Stanford Scientists Say Connecting Wind Farms Creates More Reliable Power Source

November 28, 2007 // Published as a news service by IHS

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Wind power can be groomed to become a steady, dependable source of electricity and delivered at a lower cost than at present, according to scientists at Stanford University.

The key is connecting wind farms throughout a given geographic area with transmission lines, thus combining the electric outputs of the farms into one energy source.

The findings are published in the November issue of the American Meteorological Society's Journal of Applied Meteorology and Climatology.

Wind is the world's fastest-growing electric energy source, according to the study's authors, Cristina Archer and Mark Jacobson. However, because wind is intermittent, it is not used to supply baseload electric power today.

Baseload power is the amount of steady and reliable electric power that is constantly being produced, typically by power plants, regardless of the electricity demand.

But interconnecting wind farms with a transmission grid reduces the power swings caused by wind variability and makes a portion of it just as consistent a power source as a coal power plant.

"This study implies that if interconnected wind is used on a large scale, a third or more of its energy can be used for reliable electric power, and the remaining intermittent portion can be used for transportation, allowing wind to solve energy, climate and air pollution problems simultaneously," said Archer.

"The idea is that while wind speed could be calm at a given location, it could be gusty at others. By linking these locations together, we can smooth out the differences and substantially improve the overall performance."

However, not all locations make sense for wind farms. Only locations with strong winds are economically competitive. Archer and Jacobson evaluated 19 sites in the midwestern U.S., with annual average wind speeds greater than 6.9 meters per second at a height of 80 meters above ground, the hub height of modern wind turbines.

Modern turbines are 80 meters to 100 meters high, approximately the height of a 30-story building, and their blades are 70 meters long or more.

The researchers used hourly wind data, collected and quality controlled by the National Weather Service, for the entire year of 2000 from the 19 sites. They found that an average of 33% and a maximum of 47% of yearly averaged wind power from interconnected farms can be used as reliable, baseload electric power.

These percentages would hold true for any array of 10 or more wind farms, provided the minimum wind speed and turbine height criteria used in the study were met, the authors said.

Another benefit of connecting multiple wind farms is reducing the total distance that all the power has to travel from the multiple points of origin to the destination point. Interconnecting multiple wind farms to a common point and then connecting that point to a far-away city reduces the cost of transmission.

Archer said another cost saving results when the power combines to flow in a single transmission line. For example, if a power company wanted to bring power from several independent farms each with a maximum capacity of 1,500 kilowatts (kW) from the Midwest to California, each farm would need a short transmission line of 1,500 kW brought to a common point in the Midwest.

Then they would need a larger transmission line between the common point and California - typically with a total capacity of 1,500 kW multiplied by the number of independent farms connected.

However, with geographically dispersed farms, it is unlikely that they would simultaneously experience strong enough winds to each produce their 1,500 kW maximum output at the same time. Thus, the capacity of the long-distance transmission line could be reduced with only a small loss in overall delivered power.

The more wind farms connected to the common point in the Midwest, the greater the reduction in long-distance transmission capacity that is possible.

"Due to the high cost of long-distance transmission, a 20% reduction in transmission capacity with little delivered power loss would notably reduce the cost of wind energy," said Archer, who calculated the decrease in delivered power to be only about 1.6%.

With only one farm, a 20% reduction in long-distance transmission capacity would decrease delivered power by 9.8% - not a 20% reduction, because the farm is not producing its maximum possible output all the time.

Archer said that if the U.S. and other countries each started to organize the siting and interconnection of new wind farms based on a master plan, the power supply could be smoothed out and transmission requirements could be reduced, decreasing the cost of wind energy. This could result in the large-scale market penetration of wind energy, which is already the most inexpensive clean renewable electric power source.

Source: American Meteorological Society (AMS).