Measurements of wind speed and direction are extremely important in the wind energy industry. Developers need the wind speed climatology at hub height to assess the wind resource at a potential site. Wind operators want to know how much power their farm can produce at different times of a day to determine the optimal combination of energy resources.
As turbine hubs rise, remote sensing (RS) devices, which can measure wind speeds to the rotor tip and beyond, are becoming an easier and cheaper alternative to meteorological towers that use cup anemometers and wind vanes. A survey at the 2012 AWEA Resource Assessment and Project Performance workshop showed that well over half of the attendees used RS sometimes or regularly in resource assessment.
LIDAR, a common type of RS device, uses laser light to derive the wind profile, while SODAR uses pulses of sound. Both technologies make measurements in a volume of air, instead of at a point, as a cup anemometer does. Therefore, wind speeds and turbulence measurements from met tower sensors and RS instruments may differ slightly. Even so, confidence in RS is rapidly growing as vendors and researchers show high accuracy of these devices compared to met towers.
The costs of RS campaigns are comparable to those of a met tower, while the benefits are reduced uncertainty in shear profiles, hub height and rotor plane wind speeds, and spatial variation. Reduced uncertainty can translate into improved financing terms in project development.
As with a met tower, the main outputs are vertical profiles of wind speed and direction; RS devices can also measure vertical wind speed. RS data are available at multiple user-defined heights, often up to 200 meters above the ground or higher. This allows the user to measure wind shear across the turbine rotor disk, which can have a large impact on the power produced by the turbine.
Some RS devices report the standard deviation of the component wind speeds (i.e., the turbulent, or fluctuating, portion of the wind) or the turbulence intensity (the ratio of the fluctuating portion to the average wind speed).
However, the turbulence intensity reported by an RS device is not exactly the same as that measured by a rotating cup anemometer, due to the different measurement physics of each. Thus, RS turbulence intensity data are currently used in a largely qualitative sense, although remotely sensed turbulence can still inform important site suitability decisions.
Although LIDAR and SODAR are rich sources of meteorological data, they do not provide remotely sensed measurements of the temperature, relative humidity or barometric pressure. These measurements are often provided by other instruments installed as part of the RS package, and their output is integrated in the data stream from the device. Other measurements, such as near-surface wind speed and direction, solar radiation and precipitation can be added to some devices.
To obtain reliable and accurate data from an RS campaign, it is necessary to make informed decisions at every step of the planning and execution process. This section summarizes best practices for site selection and makes recommendations for different types of campaigns.
To begin, the campaign site should have uniform flow and few obstructions. The RS device should be deployed in a flat area with a clear sky view that is not in the wakes of turbines or towers. In remote locations, an enclosure or security fence can protect against interference or wildlife. Additionally, you should plan a remote sensing campaign with the final goal in mind. Do you want to confirm wind speeds at hub height, monitor shear or examine the wind speeds at different points around the site to better understand power variability?
The ideal strategy depends on what you are interested in measuring.
Hub-height wind speed. Hub-height wind speeds are typically measured by met towers and/or by nacelle-mounted anemometers. However, anemometers that are located on met towers often do not extend to the hub height of the turbines and may be subject to tower interference (tower wake) if not mounted away from the tower itself. Cup anemometers are also adversely impacted by different types of weather conditions and suffer from poor performance during icing conditions and overspeeding in turbulent environments. Nacelle-mounted anemometers can underestimate ambient wind speeds due to the wake effect from the blades. RS devices can increase confidence in hub-height wind speed measurements, which can then be used for power performance assessment and optimization.
For optimal performance, site the RS device upwind of the turbine of interest to measure inflow conditions. In an operational wind farm, the RS device should be located upwind of the first row of turbines to avoid wake effects. Consider elevation differences between the RS device and turbine when setting the measurement heights of the RS device.
Shear profiles across the rotor disk. Wind speed and direction can vary with height through the rotor disk. The draft IEC 61400-12-1 standard for power performance testing describes a “rotor plane equivalent” wind speed that can be used instead of hub-height wind speed. The equivalent wind speed is an area-averaged wind speed across the rotor that can be a more accurate indicator of power production.
The figure shows the difference between the rotor-equivalent wind speed and the hub-height wind speed versus wind shear (the change of wind speed with height). At several sites, the difference between the two wind speed measurements tended to increase with the amount of wind shear across the rotor disk, with the difference exceeding 20% in some cases. Under low shear conditions, the equivalent wind speed tended to be lower than the hub-height wind speed, which could indicate that the actual wind resource is lower than the resource indicated by the hub-height wind speed. These results show how RS can determine more realistic measures of the power available in the wind. For optimal performance, the RS device should be positioned upwind of the turbine and set up so that it can measure the wind speed across the entire rotor disk, allowing the hub-height and rotor-equivalent wind speeds to be measured.
Wind around the site. The wind resource varies across a site due to terrain, regional flow patterns and, in operational wind farms, turbine wakes. Developers may want to monitor the wind resource and power production at different parts of an operational wind farm or determine the best locations to site new wind turbines. Often, developers can seek new financing based on actual project performance, and RS can provide some of that data. Because RS devices are mobile, they can be moved to different parts of a site to take measurements.
The locations of the RS device should avoid wakes from turbines and obstructions. Ideally, deploy two identical or similar RS devices at the same time to measure wind resource variability across a site.
LIDAR and SODAR require a reliable power supply that has more capacity than a met tower installation requires. However, many RS devices can be operated with solar power or small wind turbines and do not need to be deployed near existing power sources. The RS device may occasionally require extra power to operate a wiper blade or temperature control system. With an adequate power supply, remote sensing can be quite reliable, but just as with a met tower, you need to monitor the data coming from the device; if data stop or become suspect, a site visit could be required to verify instrument leveling and settings. Most RS devices also contain an internal wireless modem so that the data and system status can be easily monitored remotely.
Some conditions limit the usefulness of remote sensing. LIDAR performance is limited when the air is extremely clean and lacks small particles (aerosols), while SODAR can perform poorly under extremely dry or windy conditions. SODAR is also affected by ambient noise, especially certain insect or machinery noises. Prior to deployment of an RS device, it is good practice to check the ambient noise, likely wind conditions and humidity at the site to optimize performance for your instrument.
LIDAR and SODAR can give good data under conditions where cup anemometers would be affected by icing. In most cases, however, RS data are adversely affected by periods of precipitation. Many RS devices utilize precipitation sensors to activate a wiper blade when it is raining. RS devices can also suffer from beam attenuation under cloudy or foggy conditions.
Though much of this article has focused on ground-based, upward-looking RS devices, nacelle-mounted LIDAR devices are also being used at operational wind farms to measure the wind upstream of a turbine. Turbine performance assessment, plant optimization and turbine yaw error correction are assisted by this application of RS.
RS devices can also be used to study wind turbine wakes at operational wind farms. Wind turbine wakes have very different characteristics than the flow upstream of the turbine, not only in a reduced mean wind speed, but also in increased turbulence. RS devices have been used to help explain how wakes affect the performance of turbines downstream from other operating turbines.
Scanning LIDAR devices offer the possibility of measuring a 3-D flow field, rather than a vertical wind profile. The high spatial data richness of scanning LIDAR allows for visualization of turbine wake behavior and changes in wind speed across part or the entirety of a wind farm. Scanning LIDAR devices have also been used for offshore wind resource assessment and as nacelle-mounted upwind-looking devices for wind resource characterization or for use in turbine controls. w
Marketplace: Wind Assessment
How To Plan An Effective Remote Sensing Campaign
By Jennifer F. Newman & Kathleen E. Moore
Formulating the proper wind assessment strategy depends on what you’re endeavoring to measure.
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