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Abstract
This white paper is a basic overview of wind turbines used for residential power generation. It explains how a wind turbine works, the basic theory, what types of wind turbines are available, the basic parts of a residential wind turbine system and how such a renewable energy resource can be used to supplement the power needs for a residential power consumer. Also covered is how these intermittent power generation systems can affect the power grid when used in a net metering mode.
Types of Wind Turbines
Wind Turbine electric generation is increasingly popular these days for several reasons. It appeals to some due to the wind being a clean renewable energy resource. The generous federal and local rebates and credits help to reduce the overall cost of its initial installation, shortening payback time. Net metering, which is a billing mechanism that credits the wind turbine owner for producing a surplus of electricity, without the need of costly batteries along with their replacement cost and maintenance, is another financial incentive.
Wind energy is actually kinetic energy of air molecules in motion. Wind, due to its basic physics is a cubic energy resource -- as the wind speed increases, the power available increases by the cube of the wind speed. This means that it’s very important to get the wind turbine where the average wind speed is the highest; typically this is done is with the use of a tall tower. Regardless of the turbine, going higher is better, and is a reliable way to increase performance in a wind generator. The wind turbine needs to be mounted above any natural or manmade wind barriers that can block or slow the wind flow.
There are two basic types of wind turbines: the horizontal-axis configuration, similar to the farmer’s windmill, and the vertical-axis type, resembling an egg-beater. The most commonly used and most efficient wind turbine, both commercial and residential, is the horizontal-axis type. These horizontal-axis wind turbines are usually constructed with two or three blades made from composite material such as fiberglass.
The horizontal-axis type wind turbine contains the following basic components. The primary component, the rotor (or blade), allows the wind energy to be converted into rotational mechanical energy in order to turn the shaft. The blade shape, pitch, and size are critical due to the amount of wind energy that can be transferred into mechanical energy. On the system pictured in Figure 1, the blades are rigidly attached to the alternator, however they are designed to change pitch during operation by twisting. The blades start out at a pitched-up position and as the turbine speeds up, the pitch flattens out. This lowers the start-up wind speed while increasing the operating efficiency at higher speeds. The main bearing is used to stabilize the rotating shaft and allow it to turn freely with low resistance.
Figure 1 Typical 10 kW Residential Wind Turbine
The next component is the drive train, which contains the gearbox, and linkage to drive the generator (Figure 2). Some of the simpler systems incorporate a direct drive system, which does not require a gearbox. The direct drive system connects the rotor to the generator/alternator. The generator then converts the rotational mechanical energy directly to electricity.
Figure 2. Typical wind turbine
A tower is required to raise and support the rotor or blades, drive system and generator in position for the rotor to catch the wind. As mention earlier, height above ground and obstructions is critical for the wind turbine to function at maximum output. Air density is lowered slightly when the wind turbine is elevated; however this is outweighed by the improvement of wind speed in almost all cases due to getting the wind turbine above ground level and into the wind stream.
The nacelle is a streamlined housing surrounding the gearbox and generator, providing weather protection and allowing undisturbed air flow across the mechanical components. Most small wind turbines have a tail that allows the turbine to align its rotor facing into the wind. On some models the tail can be used to adjust the yaw of the turbine allowing the turbine not to face directly into the wind, slowing the turbine as a type of speed control.
There are other components required to complete the system (as shown in Figure 3), however these components may vary depending on how the system is set up. All wind turbine systems require lightning arrestors for safety reasons. A very good ground system is required for the tower and all electronics. The wind turbines can become good lightning targets due to their height.
Figure 3. Wind turbine system block diagram
Similar to the residential PV system, the wind turbine may be set up with or without batteries. In most wind turbine systems the generator’s output, if not already DC, is feed to a rectifier to convert the AC to DC and then to a controller unit. The function of the controller unit may vary a little from system to system but its main purpose is to control and perform critical system functions, such as conditioning, filtering using capacitors and inductors, and coarse voltage regulation before feeding into the power inverter. Some incorporate circuitry for the proper charge on battery systems, while other controllers can switch in load resistors quickly to manage over-speed of the turbine during high winds and gusts and provide dynamic breaking or even changing the yaw of the turbine to control the rotor speed. In some configurations, rectification is performed outside the controller, as in Figure 3, but with filtering done internally. With battery installations, the controller can also be configured to monitor battery charge level and keep the batteries from overcharging, such as in Figure 4. Controllers also can contain anti-islanding circuitry to shut down the output power feeding into the grid in case of an outside power failure. This anti-islanding circuitry is a safety lockout system to keep line workers safe.
Figure 4. Residential wind turbine installation
The power inverters used for wind turbines are similar or identical to those used in PV systems. Some residential systems use both solar and wind for power sources. As with PV systems, the inverters are designed to produce a clean sine wave, with phase and frequency locking to the incoming AC power line. This is sometimes not the case, and this is where power quality issues can occur. If the waveform is not a true sine wave, the imperfections create harmonics that, when grid tied, propagates into the power system affecting the local grid. Usually a residential wind turbine system is used only to supplement the user’s own power usage and on the average only produces a few kilowatts or on a larger system tens of kilowatts of power, so the amount that one residential wind turbine affects the grid is usually not an issue. With widespread adoption in a neighborhood or a large system that dominates a circuit, significant impacts can occur. Wind power, as with PV, is not a constant power source, so this is another factor that impacts the health of the grid. The unpredictable and quickly-changing power output may not be well handled by traditional voltage regulation schemes, and low-cost inverters may introduce significant, but subtle frequency variations and interharmonic problems.
Wind turbine generation causes many of the same power quality issues as PV systems. The amount of generated power affects the system line voltage in the opposite manner of loads – increased generation raises the line voltage. Wind power in a region can dramatically increase or decrease in just a few moments, depending on the weather. Many voltage regulation systems are not designed for these types of load/generation shifts, and require more responsive voltage regulation. In Figure 5, a graph of line voltage is shown with generated kW from a wind turbine system. The step change in voltage at the end of the graph, where the turbine is disconnected, is apparent. Also, the generated power shows an almost continuous, slow fluctuation that is translated directly to the line voltage. Although the effect is relatively small here, it can be magnified with widespread wind generation in an area.
Figure 5. Voltage generated by wind turbine system, followed by abrupt disconnect
In addition to voltage regulation issues, the process of inverting the DC and synthesizing an AC 60Hz waveform can result in increased harmonic distortion and interharmonics. The phase lock to the utility line frequency will always have some amount of error, which can result in slow beat frequencies and interharmonics.
Conclusion
Most wind turbines for residential use are of the horizontal axis type with two or three blades made from fiberglass or other types of composite material. Even though there are many factors that control the overall efficiency of a wind turbine system, the height of the turbine above the ground, potential obstructions, the aerodynamics of the rotor, and the diameter of the rotor’s sweep area are the largest overall factors.
Wind turbines are not currently as popular as PV systems, due to the higher installation cost. Some other factors are that wind turbines require a location with an ample amount of wind to make it worthwhile, and almost always require a structure such as a high tower that many people would rather not see, at least in a residential neighborhood. Solar panels tend to blend in more readily, and are not visible for miles as are wind turbine towers, so their lower profile and lower cost tends to make PV systems much more popular with most home owners. The payback time on a wind turbine is very dependent on location and wind conditions; some areas would take in excess of 15 years or longer. As with all renewable energy sources, environmentally conscience-minded people will promote the use of wind power, and with the rising cost of fossil fuels they will become more popular. As this trend continues, it will be very important to monitor the effects of wind turbines on distribution power quality and voltage regulation.
Cowles Andrus, III
Communications Specialist
candrus@powermonitors.com
http://www.powermonitors.com
(800) 296-4120