The Anatomy of a Wind Turbine

Most wind turbines on the market today are known as horizontal axis wind turbines or HAWTs (Figure 3.1a). The second type is the vertical axis wind turbine or VAWT (Figure 5.1b).

Horizontal Axis Wind Turbines

Horizontal axis wind turbines come in two basic varieties: upwind or downwind (Figure 5.2a and b). In most HAWTs, the rotor is located on the upwind side of the tower when the turbine is operating, hence the name, upwind turbine. Those in which the rotor is located on the downwind side of the turbine when the wind is blowing are referred to as downwind turbines.



Fig. 5.1: (a) Horizontal axis wind turbine (b) Vertical axis wind turbine. Although there’s a lot of interest these days in vertical axis wind turbines, they are mounted at ground level, which exposes them to unproductive low-speed winds.



Fig. 5.2: (a) Upwind turbine (b) Downwind turbine

Upwind HAWTs consist of three main parts: (1) a rotor, (2) an alternator, and (3) a tail (Figure 5.3). As noted in Chapter 3, the rotor consists of blades attached to a central hub covered by a nose cone to improve aerodynamics. Most upwind turbines have three blades. This entire assembly rotates when wind blows past the blades, hence the name “rotor.”

In many small wind turbines the rotor is attached to a shaft that’s attached to an alternator, a device that produces AC electricity. As



Fig. 5.3: Anatomy of a Wind Generator. A wind turbine consists of blades attached to a hub, forming the rotor. The rotor in this turbine is attached via a shaft (not shown) to the rotor of the alternator. When the rotor of the turbine spins, it produces electricity.

the name implies, in HAWTs the axis of the rotor is oriented horizontally, that is, parallel to the ground.

Alternators produce electricity. They consist of two main parts: a set of stationary windings, known as the stator; and a set of rotating magnets, known as the rotor. Most small wind turbines use metal magnets, rather than electromagnets. The movement of the magnets around the windings (as the blades spin) induces an electrical current in the windings. Wind turbines, therefore, first convert the kinetic energy of the wind into mechanical energy (rotation). The mechanical energy is then converted into electrical energy in the alternator.

HAWTs can also be classified according to their end use. Turbines designed to charge batteries are called “battery-charging turbines.” Turbines designed to connect to the grid are referred to as “batteryless grid-tie turbines.”

Figure 5.4 shows another common wind turbine configuration. In this design, there’s no shaft. The blades of the turbine attach to a faceplate that is attached directly to a cylindrical metal “can.” Together the blades and the face plate form the rotor of the turbine. The can to which the faceplate is attached contains magnets. They spin around a set of stationary coils of copper wire, the windings. The can is therefore also the rotor of the alternator. The windings constitute the stator of the alternator. As the rotor of the



Fig. 5.4: Anatomy of a Modern Wind Turbine. Many small wind turbines now directly link the rotor of the turbine to the rotor of the alternator, as shown here. The magnets are attached to the inside of the can, shown in the drawing. They rotate around the stationary windings of the alternator stator.

turbine spins, the magnets rotate around the windings, producing AC electricity in the windings via electromagnetic induction.

Alternators in many modern small wind turbines contain highstrength rare earth magnets. Rare earth magnets contain neodymium, iron and boron. They produce a much stronger magnetic field than conventional iron or ceramic magnets used in some small wind turbines. The stronger the magnetic field, the greater the output of a wind turbine, all things being equal. Alternators that contain magnets such as these are referred to as permanent magnetic generators.

Electricity leaves the alternator via wires that attach to the stator. In most turbines, like the one shown in Figure 5.4, these wires terminate on metal brushes. The brushes, in turn, contact slip rings, which are brass rings located near the yaw bearing. The yaw bearing allows the turbine to turn in response to changes in the wind direction. The brushes transfer electricity from the alternator to the slip rings. The slip rings, in turn, connect to a cable (electric wire) that runs down the length of the tower. Electricity flows from the alternator to the brushes to the slip rings and then down the tower.

Alternator vs. Generator

A generator is a machine that converts mechanical energy into electrical energy, either AC or DC. A generator that produces AC is called an AC generator or, more commonly, an alternator. Alternators that produce DC electricity are known as DC generators or simply generators. Most, if not all, modern wind turbines contain alternators, which produce AC electricity. Some of these turbines are equipped with rectifiers, devices that convert the AC to DC, which is then sent down the tower.

Wind generators such as the ones shown in Figure 5.4 are known as direct drive turbines because the rotor of the turbine is attached directly to the rotor of the alternator. As a result, the rotor and the alternator turn at the same speed. Although virtually all modern residential wind machines are direct drive, a few of the small wind turbines contain gearboxes. They’re located between the rotor of the turbine and the rotor of the alternator. Gearboxes increase the speed
at which the alternator spins, increasing the output of the alternator. This allows the alternator to be much smaller and also maintains a rotor speed (blade speed) that is safe and quiet. These turbines are known as gear-driven turbines.

Another important component of most horizontal axis wind machines is the tail. The tail typically consists of a boom and a vane. The tail boom connects the tail vane to the body of the turbine.

Tails keep the rotor of the wind turbine pointing into the wind. If the wind direction shifts, the tale vane turns the turbine into the wind, ensuring maximum electrical energy production. The rotation of a wind machine on a tower as it tracks the wind direction is referred to as yawing.

Although upwind turbines dominate the market, several manufacturers produce downwind turbines: Proven, Southwest Wind Power (Skystream), Entegrity, Ventera and PGE. These wind turbines contain no tails.

Downwind turbines work well. However, if the wind dies down and then reverses direction, downwind turbines can get caught in the upwind position (with blades upwind from the tower). When stuck upwind, downwind turbines are unable to spin and generate electricity. As the wind speed increases or shifts direction, however, the turbine aligns properly.

Vertical Axis Wind Turbines

There’s another type of wind machine that is getting a lot of attention these days. It is known as a vertical axis wind turbine. As shown in Figure 5.1b, the blades of a vertical axis wind turbine (VAWT) are attached to a central vertical shaft. When the blades spin, the shaft spins. The shaft is attached to an alternator generally located at the bottom of the shaft, often at ground level.

Vertical axis wind energy devices have been around for a long time, about 3,000 years. Proponents of VAWTs tout a number of supposed advantages over HAWTs, most of which are either wrong or grossly exaggerated. One of them is that they can capture wind from any direction. The machines don’t need to be oriented into the wind as the HAWTs do. In addition, proponents like to claim that VAWTs are immune to turbulence that wrecks havoc with HAWTs.

Another supposed advantage is that VAWTs can be mounted close to the ground — even on top of buildings — where they capture ground-level winds. This eliminates the need for tall and costly towers and the need to obtain the zoning variances sometimes required to install horizontal axis wind turbines on tall towers.

Yet another supposed advantage of VAWTs stems from the fact that the generator can be mounted at ground level. This, say proponents, makes it easier to access and repair the generator should the need arise. There’s no need to climb the tower — or lower a wind generator to the ground — to perform routine maintenance or to replace damaged parts.

Unfortunately, years of experience with VAWTS have been rather discouraging. “Hundreds of commercial VAWTs were installed in California in the late 1980s and early 1990s,” Bob Aram
reminds us. “They all failed and were removed from service. These were not experimental units, but production units.” In addition, VAWTs are less efficient than horizontal axis wind machines. “For a given swept area,” Jim Green notes, “they just don’t extract as much wind energy as a well-designed HAWT.” Moreover, the blades of VAWTs are prone to fatigue created as the blades spin around the central axis. The vertically oriented blades used in some early models,
for instance, twisted and bent as they rotated in the wind. This caused the blades to flex and crack. Over time, the blades broke apart, sometimes leading to catastrophic failure. Because of these problems, VAWTs have proven less reliable than HAWTs.

“The VAWT does have an advantage in dealing with wind direction shifts,” agrees Robert Preus, wind energy expert and manufacturer of Abundant Renewable Energy turbines (horizontal axis
wind turbines). However, rapidly changing wind direction that occurs in turbulent low-level winds increases fatigue on a VAWT, just like a HAWT. “Fatigue leads to equipment failure, which has been a major problem with VAWTs.”

Many VAWTs also require large bearings at the top of the tower to permit rotation of the shaft. When the top bearings or the blades need replacement, you’ve got a job on your hands.

Although VAWTs can capture ground-level winds, just like any turbine installed on a too-short tower, they are just as sensitive to turbulence and ground drag as horizontal axis wind turbines. As you learned in Chapter 2, ground-level winds are subject to friction (which creates ground drag). Both ground drag and turbulence in lower-level winds diminish the power available to any turbine mounted close to the ground — so much so that there is very little extractable energy in wind in such locations. The lower the wind speed, the less electricity a turbine will produce. In addition, dead air spaces form behind buildings and other ground clutter. Placing a VAWT in such a location renders it useless. So just because a VAWT can be mounted at ground level doesn’t mean it will produce enough electricity to be worthwhile.

VAWTs are less reliable and less efficient than HAWTs. They just don’t stack up against horizontal axis wind turbines. The few advantages they offer cannot counter the many, some say fatal, disadvantages.

How Much Electricity Will a Wind Turbine Produce?

Once you’ve determined the average wind speed at a site, it is time to determine how much electricity a wind generator could produce at the proposed tower height — and therefore whether it can meet all your needs or what percentage of your needs it will satisfy. This step is fairly easy.

Table 4.1 shows a list of wind turbines and the estimated annual output of each turbine (in kilowatt-hours) at seven different average wind speeds. To see how this table is used, consider an example.



Let’s assume that the average wind speed at a site (at hub height) is 12 miles per hour. Let’s also assume that your load analysis, after efficiency measures have been implemented, indicates you’ll need, on average, 900 kilowatt-hours per month, or 10,800 per year. In the 12 mile-per-hour column, you’ll discover two wind turbines that match your electrical requirements, the WT6000 (by Proven) and BWC (Bergey Wind Power’s) XL-S. If the wind speed at your site is 13 miles per hour, an Endurance wind turbine would meet your needs. The Proven WT 6000 and the Bergey XL-S
would produce more than you need.

Annual energy outputs used to estimate the economic performance of a wind energy system can also be found in an article Mick and Ian published in Home Power magazine (Issue 131), entitled “How to Buy a Wind Generator.”

You can also obtain annual energy output data directly from wind turbine manufacturers. While this data is useful, manufacturers tend to overstate the electric production of their turbines. As a result, we recommend derating their estimated outputs by 20 percent — just to be conservative. (Note that the data from Wisconsin’s Focus on Energy is derated annual energy output; the data in Ian and Mick’s article is manufacturer data and is not derated.)

Once you’ve found the wind turbine that meets your needs, you need to be sure it is the appropriate type of turbine. That is, you need to be sure you select a grid-tied turbine or battery-based turbine, depending on the type of system you are planning on installing.

Small-Scale Wind Energy

Humans have harvested energy from the wind for centuries. Harnessed by the Europeans as early as 900 years ago, wind was used to grind grain and manufacture goods. Wind powered ships that helped open up new territories, spurring international trade. In North America, wind energy has been used since the late 1800s. Over the years, tens of thousands of farms in the Great Plains relied on wind pump water for livestock and domestic uses — some still do.

Windmills began to emerge in the 1860s in rural America. By 1890, there were over 100 manufacturers of water-pumping windmills (Figure 1.1). All told over 8 million were installed in this country. Many of these water-pumping windmills have been restored and are still operating today with minimal maintenance.

Windmill vs. Wind Turbine

A windmill is a machine that converts the energy of the wind into other, more useful forms like mechanical energy. Early windmills were designed to grind grain and pump water. Later on, windmills were designed to generate electricity. Electricity- generating windmills are commonly referred to as wind turbines or wind generators. Waterpumping windmills are generally referred to as such or simply as windmills.

Wind energy was also extremely important to railroads in the West. Windmills were often used to fill water tanks along tracks to supply the steam engines of locomotives.

In the 1920s through the early 1950s, many Plains farmers also installed wind turbines to generate electricity. The turbines powered lights and all their appliances, many of which were ordered from the Sears catalog — including electric toasters, washing machines and radios. Radios were particularly important, as they allowed farmers and their families to keep in touch with the world.

Unfortunately, the use of water-pumping and small wind-powered electric generators began to decline in the United States in the late 1930s. Their demise was due in large part to America’s ambitious Rural Electrification Program. This program, which began in 1937, was designed to provide electricity to rural America. As electric service became available, wind-electric generators were mothballed.

Fig. 1.1: The Old and the New. Water-pumping windmills like the one in the foreground were once common in the West and Midwest. The technology hasn’t changed in 100 years. In the distance is a modern commercial wind turbine that generates electricity to power cities and towns.

In fact, local power companies required farmers to dismantle their wind generators as a condition for providing service via the evergrowing electrical grid. The electrical grid, or simply the grid, is the extensive network of high-voltage electrical transmission lines that crisscross nations, delivering electricity generated at centralized power plants to cities, towns and rural customers. A key advantage of the grid was its ability to provide virtually unlimited amounts
of electricity to customers.

Unfortunately, rural electrification drove virtually all of the manufacturers of windmills and wind-electric generators out of business by the early 1950s. However, in the mid-1970s, wind
energy made a resurgence as a result of intense interest in energy self-sufficiency in the United States, stimulated principally by backto- back oil crises in the 1970s that resulted in skyrocketing oil prices and a period of crippling inflation. Generous federal incentives
for small wind turbines, incentives from state governments, and changes in US law that required utilities to buy excess electricity from small renewable energy generators helped stimulate the
comeback.

Soon thereafter, however, wind energy took a nosedive. Conservation and energy efficiency measures in the United States and new, more reliable sources of oil drove the price of oil and gasoline down. Federal and state renewable energy tax incentives disappeared as a result of a precipitous decline in America’s concern for energy independence. As a result, all but a handful of the small wind turbine manufacturers went out of business.

In the 1990s, commercial and residential wind energy staged another comeback as a result of many factors, among them rising oil prices, global awareness of the decline in world oil production, an increase in the cost of natural gas, and growing concern for global climate change and its impacts.

Because of these factors, many believe that this time around, wind energy is here to stay. Much to the delight of renewable energy advocates, large commercial wind farms have begun to appear in numerous countries, most notably the United States, Germany, Spain

Fig. 1.2: Global Wind Energy Capacity. This graph shows the installed global capacity (in megawatts) of commercial wind turbines.

and Denmark. These facilities produce huge amounts of electricity and are changing the way the world meets its energy needs. Today, wind-generated electricity is the fastest growing source of energy in the world (Figure 1.2).

Although commercial wind farms are responsible for most of the growth in the wind industry, smaller residential-scale wind machines are also emerging in rural areas, supplying electricity to homes, small businesses, farms, ranches and schools (Figure 1.3). Most of the small-scale wind turbines “feed” the excess electricity they produce back onto the electrical grid.