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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

Readers Comments

Date Comment
George Fleming
10.12.04

Glad to see such a favorable article on wind power. Fine arguments and documentation. A Korean company called Wintec built some wind turbines with two rotors, a larger one upstream and a smaller one downstream. Prototypes were about 0.5 MW as I recall. However, I am not sure this company is still in business. I wasn't able to access their website today, www.wintec4nature.com, but printed several pages from it earlier this year.

Steve Sturgill
10.13.04

Murray, I need some help locating the UW EROEI study you mentioned. I very much want to believe that wind energy's infrastructural EROEI is high, but I need to see how the conclusion was reached. Please post a link or some additional information so that I can find the study. Thanks.

Tom Gray
10.13.04

Steve,

I think http://fti.neep.wisc.edu/FTI/pdf/fdm1063.pdf is probably what you are looking for.

Tom Gray American Wind Energy Association

Len Gould
10.13.04

Murray: A commendable article, and I must say somewhat reluctantly that I essentially agree with all your points except the feasibility of young Dayna Walker's proposal for multiple turbines. If you notice, she's not proposing counter-rotating turbines, but co-rotating turbines. It has long been well known, as even her own discussion points out, that increasing the blade coverage percent can increase power out, but it also severely restricts the upper range of wind speed which can be used before needing to shut down, witness the old prairie water pumps. Much science has already gone into sorting out the optimal figures for this and I don't see that her efforts, commendable as they are for her age, contribute anything new. However, if you suggested multiple co-rotating turbines where individual blade groups could be feathered and halted as the wind speed increased, you might have something, though there would definitely be turbulence problems as the remaining working blades passed the halted blades. Probably not worth it.

I'd always thought that wind generation was limited in reliability to a maximum of about 30% with many real-life installations not doing better than half that, but I've just come across a new contract just issued by Hydro Quebec for new wind farms (GE) on Canada's east coast. They've signed a solid contract with two developers for a total of 990 mW of wind turbines which will, by contract terms, generate 3,200 twhr per year of energy. That means they're going to generate at 36.87% on an annual basis including breakdowns, maintenance, and etc. It occurs to me that these guys have taken advantage of one of the complexities of wind power, which is that "it doesn't matter for reliability how fast the average wind speed is, what matters is how steady it is." !! Neat. If the wind speed is low but continuous, they simply need to install larger rotors, which costs very little extra. Find a location with steady, even, continuous wind speeds and you can do much better than even the places with the highest peak speeds and total power. The suppliers to Hydro Quebec state they can profitably sell wind power to the utility for Cdn$0.065 / kwhr with NO SUBSIDIES using GE turbines at an average cost of Cdn$1,800 / kw. Ov course hydro Quebec has the huge advantage of their enormous hydro generation facilities which can happily store the water whenever the wind is blowing, while releasing it for generation whenever needed. Looks like they're making an effort to become North America's greenpower supplier.

Len Gould
10.13.04

Steve: Try also the ExternE study at http://externe.jrc.es/infosys.html or the WNA at http://www.greatchange.org/bb-thermochemical-WNA_energy_analysis_of_power_systems.html . Eg at the WNA site the differences between diffusion fuel processing and centrifuge processing are laid out for you. (The fact that the Wisconsin study cited by Tom used only the obsolete diffusion process figures makes me suspicious of their hidden agenda) Overall, nuclear's input/output ratio is closer to double the number stated in the Wisconsin so-called "study".

Peter Bradford
10.15.04
An interesting and thorough article. Thank you.

However, your discussion of unsightliness did not mention a controversy that goes with the newer and larger towers, namely that they must be so tall that they are required to be topped by bright lights (in the U.S. at least) to warn off airplanes. This makes them especially controversial in mountainous areas with scenic ridgelines and tranquil night skies.

Wind turbine sighting prospects in such areas would be much improved if the wind industry would work with the Federal Aviation Administration to develop alternative warning systems. After all, the ridges are often in clouds anyway, so the lights can't be the sole warning system. A warning system that worked by sound or that turned on its lights only when a plane was in the area would reduce the intensity of sighting controversies substantially.

George Fleming
10.17.04
Mr. Gould's comment about blade coverage sent me looking for more information. The Wind Turbine Co. (www.windturbinecompany.com/technology/index.html), which has been developing two-blade downwind turbines, says: "It is well known that 2-blade turbines capture approximately 97% as much energy as 3-blade machines with the same rotor diameter." Several single-blade turbines have been built and tested, according to the book "Wind Power Plants" (Gasch and Twele: Solarpraxis AG, 2002). One of these was called MONOPTEROS (640 kW), and it looks something like a monster. These single-blade turbines worked, but the book does not say why they did not succeed in the market. I would like to see further comments on this subject by Mr. Gould and others.

Murray Duffin
10.17.04
Len, - Clearly I am not an expert on wind turbines, but I have searched for info. on multi rotor experience and find a paucity of information. What struck me about Dayna Walkers results was the sheer magnitude of the improvement. Also my experience tells me that experts often end up unable to think outside the box. It may be that Dayna's work represents nothing new and her results are just an artifact of poor or limited experimental design. I hope not. Intuitively, it seems to me that her approach can lower the cut-in wind speed, increase the low wind speed output, and that feathering of the downwind rotor could raise the cut-out wind speed. The result could be to increase the percent of time that nominal power was reached quite considerably, thus further mitigating the intermittancy problem. There was a brief note in the Scientific American a couple of months ago about an experimenter who wondered why some whales have bumps on the leading edge of their flippers. He wind tunnel tested wing sections with and without bumps and found that the bumps increased lift, reduced drag, and lowered stall speed. This seems like another outside the box idea for wind turbine design. Peter, there are always peripheral issues to address, like tower lights. Most wind farms will not be located in scenic mountain areas, and if the choice is energy scarcity or lights we will take lights, or as you suggest find other solutions. Most of the negatives that have been raised about wind are much more serious and it was those issues that I wanted to address. Murray

Len Gould
10.17.04
Murray: I agree, many experts often lock themselves into boxes with what they know and it often takes a non-expert to break out. Check Doug Selsam's multi-rotor designs at http://www.speakerfactory.net/wind.htm . This concept is particularly an interesting out-of-box development. The system eliminates costly controls by simply installing the long shaft on a spring-loaded tilt platform. In low wind each rotor sees full wind. At higher wind speeds wind pressure pushes the rearward rotors down until they are shadowed by the ones in front. I've been following this for a few years and he's making excellent progress in development. Should be cheap to mass produce.

A good discussion of blade fill percentage and count is at the Danish Wind Industry Org. site at http://www.windpower.org/en/tour/design/concepts.htm

" Two- and one-bladed machines require a more complex design with a hinged (teetering hub) rotor as shown in the picture, i.e. the rotor has to be able to tilt in order to avoid too heavy shocks to the turbine when a rotor blades passes the tower. The rotor is therefore fitted onto a shaft which is perpendicular to the main shaft, and which rotates along with the main shaft"

also at http://www.windpower.org/en/tour/design/optim.htm "The water pumping windmills to the left look very different from modern, large wind turbines. But they are quite sensibly designed for the purpose they serve: The very solid rotor with many blades means that they will be running even at very low wind speeds, and thus pumping a fair amount of water all year round. Clearly, they will be very inefficient at high wind speeds, and they will have to shut themselves down, and yaw out of the wind in order to avoid damage to the turbine, due to the very solid rotor. "

Len Gould
10.17.04
Further Wind turbines are built to catch the wind's kinetic (motion) energy. You may therefore wonder why modern wind turbines are not built with a lot of rotor blades, like the old "American" windmills you have seen in the Western movies. Turbines with many blades or very wide blades, i.e. turbines with a very solid rotor, however, will be subject to very large forces, when the wind blows at a hurricane speed. (Remember, that the energy content of the wind varies with the third power (the cube) of the wind speed). Wind turbine manufacturers have to certify that their turbines are built, so that they can withstand extreme winds which occur, say, during 10 minutes once every 50 years. To limit the influence of the extreme winds turbine manufacturers therefore generally prefer to build turbines with a few, long, narrow blades.

see http://www.windpower.org/en/tour/design/index.htm

One, two or three blades can capture as much power from a given wind as much more solid rotors simply by turning faster. Also, you can't get any energy from a wind if you slow it down too much. You wind up with static air behind your turbine and the wind just going around the whole thing. I've seen the math for this concept in a few places but can't find a reference right now.

Scott White
10.18.04
In response to Len Gould's comments about my study at the University of Wisconsin: there was no hidden agenda. When analyzing uranium enrichment, the diffusion process was used only because it was the sole enrichment process used in the U.S. at the time. While enrichment using gas centrifuges or lasers are considerably more energy efficient than gaseous diffusion, neither were being used in as of 1998, when the study was completed. At that time there were two gaseous diffusion enrichment plants in operation - one at Portsmouth, Ohio, the other at Paducah, Kentucky. As I understand it, only the Paducah plant is still operating. When enrichment plants using the gas centrifuge or lasers become built in the U.S., then this study should be updated.

As for the study you reference in the same response, I have yet to study it to see why it's results differ from my own.

Also, relating to wind energy, the study results of wind are currently being reexamined and I plan to present the updated results at the 2005 American Wind Energy Association conference in Denver next May. At the time the original study was published, 2 of the 3 windfarms analyzed did not have a full years' worth of production data to figure into the analysis, which meant the production data was based on the operators projections. Since there are now at least 6 years of generation data to base these projections, the results have changed somewhat. For more information on the wind results, see the open-file report for the wind data at: http://fti.neep.wisc.edu/FTI/pdf/fdm1092.pdf, which has more details then the refereed article Tom Gray pointed out.

Len Gould
10.18.04
Scott: Thanks for the new data. I didn't realize that the centrifuge system wasn't yet being use in the US. Perhaps to present a fair picture of the potential of nuclear power it might have been prudent to reference other countries systems

"Some reactors, for example the Canadian-designed Candu and the British Magnox reactors, use natural uranium as their fuel. "

"A number of enrichment processes have been demonstrated in the laboratory but only two, the gaseous diffusion process and the centrifuge process, are operating on a commercial scale."

Surely this last bit of data wasn't unavailable to you only 5 years ago.

"The gaseous diffusion process consumes about 2500 kWh (9000 MJ) per SWU, while modern gas centrifuge plants require only about 50 kWh (180 MJ) per SWU."

Graham Cowan
10.18.04
Actually I see White and Kulcinski's paper says the basis of its uranium enrichment energy estimates was in fact gas centrifuge enrichment. In its table 5 it shows, for fuel preparation, 1203 thermal TJ per electrical gigawatt-year. Supposing 33 percent heat-to-electricity conversion that's 1.26 percent of the yield of the fuel produced, which is indeed less than the roughly two percent that gaseous diffusion nowadays takes.

It's not as much less as I would have expected given the 50-fold reduction, mentioned here, in enrichment's electricity needs when centrifuges replace gaseous diffusion, but there are other parts to the fuel-preparation process besides enrichment, I suppose.

Windpower is favored in that paper, as it acknowledges, by its ignoring of the energy cost of storage, or as in current systems, of calling in other kinds of power plant that can pick up when the wind weakens.

If that would knock wind's lifetime EROEI from 27 down to 20, still beating the figure there computed for nuclear, that, in my opinion, would be just another illustration of the unimportance of EROEI (as long as it's above ~2).

This emerged in another discussion where solar-concentrating heat engines looked as if what I suppose is the dominant EI in making them, the mirrors, turned out to take only a few weeks' worth of their output to make. Labor was the scarce good they really wasted. Or, as a certain kind of advocate would say, job creation was one of their major benefits.

--- former hydrogen fan Graham Cowan
how personal mobility gains nuclear cachet

Scott White
10.18.04

Graham is correct in that our study did use gas centrifuge and not gaseous diffusion - I should have actually gone back and looked at it before responding earlier. My apologies. And to Len, yes we did know about the gas centrifuge and did opt to use that data instead of diffusion. I'm still not sure why the discrepancy is so large, but will not be able to revisit this until later in the week due to some other business.

And as Graham did note, we did clearly state that the wind energy results benefit from not considering energy storage. That is for another study. To compare apples to apples (and baseload to baseload) energy storage should be factored in when comparing intermittent technologies to baseload ones. However, in small enough doses, there is enough load-following capacity to both allow intermittent technologies (such as wind) onto the grid and to justify studying them as stand alone as we did.

Denholm and Kulcinski did a study on energy storage in 2003. The results here showed that the EPR of wind dropped from 23 to 17 using pumped hydro storage and to 10 with compressed air storage. This report can be viewed at: http://www.ecw.org/prod/223-1.pdf

 

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