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 transmission lines.
Freestanding towers must be strong enough to support the weight of the wind turbine, but more importantly, they need to be strong enough to withstand the forces of the wind acting on the turbine and the tower itself. Towers must have a large foundation to counteract the tremendous forces applied by the wind, forces that could easily topple a weak or poorly anchored tower. Large amounts of steel and concrete are required to accomplish this task. This makes freestanding towers the most expensive tower option.
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