THE ENERGY CHALLENGE 2004 -- Wind

10.13.04

Murray Duffin, Retired

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 15 years. In 1992 the average size of installed wind turbine was 200 kW. In 2002 it was 1.4MW. Now almost all units being installed are over 2 MW. Total world installed wind energy in 1992 was 2.5 GW. By 2003 it had reached 40 GW, a CAGR of 30%/yr.

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 50m 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 KW and blade rotation speeds were such that considerable areas were excluded for environmental reasons, i.e. bird kill. Best wind speeds were 15-25 mph and it was also assumed that only 20% of the actual wind energy/km² could be converted to electricity.

Now for 1.5 MW turbines with hub heights near 80 meters, Archer and Jacobson¹ find class 3 winds are economic, and are available for about 20% of the lower 48 land area. They also have found that near shore coastal areas with suitable winds cover more than 2x the shoreline of the 1993 paper. Today turbines being installed are up to 3 MW and up to 5 MW 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 hr. interval recorded wind speed (at 10 m average height above ground) for all areas that could provide =>1500 full load hours (FLH)/yr, i.e. minimum 17% full load factor. His analysis shows that:

• 5 minute correlation is near zero at 20 km
• 12 hour correlation is near zero at < 1000 km
• 24 hour correlation is near zero at 1800 km
• monthly correlation is near zero at 2500 km.

Of course at the 80 m hub height of a 1.5 MW turbine the FLH and correlation distances would improve significantly. Archer and Jacobson (A&J)^1,2 found that for a small area only 500 by 700 km centered in Kansas, averaged over 8 wind-farm locations, the incidence of zero power wind was zero.

One turbine might be expected to produce 30% of rated kWh during a year. Using the 8 wind farm curve of average windspeed vs % of time available, and assuming the ratings of the NEG/Micon NM82/150 turbine (nominal windspeed of 12 m/s, cut in windspeed of 3 m/s and cut out windspeed of 18 m/s) the 8 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 3 reasons:

1. A&J used measured wind speed increase from 10 m to 80 m on a few sites, generated a formula to be applied to all other sites where measurements at 80 m were not available, and generated their curve using the estimated 80 m 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 1 m/s and performance recalculated. With this adjustment, for the selected turbines, the 8 wind-farms can be expected to produce 111% of nominal energy in a year, and would be at =>100% of nominal output 48% of the time. With wind turbine costs now at about $0.90/W installed, and amortization over 30 years at 6 % the direct cost of electricity at nominal output would be 1.91 cents/kWh. If we increased the number of turbines by 33% the cost of electricity at nominal output would go to 2.54 cents/kWh, we would be at =>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/W financed at 6 %for 30 years we would be at => nominal output 75% of the time and nominal electricity would go up to 3.14 cents/kWh. 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/kWh. This cost is better than coal or natural gas at 2004 prices.

Now extend this approach to even more efficient 3 MW turbines and perhaps 3 times as many wind-farms spread over say 500 by 2000 km. 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 9th 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 2 rotor approach could enable significantly smaller rotors for the same wind regime. The 2 rotor approach might also lower turbine cost by enabling a more balanced design. There is some possibility that 3 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 2 days ahead
• local wind forecasting up to 2 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 studies³ have been done to cost the ancillary services with resulting estimates from 0.2 to 0.6 cents/kWh 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¢/kWh in 1995, remaining unchanged until 2010.¹ In a rebuttal, NREL estimated 5.3¢/kWh in 1995, going to 3.5¢ in 2010.¹ The Lake Benton Wind Farm in Minnesota, now in production, produces electricity at 4 cents/kWh unsubsidized, using 1 MW 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.

Objections⁵

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 do⁴.

Unsightliness – Surveys in Palm Springs 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 6 months, or a return of >60 for a 30 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 ANWR.
• 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/acre with livestock, and $250 per acre with crops, but can earn $1,000/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 5 of their electricity from wind by 2015 or 205 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 2 MW wind turbine with a 30% duty cycle and 95% availability will generate 5.8 million kWh/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 3MW, 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 17 million cars, trucks, and busses per year, but still, we had better get cranking. It can’t wait until after 2020.

Could the 2 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

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