Swept Area & Rotor Diameter
To help with comparisons, the various wind generator models are listed in ascending order of swept area and rotor diameter. This is a radical departure from the waymost manufacturers rate their various turbine models..You’ll see why when you read my comments on cost.
The “rotor” is defined as the entire spinning bladeassembly, including the hub to which the blades are attached. The rotor is essentially the collector of the wind generator—gathering fuel in the form of wind, and converting it into electricity by driving the generator. Think of the rotor in the same terms as we describe a solar water heater. One 4 by 8 solar hot water panel (32 square feet) will collect a certain amount of sunlight and produce a proportional amount of hot water. If you double the number of panels, you double the collector area (now 64 square feet), thereby doubling the amount of sunlight you can collect and the amount of hot water you can produce. Swept area works much the same way.
The rotor converts the movement of air passing through the two or three blades into the rotational momentum that turns the generator, thereby generating electricity. Just like a solar water heater’s area, a wind generator’s rotor size is a pretty good measure of how much electricity the wind generator can produce. The larger the swept area of the wind generator’s rotor, the more
electricity it can produce.
While manufacturers rate their products at different peak wattages, the output of a wind generator is primarily a function of its swept area. Other features will influence output, such as high-tech airfoils and more efficient generators. However, they pale when compared to the overall influence of the size of the rotor. Mike Klemen, a seasoned wind generator user and tester in North Dakota says, “Ultimately, we must realize that energy production comes from square feet.” Hugh Piggott of Scoraig Wind Electric in Scotland contends, “Swept area is easier to measure and harder to lie
about than performance.What we’d like to know is KWH per month, but until we get more independent testing done, swept area is a good guide.” Swept area is the most critical feature that will help you compare the output of one wind generator with another.

Cut-in Wind Speed
This is the wind speed at which the wind generatorbegins producing. For all practical purposes, wind speeds below about 6 to 7 mph (3 m/s) provide little or no usable energy, even though the blades may be spinning. From my perspective, a few watts does not result in usable energy. At best, this minimal output only overcomes the power losses caused by a long wire run or the voltage drop due to diodes. Note that not all wind generators are created equal, even if they have
comparable rated outputs. There is no industry standard for rated wind speed. “So what?” you ask. The listed wind generator companies rate their turbine output at anywhere from 18 to 31 mph
(8–14 m/s). This may not sound like such a big deal until you understand that there is potentially 511 percent more power in a 31 mph wind than in an 18 mph wind.

To drive home the example, let’s use 16 and 32 mph instead of 18 and 31. The power in the wind available to a wind generator is defined by the equation:
P = 1/2 d x A x V3 Where P is power, d is density of the air, A is the swept area of the rotor, and V is wind speed. Notice that wind speed is cubed. In other words, the equation really reads

P = 1/2 d x A x V x V x V.
We can simplify the relationship by stating that P ~ V3, that is, P is directly proportional to the cube of the wind speed. If we double the wind speed (V), the power (P) increases by 800 percent. So there is 800 percent more power available to the rotor at 32 mph than at 16 mph.
Viewed in reverse, there is 1/8 the power in a 16 mph wind compared to a 32 mph wind.
Let’s say we have two wind generators, both rated at 1,000 watts. Lots-o-Watts is rated at 16 mph and Mighty-Watts at 32 mph. At 32 mph, they’re both producing 1,000 watts, right? But at 16 mph, Lots-o-Watts is still producing 1,000 watts, whereas Mighty- Watts is only producing 1/8 that amount, or a paltry 125watts! All of this means that the lower the rated wind speed,
the more energy a wind generator will produce, given its rated output. As a consumer, therefore, you should be particularly interested in machines with low rated windspeeds.

Rated Output
This measurement is taken at an arbitrary wind speed that the manufacturer designs for. It tends to be at or just below the governing wind speed of the wind generator. Any wind generator may peak at a higher output than the rated output. The faster you spin a wind generator, the more it will produce, until it overproduces to the point that it burns out. Manufacturers rate their generators at a safe level, well below the point of selfdestruction. You are not necessarily interested in the rated output of a wind generator. A turbine with a high rated wind speed will invariably cost less than one with a lower rated wind speed, for the same rated output. How can this be?
Refer back to the power equation mentioned above. A higher wind speed gives a certain wattage to the manufacturer at a smaller rotor diameter, smaller physical size of the generator, and subsequently less weight. All of this means less cost for the manufacturer, and less cost to you.
But remember, it takes a higher wind speed to achieve that rating. In a 12 mph (5 m/s) average wind speed site, you will see 18 mph (8 m/s) winds a mere 3 percent of the time. Not much, you say. But you will see 31 mph (14 m/s) winds for less than 0.2 percent of the
time. Rated output comes to us from the photovoltaic industry, where panels are tested for output at a fixed light intensity and a fixed temperature. The wind industry has no such fixed standards. So, while comparing PVs based on rated wattage makes for great cost comparisons, comparing rated outputs is a poor way to compare wind generators. You are far better off comparing swept areas, or the KWH per month of electricity the different systems will produce at different
average wind speeds.
Peak Output
This figure may be the same as rated output, or it maybe higher. Wind generators reach their peak output while governing, which occurs over a range of wind speeds above their rated wind speed. Although widely touted by some marketers, it has limited relevance to the buyer. To quote Hugh Piggott, “Peak or rated output specifications for small wind turbines can be red herrings unless you take the rated wind speed into account, and yet these specs are all the customers seem to want to know about.” Wind turbines are not PVs, don’t operate in the same manner, and should not be rated in the same way. What you should be asking is what wind energy engineer Eric
Eggleston asked, “What will this wind generator do at my site in my average wind speed?”
Maximum Design Wind Speed
Bandied about by marketing departments, this term has little bearing on the expected life of a wind generator. Wind generators are designed by engineers, on paper, to survive wind speeds of 120 mph (54 m/s) or more. Unfortunately, wind turbines are not tested for these survival speeds because, quite frankly, it’s a very difficult thing to test for, or to test repeatedly. Much of the survival speed documentation we have is not from actually testing turbines at those speeds, but
from anecdotal situations. Bergey Windpower might boast that their machine survived a hurricane in Kansas that blew Toto away from Dorothy. Great, but what have we learned?
I don’t mean to demean claims like this, but again, they are difficult to test, and everybody supposedly designs their turbines for extreme winds. In fact, Bergey Windpower has actually had very good success designing their turbines to survive such high winds. How? By making their wind generators very robust, very heavy duty. Does that mean that any turbine will survive a 100 mph
(45 m/s) storm? Maybe, maybe not. A 100 mph wind that is coming straight on is fierce, I’ll grant that. But have you ever watched a wind generator sited on a short tower near trees and buildings? The poor thing hunts around continuously, all the while buffeted by the turbulence caused by the short installation height, along with the nearby ground clutter. I have seen more wind turbines destroyed by turbulence than I have seen destroyed in survival-rated high winds. Furthermore, a 100 mph wind packs an awesome wallop, and while wind generators and their towers can
be designed to withstand those winds, there's no guarantee that they will. I live in dairy country in
northeast Wisconsin. During our last 100 mph wind, cows were flying through the air! If a cow, or a 2 by 4, or a sheet of plywood hits the wind generator or tower, it will probably crumble, regardless of what wind speed the system was designed for. Flying debris is what takes
out many turbines in high winds. You can’t design for flying lumber or livestock. So what should you look for if not maximum design wind speed? I look for tower top weight, which is a pretty
good indicator of reliability. My experience is that heavy duty wind generators survive, and light duty turbines do not. While all of the units listed are rated for 120+ mph (54+ m/s) winds, in-field experience indicates that many of the lighter turbines cannot handle sites with heavier winds or turbulence. Be forewarned! Weight, by the way, will be reflected in the price.You’ll only get what you pay for.

Model

FD1KW

Whisper H40

BWC XL.1

Company

WSE Technologies

Southwest Windpower

Bergey Windpower

78.5

38.5

52.8

10.0

7.0

8.2

6.5

7.5

5.6

20.0

28.0

24.6

1,000

900

1000

1,500

900

1800

90

120

120

400

1,150

490

PM 3AC to DC

PM 3AC

PM 3AC to DC

Angle govenor

Side facing

Side facing

27

28

29

93

47

75

48

12 to 48

24

Controller and dump load

Controller and dump load

Battery controller

$1182

$1495

$1695

Rpm at Rated Output
This is the alternator or generator rpm at which rated output occurs. Generally, the smaller the rotor, the faster the blades spin. Generator rpm will have an effect on the amount of noise that the wind generator makes. High rpm wind generators also experience more stress due to centrifugal forces, which are constantly trying totear the machine apart.
Bearing life is also affected by rpm. Bearing life is dependent on the load on the bearings, plus the speed at which those bearings spin. Light duty, high-speed wind turbines typically have a shorter bearing life than slow-speed, heavier machines—yet another benefit of heavy duty machines.
Blade Material Within the last eight years, a number of new materials have become available for making wind generator blades.
While more expensive for materials and labor, wood is still considered by some to be the tried and true material of choice for blades. Blades do a lot of flexing. That’s what trees did as a side job for most of their lives, as they swayed in the ever-changing breezes. Without question, Sitka spruce is the primo material for wood blades. It has one of the highest strength-to-weight ratios of any material ever used by blade makers, as well as airplane and boat builders.
Wood blades need exceptional paint coatings to protect them, along with a durable leading edge tape to protect the blades from abrasion due to dust and insects in the air. Both paint and leading edges need maintenance. If the paint cracks or the leading edge tape tears away, resulting in wood exposed to the elements, the wood will quickly erode. Moisture entering these areas will cause an unbalanced rotor, stressing the wind generator over time.Wooden blades must be inspected annually, with repairs made as soon as damage is discovered.
Since good wood is ever more difficult to secure, as well as labor intensive to convert into quality blades, most manufacturers have moved away from wood and towards synthetic materials for their blades. A number of synthetics are currently in use. One good replacement for wood blades is fiberglass over a foam core. The foam gives body to the blade, while the fiberglass covering laid up over the foam results in an extremely durable, smooth blade surface.
The leading edge of fiberglass blades is also covered with an abrasion resistant tape to protect it from erosion. This tape needs periodic replacement. A variation on fiberglass blades is to use a carbon fiber composite for an even tougher blade surface. Yet another variation on fiberglass is to use the material, not on the outside of the blade, but throughout the entire blade. One technique, known as pultrusion, is used by Bergey.
Pultruded fiberglass blades are made in a process that resembles making spaghetti. Spaghetti dough is squeezed through a hole in a die, and then cut to length. Pultruded blades are made by pulling fiberglass through a die along with fiberglass cloth, to make the form of the airfoil. Lengths are cut, the blade butts are fabricated and added to the blades, and, voila—you
have Bergey blades. Plastics are also being used for blades. Southwest Windpower uses injection molded plastic for the blades on their Whisper H40 and H80. Proven Engineering uses a hollow polypropylene blade, another form of plastic. One potential advantage of plastic blades is that
they should be relatively inexpensive to replace when that time comes. They’re also tough and impervious to water.
Blade color is not included in the table, but should be mentioned. Most blades are white, while a few are colored (blue or gray, for example) to blend in with the sky. Plastic and carbon-fiberglass blades are black. When I first encountered black blades, I thought they would look horrendous on the landscape. Interestingly, a black rotor almost disappears in the sky when spinning. Tip Speed Ratio (TSR) The performance of a blade’s airfoil (shape) is a function
of the ratio of the speed of the tip of a blade to the wind speed. A low-speed blade will have a TSR of 5 or 6 to 1, while a high-speed blade with a TSR of 10 or 11 to 1 will
be a less efficient performer. So why use a high TSR airfoil? Faster spinning blades allow a manufacturer to build a smaller generator (therefore, lighter weight) to get a certain output.
However, the faster the blades spin, the more noise they make, especially when governing.
“Number of blades” has not been included in this version of A&O, since all of the models listed have three blades except for one, the Whisper 175. While a number f manufacturers have offered two-bladed wind generators in the past, most no longer do. Three-bladed wind generators avoid yaw chatter, which happens when a two-bladed machine yaws. “Yaw” is a term that refers to a wind generator pivoting on its bearings around the tower top to follow the continually changing direction of the wind. Regardless of the number of blades on the wind generator, proper blade balancing is critical for a smooth running machine. Severe chattering or a poorly
balanced rotor may result in the failure of the wind generator or, in extreme cases, the tower. Look for an unbalanced rotor to show up as tail wagging. All of the wind generators listed are upwind generators, with the exception of the Proven wind turbines. Upwind generators use a tail to orient the turbine into the wind. Downwind machines have no tails. With a downwind turbine, the wind blowing on the rotor literally pushes it away from the tower, thereby keeping the blades oriented into the wind. While some are biased towards either an upwind or downwind configuration, I think
either style works just fine.
Generator Type
Three types of electrical generators are used in windelectricsystems: permanent magnet (PM) alternators, DC generators, and brushless alternators. All three do afine job of generating electricity. In general, PM alternators are lighter weight, less complicated, and less expensive to manufacture than either DC generators or brushless alternators. These latter two require more copper and labor to manufacture, but they match the power curve of the rotor more closely.  All of the wind generators listed are direct drive units with the exception of the Jacobs 31-20, which uses a 6 to 1 gear box in the design. Direct drive means that theblades directly drive the generator, with no gears. Theadvantage of gear drive machines is that they can deliver kilowatt-hours at a lower cost than direct drive machines. It’s cheaper to add a gearbox than to custom design a large, slow-speed generator. The downside is that gearboxes add lots of moving parts, which
translates to more wear and tear, and more maintenance.
Governing System
Governing is necessary for two reasons. The governor protects the generator itself from overproducing and burning out, and it protects the entire system from flying apart in high winds. The governing devices used on all of these wind generators fall into two general categories—those that reduce the area of the rotor facing the wind, and those that change the blade pitch. Changing the swept area of the rotor is accomplished by tilting the rotor up and out of the wind, side facing the rotor out of the wind by moving it around the tower (Bergey and AWP), or by a combination of the two (Whisper). In all cases, the fixed-pitch rotor is offset either above or to the side of a pivot point. Wind pressure on the offset rotor causes the rotor to pivot out of the wind. These governing mechanisms are almost a foolproof method of controlling rotor speed. However, they do come at a cost. Once the rotor governs by tilting up or side facing, it often produces very little because it is no longer oriented to the wind. One exception to this is the
AWP, which maintains its power curve in the governed position. Blade-activated governors (all of the Jacobs) work by pitching the blades out of their ideal alignment to the wind. Because these governors operate due to centrifugal forces, the greater the rotor speed, the greater the degree of pitch on the blades. Having more moving parts than either the tilt-up or side-facing mechanisms, they are more complicated governing devices. More moving parts means more parts to maintain or replace sometime in the life of the turbine. However, they offer much better power output in high
winds compared to governors that reduce swept area. Finally, the Proven turbines also govern by pitching the blades, but not only due to centrifugal forces as with the Jacobs. In addition to springs, the Proven blades have a hinge built into the blade butts. In origami fashion, the blades fold and twist in high winds, changing the ideal blade pitch, stalling the blades, and thereby reducing rotor speed. In very high winds, the blades also cone back and away from the tower, cleverly resulting in a reduced swept area.

Governing Wind Speed
The wind velocity at which the governing mechanism is fully operational occurs somewhere between the wind generator’s rated power output and its maximum power output.

Tower Top Weight
This covers everything that goes on top of the tower—generator, governor, rotor, tail, and yaw assembly. You’ll notice that there is wide variation in tower top weights. Based on experience, I side with the school of heavy metal, those manufacturers that have proven that the longevity of equipment life is directly related to the beefiness of components. From my 22 years of experience rebuilding wind generators, I’ve come to realize that heavy duty, slowspeed wind generators last longer than their lightweight, high-speed cousins. Many people opt for the lighter duty wind turbines because they are invariably cheaper. They generally buy a heavy duty machine the second time around. Unfortunately, the trend in recent years has been tomake everything as cheaply as possible. Performance and reliability of the machine, while important, were overshadowed by initial cost.Why? You, dear consumer. Weight is reflected in cost. So the goal became lightweight, high-speed wind gennys. As failures accrue in the field, some manufacturers are moving back to heavy metal. I welcome that. Lateral Thrust at the Tower Top This figure is important for determining tower design specifications and choices. Lateral thrust, a critical horizontal force vector, is a function of swept area of the rotor, the resistance the tower presents to the wind, and wind speed. The greater the lateral thrust, the stronger (and therefore, more expensive) the tower must be, and the larger the concrete footings and guy wires must be.
Battery System Voltages
Available voltages for the battery systems are listed. Remember that line loss is a significant consideration for low voltage systems. Wind generators are rarely sited next to the battery bank. Line loss due to wire run (including the height of the tower) pushes people to choose higher voltages.
Controls Included
Controller, rectifier, brake, and dump load may be standard equipment that is included with the wind generator for interfacing with a battery charging system. Or, if not listed, they may be options available at an additional cost.

Cost
Note that these costs are only for the wind generator and controller or utility-tied inverter. Check under “Controls included” to determine what controllers or utility-intertie inverters are included in that price. While this may seem obvious, it never ceases to amaze me that people don’t realize that a wind-electric system’s installation costs also include such miscellaneous items as shipping for the wind turbine, a tower (of all things) and its shipping charges, maybe batteries and inverter, wiring and electrical components, backhoe and crane costs for larger turbines, concrete and rebar for some towers, sales tax, and labor and travel expenses if the job is farmed out to an installer.
Actually, depending on the system you install, the wind turbine cost represents only 12 to 48 percent of the total installed cost of the wind-electric system. In PV systems, the PV panels represent the major portion of the cost of the generating part of the system. Wind generators are mounted on towers to access their fuel, the wind. While a 120 foot freestanding tower is only
about half the cost of a Jacobs 31-20 wind generator, an 80 foot tilt-up tower can cost upwards of five times the price of a Whisper H40!

More Interesting Links

Site Analysis for Wind Generators Part 1

Site Analysis for Wind Generators Part 2

 

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