Wind Turbine Operation

Small Wind Turbine Installing wind turbine project to generate alternative home energy is a great way to offset your energy cost provided the wind blows hard enough and your electric rate is high enough.  Check out our Evaluate Wind Power page to see what it takes to install an economically viable system.  If you've been there or are just interested in how the system works, how to size it or how the power output is measured, read on...

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Conventional small wind turbines, rated at 100 kilowatts or less, utilize a propeller or rotor mounted on a horizontal shaft to drive an alternator or generator.  Newer vertical axis turbines are starting to appear on the market.  They capture the wind's energy by rotating a series of swiveling airfoils around a vertical shaft to generate power.

Let's begin by examining the components of a horizontal axis wind turbine:

Horizontal Wind Turbine Diagram

Horizontal wind turbines use a direct drive shaft to connect the rotor to the magnets rotating in the alternator section.  Power in these turbines is generated with relatively high RPM and low torque as opposed to hydro turbines that have low RPM and high torque.  The direct drive connection is the simplest and most efficient.  Avoiding gears, belts and pulleys reduces energy loss and maintenance.

The nose cone streamlines the main housing and protects the alternator section which contains permanent magnets that rotate in close proximity to stationary coils.  This rotation generates alternating current (AC) within the coils.   Due to the constant variation in wind speed this alternating current is sporadic and unsuitable for powering appliances in its raw form.

To overcome this condition, the unstable AC is converted to direct current (DC) using a rectifier.  The rectifier may reside inside the turbine housing or be located in a control panel at the base of the tower.  When converted to DC, power can be used to charge batteries directly or it can be inverted back to a stable AC by using an inverter if adequate power is available.

Regardless of where the rectifier is located, the energy must be carried from the rotating platform to a stationary tower.  This transfer occurs through a set of brushes and slip rings located next to the yaw bearing.  The yaw bearing allows the wind turbine to rotate freely so it can weather vane into the wind.  Electrical brushes track along their designated paths on the slip ring transferring current from the rotating housing to the fixed stub mast.

Small wind turbines normally work best in wind speeds of 12 to 25 MPH.   Winds above this level can over speed or burn out the turbine if speed controls are not used.  One way to control turbine speed is to feather the rotor blades to a point of near stalling to slow them down.

A simpler, more common method is to furl the tail vane.  As wind speed nears maximum, the tail vane rotates off centerline pointing the rotor into the wind at an angle.   Rotor speed decreases when the turbine is not pointed directly into the wind.

Sizing a Wind Turbine

The first thing to consider when sizing a wind generation project is the sweep area of the rotor as this determines the amount of power that can be expected from the unit.  Power is directly proportional to the to the sweep area of the rotor which is simply the disk area in which the rotor blades spin.

Wind Turbine Rotor Sweep Area Since sweep area increases at the square of the diameter we find the amount of power produced does likewise.  In other words, if you double the diameter of the rotor, sweep area quadruples and so does the power, theoretically.  Other factors such as rotor mass, type of bearings, blade aerodynamics and alternator or generator efficiency can have an affect on power output when scaling up a design.

The height and strength of the supporting tower is also affected by sweep area.   Increasing this area means the tower must be built to sustain higher wind loads and swing longer turbine blades.  Additional height may be needed to keep the full sweep area in unobstructed wind flow, especially in wooded locations.

The second consideration, based on power available from the sweep area and average wind speed, is to decide whether the wind turbine will be used to charge batteries, send its power through an inverter to power 120 volt household appliances directly or do both to provide emergency back-up power.  Our Metering Solar Power Panels page provides a good explanation of the various inverter arrangements that apply to wind power, as well.

Another important consideration is to estimate the amount of energy savings you can expect from the project when it is completed.  Knowing the amount of kilowatt-hours you consume each month, the rated power output of the wind turbine you plan to use and your average wind speed you should be able to estimate the savings and determine the pay back.

For example, let's assume your electric bills show you use an average of 1,800 kilowatt-hours per month and your cost per kilowatt-hour is 12 cents.  Wind maps, backed up with your actual measurements show your average 24/7 wind speed to be 13 MPH.  You are considering a 2.5 kilowatt unit that, when fully installed with tax credits, will cost $9,000.  What will be the expected savings and how long will it take to recover the investment?

The first step is to locate the power curve for the unit.  This will normally be found in the technical specifications part of the owners manual.  The power curve shows how electrical output varies with wind speed.

Wind Turbine Power Curve As shown in the 2.5 KW Power Curve, the red dashed line depicts the wind speed required to produce the maximum rated output.   To find the expected output follow the horizontal axis back to our measured 13 MPH and note the corresponding output on the blue dashed line which is 650 watts.  Over time, we can expect the wind turbine to produce 650 watts per hour or about 15 kilowatt-hours per day since our wind speed was based upon a 24/7 average.

Looking at our electric bill which averaged 1,800 KWh per month or 60 KWh per day, we can expect a 25 percent reduction in cost if wind generation delivers 15 KWh per day.  Using the 12 cent per kilowatt rate from our example, monthly savings would amount to $54.00.  Dividing our net project cost of $9,000 by the monthly savings shows that it will take 13.8 years to recover the investment.

Although it takes a long time to recover the investment, this example only evaluates the project from an economic standpoint.  If having an independent source of emergency back-up power at your home or lowering carbon emissions are important to you they should also be considered in your final decision.

Measuring Wind Power

If you elect to proceed with a wind generation project be sure the equipment has the ability to track and record all of the energy produced independent of the utility company's meter.   If not, we recommend installing a multi-point or a circuit level monitor that has the ability to track alternative energy generation provided the system delivers 120 volt power.

Here's a generic diagram showing how system components connect and power output is measured:

Wind Turbine Metering Diagram

Power output from the turbine must be rectified from unstable alternating current (AC) to stable direct current (DC).  From there it can be used to charge batteries or goes to an AC inverter that converts the DC back into stable 120 V. 60 Hz. AC.  This stable AC feeds back into the distribution panel which allows it to power household appliances or be sent back onto the grid.

Some configurations allow both battery charging and a direct feed into an AC inverter which provides emergency back up power.  Although quite handy during a power outage it does add a significant cost increase to the system.  Measuring the amount of energy used to charge batteries directly requires a separate DC power monitor sized to the voltage of the battery bank.

Measuring wind power output with a home energy monitor must be done at standard AC line voltages and frequencies.  Current transformers (CT's) are installed after the inverter but before any switching that routes power to your home or back on to the grid.  Total load for the home must also be recorded simultaneously with a separate set of CT's.  Utility power coming from the grid can be netted out by the energy monitor system's software.

For more information about wind power generation please visit our Wind Energy Books section for a great selection of the latest publications on the subject.

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