How Wind Power Works

The simplest possible wind-energy turbine consists of three crucial parts:

• Rotor blades - The blades are basically the sails of the system; in their simplest form, they act as barriers to the wind (more modern blade designs go beyond the barrier method). When the wind forces the blades to move, it has transferred some of its energy to the rotor.
  

• Shaft - The wind-turbine shaft is connected to the center of the rotor. When the rotor spins, the shaft spins as well. In this way, the rotor transfers its mechanical, rotational energy to the shaft, which enters an electrical generator on the other end.

• Generator - At its most basic, a generator is a pretty simple device. It uses the properties of electromagnetic induction to produce electrical voltage - a difference in electrical charge. Voltage is essentially electrical pressure - it is the force that moves electricity, or electrical current, from one point to another. So generating voltage is in effect generating current. A simple generator consists of magnets and a conductor. The conductor is typically a coiled wire. Inside the generator, the shaft connects to an assembly of permanent magnets that surrounds the coil of wire. In electromagnetic induction, if you have a conductor surrounded by magnets, and one of those parts is rotating relative to the other, it induces voltage in the conductor. When the rotor spins the shaft, the shaft spins the assembly of magnets, generating voltage in the coil of wire. That voltage drives electrical current (typically alternating current, or AC power) out through power lines for distribution. (See How Electromagnets Work to learn more about electromagnetic induction, and see How Hydropower Plants Work to learn more about turbine-driven generators.)

Now that we've looked at a simplified system, we'll move on to the modern technology you see in wind farms and rural backyards today. It's a bit more complex, but the underlying principles are the same.

When you talk about modern wind turbines, you're looking at two primary designs: horizontal-axis and vertical-axis. Vertical-axis wind turbines (VAWTs) are pretty rare. The only one currently in commercial production is the Darrieus turbine, which looks kind of like an egg beater.

Photo courtesy NREL (left) and Solwind Ltd Vertical-axis wind turbines (left: Darrieus turbine)

In a VAWT, the shaft is mounted on a vertical axis, perpendicular to the ground. VAWTs are always aligned with the wind, unlike their horizontal-axis counterparts, so there's no adjustment necessary when the wind direction changes; but a VAWT can't start moving all by itself -- it needs a boost from its electrical system to get started. Instead of a tower, it typically uses guy wires for support, so the rotor elevation is lower. Lower elevation means slower wind due to ground interference, so VAWTs are generally less efficient than HAWTs. On the upside, all equipment is at ground level for easy installation and servicing; but that means a larger footprint for the turbine, which is a big negative in farming areas.

Darrieus-design VAWT

VAWTs may be used for small-scale turbines and for pumping water in rural areas, but all commercially produced, utility-scale wind turbines are horizontal-axis wind turbines (HAWTs).

Photo courtesy GNU; Photographer: Kit Conn Wind farm in California

As implied by the name, the HAWT shaft is mounted horizontally, parallel to the ground. HAWTs need to constantly align themselves with the wind using a yaw-adjustment mechanism. The yaw system typically consists of electric motors and gearboxes that move the entire rotor left or right in small increments. The turbine's electronic controller reads the position of a wind vane device (either mechanical or electronic) and adjusts the position of the rotor to capture the most wind energy available. HAWTs use a tower to lift the turbine components to an optimum elevation for wind speed (and so the blades can clear the ground) and take up very little ground space since almost all of the components are up to 260 feet (80 meters) in the air.

Large HAWT components:

• rotor blades - capture wind's energy and convert it to rotational energy of shaft
• shaft - transfers rotational energy into generator
• nacelle - casing that holds the gearbox (increases speed of shaft between rotor hub and generator), generator {uses rotational energy of shaft to generate electricity using electromagnetism), electronic control unit (monitors system, shuts down turbine in case of malfunction and controls yaw mechanism), yaw controller (moves rotor to align with direction of wind) and brakes (stop rotation of shaft in case of power overload or system failure).
• tower - supports rotor and nacelle and lifts entire setup to higher elevation where blades can safely clear the ground
• electrical equipment - carries electricity from generator down through tower and controls many safety elements of turbine

From start to finish, the process of generating electricity from wind -- and delivering that electricity to people who need it -- looks something like this:

Unlike the old-fashioned Dutch windmill design, which relied mostly on the wind's force to push the blades into motion, modern turbines use more sophisticated aerodynamic principles to capture the wind's energy most effectively. The two primary aerodynamic forces at work in wind-turbine rotors are lift, which acts perpendicular to the direction of wind flow; and drag, which acts parallel to the direction of wind flow.

Turbine blades are shaped a lot like airplane wings -- they use an airfoil design. In an airfoil, one surface of the blade is somewhat rounded, while the other is relatively flat. Lift is a pretty complex phenomenon and may in fact require a Ph.D. in math or physics to fully grasp. But in one simplified explanation of lift, when wind travels over the rounded, downwind face of the blade, it has to move faster to reach the end of the blade in time to meet the wind travelling over the flat, upwind face of the blade (facing the direction from which the wind is blowing). Since faster moving air tends to rise in the atmosphere, the downwind, curved surface ends up with a low-pressure pocket just above it. The low-pressure area sucks the blade in the downwind direction, an effect known as "lift." On the upwind side of the blade, the wind is moving slower and creating an area of higher pressure that pushes on the blade, trying to slow it down. Like in the design of an airplane wing, a high lift-to-drag ratio is essential in designing an efficient turbine blade. Turbine blades are twisted so they can always present an angle that takes advantage of the ideal lift-to-drag force ratio. See How Airplanes Work to learn more about lift, drag and the aerodynamics of an airfoil.

Aerodynamics is not the only design consideration at play in creating an effective wind turbine. Size matters -- the longer the turbine blades (and therefore the greater the diameter of the rotor), the more energy a turbine can capture from the wind and the greater the electricity-generating capacity. Generally speaking, doubling the rotor diameter produces a four-fold increase in energy output. In some cases, however, in a lower-wind-speed area, a smaller-diameter rotor can end up producing more energy than a larger rotor because with a smaller setup, it takes less wind power to spin the smaller generator, so the turbine can be running at full capacity almost all the time. Tower height is a major factor in production capacity, as well. The higher the turbine, the more energy it can capture because wind speeds increase with elevation increase -- ground friction and ground-level objects interrupt the flow of the wind. Scientists estimate a 12 percent increase in wind speed with each doubling of elevation.

To calculate the amount of power a turbine can actually generate from the wind, you need to know the wind speed at the turbine site and the turbine power rating. Most large turbines produce their maximum power at wind speeds around 15 meters per second (33 mph). Considering steady wind speeds, it's the diameter of the rotor that determines how much energy a turbine can generate. Keep in mind that as a rotor diameter increases, the height of the tower increases as well, which means more access to faster winds.

At 33 mph, most large turbines generate their rated power capacity, and at 45 mph (20 meters per second), most large turbines shut down. There are a number of safety systems that can turn off a turbine if wind speeds threaten the structure, including a remarkably simple vibration sensor used in some turbines that basically consists of a metal ball attached to a chain, poised on a tiny pedestal. If the turbine starts vibrating above a certain threshold, the ball falls off the pedestal, pulling on the chain and triggering a shut down.

Probably the most commonly activated safety system in a turbine is the "braking" system, which is triggered by above-threshold wind speeds. These setups use a power-control system that essentially hits the brakes when wind speeds get too high and then "release the brakes" when the wind is back below 45 mph. Modern large-turbine designs use several different types of braking systems:

• Pitch control - The turbine's electronic controller monitors the turbine's power output. At wind speeds over 45 mph, the power output will be too high, at which point the controller tells the blades to alter their pitch so that they become unaligned with the wind. This slows the blades' rotation. Pitch-controlled systems require the blades' mounting angle (on the rotor) to be adjustable.
• Passive stall control - The blades are mounted to the rotor at a fixed angle but are designed so that the twists in the blades themselves will apply the brakes once the wind becomes too fast. The blades are angled so that winds above a certain speed will cause turbulence on the upwind side of the blade, inducing stall. Simply stated, aerodynamic stall occurs when the blade's angle facing the oncoming wind becomes so steep that it starts to eliminate the force of lift, decreasing the speed of the blades.
• Active stall control - The blades in this type of power-control system are pitchable, like the blades in a pitch-controlled system. An active stall system reads the power output the way a pitch-controlled system does, but instead of pitching the blades out of alignment with the wind, it pitches them to produce stall.

(See Petester's Basic Aerodynamics for a nice explanation of both lift and still.)

Globally, at least 50,000 wind turbines are producing a total of 50 billion kilowatt-hours (kWh) annually. In the next section, we'll examine the availability of wind resources and how much electricity wind turbines can actually produce.

On a global scale, wind turbines are currently generating about as much electricity as eight large nuclear power plants. That includes not only utility-scale turbines, but also small turbines generating electricity for individual homes or businesses (sometimes used in conjunction with photovoltaic solar energy). A small, 10-kW-capacity turbine can generate up to 16,000 kWh per year, and a typical U.S. household consumes about 10,000 kWh in a year.

A typical large wind turbine can generate up to 1.8 MW of electricity, or 5.2 million KWh annually, under ideal conditions -- enough to power nearly 600 households. Still, nuclear and coal power plants can produce electricity cheaper than wind turbines can. So why use wind energy? The two biggest reasons for using wind to generate electricity are the most obvious ones: Wind power is clean, and it's renewable. It doesn't release harmful gases like CO2 and nitrogen oxides into the atmosphere the way coal does (see How Global Warming Works), and we are in no danger of running out of wind anytime soon. There is also the independence associated with wind energy, as any country can generate it at home with no foreign support. And a wind turbine can bring electricity to remote areas not served by the central power grid.

But there are downsides, too. Wind turbines can't always run at 100 percent power like many other types of power plants, since wind speeds fluctuate. Wind turbines can be noisy if you live close to a wind plant, they can be hazardous to birds and bats, and in hard-packed desert areas there is a risk of land erosion if you dig up the ground to install turbines. Also, since wind is a relatively unreliable source of energy, operators of wind-power plants have to back up the system with a small amount of reliable, non-renewable energy for times when wind speeds die down. Some argue that the use of unclean energy to support the production of clean energy cancels out the benefits, but the wind industry claims that the amount of unclean energy that's necessary to maintain a steady supply of electricity in a wind system is far too small to defeat the benefits of generating wind power.

Potential disadvantages aside, the United States has a good number of wind turbines installed, totaling more than 9,000 MW of generating capacity in 2006. That capacity generates in the area of 25 billion kWh of electricity, which sounds like a lot but is actually less than 1 percent of the power generated in the country each year. As of 2005, U.S. electricity generation breaks down like this:

• Coal: 52%
• Nuclear: 20%
• Natural gas: 16%
• Hydropower: 7%
• Other (including wind, biomass, geothermal and solar): 5%

Source: American Wind Energy Association

The current total electricity generation in the United States is in the area of 3.6 trillion kWh every year. Wind has the potential to generate far more than 1 percent of that electricity. According to American Wind Energy Association, the estimated U.S. wind-energy potential is about 10.8 trillion kWh per year -- about equal to the amount of energy in 20 billion barrels of oil (the current global yearly oil supply). To make wind energy feasible in a given area, it requires minimum wind speeds of 9 mph (3 meters per second) for small turbines and 13 mph (6 meters per second) for large turbines. Those wind speeds are common in the United States, although most of it is unharnessed.

When it comes to wind turbines, placement is everything. Knowing how much wind an area has, what the speeds are and how long those speeds last are the crucial deciding factors in building an efficient wind farm. The kinetic energy in wind increases exponentially in proportion to its speed, so a small increase in wind speed is in fact a large increase in power potential. The general rule of thumb is that with a doubling a wind speed comes an eight-fold increase in power potential. So theoretically, a turbine in an area with average wind speeds of 26 mph will actually generate eight times more electricity than one set up where wind speeds average 13 mph. It's "theoretically" because in real-world condition, there is a limit to how much energy a turbine can extract from the wind. It's called the Betz limit, and it's about 59 percent. But a small increase in wind speed still leads to a significant increase in power output.

Photo courtesy General Electric Company
Raheenleagh wind farm

As in most other areas of power production, when it comes to capturing energy from the wind, efficiency comes in large numbers. Groups of large turbines, called wind farms or wind plants, are the most cost-efficient use of wind-energy capacity. The most common utility-scale wind turbines have power capacities between 700 KW and 1.8 MW, and they're grouped together to get the most electricity out of the wind resources available. They are typically spaced far apart in rural areas with high wind speeds, and the small footprint of HAWTs means that agricultural use of the land in nearly unaffected. Wind farms have capacities ranging anywhere from a few MW to hundreds of MW. The world's largest wind plant is the Raheenleagh Wind Farm located off the coast of Ireland. At full capacity (it's currently operating at partial capacity), it will have 200 turbines, a total power rating of 520 MW and cost nearly $600 million to build.

The cost of utility-scale wind power has come down dramatically in the last two decades due to technological and design advancements in turbine production and installation. In the early 1980s, wind power cost about 30 cents per kWh. In 2006, wind power costs as little as 3 to 5 cents per kWh where wind is especially abundant. The higher the wind speed over time in a given turbine area, the lower the cost of the electricity that turbine produces. On average, the cost of wind power is about 4 to 10 cents per kWh in the United States.

Many large energy companies offer "green pricing" programs that let customers pay more per kWh to use wind energy instead of energy from "system power," which is the pool of all of the electricity produced in the area, renewable and non-renewable. If you choose to purchase wind energy and you live in the general vicinity of a wind farm, the electricity you use in your home might actually be wind-generated; more often, the higher price you pay goes to support the cost of wind energy, but the electricity you use in your home still comes from system power. In states where the energy market has been deregulated, consumers may be able to purchase "green electricity" directly from a renewable-energy provider, in which case the electricity they're using in their homes definitely does come from wind or other renewable sources.

Implementing a small wind turbine system for your own needs is one way to guarantee that the energy you use is clean and renewable. A residential or business turbine setup can cost anywhere from $5,000 to $80,000. A large-scale setup costs a whole lot more. A single, 1.8-MW turbine can run up to $1.5 million installed, and that's not including the land, transmission lines and other infrastructure costs associated with a wind-power system. Overall, wind farms cost in the area of $1,000 per kW of capacity, so a wind farm consisting of seven 1.8-MW turbines runs about $12.6 million. The "payback time" for a large wind turbine -- the time it takes to generate enough electricity to make up for the energy consumed building and installing the turbine -- is about three to eight months, according to the American Wind Energy Association.

Government incentives for both large- and small-scale producers contribute to the economic feasibility of a wind-power system. Just a few of the current economic incentive programs for renewable energy systems include:

• Production Tax Credit: Basically, wind-power generators, usually businesses, receive 1.8 cents (as of Dec. 2005) per kWh of wind energy produced for wholesale distribution during the first 10 years the wind farm is up and running.

• Net metering - In this system, individuals and businesses producing renewable energy receive credits for each kWh they produce beyond their own needs. When someone produces more electricity than he needs, his power meter runs backwards, sending that excess electricity to the power grid. He receives credits for the electricity he sends to the grid, which count as payment toward any electricity he draws from the grid when his turbine can't provide enough power for his home or business. (Many large energy companies don't much care for this setup since they are essentially purchasing the individual producer's wind power at retail price instead of the wholesale price they'd be paying a wind farm.)

• Renewable-energy credits - Many states now have renewable-energy quotas for power companies, whereby those companies have to buy a certain percentage of their electricity from renewable sources. If someone with his own turbine lives in a state that has a "green credit program," he receives tradable credits for each megawatt-hour of renewable energy he produces in a year. He can then sell those credits to large, conventional-energy companies looking to meet their state or federal renewable-energy quota.

• Installation tax credits: The federal government and some states offer tax credits for the costs of setting up a renewable-energy system. Maryland, for instance, offers businesses or landlords a credit for 25 percent of the cost of purchasing and installing a wind-turbine system if the energy-supplied building meets certain overall "green criteria."

Photo courtesy NREL (left) and stock.xchng Residential wind turbine (left) and utility-scale wind turbine

While wind energy is still subsidized by the government, it is currently a competitive product and, by most accounts, can stand on its own as a viable power source. The Battelle Pacific Northwest Laboratory, a U.S. Department of Energy science and technology lab, estimates that wind power is capable of supplying 20 percent of the United States' electricity based on wind resources alone. The American Wind Energy Association puts that number at a theoretical 100 percent. Whichever estimate is right, the United States probably won't be seeing those percentages anytime soon. The American Wind Energy Association projects that by 2020, wind will provide 6 percent of all U.S. electricity. While the United States has one of the largest installed wind-power bases in the world in terms of sheer wattage, percentage-wise, it is lagging behind other developed countries. The United Kingdom has a stated goal of 10 percent wind power by 2010. Germany currently generates 8 percent of its power from wind, and Spain is at 6 percent. Denmark, the world leader in clean-energy consumption, gets more than 20 percent of its electricity from wind.

For more information on wind power and related topics, check out the links on the next page.