CHARACTERIZATION OF WIND VELOCITY DISTRIBUTIONS WITHIN A FULL-SCALE HELIOSTAT FIELD

Transcription

1 CHARACTERIZATION OF WIND VELOCITY DISTRIBUTIONS WITHIN A FULL-SCALE HELIOSTAT FIELD Jeremy Sment and Clifford K. Ho Concentrating Solar Technologies Department Sandia National Laboratories P.O. Box 800, Albuquerque, NM jsment@sandia.gov, ckho@sandia.gov 1. ABSTRACT Understanding the impact of wind loads on heliostats is important for concentrating solar power technologies employing solar power towers. The National Solar Thermal Test Facility (NSTTF) at Sandia National Laboratories is conducting full-field heliostat tests to evaluate and better understand these impacts. stats selected from around the perimeter, center, and in clusters of adjacent units are being instrumented with anemometers. The wind data is synchronized with measurements taken from accelerometers and strain gauges using National Instruments LabVIEW applications. This paper focuses on wind data collected above a single heliostat for characterization and model validation. Computational fluid dynamics (CFD) models of the wind velocity profile above the heliostat show agreement with empirically acquired average wind velocities in two recent experiments. 2. INTRODUCTION Concentrating solar power plants offer a benefit unique to the solar power industry of being able to store thermal energy in salts, water, or other medium and use this energy to generate electricity on demand including through the night. Central receiver type concentrating solar systems use enormous fields of moveable mirrors called heliostats that focus sunlight onto a central tower receiver. These fields can be over a mile in radius with thousands of heliostats. Any improvement in heliostat efficiency or reduction in materials will have a multiplied cost saving effect. Since heliostat fields are located in open desert areas they are exposed to direct wind loading. The wind-excited motion of the heliostats causes the reflected light beams to become less effective resulting in the need for more heliostats to achieve the solar energy requirements of the plant. Additionally windinduced motion of the heliostats causes significant mechanical stress on the heliostats, particularly in the azimuth and elevation drive mechanisms. (Steve Kusek, Oct. 11) Dynamic motion of heliostats is of primary consideration in determining the factor of safety of heliostat designs. Measurements of wind and heliostat vibrations throughout the field can aid predictions of fatigue factors of safety by improving estimates of magnitude and number of stress cycles caused by vortex shedding, modal vibration, and intrinsic machine fatigue. (Stahl, 198), (Yashuharu Nakamura, 1991) Prior heliostat research has been conducted using wind tunnels and CFD modeling. (J.A. Peterka, 1992), (Ying-ge Wang, 09) Some authors cite the need for full-field testing of an actual heliostat field. The NSTTF at Sandia National Laboratories in Albuquerque, New Mexico, is uniquely poised to perform such a study due to its fully functional heliostat field and mandates from the department of energy to aid in reducing the cost of solar thermal energy. 3. EXPERIMENTAL SET-UP This paper focuses on the first two wind experiments conducted in March 12 that attempt to compare CFD model results to empirical data. Anemometers located in the unobstructed flat land west of the NSTTF field were used to define the boundary conditions that were entered into the CFD model. Anemometers mounted on a rod above the facets were used to characterize points at four discrete coordinates which could be 1

2 compared to values generated by the model at approximately the same coordinates. The heliostat outlined in Fig. 1 was chosen as the best starting point for the series of full-field studies as it is expected to have the most exposure to the predominant southwesterly wind flow and least obstruction from other heliostats or ground objects. The heliostat position naming convention specifies an elevation angle determined by the angle of a line normal to the plane of the central mirror above or below horizontal. Perfectly vertical mirrors would have an elevation angle of 0 and mirrors facing directly up would have an elevation of 90. An azimuth angle is also specified as the angle from due north increasing toward the east. A heliostat facing due west would have an azimuth position of 270. Wind headings use the same convention. The two heliostat positions included in this study are STOW and 4. STOW has an elevation angle of -84 (almost facedown) and an azimuth angle of 180 (facing south). The 4 configuration has an elevation angle of 4 and an azimuth angle of 270 (facing west). 3. RESULTS Fig. 1: National Solar Thermal Test Facility, Sandia National Laboratories, Albuquerque, NM A tripod was positioned in the open land due west of the instrumented heliostat with three ultrasonic anemometers (F1- F3) mounted at, 2, and feet above the heliostat s base. I-beam Mounted The trusses and facets on NSTTF heliostats are supported by I- Anemometers beams. Four Open more anemometers (H1-H4) were mounted above the vertical Anemometers I-beam at inch intervals H1 (Fig. 2). These were H2 used to detect the boundary F1 layer above H3 the surface of the mirrors and to help validate F2 the CFD models. H4 F3 Wind data was time averaged using a sliding window algorithm. Basic statistical analysis was performed using SAS software to determine and compare the average wind speeds overall and by anemometer locations. Statistical t-testing helped to determine whether any of the sensors used were statistically identical and therefore could be reassigned to other locations Summary in STOW Position Error! Reference source not found. summarizes the data taken while the heliostat was in STOW position. From left to right, the box plots are presented from the highest to lowest anemometer with the first three comprising the group, and the last four comprising the stat group. Box plot of Speed distributions by anemometer location Min Mean Max Pooled Std Dev Min Mean Max F1 F2 F3 H1 H2 H3 H4 anem. location Fig. 2: Anemometers mounted on a tripod in the flat open field (F1, F2, F3) and I-beam mounted anemometers (H1, H2, H3, H4). Fig. 3: Box plot summary of wind data from study in STOW position. While the speed of wind near the ground is related to height, the magnitude of the speed to height relationship seen in 2

3 wind elevation (deg) speed (mph) Error! Reference source not found. exceeds that predicted by the power law. The power law posits the ratio of speeds equals the ratio of heights raised to an environmentally determined power. F1 and H1 are approximately the same height while H4 and F3 are at approximately the same height. Bonferroni t-test results show that H4 and F3 and H4 and F2 are not statistically different in this particular data set Summary in 4 Elevation Postition Fig. 4 and Fig. summarize the changes in wind velocity from the group of field anemometers to the group of heliostat anemometers. There is a % average reduction in speed from the group to the stat group and a 16 degree increase in elevation. Box plot of wind speed distributions by anemometer groups (4deg) Min Mean Max Pooled Std Dev Min Mean.998 Max MODEL VALIDATION 4.1. Model Configurations CFD modeling was performed using SolidWorks 11 Premium Flow Simulation 4.0 on a 24 CPU 98 GB computer. Solid models of the heliostats were configured and oriented relative to the initial flow conditions to resemble the configurations used in the empirical study. Two heliostat positions, STOW and 4 were modeled. In addition to the values determined from the means of the reduced timeaveraged measurements, a turbulence intensity value for open country of 14% and a roughness constant of 0.14 were specified. The initial mesh scheme used moderate resolution throughout the bulk volume of the computational domain which was many times larger than a heliostat to prevent any boundary interference. A localized mesh was produced around the volume surrounding facets with substantially higher resolution of fluid cells. There was additional refinement of the partial (part solid, part fluid) cells on all surfaces (Fig. 6). 2 1 Fig. 4: Group average speed of -group anemometers and stat-group anemometers. Box plot of wind elevation distributions by anemometer groups (4deg) Min Mean Max Pooled Std Dev Min Mean Max Fig. 6: Cut plot of mesh used on 4 configuration with 1.8 million cells. stat is rotated in azimuth from plane of cut plot Fig. : Group average wind direction (elevation) of group anemometers and stat-group anemometers Boundary Layer Test Model A CFD analysis was run prior to mounting the anemometers to help predict the height above the facets at which the wind closely resembles the boundary conditions. According to Error! Reference source not found., wind velocities measured under inches from the surface of the facets appear to have the characteristically higher turbulence and slower velocities associated with boundary layers. Anemometer H4 is located within the region marked by the red colored mounting rod in Fig. 7 and has an average velocity of 17 mph. Flow 3

4 inches above the surface of the facets is expected to closely resemble the approaching wind conditions. corner. Speeds at the position of anemometers are indicated on mounting rod. Box plot of wind speed distributions by anemometer groups Min Mean Max Pooled Std Dev Min Mean Max Fig. 7: Close-up of boundary layer streamlines seen parallel to plane of facets and flowing left to right. The red section of the mounting rod approximates the vertical extent of boundary layer at inches above center of elevation tube Model Validation in STOW with mph Winds The model results shown in Fig. 8 predict that wind velocities measured above the facets of a heliostat in STOW position should be within 1 mph of the average boundary wind velocities and have highly uniform values in the vicinity of the anemometers mounted on the I-beam. The empirical difference between the velocity group means measured at each anemometer is approximately 1 mph (Fig. 9). Furthermore, the 2 mph standard deviation of the velocities near the I-beammounted anemometers was relatively small, indicating highly uniform simulated wind velocities above the stowed heliostat. Fig. 9: Overall box plot summaries of measured speed from group and stat group anemometers in STOW position. Table 1 summarizes the differences between the modeled and actual values of the I-beam mounted anemometers. Results show that the measured velocities at lower elevations (near the facets) were less than the simulated results, indicating that the mesh may not have been sufficiently resolved in the location near the facets to capture boundary layer effects. The relative errors were approximately % or less. Table 1: Actual wind speed vs. modeled wind speeds. Actual speeds are based on Error! Reference source not found.. The modeled wind speed is based on the modeled value at the coordinates of the anemometer in Fig. 8 plus or minus the variation in the vicinity of the coordinate. STOW Position with Boundary Wind Speed mph Height Measured Modeled Relative Error (in) (mph) (mph) % ± ± ± ± Model Validation at 4 Elevation with mph Winds. Fig. 8: Velocity contours and streamlines over heliostat in STOW position. stat is oriented 22 in azimuth from plane of flow. Flow direction is from left to right. Boundary speed of mph is indicated at upper-left Fig. summarizes the empirical data from the 4 experiment, and Fig. 11 shows the modeled wind flow with predicted wind speed at the designated heights above the I-beam. The boundary velocity that was entered into the model was 21 mph. 4

5 In this model, values in the vicinity of the actual anemometers closely matched the average empirical data. Box plot of Speed distributions by anemometer location (4deg) Min Mean Max Pooled Std Dev Min Mean Max F1 F2 F3 H1 H2 H3 H4 anem. location Fig. : Average wind speed at anemometer locations during the 4 elevation test FUTURE TESTING The two experiments summarized in this paper are part of a series of planned tests to be conducted on the NSTTF heliostat field. The first tests, which characterized the wind velocity in the vicinity of a single-heliostat, will continue in tandem with row-to-row and full-field heliostat wind characterization tests. Row-to-row tests will offer insight into the gradient of wind intensity as it progresses row to row. Anemometers will be mounted on portable tripods in addition to I-beams so that similar measurements can be taken on heliostats in different positions throughout the field and compared to modeled predictions. Full-field measurements will be conducted in order to detect any general patterns of wind flow as it propagates from one side of the field to the other. 6. ACKNOWLEDGMENTS Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy s National Nuclear Security Administration under contract DE-AC04-94AL8000. The United States Government retains, and by accepting the article for publication, the publisher acknowledges that the United States Government retains, a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for United States Government purposes. Fig. 11: Contour plot of wind velocity with flow trajectories over a heliostat in 4 elevation. Flow direction is left to right. stat is rotated in azimuth from flow direction. Table 2: Actual wind speed vs. modeled wind speeds. 4 Position with Boundary Wind Speed mph Height Measured Modeled Relative Error (in) (mph) (mph) % ± ± ± ± REFERENCES (1) J.A. Peterka, R. G. (1992). Wind Load Design Methods for Ground Based stats and Parabolic Dish Collectors. Fort Collins: Colorado State University (2) J.M. Chen, F. Y.-C. (1996). Strouhal Numbers of Inclined Flat Plates. Journal of Wind Engineering & Industrial Aerodynamics, 61, (3) Stahl, W. W. (198). Some Aspects of the Flow Past a Square Flat Plate at High Incidence. Zeitschrift fur Flugwissenschaften und Weltraumforschung, 9, (4) Steve Kusek, J. B. (Oct. 11). Low Cost stat Development. HiTek Services Inc, Monthly Telecon () Yashuharu Nakamura, Y. O. (1991). Experiments on vortex shedding from flat plates with square leading and trailing edges. Journal of Fluid Mechanics, 222, (6) Ying-ge Wang, Z.-n. L.-s. (09). Wind Pressure and Wind-Induced Vibration of stat , pp