WIND MACHINE BUYING GUIDE - WIND MACHINE BASICS

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 gridconnected 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.