The key to energy independence: Go fly a kite!

Earlier this year, Big Coal got its say in “The Future of Energy” lecture series sponsored by the Harvard University Center for the Environment. Now it’s time to hear from Big Wind.

Bay area inventor and technology entrepreneur Saul Griffith envisions a wind-powered future. He’s helped design light, cheap, and efficient wind turbines mounted on kites. They harness energy from high-altitude wind currents and send it zapping earthward through cables.

In a Feb. 25 talk in Science Center Hall D, the tousled-haired Griffith introduced an audience of 200 to what he called this “little-known aspect of alternative energy” that with the right funding could be commercially available in a few years.

With venture funding from Google, his Alameda, Calif.-based company, Makani Power Inc., has built prototypes of these airborne turbines and routinely tests them in Maui, Hawaii (“Makani” is the Hawaiian word for “wind”).

On the ground, the contraptions look like large swept-wing kites fitted with compact three-bladed turbines. Aloft, they sweep in wide circles, hunting the best winds, either high up or close to the ground.

Making kite-mounted power plants requires only off-the-shelf technology, and the physics is straightforward. The same wind that flows over a wing to keep the kite aloft can be captured as energy by wing-mounted turbines.

In another scenario, the kite can create power by pulling a load on the ground. (Griffith used the example of a train on an oval track.)

Man-lifting kites were described as military observation platforms by Sun Tzu in “The Art of War” in the sixth century B.C.E.

Since then kites have been vetted as machines for pulling and lifting, and they provided aerodynamic insights for the first airplanes.

But it took “Crosswind Kite Power,” a 1980 Journal of Energy paper by engineer Miles L. Loyd, to describe the physics of capturing power from high-altitude winds.

Most of us think of kites as fragile, fluttering toys. But Griffith illustrated the raw power of high-altitude wind with a video clip of his kite-powered boat off the coast of Hawaii. It ripped through the sea like a cigarette boat, skidding over the waves at 45 knots.

“There is rather a lot of power in the wind,” said Griffith, who in the video was fighting to control the rudder. “And kites are a good way of extracting that.”

In the United States alone, wind maps show that the potential for energy generation is in the range of 8,000 gigawatts. That’s equivalent to 8,000 modern coal-fired energy plants.

But the wind’s most potent energy is out of reach for traditional ground-mounted turbines because it whips back and forth at altitudes of 500 meters to 15,000 meters. Griffith calculated that a kite turbine 10 kilometers high will capture nine times more energy than a pole-mounted turbine spinning 100 meters off the ground.

Most U.S. wind sites are rated “Class 3,” or economically marginal. (Sites rated Class 4 to 7 have the best economic potential for wind generation at ground level.)

But putting the turbines high into the sky at even Class 3 wind sites makes cost-efficient wind energy a viable option, said Griffith, and expands the territory that can be economically exploited for wind. (He calls the area of the world’s surface needed for wind and solar energy sources “Renewistan.”)

Proponents of high-altitude wind power claim that tapping just 1 percent of its potential could power everything on Earth.

Compared with their ground-mounted cousins, kite-mounted turbines are cheaper and lighter: a 1-ton wing, and 3 tons for the whole system, compared with 100 tons of concrete and steel. The kites make less noise and kill fewer birds.

And they’re more efficient, said Griffith. A traditional tower turbine spins at capacity 30 percent of the time. But wing-mounted units, with access to higher consistent wind speeds, turn out peak megawatts up to 65 percent of the time.

Griffith measures kite turbine power capacity by wingspan. The larger the wing area collecting wind, the greater the power generation.

He rates a kite with the wingspan of a Cessna at 230 kilowatts, a Gulfstream jet at 1.3 megawatts, and a Boeing 747 at 6 megawatts. (OK, since you asked: A paper airplane is rated at 15 watts.)

The world is on the verge of massive investments in the infrastructure needed for renewable energy, said Griffith, and scaling up to make enough kite turbines is possible. After all, he said, by the end of World War II, the United States was making 100,000 planes a year.

Humanity uses 18 terawatts of power every year. (A terawatt is 10 to the 12th power watts.) But so far only 0.016 percent of that comes from solar and 0.06 percent from wind.

By 2030, renewables will account for 1 terawatt of worldwide power, said Griffith. “We’d like to double that” with kite turbine technology.

There’s a “magnificent future,” he said, “for every child’s favorite plaything.”