As seemingly limitless as the air that swirls around us, wind has proven to be the world's fastest-growing source of renewable energy. Backers suggest wind power can continue growing as quickly as companies can raise turbines to capture it.
But some scientists are challenging that assumption, arguing that the laws of physics will limit wind's potential for meeting the world's energy needs. The controversy arises from the turbines themselves. "As soon as you start to put turbines into the wind, you start to change the resource," said Amanda Adams, a meteorologist who conducts atmospheric modeling at the University of North Carolina Charlotte. (See related quiz: "What You Don't Know About Wind Energy.")
In several recent published studies, Adams and other researchers have explored the issue of turbines stealing energy from the wind, creating drag or a "wind shadow" of air slowed by the spinning blades. Each turbine added to a particular landscape captures less energy. "You reach a point that if you add any more turbines, you get no more energy," Adams said.
Developers of wind farms recognize the risk, so they carefully space turbines to prevent one from robbing wind speed from another. But today's wind farms are small compared to the giant complexes needed to meet a significant slice of the world's energy needs. (See related story: "Sizing Up Wind Energy: Bigger Means Greener, Study Says.")
Adams and colleagues argue that when wind farms get big enough, they won't be able to avoid the wind-shadow effect.
Wind has a future, and even the pessimists' projections suggest that there is plenty of room to grow today's generating capacity, said David Keith, a Harvard applied physicist who recently co-authored a paper with Adams. But he argues that if the effects are as severe as projected, wind can't be counted on to provide a significant share of energy needs. The Adams-Keith paper, "Are global wind power resource estimates overstated?" was published earlier this year in Environmental Research Letters. Theirs is still a controversial view among scientists, and the American Wind Energy Association dismisses the doubts as the product of "crude theoretical modeling techniques."
But better modeling may become necessary, because the wind energy industry faces a number of well-recognized and practical limitations on where it can site farms—due to the need to capture the best resources, to steer clear of community opposition, and even bird migration routes. Research into the wind-shadow effect may grow in importance as the world seeks to expand renewable power while limiting placement of large energy installations. (See related pictures: "Flying Wind Turbines Reach for High-Altitude Power.")
High Hopes for Wind
While there is discussion and debate about how much energy we can ultimately capture from wind, the potential is far above the levels we are seeing harvested in even the most aggressive wind-energy countries today, said Daniel Kammen, founding director of the Renewable and Appropriate Energy Laboratory at the University of California, Berkeley, and an adviser to National Geographic's Great Energy Challenge Initiative. He pointed out that Denmark already gets about 20 percent of its energy from wind power, and Germany, Portugal, the United States, South Korea, and other countries have set goals at or about this level. There is no reduction in the wind resource at these levels, and it is even reasonable to achieve higher levels, he said.
Wind has proven to be the fastest-growing source of renewable energy, with worldwide capacity jumping nearly 20 percent in 2012 alone, to 282.5 gigawatts, according to the Global Wind Energy Council, an industry trade group.
Expiring tax incentives helped push the United States to add more capacity than any other country last year, placing it ahead of China for the first time since 2009. The new U.S. wind capacity in 2012 also outpaced any other source of power, including coal and natural gas. Companies plowed $25 billion into adding 13 gigawatts of U.S. wind capacity last year, an increase of 28 percent, according to the American Wind Energy Association. The industry ended the year with U.S. capacity of 60 gigawatts, or enough to power about 15 million American homes, the group said.
Many proponents of replacing fossil fuels count on wind power as the single biggest source of renewable energy for the nation's future. Industry experts, for example, have predicted wind could supply a third of the world's electricity by 2050. A recent study said New York State could convert all its energy needs to renewables by 2030, with half coming from wind.
The lead author of the New York study, Mark Jacobson of Stanford University, argues that nothing in current research convinces him that wind won't be a significant part of the world's energy mix.
Yes, there is a point at which enough turbines around the Earth would "saturate" the wind potential, and little or no more energy could be extracted, said Jacobson, a professor of civil and environmental engineering. That point was acknowledged in a study he co-authored last year, "Saturation wind power potential and its implications for wind energy," in the Proceedings of the National Academy of Sciences.
But for wind energy to become oversaturated to the point that it undermines its own effectiveness, "we're talking billions of turbines worldwide," he said. At the end of 2011, the number of turbines in operation worldwide was closer to 200,000, the wind energy council reported.
Jacobson and Archer's study projected that producing half the world's energy needs in 2030, or six terawatts of energy, would take some four million 5-megawatt (MW) turbines.
Jacobson contends that even that large number of turbines could be spaced in such a way to keep them from robbing each other of efficiency. (See related: "Going All The Way With Renewable Energy?") Even eight megasize wind farms, each with a half-million turbines, would prove efficient enough to output a total of four terawatts, or about a third of the world's projected energy needs in 2030, according to his paper last year, co-authored with Cristina Archer of the University of Delaware.
It is more likely, he said, that those four million turbines would be spread among thousands of wind farms. That greater spacing would boost the turbines' efficiency enough that the same number of turbines could provide at least half the world's energy needs, Jacobson said. (See related story: "Planting Wind Energy on Farms May Help Crops, Say Researchers.")
A Question of Spacing
Currently, though, most wind farms rely on turbines with a much smaller capacity than those envisioned in the Jacobson-Archer scenario; only a few 5-MW turbines are in use in farms offshore in Europe and China. If the industry continues to deploy smaller turbines, it will need more of them to pack the same energy punch.
The world's largest wind farm, Shepherd's Flat in Oregon, opened last fall with 338 2.5-MW turbines spread over 30 square miles (78 square kilometers) that can produce 845 MW of power, according to the American Wind Energy Association. Last month, the United Kingdom inaugurated the world's largest offshore wind farm, the London Array: 175 turbines, each rated at 3.6 MW, over 90 square kilometers (35 square miles) off the coast of Kent for a total at 630 MW of capacity. Shepherd's Flat produces enough energy to power about 235,000 U.S. homes; the London Array, though smaller in capacity, has enough juice for 500,000 homes in Europe, where energy demands are smaller.
With wind energy currently providing only 2.5 percent of world electricity, there would need to be many more farms the size of Shepherd's Flat and London Array to take a bite out of the world's coal-dominated power generation. The scientists who are working on the wind-shadow issue maintain that real-world restrictions will prevent turbines from spreading far and wide. Most of the best onshore sources of wind power, such as the Midwestern plains of the United States, lie far from population centers and would require expensive new transmission lines to bring the energy where it is needed, Adams said. But people in densely populated areas don't want large wind farms nearby, and there have been fights over offshore projects like proposed Cape Wind off the coast of New England.
Geography and politics will combine to limit turbine placement to certain areas, and the question of diminishing returns from dense arrays will become a significant factor, Adams said. "You're not going to be able to put turbines across the entire globe."
The work raises enough doubts that policymakers should be emphasizing research into other energy sources to replace fossil fuels, argues Harvard's Keith: "While they have their problems, you can clearly take nuclear and solar energy to gigantic scale."
It's not the first time Keith has raised concerns over wind power. A 2008 paper published in the Journal of Atmospheric Sciences, co-written by Keith with atmospheric scientist Daniel Kirk-Davidoff of the University of Maryland, said huge wind farms could change air patterns enough that it would also affect worldwide climate.
A more alarming conclusion came from a 2011 paper published by the Max Planck Institute for Biogeochemistry, a German research organization, that said that trying to get the maximum power from the Earth's winds could have the same impact on climate change as a doubling in atmospheric carbon dioxide. (The Planck paper, published in Earth System Dynamics, is another that argues that wind is a limited resource, concluding it is "considerably less than recent estimates that claim abundant wind power availability.")
At the least, in the next decade or so wind farms will get big enough to cause more localized climate changes, Keith said. While wind clearly is a better option than coal, "we have to get past the reflexive view that all renewables are good and have negligible impacts," Keith said.
No doubt, the models will be refined as research continues, said Adams. Less is understood about the Earth's winds than, say, the potential of solar power, where sunrises and sunsets are known and satellites can measure clouds, she said.
There is so much unknown about wind energy and how it interacts with the atmosphere, Adams said. "There are never-ending questions."