Most small-scale wind turbines are steered by a wind vane attached to the generator housing (called the nacelle). The vane works in the same way that an old-fashioned weather vane does. When the wind blows hard enough to operate the turbine, the vane orients itself to point away from the wind. Under normal operating conditions, the plane defined by the blade rotation lies perpendicular (broadside) to the wind direction.
In a small-scale wind-power system, the speed of the blade rotation vares with the wind speed, resulting in variable-frequency AC from the generator inside the nacelle. This generator resembles the alternator in a motor vehicle. (Some manufacturers call it an alternator for that reason.) The AC from the generator is converted to DC by a rectifier circuit, and the DC charges a set of storage batteries.
The electricity for household appliances comes from these batteries either directly, in which case special DC appliances must be used, or by means of a power inverter that converts the low-voltage DC electricity from the batteries to 117 V AC at 60 Hz (in the United States) or 50 Hz (in Europe and some other parts of the world).
The plane defined by the blades is normally perpendicular to the axis of the vane, so that the wind blows straight at the blades. However, in a strong wind, the plane of the blades changes, so it no longer lies perpendicular to the vane axis. This adjustment reduces the wind load on the blades but allows the turbine to keep on working. As the wind speed grows stronger yet, the angle between the plane of the blades and the vane axis decreases until, at a certain speed, it becomes zero. Then the blades rotate in a plane that contains the axis of wind flow. The variation in the angle between the plane of the blades and the wind direction is called furling. It can be done in the horizontal plane (so the blades swing, or yaw, toward the left or right) or in the vertical plane (so the blades tilt up or down).
A wind turbine can also regulate its wind load by varying the blade pitch. When the blade pitch is small (the plane of each blade’s surface is nearly the same as the plane defined by the blades), the wind produces less torque in the system, and consequently less power, than when the blade pitch is large (the plane of each blade’s surface differs greatly from the plane defined by the blades). At low wind speeds, the blade pitch is at the maximum. As the wind speed increases, the blade pitch
decreases. If the wind speed becomes great enough, the blade pitch becomes zero.
Did You Know?
In extremely high winds, the blades can turn to zero pitch, furl completely, and lock in place. This maneuver reduces the load on the blades as much as possible, minimizing the
risk of structural damage. It also shuts down the turbine.
Stand-Alone System
A stand-alone small-scale wind-power system uses rechargeable batteries to store the electric energy supplied by the rectified output of the generator. The batteries provide power to an inverter that
produces a “clean” AC wave at 117 V. The very best inverters produce “true sine waves.” The second-best ones produce “modified sine waves.”
Some stand-alone systems use the battery power directly without any inverter at all, but this
arrangement will work only with appliances and devices designed to run from low-voltage DC. Figure 4-12 is a functional block diagram of a stand-alone small-scale wind-power system that can provide 117 V AC.
FIGURE 4-12 A stand-alone small-scale wind-electric system.
The use of batteries allows the system to produce usable power even if there’s not enough, or too much, wind for the turbine to operate. A stand-alone system offers independence from the utility
company. However, a blackout will occur if the system goes down for so long that the batteries discharge and no backup power source exists.
Interactive System with Batteries
An interactive small-scale wind-power system with batteries resembles a stand-alone system, but with one significant addition. If you suffer through a prolonged spell in which wind conditions are
unfavorable for turbine operation, the electric utility can take over to keep the batteries charged and prevent a blackout. A switch, along with a battery-charge detection circuit, connects the batteries to the utility through a charger if no power issues from the turbine. When wind conditions become favorable and the turbine supplies power again, the switch disconnects the batteries from the utility charger and reconnects them to the turbine generator and rectifier.
Most interactive small-scale wind-power systems with batteries never sell any power to the electric utility, even if the wind turbine generates an excess. Power only flows one way, from the electric power line to the batteries through a charging circuit and switch, and even that happens only when the batteries require charging and the wind turbine does not provide enough power to charge them. Figure 4-13 is a functional block diagram of this type of wind-power system.
FIGURE 4-13 An interactive small-scale wind-electric system with batteries.
Interactive System without Batteries
An interactive small-scale wind-power system without batteries also operates in conjunction with the utility company. You sell excess energy to the company during times of minimum demand, and buy energy from the company during times of heavy demand. You can keep using electricity (by buying it directly from the utilities) if wind conditions remain unfavorable for a prolonged period. Because this type of system has no batteries, it can be larger, in terms of peak power-delivering capability, than a stand-alone arrangement or an interactive system with batteries.
An interactive system without batteries, like the type with batteries, is designed to function with the help of the utility company, and does not offer the independence that a purist might desire. This factor does not represent a technical drawback, but it can pose a philosophical problem for anyone who desires to live completely off the grid. Figure 4-14 is a functional block diagram of an interactive small-scale wind-power system without batteries.
FIGURE 4-14 An interactive small-scale wind-electric system without batteries.