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WIND ENERGY

Everyday across various parts of the world, a potential source of energy can be felt by millions of people.  It blows right by a person, around a car, and through the trees. It is the wind.  Wind power is a fast growing source of clean, renewable energy that can be harvested out of the air.  The key to harvesting this energy is the wind turbine and the advancements in technology that have been made and are currently being developed related to it. Wind turbines use giant rotor blades to turn the kinetic energy of the wind into electrical energy that can be used to light homes, heat buildings, and power the world.  This section of the Wikid Energy Funhouse provides details about wind energy, including: the advanced technology and engineering that has gone into the design of wind turbines; the amount of wind resources that are available and where; the advantages and disadvantages of wind power; the limitations of wind power; the technological advances that need to occur to make wind energy a viable large scale contributor to the United States', and the World's, energy needs.

Wind Energy History


Wind has been recognized and utilized as a valuable source of energy since ancient times.  The Egyptians were the first to utilize the wind to power sailboats.  Ever since then, wind has been used to perform labor intensive tasks, such as grinding grain and pumping water.  In modern times, wind was first utilized to produce electricity in the mid 1900's for rural farmhouses that were not connected to the national power grid.  However, as the grid expanded and all American homes were connected to the grid, the need for wind turbines diminished.  It wasn't until the late 1970's and early 1980's that wind made a comeback as a potential source of electricity.  This comeback was due in large part to the rising cost of oil and new government incentives to spur renewable and domestic sources of energy.  As of 2007, wind power production accounts for approximately 0.77% of the total power consumption in the United States. This number has steadily grown over the past decade and is continually increasing as the country seeks to reduce its carbon-footprint and become less depended on foreign sources of energy.

Wind turbines started out as relatively simple instruments to turn the kinetic energy of the wind into electricity.  Now, wind turbines are highly sophisticated pieces of engineering.  This level of engineering occurred as wind power became a larger, more viable source of electricity.  Higher efficiencies are continually being sought after, further increasing the engineering demands placed on wind turbine designs.  Details about wind turbines and the engineering involved in their operation can be found in the next several pages of this site.

Wind Resources

Source of Wind Energy


Wind energy comes from solar energy.  The sun emits solar radiation that causes unequal heating of the Earth's water and land.  This produces high and low pressure regions, which creates pressure gradients between the regions.  The gradients must be minimized according to the second law of thermodynamics.  The second law states that the lowest energy state is wanted in order to maximize entropy.  The lowest energy state occurs when air from the high pressure regions move to the regions of low pressure.  This is known as wind.  Moving air in the form of wind has kinetic energy.

Ideal Locations for Wind Power in the U.S


It is estimated that there is 2 x 1013 W of wind energy available in the world today.  Industry experts believe that 20% of the United States' energy needs could be met with wind energy by 2030.[#5]  In 2008, wind power generated 52TWh of electricity, which was 1.3% of the total consumption in the U.S. of 4108.6 [#6]

Figure 1 below shows the average annual wind power estimates at 50 meters above the ground.[#7]. This map shows the annual average wind power estimates at 50 m above ground. It combines high and low resolution data sets that have been screened to eliminate land-based areas unlikely to be developed due to land use or environmental issues. In many states, the wind resource has been visually enhanced to better show the distribution on ridge crests and other features.[#7]

Figure 1. Wind Power Estimates at 50 m Above Ground

Installed Wind Capacity in the U.S.


The next map, Figure 2, details the current installed wind power capacity in MW, as of January 31, 2009.  As of this date, there was 26,274 MW of wind power installed in the U.S.[#8]

Figure 2. Current Installed Wind Capacity Map at December 31, 2011 [#8]

Figure 3 shows the cumulative installed capacity in MW from 1999 to 2011.  The different colors represent different states.  California led the way in the installation of wind power, starting in 1999.  Texas, however, has caught and surpassed California as the state with the most installed wind power capacity currently in the United States.[#9]

Figure 3. Cumulative Installed Capacity (in MW) for States from 1999 to 2011 [#9]

Energy Analysis of Wind Power

Overall energy auditing can be measured in terms of life cycle analysis, which includes energy required to manufacture, install, operate, and dismantle a given energy system. Two common factors in life cycle analysis include Energy Payback Ratio (EPR) and the Energy Payback Period (EPP). EPR is the ratio between energy developed and consumed by the energy system. EPP is an estimate of the time required for an energy system to payback all of the energy it consumes. Figure 4 shows the EPR for various common energy sources and Figure 5 depicts the EPP for the same common energy sources. The life period for the coal, fusion, and fission power plants in Figures 4 and 5 above is 40 years, whereas gas and photovoltaic (PV) is 30 years, and wind is 20 years. The gas, coal, fusion, and fission power plants are assumed to generate electricity 75 percent of the time. The conversion efficiency of the PV plant is six percent. [#10] The values for fusion are based on theoretical plant designs since no fusion plants are in operation. 



Figure 4. Energy payback ratio of various energy sources[#10]

Wind turbines are far more energy efficient than other common sources of energy with an energy production to energy consumption ratio of 33:1. As a comparison, nuclear fusion has a ratio of 26:1, nuclear fission is 16:1 coal is 11:1, solar is 7:1, and natural gas is 6:1.[#10]



Figure 5. Energy payback period of various energy sources[#10]

Wind farms have an energy pay back time of approximately one year. Due to rapid advances in wind technology, however, the energy payback times are becoming shorter and shorter.[#10]



Figure 6. Life Cycle Emissions of Carbon Dioxide for Various Energy Sources[#10]



Figure 7. Life Cycle Emissions of Sulfur Dioxide for Various Energy Sources[#10]



Figure 8. Life Cycle Emissions of Nitrogen Oxides for Various Energy Sources[#10]

Figures 4 to 8 show emissions throughout the life of the energy sources. Note that life cycle emissions for wind energy account for manufacturing, installation, and maintenance.[#10] Based on a feasibility study for 20% wind energy by 2030 conducted by the US Department of Energy in May 2008, carbon dioxide produced from electricity generation will be reduced by 825 million tons by the year 2030 as Figure 9 depicts.[#10]

Figure 9: Carbon dioxide emissions reduction due to wind energy by 2030

By 2030, wind could offset 50% of natural gas generated electricity and reduce natural gas consumption in all industries by 11%. Wind would offset 18% of coal generated electricity thus eliminating 80 GW of new coal capacity. Additionally, water consumption during electricity generation could be reduced by 8%, or 4 trillion gallons, with savings of 30% in western states where water is rather scarce.[#11]

Environmental Impact of Wind

Wind can be thought of as an indirect source of solar energy since it is created when the sun's radiation creates temperature changes between air masses. Therefore, as long as the sun is heating the earth (for another 4+ billion years), the wind can be considered a renewable source of energy.[#12]

Emissions


The main advantage of wind energy is it produces no emissions once the turbines are up and running. According to EPA estimates, running a 1 MW wind turbine for one year eliminates the following pollutants from entering the atmosphere: 1500 tons of carbon dioxide, 6.5 tons of sulfur dioxide, 3.2 tons of nitrogen oxides, and 60 pounds of mercury.[#13]

Figure 10. Summary of Life Cycle Emissions for Various Power Plants

Amount of Land Needed


A utility-scale wind plant will require approximately 60 acres per MW of installed capacity in an open, flat terrain.  Only 3 acres (5%) or less of this area is actually occupied by turbines, access roads, and other equipment.  The rest of the land remains free for other uses, such as farming.  On a ridge line in hilly terrain, a wind plant will require much less space, as little as two acres per MW.[#14]

Wind Turbines - Technological Aspects

How They Work


Wind turbines use wind to generate electricity. The kinetic energy in the wind is converted into rotational motion by the rotor, which usually consists of three blades.  Then, the shaft is turned by the rotor which transfers the motion into the nacelle. Finally, the output shaft is connected to a generator that converts the rotational movement into electricity.  Figure 11 below shows the parts of a wind turbine. [#1]

Figure 11. The Components of a Wind Turbine

The generator uses electromagnetic induction to generate medium voltage electricity, which is equivalent to a few hundred volts.  The electricity can then be used immediately or stored for later use.  The electricity goes down through heavy electric cables to a transformer.  The transformer can increase the voltage to a few thousand volts.  The electricity from wind power can be sent to farms, homes, and towns.  If the electricity is to be sent to cities and factories, the electricity is sent to a substation where voltage is increased to a few thousand volts.  The electricity is then sent through transmission lines.

GE 2.5 MW Turbine


 The following figure12 is the wind turbine from General Electric Inc. It is designed of 80 meters of tower height and 100 meters of rotor diameter, with the speed of rotor at 14.1 rpm. The capacity factor of 41.9% is used. Also since it is permanent magnet generator, it has long payback period. The pitch control system increases energy capture, reduces extreme loads, and provides aerodynamic breaking.

Figure 12. GE 2.5 MW Turbine

The GE 2.5 MW turbine generates the power of 2.5 MW between the wind speeds of the cut in speed of 3 m/s and cut out speed of 25 m/s. The power curve is shown in the following figure 13.

Figure 13. GE Power Curve [#28]

Blade Design


Blade Designs:

Drag Design: Drag design is where the wind pushes the blades out of the way.  Drag design blades are pushed by the wind and are limited by the speed of the wind.  They are usually characterized by slower rotational speeds and high torque capabilities. Drag design is typically used for small scale home turbines.

Lift Design: The lift design employs the same principles that enable planes to fly.   A wind speed and pressure differential is created between the upper and lower surfaces of the blade when air flows past it.  The air sliding along the upper surface will move faster than the lower surface air.  This causes the lower surface pressure to be greater than the upper surface. This creates the lift, which is perpendicular to the direction of the wind.  When the blades are attached to a central axis, the lift causes a rotational motion.  Lift design blades typically have high rotational speeds.  Therefore, lift design blades are better for electricity generation than drag design blades.

Possible Advances in Blade Design

Advances in turbine blade design technology technology are required to make wind a viable large-scale producer of electricity. Blades need to become lighter, stronger, more efficient, and cheaper.  This will allow turbines to withstand stronger wind, produce more power, and become economically competative with coal or nuclear electricity production.

One possible advance in technology for blade design is placing tubercles, or bumps, on the edgeof the blade. This design pushes back the stall angle considerably, and the cut-out speed, allowing the turbine to operate at much greater wind speeds. An example of this possible design is shown below.[#3]

Figure 14. Model of tubercles on edge of wind turbine blade.[#3]

Betz' Law


The theoretical maximum rotor efficiency for any wind turbine is 59.3%.  This was proved in 1919 by the German physicist Albert Betz.  Betz' law states that, at a maximum, only 16/27 of the kinetic energy in the wind can be converted into mechanical energy with a wind turbine.  A proof of Betz's law can be seen at either http://www.windpower.org/en/stat/betzpro.htm or http://en.wikipedia.org/wiki/Betz%27_law

Wind Speed vs. Tower Height


Wind speeds, and hence power, vary with height.  The higher a turbine is placed off the ground, the greater the wind speeds it has to run on.  As wind moves across the surface of the Earth, it encounters friction from the turbulent flow over and around objects in its path (i.e. mountains, hills, buildings, trees, etc.).  This effects decrease with an increase in height until air is allowed to flow unhindered.[#4]Another way of viewing this is with the boundary layer approach.  As the wind travels across the Earth's surface, a boundary layer is formed that extends to some height off the ground.  The higher off the ground a turbine is placed, the higher the wind speeds will be.

The easiest way to calculate the increase in wind speed with height is with the power law method.  This power law equation is derived empirically from actual measurements.  Another method commonly used in Europe is logarithmic extrapolation.  However, while the power law method is less scientific, it is easier, it works well, and it is more conservative than logarithmic extrapolation. The power law equation is:

Wind speeds were extrapolated to a hub height of 60, 80,100,120, and 140 meters using equation shown below.  Data in table 1 shows energy production at each hub height and extrapolated wind speed. The following table 1 describes a case for estimating wind speeds at small city, of Esther ville, Iowa.

Table 1: height, mean wind speed, annual energy production

Height (m)

Annual Mean Wind Speed (m/s)

Annual Energy Production (MWh)

10

5.00

1818.28

60

6.53

4641.61

80

6.82

5776.28

100

7.05

5836.49

120

7.23

5945.47

140

7.39

6207.41

Corresponding equation used is shown below.

Wind Speed Distribution


Wind is never at a steady speed.  It is influenced by land terrain, height above the ground, and weather.  Wind speed variations over a period can be best described by the Weibull probability distribution function.

Wind speed frequency distributions are constructed to show distribution of wind speeds Figure 15 shown below represents given mean daily wind speeds from March 2011 to March 2012 taken at Estherville International Airport 10 meters off the ground.  Figure 16 shows a Weibull Distribution of those wind speeds.  Weibull Distribution is a probability density function for used for anticipating wind speeds for wind turbines.  A k factor of 2 was used for the weibull distribution.


Figure 15. Estherville International Airport wind speed frequency distribution


Figure 16. Weibull Distribution mean wind speed Estherville International Airport

Drawbacks of Wind Energy

Two of the disadvantages of wind energy are the variability of the wind and the environmental concerns that arise from the use of wind turbines.  Wind is extremely non-uniform in its patterns. Wind varies by geographic location, time of the year, and time of the day.  Since turbines can only produce electricity when the wind blows, they are useless on a calm day.  Environmental constraints include aesthetics, noise, and avian.  Large wind farms are an eyesore to some people who do not want the giant turbines in their backyard.  High-speed turbine blades produce a great deal of noise, though this is currently being improved with advances in technology.  The differential pressure gradients around the turbine blades can suck birds into the blades' path, potentially killing the wildlife.[#1]

Another disadvantage of wind turbines is the minimum and maximum wind speeds at which a turbine can successfully generate electricity.  Power produced by the wind turbine is proportional to the velocity cubed. Therefore, at small velocities negligible power is produced.  For most wind turbines, this "cut in" speed is approximately 4 m/s.  However, there also exists a "cut out" speed at which the turbine ceases to generate power and begins to operate under unsafe conditions. This "cut out" speed for average turbines is around 15 m/s.  Beyond this velocity, turbine blades begin to stall.  Wind turbines are usually recommended to operate in locations where the average wind velocity is between 7-10 m/s.

 The economic viability of wind is a fourth disadvantage of wind power.  High quality wind resources are often located in areas far from human habitation, thus requiring the generated electricity to be transmitted to populated areas, which introduces significant losses.

Limitations of Wind Energy


The limitations are both theoretical and technological.  Of all the wind energy that a wind turbine sees, the theoretical maximum amount of energy that can be converted into electricity is 60%.  For 100% efficiency, the wind speed behind the turbine blades would have to be zero.  Other limitations include the amount of power output a wind turbine can have. The power produced by a turbine is proportional to the wind velocity cubed.  This means, at low wind speeds (i.e. 4 m/s), there is a significant drop-off in the power produced versus moderate wind speeds (i.e. 7 m/s).  Also, the power produced is proportional to the swept area, or another way of saying the same thing, is its proportional to the blade length squared.  Clearly, infinite length blades are unrealistic.  Therefore finite blade lengths have to be used, which gets into the technological limitations.  Some of the technological limitations are:

Limitations on Blades: Additional limitations involve the wind turbine blades. The most common method for producing wind turbine blades is fiberglass. This involves cutting multiple sheets of fiberglass to the shape of the blade and molding them with resin between each fiberglass layer. Small imperfections develop in the surface of the blade as the resin cures which can lead to premature failure during operation.[#2]

Limitations on Structure: At high wind speeds, the turbines become unsafe to operate.  One of the main reasons for this is the vibrations caused by the high velocities. Because of this, the power output of a turbine must be limited to keep the turbine from being overloaded and/or from a catastrophic failure.

Limitations on Transmission: The modern electricity transmission grid is not so modern.  Built for the energy needs 100 years ago, the grid is not suited to transport electricity for long distances, which are usually required for wind turbines since high wind speeds are often found in less populated areas.  The problem is that transmission lines and the connections between them are too small for the amount of power companies want to squeeze through them.[#3]

Limitations on Energy Storage: Since wind turbines rely on wind to generate electricity, a calm day is a bad thing.  Currently the capability to store extra electricity when it's not needed is virtually zero.  This severely limits the practically of wind turbines as main stream power producers since they cannot provide a constant source of electricity.

Turbine Failures: Check out this wikid video to see what happens when turbines can't handle the wind.  The result is catastrophic.

Future Aspects

Grid Challenge


The grid challenge in the US, proves to be a hurdle in increasing national supply of wind energy. Since wind is greatly available in rural areas, and demand is in highly populated urban areas.

 
Figure 15. U.S. Population Density

Future Expectation

Figure 16. Expansion Possibility for the renewable sources [#27]

The U.S. wind industry experienced a banner year in 2008, once again surpassing even optimistic growth projections from years past. At the same time, the past year has been one of upheaval, with the global financial crisis impacting near-term growth prospects for the wind industry, and with significant federal policy changes enacted to push the industry towards continued aggressive expansion.

Transmission


Grid enhancement is required for all electricity-growth scenarios. Transmission is needed to relieve congestion in existing system, to improve system reliability for all customers, to increase access to lower-cost energy, and to access new and remote generation resources. Wind power requires more transmission due to remote locations of best wind.

Offshore Floating Wind Turbine

 

Figure 17. Offshore Wind Turbine Implementation

A floating wind turbine is an offshore wind turbine mounted on a floating structure that allows the turbine to generate electricity in water depths where bottom-mounted towers are not feasible. Locating wind farms out at sea can reduce visual pollution whilst providing better accommodation for fishing and shipping lanes. In addition, the wind is typically more consistent and stronger over the sea, due to the absence of topographic features that disrupt wind flow. Floating wind parks are wind farms that site several floating wind turbines closely together to take advantage of common infrastructure such as power transmission facilities.

Comparison to Solar Energy

Solar energy is the biggest sustainable competitor to wind energy; It is not simply comparing technologies, four different categories need to be analyzed and compared. The following table 2 and figure 18 show a brief comparison. It is important to notice that efficiency is given in an inverse form to make the figure possible.

Table 2. Comparison to Solar Energy

 

Emission (kg/kWh)

Cost (/kWh)

Area (ft^2)

1/Efficiency

Solar (Photo Voltaic)

0.032

$0.15

1.17*10^10

6.67

Wind

0.009

$0.09

1.02*10^7

3.33


Figure 18. Diamond Graph for Wind and Solar Energy Comparison
Emissions are calculated for the step of manufacturing, operation, transport, and removal. Cost is calculated based on the life time of the grid, capital and maintenance cost. Area is calculated with the total energy production per year and the unit production rate. Efficiency values are taken from credible sources. The smaller diamond is the better choice, so in the case wind generated electricity is currently the better choice.

Wind Turbine Implementation in Estherville, Iowa

Cost Analysis


Estherville, Iowa is one of the rural areas where the average wind speed is high, around 5~6m/s. Total area of the county is 5.2 square miles, and population is about 6,400 people. Energy rate is known at $0.075/kWh

Table 3 below shows an overall cost analysis breakdown of components. The American Wind Energy Association states that each MW of turbine capacity installed costs roughly $2,000,000.  Cost percentages of a GE 2.5 MW turbine per component was found and calculated according to this overall cost.  A Tax credit of $.01/kWh is available for wind turbines up to 50 MW resulting in a $50,000 dollar tax credit per turbine.  Overall cost of  a wind farm consisting of 18 turbines turned out to be $89,100,000.

Table 3. Cost Analysis

Land Area/Value


Wind farm of installed capacity of 42 MW needs a total of 2,110 acres.  It was then estimated that an installed capacity of 45 MW with a turbine rotor diameter of 100 meters needs an estimated 2,300 acres for a wind farm.  Pricing for an acre of farmland from farm real estate values of 2011 showed a cost per acre of farmland is $6,708 resulting in a total cost of $15,428,400 for purchasing the amount of land needed.  However, only a small fraction of this land is actually used for wind turbines.  What wind farm developers can do is enter into leasing agreements with the landowner.  This is what is called “good faith contacts” where wind developers can control the site while preliminary information is gathered for economic viability of installing a wind farm.  A wind developer will have a certain time period for this assessment outlined in the contract.  Landowners have no responsibility in this matter.  Payments in the form of single payments, fixed annual payments, share of revenue, or a combination of these payment methods can be received. 

Regulations


Some required regulations that wind farms need to go through are they must have an outside source hired to assess the environmental impacts to the surrounding land and community for installation of a wind farm.  A specific agency that handles these environmental assessments will be the Rural Utilities Service (RUS), an agency based out of the department of agriculture.  Some of the areas in which the agency assesses are listed below

--     Wildlife

--     Wetlands

--     Native Prairies

--     Plant Species

--     Floodplains

--     Cultural resources

--     Transportation

--     Noise

--     Human health and safety

--     Air quality

--     Water quality

--     Historic properties

--     Social and economic considerations

Wind farm developers also have a responsibility to the local population on increasing awareness about the impacts a wind farm will have on the community.  Public meetings are held where information is share, surveys are filled out, and public testimony is heard for either side.  Wind developer representatives are present at the meeting and would answer any questions a person may have about the project.  In some locations impacts on local Indian tribes is also taken into account.

Wind Energy Summary

Advantages of Wind Power


• Wind is a clean, renewable resource
• Wind energy is free and easy to harness with proper technology
• Energy payback ratio (EPR) - The ratio between energy extracted from the system and energy supplied to the system. 
   - A greater EPR implies a higher efficiency
   - Wind power has an EPR three times the size of a coal fired power plant
• Energy payback period (EPP) - An estimate of the time required for an energy system to payback all of the energy it consumes.
   - Wind farms pay for themselves in approximately eight months compared to about 3.5 years for a coal fired power plant.
• Pollution elimination - Running a 1 MW wind turbine for one year eliminates the following pollutants from entering the atmosphere:
   - 1500 tons of carbon dioxide
   - 6.5 tons of sulfur dioxide
   - 3.2 tons of nitrogen oxides
   - 60 pounds of mercury

Disadvantages of Wind Power


• Variability of the wind - Wind varies by geographic location, time of the year, and time of the day
• Environmental concerns -- Aesthetics, noise, and avian 
   - Large wind farms are an eyesore to some
   - High-speed turbine blades produce a great deal of noise 
   - The differential pressure gradients around the turbine blades can suck birds into the blades' path, potentially killing the wildlife
• Cut-in and cut-out wind speeds -- At low speeds, no power is produced, at high wind speeds, turbine operation becomes dangerous
   - Cut-in wind speed ≈ 4 m/s (≈ 9 mph)
   - Cut-out wind speed ≈ 15 m/s (≈ 34 mph)
   - Ideal wind speeds ≈ 7-10 m/s (≈ 16-22 mph)

Advances in Technology Required


Required Technological Advances for Blades: Investigations will be aimed at advanced materials, improved manufacturing processes, and more efficient blade designs in an attempt to make the blades lighter, stronger, and cheaper. The goals are to design a blade that can withstand high winds and maintain structural integrity for up to 30 years. These issues will become more pronounced as wind turbines become larger.[#1]

Required Technological Advances for Structures: Stronger, more durable materials are needed to make the turbines structurally sound at high wind speeds. Also, some sort of vibration dampening could be useful for limiting the amount of vibration the turbine feels.

Required Technological Advances for Transmission: Since some of the best wind energy is located in remote regions, an advance in transmission is necessary. Transmission systems need to be upgraded so the wind energy is readily accessible by regions with high electricity demands throughout the country.[#2]

Electric Utilities current electric grids are not set up to move large amounts of electricity from one region of the nation to another. Therefore, the grid needs to be redesigned to operate like an "electric superhighway" so customers across the country have access to the energy.  Also, political differences need to be worked out between states and the national government if any improvements are to be made on the grid.[#3]

Conclusions


Wind power is a fast growing source of clean, renewable energy.  The key to harvesting this energy is the wind turbine and the advancements in technology that have been made and are currently being developed related to it.  Wind energy has great potential to become a large player in the production of electricity in the U.S.  With the current technology, it is forecasted that wind could provide 20 to 30% of America's electricity needs by 2030.  New advances in technology will only increase the likelihood of this happening.

References


1. http://www1.eere.energy.gov/wind/wind_how.html

2. http://en.wikipedia.org/wiki/Wind_power

3. http://www.awea.org/faq/wwt_costs.html

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6."Wind Power." Wikipedia: The Free Encyclopedia. 9 April 2008. <http://en.wikipedia.org/wiki/Wind_power>

7.http://www1.eere.energy.gov/windandhydro/wind_potential.htm

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10. Mathew, Sathyajith (2006). Wind Energy. The Netherlands: Springer.

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15. http://www1.eere.energy.gov/windandhydro/wind_how.html#e

16. http://tfcenergy.com/the-technology.htm

17.Fairley, Peter. "Wind Turbine that Imitates Flippers Could Increase Efficiency." Discover Magazine. September 2008. 3 April 2009. <http://discovermagazine.com/2008/sep/15-wind-turbine-that-imitates-flippers-could-increase-efficiency>. 

18.Gipe, Paul. Wind Power: Renewable Energy for Home, Farm, and Business.  White River Junction, Vermont: Chelsea Green Publishing Company, 2004. 

19. Patel, R.M. (2006). Wind and Solar Power Systems. Boca Raton, FL: Taylor & Francis Group. 

20. http://www.eia.doe.gov/cneaf/solar.renewables/renewable.energy.annual/backgrnd/chap10i.htm

21. http://composite.about.com/od/inthenews/l/blsandia1.htm

22. http://www.nytimes.com/2008/08/27/business/27grid.html

24. http://composite.about.com/od/inthenews/l/blsandia1.htm

25. http://awea.org/utility/transmission.html

26. http://www.20percentwind.org/20percent_Summary_Presentation.pdf

27. 2008 Wind Technologies Market Report,” R. Wiser and M. Bolinger, US Department of Energy, Lawrence Berkeley National Laboratory, 2009

28. http://site.ge-energy.com/prod_serv/products/wind_turbines/en/2xmw/index.htm

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