TACS Wind Power

The wind may qualify as a renewable resource, but “renewable” has its limits. Exactly how much energy we can feasibly pull from the wind has been something of a controversial question in recent years, with some studies suggesting that wind power is not the planet saver it’s cracked up to be. Simulations unveiled this week by scientists in Delaware and California, though, argue that if anything, economics and politics will hold wind development back, rather than geophysical limits.

Wind-power systems work by taking the kinetic energy of wind and turning it into mechanical energy in the turbine to create electrical energy. Laws of physics tell us that the total amount of energy can’t change, so at least a minor slowing of the wind is expected as it passes through the turbine.

“Adding a turbine represents a trade-off: We get energy, but the wind is slowed down,” says Kate Marvel of Lawrence Livermore National Laboratory, in California, who did one of the analyses along with Ben Kravitz and Ken Caldeira, of the Carnegie Institution for Science at Stanford University. “At some point, adding one more turbine leads to diminishing returns: The wind is slowed to such an extent that we don’t get any more energy. This is what we call the geophysical limit.”

Marvel’s research, published Sunday in Nature Climate Change, used a climate model to estimate that limit, both for turbines placed near Earth’s surface—as they are built now—and for high-altitude turbines, such as the kite-like tethered devices currently under development. [Editor’s note: Look for a feature article on kite-power systems in the December 2012 issue of IEEE Spectrum.] They found that the geophysical limit for Earth-based turbines is 428 terawatts or more but a whopping 1873 TW for high-altitude systems. The current global power demand? About 18 TW. “While there is a geophysical limit, civilization-scale wind power is nowhere near it,” Marvel says.

This is a comforting result, given the findings of some earlier research. One study of wind power’s geophysical limits, published in the journal Energy Policy in 2011, arrived at an upper limit of about 1 TW. But Mark Jacobson, a professor of civil and environmental engineering at Stanford, says that “the calculation of 1 TW was literally done with a back-of-the-envelope single-line equation” and didn’t take actual physical properties of turbines or the atmosphere into account.

Jacobson and Cristina Archer of the University of Delaware conducted what they say is a far more detailed analysis than earlier efforts; their report is published this week in Proceedings of the National Academy of Sciences. “We remove energy from the atmosphere exactly where the turbines remove it, not near the ground at the surface,” says Archer. The model they used takes atmospheric dynamics, water-vapor effects, and other factors into account. “We even incorporated a real power curve from a modern turbine directly into the code in real time,” she says. “None of the previous studies did anything like this. Thus we believe that our results are far more reliable.”

Using a different climate model than Marvel’s group, they calculated a maximum power of more than 250 TW at 100 meters above the ground, and 380 TW at the jet-stream height of 10 kilometers. “In reality, we will never get even close to such high penetrations of wind power,” Archer says. “We humans do not even have enough cement to build so many turbines.” To get to even 100 TW of installed capacity would require somewhere around 20 million very large turbines. A somewhat more realistic 4 million turbines, installed nonuniformly around the world, could easily supply about half the world’s power.

Both new studies also address a separate issue that arises with massive numbers of wind turbines: Can they actually cause climate change? The changes in kinetic energy that result from millions of spinning turbines do have an effect, but Marvel says significant alterations to global temperature or weather patterns are only likely at “truly absurd extraction rates.” Even “civilization-scale reliance on wind power” would change mean temperatures by one-tenth of a degree Celsius and mean precipitation rates by about 1 percent, according to the California scientists. On a more local level, near-surface turbines do have a minor warming effect, and high-altitude turbines would have a small cooling effect. The former finding caused a minor stir earlier this year after reports of warming from a wind farm in Texas, but on a global level it does not appear that wind power can do much to the climate.

Marvel says the primary questions when discussing truly massive wind-power expansion are these: Will we run out of wind? And will we destroy the climate? “As far as practical implications go, no one is seriously suggesting that we blanket the surface of the Earth or the whole atmosphere with uniformly distributed wind turbines,” Marvel says. “But I’d argue that it’s reassuring to have physically defensible ‘no’ answers to [those] questions.”

About the Author

Dave Levitan is a science journalist who contributes regularly to IEEE Spectrum’sEnergywise blog. In our June 2012 issue, he reported on what’s behind the persistent efficiency gap between that of record-setting solar cells and what comes off the manufacturing line.

Geophysical limits to global wind power(pdf)