OPTICAL PERFORMANCE AND WEIGHT ESTIMATION OF A HELIOSTAT WITH GANGED FACETS

Abstract

In standard heliostat design the usual strategy of cost reduction is to increase the mirror area of the heliostats. This leads to a reduction of specific drive cost, but also to an increase of the torques caused by the wind loads, resulting in higher specific weight and higher specific drive power. The opposite strategy focuses on the reduction of the specific weight and driving power. Therefore the mirror area is decreased. In total the drive power is much lower then, but it has to be divided into more units which means a drawback for cost reduction. A combination of both strategies is to couple small heliostats so that they can be tracked by the same drive. At the Torque Tube Heliostat (TTH) the mirrors are mounted on torque tubes to simplify the coupling mechanism for the elevation angle. The optimal tracking speed of heliostats depends on the position in the field. In a heliostat with ganged facets all facets are tracked with the same speed. This leads to higher astigmatism losses compared to independently tracked mirrors. To reduce shading a certain space is foreseen between the facets of the TTH. To achieve a high mirror density and to avoid too long and therefore too flexible torque tubes this distance is limited. Thus the shading is higher than usually. Ray tracing calculations were done to obtain optimized heliostat configurations and to estimate the energy yield of a heliostat field built of 288 m² TTHs. The results are compared to a conventional heliostat field. To get an idea of the weight reduction potential of the TTH the dimensions of the torque tube, the secondary axis tube and the mirror support structure were weight optimized via FEM. The layout criterion was a mean mirror error of 5 mrad under a wind load and gravity, which was calculated from the deviations of the FE model.