Experimental investigation of peak wind loads on tandem operating heliostats within an atmospheric boundary layer

Abstract

During the operation of a concentrating solar thermal (CST) power tower plant, heliostat mirrors inclined at different angles act as bluff bodies that are exposed to large drag loads from the wind. This experimental study investigates the aerodynamic loads on a heliostat in a tandem configuration, to determine the significance of the shielding effect from an upstream heliostat. To understand the effect of turbulence on the peak wind loads, scale-model heliostats with square facets were positioned within a part-depth atmospheric boundary layer (ABL) with a Power Law velocity profile. Peak drag coefficients on the instrumented downstream heliostat in the tandem configuration were normalized with respect to those on a single (isolated) heliostat. A range of tandem configurations were tested to determine the effects of elevation angle, azimuth angle, and gap spacing between the tandem heliostats. Findings show that peak drag loads are reduced by up to 60% on the downstream heliostat relative to an isolated heliostat at an elevation angle of 90° and a gap spacing of two chord lengths, but at higher gap spacing the shielding effect is either marginal or non-existent. Peak hinge moment coefficients on a downstream heliostat in tandem are up to seven times the load on an isolated heliostat, with the maximum occurring at 90° elevation and 180° azimuth. Base-overturning moment coefficients are less affected, as the changes in the centre of pressure location are relatively small compared to the length of the support pylon. Strouhal number analysis of the fluctuating surface pressures indicated that the dominant frequency of the pressure spectra on the downstream heliostat is over three times the value on an isolated heliostat at 45° elevation and azimuth angles. Hence, both static and dynamic effects must be considered separately in the wind load design for heliostats at typical operating angles.