Strong efforts are being made to drive heliostat cost down. These efforts are summarised to give an update on heliostat technology comprising: determination of wind loads, heliostat dimensioning, solutions for the different sub-functions of a heliostat, a review of commercially available and prototype heliostat designs, canting, manufacturing, qualification, heliostat field layout, and mirror cleaning. There is evidence that commercial heliostat costs have dropped significantly in the past few years, with commercial suppliers of heliostat technologies now claiming heliostat field costs around 100 USD/m 2. With new approaches even target cost of 75$/m² seem to be realistic.
For the layout of solar trackers the wind loads on the structure have to be known. They can be calculated by using wind load coefficients given in literature. But so far these values are only valid for aspect ratios of the panel (width to height) of about 1.0. Therefore the wind load coefficients for heliostats of aspect ratios between 0.5 and 3.0 were determined to close this gap.As solar trackers are exposed to the turbulent atmospheric boundary layer the turbulence of the approaching flow has to be modelled. As a reliable method at reasonable cost wind tunnel measurements were chosen. Solar trackers of 30m² panel size were investigated at a model scale of 1:20. Wind direction and elevation angle of the panel were varied to investigate especially the constellations at which the highest wind loads are expected (critical load cases). By spires and roughness elements a wind profile and a turbulence intensity of the modelled wind according to typical sites for solar trackers were achieved. The loads were measured by a high frequency force balance placed underneath the models. Additionally measurements of the pressure distribution on a panel with aspect ratio of 1.2 were performed to better understand the effects that lead to the peak values of the wind load coefficients.A significant impact of the aspect ratio was measured. For the critical load cases the aspect ratio dependencies of the accordant peak wind load components were
The basic mathematics and structure of heliostat have remained unchanged for many decades. Following the challenge first made by Ries et al., the non-imaging focusing heliostat recently proposed by Chen et al. provides an alternative in the field of concentrated solar energy. This paper investigates the performance of a heliostat field composed of the newly proposed heliostats. In contrast to the dynamic curvature adjustment proposed in our previous work for a solar furnace, a fixed asymmetric curvature is used here with the spinning-elevation tracking method. This restriction is intended to equalize the manufacture cost of the new heliostat with that of traditional heliostats with azimuthelevation tracking and spherical curvature. Fixing the curvature results in only partial aberration correction, compared to full correction using the dynamic adjustment of curvature. Nevertheless, the case studies presented in this paper show that the new heliostat design can reduce the receiver spillage loss by 10-30%, and provide a much more uniform performance without large variations with time of day. Fig. 1 Spot size comparison between the spinning-elevation and azimuth-elevation tracking methods for June 21st. The target angle is 41.8 deg, facing angle is 10 deg to the south and the latitude is North 43 deg. Heliostat area is 25 m 2 and the slant range is 30 m.
A survey of hitherto concepts for cost reduction of heliostats is given. The survey might serve as a base for the development of low cost heliostats that are needed to meet the current challenging cost objectives. The concepts are related to the main heliostat subfunctions and to basic approaches for cost reduction found so far. Based on the main advantages and drawbacks of every concept, the most promising ones are indicated.
Several approaches for cost reduction of heliostats, which fit well to each other, are combined to a new heliostat concept to achieve the current challenging cost objectives: The wind loads are reduced by appropriate manipulators which reduces weight and cost of the heliostat structure and the ground anchor foundation. Laminated mirror facets are of high reflectivity and shape accuracy and of low weight. The low weight is advantageous for the dimensioning of the bearings and regarding energy consumption. Energy consumption is further reduced by a highly efficient drive train. Thus, small capacity of the wireless energy supply of the autonomous heliostat is sufficient which reduces significantly its cost. By the combination of horizontal primary axis with rims and winch wheels a cheap and precise solution for the drives was found. Ray tracing calculations show that the losses due to the compromised angle range are negligible. With the new heliostat concept the current cost goals seem to be achievable.
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