A wind turbine captures the force of wind velocity with rotor blades. Rotor blades are designed to accelerate air flow over one surface of the blade, which results in a low pressure system at that surface. The difference in pressure between the two surfaces of the blade causes the blade to lift to the area of lower pressure just like an airplane wing. Lift applied to each of the rotor blades causes the rotor to spin. The rotor is connected to a shaft, and as the shaft turns, it rotates a generator that produces electricity. The electricity created in the generator is transmitted through wiring from the wind turbine hub down the tower to an interconnection with the utility’s transmission system.

The power generated by a wind turbine varies according to the average wind speed and wind speed distribution. Wind turbine and blade design will influence power generation. Each wind turbine design is rated to generate electricity at particular wind speeds. The wind speed at which a wind turbine will begin to produce power is referred to as the cut-in wind speed. Once wind reaches a turbine’s cut in speed, the wind turbine produces electricity although very little at low wind speeds. As wind speed increases, power generation increases until the wind reaches what is called the rated wind speed. At the rated wind speed, the wind turbine produces the maximum amount of power it is rated at. Wind turbines are also designed to shut down at very high wind speeds, referred to as cut-out wind speeds (typically over 50 miles per hour), that may cause potential damage to the system.

The wind turbine rating is the amount of energy or the kilowatts of power a wind turbine will produce at a specific wind speed.

For example, what does it mean if a wind turbine is rated as 1 kW? First, kW stands for kilowatt or one thousand watts. Watts are a measure of electrical power. A 1 kilowatt wind turbine does not mean that you will get 1 kilowatt of power all the time.

If the wind is not blowing, you will get zero power from any turbine, no matter what the kilowatt rating. The 1 kilowatt rating is based on a specific wind speed which will typically be between 25 and 35 mph. These wind speeds are higher than most operators will experience at their property except for maybe a few hours per year. Usually turbines are rated at a wind velocity where the generator is operating at maximum capacity.

Most electric devices require the use of alternating current (AC), the type of current supplied through power lines. In order for small wind power systems to be hooked up to the utility power lines, DC electricity generated by wind turbines needs to be converted to AC electricity. This can be achieved with the installation of a power inverter, which can convert DC current into AC current.

Wind Turbine:

Wind power refers to converting wind into usable work. The wind can be used to push on a surface that causes some type of work or rotation like the water pumping wind mills of the early 1900’s or the energy from the wind can be captured with a wing such as used with modern wind turbines. The blade acts like the wing of an airplane causing lift and the rotation of the turbine. As winds rotate rotor blades, a rotating shaft can be used directly to power a mechanical system, such as a water pump or pond aeration system, or the rotating shaft can spin an electrical generator to produce electricity.

Tower:

In general, wind speeds increase with height. To capture higher wind speeds, and optimize power production, wind turbines are mounted on a tower. Mounting wind turbines on towers allows them to be raised above areas of wind turbulence, caused by trees, buildings, hills or other structures. It is recommended to install wind turbines so that the bottom of the rotor blades are at least 30 feet (9 meters) above any structure that is within 300 feet (90 meters) of the tower. Wind turbine towers are usually 80 to 120 feet (24 to 37 meters) tall for small turbines and up to 100 meters or 320 feet for large commercial wind turbines. Increasing a wind turbine’s tower height could make a significant difference in the amount of electricity a wind turbine can generate, since wind speeds increase with height and the power generated is proportional to the cube of the wind speed.

Generator:

Wind turbines generate electricity via rotation of a rotor which in turn spins a generator. Small wind turbine generators produce DC, or direct current electricity. Since most appliances use AC, or alternating current, an inverter must be used to convert DC power to AC.

Wind turbines start to generate electricity when wind speed reaches a turbine’s rated cut-in wind speed. Wind cut-in speeds range from 6 to 8 miles per hour. Winds may reach a turbine’s cut in speed intermittently, and therefore wind power can not be relied upon as a constant or consistent power supply. Wind electric systems may be connected to the utility grid which provides electricity when the wind isn’t blowing and allows excess power to flow onto the utility grid when more electricity is produced than is being consumed. Many states have “net metering” laws that require utilities to pay retail rates to small renewable energy producers for any excess power produced over a month or year time frame. All electrical components of a grid tied wind system should be UL listed and comply with the National Electric Code and any local regulations.

The above figure shows a power grid connected to a wind energy system that runs wind power through a power inverter and into the home’s electrical system. Electricity not used by appliances flows onto utility power lines.

It is also possible to operate wind-generated electric systems independent of the utility grid. A solar electric system is often used in conjunction with wind power on off-grid systems. The figure above shows a solar electric panel and wind turbine, which both create electric current. The electricity is converted to alternating current for immediate use through a power inverter or is transferred to battery storage. When electricity is demanded but there is not enough solar or wind energy, DC electricity flows from the batteries through a power inverter, where it is converted to AC electricity and then flows to appliances and lights.

If a non-grid connected system produces excess electricity it must be dissipated or stored in a battery for use when needed. Rechargeable batteries act as a reservoir for electricity and are the only means for storing electrical power. It is important to optimize the battery storage size to maximize its life and storage capability and minimize expense. This will vary according to site and system. Deep cycle-type batteries are used for battery storage. Deep cycle batteries can discharge up to 80% of their stored energy without damage to the battery. Battery selection should be based on the expected kWh need and wind power system rated output. Batteries are rated for their capacity in amp hours at a given discharge rate. For example, three 12-volt batteries, each with a 50 Ampere-hour capacity, could store 1,800 kWh. A wind power contractor will be able to help design and optimize a battery storage system. A battery system will significantly increase system costs compared to a utility-connected system. Most systems installed today are grid-connected systems.

Wind power is a good source of energy if the wind resource is strong enough. The most suitable locations are ridges, open plains, and valleys on the West coast that are affected by the Santa Ana winds. Most wind turbines have cut-in speeds ranging between 6 and 8 miles per hour where they begin to produce electricity. Even though wind turbines may begin to generate power at the cut-in speed, it is typically recommended to install wind turbines at sites which receive an average wind speed of 10 mph or higher for small turbines and 12 to 13 mph or higher for 25 kW and larger wind turbines at hub height. At higher wind speeds the payback for a wind power system becomes more reasonable compared to wind turbines sited in areas with lower wind speeds. The power generated by a wind turbine is proportional to the cube of the wind speed so a slightly higher wind speed will result in much higher power production.

At any potential site for a wind turbine, the bottom of the swept area lower poiny of the blade rotation should be a minimum of 30 feet higher than the highest point within 1000 to 1500 feet of the turbine. Trees, building and land formations can create air turbulence that will reduce the efficiency of the turbine to produce energy and can decrease the life of the turbine due to unequal forces on the turine rotors. You'll never be sorry if you installed a higher tower.

Wind maps have been developed by the National Renewable Energy Lab using long term climate data. Wind maps by state are available at the U.S. Department of Energy’s Wind Powering America Program’s web site. Some states have refined wind maps and estimate the wind speeds at different heights about the ground. Common heights would be 30, 40, 60, 70 and 100 meters (98, 131, 197, 230, and 328 feet) which would correspond to common hub heights of wind turbines.

Wind maps will provide a general overview of the average wind speeds available. It is recommended to obtain a professional wind site assessment to determine site-specific wind speeds at typical wind turbine mounting heights. A professional site assessment will consider influence of turbulence caused by buildings, trees, hills or other objects, on wind speed for a particular site. For optimal wind power production, it is recommended to erect a wind turbine on an area of at least a quarter-acre, free from obstructions.

Area wind speeds can also be determined by direct monitoring. To determine average wind speeds for a location, a monitoring tower with a wind speed monitor, called an anemometer, can be used to measure wind speeds and directions. A monitoring tower can cost $8000 to over $16,000 depending on tower height and type of monitoring equipment. Added to that is the cost to install the tower and labor to collect and analyze the data. For small wind turbine projects, this may not be cost effective. However, for a wind farm with commercial scale turbines, analyzing the wind resource and estimating profitability is required by financial institutions and investors.

Wind turbine sizing depends on the application the wind system is intended to power. If the goal is to displace all the electricity consumed at a particular site, then electric utility bills can be used to determine the amount of kilowatt-hours a wind turbine should expect to displace. Making a site or application as energy efficient as possible before a wind turbine is installed is highly recommended, as lowering the energy demand will decrease the wind system size and cost.

Wind turbine manufacturers ratings can be deceiving because there are no national standards for rating wind turbine power production. One company may rate a turbine at 10 kW while another with the same diameter blades might be rated at 6 kW. How can this be? A closer look at the ratings may reveal that the 10 kW unit is rated at a higher wind speed than the 6 kW unit. The best way to compare wind turbines is to compare rotor diameters. The same turbine rotor diameter will produce about the same amount of electricity if exposed to the same conditions.

A rough calculation of the energy output for small wind turbines is as follows:

Annual Electrical Output (kWh) = 0.01328 x (rotor diameter, in feet)² x (average wind speed, in mph)³

In general, up to 10 kW wind turbines are used to power solo applications, such as water pumping, battery charging, residential or small farms. Wind systems for larger farms or businesses may range up to 100 kW in size or larger, depending on the application. A certified wind site assessor will be able to suggest an appropriately-sized wind turbine system.

The electric systems with wind turbines can be connected one of two ways. The first would be to an independent electrical system with a battery backup. The independent electrical system would typically be used in remote locations and may be coupled with solar electric panels. The battery backup provides power when there is greater power usage than energy being generated. The electrical system is not connected to the utility electric grid.

The second connection method uses an inverter to convert the electricity produced by the wind turbine to utility grade electricity so it can flow to the electrical grid. This requires that the system meet many safety standards to prevent the chance of electrocution, fire and damage to equipment. The requirements and application process to interconnect to the utility grid may vary from state to state. Contact your local utility or the Public Service Commission in your state for more information.

Wind systems should be inspected at least once per year, and twice a year if possible. The tower needs to be inspected to be sure welds are all intact, and guy wires are not worn or damaged. Turbine access may require climbing the tower, or tilting the tower to ground height, if a tilt-up tower is used.

The turbine needs to be checked for cracked welds, worn out bearings and seals. Bolts may need to be re-torqued, wires need to be checked, and paint may need to be touched up. The turbine blades should be inspected for any cracks or other needed repairs. The wind turbine may require lubrication once or twice a year.

A wind turbine provider should provide a list of scheduled maintenance requirements, and may offer scheduled maintenance service.

Proper safety precautions should be taken when climbing towers or tilting a heavy wind turbine to the ground so there are no accidents.

Wind shear is a meteorological phenomenon in which there is an increase in wind speed with increasing height above the ground. There can also be a change in wind direction with increasing height. This common characteristic of wind can be used to advantage by wind turbines with increased hub heights to capture more wind, thereby boosting power production. Wind shear is largely affected by the ground surface roughness or amount of obstructions to wind from topography, ground cover and buildings. The wind shear will be greatest over woodland and suburban areas and lowest over open water or prairies where there are fewer obstructions. In a high wind shear area the wind will increase faster with height than it will in an area with a low wind shear value. Directional shear or the change in wind direction as the height above the ground level increases can also occur.

The Midwest Renewable Energy Association (MREA) provides certification and training for wind site assessors. The MREA website, lists MREA certified wind site assessors by state and zip code.

The price paid for electricity generated will depend on the utility tariff structure, a negotiated price or net metering laws. If you have a small wind turbine, you may qualify for net metering, otherwise you will need a power purchase agreement (contract) with the utility.

Net Metering

Net Metering or Net Energy Billing allows for the flow of electricity both to and from the utility electrical grid to a customer who generates electricity using a renewable energy source, such as wind or solar PV. When energy is generated in excess of the consumer’s needs electricity flows to the utility grid, and when the consumer is using more energy than is generated electricity flows to the customer. The grid is essentially used as an overflow or battery because the customer is credited at retail rate for the excess energy produced on a monthly or yearly basis. Compensation rates for excess energy from the utility will depend on the tariff or rate. A utility tariff is a set of terms and conditions for interconnection and parallel generation of electrical energy. Net-metering is available in 44 states but may only apply to investor owned utilities that are regulated by the state’s Public Service Commission in some cases. Net metering is usually limited to a maximum name plate generating capacity which can vary from 10 kW to 2000 kW depending on the state.

Power Purchase Agreement / Contract

If your utility doesn’t have net metering rules or your turbine is larger than would qualify for net metering, you will have to have a contract or power purchase agreement with the utility to connect to the electric grid and sell them electricity. The purchased electric rate is often referred to as the “buy-back” rate, and can vary according to the utility and energy producer contract. Sometimes utility companies will only pay the wholesale rate for each kWh of electricity generated, or they may have a rate for renewable energy supplied by a wind turbine. Their buy-back rate can greatly influence the economics of wind turbine system installation, and the size of wind turbine to be installed. If the wind turbine is connected to your farm's electrical system behind the farm's electrical meter, then any power consumed and not fed to the utility grid will offset electricity at retail rates. Only electricity fed to the grid will be affected by the "buy-back" rate.

Contact your local utility for information on their renewable energy generation inter-connection policies and rules.

To take advantage of Federal tax credits you must have taxes to pay because the credit can’t be larger than the amount of taxes that you owe. Some tax credits can be spread over multiple years so a higher credit amount can be realized.

Residential Renewable Energy Tax Credit or Business Energy Investment Tax Credit (ITC)

Small Wind Turbine Tax Credit

The renewable energy tax credit is a tax credit for small renewable energy systems including wind turbines, solar, and other renewable energy sources. A small wind turbine is defined as a unit with a name plate capacity of 100 kilowatts (kW) or less. The tax credit amount is 30% of eligible costs with no maximum limit if placed in service between January 1, 2009 and December 31, 2016. Taxpayers eligible for the federal renewable electricity production tax credit (PTC) to take the federal business energy investment tax credit (ITC) or to receive a grant from the U.S. Treasury Department instead of talking the PTC for new installations.

Renewable Electricity Production Tax credit (PTC)

The renewable electricity production tax credit is an income tax credit of 2.1 cents/kilowatt-hour for the production of electricity from utility-scale wind turbines, geothermal energy, and closed-loop biomass systems. This tax credit generally applies to the first ten years of operation. This tax credit is not available to owners of small scale or residential wind turbines and can only be applied to passive or business income but not wageincome. The PTC can only be claimed for kWh sold to an unrelated third party, not kWh used on-site. The incentive, the Renewable Energy Production Tax Credit (PTC), was created under the Energy Policy Act of 1992 (at the value of 1.5 cents/kilowatt-hour, which has since been adjusted annually for inflation). The PTC is scheduled to expire on December 31, 2012. More information can be found at www.dsireusa.org

Rural Energy for America Program (REAP) Grants

This is a grant and loan guarantee program for renewable energy projects for rural businesses and farmers. Grants can be up to 25% of eligible project costs for grants and loan guaranteed loans up to 50% of project costs. The minimum grant is $2500 with a maximum of $500,000 for renewable energy projects. More information can be found at http://www.rurdev.usda.gov/rbs/farmbill or http://www.farmenergy.org.

Step 1. Determine areas where you can conserve electricity use. Visit the energy assessment tools link to calculate potential energy savings for measures you could employ on your farm. Reducing energy use, reduces the size and cost of a wind turbine to power your business.

Step 2. Educate yourself. It is important to understand how a wind power system operates, its limitations, and how to manage the system. Evaluate the wind power potential by researching wind speed maps for your location. Look for wind speeds measured at the tower height of a proposed turbine. The annual average wind speed should be at least 8 miles per hour in order to generate usable wind power.

Step 3. Investigate the type of zoning and permitting required in your local area. Your site may be located in an area with a homeowners association, or township laws which govern wind turbine tower installation. Be aware of any building permits that may be required for installation.

Step 4. Determine the availability of tax and grant assistance programs, including their application and payment processes. Some incentive programs may require paperwork or other steps to take before the system is purchased or installed.

Step 5. Get a site assessment – Consult a professional wind turbine installer or site assessor to estimate the electrical generating potential at your site. Optimizing a wind power system requires much site specific information and calculations including weather and climate data, and area obstructions. A certified wind energy professional can be found at the Midwest Renewable Energy Association’s certified site assessor directory.

Step 6.Get quotations from two or three companies. Compare quotes to ensure they are quoting similar size and type of equipment. Ask questions.

Step 7. Consult with your tax preparer to ensure that you can take advantage of any state and federal tax credit available.

Step 8. If possible, request recommendations for other systems they have installed and question prior customers on their experiences with the installer, system maintenance, and issues they have found.

Step 9. Check with your insurance carrier to see if your proposed wind power system is insurable.

Step 10. Order the system to be installed by a professional and reputable wind power equipment installer.

Step 11. Once the system is installed, complete all applicable grant or tax incentive forms.

Wildlife impacts are minimal compared to the habitat destruction presented by other kinds of power plants. Small wind turbines (less than 20 kW rating) have very minimal impact on wildlife. Avian mortality from wind turbines is significantly less than those caused by existing radio and telecommunication towers. Individual bird mortalities from large wind turbines on wind farms average about 2 birds per turbine annually nation wide which is estimated at 0.02% of bird fatalities from collisions with man-made objects. Bird collisions with buildings, communication towers, bridges and vehicles is estimated to kill millions of birds per year. A Bird and Building conference in 2005 estimated that 1 billion birds are killed from collisions with buildings each year in the United States. It is estimated that 40 to 50 million birds are killed by communication towers per year.

Studies have indicated that bat mortality around wind turbines is higher than birds at 2.5 to 4 bats per turbine, but this is much lower than other man-made causes of bat fatalities.

Studies have found that 90% of bat fatalities occur in late summer or fall when bats are migrating or dispersing. During the breeding season bat mortality is almost non-existent, but mortality may increase during bat migration which may be due to bats turning off their echolocation to conserve energy during migration.

Wind turbines cannot “throw” ice because wind turbine control system will automatically shut down when covered with ice. With more than 6,000 MW of wind power capacity worldwide, there has never been any reported injuries from ice thrown from wind turbines. Ice shedding from a turbine tends to land near its base.

Utility-scale turbines can cause shadow flicker for a few minutes, a few days per year. Shadow flicker can occur when the sun is low in the sky (morning or evening) and the sun is bright and in-line with the turbine.

Wind turbines do not effect radio and cellular signals because the blades are made of wood, fiberglass or plastic which are transparent to telecommunication signals; interference with TV signals are extremely rare. Large commercial scale wind turbines sometimes cause distortion of TV signals, “ghosting” of the image, if the turbine is directly in the line of sight with the station transmitter. The problem can usually be overcome with a larger antenna or a reception booster.

Strict interconnection regulations must be followed that prevent safety or performance problems with electrical systems. Stray voltage is caused by a breakdown of wire insulation, corroded or loose connections, undersized wiring or extremely unbalanced loads and is usually isolated to the electrical system of a particular farm. The regulations for connection of wind turbines to the utilities' electrical system are design to prevent any negative affect to the utility or its customers. When purchasing a small wind turbine that will be grid connected, look for a turbine with an inverter that is UL-1741 listed. Note that the listing refers only to the inverter, not the whole turbine. All electrical components of a grid tied wind system should be UL listed and comply with the National Electric Code and any local regulations.