Experimental soiling assessment, characterization and modelling of a highly-compact heliostat field in an urban environment

•Soiling monitoring of a heliostat field located in an urban environment.•Analysis and assessment of main local phenomena affecting solar performance.•Statistical characterization of the intra-heliostat and spatial soiling effect.•Modelling heliostat intra-surface dynamics and averaged particle deposition effect. The soiling effect is presently assessed on heliostats of the very-high concentration solar tower facility located in Móstoles on the outskirts of Madrid, Spain, covering the lack of studies concerning Concentrated Solar Power, but also of key importance for solar-to-heat industrial applications, namely in urban areas. Specular reflectance loss was monitored with a Condor reflectometer on fifteen heliostats spaced throughout the solar field. The objectives were to evaluate the soiling effect in an urban environment during different seasons, including the measurement methodology, soiling ratio characterization, tilt angle to reduce the soiling effect and assessment of soiling intra-heliostat dynamics. Soiling rates ranging from 0.13%/day to 0.58%/day were found for this location, associated with wet and dry seasons, respectively. Special focus is given to assess long-range transports of Saharan desert dust, with soiling ratio losses of up-to 30%, and spring period due to higher atmospheric pollen concentration. Lastly, a detailed evaluation of the intra-heliostat soiling effect was performed leading to the development of soiling models to reduce reflectance loss assessment time by binding localized intra-heliostat measurements and the average soiling ratio between different heliostats with distinct tilt angles.