More than first thought?
US wind report stirs minor tempest
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 US Midwest offers a greater wind resource than has previously been thought GE WIND
Reuters picked up the story from a press release. Within five days, 10 media outlets had covered the topic, including CNN. The European wind industry was abuzz: the United States has far more wind energy potential than once thought, the American 'El Dorado' was an even richer prize than they had dreamed. A quarter of the US 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 puzzled by the collective yawn in some US circles.
It wasn't that Americans doubted the findings made by Cristina Archer,1 a doctoral student in civil and environmental engineering at the prestigious Stanford University in California, and her advisor, Mark Jacobson - it was simply that the results didn't seem that 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 … X is very large.'
Professional wind meteorologists also seemed nonplussed 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 sceptical. '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 south-eastern 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 was once thought. He was not the only one to be surprised and pleased by the report - the Stanford researchers say they received calls from wind developers who wanted more detailed data from them, especially from Texas, Florida, Louisiana, Illinois and Missouri.
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 do buttress those of several earlier studies in North America and Europe. Yet in the midst of a broad energy policy debate in the US, Archer's report could tip the balance among neo-conservative politicians who find the idea of wind energy - in fact, renewable energy in general - somehow unworthy of serious consideration, largely because it can't be turned on at will.
'Our paper shows that intermittency can be overcome by increasing the number of stations'
Intermittency has always been wind's Achilles heel. Wind can't be counted on because the wind doesn't always blow, its critics say. '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 (as can be seen in Figure 1).
Archer, also a meteorologist, undertook the study at the request of her thesis adviser and co-author, Mark Jacobson. The study was never part of Archer's dissertation, and she's somewhat overwhelmed by the attention it has generated.
FIGURE 1. Power wind speed distribution, divided into six 4-hour blocks for (a) one station (b) three stations (c) eight stations. This illustrates the decrease in low- and no-wind periods as the number of stations increase. Source: Cristina Archer1 (modified by permission of the American Geophysical Union)
In principle the study was relatively simple. Archer examined balloon sounding data from 87 stations across the US to find wind speeds at 80 metres above ground level (see Figure 2). She also compared the soundings at 80 metres with hourly wind speeds at several meteorological stations with long-term data.
FIGURE 2. Map of US wind speed extrapolated to 80 metres, averaged over all hours of the year 2000. Source: Cristina Archer1 (modified by permission of the American Geophysical Union)
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 metres than those which would be obtained by previous methods of extrapolation, such as the power law equation. Unfortunately, Archer didn't use contemporary meteorological 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 US with hourly meteorological data, and more towers are being added under a new DOE programme. Someone who would like to help Archer's study is Rory Artig, manager of Minnesota's highly respected Wind Resource Assessment Program (WRAP).
Data from Minnesota show extremely high levels of shear, and shear that varies dramatically at different heights above the ground
Artig says that he could provide Stanford with 90-metre meteorological 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', Artig adds.
Minnesota's State Energy Office has operated WRAP 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 5000 CD-ROMs of wind data. Indeed, so popular are the CDs that the state had to produce another 10,000 copies, according to Artig.
Yet Archer points out that theirs is the very first study of wind at 80 metres for the whole country, as opposed to state-wide studies, such as the Minnesotan one. 'Even though we did not use tower data to validate our findings, we directly used sounding data in our methodology and indirectly used tower data from 13 sites to derive the hourly trends of winds at 80 metres', she says.
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 that consulting meteorologists were baffled by the attention given to the study.
The tall towers in Minnesota's programme measure wind speeds at 30 metres, 60 metres and 90 metres 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 were 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.
Buffalo Ridge in Minnesota - site of the Lake Benton Wind Power Facility - is a region with high wind shear GE WIND
For example, the WRAP tower at Chandler, Minnesota recorded wind shear from the 30-50 metre heights typical of the Great Plains (0.14 or 1/7) and representative of those Archer labelled as too conservative. But at heights from 50 metres to 70 metres, shear jumped to 0.42, and this knowledge was widely disseminated in the industry.
The very first study of wind at 80 metres for the whole country, as opposed to state-wide studies
'The resource is very strong,' said Artig in a 1999 interview. 'You see quite high shear at the upper levels.' Meteorologists who have examined the Minnesota data as well as those 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'.
Equipment mounted on this gin pole collects wind speed data PHOENIX ENGINEERING
NOCTURNAL JET
High shear may be a regional phenomenon. If so, this augurs well for wind development throughout the upper Midwest; it's characteristic of Minnesota's Buffalo Ridge, said Ron Nierenberg in 1999. At exposed sites in Minnesota, he explained, wind shear is often double that of the 1/7 power law, from 0.2 to 0.3, and it is similar in Iowa and Wisconsin. It is the 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 is 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 wind turbines. The high speeds could require new fatigue margins for rotors, turbine designers worried, when they gathered at the American Wind Energy Association's 2002 conference in Portland, Oregon. After a year's 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 resources 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.' This is even more so, explains Artig, with the low-wind speed turbines becoming available, such as NEG-Micon's 1.65 MW turbine with its huge, 82-metre rotor.
The development of low-wind speed turbines is critical to the future of wind energy in the US
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-metre hub height wind speeds of about 7 m/s (see Figure 2).This is a possibility - 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-metre tower heights used in the Stanford study. Some towers in Germany reach 100 metres in height.
Consider two other manufacturers. The MD series of turbines, manufactured by several companies in Germany, can be ordered in both a 70-metre and a 77-metre version. Though both are rated at 1.5 MW, the larger turbine sweeps 20% more, by area, of the wind stream. Similarly, GE Wind offers its 1.5s model with a 70.5-metre rotor, and a 1.5sl with a 77-metre rotor. Both are rated at 1.5 MW.
The development of low-wind speed turbines is critical to the future of wind energy in the US, 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 US, the industry needs to be able to develop moderate wind sites that are also close to load centres, such as the Chicago or Minneapolis-St Paul metropolitan regions. There are 20 times as much Class 4 wind resources as Class 6 resources in the US, according to early NREL studies.
In the NREL system, Class 4 wind resources represent average wind speeds of 7.5-8.1 m/s at 80 metres (5.8 m/s at 10 metres). Class 6 resources average 8.6-9.4 m/s at 80 metres (6.7 m/s at 10 metres).Most commercial wind development in the US today is in Class 6 areas.
Most Class 6 resources are in remote areas distant from both load centres and transmission lines. Typically, Class 6 areas are 500 miles (800 kilometres) from major load centres. In contrast, many Midwestern cities are within 100 miles (160 kilometres) 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 6 resources by 2004. NREL's target for the new round of low-wind speed turbine development is 3 cents 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 isn't aggressive?'
Measuring US wind speeds: dual anemometers and single wind vane, mounted on tall tower with double-sided boom NRG SYSTEMS
In 2001, the US Department of Energy (DOE) launched its new Low-Wind Speed Turbine programme. NREL's present request for proposals is for the second round of contracts in the programme. NREL will award contracts for conceptual design, component development or full-system development. Certain 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 heyday of DOE research 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 that about half of the contract is public funds. Another contract was awarded to Enron Wind, ironically the company that Dehlsen 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 US at a critical time, when Congress was in the midst of debating a massive new energy bill. 'We certainly welcome the Stanford contribution,' says the 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 and her colleague 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.
Over the last two years, developers have proposed more than 3000 MW of offshore
With the imprimatur of one of the US's most elite universities and a skilful press office, Archer's scientific paper has successfully lodged itself in the public debate on the nation's energy future. While her results may have come as a complete surprise, her timing was impeccable.
Paul Gipe is the author of several books about wind energy including Wind Power for Farm, Home & Business, scheduled for release in winter 2003-2004.
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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