Sensitive Scaling: The amount of power a typical wind turbine produces is proportional to the cube of wind speed.
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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.”
Note: This article was originally printed with the title, "The Way the Wind Blows."
*Clarification (11/24/09): This expression was not intended literally. Turbine power generation is proportional to the cube of wind speed.
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15 Comments
Add CommentWind turbines are *not* "exponentially sensitive to changes in wind speed." An "exponential dependence" means that each constant increment in speed (say, 1 m/s) would result in the same multiplicative factor to wind power (say, a 20% increase).
Reply | Report Abuse | Link to thisIn fact, wind power is proportional to the cube of the wind speed. (At least the article gets the numbers right. A 15% increase in wind speed, i.e., a factor of 1.15, leads to a 52% increase in wind power, since 1.15 cubed is about 1.52.)
Science writers should know better than to make the mistake that any relationship that's not linear must be "exponential".
Uh, I hate to break it to you, but cubing a number makes the function exponential. Multiplying it does not.
Reply | Report Abuse | Link to thisI should clarify myself here. The function's variable is not an exponent, and in that sense you are correct. However, the lay person is not going to know the difference and to call a dependence exponential is the best way to get them to understand that your output increases by a significantly larger amount with each increase of your input.
Reply | Report Abuse | Link to thisWTF. Here in Scandinavia, we are experiencing more and more wind. I moved here in the early '70s and enjoyed the fact that it never blew hard well, almost never. Then it starting... recently (2004- 2007) we have had TWO hurricanes (never happened before). And, this year, it has blown and blown and blown. Gees, I am getting tired of it I might as well live in Flatland North Dakota! What has caused this change? Hard to say, but at the same time, we are told that the earth is getting warmer. And heat means energy and wind needs energy, so what are these smart guys talking about, exactly?
Reply | Report Abuse | Link to thisClimatge Change may mean slower winds...or not. I added that in order to make the title more accurate, because despite the hard work used to make as accurate a model as possible in hopes of predicting the future weather and climate patterns, the fact remains that with systems this large and complex, predicting something like weaker winds or for that matter stronger winds, or even no wind at all, is little more than a good guess. And "good guesses" aren't nothing. Indeed, creating the models should above all teach us that the models are not reality. To say that one predicts conditions X is fine and I hope our planners keep it in mind but based on that I wouldn't want them to spend un-limited amounts of time, effort and treasure trying to protect me from one of the many possible threats until we know more beyond mere correlation. Where 's the empirical data on climate change?
Reply | Report Abuse | Link to thisFirst we read that AGW means more powerful storms, but less wind, with cooler temps at first to be followed by a fire storm of hot co2 creating a second planet Venus.
I wonder if a certain demographic might just stop reading the "sky is falling" news from the media who see their ad revenues connected to their ability to frame the news in such a way that whateve it is they're yammering about seems urgent and critical...no matter what. Michael Jackson's lost video or the threat of crack babies or the mayan date of 1212...who can tell what craziness will be next?. It makes little difference as long as it brings in the rubes.
Koltrast, seems to me that if the average global temps are changing, up or down, that does not necessarily affect wind speeds since wind is a function of the relative differences in temp/pressure between two areas.
Reply | Report Abuse | Link to thisAs far as I could discern, the article gave no correlation between a dubiously measured decrease in wind speeds and warming at all.
Maybe all those wind turbines will end up affecting wind speed. That energy extracted does come from the wind and we all know there is no free lunch when it comes to energy.
Global warming produces more energy in the atmosphere. That will be exhibited in a number of ways, but unless you come up with better proof than you have here, I would expect wind speeds to go up on the average, not down.
Reply | Report Abuse | Link to thisAt the very least, for the next few thousand years, lunch is still free when it comes to wind energy. The wind is derived from the sun.
Reply | Report Abuse | Link to thisAnd rustyc, while global warming does produce more energy in the atmosphere, it is mainly thermal rather than kinetic. Just because something is heated does not mean that it has to move.
And overall, it is a direct conclusion of climate change as it has been geographically predicted that the global average of wind speeds will decrease. This is because the poles are expected to warm more than the equator, thereby reducing the thermal gradient, and thereby reducing the intensity of the flow between them.
On a lighter note: Thank God!!! I'm so tired of the wind messing with my toss on my serve in tennis!!!
Reply | Report Abuse | Link to thisAdmittedly my knowledge on physical things was learned many years ago, but my physics classes taught that when a gas is heated the molecules move faster, and it expands. Sounds kenetic to me.
Reply | Report Abuse | Link to thisRight, the molecules move faster randomly, but not in an organized wind sense. The random motions are basically temperature, and just blowing harder does not mean the air is warmer. It just means that the difference in temperatures (or pressures for that matter) is greater across a given area.
Reply | Report Abuse | Link to thisSounds kinetic at the molecular level, yes. But you require a flow, not just molecular erratic movement in order to move a turbine.
Reply | Report Abuse | Link to thisA hotter atmosphere will evaporate more water into humidity and clouds which are going to perturb winds with storminess. This isn't that good for wind power because most turbines have to shut down for safety in extreme high winds.
Reply | Report Abuse | Link to thisWorse yet, most wind farms are in more remote places which means miles and miles of transmission lines are necessary to get the energy to where people actually live. Transmission lines are vulnerable to storms, which suggests that greater dependence on wind energy will make emergency climate conditions worse, leaving urban people shivering in the dark while everything in the fridge melts and spoils.
The disconnect from reality here is astounding. The headlines are full of record cold temperatures across the northern hemisphere for the second year in a row and "scientists" continue to prattle on about global warming.
Reply | Report Abuse | Link to thisAnything that gets the grant $$$, eh guys?
Very sorry but no need to be a scientist to observe that the winds have been increasing since , at least, 10 years ago! meaning I totally disagree with this article. G.O.
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