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Friday, December 24, 2004

The Energy Challenge 2004 - Wind

by Murray Duffin

[Although the technical level in the section on "intermittency" of the following article might seem daunting, I am posting the article anyway because wind energy is important in the overall discussion of the Energy Challenge. Bill Totten]

The Energy Challenge 2004 - Wind

by Murray Duffin

http://www.energypulse.net (October 13 2004)


In addressing the declining availability of fossil fuels, and with nuclear energy less than popular, the remaining choices are energy efficiency and renewables. Fortunately, they are complementary choices and have the added virtue of being carbon free. Renewables include hydro, wind, solar, bio-fuels, geo-thermal, wave, and tidal energy. Of these, wind, solar, geo-thermal, and wave/tidal are abundant, but only wind is currently economical and easy to harness.

Whatever you think you know about wind, especially the negatives, if it was based on data and analysis prior to 2000 you can be pretty sure it's wrong. Wind has made enormous strides in the last fifteen years. In 1992 the average size of installed wind turbine was 200 kilowatts. In 2002 it was 1.4 megawatts. Now almost all units being installed are over 2 megawatts. Total world installed wind energy in 1992 was 2.5 gigawatts. By 2003 it had reached 40 gigawatts, a compound annual growth rate of 30% per year.


How Much Energy

Probably the best early data on the total USA wind resource is the 1993 report found at www.nrel.gov/wind/potential.html. This report estimated total potential for 25% efficient turbines, with 25% losses, and average 50 meter hub heights, and made exclusions for environmental, urban, and agricultural purposes. The result was that about 15 quads of equivalent fossil fuel energy could be replaced by class 5 to 7 winds. Adding class 4 winds, which were marginal at that time, raised the potential to greater than 60 quads. Most recent Texas wind farms are in class 4 areas.

This report was based on 1991/92 technology, when the largest envisioned turbines were 300 kilowatt and blade rotation speeds were such that considerable areas were excluded for environmental reasons, such as bird kill. Best wind speeds were 15-25 miles per hour and it was also assumed that only 20% of the actual wind energy per square kilometer could be converted to electricity.

Now for 1.5 megawatt turbines with hub heights near 80 meters, Archer and Jacobson <1> find class 3 winds are economic, and are available for about 20% of the lower 48 land area. ["Lower 48" means the United States excluding Alaska and Hawaii.] They also have found that near shore coastal areas with suitable winds cover more than twice the shoreline of the 1993 paper. Today turbines being installed are up to 3 megawatts, and up to 5 megawatts are in development. Blade rotation is much slower. Efficiencies are now above 30% and losses below 15%. Productive wind speeds are now in the class 3 range. Conservatively, total lower 48 available wind energy with 2004 technology is in the order of 150 quads, fossil fuel equivalent, or 50% more than total USA current primary energy demand. We are unlikely to want to harness more than 1/4th of that between now and 2050.


Intermittency

The primary problem usually raised by wind opponents is intermittent availability with significant daily, monthly, and seasonal variations. Probably the first person to address this issue systematically was Gregor Czisch for Western Europe. He analyzed 3 hour interval recorded wind speed (at ten meter average height above ground) for all areas that could provide 1500 or more full load hours per year, that is, minimum 17% full load factor. His analysis shows that:

5 minute correlation is near zero at 20 kilometers
12 hour correlation is near zero at less than 1000 kilometers
24 hour correlation is near zero at 1800 kilometers
monthly correlation is near zero at 2500 kilometers.

Of course at the eighty meter hub height of a 1.5 megawatt turbine the full load hours and correlation distances would improve significantly. Archer and Jacobson (A&J) <1,2> found that for a small area only 500 by 700 kilometers centered in Kansas, averaged over eight wind-farm locations, the incidence of zero power wind was zero.

One turbine might be expected to produce 30% of rated kilowatt hour during a year. Using the 8 wind farm curve of average windspeed vs percent of time available, and assuming the ratings of the NEG/Micon NM82/150 turbine (nominal windspeed of 12 meters per second, cut in windspeed of 3 meters per second and cut out windspeed of 18 meters per second) the eight wind-farms produce 85.5% of nominal annual output and operate at or above nominal 38% of the time.

However this estimate understates probable performance for three reasons:

1. A&J used measured wind speed increase from ten meters to eighty meters on a few sites, generated a formula to be applied to all other sites where measurements at eighty meters were not available, and generated their curve using the estimated eighty meter windspeed. Because wind speed increase is not linear with height, and because power is proportional to the cube of windspeed, the upper half of the swept circle has more weight than the lower half. The "virtual" windspeed at the hub is higher than the estimated.

2. Turbine manufacturers specify performance parameters conservatively.

3. Measured upper level wind speeds tend to be slightly higher then estimated.

Therefore, as a conservative adjustment, to better reflect expected performance, the A&J <8> wind-farm curve was shifted right by one meter per second and performance recalculated. With this adjustment, for the selected turbines, the eight wind-farms can be expected to produce 111% of nominal energy in a year, and would be at more than 100% of nominal output 48% of the time. With wind turbine costs now at about $0.90 per watt installed, and amortization over thirty years at 6% the direct cost of electricity at nominal output would be 1.91 cents per kilowatt hour. If we increased the number of turbines by 33% the cost of electricity at nominal output would go to 2.54 cents per kilowatt hour, we would be at more than nominal output 58% of the time and we would generate 147% of nominal output energy per year.

If we added hydrogen fueled gas turbine backup at 40% of nominal power at a capital cost of $.60 per watt financed at 6% for thirty years we would be at more than nominal output 75% of the time and nominal electricity would go up to 3.14 cents per kilowatt hour. Total output would go to 157% of nominal. If the surplus energy is used to generate and store hydrogen at 75% efficiency (feasible with existing electrolysis and compression equipment), and the backup burns hydrogen to generate electricity at only 40% efficiency (greater than 50% should be possible with a CCGT), there would be at least 70% more hydrogen than needed to run the backup generator. The cost of the hydrolysis, compression and storage might push the direct cost for total nominal electricity to 3.5 cents per kilowatt hour. This cost is better than coal or natural gas at 2004 prices.

Now extend this approach to even more efficient 3 megawatt turbines and perhaps three times as many wind-farms spread over say 500 by 2000 kilometers and nominal power will be available close to 100% of the time, so the problem of intermittence can readily be overcome. However, to get there utility management would have to think in whole system terms and would have to cooperate over a large interstate geographic area, a couple of things they are not accustomed to doing.


A Possible Surprise

If it will scale up an even more exciting potential has been illustrated by a ninth grade Canadian girl. <6> A dual rotor turbine has the potential to harness lighter winds, lowering cut-in and cut-out speeds, and greatly increasing the harnessable wind resource. Alternatively the two-rotor approach could enable significantly smaller rotors for the same wind regime. The two-rotor approach might also lower turbine cost by enabling a more balanced design. There is some possibility that three rotors would provide additional improvements, but perhaps not enough to be cost effective.

Certainly this possibility calls for immediate analysis by the wind industry, even if the source might prove to be a bit embarrassing.


Operation

To make such a system work effectively we need three additional elements, good hourly to daily wind forecasting, computerized dispatching and load matching, and a well-integrated transmission network. The keys to smooth operation that have been listed by various experts are:

<> variable speed and power factor turbines
<> modern control systems with remote sensing and control
<> regional wind forecasting up to two days ahead
<> local wind forecasting up to two hours ahead
<> minimum start and stop transients
<> good smoothing of individual turbine outputs in wind-farm outputs
<> wind-farm policy integrated into regional utility policy.

All of these are common sense, manageable requirements.

Wind antagonists raise cost issues of connections to the grid, and the costs of ancillary services due to wind variability. In many cases the output from wind-farms can serve local communities, thus reducing load on regional grids. However large scale development of wind power will require upgrades of regional and national grids. Any energy policy must strongly address upgrading and development of the transmission infrastructure. Wind should be central to such planning and execution. This is simply not a wind specific issue. Several studies <3> have been done to cost the ancillary services with resulting estimates from 0.2 to 0.6 cents per kilowatt hour with wind from 5% to 20% of the local total energy supply. The worst case includes day ahead forecast errors of 50%. With a well integrated network of wind-farms as described in 3) above, these already very small costs would decline


Cost

In a 1995 disinformation effort, the coal industry sponsored a report developed by Resource Data International and published by the Center for Energy and Economic Development, projecting wind energy costs of 6.8 cents per kilowatt hour in 1995, remaining unchanged until 2010. <1> In a rebuttal, NREL estimated 5.3 cents per kilowatt hour in 1995, going to 3.5 cents in 2010. <1> The Lake Benton Wind Farm in Minnesota, now in production, produces electricity at 4 cents per kilowatt hour unsubsidized, using one megawatt turbines. With larger turbines the cost would be lower. Of course the cost will vary with wind class and siting issues, but for developments we are likely to see by 2010, the NREL estimate is looking good. We can expect average costs in the future to be cheaper than coal fired plants, with none of coal's environmental issues.


Objections <5>

The usual objections presented by wind skeptics are:

<> Bird kill
<> Unsightliness
<> Land area
<> Noise
<> Low energy returned on energy invested
<> Future like the past

In response to these objections one can state:

Bird kill - The only place that has posed a real problem was the Altamont pass in the 1980s, with small fast rotating turbines. There is no evidence that new large turbines, with slowly rotating blades, kill even as many birds as power lines do. <4>

Unsightliness - Surveys in Palm Springs (US) and Wales (UK) show that neighbors grow to like wind farms and find them attractive. Most wind farms in the USA will be sighted in areas that vary from rural to empty, where the issue is unlikely to arise.

Land area - Class 4 and higher wind areas available for wind development are 6% of total lower 48 land area. Of this area, less than 5% would be occupied by turbines, equipment, and access roads. Cultivation can be carried out almost to the base of the turbines, and livestock like the wind shadow.

Noise - Modern turbines have noise levels below 50 dbm (like a summer breeze in the trees) at distances of about 250 yards.

Low EROEI - A recent study at the University of Wisconsin-Madison finds that wind farms generate between 17 and 39 times as much energy as is required for their construction and operation. The Danish wind energy association comes up with an energy payback time of less than six months, or a return of over 60 for a thirty year life.

Future like past - Saying that wind will never happen, because it never has is like saying a one-year-old will never walk because he never has.


Benefits

Perhaps the major benefits are environmental. There is one well documented and quantified example to support this advantage: In 2001, Ontario Canada's five coal fired power plants were responsible for 20% of all greenhouse gases released in the province, 23% of all sulphur dioxide emissions, 14% of nitrogen emissions and 23% of mercury emissions. These plants are scheduled for closure by 2007.

More specifically, one can say for wind that:

<> It's not a source of nuclear waste.
<> It's does not despoil the land like strip mining for coal.
<> It does not damage fragile habitats, like drilling for coal bed methane.
<> It does not threaten the Arctic National Wildlife Refuge.
<> It's not a source global warming greenhouse gases.
<> It's not a source of fish-contaminating mercury.
<> It's not a source of acid rain.

Apart from clean, inexpensive power, the surprise benefits to the economy can be a drop in farm subsidies. Minnesota farmers earn less than $30 per acre with livestock, and $250 per acre with crops, but can earn $1,000 per acre from land rental for wind farms, and still have the livestock or crop.

The big benefit to operators is freedom from fuel price risk, and that benefit will only grow from an already very attractive level in 2004.


The Challenge

Several states have goal of getting 10% of their electricity from wind by 2015 or 20% by 2020. With declining availability of natural gas and oil, we will have to do much better than that on a national basis. The real goal should be to get perhaps 20% of our total energy (albeit a declining total) by 2030 or 2040.

A two megawatt wind turbine with a 30% duty cycle and 95% availability will generate 5.8 million kilowatts hour per year. Fifteen quads of wind power by 2030 would require 750,000 turbines, or 30,000 per year starting now. That is five times present world production capacity, but is probably a worst-case estimate. At three megawatts, 35% duty cycle and 15 quads we would need only 450,000. Building 15,000 to 30,000 turbines per year is no big deal for an economy that can build seventeen million cars, trucks, and busses per year, but still, we had better get cranking. It can't wait until after 2020.

Could the two-rotor design mentioned above reduce dramatically the number of installations needed? The wind industry needs to address this question urgently.

References:
1. http://www.stanford.edu/group/efmh/winds/winds_jgr.pdf

2. http://fluid.stanford.edu/~lozej/winds/winds.html

3. http://www.nrel.gov/wind/pdfs/grid_integration_studies_draft.pdf

4. http://www.awea.org/faq/sagrillo/swbirds.html

5. http://www.eere.energy.gov/windpoweringamerica/

6. http://www.alumni.ca/~walk4d0/sf11.html

Copyright 2004 CyberTech, Inc.

http://www.energypulse.net/centers/article/article_display.cfm?a_id=843

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