Wind Availability and Location Optimization for Wind Turbines

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Wind Availability

Whether constructing a wind turbine is economically viable at your home or farm depends most strongly on the quality of your wind resource. Generally, average annual wind speeds of at least 4.0-4.5 m/s (14.4- 16.2 km/h; 9.0-10.2 mph) are needed for a small wind turbine to produce enough electricity to be cost-effective. A very useful resource for evaluating a site for its wind energy potential is a wind resource potential map. 

Wind Map for Eastern Ontario. (a.g.l. = above ground level) 

It may be useful to check wind speed measurements that have been recorded at a local weather station. It is important to consider that sitting factors at these weather stations, such as nearby trees and buildings, might influence any wind speed measurements. Also, keep in mind that the equipment at these stations is often located close to the ground, and that weather stations located at airports are usually sheltered from the wind.

Wind Map for Southwestern Ontario. 

This means that wind speed measurements recorded at these stations might under represent the wind potential at your site.

For the most precise evaluation of the wind speed at your site, you need to purchase a wind resource evaluation system. While wind resource evaluation systems can be expensive, if your property is hilly and has unusual terrain features then it might be worth obtaining one.

The most important component of a wind resource evaluation system is an anemometer. Anemometers are typically designed with cups mounted on short arms that are connected to a rotating vertical shaft.

The anemometer rotates in the wind and generates a signal that is proportional to the wind speed. If you do purchase an anemometer, you will also need to purchase something to record the readings made by the anemometer, and a tower or tripod to mount the whole system on.

For as little as $500 you might be able to purchase a wind totalizer, which is a very simple type of wind resource evaluation system where the anemometer is linked to an odometer. The odometer is similar to those found in cars. After a period of time, the number recorded on the odometer, which represents the total "distance" the anemometer has turned, can be divided by the time passed since the odometer was last checked in order to determine the average wind speed over a period of time at a location.

If there is a small wind turbine system in your area, you may be able to obtain useful information from its owners about the annual electrical output of the system and, possibly, wind speed data. Such information could be extremely valuable as an alternative to installing a wind resource evaluation system.

 Wind turbine schematic. (Modified image from Natural Resources Canada)

Picking the Best Location for a Wind Turbine

Where you choose to build your wind turbine is important. Remember that if nearby houses, tree lines and silos obstruct the full force of the wind from your wind turbine, you will not be able to generate as much power.

Also keep the following in mind:

• Wind speeds are always higher at the top of a hill, on a shoreline, and in places clear of trees and other structures.
• Remember that trees grow over the years; wind turbine towers do not.
• Inform neighbours of your plans to avoid conflict later on.
• Be courteous. Keep the turbine as far away from neighbours as possible. 250-300 m away is typical.
• Check with the local government for any other laws and regulations about zoning.

Wind speeds tend to be higher on the top of a ridge or hill, and for that reason it is a good idea to locate wind turbines at hilly locations. Just remember to keep your turbine away from high turbulence. Neighbours must also be taken into consideration when picking a spot to build your turbine. The farther your wind turbine site is from neighbouring houses, the better.

Do not expect your wind turbine to generate the same amount of power all the time. The wind speed at a single location may vary considerably, and this can have a significant impact on the power production from a wind turbine. Even if the wind speed varies by only 10%, the power production from a wind turbine can vary by up to 25%!

 Example of wind speed distribution by hour of the day. Values shown are monthly averages of measurements made by anemometers. 

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Wind Generator

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The wind turbine generator converts mechanical energy to electrical energy.
Wind turbine generators are a bit unusual, compared to other generating units you ordinarily find attached to the electrical grid. One reason is that the generator has to work with a power source (the wind turbine rotor) which supplies very fluctuating mechanical power (torque).
These pages assumes that you are familiar with the basics of electricity, electromagnetism, and in particular alternating current. If any of the expressions volt (V), phase, three phase, frequency, or Hertz (Hz) sound strange to you, you should take a look at the Reference Manual on Electricity and read about alternating current, three phase alternating current, electromagnetism and induction before you proceed with the following pages.

Wind genearator

Generating Voltage (tension)
On large wind turbines (above 100-150 kW) the voltage (tension) generated by the turbine is usually 690 V three-phase alternating current (AC). The current is subsequently sent through a transformer next to the wind turbine (or inside the tower) to raise the voltage to somewhere between 10,000 and 30,000 volts, depending on the standard in the local electrical grid.
Large manufacturers will supply both 50 Hz wind turbine models (for the electrical grids in most of the world) and 60 Hz models (for the electrical grid in America).
Cooling System
Generators need cooling while they work. On most turbines this is accomplished by encapsulating the generator in a duct, using a large fan for air cooling, but a few manufacturers use water cooled generators. Water cooled generators may be built more compactly, which also gives some electrical efficiency advantages, but they require a radiator in the nacelle to get rid of the heat from the liquid cooling system.
Starting and Stopping the Generator
If you connected (or disconnected) a large wind turbine generator to the grid by flicking an ordinary switch, you would be quite likely to damage both the generator, the gearbox and the current in the grid in the neighbourhood.
You will learn how turbine designers deal with this challenge in the page on Power Quality Issues , later.

Small wind generator

Design Choices in Generators and Grid Connection
Wind turbines may be designed with either synchronous or asynchronous generators, and with various forms of direct or indirect grid connection of the generator.
Direct grid connection mean that the generator is connected directly to the (usually 3-phase) alternating current grid.
Indirect grid connection means that the current from the turbine passes through a series of electric devices which adjust the current to match that of the grid. With an asynchronous generator this occurs automatically.

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Direct-Drive Wind Turbines

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A Different Drive Train Design That Eliminates The Gearbox Between A Turbine’s Rotor And Generator Is Attracting Wind Turbine Manufacturers In The Quest For Higher Power Output, Increased Offshore Reliability, And Potential Cost Savings Over The System’s Lifetime. 
The majestic, low-speed rotation of wind turbine blades, our usual first sight of a wind farm, is largely dictated by laws of physics—the need to limit detrimentally high blade tip speeds at blade lengths of 50 m or more. However, to produce electricity, traditional wind turbine design must introduce a gearbox (or speed incremental) between the turbine’s rotor, spinning at 4-20 rpm, and the standard generator that requires higher rotary speed. Gear ratios are on the order of 100:1 or more. A newer technology for utility-scale wind turbines eliminates the gearbox by use of a low-speed, permanent magnet generator, as the term "direct drive" suggests. Besides simplifying the turbine’s drive train, direct drive (DD) offers decreased system weight, potentially higher reliability due to fewer parts, and reduced maintenance. A growing number of wind turbine manufacturers produce DD machines, with current focus on higher power ratings and offshore applications. DD and geared wind turbines differ little from the controls viewpoint. Either approach employs pitch control of individual blades and yaw control of the nacelle relative to wind direction. Generator output is regulated to voltage and frequency requirements of the grid by the power converter and transformer. The turbine controller handles system functions and interacts with a supervisory control used in multiple turbine wind farms.

Direct drive wind generator

Virtually all direct drive wind turbine vendors also offer geared turbines, which are expected to remain competitive in the foreseeable future due to their large installed base and excellent overall performance. (See table of manufacturers, online.) Ultimate application sectors for geared and DD wind turbines will be determined by further operating experience and system cost.

“Direct-drive technology has an advantage that increases with size. Hence, it is likely that geared turbines will remain competitive at low power values,” said Henrik Stiesdal, CTO of Siemens Wind Power. Meanwhile, vendors are quite tight-lipped about comparative power generation costs and system reliability figures for the two design approaches.

No gearbox = 12 tons less !!!

Eliminating the gearbox, the heaviest element in the turbine nacelle, provides a definite weight advantage.

“Using the same 101-m rotor, the nacelle of Siemens’ 3.0 megawatt (MW) direct-drive turbine is 12 metric tons lighter than Siemens’ 2.3 MW geared turbine,” Stiesdal noted. The direct drive wind turbine (SWT-3.0-101) is shown in the Siemens photo. Gearbox reliability is a concern for the industry. The high dynamic load environment and number of moving parts require proactive monitoring and maintenance. While some recent gearbox failures have been reported, wind turbine manufacturers stand firmly behind the product’s reliability.

Direct drive generator

“Siemens geared turbines are currently setting the [reliability] benchmark for the wind industry, and it is not easy to improve on this,” explained Stiesdal. “However, in principle the direct-drive turbine with its much lower number of moving parts and 50% less components should offer yet a small nudge upwards on reliability.”

Some gain in drive-train efficiency comes from removing the gearbox. In addition, turbine nacelle length becomes noticeably shorter. One indication of efficiency gain and other benefits comes from MagnetDrives AG of Switzerland in a presentation at Motor Summit 2010 in Zurich. (For more on Motor Summit, see online reference 2.)

Dr. Stefan Berchten, principal of MagnetDrives, compared 2 MW gearless and geared wind turbine designs, showing five metric tons less weight and 94% versus 92.4% system efficiency in favor of DD. For an additional 5% direct drive project investment, Berchten forecasted substantial savings in maintenance cost, energy, and total cost over a seven-year payback period.

Low-speed generator
A different generator design is needed to produce electricity at low rotary speeds. Permanent magnet (PM) synchronous generators provide that need most efficiently and, for direct drive, they take a “doughnut” configuration rather than the cylindrical shape of traditional generators. A substantially larger diameter generator is necessary to increase the effective rotary motion of the PMs relative to the stator coils so that the required high torques can be developed.

Hub connection

The Switch Controls & Converters Inc., a provider of megawatt-class PM generator and full-power converter packages, noted the advantageous efficiency of PM synchronous generators for DD turbines. Importantly, PM generator efficiency remains high, close to nominal value, even at partial loads—where turbines must often operate due to wind inconsistency, explained Anders Troedson, vice president and GM of The Switch. For example, a generator rated 2.2 MW at 18 rpm (see photo) has full-load efficiency of 94.4%, while at 25% load its efficiency is a remarkable 92.9%. Similar values, even slightly higher 25% load efficiencies, apply to the company’s other DD generator models.

In addition, PM generators eliminate the need for separate excitation, slip rings, and rotor windings with associated losses, and require less m

aintenance compared to double-fed induction generators (DFIGs). While the most numerous generators in service, DFIGs aren’t applicable to DD operation because of power factor restrictions and lack of fault ride-through capability, among other reasons. Over a 20-year turbine lifespan, PM generators are reportedly also more cost effective.

“Direct-drive generator design requires special magnet shapes and arrangements to match specific wind conditions and optimize efficiency,” said Troedson. Another feature is a patented stator construction with several independent segments that are controllable by different converters—which translates to drive redundancy. “The turbine may even remain operable in case of minor faults in one of the segments,” he stated.

As for other generator types, “the separately excited synchronous generator is a viable but less efficient type of generator, which can be used for direct drive,” added Troedson. “Largest user of this technology is German wind turbine manufacturer, Enercon. In addition to low-speed generators, The Switch offers medium- and high-speed units optimized to work efficiently and reliably with its full-power converters.

More power offshore, onshore

Intended for higher power output, including offshore applications, DD generators rely on rare-earth PM materials—typically neodymium iron boron (Nd-Fe-B). However, technology offers no free lunches. A trade-off for eliminating the gearbox is the need for large quantities of these costly magnet materials, which are becoming subject to supply shortages. Approximately 650 kg of PMs is needed per MW wind turbine capacity, according to Siemens, of which 25%-30% is rare-earth magnet material.

“The cost of these materials is accounted for in the competitive price of the direct-drive concept.” Stiesdal noted. Recent strategic sourcing issues for rare-earth PMs have caused user companies to consider alternatives.

“Generators currently offered use permanent magnets with neodymium and dysprosium elements,” added Stiesdal. “A variant design magnetized with a system not using rare-earth magnets will be available in case of shortage of these rare-earth elements.” (See more on supply and cost issues of rare-earth magnets in an online article extension.)

Growing power output of DD wind turbines is reflected in recent introductions and announcements. GE Energy, reportedly the leading producer of wind turbines in the U.S. and the second worldwide, introduced a 4-MW turbine in March 2011, optimized for offshore use. Design of the 4.1-113 direct drive turbine is said to have built-in redundancy and partial operation capability for its major components. DD technology focuses on “keeping turbines operating reliably at sea…and relies on a modular approach to maximize in-situ repair and reduce the need for large repair vessels,” according to the company.

The 4.1-113 has rated wind speed of 14 m/s (31.3 mph) and 113 m rotor diameter. It builds on the evolution of GE’s 3.5 MW DD turbine, and its base design that has operated since 2005 in the high wind speed and high turbulence environment of a Norwegian coastal site. The turbine is going to Sweden’s Göteborg Energi for installation in Gothenburg harbor in the second half of 2011.

Offshore direct drive

GE also recently announced an investment of € 340 million for plants in Germany, Norway, Sweden, and the U.K. to produce 4-MW gearless wind turbines for offshore use in 2012. The company boosted its DD technology expertise with the acquisition of ScanWind of Norway in 2009.

Siemens Energy and Alstom are among other companies that have announced large DD wind turbines. In June 2011, Siemens installed a prototype of its latest offshore turbine with DD rotor technology at a Danish site. Trial operation has started for the 6 MW machine with a 120-m rotor diameter.

Meanwhile, Alstom has begun manufacture of its prototype 6 MW offshore turbine also featuring direct drive technology (see photo). The first prototype machine is to be installed before year-end 2011 on the west coast of France. Both Siemens and Alstom expect their wind turbines to reach serial production in 2014, according to the companies. (These developments are covered further online.)

In contrast, Denmark’s Vestas—reputed to have built the most wind turbines—has chosen a geared design for its prototype 7-MW offshore turbine announced earlier in 2011. However, prototype production of the machine with 164-m rotor diameter is not expected until 4Q 2012.

Not for all applications

While higher power is now the “sweet spot” for DD wind turbines, sheer physical size can impose mechanical constraints.

“Several challenges arise as we approach 8 to 10 MW rating with direct drive,” said Troedson. “The generator grows rapidly larger and heavier, due to high torque requirements.” Besides design limits such as material strength, manufacturing tolerances, and bearing requirements, Troedson mentioned the following constraints:

• Generator weight and size—must be limited to avoid upgrading the tower structure to handle the higher nacelle weight
• Air gap design—to handle flux density requirements and prevent rotor-stator rubbing under all operating conditions
• Dynamic short circuit forces—must be accommodated within limits of the minimum air gap.

Some of these factors can become too dominant and affect generator choice, Troedson explained. “A single-stage gearbox enables a much smaller, lighter generator and still offers some of the same advantages as a direct-driven generator,” he stated. “Therefore we see some of our customers for very large generators favoring a single-stage gearbox over direct drive.”

In view of the above, Troedson emphasized the need for generator and turbine designers to work together to optimize the entire turbine design, rather than a particular component.

Exciting times appear to lie ahead for wind turbine technology.

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