Posts Tagged Great Plains

Wind Power and Wind Speed

I received serious skepticism to my idea that wind turbines could significantly slow the wind speed on the Great Plains. One of the criticisms came from an atmospheric scientist I asked to weigh in on the matter. The problem is, I did not find either of their arguments convincing, although I concede Daniel knows more about it than I do.

In responding to them, I came up with an approach for calculating the total power of the wind in the Great Plains. Wind is caused by differences in temperature and pressure as a result of uneven solar heating. Hence the total energy of the wind is a small fraction of the total solar flux. I’m guessing that the amount of solar flux that is actually converted into wind energy is below 1%, probably far below that, but I’ll use 1% until someone gives me a better number.

The Great Plains is 1.4 millions square miles in area, including parts in Canada and Mexico. The average solar flux is about 4 MWh/day/m2 (using numbers for Des Moines, IA.) There are 2.6 million square meters per square mile, making the total solar flux on the Great Plains about 14 trillion MWh/day. Using my 1% conversion efficiency into wind, and 24h in a day, we get total average wind power on the great Plains of 6,000 million MW. That energy is currently absorbed by objects on the ground and internal frictional losses in the air. To create significant wind speed drops, a significant fraction of that 6,000 million MW would have to be absorbed by wind turbines.

In my previous article, I used another approach to calculate that 1 million MW of wind turbines would be enough to significantly slow the wind on the Great Plains. Hence, unless my 1% solar-to-wind conversion efficiency is too high by three orders of magnitude, it looks like the skeptics were right.

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Wind Power and Soil

I recently had the somewhat questionable pleasure of driving across most of the Great Plains.  It has been over a decade since I last did a long distance drive across the Plains, and a new feature is starting to pop up: Wind turbines.  Sometimes in ones and twos, sometimes by the tens or hundreds.  I may be biased, but I find modern wind turbines to be among the most beautiful built structures in the world.  They have a slow, graceful motion that belies their Brobdingnagian scale.  They were particularly beautiful on a foggy evening driving as I drove through a wind farm near dusk, when I could see only the bottom half of the giant blades as they swept gracefully down out of the mist in a slow motion appearing and disappearing act.

The other feature of the generally broad and open landscape were lines of trees, sometimes bordering the interstate, and sometimes bordering fields.  I recall from a US history class in high school that these wind breaks were planted in response to the 1930’s dust bowl.  A little web research led me to the Shelterbelt Project, which seems to be what I recalled (somewhat inaccurately) from high school:

Established by President Franklin D. Roosevelt under executive order on
July 21, 1934, the Shelterbelt Project provided for a tree barrier one hundred miles wide extending twelve hundred miles north to south from the Canadian border through the Texas panhandle. It was designed to
reduce wind velocity, which had occasioned severe soil erosion across the Midwest and dust storms to the eastern seaboard.

In some ways, the Shelterbelt project can be seen as an early experiment in geoengineering. I sincerely hope that any future projects are so successful and benign.

Wind turbines, too, reduce wind velocity.  After all, a wind turbine’s function is to take wind energy, and convert it to electricity.  This led me to wonder just how many turbines would it take on the Great Plains to significantly lower the average wind speed in the region?

According to FTExploring, a wind turbine can extract about 35% of the wind energy passing through the swept area of its blades.  A typical 2.5 MW wind turbine from General Electric (GE) has a rotor diameter of 100m.  To get a ball-park figure, imagine two rows of GE 2.5MW turbines were installed from north to south along the Shelterbelt project (1200 miles) with rotor blade tips inches apart.  If the two lines were offset, wind blowing from east to west or west to east would have to pass through one or two rotors, losing 35% to 58% of its energy along the way, and exiting the back of the turbines 15% to 25% slower.

Such a double row of turbines would require about 386,000 turbines, or about 1 million MW of wind.  So, according to this back-of-the-envelope calculation, 1 million MW of wind installed in the Great Plains (even if not installed in a north-south line) should be enough to noticeably decrease the overall wind speeds in the region, and not only reduce soil loss from wind, but also reduce the cost effectiveness of installing more turbines.  Assuming a 35% capacity factor, this equates to about 3,066 million MWh.  In 2008, the US produced 4,119 million MWh of electricity, so 1 million MW of wind represents about a 75% of electricity production from wind in the Great Plains.  Even if there were sufficient transmission to distribute the power across the country, and geographic diversity greatly moderated the the overall variability of wind, such high penetrations would be impossible without prohibitive investments in electricity storage. 

With current storage technology, a greatly enhanced national grid, and a full roll out of smart grid technology used to better match demand to supply, I would guess that the upper limit for wind penetration would still be only 50% (and considerably lower if any of these things fail to materialize, especially the diversifying benefits of a robust national grid.)   This upper limit (and the fact that only a fraction of wind power is likely to be generated on the Great Plains) means that we’re probably unlikely to need to cut down any trees on the Great Plains in the hope of increasing the wind output of out turbines.

The United States had a cumulative 35,000 MW of wind installed by the end of 2009 (about 3.6% national penetration using the numbers above) so we’re still a long way from slowing the wind significantly on the Great Plains, or anywhere else.

I wonder if farmers who lease some of their land to wind farms notice any local slowing of the wind?  Is that a positive externality worth accounting for?

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