Session: 18-02 HelioCon Windload

Paper Number: 142158

142158 - Wind Tunnel Study on the Effects of Ground Clearance Ratio on Heliostat Dynamic Wind Loads 

Abstract: 

The fluctuation in surface pressures and loads on heliostats arising due to vortex shedding impact their optical performance and potential structural failure resulting from cyclic loading. The forces generated by vortex shedding and self-excited oscillation of the structure by vortex-induced vibration influences the structural design of the heliostat and operational performance of a concentrating solar field. Wind tunnel studies relating to dynamic wind loads on solar panels have identified the dependence of Strouhal number of a flat plate on elevation angle, Reynolds number and aspect ratio based on wake flow measurements. The effect of ground clearance ratio has been shown to influence the drag forces and Strouhal number of upright panels. This study investigates the relationship of dominant frequencies of fluctuating loads and the relationships between wind and load spectra at different elevation angles and ground clearance ratios in uniform and turbulent flows in the University of Adelaide wind tunnel.

Surface pressures on a single rectangular heliostat model with 0.53 m width and 0.33 m height were measured using 24 board-mount differential pressure sensors with a range of ±248.8 Pa and an accuracy of ±0.2%. Base force balance load measurements were analyzed using a JR3 six-axis load cell with ±100 N capacity in horizontal and ±200 N in vertical directions and an accuracy of ±0.25%. Surface pressures and base force and moments were sampled at 1000Hz for a duration of 120 seconds. To examine the impact of ground clearance ratio, an adjustable pylon height from 200 mm (GR=0.106) to 325 mm (GR=0.485) was tested. The elevation angles of the heliostat model were varied from α=15° to 90°, with intervals of 15° at H=230 mm (GR=0.196). The heliostat was positioned in (1) a uniform flow with a longitudinal turbulence intensity of 2% and freestream velocity of 11.1 m/s with a Reynold number of 2.36 × 10⁵ based on the heliostat chord length (height), and (2) an atmospheric boundary layer (ABL) flow with 11.1 m/s freestream velocity and 14% longitudinal turbulence intensity at the hinge height, H=230mm, of the heliostat model.

PSD analysis of load spectra from load cell measurements showed that the Strouhal number corresponding to the peak fluctuation frequency of the drag force on a vertical heliostat model (α=90°) in uniform flow decreases marginally from 0.13 to 0.12 as GR increases from 0.1 to 0.26 and remains constant at GR>0.26 in agreement with Mammar et al. (2018). In contrast in turbulent flow, the Strouhal number decreases significantly from 0.17 to 0.1 with increasing GR from 0.1 to 0.5. Wake velocity data in uniform flow showed that the Strouhal number decreases logarithmically with increasing elevation angle from 30° to 90°. There was some discrepancy observed between the peak frequencies of load spectra and wake flow measurements due to the influence of the pylon and structural components of the heliostat model. Nevertheless, the results indicate that the frequencies corresponding to dynamic loads are more sensitive to turbulent flows in the ABL compared to a uniform flow and vary depending on the ground clearance ratio of a heliostat.

Presenting Author: Matthew Emes The University of Adelaide

Presenting Author Biography: Dr Matthew Emes is a Postdoctoral Research Fellow in the School of Electrical and Mechanical Engineering at the University of Adelaide. His research interests are in experimental fluid mechanics, aerodynamics and wind engineering of renewable technologies. He has expertise in wind tunnel experiments and field measurements of atmospheric boundary layer turbulence parameters and their influence on wind load design of single heliostats and heliostat arrays. He leads the wind load subtopic within Task 7 Field Deployment in HelioCon working on measurement, characterization and prediction of wind loads on heliostats to develop wind site characterization and heliostat wind load design guidelines with the aim to reduce cost and increase performance of heliostat fields.

Authors:

Sahar Bakhshipour The University of Adelaide
Matthew Emes The University of Adelaide
Azadeh Jafari The University of Adelaide
Maziar Arjomandi The University of Adelaide