Fig 3 - uploaded by Minghuan Guo
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Heliostat tracking geometry with respect to the sun, the target plane and the intersection, T 0 , of the reflected central solar ray from the mirror surface centre, M 0 with the target plane.
Source publication
For a heliostat with geometric errors, the reflected central solar ray from the mirror surface center forms a curved error trace on the target plane during the day rather than staying fixed on one target point. A general azimuth-elevation tracking angle formula has been developed for a heliostat with a mirror-pivot offset and other typical geometri...
Citations
... Several investigations have been carried out to study drift effects in CST facilities having heliostats with the most commonly used tracking system based on azimuth-elevation movements [6][7][8]. For instance, simulations only taking into account the reflected central ray from the heliostat have been performed [2,8]. ...
In this work, we experimentally and numerically evaluate the drift errors in a solar tower facility with tilt-roll tracking-based heliostats. For this investigation, the solar tower facility located at IMDEA Energy in Móstoles (Spain) is used as a test case. By acquiring flux maps at different times of the day and representing their maximum irradiance point as a function of the time, the drift is evaluated experimentally. We observed that the experimental drift can be divided in two components, vertical and horizontal, with a constant velocity of the maximum irradiance point of -7.4 cm/h and 2.2 cm/h, respectively. In order to investigate the source of the drift observed in the experiment, detailed Monte Carlo ray-tracing simulations including possible misalignments in the heliostat tracking system were performed. We found out that by considering rotations of the heliostats pedestal, the experimental drift is very well reproduced by the simulations. In particular, for a rotation of the pedestal of 17 mrad towards the east, the velocities of the maximum irradiance point in the simulations are -7.6 cm/h and 1.1 cm/h for the vertical and horizontal drift components, respectively.
... For this study, a Monte-Carlo ray-tracing software, SolTrace (Wendelin, 2003), was used, instead of the commonly used central-ray approximation, in which only the ray coming from the center of the heliostat facet is considered to calculate the drift curves. Guo et al. derived a formula for azimuth-elevation tracking systems (Guo et al., 2011;Guo et al., 2013a), then using it to study the drift also for different errors of a heliostat located in Beijing (Guo et al., 2013b). In 2014, Escobar-Toledo et al. analyzed the same three errors treated by Stone et al. (1999), but for the solar field located in Hermosillo, Mexico. ...
... For all the cases, flux maps are simulated, and the drift curves are determined using their centers of gravity. This approach provides a more detailed description of drift behavior with respect to reported studies where only the central ray was used (Guo et al., 2013b;Escobar-Toledo et al., 2014;Lara-Cerecedo et al., 2016). In fact, it leads to observe an intrinsic drift that appears even when there are no misalignments in the tracking system of the heliostat, which is not detected with the approximation of the central ray. ...
Drift is a relevant issue in concentrated solar tower facilities since this time-dependent pointing error severely affects their performance. This work deals with the optical analysis of drift for tilt-roll heliostats in terms of the geometrical parameters associated to the heliostat mechanical structure and the local time. For each drift source or error, flux maps have been obtained by means of Monte Carlo ray-tracing calculations as a function of the source magnitude and daily time in winter and summer solstices. Then the corresponding pointing errors and the drift curves are determined from the displacement of the center of gravity of the flux maps. As a result, drift related to each source is identified and characterized. Additionally, drift sources are compared in terms of the standard deviation and the mean of their daily pointing errors. The analysis evidences that main drift sources are the misalignments of the pedestal like the one of an inclination with respect to the zenith.
... Numerical results are also shown, illustrating daily drift trajectories for a fixed value of the deviation angle. In [7], Guo et al introduces a simulation model of sun-beam traces over the target plane for an azimuth-elevation tracking heliostat, also considering fixed geometric error sources. Other models as [6] introduce Montecarlo distributions to predict random deviation error values over the course of the day. ...
Heliostat alignment evaluation is among the main issues in solar tower concentration plant operation and maintenance. This paper describes a novel method used to evaluate heliostat misalignment and its experimental verification. This method provides a different way of visualizing beam centroid pointing errors by generating the complete deviation curve for each axis. This, for example, would be useful for verifying a heliostat’s correct alignment by using a measurement performed out of the receiver target, using these traces to calculate its reflection’s expected location, given a known misalignment. This measurement could be performed during operation simply by including a reflective element in the heliostat and two detector arrays on the tower’s surface. This model has been tested for various types of misalignments of a heliostat at different hours, dates, and heliostat locations. The simulation results have been validated by using an experimental setup at a scale of 1:100.
The dish concentrator is an indispensable optical device for solar dish/stirling power system, which requires accurate tracking sun’s position. However, both the foundation settlement caused by long-term service and installation error can cause the support column tilt, thus destroying the vertical relationship between the column axis (i.e. azimuth axis) and ground plane resulting tracking errors. In this paper, a tracking error model of dish concentrator with elevation–azimuth dual-axis tracking considering column tilt is developed, the radial error angle θ1 and circumferential error angle φ1 are used to describe column tilt angle and tilt direction. The concentrator tracking error and focused spot position under column tilt error are analyzed using several dimensional evaluation indexes for the whole year, every day and every moment, and are verified by a novel virtual experiment of the concentrator tracking and optical simulation. When θ1 is constant, the tracking performance is worst throughout the year at φ1 of 90° and 270°, while the influence of column tilt can be effectively reduced at φ1 of 0° or 180°. The maximum daily tracking error can reach 1.91∼3.49 mrad in whole year at θ1=0.1° and φ1 in 0°∼360°, so θ1 should be controlled within 0.1°.
