Pinpoint accurate adaptive structures for heliostats made from high performance concrete for central tower power plants

• Applicants: Professor Dr.-Ing. Peter Mark; Professor Dr.-Ing. Jürgen Schnell
• Subject Area: Structural Engineering, Building Informatics and Construction Operation
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 389020360

In the second funding period of SPP 1542, the applicants recently work on "Light deformation-optimised shell structures made from micro-reinforced UHPC for parabolic trough power plants". The investigations revealed high-performance concrete being promising for utmost precise and stable structural members. So far, specific action effects, determined by wind tunnel testing and a holistic accuracy criterion for solar collectors were derived. Additionally modular FE-models were set-up, that respect an effective stiffness of complex structures e.g. by means of local void formers placed according to experimental results. These are complemented by multi-level optimisation with a priori variance-based sensitivity analysis. Finally a specific concrete has been composed matching all demands of persistent shape and tensile strength to realize a prototype (3,2 x 2,2 m). Now, these insights and strong interest from the solar industry encourage a transfer to similar fields of application namely heliostats, envisaged to be manufactured in serial production and industrial precision. The objectives are to further increase the accuracy, to establish adaptive collector shapes consequently and to augment the entire production chain. Conventional reinforcement shall be reduced or even avoided by pre-stressing. That way cost-efficient production for a wide range of applications can be achieved. Among these, heliostats are solar collectors able to biaxially track the sun and to focus solar irradiation on a central receiver at the solar tower. Regarding reflection properties up to tenfold precision has to be achieved compared to parabolic troughs. However, adaptive manufacturing is indispensable, since the heliostats geometry, more precisely its paraboloidal surface, widens with the distance to the focal point. Envisaged are circular shells with outer bracings composed from repeated segments of similar shape. In order to realise a fully equipped and geometrically verified full-scale prototype in cooperation with industrial partners, theoretical and production-based prerequisites must be met before. Key is a multi-scale optimisation-based structural design that is sustainable, highly accurate and robust against uncertainties arising from geometry and material. Governing geometric variations are identified by means of few control parameters and enable to design an adaptive and cost-effective precision formwork allowing for finishing inner surfaces subsequently. The prototype will be circumferentially pre-stressed and possesses reflectors as well as mature links to the substructure equipped with measuring devices. Expected is a shell surface with an effective thickness of 2-3 cm (average of shell and bracings) that is highly accurate up to the submillimetre range.

• DFG Programme: Research Grants (Transfer Project)
• Application Partner: Almeco GmbH; Innogration GmbH; Deutsches Zentrum für Luft- und Raumfahrt (DLR); Institut für Solarforschung; durcrete GmbH
• Co-Investigator: Professor Dr.-Ing. Christian Glock