Computer modelers say planet has enough wind to meet energy demand sevenfold

Source: Nathanael Massey, E&E reporter • Posted: Tuesday, September 11, 2012

Like all current and proposed energy resources, wind power faces a long list of challenges: connection to inflexible grids, inconstant generation and stiffly competitive energy markets, to name just a few.

One problem the sector is unlikely to face, however, is scarcity of supply. According to the most detailed model of wind’s potential to date, undertaken by researchers at Stanford University and the University of Delaware, there’s enough wind on Earth to meet world power demand seven times over.

That figure is, of course, strictly hypothetical — upscaling wind power to such a degree would require peppering almost every square mile of the Earth’s surface, and much of its oceans, with turbines.

With only 4 million turbines dispersed over the globe, however, the world could meet half its energy needs within the next two decades, the researchers found.

“We’re not saying, ‘Put turbines everywhere,’ but we have shown that there is no fundamental barrier to obtaining half or even several times the world’s all-purpose power from wind by 2030,” said author Mark Jacobson, professor of civil and environmental engineering at Stanford. “The potential is there, if we can build enough turbines.”

Jacobson and co-author Christina Archer, an associate professor of geography and physical ocean science and engineering at the University of Delaware, adapted a 3-D model of the Earth’s surface and atmosphere to account for wind reduction from turbines.

Their findings contradict previous studies that see wind power curtailed as the number of installed turbines increases.

They corroborate the results of another study, conducted by researchers at the Lawrence Livermore National Laboratory and published in the journal Nature Climate Change, which found that the geophysical parameters of the Earth could yield up to 400 terawatts of surface wind power.

Turbines absorb kinetic energy, but at manageable levels

Central to the question of wind energy’s potential is the concept of saturation, or the idea that, in high enough numbers, wind turbines will fully absorb the wind’s energy potential, rendering any excess turbines inoperable.

Like any power source, wind must give up energy in order to generate electricity. Saturation occurs when so many turbines have been placed in a particular area that they exhaust or absorb the full kinetic energy of the wind, similar to the way a windbreak can shield houses or fields.

Contrary to previous studies, Jacobson and Archer’s model determined that this point of saturation is unlikely to occur in the near or conceivable future, even given a dramatic upscaling of the wind sector.

The difference in their approach stemmed from the level of detail in their models and their method of accounting for the resistance of wind turbines to the air.

Where most previous models applied a general “roughness parameter” — accounting for friction at ground level, as would be done for a forest or uneven ground — the researchers’ model allowed them to measure wind resistance at various heights, accounting for higher-elevation wind speeds and the varying levels of resistance along turbines’ blades and hubs.

“We actually tried both approaches, comparing what we could get with a surface roughness approach to a turbine extraction approach,” Archer said. “We were surprised by how much the results of the two differed.”

She added, “When you use a surface roughness approach, too much energy is removed from the surface boundary layer, including most of the kinetic energy.”

Still, spacing turbines apart can contribute positively to the overall performance of an installation, the authors note.

No significant contribution to warming

Another charge leveled at wind power by critics and skeptics is that turbines disrupt global meteorological patterns, drawing warm air down from the atmosphere and perpetuating global warming, the chief problem they are meant to alleviate.

Indeed, a study published in Nature Climate Change in May found that, on a localized level, wind farms did increase the surface temperature of their immediate area by around 1.3 degrees Fahrenheit per decade.

The study did not link the farms to global climate change, although that didn’t stop several local media outlets from drawing equivalent conclusions.

According to Jacobson, however, this phenomenon is most likely due to a reduction in evaporation at wind farm sites, rather than atmospheric mixing.

“When you evaporate water from the surface, you get a cooling effect,” he said. “Wind turbines reduce wind at the ground level, which reduces evaporation from the soil, and thus you get a localized warming effect.”

Still, he said, “evaporation doesn’t get rid of energy — it just moves it up to the clouds” to be lost during condensation. That energy remains trapped beneath the atmosphere, and the net effect on planetary warming is not lowered, he said.

“In fact, when you factor in the fact that water vapor is itself a heat-trapping gas, more evaporation can lead to a net warming effect,” he added.