By Sarani Rangarajan, Robert Mills, and Sara Parks, Iowa NSF EPSCoR
Over the past two years, Gene Takle and his team have been taking measurements around a wind farm in Iowa to find out how turbines change the micro-climate downwind. Takle is leader of the resource characterization plank of the Iowa NSF EPSCoR’s wind energy platform and a professor of agricultural meteorology in the department of agronomy at Iowa State University.
There are many advantages to conducting wind energy research at this site. Takle says, “There’s not a lot of research anywhere on the effect of wind turbines on crops. It’s a relatively new technology. The wind farm we are working in is not a toy wind farm. It’s a real wind farm. We can do things that other people can’t do.”
The Midwest is also special in that farmers engage in intensive agriculture next door to wind turbines. Takle is interested in this unique interface and the unique climate of Iowa. In other wind-energy states such as Texas, Takle explains, “they don’t raise the kinds of crops we raise here, and they don’t have the kind of conditions, ice and snow, that we have here. This is a good place to do research on weather extremes on wind turbines.”
To determine the effect of turbines on local microclimates, Takle oversaw the measurements taken upwind and downwind of turbines by his graduate student, Dan Rajewski, and scientists from the National Laboratory for Agriculture and the Environment and the National Center for Atmospheric Research. They took measurements of temperature, moisture, turbulence and wind speed 20 times a second, 24 hours a day for five months during the summers of 2010 and 2011.
Their data indicate that the turbines act like giant fans. Temperatures downwind from the turbines are slightly cooler during the day and warmer at night than upwind. They also found that there was more turbulence downwind from the turbines. While these findings were as expected, Takle says, “You can’t assume. Nobody had measured the turbulence, so we did.”
The team also measured the amount of carbon dioxide that moved through an imaginary flat surface (flux) at crop height. There is about 10 percent more carbon dioxide going into the crop downwind than upwind. Takle says, “If the sun is shining and there’s a lot of carbon dioxide, the plant just makes more biomass. But a 10-20 percent difference in some cases, we don’t know if that will make a difference on plant growth or yield over the whole season.”
There isn’t a simple answer to the question of how the carbon dioxide flux affects crop growth. Fernando Miguez an assistant professor in the department of agronomy at Iowa State University says, “I would have to model it to say for sure.”
Like grasses and sugarcane, corn (classified as a C4 plant) has a way of capturing carbon dioxide that is more efficient in producing dry biomass. According to Sotiris Archontoulis, a researcher with the Iowa State agronomy department, “With a 10 percent change in carbon dioxide, I would expect very marginal changes in biomass with corn. If the same effect was seen with soybean, then I would expect to see greater biomass changes.”
Miguez agrees, “In theory, we know that a carbon dioxide change in concentration in this range should not have an effect on yield for corn. My speculation is that he’s seeing more turbulence.” And turbulence can mix things up.
It’s not only the overall flux that matters, but also how the concentration is distributed inside the crop canopy. If the profiles change, as they can with increased mixing due to turbulence, the impact on crop yields becomes more complicated.
How dry the season has been also factors. “Corn, as a crop, is very sensitive around flowering. Since corn grows in very dry climates,” Miguez says, “there should be no difference in a year with ample water. In a dry year it could have a positive effect. And in a very dry year, it could be detrimental.”
For corn, the bottom-line appears to be that the turbines would theoretically not have much effect on yield. Research to investigate whether we can expect changes continues.
Takle says, “The EPSCoR program’s job is to provide the research infrastructure to make the state of Iowa competitive in terms of getting research grants. And that’s already working. We have a $300,000 joint project with Anemometry Specialists, Inc, an Iowa company, to develop a new method to measure wind speeds in the vicinity of wind farms using balloons. They’ll come in and put in these 50 meter towers, fly the balloon system, and we will evaluate the data quality.”
The balloon system will have measurement packages suspended from the tether of a balloon, which will float in the vicinity of 50-meter towers. The measurement packages will be tethered with a cable. The new measurement system will be compared to data from the meteorological towers to evaluate the data quality.
While Takle’s present research shows mostly a decrease in wind behind turbines, they do see some increases as well. He explains that, “We’re going to make some very detailed pressure measurements this winter with very sensitive barometers. When the wind is blowing, there’s a high-pressure area upwind of the turbine. These influences cause the wind to accelerate underneath the turbine. The speed is higher than the speed upwind due to pressure effects. And we want to measure that.” In addition, Takle says, “The wind farm owner also is interested in what we are doing, and they want to bring in their own sonic detection and ranging (SODAR) instruments to measure wind speeds.”
With these towers in place, new avenues of research open up. “We haven’t measured in the winter with anything yet. So measuring towers will go up after harvest, and they’ll come down before they plant again,” says Takle, talking about immediate future plans. “We’ll also be working with the impact of the whole wind farm. We’ve been studying single turbines, and we want to study aggregate effects of wind farms,” he adds.
There is a wealth of data to be mined from the measurements that have already been taken, and a number of questions that Takle and his teams are working on answering. For example, they hope to find out the impact of idle turbines. To investigate this, they can use measurements taken in 2010 over a ten-day period when the whole wind farm was shut down. They can compare that data to periods when just one turbine was dormant.
In the summer, wind in Iowa blows primarily from the south, southwest or southeast. So, the Takle team’s measurement towers are placed north and south of the southwestern-most turbines of the wind farm. On the rare days that wind flows directly from the west, the towers’ measurements can be used as a control; they can tell the team what these meteorological measurements would be without the wind farm and how the turbines interact with each other.
Farmers, Takle says, like to support education and research. They understand the value of finding out the effect of this new technology. Takle gives the example of a major question that comes up around wind energy turbines, which is the impact on wildlife, particularly bats and birds. Takle says, “Well, we see no evidence that turbines are killing bats – not here. In California, they are. Here, we haven’t seen a single dead bat or bird.”
As a land grant university, part of Iowa State’s job is to answer scientific questions relating to what the Iowan needs. Takle explains, “In an extreme year, there are all kinds of speculations and rumors. And we want to be out there measuring before any rumors get started about benefit or harm of windmills. When people come to Iowa State with questions, we want to have answers.”