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July 29, 2003
Paul Gipe

More Wind than First Thought?


 

A recent report from Stanford University points to a far greater US wind resource in the Midwest than was previously estimated. This is not exactly breaking news, according to consulting meteorologists, but as Paul Gipe observes, the report is a timely indicator of the future direction of the US wind industry.


The following article appeared in an edited form in Renewable Energy World's September-October 2003 (Vol. 6 No.5) issue.


by Paul Gipe

Reuters picked up the story from a press release. Within five days ten media outlets had covered the topic, including CNN. The European wind industry was a buzz: the United States has far more wind energy potential than once thought. The American Eldorado was even a richer prize than they had dreamed. One-quarter of the United States was windy enough to compete with coal-fired generation, or so said the report. By the end of the news cycle Europeans were excitedly calling their Yankee colleagues for more details. They were surprised by the collective yawn in the states.

Not that Americans doubted the findings by Cristina Archer, a doctoral student in civil and environmental engineering at the prestigious Stanford University. Simply that the results didn't seem new or surprising. "No matter how you look at it there's a hell of a lot of wind in the lower 48 states," says the Department of Energy's Jack Cadogan. "Whether it's X or 2X doesn't matter, that X is very large."

Professional wind meteorologists too seemed nonplused by the media frenzy. "It's not jaw dropping," says Jack Kline. "No surprise that the Dakotas are windy, that's nothing new." Some wouldn't be quoted, others were openly skeptical. "It's nice to see some academics that have no feel for our science take an interest," says Ron Nierenberg, a cantankerous meteorologist with 26 years in the wind business. "It's a fresh, but primitive approach."

On the other hand, Florida renewable energy activist Frank Leslie was elated with the report's findings, especially that the southeastern coast and the Gulf of Mexico held a potential bounty of wind energy offshore. Leslie hopes to use the report to counter the conventional wisdom "that there's no wind in Florida." He was also excited that the Stanford report found much greater wind shear than once thought.

Overlooked in the brouhaha was Archer's potentially more significant finding that large numbers of geographically dispersed wind plants would provide significantly less hourly deviation in power generation than a single plant. Again, not a new finding. The results buttress those of several earlier studies in North America and Europe. But in the midst of a broad energy policy debate in the United States, Archer's report could tip the balance among neo-conservative politicians who find the idea of wind energy, in fact renewable energy in general, as somehow "unmanly" and not worthy of serious consideration largely because it can't be turned on at will.

Intermittency has always been wind's Achilles heel. Wind can't be counted on because the wind doesn't always blow its critics says. "Intermittency is an important issue," says Archer. "Our paper shows that this can be overcome by increasing the number of stations." In meteorological jargon, the standard deviation decreases as the number of stations increases. She says that the results of the study are counter intuitive. Wind can indeed be counted upon--there will always be some wind somewhere--if there are enough stations widely dispersed geographically.

Archer, also a meteorologist, undertook the study at the request of her thesis adviser, Mark Jacobson, himself the author of a controversial paper on wind energy's new found competitiveness. Archer's study was never part of her dissertation and she's somewhat overwhelmed by the attention it has generated.

In principle the study was relatively simple. Archer examined balloon sounding data from 87 stations across the United States to find wind speeds at 80 meters above ground level. She also compared the soundings at 80 meters with hourly wind speeds at several met stations with long-term data.

One of the more contentious findings in the Stanford study, was Archer's bold assertion that she had discovered much higher wind speeds at 80 meters than that which would be obtained by previous methods of extrapolation, such as the power law equation. Unfortunately, Archer didn't use contemporary met data from tall towers to validate her technique and her projected wind shear. "We wanted reliable, official data," she argues. "You could spend years collecting data from various sources." Instead she relied solely on data easily accessible from the web. "We would certainly want to verify our results in the next phase," she says.

 

Tall Tower Data

Unbeknownst to Archer, there are a number of very tall towers in the United States with hourly met data. And more towers are being added under a new DOE program. One who would like to help Archer's study is Rory Artig, manager of Minnesota's highly respected Wind Resource Assessment Program.

Artig says that he could provide Stanford with 90-meter met data from around the state. Archer could then work with actual hourly data at the heights needed instead of relying on either airport data or the limited sounding data available from balloons. Minnesota operates seven such towers and "we'll have two more by mid 2003," says Artig.

Minnesota's State Energy Office has operated the WRAP program for nearly a decade and has placed all the data collected in the public domain. The state's outreach has been so successful that the Energy Office has distributed more than 5,000 CD-ROMS of wind data--so popular are the CDs that the state had to print another 10,000 copies, says Artig.

 

High Shear

If the Stanford researchers had examined Minnesota's data they would have found extremely high levels of shear and shear that varies dramatically at different heights above the ground. This is one reason consulting meteorologists were baffled by the hoopla surrounding the study.

The tall towers in Minnesota's program measure wind speeds at 30, 60, and 90 meters above the ground. What Artig found is startling: surface friction coefficient exceeding 0.40 at some tower heights. Meteorologists had suspected that such a strong wind shear existed from the so-called nocturnal jet, but it wasn't until data was collected that it became apparent how beneficial it might be. Wind farm developers in the Midwest were incorporating these high shear values in their performance projections by the late 1990s.

For example, the WRAP tower at Chandler, Minnesota recorded wind shear from the 30 to 50 meter heights typical of the Great Plains (0.14 or 1/7) and representative of those Stanford's Archer labeled as too conservative. But at heights from 50 to 70 meters shear jumped to 0.42 and this knowledge was widely disseminated in the industry.

"The resource is very strong," said Artig in an 1999 interview. "You see quite high shear at the upper levels." Meteorologists who have examined the Minnesota data as well as that from private sources agreed with Artig. "We've seen high shear, particularly in the wintertime," said consulting meteorologist Kline at the time. The Upper Midwest is "not a particularly robust wind regime otherwise."

 

Nocturnal Jet

High shear may be a regional phenomena. If so, this augers well for wind development throughout the upper Midwest. It's characteristic of Minnesota's Buffalo Ridge said meteorologist Nierenberg in 1999. At exposed sites in Minnesota, explained Nierenberg, wind shear is often double that of the 1/7 power law, from 0.2 to 0.3. It's similar in Iowa and Wisconsin. It's use of the 1/7 power law that Archer's paper called into question.

During summer months when wind speeds are typically low in a continental wind regime, a "nocturnal jet" may occur at a certain height above ground where the friction coefficient in the power law equation can reach 0.4. This "jet" has nothing to do with the jet stream, elaborated Nierenberg. it's simply a layer of fast moving air. "There are lots of places in the world where there's a localized zone of high winds, a so-called jet," he says.

In fact the awareness of this high speed jet has led to consternation. The wind speeds at current hub heights in the Midwest may be so great at times that they exceed the design margins for today's crop of wind turbines. The high speeds could require new fatigue margins for rotors, worried turbine designers at the American Wind Energy Association's 2002 conference in Portland, Oregon. After a years worth of additional data to delineate the problem, DOE has happily found that the problem isn't as severe as initially thought, but bears watching as the powerful gusts could shorten the lifespan of turbines. If that happened, it would jeopardize all the rosy economic scenarios which depend on the bulk of wind farm profits occurring in later years.

The wind resource at today's hub heights "are substantial around the state," concludes Minnesota's Artig. "The developable area is much greater than--possibly double--that we once thought." Even more so, explains Artig, with the low-wind speed turbines becoming available, such as the NEG-Micon's 1.65-MW turbine with a huge 82-meter rotor.

 

Low-Wind Speed Turbines

Archer's paper contends that wind is competitive with new coal and gas-fired power plants in Class 3 wind resources, the equivalent of 80-meter hub height wind speeds of about 7 m/s. Possibly. But only if new low-wind speed turbines, widely used in Germany, can prove themselves.

NEG-Micon isn't the only manufacturer to field so-called low-wind speed turbines. Most major manufacturers provide the option, whether it's for the American heartland or Germany's Mittelgebirge. In essence, low-wind speed turbines incorporate a large diameter rotor relative to generator rating and are installed on very tall towers, such as the 80-meter tower heights used in the Stanford study. Some towers in Germany reach 100 meters in height.

Consider two other manufacturers. The MD series of turbines, manufactured by several companies in Germany, can be ordered in both a 70-meter and a 77-meter version. Though both are rated at 1.5 MW, the larger turbine sweeps 20 percent more area of the wind stream. Similarly, GE Wind offers its 1.5s model with a 70.5 meter rotor and a 1.5sl with a 77-meter rotor. Both rated at 1.5 MW.

The development of low-wind speed turbines is critical to the future of wind energy in the United States, where the commodity price of electricity determines what is built. For this reason, the National Renewable Energy Laboratory has issued a request for proposals to develop new low-wind speed turbine designs, says NREL's Paul Migliore.

To make wind work widely in the United States, the industry needs to be able to develop moderate wind sites that are also close to load centers like the Chicago or Minneapolis-St. Paul metropolitan regions. There are twenty times as much Class 4 wind resources as Class 6 resources in the United States, according to early NREL studies.

In the NREL system, Class 4 wind resources represent average wind speeds 7.5-8.1 m/s at 80 meters (5.8 m/s at 10 meters). Class 6 is 8.6-9.4 m/s (6.7 m/s at 10 meters). Most commercial wind development in the United States today is in Class 6 areas.

Most Class 6 resources are in remote areas distant from both load centers and transmission lines. Typically Class 6 areas are 500 miles (800 kilometers) from major load centers. In contrast many Midwestern cities are within 100 miles (150 kilometers) of Class 4 wind resources. At these distances the transmission network is denser and transmission is less likely to become a stumbling block to greatly expanded wind development.

For NREL, low-wind speed designs can be more than simply larger diameter rotors and tall towers. They can incorporate new blades or control strategies, or "technology improvement opportunities" in NREL-speak.

The interim milestone for the new turbines is NREL's infamous 3-cent turbine. That is, the turbine must produce wind-generated electricity for 3 cents per kilowatt-hour in Class resources 6 by 2004. Their target for the new round of low-wind speed turbine development is 3-cent per kilowatt-hour in Class 4 wind resources by 2012. While Europeans may raise an eyebrow at Yankee chutzpah, NREL argues "why have a target if it's not aggressive."

In 2001 DOE launched its new Low-Wind Speed Turbine program. NREL's present RFP is for the second round of contracts in the program. NREL will award contracts for either conceptual design, component development, or full-system development. Some companies bidding on the contracts hope to develop new wind turbine designs, some for offshore. In the previous round, some proposed direct-drive generators, others simple one-stage gearboxes with permanent-magnet generators. A few are outlandish and reminiscent in their off-the-wall approach of those proffered in the heydey of DOE rearch and development in the 1970s and early 1980s.

In the first round, one contract for US$16 million was awarded to Jim Dehlsen's Clipper Wind for a multiple-generator drive train. The contract includes cost-sharing, but NREL acknowledges about half of the contract is public funds. Another contract was awarded to Enron Wind, ironically the company that Dehlsen's once led. Theoretically, foreign firms can apply for the contracts, says NREL, but Congressional limitations thwart participation to all but the most determined.

 

Stirring the Pot

The Stanford report stirred the professional and political pot in the United States at a critical time when Congress was in the midst of debating a massive new energy bill. "We certainly welcome the Stanford contribution," says DOE's Cadogan. He could add that the report is particularly useful at this time.

Archer's report once more focuses attention on the nation's abundant wind resources. In doing so she forced DOE and NREL to drag out their studies, new and old, to confirm to new media and political queries--yet again--that yes, it's windy out there. "We're finding a lot more wind just by refining the resolution of our mapping, says Cadogan.

With the imprimatur of one of the United States's most elite universities and a skillful press office, Archer's scientific paper has lodged itself in the public debate on the nation's energy future. While her results may have not been surprising, her timing was impeccable.

Reference

1. Archer, C. L., and Jacobson, M. Z. 'The spatial and temporal distributions of U.S. winds and wind power at 80 m derived from measurements'. In Journal of Geophysical Research, Vol. 108, No. D9, 4289. American Geophysical Union. 2003.


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