Wednesday, June 23, 2010

The Way the Wind Blows

The wind power industry requires stiff gusts. Global warming may not deliver

This summer scientists published the first study that comprehensively explored the effect of climate change on wind speeds in the U.S. The report was not encouraging. Three decades’ worth of data seemed to point to a future where global warming lowers wind speeds enough to handicap the nascent wind industry. But the real story, like so much in climate science, is far more complex.

The study of decreased wind speeds came from a team led by Sara Pryor, professor and chair of the atmospheric science program at Indiana University. It examined wind speed data from hundreds of locations across the U.S. The team attempted to correct for any change in instrument position (such as what would happen if an airport places its anemometer atop a new control tower) and calculated for each site the average annual wind speed. Pryor and her colleagues found that in most of the U.S. wind speeds appear to be waning, in many locations by more than 1 percent a year.

The decline has the potential to be especially pernicious because turbines are exponentially sensitive to changes in wind speed. If the wind blows just 15 percent faster, a turbine will produce 50 percent more power. Conversely, a drop in average wind speed will significantly reduce the power output. Most of the locations that showed the most prominent decreases in wind speeds are strung along a corridor stretching from Texas to the Great Lakes that is home to 60 percent of the nation’s installed wind power.

Yet the situation may not be as dire as the data imply. Direct observations of wind speeds are inherently problematic. Anemometers are far less accurate and consistent than thermometers, Pryor says. In addition, almost all the locations used in the study are close to fast-growing urban areas that can alter wind patterns in unpredictable ways. And unlike temperature measurements, which in some locations stretch back 150 years, relatively accurate and widespread wind measurements began only in the 1970s—hardly enough time to pluck a subtle trend out of noisy data.

Because direct measurements of wind speeds are so unruly, Pryor’s team also tracked indirect measurements. These come from surface temperature and pressure records as well as balloon and satellite surveys. Computers crunch the data to produce a rich series of atmospheric portraits— a way to measure wind speeds without measuring the wind. This “reanalysis” data showed no change. Says Pryor: “If you have a mechanism causing your wind speeds to change”—global warming, for instance—“it should be evident in both the observations and in the reanalysis data.” If only one out of the two shows an effect, no one can say for sure what is going on.

For the wind industry, the most important change would be to peak wind speeds, because a turbine delivers most of its power only once the wind blows faster than about 25 miles per hour. Although the conclusions are preliminary, global climate models suggest that in the Northern Hemisphere, storm tracks should migrate northward, bringing more gusty storms to higher latitudes. “The northern part of the U.S. into Canada may see an increase” in peak wind speeds, Pryor says, “whereas the southern regions may see a decline.”

Yet any decline should still leave wind farms with plenty to work with. A recent study by Xi Lu of Harvard University calculates that wind power in the U.S. could potentially generate 16 times the nation’s current electricity production. The study limits potential wind farm locations to rural, nonforested sites (both on land and offshore) with high wind speeds. Worldwide, wind energy under the same constraints could supply at least 40 times the current electricity consumption.

According to Ryan Wiser, a staff scientist at Lawrence Berkeley National Laboratory and author of an upcoming special report on renewable energy and climate change by the Intergovernmental Panel on Climate Change, Lu’s study simply confirms that “there is absolutely no resource constraint for wind in the U.S.” Or, as Pryor puts it, “there may be regional winners and losers, but the winds are going to continue to blow.”

Source of Information : Scientific American October 2009

No comments: