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Wind is energy in motion, and wind power, the extraction of kinetic energy from wind and its conversion to useful energy, most often electrical, is also a form of solar energy as wind is created by the sun's heating the earth unevenly. As the sun heats the earth, hot air rises and cooler air rushes in, creating a pressure differential that drives air from one point or location to another, causing wind to blow. The magnitude of the blowing wind is the consequence of the pressure gradient between two locations. When the wind blows, its energy can be captured. The operational term is "when" —as the wind doesn't always blow. Consequently, wind energy must be viewed as an intermittent source. Nevertheless, wind and wind power have an ancient and honorable history.

Windmills were used in the Middle East by the eleventh century; in Europe, by the thirteenth; and extensively by the Dutch, in the fourteenth. But it was in Denmark by the 1890s that literally thousands of windmills were being used for generating electricity [2].

In the American West, from the 1880s onward, farmers and ranchers used small water-pumping windmills, and in rural areas in the 1920s, small windmills generated electricity for home appliances. However, with President Franklin D. Roosevelt's Rural Electrification Administration, in the 1930s, which electrified the countryside, windmills were laid to rest. The actual demise of windmills was due to an accident to a windmill operated by a public utility in Rutland, Vermont. In 1945, this windmill lost a blade while spinning, and the blade was hurled almost a thousand feet. That was the end of windmills until

Rutland Vermont Windmill
Figure 7.1. An offshore farm of hundreds of wind turbines in the North Sea off Denmark's northwest coast. (Figure courtesy of Vestas Wind Systems—Vestas Americas.)

the early 1970s, when the worldwide oil embargo sent oil prices soaring, and with it, worldwide interest in wind power revived [2].

A turbine, no longer a typical mill, now uses the mechanical energy provided by the wind to produce electricity. The turbine works the opposite of an electric fan. Instead of using electricity to move air, producing a cooling breeze, the wind turbine uses wind to make electricity. It's the wind that turns the long blades (not referred to as "propellers") that are connected to a drivetrain or shaft that drives a generator within the turbine housing, producing electric current. Figure 7.1 shows a field of turbines or mills, and Figure 7.2 displays inner workings of the turbines.

All turbines operate within a range of windspeeds, from about 8-16 to 65 miles per hour (mph). Under 8 mph, the rotors (blades and hub together) cannot turn the blades fast enough to generate electricity. The controller, (see Fig. 7.2) starts up the turbine at windspeeds of about 8-16 mph, and shuts off at about 65 mph, as turbines cannot operate at windspeeds of 65 and higher, as the generators overheat [3,4].

The energy sent to the generator by the spinning blades increases the voltage from 480 to 65,000 Volts. This current is sent through cables down the turbine tower to an underground transformer that boosts the voltage higher still—up to 400,000 volts (V). Higher voltages are far more efficient in trans-

Inside the Wind Turbine

Inside the Wind Turbine

Anemometer. Measures the windspeed and transmits windspeed 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 windspeeds of ~8-16 miles per hour (mph) and shuts off the machine at ~65 mph. Turbines do not operate at windspeeds above ~65 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 ~30 to 60 rotations per minute (rpm) to about 1200-1500 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 30-60 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 directions: 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.

Wing vane: Measures wind direction and communicates with the yaw drive to orient the turbines 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.

Figure 7.2. An exploded view of a wind turbine 's inner workings. Note the controller toward the rear. (Source: Department of Energy.)

porting over long-distance powerlines. In the underground cables this high voltage is moved to substations where the current is reduced. It then moves to above-ground powerlines and on for use in homes, where the voltages are 110 V for lights, TV, and small appliances and 220 V for clothes washers and dryers.

A small computer housed in a metal container attached to a tower connects to a central office and tracks the current running through the cables, recording the amount of electricity produced. It also records the speed of the spinning rotor, the temperature of the generator, wind direction, and windspeed [2]. To be effectively productive, 50-100 or more turbine towers must be situated together on what are now referred to as wind farms - requiring great parcels of real estate as each tower requires about 2 acres of land—-n the windiest areas. As windspeed increases with altitude, the taller the tower (turbine), the more wind it will capture. Consequently turbine towers now rise a hundred feet or more. And to generate greater amounts of electricity, today's largest, commercial wind turbines have blade spans of 104 meters—343 feet from tip to tip, and can produce 5.6 megawatts (MW) of electricity—enough to power 1000 average-size homes. Recently, an experimental turbine in Germany used blades spanning 416 feet, and the General Electric Company is developing blade designs topping 462 feet. Such massive units are anticipated to produce as much as 7 MW, but these are some years away [5].

Wind turbines are generally divided into two major categories: horizontal-axis turbines, which resemble a windmill, and the vertical - axis, or Darrieus, turbine, which looks for all the world like a huge eggbeater. Figure 7.3 shows realistic representations of each. Currently, the horizontal-axis turbine is the most common. To generate electricity this type of turbine captures wind energy with two or three propeller like blades mounted on a rotor sitting on top of a tower often well over 100 feet in the air. The blades are made of a resin-coated fiberglass to withstand years of battering by heavy winds. These turbines have variable-speed generators, as turbines need to run at constant speed to produce a constant flow of electricity. The generator regulates this flow. Should windspeed fall below 8 mph, as it often does, the generator will shut down as it cannot maintain a constant flow. All variable-speed generators can increase the range of windspeeds under which a turbine can operate, by switching gears when windspeed changes. This type of turbine has a yaw drive, used to keep the rotor facing into the wind, as wind direction changes. These are also referred to as "upwind turbines" with their blades facing into the wind [6] .

Vertical-axis turbines can accept wind from any direction. Furthermore, the eggbeater has its mechanical components at ground level, making them much easier to inspect and repair. One of the only eggbeater wind forms in the United States is at Altamount Pass, near Livermore, California. Because the vertical-axis wind turbine has its gearbox and generator at ground level, maintenance is simplified, but at ground level it cannot take advantage of the greater windspeeds and lower turbulence at the higher altitudes.

Horizontal and Vertical Axis Wind Turbines

Generator Nacelle —

Tower

Vertical axis wind turbine

Horizontal axis wind turbine

Figure 7.3. Horizontal- and vertical-axis wind turbines in profile. (Figures courtesy of GAO-04-766, Renewable Energy.) (Source: Izaak Walton League of America.)

TABLE 7.1. UnitsofMeasurementofElectricalPower

A (amperes) x V (volts) = Wa (watts) 1000 watts = 1 kW = kilowatt 1 kilowatt = (kW) = 1000 watts 1 megawatt = (MW) = 1000 kW or 1 million watts 1 billion kW = 1 terrawatt = TW 1 kW x 1 hour = 1 kwh (kilowatt-hour)

a The watt is the basic unit used to measure electric power.

Utility-scale turbines, those that contribute (sell) power to a regional grid, range in size from 50 kilowatts (kW) to as large as several megawatts. Table 7.1 indicates how electrical power is measured.

Electricity production and consumption are measured in kilowatt- hours (kWh), while generating capacity is measured in kilowatts or megawatts. If a power plant that has 1 MW of capacity operates nonstop for the 8760 hours in a year, it will produce 8,760,000 kWh. An average US household consumes about 10,000 kWh a year. On average, however, wind turbines operate at 40% of their peak total hours per year because of the intermittent nature of wind and time of year. This is the motivation for higher towers, longer blades, and more units per farm, or larger farms. Wattage production is their reason for being. General Electric ' s Wind Energy 3.6 - MW wind turbine is one of the largest ever erected. The larger the turbine, the more efficient and cost-effective. But they are also expensive.

In 2003, American wind farms generated 13 billion kWh—enough to light and serve 1.3 million households—but this was 0.3 of one percent of all the electricity generated in the United States, which represents a quadrupling of generating capacity between 1990 and 2003, to 6400 MW, and the Department of Energy projects continued growth through 2025. On a percentage basis, wind power capacity has been growing at a higher rate than other electricity generating sources. Additionally, according to the DOE, the US Midwest theoretically has enough wind power potential to meet a significant portion of the nation's electricity needs—but remains largely untapped. Two additional projections are worth contemplating. Wind energy potential is estimated at over 10,000 billion kilowatt-hours annually—that's 1 x 1013 or 10 trillion kWh— more than twice the total generated from all sources in the United States today. The potency of such numbers does inspire mulling [6]. So, for example, it is also worthy of note that annual nuclear energy production runs at about 765-770 billion kWh, and if wind power were just to equal that, it would require an area equal to the state of Minnesota. How many states would it take to achieve 10,000 billion? It does seem a bit of a stretch.

It has also been suggested that wind energy could easily generate 6% of the nation's electricity by 2020, as much as hydroelectric power does currently. Is that realistic? After all, if wind energy contributed about 0.3 of one percent of the country's total electricity in 2003, it would be necessary to increase its production 20-fold in the coming 15 years. That, too, seems like pushing the envelope, even though it has been shown that with regular maintenance, turbines should work at least 90% of the time. But it is also well known that wind doesn't always blow, and doesn't always blow at optimum speed. From Table 7.2, we see that of the seven wind power classes, only four are suitable. North and South Dakota, with the greatest wind power potential, have done little to utilize their wind's kinetic energy. The winds intermittancy and fickleness was vividly brought home to me with the account of the four- day delay in the America ' s Cup Race because of the lack of wind off the coast of Valencia, Spain, in April, 2007. Another major limiting factor is space. With even higher towers and larger blades, towers require greater space between one another or they will steal each other's wind. If turbines are placed in long, straight rows on flat farmland, those in front will grab the most wind, while those in back will be deprived.

Is windspeed the be all and end all? What about the overall environment? Tornadoes cut a swath through southern Indiana and northern Kentucky, flattening everything with a 20-mile-long, 0.75-mile wide path. And hurricanes are still fresh in our minds. Ice storms are no strangers to the US Northeast, as well as states bordering Canada. Adverse environmental conditions can damage turbines and increase downtime. Furthermore, although turbines can generate electricity, are they located near population centers where electricity

TABLE 7.2. Wind Energy Classification3

Wind Power Class

Potential

Windspeed at 50 m (mph)

1

Unsuitable

>12.5

2

Marginal

12.5-14.3

3

Fair

14.3-15.7

4

Good

15.7-16.8

5

Excellent

16.8-17.9

6

Outstanding

17.9-19.7

7

Superb

19.7-24.8

a Estimates of wind resources are expressed in wind power classes, at 50 m above ground level. Class 1, with 11 Eastern states is wholly unsuitable as a resource. Class 2, with 14 states from the Atlantic seaboard to the Midwest, is marginal, and 5 states are fair, leaving 20 states with good to supberb sites scattered west of the Mississippi River where population centers are few and far in between. The only superb areas are the Aleutian Islands off Alaska.

a Estimates of wind resources are expressed in wind power classes, at 50 m above ground level. Class 1, with 11 Eastern states is wholly unsuitable as a resource. Class 2, with 14 states from the Atlantic seaboard to the Midwest, is marginal, and 5 states are fair, leaving 20 states with good to supberb sites scattered west of the Mississippi River where population centers are few and far in between. The only superb areas are the Aleutian Islands off Alaska.

is needed? Long-distance powerlines can be more expensive than the towers. Also, how expensive are they to maintain? Is there sufficient experience with wind turbines to determine their price competitiveness per kilowatt-hour with other renewables, but especially the nonrenewables, oil and coal? Answers need to be pinned down before decisions are finalized.

Wind energy appears to be the fastest-growing energy source in the world because of its many advantages, including the fact that it's a clean source with no polluting byproducts. It is, of course, home-grown, so to speak; it does not need to be imported. No one owns it, and it is abundant. As long as the earth is heated unevenly, and there is no reason to think that that will change, there will be wind. That being the case, it can be considered inexhaustible. It also appears to be price-competitive at 0.4-0.6 cents per kilowatt-hour. For farmers and ranchers, on whose vast acreages wind farms are being located, this means additional rental income, while crops and herds go undisturbed, as both are well beneath the spinning blades.

With all that, there is a downside. Wind power must compete with traditional energy sources on a cost basis. Depending on the wind site, and its energy, the farm may not be competitive. Although the cost of wind power has decreased substantially over the past decade, the technology requires higher initial investments than do fossil-fueled plants. Of course, the major downside is the inter-mittency of the wind, which may not blow when electricity is needed, and wind energy cannot be stored, nor can it be harvested to meet the timing of electricity demands. Moreover, some of the best wind sites are in remote areas, far from population centers, where the demand is. In addition, as noted earlier, long-distance lines are expensive. Expense must also be considered when wind farms must compete for available land that can offer other lucrative uses, which may be more valuable than the generation of electricity.

Although wind farms appear to have little adverse impact on the environment, there is growing concern about the noise produced by the whirling blades, the aesthetic impact of hundreds of turbines, and the birds killed flying into the rotors. In fact, at the moment, a number of events typify the pros and cons of wind power.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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