SHORT is a patented (PCT/ES2016/070681 – WO 2017/055663) automatic heliostat calibration system based on lowcost artificial vision systems mounted on each heliostat which is used to observe a number of targets with known positions distributed throughout the solar field. During the calibration, the heliostat is moved to sequentially capture such targets with the camera. Once sufficient observations are made, the parameters of a detailed kinematic model of the heliostat are adjusted through an iterative optimization process. This model allows predicting the precise position and orientation of the heliostat for any given axis’s angles and, by means of inverse kinematic, calculating the angles required to move the heliostat into a specific desired orientation. SHORT has been thoroughly validated at the Plataforma Solar de Almeria with a heliostat equipped with a low-cost camera observing artificial infrared light sources – targets – distributed throughout the solar field with accurately measured positions. The experimental error in movement prediction resulted below 0.25 mrad (RMS). To successfully implement SHORT in commercial Solar Towers and face the challenge of applying the system in large heliostat fields (thousands of heliostats) a dedicated methodology to find a minimum set of suitable infrared lights distribution has been developed. Results show that no more than 6-8 targets surrounding the field are enough to reach the aimed accuracy. In addition, a techno-economic analysis of Solar Tower plants according to the solar field optical error has been performed to estimate the impact of SHORT implementation in LCOE reduction with a promising result of 5% cost savings.
Heliostat’s reflected beam quality has been estimated using the target-camera method for years due to being the only method that can be realistically implemented in a commercial plant. But this methodology is prone to errors such as those derived from target surface flaws and its limited size, and the sensitivity and dynamic range of the camera. To reach a high quality characterization, thus reducing required security margins and boosting plant profitability, a novel system and methodology have been developed. This is a scanner-based methodology in which the spot reflected by a static heliostat, no matter how far it is from the measurement system, is captured simultaneously by two subsystems, a vertical array of detectors and a group of cameras, in order to produce a high quality representation of the reflected beam and a precise characterization of the normal vectors along the whole heliostat reflective surface. The use of optoelectronic detectors allows capturing the solar beam with reduced optical and electronical noise and wider dynamic range with respect to the state-of-the-art methodology. At once, the camera subsystem is used as a scanner to perform an accurate normal vector estimation of the heliostat surface. The combination of both approaches lead to the most precise heliostat characterization to date. This system can be implemented at low cost in any commercial plant, planned, under construction or under exploitation with any size of heliostat field and any number and typology of heliostats.
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