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Together Better than Alone: Network of Wind Farms Can Generate Smoother Power Supply
7 Dec, 2007 06:24 pm
Wind is a clean and never ending source of energy, but also an unreliable one, because it blows intermittently. This intermittency and unpredictability of wind patterns has always prevented wind power from being used as a regular electricity supply, but now two researchers at Stanford have shown that linking distant wind farms together would result in a smoother power source and could also reduce the final cost of wind-generated energy.
The explanation to this improved reliability is that connecting wind farms located in distant geographical locations with different wind patterns increases the chances that, at any given moment, the wind will be blowing in at least some of the sites. Thus the joint power supply will be steadier than that from each wind farm.
Archer and Jacobson calculated that 33% to 47% of the wind power generated during a year in a wind-farm array could be used as a “firm” power source, as dependable as the power produced by a coal-fired power plant.
Jacobson, an associate professor of civil and environmental engineering at Stanford, said that, although other researchers had studied the interconnection of wind farms, no one had ever quantified how reliable the combined energy output would be.
“This is the first time the reliability of wind has been gauged against coal-fired power plants,” Jacobson said.
In their study, Archer and Jacobson used real wind data from 19 sites in the Midwest of the United States of America. At all the sites, the average annual speed of the wind was at least 6.9 meters per second at the height of the hub, which is 80 meters above the ground for modern turbines.
The hourly and daily-averaged wind speed measurements the researchers used had been collected during the year 2000 by the National Weather Service. Archer, a consulting assistant professor of civil and environmental engineering at Stanford and a research associate in the Department of Global Ecology of the Carnegie Institution, said the information was originally collected at 10 meters above the ground, so she and Jacobson had to design a technique to extrapolate the data to the hub height.
Archer said she tried an enormous amount of combinations among the 19 sites to compare the resulting benefits. She concluded that, to obtain a significant amount of base load power, the array had to include 10 or more wind farms.
As an additional and hidden gain, Archer found that the interconnection reduced the transmission needs by one fifth, which would make the final cost of wind power cheaper.
In the Stanford researchers’ model, all the wind farms in the array would connect to a common point in the grid that would then distribute the electricity in a single, powerful channel. But the number of transmission cables needed to transmit the electric power from the collecting point to the consumers would be less than the number of wind farms in the array.
“If you had ten wind farms connected to that common point, the chance that the wind would be maximum at all ten farms at the same time is basically zero,” Archer said. Because all the connected wind farms would never be operating simultaneously at full capacity, the number of cables needed to transmit the wind-generated electricity could be reduced.
Archer and Jacobson calculated that this reduction in transmission cables would be 20 percent.
“That is: you can connect ten wind farms for the cost of eight,” Archer said. The associated loss of energy would be of only 1.6 percent.
“Right now, transmission is the biggest barrier for the large-scale implementation of wind farms,” said Jacobson, who also pointed out that the cost of transmission increases when the wind-generated electricity has to travel longer distances.
The researchers said the wind farms in the array could be up to 850 km away and still be economically viable. Further than that, “it would be an economical decision: am I willing to pay more in transmission costs to connect them from farther away because I get more smoothing?” said Archer.
Archer, Cristina and Jacobson, Mark. "Supplying baseload power and reducing
transmissions requirements by interconnecting wind farms." Published in the November 2007 issue of the American Meteorological Society's Journal of Applied Meteorology and Climatology