Heliostat Testing according to SolarPACES Task III Guideline

The SolarPACES Guideline for Heliostat Performace Testing finally provides a solid base for standardized testing and comparison as well as the definitions of essential heliostat parameters such as slope and tracking errors. SBPS is running an extensive test program for their 4 Stellio preseries heliostats at the DLR Solar Tower in Juelich, Germany until summer 2019. Additional objective is to accumulate operating hours and evaluate long-term effects on the Stellio performance quality. Slope error measurement has been performed by CSPS and is repeated every 3 months. First results show 1D slope errors of 0.7 to 1.2 mrad. Tracking performance could not have been concluded due to missing final measurements of the kinematic system of each heliostat which is necessary for calibration. However, beam centroid evaluation software has been tested with first uncalibrated tracking hours and is prepared for normal operation. First photogrammetric measurements have been performed to characterize the dead weight deflection of the heliostat in 15 different azimuth and elevation combinations. This has been prepared and implemented in Rhino CAD. Adaptions may be necessary to include pylon deflection as well

Review of heliostat calibration and tracking control methods

Large scale central receiver systems typically deploy between thousands to more than a hundred thousand heliostats. During solar operation, each heliostat is aligned individually in such a way that the overall surface normal bisects the angle between the sun’s position and the aim point coordinate on the receiver. Due to various tracking error sources, achieving accurate alignment ≤1 mrad for all the heliostats with respect to the aim points on the receiver without a calibration system can be regarded as unrealistic. Therefore, a calibration system is necessary not only to improve the aiming accuracy for achieving desired flux distributions but also to reduce or eliminate spillage. An overview of current larger-scale central receiver systems (CRS), tracking error sources and the basic requirements of an ideal calibration system is presented. Leading up to the main topic, a description of general and specific terms on the topics heliostat calibration and tracking control clarifies the terminology used in this work. Various figures illustrate the signal flows along various typical components as well as the corresponding monitoring or measuring devices that indicate or measure along the signal (or effect) chain. The numerous calibration systems are described in detail and classified in groups. Two tables allow the juxtaposition of the calibration methods for a better comparison. In an assessment, the advantages and disadvantages of individual calibration methods are presented

Combined Measurement of Thermal and Optical Properties of Receivers for Parabolic Trough Collectors

Two new test benches have been developed for the combined measurement of the thermal properties and overall optical efficiencies of receivers for the use in parabolic trough collectors. In the first test bench two receivers are simultaneously irradiated by natural sunlight via parabolic troughs that are mounted on a two axis tracking stage. Shutters enable the variation of the aperture for quasi constant solar input power over the test period. The second, the solar simulator test bench consists of an elliptical cylinder with flat end reflectors. Metal halide lamps (HMI) and the receiver are positioned in the two respective focal lines. The test benches are operated in two modes: Cold performance tests with water flow through the receiver at near ambient temperature in order to measure the optical efficiency in the stationary enthalpy increase, secondly hot testing with the empty receiver at typical operating temperature in order to compare irradiant power and steady state thermal loss power and calculate the efficiency. The test benches are operated in combination with classic heat loss measurement, together the three test benches form a complementary set. First comparative measurements with cold testing on the solar simulator test bench show high reproducibility and a measurement precision relevant for qualification and comparison of different receiver types at industrial and research level

An Automated Model-Based Aim Point Distribution System for Solar Towers

Distribution of heliostat aim points is a major task during central receiver operation, as the flux distribution produced by the heliostats varies continuously with time. Known methods for aim point distribution are mostly based on simple aim point patterns and focus on control strategies to meet local temperature and flux limits of the receiver. Lowering the peak flux on the receiver to avoid hot spots and maximizing thermal output are obviously competing targets that call for a comprehensive optimization process. This paper presents a model-based method for online aim point optimization that includes the current heliostat field mirror quality derived through an automated deflectometric measurement process

Dynamic photogrammetry applied to a real scale heliostat: Insights into the wind-induced behavior and effects on the optical performance

Wind loads may decrease a heliostat’s optical performance by affecting its shape and orientation. As small scale wind tunnel or numerical models cannot reflect a heliostat’s dynamic behavior entirely, in this study we investigated a real scale Stellio heliostat installed on the DLR heliostat testing platform where it was exposed to the natural wind. The wind-induced response was captured with a dynamic photogrammetry system that combines a high spatial and high temporal resolution. As its application to a heliostat is novel, the first focus of this paper is to validate the applied setup and the accuracy of measured displacements. The second focus is to discuss the wind-induced Stellio behavior. A method is presented which allows for separating the total wind-induced behavior into a tracking- and a slope-relevant part. Based on this separation, the wind-induced tracking deviation (0.44mradRMS) during the investigated measurement period (mean wind speed ≈ 4.8m/s, mean turbulence intensity  ≈ 26%) reached a level of approximately one third of a heliostat’s typical total tracking deviation. Likewise, during the time step of largest deformations, the wind-induced slope deviation (0.75mradRMS) of the most affected facet reached a level of approximately one third of a Stellio facet’s total slope deviation. However, wind-induced slope deviations occurred only locally and temporally. Furthermore, wind-excited eigenfrequencies were revealed to have a negligible impact on both the tracking and slope deviation in case of the Stellio. Oscillations and deformations related to frequencies below the eigenfrequencies were rather found to have a predominant impact on the optical performance

Optische Qualifizierung von Heliostatfeldern für Forschung und Industrie

Es werden im DLR entwickelte Messmethoden vorgestellt, die der Qualifizierung von Heliostatfeldern dienen. Anwendungsbeispiel ist das Solarthermische Versuchskraftwerk Jülich. Des Weiteren werden Verwendungsmöglichkeiten der gewonnenen Daten (bspw. für Simulationen im Rahmen von Auslegungen und Betriebsbegleitung) aufgezeigt

Testing and Validation of Innovative on-Site Solar Field Measurement Techniques to Increase Power Tower Plant Performance: The LEIA Project

The LEIA project aims to contribute to the development of the next generation of central receiver power plants focusing on validating a combination and integration of pre-commercial solar field control and O&M solutions for the central tower receiver technology using molten salts, as the most promising cost-effective solution with the highest market penetration potential. To effectively remove the existing technical and industrial barriers to optimize central receiver and heliostat field operation & maintenance and thus to improve overall CSP performance, the following innovations are being developed: 1) Smart heliostat field control, 2) Smart control systems, 3) Solar Field Operation and Maintenance control strategies. These developments will be tested and demonstrated in three flagship operational environments: a) Cerro Dominador (Chile), b) CIEMAT-PSA (Spain), and c) CENER-Tudela (Spain)