The terms wind energy or wind power describe the process by which the wind is used to generate mechanical power or electricity. Wind turbines convert the kinetic energy in the wind into mechanical power. This mechanical power can be used for specific tasks (such as grinding grain or pumping water) or a generator can convert this mechanical power into electricity.
So how do wind turbines make electricity? Simply stated, a wind turbine works the opposite of a fan. Instead of using electricity to make wind, like a fan, wind turbines use wind to make electricity. The wind turns the blades, which spin a shaft, which connects to a generator and makes electricity. The electricity is sent through transmission and distribution lines to homes, businesses, schools, and so on.
Types of Wind Turbines
Modern wind turbines fall into two basic groups: the horizontal-axis variety, and the vertical-axis design, like the eggbeater-style Darrieus model, named after its French inventor.
Horizontal-axis wind turbines typically either have two or three blades. These three-bladed wind turbines are operated "upwind," with the blades facing into the wind.
Sizes of Wind Turbines
Utility-scale turbines range in size from 100 kilowatts to as large as several megawatts. Larger turbines are grouped together into wind farms, which provide bulk power to the electrical grid.
Single small turbines, below 100 kilowatts, are used for homes, telecommunications dishes, or water pumping. Small turbines are sometimes used in connection with diesel generators, batteries, and photovoltaic systems. These systems are called hybrid wind systems and are typically used in remote, off-grid locations, where a connection to the utility grid is not available.
Inside the Wind Turbine
Anemometer: Measures the wind speed and transmits wind speed data to the controller.
Blades: Most turbines have either two or three blades. Wind blowing over the blades causes the blades to "lift" and rotate.
Brake: A disc brake, which can be applied mechanically, electrically, or hydraulically to stop the rotor in emergencies.
Controller: The controller starts up the machine at wind speeds of about 8 to 16 miles per hour (mph) and shuts off the machine at about 55 mph. Turbines do not operate at wind speeds above about 55 mph because they might be damaged by the high winds.
Gear box: Gears connect the low-speed shaft to the high-speed shaft and increase the rotational speeds from about 30 to 60 rotations per minute (rpm) to about 1000 to 1800 rpm, the rotational speed required by most generators to produce electricity. The gear box is a costly (and heavy) part of the wind turbine and engineers are exploring "direct-drive" generators that operate at lower rotational speeds and don’t need gear boxes.
Generator: Usually an off-the-shelf induction generator that produces 60-cycle AC electricity.
High-speed shaft: Drives the generator.
Low-speed shaft: The rotor turns the low-speed shaft at about 30 to 60 rotations per minute.
Nacelle: The nacelle sits atop the tower and contains the gear box, low- and high-speed shafts, generator, controller, and brake. Some nacelles are large enough for a helicopter to land on.
Pitch: Blades are turned, or pitched, out of the wind to control the rotor speed and keep the rotor from turning in winds that are too high or too low to produce electricity.
Rotor: The blades and the hub together are called the rotor.
Tower: Towers are made from tubular steel (shown here), concrete, or steel lattice. Because wind speed increases with height, taller towers enable turbines to capture more energy and generate more electricity.
Wind direction: This is an "upwind" turbine, so-called because it operates facing into the wind. Other turbines are designed to run "downwind," facing away from the wind.
Wind vane: Measures wind direction and communicates with the yaw drive to orient the turbine properly with respect to the wind.
Yaw drive: Upwind turbines face into the wind; the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. Downwind turbines don’t require a yaw drive, the wind blows the rotor downwind.
Yaw motor: Powers the yaw drive.
Advantages and Disadvantages of Wind Energy
Wind power offers many advantages, which explains why it’s the fastest-growing energy source in the world. Research efforts are aimed at addressing the challenges to greater use of wind energy.
Advantages
Wind energy is fueled by the wind, so it’s a clean fuel source. Wind energy doesn’t pollute the air like power plants that rely on combustion of fossil fuels, such as coal or natural gas. Wind turbines don’t produce atmospheric emissions that cause acid rain or greenhouse gasses.
Wind energy is a domestic source of energy. The nation’s wind supply is abundant.
Wind energy relies on the renewable power of the wind, which can’t be used up. Wind is actually a form of solar energy; winds are caused by the heating of the atmosphere by the sun, the rotation of the earth, and the earth’s surface irregularities.
Wind energy is one of the lowest-priced renewable energy technologies available today, costing between 4 and 6 cents per kilowatt-hour, depending upon the wind resource and project financing of the particular project.
Wind turbines can be built on farms or ranches, thus benefiting the economy in rural areas, where most of the best wind sites are found. Farmers and ranchers can continue to work the land because the wind turbines use only a fraction of the land. Wind power plant owners make rent payments to the farmer or rancher for the use of the land.
Disadvantages
Wind power must compete with conventional generation sources on a cost basis. Depending on how energetic a wind site is, the wind farm may or may not be cost competitive. Even though the cost of wind power has decreased dramatically in the past 10 years, the technology requires a higher initial investment than fossil-fueled generators.
The major challenge to using wind as a source of power is that the wind is intermittent and it does not always blow when electricity is needed. Wind energy cannot be stored (unless batteries are used); and not all winds can be harnessed to meet the timing of electricity demands.
Good wind sites are often located in remote locations, far from cities where the electricity is needed.
Wind resource development may compete with other uses for the land and those alternative uses may be more highly valued than electricity generation.
Although wind power plants have relatively little impact on the environment compared to other conventional power plants, there is some concern over the noise produced by the rotor blades, aesthetic (visual) impacts, and sometimes birds have been killed by flying into the rotors. Most of these problems have been resolved or greatly reduced through technological development or by properly siting wind plants.
The Benefits of 20% Wind Energy by 2030
According to the American Wind Energy Association, if we increase our nation’s wind energy capacity to 20% by 2030, it would…
Reduce Greenhouse Gas Emissions
A cumulative total of 7,600 million tons of CO2 would be avoided by 2030, and more than 15,000 million tons of CO2 would be avoided by 2050.
Conserve Water
Reduce cumulative water consumption in the electric sector by 8% or 4 trillion gallons from 2007 through 2030.
Lower Natural Gas Prices
Significantly reduce natural gas demand and reduce natural gas prices by 12%, saving consumers approximately $130 billion.
Expand Manufacturing
To produce enough turbines and components for the 20% wind scenario, the industry would require more than 30,000 direct manufacturing jobs across the nation (assuming that 30% – 80% of major turbine components would be manufactured domestically by 2030).
Generate Local Revenues
Lease payments for wind turbines would generate well over $600 million for landowners in rural areas and generate additional local tax revenues exceeding $1.5 billion annually by 2030. From 2007 through 2030, cumulative economic activity would exceed $1 trillion or more than $440 billion in net present value terms.
History of Wind Energy
Since early recorded history, people have been harnessing the energy of the wind. Wind energy propelled boats along the Nile River as early as 5000 B.C. By 200 B.C., simple windmills in China were pumping water, while vertical-axis windmills with woven reed sails were grinding grain in Persia and the Middle East.
New ways of using the energy of the wind eventually spread around the world. By the 11th century, people in the Middle East were using windmills extensively for food production; returning merchants and crusaders carried this idea back to Europe. The Dutch refined the windmill and adapted it for draining lakes and marshes in the Rhine River Delta. When settlers took this technology to the New World in the late 19th century, they began using windmills to pump water for farms and ranches, and later, to generate electricity for homes and industry.
Industrialization, first in Europe and later in America, led to a gradual decline in the use of windmills. The steam engine replaced European water-pumping windmills. In the 1930s, the Rural Electrification Administration’s programs brought inexpensive electric power to most rural areas in the United States.
However, industrialization also sparked the development of larger windmills to generate electricity. Commonly called wind turbines, these machines appeared in Denmark as early as 1890. In the 1940s the largest wind turbine of the time began operating on a Vermont hilltop known as Grandpa’s Knob. This turbine, rated at 1.25 megawatts in winds of about 30 mph, fed electric power to the local utility network for several months during World War II.
The popularity of using the energy in the wind has always fluctuated with the price of fossil fuels. When fuel prices fell after World War II, interest in wind turbines waned. But when the price of oil skyrocketed in the 1970s, so did worldwide interest in wind turbine generators.
The wind turbine technology R&D that followed the oil embargoes of the 1970s refined old ideas and introduced new ways of converting wind energy into useful power. Many of these approaches have been demonstrated in "wind farms" or wind power plants — groups of turbines that feed electricity into the utility grid — in the United States and Europe.
Today, the lessons learned from more than a decade of operating wind power plants, along with continuing R&D, have made wind-generated electricity very close in cost to the power from conventional utility generation in some locations. Wind energy is the world’s fastest-growing energy source and will power industry, businesses and homes with clean, renewable electricity for many years to come.
Wind Energy Resource Potential
The United States has enough wind resources to generate electricity for every home and business in the nation. But not all areas are suitable for wind energy development. The Wind Energy Program measures the potential wind energy resources of areas across the United States in order to identify ideal areas for project development. For information on the program’s mapping activities and individual state maps, visit the Wind Powering America Web site.
Wind Energy Resource Potential and Wind Energy Projects
One of the first steps to developing a wind energy project is to assess the area’s wind resources and estimate the available energy. Correct estimation of the energy available in the wind can make or break the economics of a project.
To help the wind industry identify the areas best suited for development, the Wind Energy Program works with the National Renewable Energy Laboratory (NREL) and other organizations to measure, characterize, and map wind resources 50 meters (m) to 100 m above ground.
This map shows the annual average wind power estimates at 50 m above ground. It combines high and low resolution datasets 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.
Estimates of the wind resource are expressed in wind power classes ranging from Class 1 to Class 7, with each class representing a range of mean wind power density or equivalent mean speed at specified heights above the ground. This map does not show Classes 1 and 2 as Class 2 areas are marginal and Class 1 areas are unsuitable for utility-scale wind energy development.
Wind Energy Research and Development
The United States faces many challenges as it prepares to meet its energy needs in the twenty-first century. Electricity supply crises, fluctuating natural gas and gasoline prices, heightened concerns about the security of the domestic energy infrastructure and of foreign sources of supply, and uncertainties about the benefits of utility restructuring are all elements of the energy policy challenge. Wind energy is an important part of the diverse energy portfolio that is needed for a stabile, reliable energy sector in the United States.
The promise of wind energy is immense; however, reaping the full benefits from this technology rests heavily on sustaining aggressive research, development, and support programs.
In order to expand wind energy’s contribution to the nation, the Wind and Hydropower Technology Program’s wind energy research focuses on the two elements of its mission:
* Increasing the technical viability of wind systems, and
* Increasing the use of wind power in the marketplace.
What are wind turbines made of?
The towers are mostly tubular and made of steel. The blades are made of fiberglass-reinforced polyester or wood-epoxy.
How big is a wind turbine?
Utility-scale wind turbines for land-based wind farms come in various sizes, with rotor diameters ranging from about 50 meters to about 90 meters, and with towers of roughly the same size. A 90-meter machine, definitely at the large end of the scale at this writing (2005), with a 90-meter tower would have a total height from the tower base to the tip of the rotor of approximately 135 meters (442 feet).
Offshore turbine designs now under development will have larger rotors—at the moment, the largest has a 110-meter rotor diameter—because it is easier to transport large rotor blades by ship than by land.
Small wind turbines intended for residential or small business use are much smaller. Most have rotor diameters of 8 meters or less and would be mounted on towers of 40 meters in height or less.
How much electricity can one wind turbine generate?
The ability to generate electricity is measured in watts. Watts are very small units, so the terms kilowatt (kW, 1,000 watts), megawatt (MW, 1 million watts), and gigawatt (pronounced "jig-a-watt," GW, 1 billion watts) are most commonly used to describe the capacity of generating units like wind turbines or other power plants.
Electricity production and consumption are most commonly measured in kilowatt-hours (kWh). A kilowatt-hour means one kilowatt (1,000 watts) of electricity produced or consumed for one hour. One 50-watt light bulb left on for 20 hours consumes one kilowatt-hour of electricity (50 watts x 20 hours = 1,000 watt-hours = 1 kilowatt-hour).
The output of a wind turbine depends on the turbine’s size and the wind’s speed through the rotor. Wind turbines being manufactured now have power ratings ranging from 250 watts to 5 megawatts (MW).
Example: A 10-kW wind turbine can generate about 10,000 kWh annually at a site with wind speeds averaging 12 miles per hour, or about enough to power a typical household. A 5-MW turbine can produce more than 15 million kWh in a year–enough to power more than 1, 400 households. The average U.S. household consumes about 10,000 kWh of electricity each year.
Example: A 250-kW turbine installed at the elementary school in Spirit Lake, Iowa, provides an average of 350,000 kWh of electricity per year, more than is necessary for the 53,000-square-foot school. Excess electricity fed into the local utility system earned the school $25,000 in its first five years of operation. The school uses electricity from the utility at times when the wind does not blow. This project has been so successful that the Spirit Lake school district has since installed a second turbine with a capacity of 750 kW.
Wind speed is a crucial element in projecting turbine performance, and a site’s wind speed is measured through wind resource assessment prior to a wind system’s construction. Generally, an annual average wind speed greater than four meters per second (m/s) (9 mph) is required for small wind electric turbines (less wind is required for water-pumping operations). Utility-scale wind power plants require minimum average wind speeds of 6 m/s (13 mph).
The power available in the wind is proportional to the cube of its speed, which means that doubling the wind speed increases the available power by a factor of eight. Thus, a turbine operating at a site with an average wind speed of 12 mph could in theory generate about 33% more electricity than one at an 11-mph site, because the cube of 12 (1,768) is 33% larger than the cube of 11 (1,331). (In the real world, the turbine will not produce quite that much more electricity, but it will still generate much more than the 9% difference in wind speed.) The important thing to understand is that what seems like a small difference in wind speed can mean a large difference in available energy and in electricity produced, and therefore, a large difference in the cost of the electricity generated. Also, there is little energy to be harvested at very low wind speeds (6-mph winds contain less than one-eighth the energy of 12-mph winds).
How many turbines does it take to make one megawatt (MW)?
Most manufacturers of utility-scale turbines offer machines in the 700-kW to 2.5-MW range. Ten 700-kW units would make a 7-MW wind plant, while 10 2.5-MW machines would make a 25-MW facility. In the future, machines of larger size will be available, although they will probably be installed offshore, where larger transportation and construction equipment can be used. Units up to 5 MW in capacity are now under development.
How many homes can one megawatt of wind energy supply?
An average U.S. household uses about 10,655 kilowatt-hours (kWh) of electricity each year. One megawatt of wind energy can generate from 2.4 to more than 3 million kWh annually. Therefore, a megawatt of wind generates about as much electricity as 225 to 300 households use. It is important to note that since the wind does not blow all of the time, it cannot be the only power source for that many households without some form of storage system. The "number of homes served" is just a convenient way to translate a quantity of electricity into a familiar term that people can understand. (Typically, storage is not needed, because wind generators are only part of the power plants on a utility system, and other fuel sources are used when the wind is not blowing. According to the U.S. Department of Energy , "When wind is added to a utility system, no new backup is required to maintain system reliability." Wind Energy Myths, Wind Powering America Fact Sheet Series, http://www.nrel.gov/docs/fy05osti/37657.pdf.)
What is a wind power plant?
The most economical application of wind electric turbines is in groups of large machines (660 kW and up), called "wind power plants" or "wind farms." For example, a 107-MW wind farm near the community of Lake Benton, Minn., consists of turbines sited far apart on farmland along windy Buffalo Ridge. The wind farm generates electricity while agricultural use continues undisturbed.
Wind plants can range in size from a few megawatts to hundreds of megawatts in capacity. Wind power plants are "modular," which means they consist of small individual modules (the turbines) and can easily be made larger or smaller as needed. Turbines can be added as electricity demand grows. Today, a 50-MW wind farm can be completed in 18 months to two years. Most of that time is needed for measuring the wind and obtaining construction permits—the wind farm itself can be built in less than six months.
What is "capacity factor"?
Capacity factor is one element in measuring the productivity of a wind turbine or any other power production facility. It compares the plant’s actual production over a given period of time with the amount of power the plant would have produced if it had run at full capacity for the same amount of time.
Actual amount of power produced over time
Capacity Factor = Power that would have been produced if turbine operated at maximum output 100% of the time
A conventional utility power plant uses fuel, so it will normally run much of the time unless it is idled by equipment problems or for maintenance. A capacity factor of 40% to 80% is typical for conventional plants.
A wind plant is "fueled" by the wind, which blows steadily at times and not at all at other times. Although modern utility-scale wind turbines typically operate 65% to 90% of the time, they often run at less than full capacity. Therefore, a capacity factor of 25% to 40% is common, although they may achieve higher capacity factors during windy weeks or months.
It is important to note that while capacity factor is almost entirely a matter of reliability for a fueled power plant, it is not for a wind plant—for a wind plant, it is a matter of economical turbine design. With a very large rotor and a very small generator, a wind turbine would run at full capacity whenever the wind blew and would have a 60-80% capacity factor—but it would produce very little electricity. The most electricity per dollar of investment is gained by using a larger generator and accepting the fact that the capacity factor will be lower as a result. Wind turbines are fundamentally different from fueled power plants in this respect.
If a wind turbine’s capacity factor is 33%, doesn’t that mean it is only running one-third of the time?
No. A wind turbine at a typical location in the Midwestern U.S. should run about 65-90% of the time. However, much of the time it will be generating at less than full capacity (see previous answer), making its capacity factor lower.
What is "availability" or "availability factor"?
Availability factor (or just "availability") is a measurement of the reliability of a wind turbine or other power plant. It refers to the percentage of time that a plant is ready to generate (that is, not out of service for maintenance or repairs). Modern wind turbines have an availability of more than 98%–higher than most other types of power plant. After more than two decades of constant engineering refinement, today’s wind machines are highly reliable.
How much does wind energy cost?
Over the last 20 years, the cost of electricity from utility-scale wind systems has dropped by more than 80%. In the early 1980s, when the first utility-scale turbines were installed, wind-generated electricity cost as much as 30 cents per kilowatt-hour. Now, state-of-the-art wind power plants can generate electricity for less than 5 cents/kWh with the Production Tax Credit in many parts of the U.S., a price that is competitive with new coal- or gas-fired power plants.
The National Renewable Energy Laboratory (NREL) is working with the wind industry to develop a next generation of wind turbine technology. The products from this program are expected to generate electricity at prices that will be lower still.
Why does the cost of wind energy vary from place to place?
The most important factors in determining the cost of wind-generated electricity from a wind farm are: (1) the size of the wind farm; (2) the wind speed at the site; and (3) the cost of installing the turbines. Each of these factors can have a major impact. Generally speaking:
* The larger the wind farm, all other factors being equal, the lower the cost of energy;
* The higher the wind speed, the lower the cost of energy;
* The less expensive construction costs are, the lower the cost of energy.
On New England ridgelines, for example, wind farms are likely to be smaller, to experience lower wind speeds, and to cost more to install than in the flat terrain of northern Plains states. While wind power may cost less than 5 cents/kWh in the northern Plains, it may cost 6-7 cents/kWh in New England.
In the case of offshore wind farms, the distance that power must be transmitted to shore is a fourth potentially significant cost element.
How do utility-scale wind power plants compare in cost to other renewable energy sources?
Wind is the low-cost emerging renewable energy resource.
What is the "production tax credit" for wind energy?
1.5-cent per kilowatt-hour1 production tax credit (PTC) for wind energy was included in the Energy Policy Act of 1992. Passage of the PTC reflected a recognition of the important role that wind energy can and should play in our nation’s energy mix. It also was intended to partially correct the existing tilt of the federal energy tax code, which has historically favored conventional energy technologies such as oil and coal.
Generally, the credit is a business credit that applies to electricity generated from wind plants for sale at "wholesale" (i.e., to a utility or other electricity supplier which then sells the electricity to customers at "retail"). It applies to electricity produced during the first 10 years of a wind plant’s operation. The company that owns the wind plant subtracts the value of the credit from the business taxes that it would otherwise pay.
The U.S. Congress recently (July 2005) extended the wind PTC to expire for the fourth time since it was created, through December 31, 2007. While the U.S. wind industry welcomed the extension, it noted that a longer term for the PTC is needed to provide a stable financial environment industry. Such a stable financial environment would allow the industry to reduce wind energy’s cost—for example, by allowing wind farm development companies to order wind turbines in larger quantities.
An incentive similar to the PTC is made available to public utilities (which do not pay taxes and therefore cannot benefit from a tax credit). The incentive is called the Renewable Energy Production Incentive (REPI) and it consists of a direct payment to a public utility installing a wind plant that is equal to the PTC (1.5 cents per kilowatt-hour, adjusted for inflation). However, since the REPI involves the actual spending of federal funds, money must be "appropriated" (voted) for it annually by Congress. It is sometimes difficult to obtain full funding for REPI because of competing federal spending priorities.
If wind energy is competitive, why does it need a tax credit subsidy from the government? Isn’t this government interference in the free market?
The energy market has never been free — large energy producers such as coal and oil have always been able to win government subsidies of various kinds. To take just one example, the federal government has paid out $35 billion over the past 30 years to cover the medical expenses of coal miners who suffer from "black lung disease." These subsidies mean that the true cost of coal is not reflected in its market price.
As the previous answer indicates, the wind PTC was passed by Congress to give wind a "level playing field" compared with other subsidized energy sources. More information on energy subsidies is available from the Renewable Energy Policy Project,
More generally, coal receives a huge hidden subsidy resulting from the fact that its full environmental and health costs are not accounted for. The hidden environmental and health costs of coal and other fossil fuels are also confirmed by a major 10-year study by the European Union.
Nuclear power and oil also benefit from hidden subsidies. The potential cost of damages that might result from an accident at a nuclear power plant are too large for the insurance industry to cover, so the federal government has pledged to act as "insurer of last resort" above a certain level of cost. The cost of oil does not reflect government military expenditures that are required to make sure that the shipping lanes to the Persian Gulf remain open.
If wind energy is competitive, why do "green power" or "green pricing" programs charge extra for it?
There are several reasons for the cost premium (typically 2 to 3 cents per kilowatt-hour) that most green marketers charge for wind-generated electricity. Among them:
1. If the power is being sold by a marketing company, it has to recover the cost of its marketing campaigns;
2. Whether the power is being sold by a marketing company or a utility, the sale is being done on a piecemeal basis. Often one turbine’s output is sold, then another’s, and another’s. Also, the term of the sales to retail customers is short, typically a year or two. This is more expensive, and risky, than buying all of the power from a 50-megawatt or 100-megawatt wind farm for 10 years.
If my utility uses more wind energy, will that make my electric rates go up?
Yes, probably, but not much. Let’s say that wind energy costs 2 cents more per kilowatt-hour (2 cents/kWh) than the rest of the electricity your utility is generating or buying—a conservative estimate. If your utility were to decide to use wind energy to generate 10% of its electricity (more than nearly all utilities in the U.S.), then the added cost to you would be 0.2 cents/kWh. An average U.S. home uses about 800 kWh per month, so you would pay an extra $1.60 per month, or about a nickel a day.
With the price of natural gas, oil, and other fuels soaring today (August 2005), wind energy is becoming more of a bargain than ever. A recent landmark study of wind integration into the New York State electric power system, looking at a 10% addition of wind generation (3,300 MW of wind in a 34,000-MW system), projected a reduction in payments by electricity customers of $305 million in one year.
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