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Wind in a Bottle PUBLIC ACCESS

Like Everything Else, Wind Power is Bigger in Texas. But Plans to Erect Thousands of Large Turbines Have Utilities Scrambling to Build a New Energy Infrastructure.

[+] Author Notes

Bridget Mintz Testa is a freelance writer based in Houston.

Mechanical Engineering 130(05), 22-25 (May 01, 2008) (4 pages) doi:10.1115/1.2008-MAY-1

This article highlights that like everything else wind power is biggest in Texas. But plans to erect thousands of large turbines have utilities scrambling to build a new energy infrastructure. To take advantage of its wind potential, address the state’s fuel diversity problem, and demonstrate sensitivity to regional environmental issues, the 1999 Texas legislature established a “renewable portfolio standard.” This standard sets a goal of 2000 MW of new renewable electricity generation by 2009. Compressed air energy storage (CAES) uses off-peak electricity from the grid to compress air that is stored at high pressures in natural underground repositories. When needed, the compressed air is raised to the surface, expanded in two phases, and then mixed with natural gas. The combination is ignited, turning a gas turbine and producing electricity. Naturally arched salt domes of the type used in the Huntorf and McIntosh plants are not common in West Texas, so the Shell-Luminant CAES plant will use salt beds instead.

Texas and oil go together like hound dogs and huntin', but petroleum wont'keep the lights on. The state is facing a possible shortfall in electrical power generation capacity as early as 2009, when its reserve margins could drop below the minimum 12.5 percent level. Due to a peculiarity in the state's electrical system, all the power that Texas needs must be generated in the state. The conventional options for new generation are limited. New nuclear plants are being planned, but it will be years before they contribute to the Texas electrical grid. Coals plants have become even more controversial than nuclear, and, like nuclear plants take years to build. Almost half of the Texas grid is powered by natural gas. This was fine a decade ago when natural gas was cheap, but the era of $2 per million Btu gas is over and as the price of the fuel goes up, so does the price of power. Consequently,Texas badly needs to diversify the energy sources it uses to generate electricity. How lucky it is,then that Wast Texas, the home of miles and miles of miles and miles,also happens to possess some 100,000 megawatts of potential wind generation, second only to North Dakota. Indeed, in 2006, the Lone Star State surpassed California as the number one producer of wind power in the country.

To take advantage of its wind potential, address the state's fuel diversity problem, and demonstrate sensitivity to regional environmental issues, the 1999 Texas legislature established a "renewable portfolio standard." That standard set a goal of 2,000 MW of new renewable electricity generation by 2009. After that goal was exceeded by new wind generation in 2006, the legislature reset the standard to 5,880 MW by 2015 and 10,000 MW by 2025. By the end of 2007, Texas had about 4,200 MW of installed wind generation capacity in service, and another 2,600 MW lined up to join the grid soon.

In spite of the rapid increase in wind power, the fuel diversity problem and reserve shortfall still loom. Adding a few more wind turbines here and there won't make much of a dent. But last August, Shell WindEnergy, a division of Shell Oil, and Luminant, a subsidiary of electric power utility TXU, announced a project that just might make a difference In spite of the rapid increase in wind power, the fuel diversity problem and reserve shortfall still loom. Adding a few more wind turbines here and there won't make much of a dent. But last August, Shell WindEnergy, a division of Shell Oil, and Luminant, a subsidiary of electric power utility TXU, announced a project that just might make a difference: The companies would partner to build a 3,000 MW wind energy plant in Briscoe County, about an hour's drive southeast of Amarillo. Meanwhile, oilman T. Boone Pickens' private equity firm, BP Capital Management, plans to enter a project that will build a 4,000 MW wind energy plant in Pampa, about an hour east of Amarillo.

Together, the 7,000 megawatts of wind capacity are impressive. But the Shell-Luminant plant also features something that 's almost unique: a method of storing wind energy that allows it to be used when it's most needed. Known as compressed air energy storage, the system will significantly enhance the integration of wind onto an electrical grid, because one of wind's most annoying characteristics is that it blows hardest and most energetically when it's least needed or wanted.

Wind Power has a long history in the Texas PanHandle. At one time, it was used to draw water for irrigation. Increasingly, large wind frames, such as the one in White Deer(below), are generating electricity.

Grahic Jump LocationWind Power has a long history in the Texas PanHandle. At one time, it was used to draw water for irrigation. Increasingly, large wind frames, such as the one in White Deer(below), are generating electricity.

Oklahoma, as the song says, may be where the wind comes sweeping down the plains, but that sweeping wind continues into the panhandle of Texas. "If you look at a wind map of Texas , Briscoe County stands out," said Mark Wilby, a senior business developer for Shell WindEnergy. "Wind is substantial in the Panhandle, and Briscoe is in the southern area of it. The wind terminates there just before going into Palo Duro Canyon, so the wind energy peaks there. The wind resource in Briscoe County is one of the better areas in the entire country."

But "windswept" often goes hand in hand with "underpopulated" or even "desolate," so there hasn't been much call for building up the electrical grid in West Texas. In fact, the region has only about 5,000 MW of the high-voltage transmission lines necessary to move wind-generated electricity east to the energy-hungry markets of Houston, Dallas, Austin, and San Antonio.

Texas's current installed wind capacity already needs almost all of that. So before the state can take advantage of the wind bonanza in the west, it's going to have to spend a few billion dollars to build new transmission lines. But first, Shell and Luminant (and other builders) must harness the wind.

Most plant details are still up in the air, since the project is still in the early development stage, and much depends on ongoing studies and decisions by the Public Utility Commission of Texas. Still, a few choices have been made. Wilby said a site has been acquired "in terms of appropriate leasing activity." The plant will generate a n1aXimum of 3,000 MW, but it won't be built all at once, nor will its output feed into the Texas grid in one giant gulp.

"Due to the sheer size, the point would be to ,divide up the Briscoe plant," Wilby said. Then it could feed power to the grid from different substation s. Land area also hasn't been determined, but Wilby said average 2 MW wind turbine "requires about 80 acre;;, depending on wind and topography." So for 3,000 MW, one would need 1,500 2-megawatt turbines covering/some 120,000 acres-or about three times the area of the District of Columbia. The facility might be broken into small units spread over an even greater expense,

Location isn't the only challenge facing a "big wind" project like the proposed Shell-Luminant plant. Another is timing: Average peak demand on the Texas Interconnect, the state's main grid, is about 62,000 MW on summer afternoos:(The all-time high was 63,239 MW set between 4 ahd 6 p.m. on Aug. 23, 2005.) "The wind in West Texas is highest in the morning, especially before dawn" and it drops around 8 a.m. to 10 a.m.," said Bill Bojorquez, vice president of system planning for the Electric Reliability Council of Texas, the organization responsible for managing the state's electrical grid. " It's the opposite of when demand is up." Electricity can't be stored on the grid, so wind generators must shut down J ust when their power production is peaking.

The electrical load on the ERCOT grid (left) is concentrated in eastern and southern Texas. The greatest potential for wind power (red and orange areas on the map at right) ERCOT is building transmission lines to connect new wind farms to cities.

Grahic Jump LocationThe electrical load on the ERCOT grid (left) is concentrated in eastern and southern Texas. The greatest potential for wind power (red and orange areas on the map at right) ERCOT is building transmission lines to connect new wind farms to cities.

"There are challenges with the 6,000 MW of wind available today," Bojorquez said. "So this 3,000 MW plant would be a significant challenge, especially when concentrated in one area." If the wind slows or stops when the grid is relying on that power, then other generation capacity must quickly kick in to "follow the wind"-that is, pick up the slack.

That capacity can't be coal or nuclear, because " quick" is not in those facilities' start-up or shutdown vocabularies .Instead, additional natural gas facilities, which can start and stop fast, would have to take up the slack, "almost megawatt for megawatt, " Bojorquez said. New wind power in Texas might increase total available megawatts, "but it's not a great help in terms of having to build other sources for peak load and for following the wind," Bojorquez said.

Could Bojorquez be overestimating the difficulty of dealing with wind's variability? In the past, electric utilities, which had lots of small generating plants, couldn't easily absorb any variability. Today's larger, regional utilities are much more efficient in terms of generation and transmission assets. "Large regional systems can absorb a lot of wind," said Randall Swisher, executive director of the American Wind Energy Association.

Additional costs because of wind 's variability might amount to 10 percent on large regional grids, according to Brian Parsons, project leader for grid integration at the National Renewable Energy Laboratory. "The grid is amazing," he said. "It has a lot of flexibility. The system can handle wind very easily."

But even if large regional grids can absorb wind's variability, ERCOT, which is limited to Texas, might not be big enough. " It would be beneficial to the grid to have a place to store the energy," Bojorquez said.

Electricity can't be stored on the grid, but energy can be stored off the grid and brought back later to generate power. One major storage method is "pump-back" hydroelectricity: Excess electricity pumps water uphill to areservoir; when demand is higher (or power production lower) , the water is sen t dovvn through turbines to make electricity. Other systems store natural gas in tanks to be released to power gas turbin es when needed.

Those storage methods don't match up well with the West Texas wind projects, however. Water is scarce there, and the Texas grid already uses too much natural gas. For these reasons, Shell and Luminant now plan to build a third type of energy storage system on the wind farm, one that relies on the energy in compressed air.

Compressed air energy storage, or CAES, uses off-peak electricity from the grid to compress air that is stored at high pressures in natural underground repositories. When needed, the compressed air is raised to the surface, expanded in two phases, and then mixed with natural gas. The combination is ignited, turning a gas turbine and producing electricity.

"There are a large number of gas turbine makers, but only a few are willing to work with CAES because it only uses the back end of a conventional turbine," said Paul Denholm , a senior analyst in energy forecasting and modeling at the National Renewable Energy Lab in Golden, Colo. "The front half of a gas turbine includes a compressor, which isn't used with CAES because you're using off-peak electricity to do the compression."

Until now, there have been just two CAES facilities: a 30- year-old plant in Huntolf, near Bremen, Germany, and one built in 1991 in McIntosh, Ala. Both plants store the compressed air in salt domes, and both use the technique mainly for electricity price arbitrage-compressing air using cheap, off-peak electricity and releasing air to generate electricity at intermediate or peak-rate periods.

"Our load is mainly residential, which means there are very high swings between peak and off-load," said Lee Davis, manager of the McIntosh plant, which is owned by the PowerSouth Energy Cooperative. "So to complement our coal plant in Lohman, CAES was chosen to help stabilize the grid."

With the CAES plant, PowerSouth can generate additional intermediate or peak power without having to build another baseload coal or nuclear plant. " It gives us better ramp-up for generation," Davis said. "We can go from turning gears to 110 MW in 14 minutes, and faster in an emergency."

Besides the proposed Shell-Luminant CAES plant, two others are in early development. One, a project of the Iowa Association of Municipal Utilities, is the Stored Energy Park in Dallas Center, about 25 miles northwest of Des Moines. The other, planned by Haddington Ventures, will be situated in Norton, Ohio.

Why hasn't CAES been tried in the United States since the McIntosh plant was built in 1991, It's all economics. "Look over the last 20 years, and you Will see only combined- and simple- cycle natural gas plants no coal and no nuclear," Davis said. "You could only put in gas plants." Natural gas was cheap, and CAES required expensive repository development. Today, with natural gas prices at sustained highs, CAES is suddenly an attractive option.

Naturally arched salt domes of the type used in the Huntorf and McIntosh plants aren't common in West Texas, so the Shell-Luminant CAES plant will use salt beds instead. "With a salt bed, you're looking at a big pancake, so subsurface collapse is possible," Wilby said. "You can't mine out a big area; you need multiple beds."

The geology affects the way the air is stored. Salt domes store compressed air in roughly elliptical or pear-shaped pockets mined out of the salt. With salt beds "we're creating flat pockets," Wilby said.

The Shell-Luminant CAES plant is planned to co-locate with the wind farm, but the benefits won't accrue just to the two partners. "CAES has big advantages for the whole grid," Parsons said. "Cheap storage and flexibility for wind are good things for the grid. If you have a market with a big differential between peak and off-peak power, wind plus CAES is good. If you're building a new transmission line to a wind plant, then you can use CAES to smooth the output of the wind plant, and you can size the transmission line for the peak."

In Briscoe County, Shell and Luminant's CAES plant will store 1,000 MW of power "To maximize the use of transmission lines, the facility will run at 2,000 MW consistently rather than 3,000 MW sometimes and zero MW the rest of the time,"Wilby said. "This improves the economics for the rate-payers."

CAES makes a big difference in integrating wind onto the ERCOT grid. For one thing, it will reduce the need for more natural gas plants to' compensate for wind variability. " From an operational, perspective, [storage] allows wind to produce energy and hot be subject to curtailmelHs," Bojorquez said. "It allows us to integrate more wind onto the grid when we need it and not waste it."

Since 2005, ERCOT has build more than 2,500 circuit miles of transmission lines. In early 2008, the organization announced plans to spend $3 billion over the next five years to add another 2,500 miles of transmission lines. Most, if not all, of this new ,capacity is due to the rapid growth in wind-generation capacity in West Texas and the need to move that power to 'the large cities in the eastern half of the state.

However, the new 2008 plans could be superseded by the public utility commission, which is evaluating the transmission needs of various areas at the state designated as competitive renewable energy zones." The evaluation includes scenarios involving some 35,000 MW of proposed wind energy generation (although that total includes duplication, such as multiple possible sites for a single proposed generating plant). Once the conmission finishes 'ts evaluation, sometime later this year, it will order new transmission lines to support the new wind generation.

For now, then, Shell and Luminant are in a holding pattern. They need to find out how much capacity the,they'll get, and they're also waiting on the transmission line decisions. "Once the final order is given, there are two more dockets to go through for selection of a transmission provider and then which generator would have priority access," Wilby says. "It will be about three years [from then] that construction would start." Once construction begins, the wind energy plant should be up and running in approximately 18 months.

"Shell firmly believes that to capture the economies required, scale is the way to go," Wilby says. "Wind is here; it's a strategic direction".

Copyright © 2008 by ASME
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