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SPIE Professional January 2011

Optics and Wind Power

Advances in optics and photonics make wind energy more viable

By Beth Kelley

Interest in wind energy has picked up in the last few years, thanks in part to advances in optical sensing and communications technologies. Wind power production utilizes laser-based atmospheric measurement and sensing, pyrometry to map and monitor turbine blades, fatigue-related measurements through thermography or embedded fiber optics, and optical position sensing.

From creation to system maintenance to transmission, photonics technologies have made wind power competitive with other forms of alternate energy on a large scale.

Wind on land and sea

Wind power is as old as the sails that propelled our ancestors across water. The current need for alternative energy sources has made people revisit wind, and rural areas that once used windmills to pump water and produce power are again leading the way in wind production.

Historical farmland territories across the United States and Canada are becoming hot spots for wind power because they are known for consistent winds and there is space to build. According to the nonprofit Renewable Northwest Project, wind turbines in the U.S. Pacific Northwest currently generate about 4300 megawatts of electricity, enough to keep the lights on in 1.1 million homes.

Optical sensors to increase the efficiency of the turbine output are mounted on the back of the wind turbine. Photo courtesy of Micron Optics.

The United States is currently the largest producer of this power, generating 35 gigawatts (GW) of wind energy, but it has some major competition. A 2010 Global Wind Energy Council report found that China has become the second largest producer of wind power, with 25.8 GW. Germany came in third, with 25.77 GW.

Spain, Holland, France, Romania, Turkey, Belgium, and 15 other European countries have already begun to develop wind power. Wind farms are also being built in open spaces of Morocco, Ethiopia, Kenya, Japan, and Australia. The governments of all these nations are providing funding and other resources for developing wind power.

Bigger, faster, stronger

Other reasons for renewed interest in wind technologies are improvements in wind turbine efficiency and power production and expanded transmission capabilities.

Wind turbines rely on anemometers to optimize the blades’ configuration to produce the most energy. Mounted to the back of the turbine’s gear-housing unit, today’s anemometers are a big improvement over the old analog cups designed in the 1930s. With advanced optical sensors, wind propellers are able to sense how much wind they are receiving and adjust accordingly to increase the efficiency of the turbine output.

The sensors are the control variables for feedback in the control systems, says Alan Turner, optical sensing specialist at Micron Optics. “As the industry goes forward, smart blades with active control surfaces will increase efficiency even more,” he says.

Laser-based anemometers seem especially promising. Lidar instruments are capable of measuring with high resolution (~0.1 ms–1) at different heights and allow wind turbines to “see” the wind before it hits the turbine blades. The National Laboratory for Sustainable Energy at the Technical University of Denmark (Risø DTU) recently completed testing on a lidar-based wind turbine system that can predict wind direction, gusts of wind, and turbulence. In various studies, this kind of sensing has allowed turbines to increase productivity by 5 to 10%.

Sense and protect

Lidar, which has seen rapid improvements in cost, compactness, and reliability the last five years, could also be used for sensing and monitoring bearing or gearbox failures, delamination of blades, or out-of-balance blades or systems, Turner says.

photo of Micron Optics strain gauges
Fiber-optic strain gauges are installed during construction of the wind turbine blades.Photo courtesy of Micron Optics.

New sensing technologies make the job of monitoring distant towers, with lots of moving parts, easier. Two of the most important monitoring tools are the tower’s strain and temperature sensors. Conventional electronic sensors are prone to failure from the harsh environmental conditions wind turbines normally operate in.

“Wind turbines are often struck by lightning and this has had adverse effects on conventional electrical sensors,” says Jason Kiddy, principal at Aither Engineering near Washington, DC. Optical fiber sensors have gained popularity in wind turbines because of their resistance to electromagnetic interference, lightning, and electric noise. Manufacturers see them as a practical, reliable, and cost-effective monitoring tool.

“Fiber-optic monitoring of the wind turbine blades helps primarily by reducing stresses on the blades and thereby increasing the blade’s life,” Kiddy says. It can also detect blade problems before they become huge.

Many different types of optical fibers are used, each with its own advantages and disadvantages. “POF (power-optical fiber) is easy to handle and easy to terminate, and no injury is possible when using polymer vs. glass,” says Mickael Marie, marketing manager for Avago Technologies. However, Marie says hard-clad silica (HCS) offers better bandwidth and distance than POF.

Fiber Bragg-Grating (FBG) sensors are also gaining popularity in wind turbine monitoring. Their primary advantage is their immunity to lightning strikes. Kiddy adds that “FBGs allow for many more measurements compared to other fiber-optic sensor types.”

Motion sensors also allow wind towers to monitor other objects in its airspace, like birds or planes. Norway-based OCAS (Obstacle Collision Avoidance Systems) developed a ground-based, low-power radar that will detect and track an aircraft’s proximity to a wind tower and illuminate the tower’s visual warning lights when an aircraft is detected.

Offshore investments

Many people believe that offshore wind turbines are the future of wind power production.

Spain, the UK, and other European countries have several offshore wind projects in place or in development. In the United States, developers of a 130-turbine project proposed for the waters off Cape Cod in Massachusetts won federal permits for the country’s first offshore wind farm in 2010 and hope to pave the way for more offshore wind projects.

photo: Avago's semiconductor for wind turbines
Avago’s HFBR-3810Z is a semiconductor that can deliver data transmission from the wind turbine at signal rates of DC to 10 MBaud.

Photo courtesy of Avago Technologies.

Offshore wind turbines need to be more powerful than those on land because of higher maintenance costs and wind speeds on the high seas. This was tricky until sensing and monitoring technologies caught up and components became more reliable.

Marie explains that the durability and reliability of optical sensors are important “because it is difficult to access offshore wind turbines.” He believes they will find a niche market in offshore sensing.

“Another area of monitoring, especially as wind moves offshore, will be sensors in the towers’ base and nacelles,” says Turner.

Lidar is also applicable to offshore monitoring because it works well over large open areas, like the ocean, and is most cost effective when used over larger areas.

Power to the people

One drawback to siting wind farms in rural settings or offshore is the long distances the power transmission lines must traverse. The wind boom in the U.S. states of Washington and Oregon, for example, has led to a shortage of substations and transmission lines to carry electricity to cities where the usage is.

Optical transmission and communication networks using fiber optics have turned out to be a great fit for this need. A consortium backed by Google, Green Energies, and Tokyo-based Marubeni announced a $5 billion undersea transmission cable project off the East Coast of the United States in September. Upon completion in 2020, the new 350-mile cable will have the capacity to carry 6,000 MW of energy – enough to power 1.9 million homes, according to a press release. The project, dubbed the Atlantic Wind Connection, will have an underwater cable approximately 17 miles off the coastline, from northern New Jersey to southern Virginia.

As well as getting power to homes and industries, communication among far-flung towers, power stations, and data centers is critical. Ethernet protocols for wind turbine applications for real-time monitoring are currently being explored by Avago and Phoenix Contact. Single-mode transceiver solutions for networking turbine router switches are also being explored by several companies. Transformers and converters using high-frequency cores and silicon switching devices have proven extremely robust in wind and wave energy applications.

Making sure the lines are open, both for communication and transmission, will allow wind power to continue its growth as a viable alternate energy option now and into the future.

Turbine maintenance

Sensing capabilities on wind turbines are still fairly basic.

“Right now, only relatively large problems can be detected, perhaps not as early as desired,” says Jason Kiddy of Aither Engineering.

Future wind turbine manufacturers will need improved monitoring as these new turbines start to break down and need maintenance.

Offshore wind turbines also need more monitoring than land-based turbines due to their size and remoteness.

Tilting at windmills

As happens with any technological change, groups have sprung up to oppose wind power.

Some people complain about noise produced by turbines in rural areas while others do not like the look of wind farms disrupting the landscape or ocean views.

Some people have voiced concerns about the impact on migratory birds, fisheries, and tourism.

Another concern is space, since wind farms typically need large open areas.

SPIE Smart Structures/NDE

Numerous papers on optical technologies for wind applications will be presented in March at SPIE Smart Structures and Non-Destructive Evaluation in San Diego.

Have a question or comment about this article? Write to us at spieprofessional@spie.org.

Beth Kelley 
is a science and technology writer.

DOI: 10.1117/2.4201101.07

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