Wind Energy

By this point in the course, you have been told repeatedly that our energy and electric power systems are dominated by fossil fuels. And this is true. But you may be surprised to know that in the United States and many other countries, wind is among the fastest-growing sources of new power plant investment, as measured by megawatts of new capacity. In several areas, including Texas and the Mid-Atlantic (where a boom in fossil fuel production is currently underway), wind power is the largest source of new electrical generation capacity, making up a majority of new plants. That’s right – in oily Texas, more than 50% of new electrical generation in recent years has been from wind. In fact, Texas is the US leader in wind energy generation – much more than even California, which has somewhat greener political leanings.

In this section of the course, we’ll take a look at what’s going on in all those tall towers sprouting up along ridgetops and plains – and out in the middle of the ocean, in some places. Humans have been harnessing the wind to do useful work in one fashion or another for many thousands of years – the first “wind energy” systems were actually sailboats. Humans have also been smart enough to realize that wind is a very useful cooling mechanism on hot days. So in some sense, the windows in our houses are a form of wind energy. Windmills (the precursor to today’s wind turbines) appear to have first been used in Greece around two thousand years ago.

Picture of old and new windmills in the Netherlands. Details in caption.

An old windmill against the backdrop of a modern wind farm, Netherlands

Source: Wikimedia Commons CC BY-SA 3.0 NL (Creative Commons [1])

How Wind Turbines Work

In a conventional power plant (fueled by coal or natural gas), combustion heats water to steam and the steam pressure is used to spin the blades of a turbine. The turbine is then connected to a generator, which is a giant coil of wire turning in a magnetic field. This action induces electric current to flow in the wire. The workings of a wind turbine are much different, except that instead of using a fossil fuel heat to boil water and generate steam, the wind is used to directly spin the turbine blades to get the generator turning and to get electricity produced.

The inner workings of a wind turbine consist of three basic parts, seen in the figure below. The tower is the tall pole on which the wind turbine sits. The nacelle is the box at the top of the tower that contains the important mechanical pieces – the gearbox and generator. The blades are what actually capture the power of the wind and get the gears turning, delivering power to the generator. The direction that the blades are facing can be rotated so that the turbine always faces into the wind, and the pitch of the blades (the angle at which the blades face into the wind) can also be adjusted. Pitch control is important, especially in very windy conditions, to keep the gearbox from getting overloaded.

Modern wind turbines sit upon towers that are typically 80 meters high or taller, with rotating blades that are 50 meters or longer.

Source: Wikipedia: Lamma wind turbine [2] CC BY-SA 3.0 (Creative Commons [3])

The amount of power (in Watts) collected by a wind turbine is explained in the following equations:

The physics of wind power

Click here for a text description of the physics of wind power equation.

Source: David Bice

The Kinetic Energy (KE) of the wind is:

K E = \frac{1}{2} m v^{2}

Where m = mass, and v = velocity of wind.

Power (P) in the wind is the KE per unit time, so we replace the mass(m) with the mass flux rate dm/dt:

P = \frac{1}{2} \frac{d m}{d t} v^{2}

\frac{d m}{d t} = ρ A v

Where p = air density, and A = swept area of blades.

So the wind Power(P) is:

P = \frac{1}{2} ρ A v^{3}

If the wind turbine collected all of this power, the wind would have to stop and the blades would stop spinning. If you want the blades to keep spinning, it turns out that you can collect about 60% of the power (called the Betz limit).

So, collectible Power(P) is:

P = 0.3 ρ A v^{3}

How much power could we get with a turbine whose blades are 100m long, with a wind speed of 10m/s (about 22mpg>, with an air density of 1.2kg/m2?

P = 0.3 \times 1.2 \times π \times 100^{2} \times 10^{3} = 11, 304, 000 Watts = 11.3 MW

This is clearly a lot of power! But, mechanical inefficiencies related to the gears and the generator mean that we might only get 30% of this figure, but that is still a lot of power from one turbine.

All wind turbines have a minimum wind speed that differs depending on the size but is typically about 4-5 m/s (10 mph) and maximum wind speed above which they shut down to avoid damage, usually around 20-25 m/s (about 50 mph). Most wind turbines have a maximum spinning rate, reached a bit above the minimum velocity, and when the wind speeds up, the pitch of the blades is adjusted so that the rate of spinning remains more or less constant. The figure below shows a typical "power curve" for a small wind turbine.

Power curve for 1.5 MW wind turbine. Wind on x-axis, power on y-axis. Power increases with wind speed til 50 mph where power drops to 0

Power curve for a 1.5 MW wind turbine

Click here for a text description of the power curve for 1.5 MW wind turbine.

Source: US Department of Energy, Idaho Office

The wind, as you may have noticed, is highly variable in any given place, but as a general rule, it is stronger and steadier as you rise up above the ground. This is because friction between the wind and the land surface slows the wind. But there is also a lot of regional variation in the wind velocity. Both of these factors (elevation above the ground and location) can be seen in the maps below, showing the average wind speed in the US at two different heights.

graph showing annual average wind speed at 30 meters in U.S.

United States - Annual Average Wind Speed at 30 m

Click here for a text description of the U.S. Annual Average Wind Speed at 30 m.

These two maps of the United States show the average annual wind speed at two different heights above the surface. The upper map shows the wind speed at 30 meters height and the one below shows it at about a hundred meters. You can see a couple thing right away. One is that there is just a lot more wind at greater velocities at this higher elevation above the lands surface. You get to 100 meters and there are a lot of places in the central part of the US where you get wind speed from 8 to 10 meters per second, which is really moving along quit fast. And you also see this lower map of 100 meter of wind speeds of the offshore regions everywhere on the west coast and the east coast and around the Gulf of Mexico there are very high wind speeds. Also the Great Lakes are like this. The primary reasons these offshore regions have such high wind speeds and also why higher up you have such wind speeds are because there are less friction in those settings. So you go higher up from the surface there is less friction from the air and all the trees and the roughness of the land surface. That roughness slows the wind down and as you rise above that to 100 meters you get away from that disturbance and have higher wind velocities. You can also see that in the mid-continent region, both the 30 meter and the 100 meter heights, that’s the area with the greatest wind potential. You have these annual average wind speeds that are quit high and this is primarily because this is flat part of the country. There are not a whole lot of topography in those areas so the winds can really get going and be maintained. They do not encounter mountains and valleys and the sort of complexity that you see in other areas where further out west the wind speeds are not that high. So you can look at this and see right away that if you wanted to develop wind power, the best places are in the middle of the continent and at a high elevation 100 meters above the surface. That is why you see so many tall wind turbines to get up that high.

Credit: National Renewable Energy Laboratory (NREL) [4]

graph showing annual average wind speed at 100 meters in U.S.

United States - Land-based and Offshore Annual Average Wind Speed at 100 m

Click here for a text description of the U.S. Land-based and Offshore Annual Average Wind Speed at 100 m.

These two maps of the United States show the average annual wind speed at two different heights above the surface. The upper map shows the wind speed at 30 meters height, and the one below shows it at about a hundred meters. You can see a couple thing right away. One is that there is just a lot more wind at greater velocities at this higher elevation above the land's surface. You get to 100 meters and there are a lot of places in the central part of the US where you get wind speed from 8 to 10 meters per second, which is really moving along quite fast. And you also see this lower map of 100 meter of wind speeds of the offshore regions everywhere on the west coast and the east coast and around the Gulf of Mexico there are very high wind speeds. Also, the Great Lakes are like this. The primary reasons these offshore regions have such high wind speeds and also why higher up you have such wind speeds are because there are less friction in those settings. So you go higher up from the surface, there is less friction from the air and all the trees and the roughness of the land surface. That roughness slows the wind down and as you rise above that to 100 meters you get away from that disturbance and have higher wind velocities. You can also see that in the mid-continent region, both the 30 meter and the 100-meter heights, that’s the area with the greatest wind potential. You have these annual average wind speeds that are quite high, and this is primarily because this is a flat part of the country. There are not a whole lot of topography in those areas, so the winds can really get going and be maintained. They do not encounter mountains and valleys and the sort of complexity that you see in other areas, where further out west the wind speeds are not that high. So you can look at this and see right away that if you wanted to develop wind power, the best places are in the middle of the continent and at a high elevation 100 meters above the surface. That is why you see so many tall wind turbines to get up that high.

Credit: National Renewable Energy Laboratory (NREL) [4]

The graphs above show annual average wind speeds in the US at 2 different heights above the ground surface. For reference, 10 m/s is 22.3 mph. You can see that the wind speeds at 100 m are far greater than at 30 m — this is the friction effect of the land surface (which is minimal above large water bodies). As you can see, the Great Plains have great wind potential, as do the Great Lakes and offshore areas on both coasts.

The area covered by the turbine’s blades is another important factor in determining power output. While wind turbines are available in a wide variety of capacities, from a few kilowatts to many thousands of kilowatts, it’s the larger turbine sizes that are being deployed most rapidly in wind farms. Several years ago the image on the right side of the figure below of a Boeing 747 superimposed on a wind turbine gave an astonishing representation of the scale of the state-of-the-art wind technology. Now, turbine rotor diameters are approaching the size of the Washington Monument!

Graph shows increase in wingspan of turbine blades over 10 years, explained in caption.

Over the past decade, wind turbine blades have grown from being about the size of the wingspan of a jumbo jet to being about as long as an American football field.

Click for a text description of the Wind Turbine graph.

Credit: UpWind: Design Limits and Solutions for Very Large Wind Turbines [5], 2011. European Wind Energy Association

Activate Your Learning

Given that the area of wind captured by the turbine is proportional to the square of the radius (essentially the length of the blade), if you were to double the length of a wind turbine's blade, how much more power would that turbine generate? Assume that wind speed and all other variables remain the same.

Click for the answer.

Market Deployment of Wind Energy

Over the last 20 years, growth in the total installed capacity of wind energy generation across the globe has been growing rapidly. Germany was the first country to lead the development of wind power, but the US and China have dominated the growth since 2010. China is especially impressive in terms of its recent growth.

Graph of cumulative installed wind energy capacity, gigawatts

The growth in wind power capacity over the last 20 years. Both China and the US are growing exponentially.

Click here for a text description of the growth in wind power capacity over the last 20 years graph.

Credit: BP Statistical Review of Global Energy, from Our World in Data [6] (CC BY 4.0 [7])

Part of the reason for this growth is the steady decline in the cost of wind energy, as discussed in the previous section on solar energy. But government policies are another important factor. The United States has one of the most volatile markets for wind energy in the world, while those in Europe and China have been among the most stable. This is due in part to differences in how governments in these countries treat wind energy. In many parts of Europe, wind energy (and other renewable generation technologies) enjoy subsidies and incentives known as feed-in tariffs. The feed-in tariff is essentially a long-term guarantee of the ability to sell output from a specific power generation resource to the grid at a specified price (typically higher than the prices received in the market by other generation resources). The United States, on the other hand, has favored a system of tax incentives called the “Production Tax Credit” (PTC) to encourage renewable energy deployment. In theory, a tax incentive should not work much differently than a feed-in tariff (both are just payments based on how many kilowatt-hours are generated). But the PTC has historically needed to be re-authorized frequently by the US Congress – this “on-off” policy strategy has been a major factor in the volatility of wind energy investment in the US as shown in the figure below. It is worth noting that the PTC was recently renewed for 2013, but will lapse again at the end of 2019, so it is difficult to say what impact it will have on wind investment going forward.

US wind energy annual installations, showing the relationship between lapses in the Production Tax Credit (PTC) and declines in new installations.

Click here for a text description of the US wind energy annual installations graph.

Credit: David Bice, data from American Wind Energy Association (AWEA) [8]

The above clarifies that government policies are important to the growth of renewable energy production (both wind and solar). In a very real way, you can think about these policies (feed-in tariffs or tax credits) as a form of investment. Governments can also provide investments in the form of funding for basic research related to these technologies. In general, these investments do not add up to a huge amount when seen in the context of a country's gross domestic product (GDP), which is a measure of the size of the economy, as seen in the figure below.

Renewable Energy Investment (% of GDP) 2015

A comparison of some leading countries in terms of the percentage of their gross domestic product (GDP) is committed to the development of renewable energy technologies.

Click here for a text description of the graph of renewable energy investment (% of GDP) for 2015.

Credit: Bloomberg New Energy Finance; World Bank, from Our World in Data [6] (CC BY 4.0 [7])

The Potential Wind Energy Resource

A quick look at an annually-averaged wind map of the world (below) shows the regions of the world that are best suited for the production of wind energy in colors ranging from yellows to red (where the average winds are at least 9.75 m/s or 20 mph). The offshore regions are clearly the best in terms of the energy potential, but not all of these offshore regions are close to where people live. Even for onshore portions of the world, the wind energy potential does not always coincide with where the people are concentrated. This points to the necessity of new transmission lines to deliver this wind energy to major population centers.

Map of global distribution of onshore and offshore wind resources

The global distribution of onshore and offshore wind resources, showing the annually-averaged wind speed at a height of 100 m. For reference, the blue regions have speeds of 2.5-4 m/s (too low for good wind energy), green regions have velocities in the range of 5-6 m/ (also a bit too low for big turbines), yellow regions have speeds of 6.5 m/s (about 15 mph), which is enough to get most large modern turbines spinning, orange areas range in speed from 7-8.5 m/s, and the red areas range from 8.5 to more than 9.75 m/s, which is plenty of speed to generate wind energy at the full capacity of big turbines.

Credit: Global Wind Atlas [9], CC BY 4.0 [10]

So, just how much energy could be produced by the wind? In 2009, a group of scientists makes some calculations to estimate the potential for the world and the US, using wind data and some assumptions about the size and spacing of the turbines. They assumed 2.5 MW turbines on land, and 3.5 MW turbines offshore, which were big for that time. They assumed that you could only place the turbines in unforested, ice-free, nonmountainous areas away from any towns and that the turbines had to be spaced by several hundred meters so they do not interfere with their neighbors. They further assumed that each turbine generated just 20% of its rated capacity to account for mechanical problems and intermittent winds. What they came up with is summarized in the table below, and it is pretty remarkable. The units here are exajoules (EJ = 1 x 10¹⁸ Joules) of energy over the course of a year. For reference, in 2018, the US total energy consumption (not just electrical energy) was 106 EJ and the global consumption was about 600 EJ. So, with just onshore wind energy, the potential is more than twice what we consume in the US, and more than 4 times the global consumption. But getting there is a matter of installing a lot of wind turbines!

Table 1: Wind Energy Potential in EJ
Region	World	Contiguous US
Onshore	2484	223.2
Offshore 0-20m	151	4.32
Offshore 20-50m	144	7.56
Offshore 50-100m	270	7.92
Total	3024	244.8

Now let's consider a more practical question — how much wind energy have we managed to produce, and can we somehow project the past trends into the future? The figure below shows the global history of wind energy (solar is plotted too just for comparison), and you can see that it is growing fast.

Graph of global solar and wind energy generation history

The history of wind and solar energy production for the world have both grown dramatically over the past 20 years or so. Both curves are following an exponential growth trajectory, increasing by an average of 25% per year.

Click for a text description of the Global Solar and Wind Energy Generation History graph.

Credit: David Bice, data from BP Statistical Review of Energy, 2018

Both of these curves are growing exponentially, and the history so far suggests a growth of about 25% per year on average. If we assume that they continue to grow in the further following this exponential growth, we can project where we'll be at any time in the future. Below, we see where we might be in the year 2030, just eleven years from now. What you see is that we end up with vast amount of wind energy by 2030 — if it grows at the same rate it has been growing at, we end up with almost 300 EJ per year, about half of the current global energy consumption, and if it grows at a smaller rate of 20% per year, we still end up being able to supply about 20% of the total global energy demand.

Graph of global wind energy generation history and projection

The history of wind energy production for the world projected into the future, increasing by an average of 25% per year and 20% per year.

Click here for a written description of the Global wind energy generation history and projection graph.

Credit: David Bice, data from BP Statistical Review of Energy, 2018

Barriers to Additional Wind Energy Development

It is worth noting that, as with solar, wind investments are not always happening in the windiest areas. The reality is that there are a large number of factors that influence the development of wind energy globally. As the technology for wind energy has improved, other factors have also come together to create market drivers for wind power. These drivers include:

Declining Wind Costs
Fuel Price Uncertainty
Federal and State Policies
Economic Development
Public Support
Green Power
Energy Security
Carbon Risk

Earth: The Operator's Manual

Despite all of these barriers to wind energy deployment, wind is, in fact, one of the fastest-growing sources of power generation in the world. Wind energy is being embraced in areas that have traditionally favored low-carbon energy development as well as in areas that have a long history of fossil fuel extraction and use. The following video explains how two very different regions - Denmark and Texas - have embraced wind energy.

Video: Yes, in My Backyard (aka YIMBY!): (9:08)

Click here for a video transcript of "Yes, In My BackYard".

NARRATOR: Are there other examples of communities and nations that have begun the transition away from fossil fuels? What does it take to welcome the turbines and solar farms of the new energy system, and say, "Yes, In My Backyard." This is the story of two communities that at first look very different. Samso is a small island off the Danish mainland. West Texas is a vast, dry expanse in America's South. What both have is abundant wind. At times, Samso produces more electricity than it uses, exporting surplus power to the Danish mainland. And Texas wind now generates as much power as the next three U.S. states combined. Samso and West Texas both solved the NIMBY, not in my backyard challenge that has stymied so many renewable energy projects. It's not easy, but with patience, and persistence, and the efforts of the right people, it can be done.

SOREN: Okay-- My name is Soren Hermansen, and I am the Director of the Samso Energy Academy. Samso means, in Danish, means the Meeting Island-- when you make a circle around all of Denmark, then Samso is right in the center of the circle.

NARRATOR: Narrator: But it wasn't geography that brought Lykke Friis, then Denmark's Minister of Climate and Energy, here in mid-2011. It was why and how this community had turned NIMBY into "Yes, in my backyard."

LYKKE FRIIS: Well, Samso is a pioneering project, in the sense that Samso, way back, decided that Samso should become independent of fossil fuels. Narrator: Before its transformation, people thought of Samso as just a cute tourist community, busy in summer, empty and desolate in winter. Now people come here not just to see the turbines, but to understand the process that got the community to welcome wind energy. After a national competition, Samso was selected by the Danish government to be a proof of concept for how to transition from fossil fuels. But it was up to individuals like Soren Hermansen, with the passion and skills to effect change, to figure out just how. Soren: So when we won, the normal reaction from most people was, "Yeah, you can do this project, that's OK, but just leave me out of it."

NARRATOR: Samso has a deep attachment to its past and values its traditional way of life.

SOREN: But gradually we won their confidence in establishing easy projects to understand, and also easy projects to finance. Because basically, it's all about, "What's in it for me?" Because it's not convinced idealists or green environmental hippies who lives here.

NARRATOR: Soren, a native of the island, convinced some of his neighbors to become early adopters. They found success and spread the word. Jorgen Tranberg operated a large and profitable herd of milk cows. After initial reservations, he invested in a turbine on his own land. When that went well, Jorgen became part owner of one of the offshore turbines.

SOREN: Farmers, they have to invent new things and be ready for changes. So when they see a potential, they look at it, no matter what it is. They look at it, say, "Could I do this?" And if they see fellow farmers do the same thing, they are quick to respond to that. So even being very traditional and conservative in their heads I think they have this ability of making moves and do things because they have this independency in them. A farmer is a free man-- maybe he owes a lot of money to the bank, but he's still a free man in his thinking.

NARRATOR: It was seeing what was in it for them and for their community, that won over landowners in West Texas. And it took one of their own, a man whose family had deep roots in Roscoe's cotton fields, to educate them about wind farming. Cliff Etheredge: Well, I'm really a farmer-farmer, you see. I farmed for almost over 40 years. We're in-- right in the middle of the Roscoe Wind Farm. And we've got about 780 megawatts of production, that's per hour, enough electricity for about 265,000 average homes. Narrator: Roscoe had no oil and faced hard times in the early '90s, but it did have wind.

CLIFF: When this land was acquired there was absolutely no value to the wind. Fact is, it was a severe detriment, because of the evaporation of the moisture.

NARRATOR: Cliff, like Soren, had to work with his neighbors to get them ready to accept wind turbines.

CLIFF: The first thing farmers want to know is, "Well, how much is it going to cost me?" It costs them nothing. "What's it going to hurt?" Three to five percent of your farmland is all it's going to take up. You can do what you want to with the rest of it. Then it came down to, "Well, how much money is this going to make me?"

NARRATOR: Cliff did his research and checked his numbers with wind experts and the Farm Bureau.

CLIFF: Then I was able to go to our Landowners' Association and show them, where they had been receiving 35 to 40 dollars an acre, then the landowners could expect somewhere in the neighborhood of three times that.

NARRATOR: In fact, farmers stand to make 10 to 15 thousand dollars a year, per turbine, just from leasing the wind rights.

CLIFF: There was no guarantee in it from the very beginning, but sure enough we've got, I think, in the neighborhood of 95 or more percent of our area that accepted the wind farm.

NARRATOR: In both Samso and West Texas, individuals saw economic benefits. But the whole community, beyond the investors and land-owners, benefited too.

CLIFF: Because of the wind farm, now, and the people working in the wind industry, now we've got jobs available and opportunities for young people to come back from college or from technical school or from whatever. It's just been a Godsend.

NARRATOR: For Kim Alexander, superintendent of the Roscoe school district, that godsend translates into dollars.

KIM ALEXANDER: In 2007, prior to the wind values coming on our tax roll, our property values were at about 65 million dollars. And then, that wind development, they jumped to approximately 400 million dollars, to 465 million dollars.

NARRATOR: The school district will get more than 10 million dollars over a decade. That guaranteed revenue stream unlocked additional funding. School buildings, some dating from the 1930s, could be updated, and computer labs added.

CLIFF: This is an indication to me of what can be done for rural areas, and will be done, all the way to Canada-- bringing life and prosperity back to these rural communities that are suffering just like we have.

NARRATOR: The same oil shock that got Brazil started on ethanol, got Denmark started on manufacturing wind turbines, just in time to compensate for a decline in its shipbuilding industry.

LYYKE: And it's also good for the economy, in terms of export. I mean, 10 percent of Danish exports comes from the cleantech area.

NARRATOR: Energy and environment always require tradeoffs, such as clear vistas versus clean energy. It's something that communities have to make time to work through. Cliff, for one, believes it's worth it.

CLIFF: Everything, the schools, the churches, the civic organizations, all the businesses will benefit from this. It will increase, hopefully, our town's populations, and our economics.

KIM ALEXANDER: My granddad used to say, not realizing he was prophetic, but "If we could sell the wind, we'd be wealthy." Well, who would have ever thought we'd be able to sell the wind?

NARRATOR: For Samso, Denmark, and Texas, clean energy brought economic benefits and energy security. But replacing fossil fuel emissions with wind power has other advantages.

LYKKE: And let's not forget, also good for climate and health, and such, and that's a very important argument.

CLIFF: We've got a constant wind resource here, that's tremendously valuable, and as opposed to oil and gas, it'll last forever, and it doesn't pollute anything.

Credit: Earth: The Operators' Manual [11]. ""Yes, In My BackYard" (aka YIMBY!) [12]." YouTube. April 22, 2012.

We have already mentioned the US Production Tax Credit, which is responsible for a good amount of the trend in US wind energy investment – both up and down! A decline in wind investment in 2010 and 2011 was due in part to the global financial crisis. A drop in natural gas/wholesale electricity prices has made some planned projects less competitive than originally expected and halted development. There has also been a slump in the overall demand for energy. Another factor that limits the growth of wind power capacity is the constraint on the transmission infrastructure. As can be seen in the wind capacity map on the previous page, many of the locations that experience the windiest conditions are not close to coastal population centers. The cost of upgrading this infrastructure is significant — perhaps $30 to $90 billion in the US by the year 2030 according to some estimates. This seems like a huge amount, but consider that our government spends about \$20 billion each year in direct subsidies to the fossil fuel industry, which would sum up to \$200 billion by the year 2030. In light of that, the upgrade cost for better transmission lines is a bargain!

Note

A great resource for information on the current state of the US wind market and the wind industry, in general, is the US Wind Technologies Market Report [13]which is annually published by the Mark Bolinger and Ryan Wiser of the Lawrence Berkeley National Laboratory.