Operation in Wind
Deflection of the beam on the target due to wind gusts was initially anticipated to be major concern. Tests were conducted during windy/gusty conditions up to 20 mph. Video recordings of the movement of the beam on the target during windy conditions were made. The video that accompanies this report shows one such test. The maximum movement of the beam on the target for winds and wind gusts up to 20 mph was approximately plus/minus 1.5 inches (approximately plus/minus0.12 deg) in the vertical direction, and mush less in the horizontal direction. While this maximum movement is relatively small, the average displacement of the beam from its nominal position is much less. The accompanying video illustrates that there are occasional maximum movements, but the time averaged displacement is much less.
There is more movement of the beam in the vertical direction than the horizontal direction because the heliostat mounting is less stiff perpendicular to the polar axis than parallel to the polar axis (see Force/Deflection curve below). Although the current design probably provides adequate stiffness, changes could be made in the production design to provide additional stiffness perpendicular to the polar axis to reduce vertical deflections.
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noted that: The wind velocity was measured at a height of 5 ft above ground level. If this were corrected to the standard wind measurement height of 33 ft, the wind velocities would be increased by a factor of approximately 1.46. This means that the 20 mph maximum wind speeds that were measured in our tests at a height of 5 ft are roughly equivalent to 29 mph at the standard wind reporting height of 33 ft. The wind measurement instrument (a Kestrel 1000) reports 3 second averages of wind velocity – so high wind gusts of shorter duration get averaged down to lower values.
As can be seen in the video, the focused beam shape on the target was changed very little as wind gusts occurred.
In addition to the beam on target tests described above, tests were conducted in which a laser beam was mounted approximately 5 ft in front of the heliostat. The laser beam was reflected off the heliostat mirror, and onto a target located 55 ft in front of the heliostat. Defections of the heliostat resulted in changes in the laser spot on the target in much the same way as the sun beam was deflected in the tests above. The result of this testing confirmed the results described above.
In summary, the prototype exhibited the ability to keep the beam well focused on the target while operating in wind velocities up to 20 mph.
The heliostat has been exposed to and operated in an outdoor environment since August of 2002. The test area is just North of Bozeman, MT. Bozeman experiences a wide variety of weather conditions including a wide range of temperatures, wide daily temperature swings, strong winds, precipitation in the form of rain, snow and hail, and long periods of direct sun exposure at 5000 ft elevation. Our plans call for continuing the weather exposure through the winter of 2002-2003.
November 24, 2002