Novel imaging closed loop control strategy for heliostats

Abstract

Abstract. Central Receiver Systems use up to thousands of heliostats to concentrate solar radiation. The precise control of heliostat aiming points is crucial not only for efficiency but also for reliable plant operation. Besides the calibration of open loop control systems, closed loop tracking strategies are developed to address a precise and efficient aiming strategy. The need for cost reductions in the heliostat field intensifies the motivation for economic closed loop control systems. This work introduces an approach for a closed loop heliostat tracking strategy using image analysis and signal modulation. The approach aims at the extraction of heliostat focal spot position within the receiver domain by means of a centralized remote vision system decoupled from the rough conditions close to the focal area. Taking an image sequence of the receiver while modulating a signal on different heliostats, their aiming points are retrieved. The work describes the methodology and shows first results from simulations and practical tests performed in small scale, motivating further investigation and deployment.

A concise overview of heliostat fields-solar thermal collectors: Current state of art and future perspective

“…Other techniques recommended by Kribus et al and Coquand et al involve using backward gazing cameras positioned on the top of the tower. Bern et al analyzed a time series of receiver images to determine the contribution of a heliostat under oscillation. An alternative to previously mentioned calibration methods is to use sensors mounted on the heliostat in order to determine its orientation.…”

Progress in heliostat development

“…18. An advanced version is proposed by Bern et al (2016) where a well-defined oscillation is imposed on a heliostat and a frequency analysis of a time series of images of the receiver extracts the contribution of the heliostat in question.…”

“…Mechanical vibrations having small amplitudes and high frequencies (kHz) can be introduced into the heliostat reflector surface using piezoelectric actuators (Convery, 2011). Alternatively, the whole concentrator can be excited with higher amplitudes and lower frequencies (Hz) using the heliostat drives (Bern et al, 2016). Both concepts should allow identification of the light beams of the excited heliostat in the focus by its specific frequency.…”

A method for calibrating a single facet small heliostat based on extracting invariant moments

“…The proposed model has the ability of detecting movements in the tower with ±0.32 degrees' average error. An approach for tracking heliostat using image analysis and signal modulation is proposed in Bern et al [4]. The proposed approach aimed to extract the focal spot position within the receiver.…”