ValidWind applications: Wind power prospecting, aerosol transport
T. Wilkerson*, A. Marchant, T. Apedaile, D. Scholes,J. Simmons,and B. Bradford
Energy Dynamics Laboratory, 1695 North Research Park Way, North Logan, UT 84341
ABSTRACT
The ValidWind™ system employs an XL200 laser rangefinder to track small, lightweight, helium-filled balloons (0.33 meters, 0.015 kg). We record their trajectories (range resolution 0.5 meters) and automatically produce local wind profiles in real time. Tracking range is enhanced beyond 2 km by applying retro-reflector tape to the balloons. Aerodynamic analysis shows that ValidWind balloon motion is well coupled to the local wind within relaxation times 1 second, due to drag forces at subcritical Reynolds numbers Re < 2×105. Such balloons are Lagrangiansensors; i.e., they move with the wind as opposed to being fixed in space. In a field campaign involving many balloons, slight variations in ground level winds at launch lead to trajectory patterns that we analyze to derive 3D maps of the vertical and horizontal wind profiles downwind of the launch area. Field campaigns are focused on likely sites for wind power generation and on facilities from which airborne particulates are emitted. We describe results of wind measurements in Utah near the cities of Clarkston, Logan, and Ogden. ValidWind is a relatively inexpensive wind sensor that is easily and rapidly transported and deployed at remote sites. It is an ideal instrument for wind prospecting to support early decisions required, for example, in siting meteorology towers. ValidWind provides high-resolution, real time characterization of the average and changing 3D wind fields in which wind power turbines and other remote sensors must operate.
Key words: wind, wind power, wind profiles, balloons, lidar tracking
1. Introduction and Background
In response to needs for reliable lower atmosphere wind data for studies of aerosol transport and wind power resources, our laboratory has initiated an R&D program called VisibleWind, the first phase of which has been to develop the ValidWind™ sensor. The sensor method consists of optically enhanced lidar tracking of lightweight, helium-filled balloons. The advantages of this system include low cost, easy deployment, rapid data turn-around, and operation by a single person, in daytime and nighttime. Balloon trajectories r(t) are automatically recorded and analyzed for profiles of wind speed, wind direction, and wind shear as functions of time, altitude, and geolocation.
Previous accounts1,2 of ValidWind have described the phases of its development, early results, and the rigorous The theoretical basis for the authenticity of balloon motion as a wind sensor is treated in a forthcoming journal article3. Here we describe the development of an automatic balloon tracking system (Section 2), results of recent measurement campaigns (Section 3), and new data products of ValidWind operations (Section 4). Further applications to air flow in complex terrain and in inversion-dominated situations are underway.
2. Auto-tracking.
The ValidWind sensor has recently been has been enhanced by incorporation of an automatic target tracking system. The rangefinder and compass are now mounted to a motorized gimbal (QuickSet Gemineye) along with a digital camera (Sony, 36x optical zoom). An image recognition system (PerceptiVU PVU-TT-M6) tracks the balloon within the field of view and controls the gimbal to keep the sensor centered on the balloon. Figure 1 shows the ValidWind sensor mounted to the autotracking gimbal. The purposes of the autotracker are to eliminate operator fatigue from tracking balloons, to maximize the tracking accuracy, and thereby to minimize trajectory errors. In this configuration, ValidWind requires only a single operator for setup, balloon preparation, and tracker operations.
The integration of automatic tracking into the ValidWind architecture is explained by reference to the system block diagram in Figure 2. Operation begins when the laptop controller commands the rangefinder and compass (across a wireless link) to begin seeking valid range data. The operator releases a balloon and manually (by joystick) guides the gimbal to center the camera view on the balloon. The operator sends a joystick trigger command to the image recognition subsystem to identify the centered image object as the target. The image recognition software
*; 435-760-2468 (mobile); 435-797-4686 (fax).
Figure 1. ValidWind sensor mounted to its autotracker.
Figure 2. Architecture of the ValidWind system.
the gimbal to center the camera view on the balloon. The operator sends a joystick trigger command to the image recognition subsystem to identify the centered image object as the target. The image recognition software begins autonomously tracking the balloon within the camera field of view. The subsystem continuously feeds the target offset back to the gimbal as an error signal to keep the camera centered. Because they are optically co-aligned, the rangefinder tracks the balloon along with the camera. The rangefinder sequentially delivers range and directional data back to the laptop as long as the tracking remains locked and the balloon is within range. The typical data collection rate is one trajectory point every 3 seconds.The camera has 32x optical zoom so that the balloon can be tracked out to an extended range. The operatorincreases the zoom (manually, using the joystick) as the apparent balloon size decreases. Nightime use ofValidWind is facilitated by a bright flashlight (40W high-intensity discharge lamp) mounted on the gimbal next to the rangefinder. The retroreflectors on the balloon create a bright return that can be tracked to a range of at least 1.5 km. We note that the system’s maximum range can easily be extended with the use of a higher laser pulse energy.
ValidWind is subject to system limitations under some conditions of use.Structure in the background image can interfere with image recognition, causing tracking errors if the balloon drifts in front of a background scene (e.g., a mountain) or high-contrast clouds. High levels of dust or insects (attracted by the flashlight) can cause false readings from the rangefinder. And image tracking offsets when the balloon has high proper motion increase the trajectory errors and reduce the rangefinder range. This latter problem is currently being addressed by an upgrade to the controller software. An integral control term will be added to the error signal sent to the gimbal so that the tracker will follow the target balloon more closely and will be less susceptible to transient structure in the background image.
3. Recent applications of ValidWind
3.1. Canyon winds.
August 19/20, 2009: An overnight study of catabatic canyon winds in Logan, UT was conducted at the mouth of Logan Canyon on the Logan River. This location has been proposed as a possible wind turbine site for Utah State University. Figure 3 shows the time dependence of the wind speed at selected altitudes of 75 – 200 meters AGL. Wind speeds above 15 mph occur during the eleven hour period 22:00 (MST), August 19 – 09:00 (MST), August 20. High winds exceed 30 mph. The optimum altitude range (for maximum wind duration) is 75 – 125 meters. The 3D trajectories demonstrate a very steady direction throughout this period. The nocturnal jet develops from the ground up, and the optimum height for a wind turbine would be about 100 meters.
Figure 3. Logan Canyon winds, Logan UT, at altitudes of 75 – 200 meters for 13 hours, August 19-20, 2009, showing optimum wind layer 75 – 175 meters high.
3.2. Ridge winds.
September 19, 2009: Terrain-coupled wind fields showing consistent wind speeds above 15 mph were monitored during 6 daylight hours above a mountain ridge near Clarkston, UT. Figure 4 shows the set of nine balloon trajectories collected from 15:50 to 16:41 MDT projected onto a terrain topographic map (brighter shades correspond to higher altitude). Seven of these balloons were launched at 5-minute intervals during the period 16:14 – 16:41 in a saddle of the ridge andfollowed a northeast compass course at roughly +52˚. Projecting this set of trajectories onto that vertical plane, we found the corresponding set of velocity (tangent) vectors shown in Figure 5. This demonstrates the ability of the ValidWind method to capture cross sections of the wind field. The horizontal velocity component is relatively steady at 7.8 ± 1.3 m/s, while the vertical component shifts dramatically from + 1.6 m/s to – 3.5 m/s in passing leeward from the ridgeline. Ridgeline behavior is an important consideration in choosing the locations for not only wind turbines but also the meteorology towers used to test for turbine site suitability.
Figure 4. Terrain map of two groups of ValidWind launches from ridge above Clarkston, UT
followed a northeast compass course at roughly +52˚. Projecting this set of trajectories onto that vertical plane, we found the corresponding set of velocity (tangent) vectors shown in Figure 5. This demonstrates the ability of the ValidWind method to capture cross sections of the wind field. The horizontal velocity component is relatively steady at 7.8 ± 1.3 m/s, while the vertical component shifts dramatically from + 1.6 m/s to – 3.5 m/s in passing leeward from the ridgeline. Ridgeline behavior is an important consideration in choosing the locations for not only wind turbines but also the meteorology towers used for qualifying the suitability of turbine sites.
The other two trajectories at the top left of Figure 4 were launched at a point 300 meters windward of the ridge. Because of the complex air flow they rose more toward the north (+33˚), providing a picture of the rising flow over higher elevations at a horizontal speed of 7.2 ± 0.8 m/s with an updraft speed of 1 – 2m/s.
Yet another group of eight Clarkston balloon flights on September 19, 2009 between 10:43 and 14:56 were launched from a higher elevation south of the above site, for comparison with anemometer data being collected from a 60 meter tower. These data sets showed excellent agreement at 60 meters AGL, with winds averaging 8.5 m/s with a maximum of 11 m/s at 13:30.
Figure 5. Wind velocity vectors: Seven Clarkston trajectories, projected ontoplane at + 52˚ and averaged
Horizontal velocity steady at 7.8 ± 1.3 m/s;Vertical componentdrops from + 1.6 to – 3.5 m/s leeward.
4. New data products
4.1. Software
The laptop controller for ValidWind includes quick-look software to organize and analyze balloon trajectories in the field. The quick-look software (Figure 6) is a MatLab application that includes BlueTooth communication with the rangefinder, data logging, trajectory processing, and graphical display of wind data products.
For a each experimental session, the raw data is recorded as a sequence of 4-vectors. Each vector consists of a time-tag, range, altitude angle, and azimuth angle. Between balloon flights, the data file is padded with special values that enable its segmentation into individual balloon flights. Trajectory analysis and wind characterization may be requested for one or more balloon flights without interrupting the data session.
The first task of trajectory analysis is elimination of false readings. These are 4-vectors for which there is no valid range (e.g. the rangefinder was not pointed accurately enough) or the range does not correspond to the balloon (e.g. interference from the background scene or foreground dust particles). False readings are identified by the occurrence of invalid range values or by non-physical increments in the range value.
Figure 6. Quick-look software for ValidWind
The valid data points aretransformed into local Cartesian coordinates (relative to the tracker location) and then smoothed by a Gaussian-weighted Quadratic Least-squares Filter (GQLF), a variant2of locally estimated scatterplot smoothing (LOESS). The timing of the raw trajectory points is irregular, but GQLF produces a regular time sequence of trajectory estimates. At each estimate point, GQLF simultaneously estimates vectors for the balloon location, velocity, and acceleration. The scale parameter for Gaussian weighting is typically set to = 10s. At a sample rate of 3 readings/second, approximately 8 readings contribute to each trajectory estimate. If the number of nearby data readings is insufficient at a given estimation time, a blank point is inserted in the smoothed trajectory.
4.2. Weber Canyon, UT
August – September, 2010: A new and more comprehensive study of canyon airflows is being undertaken in Weber Canyonnear Ogden, UT The real-time software allows us to assess the state of the wind field to decide how frequently to sample. The quick-look software displays the ValidWind data in three graphical formats: 3D trajectory, wind distribution, and wind profile. The trajectory plot displays the balloontrack in a 3D viewer with full user control. The wind distribution shown in Figure 7is an intensity plot indicating the extent to which each wind speed and direction was experienced by the balloon(s). This display provides the same information as a traditional Wind Rose plot, with a more intuitive interpretation. The wind profile plots for August 4 shown in Figure 8 display the balloon speed and direction (primarily to the west)as a function of altitude (AGL) for a set of 4 sequential balloon flights. These data are generated directly by the GQLF smoothing algorithm and do not require any secondary differentiation or smoothing. Further ValidWind results will be reported in the near future.
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Figure 7. A wind probability distribution. Figure 8. Wind profile plot (August 4, 2010).
References
[1]Wilkerson, T., B. Bradford, A. Marchant, C. Wright, T. Apedaile, E. Fowles, A. Howard, T. Naini, "VisibleWind: A rapid-response system for high-resolution wind profiling," Proc. SPIE Lidar Remote Sensing for Environmental Monitoring X, Ed. U. N. Singh, 746008 ( 2009).
[2]Wilkerson, T., B. Bradford, A. Marchant, T. Apedaile, C. Wright, "VisibleWind: Wind profile measurements at low altitude," 74790L, Proc. SPIE Lidar Technologies, Techniques, and Measurements for Atmospheric Remote Sensing V, Eds. U. N. Singh, G. Pappalardo, 74790L (2009).
[3]Wilkerson, T., A. Marchant, “Wind-field characterization from the trajectories of small balloons”, submitted to J. Appl. Remote Sensing, (2010).
ACKNOWLEDGEMENTS
Development of ValidWindTM has been supported by funding from the Utah Science, Technology, and Research Initiative (USTAR). Valuable advice has been provided by Ryan Pierson of Electronic Data Systems and Michael Wojcik of the Energy Dynamics Laboratory.