, or something like one, that flies day and night and generates energy from the wind. The prize: potential riches, and renewable energy that proponents say will be cheaper, safer and more plentiful than fossil fuels or nuclear power.

As any kite-flier knows, getting a kite off the ground can involve some effort. But once aloft, a kite can fly seemingly forever. That’s because winds even a hundred feet or so above the ground are stronger and steadier than those close to the surface.

In the jet streams, which flow about six miles above the Earth, winds often exceed 100 mph. Those powerful air currents contain about 100 times as much energy as the world now uses, according to experts.

Even at 600 or 700 feet above-ground, the wind can move twice as fast as at 150 or 200 feet, and it contain eight times as much physical energy.

Modern wind turbines, however, are typically mounted on towers 200 to 300 feet tall. As a result, those icons of renewable energy stand idle for the equivalent of months every year for lack of sufficient wind.

Kite-inspired aircraft could change the equation. The concept behind what’s known as airborne wind energy is to liberate the power-generating component of a wind-energy system — today, the turbine — from the Earth-bound structure that anchors it to the power grid, allowing it to move higher and take advantage of the greater wind velocity up there.

One option that entrepreneurs are exploring is to deploy a kite or gliderlike aircraft that, as it rises, pulls on a cable attached to a generator on the ground. The generator can convert the cable’s unwinding motion into electricity. And when the aircraft reaches the end of its rope, it can glide almost effortlessly back toward the ground, then repeat.

Another approach is to use a wind-powered, unmanned aircraft that carries a turbine-and-generator unit onboard. As the airborne turbine catching the high-altitude breeze and spins, the electricity it generates gets routed down a sturdy electrical wire that tethers the vehicle to a ground station.

Both approaches have been shown to work in principle. Half a dozen or more companies have flown self-piloting prototypes and are working to build bigger, better versions for commercial use.

The biggest challenge, the entrepreneurs say, is perfecting the flight software needed to keep the vehicles flying safely day and night, and to take off and land when necessary. An industrial-scale airborne wind energy plant would need to have many craft in the air, and having human pilots at the controls of each one isn’t practical. So the unmanned aircraft would need to fly expertly enough to stay out of one another’s way, out of airspace used by commercial and military aircraft, and safely off the ground.

A company called KiteGen has flown nearly 1,000 hours with its C-shaped kite, which looks like those used in kitesurfing. While that prototype generates just 40 kilowatts, enough to power 400 100-watt bulbs, the Italian firm is working on a commercial-scale, three-megawatt system that could power perhaps 1,500 homes. That requires refined flight software and a more aerodynamically efficient kite made of rigid composite materials rather than fabric.

KiteGen’s kite would be able to fly as high as the jet stream, says the company’s research-and-development chief, Massimo Ippolito. But the company plans to restrict its operations to a maximum altitude of 11 / 4 miles, partly to stay out of the way of air traffic.

At that altitude, the kite would enjoy steady breezes of more than 20 mph, according to Ippolito.

Another European start-up, Ampyx Power, of The Hague, is flying its prototype glider just 1,000 feet off the ground, in order to minimize the weight and aerodynamic drag of the cable connecting it to its base station. The Ampyx PowerPlane can fly faster than the wind that’s lifting it, says the company’s Richard Ruiter-kamp.

When the cable has played out, power generation temporarily ceases while the PowerPlane glides back toward the base; that phase uses about one-tenth of the energy generated during the glider’s outbound flight.

The biggest challenges ahead for the company, says Ruiter-kamp, are achieving autonomous launch and landing, and flying for weeks at a stretch without human intervention. Once it solves those problems, Ampyx plans to build a one-megawatt-generating glider with a 100-foot wingspan, he said.

In the United States, California is a hotbed of innovation in altitude wind energy. At least three companies with operations there — Makani Power, Joby Energy and Sky WindPower — have built wind-powered aircraft outfitted with onboard generators and multiple turbines. Another, Magenn Power, has developed a helium-filled balloon that can go as high as 1,000 feet and spins like a waterwheel in the wind stream, powering a generator inside.

Some independent experts say Makani, which has attracted investments from the Department of Energy (through a $3 million grant) and Google, is leading the pack technologically. One of its prototypes looks like an airplane sporting six wing-mounted propellers.

An advantage of sending a generator aloft is that power can be produced continuously, in contrast to the yo-yo action that powers KiteGen’s and Ampyx’s ground-based generators. On the other hand, generators are heavy and expensive, increasing the size and cost of the aircraft.

It’s too early to tell which approach will win out or whether any company in the nascent industry can succeed in making airborne wind the power source of the future.

“In any case,” says KiteGen’s Ippolito, “the energy is up there.” Humanity just needs to master the task of pulling it down.

Harder is the general manager of Health and Science at U.S. News & World Report.

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