The Sky Is the Limit for Wind Power

Wind turbines on land and offshore could readily provide more than four times the power that the world as a whole currently uses. Throw in kites or robot aircraft generating electricity from sky-high winds and the world could physically extract roughly 100 times more power than presently employed?and the climatic consequences remain minimal.

Two new computer-model analyses suggest there are few limits to the wind's potential. Although "there are physical limits to the amount of power that can be harvested from winds, these limits are well above total global energy demand," explains climate-modeler Kate Marvel of Lawrence Livermore National Laboratory, who led the analysis published September 9 in Nature Climate Change. (Scientific American is part of Nature Publishing Group.) Current global demand is roughly 18 terawatts. (A terawatt is one trillion watts.)

Given the desire to reduce greenhouse gas emissions from electric generation, a growing number of wind farms are cropping up from the U.S. to China?more than 239 gigawatts worth of wind turbines have been installed globally. But the ultimate limits of wind power's potential contribution remained unclear. One complication, for example, is that any effort to harvest wind power ends up having an impact on the wind itself, reducing its speed?as well as influencing both local weather and global climate.

Using a global meteorological and sunlight-chemistry computer model paired with power generation information from turbine manufacturers, environmental scientists Cristina Archer of the University of Delaware and Mark Jacobson?of Stanford University analyzed when wind turbines might reach a saturation point?the point at which the addition of more turbines would reduce the amount of power generated, rather than increase it. At 100 meters up?roughly the hub height of a modern large-scale wind turbine on land?that saturation point would allow more than 250 terawatts of power to be generated, according to the results published online in Proceedings of the National Academy of Sciences on September 10. "We calculated how much electrical energy can be generated from the atmosphere," Archer explains, noting that four million turbines spread around the globe could easily and sustainably produce 7.5 terawatts of power, or nearly half of all power used today.

For their part, Marvel and her colleagues examined the geophysical limits of wind power, or how much energy can be extracted from global winds without major impacts. Surface winds below 395 meters, which ultimately dissipate anyway, could provide at least 400 terawatts of power, whereas those at higher altitudes could offer more than 1,800 terawatts based on atmospheric physics.

The researchers then used a computer model to simulate the global climate over a century to find out what impact such power extraction might have. If humans could figure out how to extract all that power, global temperatures could rise by as much as one degree Celsius and precipitation decreased by roughly 10 percent. Of course, that's more than 100 times more energy than presently consumed by the entirety of human civilization, suggesting that the actual impacts of wind power would be far smaller. "At the scale of civilization, the climate consequences of widely distributed wind turbines are negligible," says climate-modeler Ken Caldeira of Carnegie Institution's Department of Ecology at Stanford, a co-author with Marvel.

Big if
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, such as in the jet stream?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."

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Source: http://news.yahoo.com/sky-limit-wind-power-210100483.html

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