Wind Speed and Height: Why do Wind Turbines Have to be So Tall?
In this lesson, students will investigate the relationship between wind speed and height, through both experiments and quantitative analysis. Students will be able to explain why turbines are built at heights of 50-80 m, rather than being taller or shorter.
Time Required: 1-2 lessons
People, Place and Environment:
• Students will be expected to demonstrate an understanding of the interactions among people, places and the environment.
• Students will use maps, globes, pictures, models and technologies to represent and describe physical and human systems.
• Students will use location, distance, scale, direction, and size to describe where places are and how they are distributed.
• Students will be expected to demonstrate an understanding of the interdependent relationship among individuals, societies, and the environment and the implications for a sustainable future.
• Identify and describe examples of positive and negative interactions among people, technology and the environment.
By the end of the lesson, students will be able to:
• Understand why turbines are built at heights of 50-80 m, rather than being taller or shorter.
• Understand the relationship between wind speed and height above the ground surface.
1. For background preparation, direct students to the information on the Canadian Atlas Online site:
“Does it matter where wind turbines are located”?
“Why is it necessary for the turbines to be mounted so far above the ground?”
“How much difference does height make?”
Note: Wind turbines typically are constructed 50-80 m above the ground surface. Traditional windmills (especially those in The Netherlands) are not so high.
3. Review the following concepts through guided questioning:
a.) Winds are horizontal motions of air produced by pressure differences.
b.) Three parameters determine speed and direction of wind:
Coriolis Effect: movement rotates to the right in the Northern Hemisphere. However, this is only notable over a large region.
Pressure gradient force: winds blow from high to low pressure. Larger differences in air pressure produce stronger winds.
Friction Force: interaction with the ground and obstacles. Winds are subject to drag at the surface due to roughness (mountains, trees, buildings). This causes winds to slow down and counters the other effects.
4. Indicate that in an enclosed space such as a football stadium wind directions can vary with altitude due to the actions of friction and pressure in combination. The wind can blow in a different direction at the top of the stadium (as indicated by the flags) than at field level (as indicated by passes and punts). Ask,“Have you observed this?", “Does the height above ground make some difference?”, “Is the wind direction indicated by flags at your school (or elsewhere in town) always the same as that felt while standing on the ground?”
5. Use one or more of these techniques to demonstrate:
a.) Set an electric fan on the edge of a desk. Estimate the speed of this artificial wind by launching small paper balls from a short distance in front of the fan. (This can provide amusement). See how far the breeze extends below the level of the fan.
b.) If you have a tall flagpole available, fly several flags at different heights on the pole and observe the differences.
c.) Using either a commercial or homemade anemometer, measure the wind speed at several heights: directly above the ground, at waist height, above your head, from a second story window (if possible and safe).
d.) If possible, take measurements from different stories of a taller building (in a safe fashion).
6. Instruct that through a combination of measurements and physics research, if one knows the speed of the wind at 1m bove the surface, the speed of the wind at greater heights above the ground surface can be estimated.
Note: The speed of the wind increases proportional to the 1/7 power of the height above the ground. (For heights less than approximately 300 m above the ground surface. For greater elevations, this relationship is not valid.)
7. Open the following link to reinforce: http://www.windatlas.ca/en/maps.php
Notice that the wind speeds vary with height above the ground surface. Wind speeds are given in m/s;
1 m/s = 60 m/minute = 3600 m/h = 3.6 km/h
a) “If the wind speed is greater at greater heights, why not build turbines that are 128 m high, instead of 65 m high?”
b) “Why not build turbines that are only 20m high?”
c) ”How does the wind speed vary in your area throughout the year?”
9. Prompt students to synthesize and evaluate their knowledge.
Instructions for Students
1. Gather background information from:
2. Discuss in small groups or individual short written responses with suggested responses to these questions. These suggestions will be explored throughout the course of the lesson.
3. Conclude that interaction between Coriolis Effect and Pressure gradient force can be seen on weather maps and television weather broadcasts – winds do not move in straight lines from high to low pressure, but
rotate counter clockwise around lows, clockwise around highs.
(Observe the regional pattern by watching (recorded) TV weather broadcast; or from downloaded weather maps from Environment Canada website.)
Animations: The Coriolis Effect / Pressure Gradient Force
Think about consequences: To get out of the wind, get low.
4. Respond to example and questions to and conclude that the height above the ground makes some difference.
(If it didn’t, you could launch a kite at ground level by just letting it go.)
5. In groups or whole class demonstration, explore experiments.
Note: Hold the anemometer at a 90° angle from the body to ensure that you do not block the wind. To measure directly above the ground, lie down and hold the anemometer upright.
(This could also be done outside of normal class hours or as a project/assignment if time does not permit an exploration of all.)
6. For measured wind speeds at each height, calculate the wind speed at 65 m above the ground surface.
Note: If you measured the wind speed at 2 m above the surface:
divide your value by 1.1, to give an estimate of the speed at 1 m above the surface, then use that value in the equation. (Current version used here from: http://www.windatlas.ca/en/maps.php)
7. Note that this map allows one to see the differences with height above the ground, as well as from season to season.
8. Respond to questions:
a) The advantage in increased wind
speed does not offset the difficulties in construction or maintenance or the potential risk to low-flying aircraft.
b) There would be insufficient clearance between the rotor blades and the ground
c) Study the maps in the online atlas.
9. Conclude that wind speed is related to height and turbines are constructed at 50-80 m height.
In-class discussion, process journal, or short assignment detailing observations or numerical calculations.
Web Pages Used
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