WIND GENERATORS for HOME USE




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BASIC FACTS



If you are located in a rural area with average annual wind speeds of at least 8-10 mph, it is quite possible to use a small wind power generator to provide electricity to your house, lower the utility bills 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.
Most small wind turbines suitable for homes are horizontal axis fixed-blade three-phase permanent magnet systems. The rotation of their rotor varies with air speeds and therefore produces AC voltage with variable amplitude and frequency. This voltage is not usable directly by conventional household electrical appliances. It has to be rectified and then converted into a regulated constant-frequency AC compatible with the utility power. The conversion is done by a solid-state inverter, which operates as a switch mode power supply (SMPS). The reason wind systems usually use a 3-phase alternator is because rectified 3-phase voltage has ten times lower ripple factor than that of a single-phase circuit. Therefore it requires much lower value of smoothing capacitance to produce DC output with a given amount of ripple.

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 had no other power source. To reduce their size, off-grid wind systems are normally supplemented by solar electric systems or by auxiliary generators that are fueled from diesel or propane tanks.

Grid-Tied setups are connected parallel to the existing electric service. The energy they generate is fed directly into the household wiring, which reduces the electricity consumption from the utility. A special grid tie inverter synchronizes its operation with the mains. Whenever the power produced by the turbine is greater than your needs, the inverter will send the surplus to the grid. However contrary to common misconception, such a batteryless system will not provide any back up during blackouts even when there is a sufficient air flow. The frequency of the inverter in such a system is set by the power line. During power outage the inverter gets no reference voltage to operate. In addition, it is required to automatically disconnect from your wiring in order to prevent backfeeding into "dead" utility lines.

Wind generator schematic diagram This is a diagram of a typical wind-powered generator with a transformer-isolated output. Such batteryless configuration can be used if you are on grid or you have other sources of power.

Grid-Tied systems with battery backup use special grid-tie inverters with an additional built-in transfer relay. They can 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, an inverter and possibly an optional battery charger. Some residential-grade turbines rectify the output AC voltage at the tower top, and supply it down as DC. Commercially available home wind generators normally come with a control box that combines the functions of a rectifier and a charger. It provides a DC typically suitable for 12V or 24V batteries or for inverters with low input voltage. If you see a model for sale whose description specifies a DC output, it means an inverter is not included and has to be purchased separately. Higher power systems (>5 kW) are usually sold with an inverter. To reduce conduction losses, such systems often have higher-level DC-link voltage (up to 600VDC).

The generic requirements for wiring and installation are provided in National Electric Code® 2014. Its Article 694 covers specifically small household wind electric systems up to 100kW. Other articles, such as 110, 250, 300, 310, 480, and 702 may also apply. Keep in mind that the installation and operation of any turbine presents certain degree of risk. Consult with the latest 2014 NFPA, local codes and manufacturer recommendations 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 many 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. Also be sure to check local noise restrictions, which could be 10dB above a background noise. 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. I would 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 output varies all the time, you need a voltage regulator. In the above diagram it is a boost converter. If you feed a rectified DC voltage into a battery, it will act as a regulator. However, this is the least efficient approach, which requires carefully designed overcharge protection. It is always preferred to use PWM regulators- see schematics of various configurations. Note that if the battery is fully charged, and your electric loads consume less than what's 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" (NEC® calls it "diversion 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 a turbine can handle. It should stops spinning at the air streams 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 blow relatively steady. For more information see wind power system's calculation.

Wind generators remain relatively expensive. A small turbine alone runs for $1,500-2,000 per kilowatt capacity. At this, as I mentioned above, the capacity is usually specified at unrealistically high winds. A complete professionally installed residential system typically costs $4,000-$6,000 per rated 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 somewhat lower than an average cost of a solar system, which is $6,000/kW in 2013, but it's still not cheap. You just need to remember that a wind system will rarely provide its rated wattage. Nevertheless, 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 lower 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