Renewable energy, particularly solar energy is at the forefront of the fight against fossil fuels. Concentrated Solar Power plants utilizing heliostats, large reflecting mirrors, to concentrate the sun’s solar energy onto a central tower are one are the main solar technologies in use today. These plants consist of hundreds to hundreds of thousands of heliostats. The heliostats in most cases make up the largest portion of initial capital expenditure of a solar plant. Consequently, the design of these heliostats is an important area of research to enable Concentrated Solar Power to be a viable competitor to not only fossil fuels but also photovoltaic solar technologies.
Vortex shedding and the resultant transient loadings on a medium sized heliostat are investigated in this paper. Reynolds-Averaged Navier-Stokes (RANS) Computational Fluid Dynamics (CFD) of an operational heliostat is used as a validation case. The Atmospheric Boundary Layer (ABL) is characterized and optimized. The ABL is implemented as the inlet flow boundary condition for both the RANS and Stress-Blended Eddy Simulation (SBES) simulations. The SBES Scale-Resolving Simulation (SRS) model was used which provided the transient peak wind loadings necessary to investigate the structural response of the heliostat. The synthetic turbulence technique used at the inlet of the SBES simulation was the Vortex Method. This method appears to produce unphysical pressure spikes in the flow but their effect appears to be negligible. The SBES results show a strong likeness to the experimental results of Peterka (1986) with a comparable mean and peak loading distribution. The SBES results couldn’t be accurately compared to the experimental results of Huss et al (2011) due to the uncertainty of the turbulence intensity in the experimental values. The transient SBES CFD pressure was implemented in a one-way FSI simulation. These simulations shed light on the structural response of the heliostat to the transient wind loading. The results showed that the response of the heliostat conformed to and depended on the mode shapes and frequencies of the heliostat structure more so than the vortex shedding frequencies. The results from the transient structural analysis using the temporal SBES heliostat surface pressure fields as input indicate that the method holds promise in predicting the transient response of heliostats. Importantly it can be concluded that due to the difference in frequencies between the vortex shedding and modal frequencies, the structure is safe from self-excitation.
Dissertation (MEng)--University of Pretoria, 2018.