MDBA: An accurate and efficient method for aiming heliostats

In a solar power tower plant, the role of the heliostat aiming strategy is to control the radiative flux distribution at the receiver surface to avoid thermally induced damage, while minimising spillage losses and maximising the receiver thermal efficiency. Flux limitations arise from factors including the heat transfer fluid stability limits, and thermo-mechanical stress limits in receiver pipes. Maximised flux, as close as possible to local flux limits, facilitates an overall smaller receiver with lower thermal losses. Methods exist to sequentially optimise aiming points of single heliostats, using fast convolution-based optical simulations to evaluate individual flux maps. However, to accurately determine receiver flux distributions, ray-tracing is preferred. Ray-tracing is computationally expensive and determination of the aim-points for every heliostat independently potentially leads to impractical simulation times. In this study, a new parameterisation of heliostat aim-point locations is introduced that significantly simplifies the aiming problem. The new aiming model, named Modified Deviation-Based Aiming (MDBA), enables efficient use of ray-tracing to optimise the aiming strategy and, together with receiver thermal and mechanical models, is able to closely match the flux distribution to local values of allowable flux on the receiver. This new parameterisation enables accurate aiming strategy interpolation, used to dynamically predict full field aim-points at different sun positions and values of direct normal irradiance (DNI). A reference case with a surround field and a cylindrical external receiver compatible with the Gen3 Liquid Pathway project is presented to test the capability of the method developed in this study.