Harnessing that high-altitude wind remains tantalizingly out of reach at present, although companies such as California-based Makani Power or Italy's KiteGen are working on it. Makani employs computer-controlled aircraft to harvest 300-meter-high winds offshore and send electricity back to the ground via a carbon fiber and conducting metal tether. With funding from Google and the Advanced Research Projects Agency–Energy, Makani hopes to have a commercial product within a few years. Essentially, the autonomous aircraft functions as the tip of a conventional wind turbine blade, minus everything else. "It's a huge reduction in mass and an increase in the consistency of power production," says Makani CEO Corwin Hardham, a mechanical engineer. "All lead to a much lower cost of energy." KiteGen, for its part, employs a kite that continuously flies up to roughly 500 meters high, mechanically pulling up its tether behind and thereby spinning an electric generator at ground level. Then the kite is reeled back down to begin the process anew.
Going even higher—to roughly two kilometers, where the wind blows even more steadily—begins to stretch the ability of present materials or technology, whether it be to create a tether that can withstand such forces and still transmit energy back to the surface or an aircraft that can reliably fly autonomously under such conditions. "A very small percentage of our advantage comes from flying at altitude," Makani's Hardham notes. "It's low mass and interacting with more of the sky," which is actually a key challenge given all the other uses for that airspace and Federal Aviation Administration regulations.
Whether high in the sky or closer to the ground, as wind power proliferates proper siting will become a challenge. Clustering turbines in one particularly windy location, whether the U.S. Great Plains or China's grasslands or desert, can also cause turbines to reach a local saturation point, cutting down on each turbine's ability to produce power. "Good windy sites are the best choice initially," Delaware's Archer notes. But "it is not smart to focus on a few 'prime' sites if we want to have large-scale penetrations of wind [power]."
"The location of turbines matters," Livermore's Marvel adds. "We are currently investigating the regional climate effects that would result from more localized wind energy extraction," such as local temperature rises at night.
In addition, manufacturing so many wind turbines—whether carbon-fiber wing's like Makani's or more traditional turbines made of plastic, steel and mounted in cement—will require lots of energy itself, most of it supplied by fossil fuels. Yet the climate change debt of a wind turbine is smaller than that of other energy technologies that do not emit greenhouse gases, such as nuclear reactors. "It takes only a few months—less than nine—for a turbine to offset its greenhouse gas emissions" related to construction, Archer notes. "Good luck building anything else that can produce all that power with such small carbon emissions."
At present, conventional land-based wind turbines produce roughly 3 percent of U.S. electricity—and even less of the global supply. The limits on wind power remain political, social and economic—in short, everything but geophysical. And burning natural gas to generate electricity remains cheaper than harvesting the wind, as abundant as it is—although a cost will be paid in terms of adding to climate change in future. "I would like to find sustainable clean energy sources that can power economic growth and development without substantial adverse side effects," Caldeira says. "In this light, wind power seems to present an attractive opportunity."