Measuring Ocean Winds from Space
Pimenta’s professor Willett Kempton pointed out two limitations with QuikSCAT’s measurements of wind speed. First, the satellite only measures winds in any given place once per day, far less information than buoys provide. “With buoy data, you can get down to five-minute or even one-minute time resolution,” says Kempton. Since winds are variable, power companies need to know how frequently they can expect lulls when power can’t be produced and how long those lulls might be.
The other limitation of QuikSCAT was the nature of the measurement itself. “Satellite data is inferred. It’s one more step away from what is actually happening,” says Kempton. By contrast, a buoy records wind speed directly. “You’ve got the wind turning the shaft of an anemometer, and then the anemometer shaft speed tells you the wind velocity,” says Kempton. The satellite observes waves on the ocean to calculate wind speed. “There are more things that can intervene and throw off that measurement.”
For all that, satellite wind data had tremendous potential. Most importantly, QuikSCAT provides wind speed estimates for the entire ocean, worldwide, including coastal Brazil. Second, the data are publicly available for free. Finally, QuikSCAT had been observing winds for more than eight years. “Eight years of global coverage of wind is amazing,” says Pimenta.
“Eventually, we just tried QuikSCAT,” says Pimenta. He compared QuikSCAT measurements of wind speed to measurements taken at the same time by the one buoy that had been in operation during the QuikSCAT record. The two matched reasonably well—close enough, his professor told him, to do a first estimate of the power potential. He extrapolated QuikSCAT measurements of wind speeds at 10 meters above the surface of the ocean to wind speeds 80 to 100 meters up, the height of the wind turbine’s hub. He calculated the amount of power existing turbines would produce at those speeds, and then averaged these measurements to get an estimate of power production throughout the year. In the end, he came up with a map of turbine power output for a wide swath of coastal waters off southeast Brazil.
Southeast Brazil’s Offshore Potential
But knowing how much power the winds could provide was not enough to make a practical estimate of the wind potential of the area. Pimenta also had to know how many turbines could actually be built off the Brazilian coast. Wind farms can’t be built in shipping lanes or bird flyways or near scenic coastlines. Current turbine technology can’t be installed in waters deeper than 50 meters, though companies are beginning to invest in floating turbines. Pimenta superimposed the wind data onto a map of the ocean floor to identify suitable locations for wind farms. This allowed him to calculate how many turbines could be installed off the southeast coast of Brazil. If the suitable areas were fully developed with existing technology, Pimenta estimated that offshore wind farms could produce 102 gigawatts of power on average. Brazil’s average national power need in 2008 was about 100 gigawatts.
After turning in his final project, Pimenta published his results in Renewable Energy in June 2008. No one has contacted him yet about developing an offshore wind farm in Brazil, but the possibility is real. It has happened to other students who have taken Kempton’s course. “A developer decided where to site a billion dollar plus project based on data that was produced in one of our classes,” says Kempton. “We have some very good students. I show them how to do it and let them go.”
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