Best practices for using Sodar and Lidar remote wind sensors

DNV has published a new Recommended Practice document that provides the wind power industry with in-depth information regarding use of remote wind sensing equipment used to characterizing wind resources. Document (DNV-RP-J101) addresses the use of Sonic Detection And Ranging (SODAR) and Light Detection And Ranging (LIDAR) equipment. These have been developed by DNV’s specialists in conjunction with equipment manufacturers and project developers.
“As the size of wind turbines continues to increase, both in terms of blade diameter and hub height above ground, remote sensing is being increasingly relied on to gain vital insight into resource conditions at operating heights well above traditional meteorological towers. As long as this sensing technology is used correctly, it can provide a cost-effective technique to obtain the required data,” says Dr. Tony Rogers, Principal Specialist and a remote sensing expert at DNV.

“DNV’s independent role in developing technology standards together with the key players and authorities is especially valued by the wind industry. This supports the rapid development of the industry, ensuring the highest levels of safety and added business value,” says Robert Poore, Vice President of Business and Service Development at DNV’s Cleaner Energy operations in the U.S.

“Therefore, to provide the wind industry with expert guidance on the optimal use of remote sensing technology, DNV and its key partners now have developed this Recommended Practice (RP) document. Entitled ‘DNV-RP-J101 Use of Remote Sensing for Wind Energy Assessments’ it is based on 10 years of operational experience with remote sensing instrumentation,” he says.

The new RP intends to give the industry and financing community greater confidence in the use of sodar and lidar data in

energy assessments.

Poore continues, “A better understanding of the conditions above typical measurement heights is a key part of reducing uncertainty in energy assessments. The RP, therefore, addresses the planning and implementation of remote sensing measurement campaigns, as well as using the data obtained by remote sensing in energy assessments. Key topics include siting of sensors to avoid noise and echoes; complex flow effects, correlation with other measurement sources, to name a few.”

For additional DNV standards, link to:

http://www.dnv.com/industry/energy/segments/wind_wave_tidal/standards_guidelines/

DNV

DNV.com

Program gets community wind access to better wind sensors

A manufacturer of wind measurement equipment has announced a program to fuel the growth of community-scale wind power projects. The Community Wind Information Service uses leading-edge wind measurement technology from Second Wind Inc, to provide comprehensive wind resource analyses to individuals and groups considering wind turbines.

The service will let community wind developers decide quickly whether the wind resource at their site will make their proposed wind power project economically viable. The process begins with a wind measurement campaign using the company’s Triton Sonic Wind Profiler. Second Wind then analyzes the wind data and other information to produce a Wind Information Report. The report includes detailed wind resource data, energy estimates, and capacity factors for the turbines under consideration. With minimal environmental impact, few permitting requirements, and a fast installation process, Triton can accelerate the development process. Because Triton units are easily relocated, Second Wind can evaluate multiple sites in a single community faster and at a lower cost than meteorological towers.

“Community wind project economics are tight,” says Matthew Cumberworth Sr., VP wind energy at WPCS, an international design-build engineering firm that provides meteorological towers, Triton installations, maintenance, and data services to wind-farm developers. “Many projects don’t have a budget for consulting and equipment purchases for site evaluation. A service like Second Wind’s can make the difference between a productive project and one that shuts down after a year.”

Triton is an advanced remote sensing system that uses sodar to measure wind at higher heights than the previous tower-based standard. By measuring wind speeds at the turbine rotor’s hub height and beyond (up to 200 m), the measurements reduce uncertainty in annual energy production forecasts. Ease of deployment also streamlines wind farm development.

“Community wind projects need a credible, low-cost, and efficient method for evaluating their sites,” says Michael Wiltshire, Triton account manager at Second Wind. “The Community Wind Information Service is a turnkey proposition. The customer doesn’t have to hire multiple parties or go through the onerous permitting procedures that met towers require. After the measurement campaign is finished, they get a report that details all of our findings so they can make informed decisions about their project.”

The service is available to municipalities, private landowners, engineering firms, or anyone developing community wind projects. It is cost-optimized for projects with small capital budgets involving low numbers of small turbines – one or two megawatts. The assessment period can last anywhere from three to 12 months, and all service options are priced under $50,000.

Second Wind Inc.
secondwind.com/community-wind

Sodar measures wind higher than tower and with lower uncertainty

AWS Truepower says it has Sodar units available for lease with support from its team of professional meteorologists and engineers. Client units are also welcomed and supported. Sodar, or Sonic Detection and Ranging, is acoustic equipment that is easily deployed at existing or proposed wind project sites to accurately measure the boundary layer’s wind profile and turbulence structure at heights above conventional meteorological towers. Acting as a virtual tower, Soday reliably measures the wind profile up to heights of 150m. This information provides definitive information about the winds at the hub heights of today’s large wind turbines. Measurements at hub height, and also across the entire rotor plane, give the most accurate predictions of wind plant output.

The company says Sodar equipment can:

• Provide high-resolution wind speed and direction data throughout the rotor plane

• Directly measure the wind flow within the turbine rotor layer

• Significantly reduce uncertainty related to wind speed and energy production

• Quantify individual horizontal and vertical wind flow components

• Measure turbulence levels throughout the rotor plane

• Identify flow discontinuities that fixed met towers miss

• Measure wind speed in a volume of air, not just at one point

• Confirm or revise the wind shear aloft defined by on-site fixed towers

• Reduce the number of conventional met towers needed to qualify a site.

AWS Truepower
awstruepower.com

Leasing comes to large wind sensors

A wide range of wind-energy companies can measure hub-height wind speeds by using reliable and dependable remote sensing systems in wind resource assessments. Second Wind has announced a flexible leasing program for its Triton Sonic Wind Profiler.

The leasing program, operated through recently-established Second Wind Financial Services, is the first manufacturer-supported third-party leasing program for remote sensing. The company Wind will also rent Tritons directly to customers for as short as one month and as long as twelve months.

“Our goal is to streamline the development of wind energy,” says Second Wind CEO Larry Letteney. “This means giving wind farm developers access to industry-leading technology to get their sites approved and working as soon as possible. Rental and leasing gives growing companies tools to use industry best practices, while managing their cash flow.”

The company says its Triton, a ground-based remote sensing system, uses sodar (sound detection and ranging) to measure wind up to and above the 140-meter blade tip height of utility-scale wind turbines. The unit is for wind energy applications including wind resource assessment, micro-siting of wind turbines, and ongoing monitoring of wind conditions on working wind farms. It has been in commercial use since April 2008.

Second Wind Inc.

secondwindfinancialservices.com

Sound intelligence in the search for wind power

Reprint Info >>

The wind industry’s need for wind measurement has grown beyond the 60-m reach of standard meteorological (“met”) masts. To reduce uncertainty for wind projects that can cost anywhere from $100 million to $1 billion, the industry needs data from the entire rotor sweep that can’t be gleaned from 60, 80, or even 100-m met masts.

A 60-m met mast outfitted with sensors measures only about a quarter of the rotor sweep of a typical commercial turbine mounted on an 80-m tower. Taller masts are obtainable, but permitting and aircraft obstruction regulations make them challenging to site. And even where an 80-m mast is feasible, it only monitors the lower half of the wind powering the turbine. Is there a solution to met mast shortcomings?

Remote sensing is a credible alternative to mast-based measurements. Remote sensors are ground-based, and use sound or light to measure wind speed and direction at various heights. They can reveal the extent of wind shear events and wake effects. Developers prospecting new sites can use remote-sensing equipment to quickly identify the most promising turbine locations while eliminating marginal locations. With that intelligence, they can use fewer met masts and site them more effectively.

Beyond their height limitations, masts are firmly anchored to their location. If they’re not set up in the right area to measure the best wind on the site, it’s a cumbersome process to move them. Remote sensors, on the other hand, can be relocated in just a few hours. That makes them increasingly used for rapid wind prospecting, detailed wind resource assessments, and operations and maintenance studies.

Remote sensing

The “remote” in remote sensing refers to the separation between the devices and what they measure. Remote wind sensors operate on similar principles, whether radar, sodar, or lidar. They all emit signals of one form or another in a beam pattern. The emitted signals encounter variations in the air, reflecting back to the sensing equipment. Frequency changes from the original signals are interpreted as Doppler shifts, indicating that the sensed winds are moving towards or away from the instrument. Processing the Doppler shifts, elapsed times, and geometries of the emissions and reflections yields a profile of wind conditions.

Remote sensors used in the wind industry are based on either sodar (SOund Detection And Ranging) or lidar (LIght Detection And Ranging) technology. Radar isn’t used in wind energy applications because it can’t resolve wind speeds within a few hundred meters of the ground.

Sodar is a form of sonar, like that used for echolocation by dolphins and bats. Sodar sends audible acoustic pulses into the air, which reflect off encountered temperature differences. Microphones detect the resulting “back-scatter.” Calculating the time it takes for the sounds to travel back to the microphones yields the heights where the reflections occurred. Measuring the frequency change from the emitted pulse allows calculating wind speed towards or away from the instrument. Sending acoustic pulses in three or more different directions allows translating steeply angled wind speed measurements into horizontal wind speeds and directions over the measurement range.

Second Wind’s Triton is one example of a next generation sodar system. The Triton is built around a 36-element array of piezoelectric trans-ducers, which operate as either highly efficient speakers or microphones. In speaker mode, the array emits sonic “chirps” that reflect off an internal sound mirror and into the atmosphere. Switching to microphone mode, the arrays record echoes from the sound scattering.

The array is called “phased” because individual transducers are operated in acoustic phase with each other. Phased arrays are used in radio astronomy and other precise imaging applications, because they substitute inexpensive and efficient small transducers for more costly, larger equivalent devices. The transducers can create a better-directed beam and listening cone than a single speaker-microphone, thanks to the physics of interference patterns. Only one array is needed for three different beam directions. Triton achieves this by introducing time delays between rows of transducers, there being three-row orientations 120° apart in the hexagonal array pattern.

The U.S. Department of Energy’s National Renewable Energy Lab reports that sodar delivers accuracy comparable to tower-mounted meteorological instruments. The lab recently tested a Triton and found its wind speed and direction data correlated well to instruments mounted on meteorological towers.

“We see Triton as a valid stand-alone system for wind measurement studies,” says NREL Principal Scientist Dennis Elliott. “In addition, the sodar unit was reliable, with an uptime of more than 98%.”

The Energy Research Center / Netherlands came to a similar conclusion after comparing Triton data to data from a 100-m meteorological tower mounted with four anemometers and wind vanes at different heights. While maintaining 98.85% operational availability, the sodar unit provided data on wind speed, direction, shear, and turbulence intensity that qualified it as “a stand-alone system for wind resource assessments, especially given the industry’s tendency toward higher hub heights,” states the report.

Lidar, like sodar, measures wind speeds by processing the Doppler shifts of its emitted beams. A lidar either pulses or continuously fires a solid-state infrared laser while motorized mirrors or optical waveguides maneuver the beams. These strike particles in the air, “aerosols,” which reflect back to the source instrumentation where photosensors detected them.

In the case of lidar, the Doppler shift takes the form of a slight color change in the radiated laser light. The shift is generally not directly measurable due to light’s ultra-high frequency. Measurements are using various electronic and optical modulation and detection techniques.

Like sodar and sonar, lidar technology has been in use for years in applications other than wind power. Meteorological lidar has been used successfully to measure narrow regions of air, sometimes to distances as great as 15 km. Such systems have been built with color-tunable lasers, high-power requirements, and great expense, typically upwards of $1 million.

Lidar systems now at work in wind-energy applications use lasers and optical detectors developed for fiber-optic communications, so they are not tunable for atmospheric applications. They have limited power – typically in the hundreds of watts instead of kilowatts – giving them a range comparable to sodar: hundreds of meters instead of kilometers.

Sodar and lidar each have advantages and disadvantages. Lidar’s strong suit is response time. Light speed permits many more measurements than can be made in the round-trip time of sound waves traveling a few hundred meters. Lidar can make second-by-second wind-speed measurements, which are useful for wind turbine controls. For resource assessments, however, the high response speed possible with lidar has no particular advantage.

Sodar’s technical advantages include an inherently lower cost and power of basic transducers. These are not academic considerations. There are several sodar systems available from $50,000 to $75,000, while the lowest cost complete lidar systems carry price tags of $150,000 to $250,000. The lowest power sodar, Triton, consumes 7W on average. The lowest power lidar system is reported to be 45W. For remote applications this is a substantial difference.

For the wind industry, sodar is inherently better at making precise frequency shift measurements than the lidar technology used in wind applications. Audio frequency measurements of hundreds of parts per million is commonplace. It is difficult to make, within a percent, accurate measurements of fiber-optic laser emissions that have been Doppler shifted by wind speeds of just a few meters per second.

Both sodar and lidar have characteristics that can limit the range and reliability of their data. For example, sodar is susceptible to ambient noise in the range of its operation, especially at lower frequencies. Dry, thermally homogenous air affects sodar’s performance and it can’t compete with a hard rain. Lidar cannot work through a dense fog and its performance drops in clean air, including after rainfall where the aerosols have been washed away. Lidar can also have issues with sunlight and cloud cover.

However, anemometers and wind vanes are also prone to malfunctions and failure from rain, freezing, and dust. They perform nonlinearly in the presence of non-horizontal flow, and can’t measure inflow angles. Mast-mounted sensors are vulnerable to lightning strikes and other environmental hazards – much less of an issue with ground-mounted remote sensors. The reliability of remote sensing systems’ can rival that of the best conventional mast-based systems. And the reliability of 100 m-plus lidar or sodar is greater than extrapolations made from measurements from a 60-m mast.

Reducing uncertainty

The current political and social climate forecasts big growth for wind power. That growth will not occur, however, unless developers can convince investors that wind power is a reliable, profitable power source. There is inherent uncertainty in banking on the wind. Investors don’t like uncertainty, so it’s incumbent on developers to reduce it.

Meteorological towers don’t yield enough data. They don’t reach high enough to measure a modern turbine rotor’s full sweep, which forces developers to rely on extrapolations. There’s no reason to do that when remote sensing can provide the missing data.

Furthermore, the wind-power industry would benefit from a new equation between fixed and remote sensing. The current standard is to erect three to five masts for every remote sensor used on a site. That ratio should be reversed: three to five remote sensors for every mast. Remote sensing equipment should be the primary data gatherer. They’re just as accurate as mast-mounted sensors, gather more data, and are more cost effective, and versatile. Mast-mounted sensors’ proper role right now is providing reliable data baselines for correlating remote units.

Combining remote and fixed sensing in these roles adds up to more comprehensive wind data sets that reduce the uncertainty inherent in wind farm development. Combined, they offer wind-farm developers persuasive facts to attract investors, while offering investors an assurance of a return on their investment. WPE

-Susan Giordano/Second Wind Inc, Somerville, Mass./www.secondwind.com

Its official: NREL says sodar data as good as from met towers

Sodar or sonic detection and ranging, is a relatively recent tool for measuring wind speed and direction up to 200-m up. The task was usually relegated to meteorological tower that had to be built and often reached only 80-m up. Portable sodar units simplify the task. Recently, as part of continuing scientific research to understand wind resources, the National Renewable Energy Laboratory (NREL) and sodar developer Second Wind Inc, Somerville, Mass., partnered to characterize the performance of Second Wind’s Triton Sonic Wind Profiler.

The following is an edited version of conclusions reached by NREL authors. Their analysis (http://tinyurl.com/wpenrel) of the wind resource data provided by Second Wind Inc., came from 10-min data samples over a 198-day period. It was collected from an 80-m meteorological tower and a Triton sodar system from a measurement program conducted near an operating wind farm in western Texas. NREL did not participate in the measurement phase of the study. The tower configuration, specs of the locations of the met tower and sodar unit, and the data collection were done by the wind-farm developer with assistance from Second Wind. The collected data were then sent to NREL for analysis.

“Our analysis of the sample Triton data set shows excellent agreement with the tower measurements,” says the NREL report. “Given the 200 m distance between the sodar and the met tower, it would be unreasonable to expect a perfect correlation between the two datasets.” From the data provided, NREL engineers make these observations:

• The operational uptime was greater than 98%, demonstrating sodar’s operational reliability during this 198-day period.

• Triton’s (sodar) measured wind speeds correlated well to the meteorological tower, but were generally slightly lower. The correlation coefficients were greater than 0.983 at both heights (50m and 80m).

• The Triton’s measured wind direction also correlated well to the tower data when the sector containing the wind farm was removed. At 80 m, the correlation coefficient calculated between the Triton and the wind vane was 0.994.

• The wind direction distributions as measured by the Triton and the tower were consistent. A slight rotation of the directions may be attributable to instrument alignment, or may reflect an actual difference in the wind direction over the horizontal distance of 200 m between the meteorological tower and the Triton.

• The percent of valid data (Q >= 90%) measured by the Triton was greater than 95% at 80m and was approximately 81% at 120m.

• Shear exponents calculated from the tower and Triton data were comparable in terms of overall shear as well as daytime and nighttime shear.

• A discrepancy in the average turbulence intensity was measured by the two instruments. At 80m, the Triton measured an average TI of 0.100, while the average 80m TI on the meteorological tower was 0.132.