With nearly 47,000 MW of wind power capacity in the U.S. and more than 8,000 MW expected to go online in 2012, wind power's rapid growth poses a tremendous challenge for utilities and grid managers charged with managing a growing source of intermittent electricity.
It’s a high-stakes chess game with Mother Nature as grid operators perform a sophisticated balancing act using advanced turbine technology, best practices, forecasting techniques, modeling and other tools to get the most out of North America’s wind energy resources. The best solution for integrating large amounts of variable wind power is increased transmission capacity, but the siting, permitting and construction of new transmission can take many years to achieve.
Meanwhile, the industry has made great progress in managing the variable nature of wind energy. Higher capacity factors and penetration levels for wind energy mean the industry is getting better at predicting wind power output hours and days in advance with increasing accuracy and confidence. The increased output also stems from larger rotors and turbines and building wind farms in strategic areas to maximize output.
On the grid, there must be balance between load and generation. Achieving that balance while integrating larger amounts of variable wind power is one of the biggest challenges facing the power sector. But better forecasting methods, advanced computer software, the use of spinning and non-spinning reserves, and more grid interconnections have given the industry the tools it needs to accommodate large amounts of wind power.
During the early development of wind power in the U.S., integration was not a high priority for utilities and grid managers, said Charlie Smith, executive director of the Utility Wind Integration Group (UWIG). “In fact, if there were any system disturbances, the typical requirement was that the wind plant get off the system.” Smith said. “Today, wind plants are an integral part of the system and are expected to ride through disturbances and provide power control and voltage control just like any other power plant.”
In 2011, the U.S. installed nearly 7,000 MW of new wind power. That's a 31 percent increase over 2010, according to the American Wind Energy Association (AWEA). Last year, construction began on more than 8,000 MW of wind power capacity. What’s more, U.S. wind generation rose 27 percent in 2011 to 120 terawatthours, up from 27 terawatthours in 2006, according to the Energy Information Administration, the statistical arm of the Department of Energy.
Although utilities can’t directly control wind energy output, changes in output tend to be gradual and predictable. According to AWEA, wind power accounted for nearly 8 percent of the electricity placed on the grid in Texas in 2010. At one point in 2010, wind power accounted for 25 percent of the power on the Texas grid, AWEA said.
“Texas serves as a powerful example of how wind is being reliably and cost-effectively integrated,” the association said.
Telvent DTN, a leading provider of weather forecasting services, provides data to more than 100 wind power facilities in North America. Don Leick, senior energy project manager for Telvent, said the accuracy of wind power forecasting has greatly improved but requires sophisticated techniques and data crunching to devise an accurate forecast.
“It’s a complex challenge to do an accurate wind power forecast for such a highly variable source,” Leick said. “You’re predicting wind at a height that’s not at the surface level, which makes it more complicated. It’s typically 80 meters.”
Telvent provides utilities customized forecasts based on the characteristics of the wind farm and historical generation data.
“It’s a trained forecast that ends up being highly customized to the characteristics of that particular wind plant and how wind direction affects that plant,” Leick said. “That’s the approach we take. It discovers the variables that matter.”
Wind farms generally operate at 20 percent to 40 percent of their maximum capacity. Average capacity factors have improved, thanks to better estimates on wind speeds and direction.
“Relatively small increases in wind speed can mean fairly large increases in power because it’s a cube relationship,” Leick said.
A relatively new technique in wind power forecasting uses a series of likely probabilities to determine wind power output and the amount of stand-by reserves required. “Not many utilities are doing it,” Leick said. “They’re learning how to do this – the technique of probabilistic forecasting.”
The technique uses historical information on wind speeds and generation as well as variations, or errors, in the weather forecast. For example, probabilistic forecasting can tell grid operators that there is an 80 percent probability that there will be X amount of wind power at a certain time of day.
“I can base my reserves off of that,” Leick said. “Probabilistic forecasting allows you to be more intelligent about your reserve planning. It’s something people are beginning to see the value of.”
Turbine and equipment manufacturers are responding to the power sector’s need to improve wind power integration by engineering new quick-firing gas turbines and technologies that give utilities more control over the flow of electricity.
General Electric’s aeroderivatives gas turbine is capable of ramping up to full production in just minutes, allowing power producers to offset lost generation when the wind stops blowing. “The turbine can go from cold iron to 50 MW in just 10 minutes and make up for the lost electricity,” GE said.
The average size of a U.S. wind turbine in 2011 was 1.97 MW, up from 1.77 MW in 2010, according to AWEA.
“For a while, there was a strong trend of capacity factor improvements that were driven by two things,” UWIG’s Smith said. “One was locating wind plants in better wind regimes. The other was larger rotors for extracting more energy.”
But rising capacity factors have leveled off because transmission limits are preventing developers from building wind farms in the best places for harnessing the wind. Also, the development of larger rotor technology has slowed.
“When you talk about system practices and policies that are important for integrating large amounts of variable generation, I think transmission is No. 1,” Smith said. “You’ve got to be able to move the wind from where it’s generated to where it’s used. You’ve got to aggregate wind output over very broad geographical regions.”
The Federal Energy Regulatory Commission’s Order 1000 may be the best tool for integrating large amounts of wind power. Issued last July, the new rule requires transmission plans to reflect state and federal standards for renewable power and establishes a method for allocating the cost of building new transmission broadly to the beneficiaries of transmission projects. The rule is expected to remove the barriers to the development of new transmission capacity, Smith said.
The new rule is “providing a strong framework for inter-regional cooperation and looking at regional solutions, including cost allocation,” he said.
AWS Truepower is a consulting company that provides modeling and wind farm siting services to independent system operators and utilities. To help utilities and developers select the best sites for wind farms, the company creates hypothetical facilities and then simulates weather data to determine the potential wind power output. “From that, we can extract the variables necessary to run through a power conversion based on whatever the technology might be,” said Ken Pennock, forecasting business manager for AWS.
Because supply must equal demand, power schedulers have to be certain the power will be there. Forecasting by itself doesn’t provide the certainty required to bring balance to the grid. “It’s a matter of having a system that is intelligent enough to blend together all of those probabilistic values to get to a final answer,” Pennock said.
Utilities and grid managers need better data to improve the integration of wind power, Pennock said, adding that government could enhance the data used in U.S. weather models.
“Europe by far has developed a better system than the U.S.,” he said. “Data gathering for weather models in the U.S. is very limited and it needs to be enhanced. Private companies can’t deploy the number of sensors that would be necessary to enhance the input into weather models, but the government can.”
Last year, wind power was the No. 2 source of new electric capacity in the U.S., representing 35 percent of new capacity, according to AWEA. What’s more, turbine manufacturing continued to grow last year in the U.S., home to 23 turbine manufactures. The U.S. had only five turbine manufacturers in 2005.
“As the market expands, we’re getting more companies involved,” said Elizabeth Salerno, director of data and analysis for AWEA.
The biggest concern for the U.S. wind power market is the expiration of the Production Tax Credit at the end of this year. Wind turbine manufacturers have not received any order for new projects in 2013 due to the uncertainty surrounding the PTC, Salerno said.
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