WIND GENERATORS for HOME USE
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BASIC FACTS
If your residence or business is located in a rural area with average annual wind speeds of at least 8-10 mph, a small wind power generator can be used to provide electricity to your house, lower your utility bill or supply an emergency backup power. This guide will tell how it works and things to know to make the right choice. Let's just start with a quick technical reference. Wind generators for home use range in size from several hundreds watts to tens of kilowatts with rotors typically up to 25 feet in diameter.
TYPES OF THE SYSTEMS
In general, there are three basic types of renewable energy power systems: Off grid, Grid-tie, and Grid-tie with battery backup.
Stand-alone (or off-grid) systems operate independent of the electric utility grid. Since the wind turbines do not store energy and can generate electricity only when there is sufficient air movement, for continuous power flow to your home the generated energy have to be stored in the batteries. Due to erratic energy flow from the turbines, the battery bank would have to be significantly oversized if your installation has no other power source. To reduce its size, off-grid wind systems are normally supplemented by a solar electric system or by auxiliary generators that are fueled from diesel or propane tanks.
Grid-Tied systems are connected parallel to the utility grid. The energy they generate is fed directly into the household wiring, which reduces the electricity consumption from the utility. Whenever the power produced by the turbine is greater than the household needs, the inverter will send the surplus to the grid. However contrary to common misconception, a batteryless grid tie system will not provide any back up during blackouts even when there is a sufficient air flow. During a power outage the inverter will automatically disconnect from the mains in order to prevent backfeeding into "dead" utility lines.
Grid-Tied systems with battery backup use special grid-tie inverters with an additional built-in transfer switch. They will reduce your utility bills similarly to grid tie systems. Under normal conditions, a portion of the energy in such systems is used to keep the storage batteries charged. During a blackout, the transfer switch will automatically disconnect the grid and will continue powering the whole house or selected loads from the energy accumulated in the battery bank.
The wiring routed from the turbine down the tower goes to the tower base junction box, from which you can run a cable to the home entrance junction box, and then to the rectifier, optional battery bank and an inverter. Some residential-grade turbines rectify the output AC voltage at the tower top, and supply it as DC. Commercially available home wind generators normally come with a control box that combines the functions of a rectifier and a battery charger. It provides a DC output typically suitable for 12V or 24V batteries or for inverters with low input voltage. If you see a model for sale whose description lists a DC output, it means an inverter is not included and has to be purchased separately. Higher power systems (>1 kW) are usually sold with an inverter. Such systems often use higher level rectified DC voltage, up to 600VDC to reduce conduction losses.
The wiring and installation has to be done according to National Electric Code® (NEC) published by NFPA. The NEC Article 694 covers specifically small wind electric systems up to 100kW. Other articles, such as 110, 250, 300, 310, 480, and 702 may also apply. Consult with the latest NFPA and local codes for all your design decisions.
HOME MADE WIND GENERATOR
If you are going to build a homemade system, consider the following. The commercially available wind turbines for home use are generally designed to generate maximum output at air speeds around 24-36 mph (10.5-16 m/s). In reality in most areas you rarely get such speeds. At the lower air velocity you obviously get less power. So, don't count much on the advertised ratings.
A horizontal axis turbine have to be mounted on a toll tower to harvest more energy. It is generally recommended to place it at least 10 feet (3 meters) plus the blade length above the top of any obstruction, such as a tree or a building within 300 feet (100 meters). That is why home wind generators are used primarily in rural areas. You need to check your local zoning rules which may impose certain height restrictions on any structures. Many zoning ordinances in US have a height limit of 35 feet or so. The horisontal wind power systems for homes are usually built with guyed towers, which consist of lattice sections, a pipe, and supporting guy wires. The wires are anchored to the ground to hold the tower erect. Such design lets you raise and lower the tower easily, but it occupies more land area.
Vertical axis (VAWT) systems are less efficient. However you don't have to deal with the tower. There is a lesser-know type of VAWT called Savonius, which is probably the easiest one to build at home. By the way, stay away from the blades made of metal to avoid an interference with TV and other sensitive electronics. Use materials that are transparent to electromagnetic waves, such as fiberglass or wood.
Since any inverter can operate only within a specific input voltage range, and the turbine's voltage vary all the time, you need a voltage regulator. If you feed the rectified DC voltage into a battery, it will act as a regulator. However, if the battery is fully charged, and your loads consume less energy than is generated by the turbine, the excess of the energy has to go somewhere. This is normally accomplished by diverting electric current from the alternator into a so-called "dump load".
Turbine manufacturers data sheets should provide power curves and specify cut-in and cut-out speeds. Cut-in speed is the minimum air speed needed to turn a wind turbine and produce electricity. Generally, for small installations an average air speed of at least 9 mph (4 m/s) is required . Cut-out speed is the maximum speed that a turbine can handle. A turbine should stops spinning at the speeds greater than the cut-out to prevent its damage. There are atlases that provide data on annual average wind speed for various geographical areas. A power curve however will not tell you how much kilowatt-hours you will produce at a certain average air speed. It all depends on how that average came about, i.e. if winds vary a lot, or if they blows at a relatively constant speed. For more information see wind power system's energy calculation.
Wind energy generators remain relatively expensive. A small turbine alone runs for $1,500-2,000 per kilowatt capacity. A complete professionally installed residential system typically costs $4,000-$6,000 per kW before credits and rebates. Generally, the higher capacity the lower price per kW. Depending on the rated output, the typical installation cost for a small home may be $15,000 to $40,000 before incentives. This is lower than an average cost of a solar electric system, which is $7,000/kW, but it's still not cheap. You just need to remember that a wind system will rarely provide its rated wattage. However, in a remote location without easy access to the grid, it may help you stay off-grid and avoid the high costs of extending utility power lines to your place. Of course, the cost will be less if you build your system by yourself, but then you may not get a warranty (except for the warranty on individual parts) or rebates.
REFERENCES AND ADDITIONAL INFORMATION
Small Wind Generators for Home - a consumer's guide
Homebrew Wind Power - a hands-on guide to building a home made wind system
Small Wind Generators for Home - a consumer's guide
Homebrew Wind Power - a hands-on guide to building a home made wind system
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