Wind gust distribution analysis and potential effects on heliostat service life

Intense gust conditions associated with storms are shown to occur in the range of heliostat natural frequencies and can thus induce high dynamic coupling loads. Wind data taken during a severe storm on an instrumented tower at Savannah River National Laboratory shows that conservative wind speed differences of at least ±4.5 m/s (±10 mph) relative to the average speeds occurred a substantial fraction of the time over periods of the order of 1 s. Although these results were determined for a specific site and a particular storm, they are representative of storm conditions, which are governed primarily by buoyancy effects, as opposed to essentially boundary layer shear effects associated with time-averaged winds speeds that dominate meteorological site data sets. Since heliostats typically have relatively low damping ratios and natural frequencies of the order of 1 Hz, resonant dynamic coupling could occur with significantly higher loads than those predicted from design requirements for steady state winds. This effect reduces service life and impacts reliability through both possible near-instantaneous failures for excessively high dynamically coupled load and the additional high-load cycles that increase cumulative fatigue damage, even if relatively few in number, given the characteristics of fatigue life and loads. Therefore, the effect of these gust-induced cyclic dynamic effects on fatigue life and survival deserve consideration as part of heliostat design and operation. In particular, increased damping of heliostats to mitigate dynamic coupling deserves consideration.