Proper Siting of a Wind Machine

A wind turbine must be mounted in a good wind site, well above ground clutter in the strongest, smoothest winds. Wind site assessors begin the process of siting a wind turbine by determining the prevailing wind direction at a site. Although winds blow in different directions at different times of the year, or even within the same day, they arrive from one or two directions predominantly over the course of the year. In many places in North America, winds come predominantly from the southwest — thanks to the Coriolis effect. They often blow from the northwest in the winter.

To determine the predominant wind flow, ask the advice of farmers, who work outdoors and hence are familiar with wind patterns, or contact a local airport. They may be able to provide you with a wind rose, a graphical representation of wind direction (Figure 6.14). In a wind rose, the length of the spokes around the circle is an indication of how frequently the wind blows from a particular direction. The longer the line, the greater the frequency. In the wind rose in Figure 6.14, the winds blow predominantly from the southwest. A wind rose also indicates the percentage of total wind energy from each direction, which is very helpful.

In an open site, with little ground clutter, a wind turbine can be located almost anywhere — so long as the entire rotor is mounted 30 feet above the tallest obstacle within a 500-foot radius and you’ve taken into account future tree growth, if trees are the tallest objects.



Fig. 6.14: Wind Rose. This unique graph shows how often winds blow from various directions and the percent energy of the wind for various directions. The wider white bars represent the percent of total energy from different directions and the narrower, shaded bars illustrate the percent of total time from each of the sixteen different direction sectors.

Unfortunately, very few of us live on ideal sites. There’s almost always some major obstacles.

To site a wind turbine, first determine the prevailing wind direction, then look for a location for the tower that’s upwind of major obstacles. Although winds will shift so that upwind temporarily becomes downwind, situating your wind turbine and tower this way will ensure that it can take advantage of the strongest prevailing winds.

When siting a wind machine, it is also a good idea to minimize wire runs from the turbine to the controller and inverter to reduce line loss. As a rule, the higher the wind turbine’s voltage, the farther it can be sited from the point of use. When installing a turbine, contact the manufacturer or an experienced installer for recommendations.

Building Your Own Wind Machine

If don’t have the cash to buy a wind turbine or want the challenge of making your own turbine, you may want to consider building your own wind turbine (Figure 5.11). Building your own wind generator provides invaluable experience.

Directions for how to build a wind machine are available online. At sites like Hugh Piggott’s (scoraigwind.com) or Dan Bartmann’s

Fig. 5.11: This amazingly quiet wind turbine designed and built by the folks from otherpower.com is remarkably reliable and efficient. Other Power sells kits that require some assembly for independent and budget-constrained buyers.

(otherpower.com), you’ll find a listing of other valuable websites and resources.

Homebrew wind turbine master Hugh Piggott’s website sells step-by-step plans for building axial-flux alternator turbines.

There are even a couple of books on the subject: Windpower Workshop by Hugh Piggott and a more recent title, Homebrew Wind Power: A Hands-On Guide to Harnessing the Wind by Other Power’s Dan Bartmann and Dan Fink, which is an extremely well-written and thorough treatise on the subject. Rather than provide a full description of this subject, which would take a chapter or two, maybe even an entire book, we recommend you consult these resources. We also recommend that you sign up for a workshop, for example, through The Evergreen Institute or otherpower.com.

Building a wind generator is not that difficult, if you are mechanically inclined and persistent. You will need a shop and some common hand and power tools. “The hardest part of designing a small
windmill for electricity production is to find a suitable generator,” writes Piggott in Windpower Workshop. “For best performance you’ll need a reliable, low-speed generator that’s pretty efficient in light winds,” he adds. Piggott strongly recommends permanent magnet alternators. “Unfortunately, permanent magnetic alternators are difficult to find.” Piggott lists some sources, among them motorcycle alternators and welders. Unfortunately, each one has a significant downside. See his website and book for more on this and other options.

Car alternators are popular among do-it-yourselfers; however, they require high rpm and are not always very efficient. They also require a lot of electricity to power the electromagnets that create the magnetic field. You’ll also need to rewind the stator coils so they produce electricity at the lower speeds common in wind turbines. This is a big job for those without experience. Most people who’ve tried this route give up in failure.

When building a wind turbine, you’ll also have to design and build blades and install controls. Blades and rotors are particularly tricky. They need to be carefully matched to the generator. Most homemade wind turbines use wooden blades, which must be hand carved.

You will also need to incorporate overspeed controls and a shutdown mechanism. And don’t forget you will need an inverter matched to the turbine. Be wary of cheaply made wind turbines like those with blades fashioned from PVC pipe. Remember, too, that you need to install the turbine on a tower. Tower designs can also be found online. Be careful, however; homemade towers often leave much to be desired. Towers are far more complicated to design than wind
turbines. Your best and safest bet is to buy a new or used tower.

What to Look for When Buying a Wind Machine

While there are many turbines on the market, careful load and site analysis will narrow the field considerably. Once you have determined your average monthly electrical load and the average wind speed on your site, you can select a wind turbine that will produce enough electricity to meet your demands.

Manufacturers provide a plethora of technical data on their wind machines that can be used to make comparisons. Unfortunately, most of it is useless. Further complicating matters, “There can be a big difference in reliability, ruggedness, and life expectancy from one brand to the next,” according to Mike Bergey, president of Bergey Windpower.

So how do you go about selecting a wind machine?

Although wind turbines can be compared using many criteria, there are only a handful that really matter: (1) swept area, (2) durability, (3) annual energy output, (4) governing mechanism, (5)
shut-down mechanism, and (6) sound.

Swept Area

Swept area is the area of the circle described by the spinning blades of a turbine. Because the blades of a wind turbine convert wind energy into electrical energy, the swept area is the collector area of the turbine. The greater the swept area, the greater the collector
area. The bigger the swept area, the more energy you’ll be able to capture from the wind. To get the most out of a wind turbine — to produce the most electricity at the lowest cost — select a wind turbine with the greatest swept area. Swept area allows for easy comparison of different models.

Swept area is determined by rotor diameter. The rotor diameter is the distance from one side of the circle created by the spinning blades to a point on the opposite side or about twice the length of the blades. When comparing wind turbines, then, the rotor diameter is a pretty good measure of how much electricity a turbine will generate. Although other features such as the efficiency of the generator and the design of the blades influence energy production, for most turbines they pale in comparison to the influence of rotor diameter and, hence, swept area.

Manufacturers list the rotor diameter in feet or meters — often both. The greater the blade length, the greater the rotor diameter and the greater the swept area.

Most manufacturers also list the swept area of the rotor. Swept area is presented in square feet or square meters — sometimes both.

Annual Energy Output

Another, even more useful, measure is the annual energy output (AEO) or annual energy production (AEP) at various wind speeds. The AEO of a given wind turbine is presented as kilowatt-hours of electricity produced at various average wind speeds. Like the US EPA’s estimated gas mileage for vehicles, AEO gives buyers a convenient way to compare models. As in the estimated gas mileage rating, however, AEOs won’t tell you exactly how much electricity
a wind machine will produce at a site. Performance varies depending on a number of factors such as turbulence and the density of the air.

Durability: Tower Top Weight

Another extremely important criterion is durability. The most important measure of durability is tower top weight — how much a wind turbine weighs. Four turbines that produce about the same amount of electricity are for example, the Proven WT2500 (419 pounds), the ARE110 (315 pounds), the Skystream 3.7 (170 pounds) and the Whisper 500 (155 pounds). The weight differences are in some cases substantial.

In our experience, heavyweight wind turbines tend to survive the longest — sometimes many years longer than medium or lightweight turbines. Weight is usually reflected in the price. Remember, however, that you get what you pay for. Producing electricity on a precarious
perch 80 to 165 feet above the ground isn’t a job you want to relegate to the lowest bidder, which is invariably the lightest turbine.

Balance of System Cost

Before you buy a machine, consider the total system cost. You’ll need to purchase a tower and pay for installation, unless, of course, you install the tower yourself. Even then, you’ll need to pay for concrete, rebar and equipment to excavate the foundation and anchors. You’ll also need to run electrical wire from the turbine to the house and purchase an inverter (although they’re included in most batteryless grid-tie wind turbines). If you’re going off-grid or want battery backup for your grid-connected system, you’ll also need to buy batteries. All of this will add to the cost. The cost of the turbine itself may range from 10 to 40 percent of the total system cost.

Governing Systems

Found in all wind generators worth buying, governing, or overspeed control, systems are designed to prevent a wind generator from burning out or breaking apart in high winds. They do this by slowing down the rotor when the wind reaches a certain speed, known as the governing wind speed. Why is this necessary?

As wind speed increases, the rotor of a wind turbine spins more rapidly. The increase in the revolutions per minute (rpm) increases electrical output. Although electrical output is a desirable goal, if it exceeds the machine’s rated output, the generator could overheat and burn out. In addition, centrifugal forces in high wind speeds exert incredible forces on wind turbines that can tear them apart if the rotor speed is not governed.

A governing system is essential because it allows the turbine to shed extra energy when the winds are really strong. Not all wind turbines come with governing mechanisms, however. Many of the smallest wind turbines, the micro-turbines, with rated outputs of around

Fig. 5.5: Microturbines. Many microturbines like the Marlec (shown here) have no governing mechanism to slow the rotor in high winds. They rely on the relatively low rotor speed and rugged construction to endure high winds.

400 watts, for example, have no governing mechanisms (Figure 5.5). (These turbines are too small to produce a significant amount of electricity for most applications.) Larger wind turbines, those with swept areas over 38 square feet, however, come with overspeed controls. Two types are commonly found: furling and blade pitch.

Furling

Most manufacturers protect their wind turbines by furling. Furling is accomplished in one of two ways, both of which shift the position of the rotor (hub and blades) relative to the wind. This turns the blades out of the wind, decreasing the amount of rotor swept area that intercepts the wind. Reducing the swept area reduces the speed at which the rotor turns and the energy collected. Slowing the rotor will protect the wind turbine from damage.

Manufacturers employ two main types of furling: horizontal and vertical. In horizontal furling, the rotor turns out of the wind by turning sideways. For this reason, horizontal furling is also
known as side furling. In vertical furling, the rotor rotates upward with the same effect. “Angle furling” is a combination of the two.

Horizontal or side furling is achieved, in part, by hinging the tail. In side-furling turbines, a hinge is located between the tail boom and the body of the turbine. As you can see from Figure 5.6,
the turbine is also slightly offset from the yaw axis — that is, the yaw bearing is attached to the side of the turbine body, not its center so the turbine is not directly over the tower. Because the turbine is offset from the yaw axis, the force of the wind on the blades tends

Fig. 5.6: Side Furling. This wind turbine is not broken, it is side furling in high winds, which slows the rotor and protects the machine from damage.

Fig. 5.7: Vertical Furling

to rotate the machine around the yaw axis. However, the tail resists this rotation and keeps the rotor facing into the wind.

In light winds, the forces on the rotor and tail are small and the wind holds the tail in its normal position — straight behind the turbine. However, in strong winds, the increasing forces on the rotor overcome the force of the wind on the tail. Since the tail creates more force than the offset rotor, the tail stays mostly aligned with the wind and the turbine turns away from the wind. As a result, the turbine folds on itself. This slows the rotor.

Vertical furling is achieved by moving the hinge in front of the yaw axis and rotating it slightly. In high winds, the force of the wind tilts the rotor up, while the tail stays oriented downwind. As in side furling, this reduces its speed (Figure 5.7).

When fully furled, the rotor of a vertical furling turbine resembles a helicopter rotor. When wind speed declines, however, the rotor returns to its normal operating position. Shock absorbers are
often used to ease the rotor back into position.

Furling reduces the amount of energy collected by the rotor. Although electrical output typically continues, it usually occurs at a lower rate, as shown in the power curve of the ARE442 in Figure 5.8.

Changing Blade Pitch

Fig. 5.8: Power Curves. The electrical production of the ARE442, like that of many other turbines, declines significantly in high wind speeds as a result of overspeed controls that protect the wind turbine from damage. The power production of the ARE110 plateaus, so the machine continues to produce a significant amount of energy in high winds.

Fig. 5.9: Pitch Control — Blade-Actuated Governor. Numerous ingenious methods of blade pitch control have been devised. In this turbine, a Jacobs 31-20, the springs are part of a complex and effective blade pitch control mechanism.

pitch changes automatically in these turbines as wind speed increases over a certain level. The greater the wind speed above the operating range of the machine, the more the blades rotate (pitch). Changing the angle of the blade reduces rotor speed.

Pitch control typically requires springs, gears and weights ingeniously
arranged to produce the desired effect (Figure 5.9). Some machines, like the Jacobs, use the weight of the blade itself to change the pitch.

Blade pitch functions admirably, but is not as widely used as horizontal and vertical furling mechanisms. Of the two, blade pitch control is more expensive, but provides better control of blade speed and is more reliable. Bottom line: although furling mechanisms are cheaper, cheap is not necessarily better when it comes to a wind machine. The goal in buying a wind machine is to purchase the most reliable and most durable turbine. That said, you may only have a few choices among the turbines that produce the amount of electricity you need and most of home-scale wind turbines use furling.

Shut-Down Mechanisms

Small wind turbines should include a reliable shut-down mechanism. They allow a turbine to be turned off so operators can maintain and repair a wind turbine without fear of injury. They also provide a means of shutting a wind machine down when extremely violent storms, especially thunderstorms, are approaching. Maintenance personnel engage the shut-down mechanisms when they need to work on a turbine, but they also typically secure the blades with rope — just in case the wind comes up while they’re servicing a turbine.

Wind turbines contain two types of shut-down mechanisms: mechanical and electrical. Mechanical systems include disc brakes and folding tails. Both are manually activated. They’re attached to a cable that runs down the tower. Tightening the cable activates the brake or folds the tail (side furling the machine), stopping the rotor (Figure 5.10).

Fig. 5.10: Cable Winch on Tower. Clay Sterling, MREA’s Education Director, shuts down a Jacobs wind turbine by tightening the cable attached to the tail of the turbine.

Although disc brakes may seem like a good idea, they are not fail-safe. If the cable breaks in violent storm, for example, an operator would be helpless to stop the turbine. There’s no way to apply the brakes!

Although folding the tail protects the rotor from overspeeding, it doesn’t stop it from rotating. This presents a potential risk to service personnel working on the tower, unless another means of stopping the rotor, such as a disc brake, is available. Furthermore, if the cable breaks in high winds, when the machine is shut down, the tail will swing back into the wind and the wind turbine will start back up. If the winds are strong enough, this could seriously damage the turbine.

Some wind turbines come with electrical brakes, a.k.a. dynamic brakes. Dynamic braking is the least expensive option and is found in many small-scale wind turbines.

Dynamic braking is a fairly simple approach that is found in turbines equipped with permanent magnet alternators. It consists of a switch inside the house or at the base of the tower. When the
brake switch is closed, it short-circuits the wind machine, rapidly slowing the rotor.

In dynamic braking, the braking force is proportional to the rotor speed. As the rotor slows down, the braking force diminishes. As the rotor speed approaches zero, so does the braking force. In low to moderate winds, dynamic braking should either stop the rotor or slow it down considerably. However, dynamic braking may not completely stop the rotor in high winds. In winds blowing over 20 miles per hour, for instance, dynamic brakes can’t be counted on. If a wind machine is shut down prior to a storm’s arrival, strong winds may overpower the brakes, causing the rotors to start turning. In high wind speeds that force the blades to start spinning
slowly, energy is dissipated in the windings of the alternator, which could cause it to burn up. Not all dynamic brakes are created equal. Those found in Southwest Windpower’s Skystream 3.7 and turbines made by Proven and Abundant Renewable Energy are 100% reliable, as far as we can tell.

Shut-down mechanisms of a wind turbine should be high on the list of considerations, right up there with swept area and tower top weight. If the turbine is to be serviced on the tower, the shut-down mechanism should be capable of completely stopping the rotor. Don’t buy a turbine without a shut-down mechanism. Inexpensive wind turbine designs without a reliable shut-down mechanism are a short-sighted gamble, at best.

Sound Levels

The sound a turbine produces is another important factor to consider, both for your own peace of mind and your neighbors’. All residential wind machines produce sound. Sounds emanate from
the blades as they spin. They produce a swooshing sound. Sound is also produced when a turbine furls in high winds. Rotation of the rotor in the alternator also produces sound, as do gears in geardriven wind turbines. (Sound test reports can be found at the National Renewable Energy Laboratory’s website.)

Sound levels increase as wind speed increases. However, sound from a wind turbine is often difficult to detect and is rarely a nuisance. Remember, too, that mounting a turbine high off the ground — typically 80 to 120 feet — to reach the smoothest, most powerful winds significantly reduces sound levels at ground level.

Even so, it is important to consider sound levels. One way is to observe turbines you are considering in operation under a variety of wind speeds. If you can’t, you may want to ask homeowners or business owners who have installed the turbines you are considering for their experiences.

Another method is to check out the rpm of the turbines at their rated outputs. Rated output is the output in watts at a certain wind speed, known as rated speed. Knowing this gives an idea of how much sound they’ll produce — the higher the rpm, the more sound. The rpm of a wind turbine also give an indication of quality. Generally, less expensive and less durable turbines spin at a higher rpm. They rely on less expensive generators that operate at high speeds to produce energy. In addition, higher rpm machines are subject to more wear and tear and tend not to last as long.