How to Test Wind Turbines

A typical spinning windmill generating power.

Windmills, or wind turbines as they are also known, convert energy from the wind into electrical power. The wind's force turns the windmill's blades, which then spin a generator to produce energy. Power is the rate of energy production and is measured in watts. The theoretical power output of a windmill is related to the wind speed and the length of the windmill blades. The windmill's power output may be tested by comparing the theoretical power to the actual power generated measured by a watt meter.

Things You'll Need

• Watt meter
• Tape measure
• Calculator

Instructions

1. Record the local wind speed in miles per hour in your area every day for 30 days. Consult a weather report for the information. Also, plug the windmill into a watt meter and record the power output once a day.

2. Add together all the wind speeds and then divide by 30 to obtain an average wind speed for your area. Also, add together all the watt-meter readings and divide by 30. This is the average actual power for the windmill. For example, the average wind speed might be 12 miles per hour and the average actual power may be 775 watts.

3. Convert the average wind speed to meters per second by multiplying by 0.44703, since a mile per hour equals 0.44703 meters per second. Performing this step--12 times 0.44703--equals a result of 5.4 meters per second.

4. Measure the length of one of the windmill blades in meters. For example, the length could be 3.0 meters.

5. Multiply the square of the blade length by 1.88 kilograms per cubic meter. The factor of 1.88 kilograms per cubic meter is related to the density of air. Now you have 3.0 meters times 3.0 meters times 1.88 kilograms per cubic meter. Call this result "x." The result is 16.9 kilograms per meter.

6. Multiply "x" by the cube of the average wind speed to arrive at the ideal power in watts. Ideal power assumes that all of the available energy from the wind is changed into electricity. Performing this step leads to 16.9 kilograms per meter times 5.4 meters per second times 5.4 meters per second times 5.4 meters per second, or 2661.1 watts.

7. Multiply the ideal power by the efficiency of the windmill design to get the theoretical power output. Assuming a three-bladed rotary design with 30 percent efficiency, you have 2661.1 watts times 0.30 which equals 798.3 watts.

8. Compare the average actual power to the theoretical power to determine how the windmill is performing by percent difference. Subtract the actual average power from the theoretical, take the absolute value, dividing by the theoretical power and then multiply by 100. Generally a percent difference less than 5 percent indicates a properly functioning device. Completing the exercise you first have 798.3 watts minus 775.0 watts, or 23.3 watts. Now dividing by 798.3 and multiplying by 100 results in a percent difference of 2.9 percent.

References

• "Physics for Scientists and Engineers with Modern Physics"; Raymond A. Serway and John W. Jewett; 2009
• HyperPhysics at Georgia State University: Power
• Engineering Tool Box: Speed
• University of Oregon Physics Department: Wind Energy