It should be noted that some of the materials and equipment used in the prototype were chosen to keep the cost and construction time for the prototype within the scope of the project. The production design will substitute materials and equipment with a long service life in mind.
Temperature Range Exposure: To date, the heliostat has been exposed to temperatures ranging from 0 F to 90 F, with daily
variations up to 50 deg F. to date.
No temperature related failures or deterioration have been observed
Winds Exposure: Demonstrating the capability of the heliostat design to survive in high winds is a major goal of the project. One of the primary reasons for choosing the spherical shape for the heliostat is its low drag coefficient, and its lack of sensitivity to wind direction and heliostat orientation.
To date, the heliostat has been exposed to winds up to approximately 30 mph with no failures. Its behavior to date can be characterized as follows:
There is no observed tendency for inflated (or non-inflated) surfaces to flutter.
Deflections due to wind are small, and well controlled.
Small oscillations in gusty winds have been observed – these are low in magnitude and well damped.
The behavior of the heliostat is consistent regardless of wind direction and heliostat orientation (as would be expected for a spherical shape).
While our plan calls for direct observation the behavior of the heliostat in winds well in excess of 30 mph, a rough estimate of its performance in higher winds has been attempted in the interim:
A force-deflection curve has been determined by test. This provides the relationship
between a force applied horizontally to horizontal deflection of the mirror ring.
the mirror support ring and the resulting This relationship was established for forces
acting along the polar axis, and perpendicular to the polar Figure 9d. The horizontal force is believed to be roughly by a wind from the same direction.
axis. The results equivalent to the
are shown in force applied
The equivalent horizontal deflection for a wind speed of 18 mph was also measured, and found to be 0.25 cm. Using 1) the measured deflection for this wind speed, 2) the force deflection relationship discussed above, and 3) assuming that the wind dynamic pressure and wind force on the heliostat increase with the square of the wind velocity, the following rough estimate of severe wind survival performance of the prototype heliostat is provided:
The approximate wind force on the heliostat at a wind speed of 18 mph is 7 lb. (from the 0.25cm deflection at 18 mph and the force-deflection curve).
The approximate force on the heliostat for a design wind speed of 60 mph is 77 lb (from the dynamic pressure relationship, and estimated wind force at 18 mph)
Forces of 60 to 80 lb were applied to the heliostat in the building of the Force/Deflection curve – this would imply a wind survival speed of at least 50 to
November 24, 2002