Wind Power Basics. Dan Chiras

in swept area. As an example, a wind generator with an 8-foot blade has a swept area of 200 square feet. A wind generator with a 25 percent longer blade, that is, a 10-foot blade, has a 314 square-foot swept area. Thus, a 25 percent increase in blade length results in a 57 percent increase in swept area and, theoretically, a 57 percent increase in electrical production.

      Wind Speed

      Although swept area is more important than density, wind speed is even more important in determining the output of a wind turbine. That’s because the power available from the wind increases with the cube of wind speed. This relationship is expressed in the power equation as V³ or V x V x V — wind speed multiplied by itself three times.

      Consider an example: Suppose that you mount a wind machine 18 feet (5.5 meters) above the ground surface on the grasslands of Nebraska. Suppose that the wind is blowing at eight miles per hour (3.6 meters per second). A friend, who knows how important it is to mount a wind machine on a tall tower, installs an identical wind turbine on a 90-foot (27-meter) tower. When the wind is blowing at 8 mph where your turbine flies at 18 feet, an anemometer on your friend’s 90-foot tower indicates that the wind is blowing at 10 miles per hour. Wind speed is 25 percent higher. What’s the difference in available power?

       Shopping Tip

      Because swept area is such an important determinant of the output of a wind turbine, we strongly recommend focusing more on the swept area of a wind turbine than on its rated power — at least until the industry can come up with a standardized way of measuring and reporting rated power.

      The power available in the wind can be approximated by multiplying the wind speeds by themselves three times. (Units aren’t important for this comparison.) For the lower turbine the result is 8 x 8 x 8 or 512. The power available to the wind turbine mounted on a 90-foot tower is 10 cubed or 10 x 10 x 10 which is 1,000. Thus, a two-mile-per-hour increase in wind speed, a paltry 25 percent increase, doubles the available power. Put another way, a 25 percent increase in wind speed yields an increase of nearly 100 percent. The important lesson is that because power is function of V³, a small increase in wind speed results in a very large increase in the power available to a wind turbine. This can result in a very large increase in the electrical output of a wind turbine.

      Although winds are out of our control, homeowners can affect the wind speed at their wind turbines by choosing the best possible site and by installing their machines on the tallest towers.

Closing Thoughts

      Wind is a tremendous resource available in many parts of the world thanks to the Sun’s unequal heating of the Earth’s surface. Depending on your location, you may be able to take advantage of offshore and onshore winds or perhaps mountain-valley winds. Prevailing winds and winds that are created between high-pressure and low-pressure air masses could become your ally in reducing your carbon footprint and meeting your own needs sustainably. Don’t forget to take into account ground drag that slows wind speed and robs you of additional energy and damages your wind turbine. Site wisely and you’ll be repaid day after day after day.

       CHAPTER 3

       WIND ENERGY SYSTEMS

      Wind-electric systems fit into three categories: (1) grid-connected, (2) grid-connected with battery backup, and (3) off-grid. In this chapter, we’ll examine each system and discuss the pros and cons of each. We’ll also examine hybrid systems, consisting of a wind turbine plus another form of renewable energy. This information will help you decide which system suits your needs and lifestyle. To begin, let’s take a look at two of the main components of wind systems, wind turbines and towers. Subsequent chapters contain more detailed discussions of these and other components.

Wind Turbines

      Most wind turbines in use today are horizontal axis units, or HAWTs, (explained shortly) with three blades attached to a central hub. Together, the blades and the hub form the rotor. In many wind turbines, the rotor is connected to a shaft that runs horizontal to the ground, hence the name. It is connected to an electrical generator. When the winds blow, the rotor turns and the generator produces alternating current (AC) electricity. (See the accompanying box for an explanation of AC electricity.)

      One of the key components of a successful wind generator is the blades. They capture the wind’s kinetic energy and convert it into mechanical energy (rotation). It is then converted into electrical energy by the generator.

The generators of wind turbines are often protected from the elements by a durable housing made from fiberglass or aluminum (Figure 3.1a). However, in many modern small wind turbines, the generators are exposed to the elements (Figure 3.1b).

      Most wind turbines in use today have tails that keep them pointed into the wind to ensure maximum production. However, some very successful turbines like those made by the Scottish company Proven (pronounced PRO-vin) are designed to orient themselves to the wind without tails. (More on them in Chapter 5.)

       AC vs. DC Electricity

      Electricity comes in two basic forms: direct current and alternating current. Direct current (DC) electricity consists of electrons that flow in one direction through the electrical circuit. DC electricity is the kind produced by flashlight batteries or the batteries in cell phones, laptop computers, or portable devices such as iPods.

      Most wind turbines produce alternating current electricity. In alternating current, the electrons flow back and forth. That is, they switch, or alternate, direction in very rapid succession, hence the name. Each change in the direction of flow (from left to right and back again) is called a cycle.

      In North America, electric utilities produce electricity that cycles back and forth 60 times per second. It’s referred to as 60-cycle-per-second — or 60 hertz (Hz) — AC. The hertz unit commemorates Heinrich Hertz, the German physicist whose research on electromagnetic radiation served as a foundation for radio, television and wireless transmission. In Europe and Asia, the utilities produce 50-cycle-per-second AC.

      AC electricity is also produced by electrical generators in hydroelectric and power plants that run on fossil fuels or nuclear fuels. No matter what form of energy is used to turn a generator, all of them operate on the principle of magnetic induction — they move coils of copper wire through a magnetic field (or vice versa). This causes electrons to flow through the coils, producing electricity.

Fig. 3.1a and 3.1b: Wind Turbine Design. (a) The generators in many small wind turbines are housed in a protective case made from aluminum or fiberglass. (b) Others, like this one, are not.

Towers

      Another vital component of all wind systems is the tower, discussed in more detail in Chapter 6. Residential wind generator towers come in three varieties: (1) freestanding, (2) fixed guyed, and (3) tilt-up (Figure 3.2).

      Freestanding towers may be either monopoles or lattice structures. Freestanding monopole towers consist of high-strength hollow tubular steel like that used for streetlight poles. Lattice towers consist of tubular steel pipe or flat-metal steel bolted or welded together to form a lattice structure like the Eiffel Tower or transmission towers used for high-voltage