By Lindsay Morris, Associate Editor
It has been used to detect air patterns at airports and turbulence on planes, adopted by cartographers and used to develop ranging applications and has been used to detect atmospheric pollutants. It seems that LIDAR, or light detection and ranging, is being used everywhere, even on Mars, where NASA uses it to detect ice and dust clouds.
LIDAR is an optical remote sensing technology that uses lasers to measure properties of scattered light to find range and/or other information of a distant target. LIDAR has been implemented in a number of fields since its development in the 1990s and the last decade has seen this technology adapted for use in the wind energy industry. LIDAR has progressed into an even more robust, compact, silent and wireless device that is installed in wind turbines or ground-based.
Changes in the wind energy sector make the use of LIDAR increasingly attractive. The size of turbines is increasing: Wind turbines with 400-foot rotor diameters and nearly 330-foot hub heights are expected to become available soon. While these larger turbines will help generate more wind power, they also pose potential challenges for installations, as well as manufacturing concerns and expenses.
The wind industry was founded largely on single point wind speed measurements that are traditionally provided by cup anemometry and meteorological masts, or MET. However, larger wind turbines require wind speed measurement at greater heights. Installing MET masts that can monitor wind at such great heights is expensive.
The implementation of LIDAR at a wind farm provides a volume measurement, recording exactly what is experienced. LIDAR is also capable of acquiring measurements with high resolution (~0.1 ms-¹ (mean hourly wind speed)) at different heights. By providing indirect distant measurements, LIDAR does not affect the flow of the wind while acquiring measurements.
Wind LIDAR systems can be either ground-based, used for measuring the wind speed at different heights above the ground, or mounted on wind turbines to measure wind speed at various distances in front or behind the wind turbine rotor.
LIDAR can investigate not only the incoming flow wind approaching the rotor plane, but also the wake produced by the wind turbine. Until the implementation of LIDAR, wind turbine control has been based on the measurement of instruments like wind vanes and anemometers installed on top of the nacelle, in the hub of a wind turbine. But errors can be caused as a result of the effect of the rotating blades on the wind. Since LIDAR can be installed in a turbine or ground-based, blade effects can be avoided.
LIDAR can be used in project planning for wind energy operators to determine a prime location to build a wind farm or wind plant. LIDAR devices can be rented, allowing an operator to move the device to various locations, testing to find the best wind activity location. Through the use of LIDAR, plant operators may decide to relocate some wind turbines in order to maximize wind intake.
“You can do with LIDAR what you can’t do with a mast to validate wind flow models or get to heights you can’t reach with a mast,” said Peter Clive, technical development officer with SgurrEnergy.
Clive said the most effective way of spending data acquisition dollars involves a variety of techniques with different and complementary strengths.
“LIDAR has a vital role to play in any measurement campaign that is well designed and implemented. The aim is to reduce the risks in the project planning process, resulting in positive financial benefits.”
LIDAR offers a potential for reducing total project costs and data uncertainty risks associated with the siting of more traditional fixed met masts, said Alex Woodward, product development and marketing manager for Natural Power.
“Because of its portable and easily re-deployable nature, LIDAR also represents a significant cost savings when used either across large sites often found in the U.S. or across an owner’s portfolio of sites,” Woodward said.
A further benefit of LIDAR is the potential of accelerating the development process by side-stepping any obligation for permitting rights as needed with traditional tall mast anemometry. Offshore wind farms can benefit from even greater cost and time savings, Woodward said, with fixed platforms or floating platforms that are lighter and cheaper in structure. These devices allow developers to start collecting wind data almost immediately on a potential wind farm site without the need to construct a platform.
Natural Power was one of the first companies to produce a commercially available LIDAR system when its ZephIR was released in April 2005. The UK-based company’s LIDAR is currently operating in over 25 countries, deployed in North America with Naikun Wind Energy Group, NANA Regional Corp, the Wind Energy Center at the University of Massachusetts, Canada Centre for Mineral and Energy Technology, GA Developments Ltd., Aeolis Wind and Deep Water Wind.
Virginia-based Catch the Wind was spun off from Optical Air Data Systems in 2008 to develop fiber-optic laser wind sensors for the wind energy industry. Optical Air Data Systems has been developing fiber optic laser sensors for the U.S. military and aerospace industry for more than two decades.
Since July 2009, Catch the Wind has been controlling the Vestas V-82 wind turbine at Nebraska Public Power District’s Ainsworth Windfarm. Through the use of the Catch the Wind LIDAR, the Ainsworth wind farm has experienced an average increase in power of more than 10 percent throughout the trial period.
Choices in LIDAR
Two different types of LIDAR exist: continuous wave (CW) and pulsed. Pulsed LIDAR emits pulses of laser light into the atmosphere and uses time of flight to calculate distance. Essentially, since the length of the laser pulse and the speed of light are known, the height based on the time it takes the transmitted pulse (photons) to reach the height of interest, backscatter and return can be calculated. This is known as time gating.
CW LIDARs do not use time gating and rely on focusing the beam at the height of interest to measure the wind speed. Although the CW beam is focused at this height of interest, it still continues on through the atmosphere and can lead to problems when the beam encounters a cloud base at a higher altitude.
For the needs of the wind industry, CW has proved advantageous for many reasons, Woodward said. The sensitivity of CW remains constant at all heights, ensuring high data availability in all conditions. The emitted laser beam is focused at each user-configured height; therefore the sensitivity does not degrade through fluctuating height or range.
Woodward said that CW has greater sensitivity to changes in wind patterns in comparison to pulsed LIDAR. The emitted pulsed beam is focused at each user-configured height. The laser power does not change at each height or with range, therefore, the sensitivity does not degrade.
However, pulsed LIDAR can be advantageous over CW in a number of ways, said Clive of SgurrEnergy. “While CW can only measure one height at a time, pulsed LIDAR can measure all heights simultaneously. In addition the distance resolution of pulsed LIDAR is constant, enabling long range measurements.”
The LIDAR developed by SgurrEnergy, known as the Galion, is capable of directing its light beam to measure from any direction out to four kilometers, introducing the opportunity to “exploit an extended range,” Clive said. The Galion is a second-generation LIDAR that provides direct velocity measurements, Clive said, which is helpful for understanding losses and loads.
|SgurrEnergy’s Galion is a continuous wave LIDAR in its second generation of production.|
“Until now losses and loads have only been assessed using model approximations. We’re using this device to reduce risk by acquiring direct measurements,” Clive said.
NRG Systems, a U.S. company that merged with Leosphere of France in 2009, released its WindCube V2 at the beginning of 2010. The WindCube V2 is a pulsed LIDAR system. Larry Jacobs, marketing manager for NRG Systems, said NRG chose pulsed LIDAR for several reasons, one being the capability of pulsed LIDAR to track the length of the waves emitted.
Also, CW LIDAR has encountered challenges in regards to cloud cover that do not exist with pulsed LIDAR. Under certain conditions, Jacobs said, the backscatter Doppler signal from cloud cover can contaminate the backscatter from the aerosols at the height of interest. The severity of this contamination depends on a number of factors including low cloud height, high height setting of the CW LIDAR, low aerosol content at the height of interest and high wind shear.
With LIDAR marketing nearly a decade in the wind energy industry, most developers have now released second-generation systems that address most of the limitations encountered using first-generation LIDAR. First-generation LIDAR systems met boundaries when faced with challenges such as wake effects, meteorological conditions like fog, and terrain characteristics, such as wind parks located in forests or mountain areas. Fog in particular proved to be a challenge, as LIDARs can be blinded, not allowing the emitted beam to break through the dense layer of collection of water droplets or ice crystals suspended in the air at or near the ground. This can cause some LIDARs to misread information and present it as a valid stream of data, said Woodward. However, Natural Power’s LIDAR has found a solution for errors caused by fog interference: a patented cloud recognition algorithm which detects the erroneous source of wind signal and removes it from the data, resulting in a more accurate data set.
NRG Systems’ Jacobs said that while terrain characteristics were a concern with wind LIDAR up until about three years ago, improvements have been made in the last few years through the use of post-processing the Constant Fraction Discriminator (CFD) data. CFD is an electronic signal processing device used to determine the precise timing of pulse transmission and reception.
Wind LIDAR is a growing market, and research is underway to make the products more valuable. Risø DTU, a Denmark-based National Laboratory for Sustainable Energy, introduced its LIDAR WindScanner product as part of the European Union (EU) joint research infrastructures in November 2010. In collaboration with six European Energy Research Alliance (EERA) partners, which include Leosphere and Natural Power, the research program aims to support the creation of large-scale transnational research infrastructures in Europe. The Risø WindScanner will now be made available to EU sustainable energy research laboratories and companies via the European research infrastructures.
A windscanner system developed from Risø DTU with help from Natural Power and Leosphere. These systems will be used as research tools.
By implementing research results based on the WindScanner facility, Risø DTU proposes that commercial companies will be able to provide a wind turbine of a given size and cost, with a better performance. While Leosphere and Natural Power are European-based companies implementing the research gathered by Risø DTU, the results of the research will affect U.S. companies that purchase Leosphere and NRG Systems’ wind LIDARs.
LIDAR has been used in the early 2000s to help wind energy operators target more effective operations and in the long term, more affordable methods. This device can lead to a more strategic use of resources if it continues to be adopted by wind energy operators. Use of LIDAR can lead to a more pro-active approach to monitoring operational parameters, helping to achieve an optimum rpm (revolutions per minute), yaw and pitch control of the wind turbine. LIDAR can also be used to increase a wind turbine’s lifetime through the advance detection of wind gusts, leading to adjustment of the operational parameters of the machine. All of this could lead to increased power production while minimizing wear and extending the operational lifespan of the turbine.
Bill Fetzer, vice president of business development for Catch the Wind, said that with advanced knowledge of the wind’s path and gusts, turbine designs can be revolutionized.
“Future turbine designs could be impacted by the fact that you can implement advance turbine control designs to reduce the stress loading using proactive yaw and pitch control,” Fetzer said.
More accurate output of data can in turn allow wind farm operators to take advantage of gusts, rather than avoiding them.
“Right now many turbines are designed to shed those gusts by feathering the blades. But if you can see the gusts coming and know their intensity and duration, you can take that windflow information and transfer it into more energy without causing damage,” Fetzer said.
Integrating laser wind sensors for improved, more proactive wind turbine control and stress load reduction will become a standard industry practice in a few years, Fetzer said. “With LIDAR leading to improved efficiency and effectiveness of large wind turbines, “renewable wind energy will become a more cost-effective method of generating electricity.”
Clive of Sgurr Energy said that instrumentation in wind energy in the past has been “quite limited” until the implementation of LIDAR. “LIDAR removes the limitations. Now we can go from saying, ‘What can we measure?’ to ‘What do we want to measure?’”
The wind energy industry experienced a slow 2010: Installations for the first three quarters were down 72 percent from 2009, according to the American Wind Energy Association. While the decrease in installations is credited to the economic downturn and instability in regulations, increased predictability in wind forecasting could help boost the industry into an era of greater stability.
Laser wind sensor control could “result in recapturing much of the underperformance,” said Fetzer. With a drive to help wind producers position themselves to produce as much energy from the wind as possible, the new generation of LIDAR may be just the answer the wind energy industry needs.
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