For the InstructorThese student materials complement the Renewable Energy and Environmental Sustainability Instructor Materials. If you would like your students to have access to the student materials, we suggest you either point them at the Student Version which omits the framing pages with information designed for faculty (and this box). Or you can download these pages in several formats that you can include in your course website or local Learning Managment System. Learn more about using, modifying, and sharing InTeGrate teaching materials.
Student Reading: Using Wind to Do Work
Learning Goals: Students will be able to:
- Recount the historical use of wind energy to include power for running boats and ships, pumping water, processing grain and sugar cane, and making electricity.
- Evaluate the impact of each of these technologies on the history of social development to include trade, agriculture and human dispersal.
- Use data they collect to test the relationship between airfoil design and energy harnessed.
- Explain the basic principles involved in transferring wind energy to mechanical energy, to include the roles of lift, drag, velocity, and ways to link foils to rotating shafts.
- Use published data sets to evaluate specific locations for siting of wind turbines, to include fluctuations in wind velocity associated with altitude, latitude, and daily and seasonal cycles. Interpret US maps of wind fields—spatial and seasonal. Evaluate their state for wind farms.
- Articulate the potential negative and positive environmental effects of wind turbines, including economic cost and environmental impacts of wind farms.
- Compare electrical-generating capacity between a wind farm and natural gas-fired power plant to include the issue of reliable base load generation.
- Diagram the major circulation pattern of wind on the planet and detail the underlying principles involved.
What makes the wind blow?
For fluids (gases and liquids), less dense substances tend to float up, and more dense substances tend to sink. Consider a glass of water. If you drop a penny in the glass, it will sink. That is because the penny is denser than the water. If you then dropped in the water a piece of cork with the same mass as the penny, the cork would float. Even though the cork weighs the same as the penny, its mass is in a much larger volume, and thus has a lower density than the water.
While it is easy to see the difference in density for solids like corks and coins, it is less obvious that liquids like air and water also differ in density. If you heated a pan on the stove and placed a feather above the hot pan, you would notice the feather floats up. It is being carried up by a current of air warmed by the hot pan below. That warm air is less dense than the surrounding cooler kitchen air, so it floats up — just like the cork in the glass of water. But the warm air is different from the cork floating in water in an important way. As the warm air floats up, it cools and mixes with the other air in the kitchen. The feather that floated up will now sink off to the side of the hot pan.
The warmer places will have warmer air floating above the land. And warmer air has lower density. The cooler places on Earth will have cooler air above. And the cooler air will have higher density. The amount of pressure exerted by the columns of air is dependent on the temperature. Warmer air will exert less pressure than colder air. The columns of cooler air will exert more pressure, owing to their higher density. The higher-pressure air will tend to flow toward the lower-pressure air. Think of blowing up a balloon. The air in the inflated balloon is under high pressure from the elastic rubber of the balloon. As soon as you let your fingers open the balloon, the high-pressure air will rush out. In nature, the flow of cooler, high-pressure air toward the warmer, low-pressure air creates wind.
The picture gets a bit more complicated on a global scale. Look at the figure below. It shows the location of the prevailing winds around the planet. Note that the winds come from the east in the tropics and near the Arctic and Antarctic Circles, and from the west in the mid temperate zone (around 40 degrees). Why?
To understand these prevailing winds, you need to apply your knowledge of rising and sinking air on a global scale. Near the equator Earth gets maximal solar radiation and it is hot. This causes a constant rising column of hot air. As the air floats up, it cools and becomes denser, but it cannot fall directly down, as more warmer air is constantly on the rise below. So the cooling air flows to a higher latitude (north or south) before it starts to descend. When the now cooler air comes back down to ground level, it is under high pressure, so it will flow in the direction of the lower-pressure warm air, essentially going back to where it started its journey. Once there, it too will warm and flow back up to continue the cycle. This pattern of circulation is called a convection cell, and three such cells are found in the Northern Hemisphere, and another three in the Southern Hemisphere. The two circulations in the tropical regions are called Hadley Cells.
Now you know the origin of the prevailing easterlies and westerlies. Understand that landforms like mountain ranges and bodies of water will alter these flows in many places. Consider mountains. As the prevailing winds encounter mountains, the flow of air is forced up and over these obstructions. This increases the velocity of the wind, making such places prime candidates for wind turbine installation.
Mountains and adjacent valleys also create a different kind of predictable wind that relies on local patterns of heating and cooling. Anabatic winds develop from the daytime warming of air in valleys and sun-facing slopes. Solar-heated slopes warm the overlying air, reducing its density, which results in upslope flow. Catabatic (or Katabatic) winds flow from the tops of mountains into the adjacent valleys. These may follow a daily cycle. At night, the air mass and underlying land at the top of the mountain back-radiates heat to the atmosphere. This creates a cold and dense layer of air, which gravity drives downslope. The presence of persistent snow or ice on the mountaintop may create long-lived downslope flows. At some high latitudes, coastal locations' catabatic flows routinely reach the velocity of strong hurricane winds (140 mph or 220 km/h).
Coastal environments create the conditions for the development of sea breezes. These occur as land heats up during the morning. The warmed earth heats the overlying air mass, lowering its density, which causes it to float up. The ascending air mass is replaced by cooler air that is overlying an adjacent body of water. Typically sea breezes flow from midday until near sundown. At night a land breeze often develops in the opposite direction. Since land has much lower specific heat than water, it heats and cools much more quickly than the adjacent ocean. The rapidly cooling land at night drops in temperature enough to make the now relatively warmer sea the source of rising air, and the land the source of replacement air.
Storms also create temporary changes in wind patterns. The high intensity and short duration of storm winds are not suitable for powering wind turbines. Indeed, most wind turbine designs quit spinning in high winds to prevent electrical and mechanical damage.
In the end, the thermal energy that drives the wind comes from the sun. So wind energy is in its essence solar energy.
Harnessing the energy of wind
In fact, modern sailboats can sail faster than the true wind speed! Say a boat is sailing with the true wind coming from ahead at about 45o at 10 knots (kts, which means nautical mile per hour, about 10% faster than a statute mile per hour). The boat could be making 6 kts of speed through the water. Using vector addition, the actual flow of the wind over the boat will be about 14 kts of apparent wind. If you bicycle on a still day at the rate of 10 mph, you would feel on your face an apparent wind force of 10 mph. If you bike at 10 mph into a 10 mph wind, you would experience a 20 mph apparent wind and your legs would get tired fast!. But if you pedal at 10 mph with a 10 mph wind behind you, the apparent wind would be zero, and you would barely need to move your legs.
How airfoils work
Lift, or the amount of force generated by a sail (or foil) depends on various factors. If the lift coefficient for a wing at a specified angle of attack is known, then the lift produced for specific flow conditions can be determined using the following equation: L = CL x A x (1/2 x ρ x v2) where:
L is lift force,
ρ is air density
v is true airspeed,
A is planform (sail) area, and
CL is the lift coefficient at the desired angle of attack (other stuff goes into this too)
Note that lift goes up with the square of the wind velocity — so a doubling of wind velocity will create four times the lift.
Using the formula, would it be easier for an airplane to take off on a day when the air is cold than when the air is hot? Why?
Drag vs Lift
As wind or water flows over an object, it creates lift, as explained above. But it also creates drag, a force opposite to lift. There are different kinds of drag. Lift-induced drag is created by foils (sails, wings, turbine blades) as air flows over them. The air leaving the foil is disturbed, creating vortexes (spinning masses of air) and other patterns of flow that cause drag. Form drag is caused by the body (boat hull, airplane fuselage, wing or sail) displacing the fluid as it moves forward. A streamlined shape helps reduce form drag. Skin friction drag is caused by the rough surface of an object moving through a fluid. The roughness tends to trap portions of the fluid and make for disturbed flow patterns.
Now that you know something about drag, revisit the equation for lift above. The formula also shows that larger sails or wings will create more lift. There is a downside to a larger airfoil. The air of thrust that exits the foil starts to swirl around. If you move your finger or hand through water, you can see such swirls forming behind it. The same is true for air. This disturbed airflow creates drag, a force opposite to the direction the boat or airplane is moving. Larger sails and wings create more drag. If there is plenty of wind, a smaller sail can actually make a boat go faster, as it has plenty of lift and less drag. Large jumbo jets have relatively small wings because their engines are so powerful that sufficient lift is generated by their fast speeds through the air. A small and slow propeller airplane has to have proportionally larger wings, since the velocity of the air passing over the wings is so slow.
The same advances that have made aircraft fly faster and more efficiently and made wind turbines better have also helped modern sailboats reach amazing speeds. In 1972, the world record for sailing fast was 26 kts, and by 2012 it jumped to 65 kts (that is 75 mph!). In the 2013 America's Cup competition, futuristic catamarans on hydrofoils raced around the course at speeds as high as 44 kts! To see how fast those boats go visit: Foiling sailboats. Think about this. Those boats were going 44 kts in winds of just 15–20 kts. They sailed faster than the wind! How is that possible? The lift created by their sails was much greater than the drag of the hydrofoils skimming along the top of the water. You will see below that wind turbines use this same principles to spin faster than the true wind.
From sailboats to windmills and wind turbines
Drag vs. Lift wind turbines
Drag-based turbines are simple to construct but much less efficient than lift-based versions.
The latest generation of wind turbines use blades (airfoils) made of very strong and light composite materials consisting of carbon fiber and epoxy resins. The shape of the blades are optimized for rotation speeds of about 15 revolutions per minute (RPM). Look at the shape of one of these modern foils (see images on this page) and note that they are wider near the base and narrow toward the tip. Why? You learned the reasons above. First, look back at the paragraph that explains the Coriolis effect at the beginning of this module. Recall that an object on the equator is moving much faster than one near the Arctic Circle, as Earth rotates.
A power equation is used to estimate the amount of energy per unit time produced by a wind turbine. Power = 0.5 x Swept Area x Air Density x Velocity3. Power is given in Watts, the circular area swept by the blades in m2, the air density in kilograms per m3, and the velocity in m/s. Because the power generated is a function of the cube of velocity, increases in wind speed means significantly more electricity generated over time. Let us compare the power generated at 4 m/s (9 mph) and 8 m/s (18 mph) by a wind turbine that sweeps 10,000 m2. Lets assume that the turbine is at sea level, so the density of air is 1.2 kg/m3. The power equation would be 0.5 x 10,000 m2 x 1.2 kg/m3 x (4 m/s)3= 384,000 Watts (= 384 kw). Doubling the wind speed would produce 3,072,000 Watts (= 3,072 kw = 3.072 mw). So doubling the wind speed caused an eight-fold increase in production of electricity.
The typical cut-in wind velocity for turbines is about 3.5 m/s (7.8 mph). This is when they begin to produce electricity. The cut-out velocity is 25 m/s (60 mph). At that speed, turbines are braked or their blades are feathered (rotated to stop gaining lift). Allowing the turbines to continue to rotate at such high wind speeds would damage the bearings and other mechanical components, and the heat produced by the generator would cause it to ignite. To view a video of a burning wind turbine go to: https://www.youtube.com/watch?v=Q5COAi6KM8o
The Betz law states that only about 60% of the wind's energy can be turned into usable energy by a turbine. A turbine works by taking energy from the wind and converting it to electricity. That means the wind is slowed as it looses energy to the blades. If all the energy were taken from the wind, it would not flow and could not spin the blades. In practice the modern turbine can reach about 80% of the Betz limit, so that means that roughly half the energy in wind can be turned into electricity.
Key: 1. Foundation; 2. Connection to electric grid; 3. Tower; 4. Access ladder; 5. Wind orientation control; 6. Nacelle; 7. Generator; 8. Anemometer; 9. Brake; 10. Gearbox; 11. Rotor blade; 12. Blade pitch control; 13. Rotor hub
The modern wind turbine is rotated to face into the wind. Older designs faced downwind, and it was discovered that disturbed air created by the wind flowing around their towers caused the turbine to vibrate and make a "whomp" sound every time the blade passed in front of the tower. The newer, upwind facing design is much quieter, producing a "woosh" from the blades and "humming" from the electric generator.
During the Obama administration (2009–2016), the installed capacity for generating electricity from wind in the United States tripled (Green Tech Media). This was despite two lapses in the Production Tax Credit (a tax incentive system for renewable energy) from a resistant Congress (Union of Concerned Scientists). While the current Trump administration is focused on fossil fuel production, the wind industry is now well established and has many industrial and environmental advocates. Indeed, the largest producer of wind energy in the United States as of 2017 is Texas, a state known for fossil fuels and a conservative political landscape somewhat hostile to environmental concerns. Now many farmers and ranchers in Texas and across the United States supplement their incomes by providing space for wind turbines on their lands. In 2016, rural land owners earned $222 million from leasing land for wind turbines (AWEA.org).
Optimizing wind turbine design and location
While a fossil fuel power plant could be placed mostly anywhere with access to the fuel and needed cooling water, wind turbines require predictable breezes of sufficient strength. Such locations typically include coastal areas (both on and offshore siting), mountaintops, and continental plains. The early builders of windmills understood that wind strength varied between locations. The Dutch built their windmills along the coast to capture sea breezes. When Europeans colonized the Caribbean for sugar plantations, they sited the windmills for crushing the cane atop mountains and hills. One can see the ruins of those early mills throughout Caribbean.
All wind turbines are designed to swivel, to keep their blades perpendicular to the flow of wind. For those located on or near mountains, this may also include the ability to face the blades upslope or downslope to take full advantage of the anabatic or catabatic winds (Hitachi).
Recall that the first widespread use of wind to generate electricity came on a small scale as farmers modified their water-pumping windmills to rotate small generators. Small- scale generation is still important. Modern cruising sailboats typically sport small wind generators and solar panels to charge banks of batteries. The stored electricity is used for lights, radios, navigation instruments, and refrigeration. Many companies produce small marine wind generators, with prices ranging from $500 to $2,000. People living remotely from the electrical grid in rural areas may install modest-sized wind turbines to charge batteries for lighting and communication. To see an image of a wind generator installed on a sailboat, go to: (The Solar Store) and to see a backyard installation see these: (Google images). Businesses wishing to project a "green" image may place small wind turbines atop parking lot light poles to make their statement (for image go to: these Google images ).
Windmills versus wind turbines
At this point, be sure you understand the fundamental difference between a windmill and a wind turbine. Windmills convert the kinetic energy of wind to mechanical energy (still kinetic) to rotate a shaft. Through a system of gears, that shaft rotates other shafts with attachments to do work such as grinding grain or pumping water. The wind turbine also uses wind to rotate a shaft, but it is ultimately linked to an electrical generator. The rotation of magnets across bundles of wires induces the flow of electricity. So wind turbines convert wind energy to mechanical energy (like windmills), and go the next step to turn that into electrical energy.
Look at the graphs on installed wind capacity. Which nations are leading? Why do you think?
Which states in the United States are producing the most wind energy? To find the latest information go to: https://cleanpower.org/facts/state-fact-sheets/ After Texas, who is number two?
Impediments to the Growth of the Wind Industry
Wind turbines do have an impact on the environment. They may kill large numbers of birds and bats, and their vibrations may harm marine life in offshore settings (https://rewi.org/). Many people find them displeasing to view, and they may produce annoying sound pollution. Better designs have reduced bird and bat deaths. Newer models rotate more slowly (3 m/s), which is safer for flying wildlife. More recent designs are also quieter.
Another issue is competition from established fossil fuel industries. Coal and natural gas companies view wind (and solar) energy as direct threats and spend much money lobbying against the wind industry. Since the 1970s the US Congress has gone back and forth on awarding a tax incentive to renewable energy companies. In general, Democrats support the tax incentives and Republicans do not. This creates uncertainty for investors as the controlling government party frequently changes hands. This was evident in 2013, the first time in twenty years that the rate of global-installed wind energy decreased. This traced primarily to a huge drop in US wind installation. In 2012 a record 13 GW (gigawatts) was installed, but that dropped to just 1 GW in 2013. And that drop was a consequence of the tardiness of the US Congress in reauthorizing tax incentives for the industry.
There is a current worldwide boom in natural gas production stimulated by hydraulic fracturing (fracking) techniques. Fracking may poison groundwater and pollute the air with escaped methane, but it has led to a super-abundance of cheap natural gas. With gas so inexpensive, it is hard to attract the capital and political will to invest more in wind energy. Yet both the wind and solar industries have matured to the point that refined mass production has brought down the cost of renewable energy installation so that it is now the cheapest way to make electricity (https://energy.gov/eere/wind/advantages-and-challenges-wind-energy). Even fossil fuel-rich Canada is making the transition to wind and solar energy (see short video: https://www.youtube.com/watch?v=bKwUeyoEe70). In the United States for 2016, there were 216 gigawatts of new production capacity installed, 9.5 GW from solar, 8.0 GW from natural gas, 6.8 GW from wind, 1.1 from nuclear, and 0.3 from hydro (https://www.eia.gov/todayinenergy/detail.php?id=25172). The trend to wind and solar is worldwide (http://www.greenpeace.org/international/en/news/Blogs/makingwaves/10-facts-about-renewable-energy/blog/51143/).
Wind energy will not work everywhere. Look at the image below showing average annual wind speeds for the continental United States. A minimum average wind speed of 6.5 m/sec (14.5 miles per hour) is required to support wind turbines.
Collecting your thoughts: Systems thinking and reflection
You have just learned about the history of using wind energy and the scientific principles underlying the various technologies involved. Take a few moments to consider how this all fits together. Consider the circulation of the atmosphere as a system. What drives that circulation? Will global warming increase or decrease the rate of circulation?
Wind turbines are connected to the electrical grid. Because the wind velocity varies over time, so will the electrical output from turbines. Considering this, how does the grid maintain a stable supply of electricity over time?
What keeps a wind turbine from overheating during high wind events? Is that an example of positive or negative feedback?
What would be the pros and cons of locating a wind turbine on your college campus?
Exercises for Energy from Wind
1. Would wind turbines work where you live?
Having sufficient wind energy to invest in wind turbines is only part of the puzzle. Is the windy land really available for development? Do current land uses prevent building wind generators? Some development, like farms, is compatible with wind power, while others like cities and protected parklands are not. Go to the US DOE's Potential Wind Capacity map and open the tab for the Excel table shown in the second paragraph.
What percent of the land in your state has sufficient wind for turbines and is actually available for development?
Now go to the Offshore 90-Meter Wind Maps and Wind Resource Potential site from US DOE to view potential sites for offshore wind turbine placement. In general, how does offshore wind power potential compare to that for on-land locations?
2. Testing the influence of sail area on boat speed.
Selecting the correct size and shape sail for a boat or blade for a wind turbine is complex stuff. There are all sorts of considerations regarding the three-dimensional shape of the sail. However, we can perform a simple experiment to see how sail size influences the speed of a boat and by inference the amount of lift generated by the sail. Keep in mind that while bigger sails create more lift, they also create more drag. A sailboat with a displacement hull can only go up to its "hull speed" before it trips over its own wave and falls sideways. So a bigger sail is not always better!
We will use three different-sized headsails on a 30-foot sailboat to measure the effect of sail area on speed. We will sail the boat on a beam reach (apparent wind coming from 90 deg.) for 5-minute trial runs.
Determining the area of each sail:
- Lay the sail on the ground and measure each side with a meter tape.
- Use the formula below to calculate area in m2. Area = SqRoot [p(p-a)(p-b)(p-c)], where a,b and c are the lengths in m of each side of the sail.
- Make a graph of Average Boat Speed (Y-axis) versus sail area (X-axis). Choose runs where the true wind speed was about the same for each sail size.
- What can you conclude about the effect of sail area on wind speed?
- How is this information important for designers of wind turbines?
- Are sails only for recreational boats in the modern era, or can they be part of commercial shipping once again? What keeps modern commercial shipping from making use of wind power today?
Experiment #3. Designing a windmill
The classic Dutch windmill had four cloth blades. The one typically found on farms in the late 19th and early 20th centuries had numerous galvanized steel blades. Today's modern wind turbines use either two or three long, thin blades. In this experiment we will test the effect of increasing the surface area by varying the number of blades on the rotor.
You will be provided with a windmill kit that has an LED. The kits come with rotors containing six blades. In this experiment you will change rotors to test the effect of the number of blades (surface area) in producing electricity. You will compare six-, three- and two-blade configurations.
1. Use the electric fan on the high setting to create artificial wind. Place the six-blade rotor on the turbine and put in front of the fan. Move the turbine away from the fan and measure the maximum distance from the fan that the LED light can be seen. Record that distance. Do the same for the three- and two-blade configurations. You may have to move the turbine back and forth several times to find maximum distance for LED illumination.
Maximum Distance (cm) at which the LED produces light.
What can you conclude about the role of surface area (number of blades) and the electricity produced by wind turbines? Is this consistent with modern wind turbine design? What is the difference?
2. Use the six-blade configuration to determine the effect of orientation toward the wind. Place the turbine close enough to the fan to produce a strong light. Next orient the turbine away from the wind 45 degrees. Then turn it 90 degrees to the wind. What effect does orientation have on speed of the windmill? Use sound as a clue to how fast the turbine is spinning.
Web Investigation — Find the latest facts online! Be sure to include the URLs in your answers.
1. What is the most recent estimate of the % of electricity in the United States that is produced by wind power?
2. Which nation produces the most electricity by wind?
3. Which nation produces the largest % of its electricity use by wind?