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.
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:
- swept area,
- durability,
- annual energy output,
- governing mechanism,
- shut-down mechanism, and
- 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.
AEO gives buyers a convenient way to compare models. 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. 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.
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 400 watts, for example, have no governing mechanisms. 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 hori- zontal 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. 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 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 .
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.
Changing Blade Pitch: The second type of overspeed control involves a change in the angle of the blades, known as blade pitch, to reduce rotor speed. Blade 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. Some machines, 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 with- out 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.
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.
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 gear- driven wind turbines.
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.
Other Considerations
Although swept area, weight, annual energy output, governing mechanisms, shut- down mechanisms and sound levels are the most important factors to consider when buying a wind machine, there are other details that manufacturers provide. We think readers should be familiar with them, but not let them overly influence their judgment.
Cut-In Speed: One factor that is of little relevance is the cut-in speed — the wind speed at which a wind generator starts producing electricity. Most turbines don’t produce appreciable amounts of electricity until wind speeds reach 10 miles per hour. They produce full power at speeds from about 23 to 30 miles per hour.
Some manufacturers recommend installing a turbine with the lowest cut-in speed possible in areas of low wind to make better use of the wind resource. However, because low wind speeds hold very little energy, low speed cut-in isn’t important.
Power Curves: Another useless bit of information provided by wind turbine manufacturers is the power curves. Power curves are graphical representation of the power production of a wind turbine (in watts) at different wind speeds.
While power curves make for good visuals, they aren’t always accurate. They’re not always obtained under the most ideal test conditions. Moreover, they are a bit misleading to the uninitiated. That’s because most wind turbines operate at relatively low wind speeds — around 10 to 20 miles per hour — and very rarely operate at the peak of the power curve.
As you can see, wind turbines produce much less electricity at lower wind speeds. When considering a wind turbine, what’s important is how much energy it will produce at your site on your tower at your average wind speed — that is, the AEO.
Rated Power: Last but not least is a measurement known as rated power, one we’ve avoided using as much as possible in our articles. Rated power is the output in watts at rated wind speed. Much like the output of solar electric modules, rated power of wind turbines was devised to give buyers a way to compare products. Buyers, for instance, typically compare one kilowatt or ten kilowatt turbines as they would compare solar modules.
Rated power is of limited usefulness in large part because there are currently no standards in the wind industry (as there are in the solar industry) for determining rated power. Manufacturers determine rated power in different ways and at different wind speeds — the rated wind speed. This situation will change because the small wind industry is developing a standard technique of measuring performance.
Final Factors
When shopping for a wind turbine, be sure to check into the company’s customer service record — how well it supports its dealers and customers. Be sure that the company you buy from offers technical advice should you have trouble and that their technical support staff speaks your native tongue.
Another factor to consider is how long the company has been in business. Doing business with a company that been in business for a while, say ten or more years, is a good idea. A related criterion to take into consideration is how long a wind turbine has been on the market. The longer a wind turbine has been on the market, the better. It has been tried and tested and improved upon.
It’s also important to check on the availability of parts. Parts shipped from foreign countries may take months to arrive. Meanwhile, your wind energy system sits idle. When shopping, check to see if the importer of the wind turbine you are interested in stocks spare parts. It’s a good idea to consider the number of moving parts and wear points in a turbine.
Some designs, contain a lot of moving parts, which results in numerous wear points — up to 300. These machines may require a lot of tinkering at 100 feet. Be sure to check out the warranty. Warranties typically run five years. The longer the better.
Also be sure to determine what warranties cover. Is it materials and workmanship or parts only? Does it include shipping? Unfortunately, most warranties do not cover the cost of labor.
We recommend buying from a local dealer/installer, too. That way, you’ll have local support, even if you opt to do the installation yourself. Be sure to ask for references and interview them.
If you buy from an online wholesaler, be aware that most of these companies do not offer technical support, installation advice or assistance. Nor do they offer replacement parts or repair services. If you buy cheap, you are on your own when you need help.
We believe that it is a good idea to stick with name brand wind turbines and avoid newcomers and foreign imports, especially Chinese-made machines, at least at this time.
Buying a used or reconditioned wind turbine may be an economical option, but it’s not a very good idea. The problem with used wind turbines is that you don’t often know what you’re getting. If you buy a re-manufactured wind turbine, you want to be sure that the machine has been fully reconditioned, not just painted and with new blades installed.
Make sure that the equipment supplied with the turbine, like the controller or inverter are code compliant. You may also want to obtain an extended warranty. Also be sure you can find a tower for the turbine before you purchase it. Suitable tall towers for the larger small-scale wind machines are hard to find.