This PDF file contains the front matter associated with SPIE Proceedings Volume 12671, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Achieving a decarbonized energy sector by 2050 will require the development of cost-effective technologies beyond today’s commercial concentrating solar-thermal power (CSP) technologies. Achieving the 2030 target will depend heavily on reducing the cost of heliostats, while improving technical performance. There are several opportunities for metrology, in addressing heliostat optical error, thus increasing the overall heliostat efficiency. In addition, opportunities in reducing the costs of manufacturing, assembly, calibration, or operations and maintenance exists.

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.

Concentrated solar power (CSP) plants need to monitor the surface slope error of thousands of heliostats with sub-milliradian accuracy. We present grating embedded mirrors (GEMs) and the accompanying Diffractive AutoStigmatic Hartmann Camera (DASHCam) metrology system for measuring heliostat surface slopes. GEMs have multiple phase diffraction gratings written within a glass mirror substrate using an ultrafast laser. The gratings direct a small fraction of incident light to non-specular directions, which the DASHCam senses from a virtual center of curvature to measure the facet slopes at each grating location. Our results show, in a laboratory environment, 24 μrad RMS measurement repeatability and 47 µrad RMS accuracy, with single-shot image capture. GEMs and DASHCam are a compact, accurate, and high-speed heliostat slope error metrology system that is robust to the harsh environmental conditions at CSP plants.

Warpage of mounted mirrors, pointing adjustments and mirror to mirror relationships in solar thermal systems require a scale of measurement of many meters, larger than most other optical systems, but still with micron level accuracies. The tools used in automotive and other durable goods industries contain years of application experience that could provide valuable production line fast and efficient shape and alignment measurements for large solar heliostats. This paper will explore a number of these industrial tools and suggest where they might help reduce cost and time in the operation of building of thermal solar installations.

Power tower or central receiver systems use hundreds to ten thousands of two-axis tracking mirrors, so called heliostats, which reflect and focus the sunlight onto a receiver. The final quality of each individual heliostat and its interaction as a heliostat field determine the performance of a solar tower system. In recent years, the SolarPACES Guideline for Heliostat Performance Testing and prototype characterization has been developed to ensure the quality of individual heliostats. The paper gives a short introduction to the most relevant aspects of the recently published version 1.0 of this guideline. Its content is currently being incorporated into the international standardization activity IEC-TC-117 62862-4- 3 (design qualification of heliostats). The total energy provided to the receiver is generated by the interaction of all heliostats acting together as a heliostat field. For this, a second guideline, the Heliostat Field Acceptance Guideline, has been developed in a German national project to be able to measure the performance of the solar field. This national draft guideline will be disseminated and peer reviewed internationally through the SolarPACES task III in October 2023. In this manuscript, first insights into the new field guideline are given.

Solar radiation attenuation in the path from the heliostats to the receiver is one of the main contributions to production estimation uncertainty in the operation of central-receiver concentrated solar plants (CSP). Few systems are commercially available to monitor this phenomenon and those available have high uncertainties due to the non-uniform attenuation pattern across the wavelength range of the useful solar radiation. In this work, we report the results from a 6-month measurement campaign carried out in a commercial CSP tower plant using a prototype system to measure the spectrally-resolved solar radiation attenuation. The system measures the differential spectrum between two pairs of high-resolution spectrometers (VIS and IR) separated approximately 800 meters and each coupled to a telescope system. Both systems are pointed at a white Lambertian target and regularly take a baseline measurement pointing at a black target to eliminate contributions from the diffuse light present in the solar field. The system is calibrated to <1% uncertainty using the reference of a portable spectrometer at both locations. Full-spectrum measurements were taken every 5 minutes. Spectral characteristics of different atmospheric conditions (suspended dust, fog, humidity) and their intra-day and seasonal evolutions are observed and analyzed.

While deep learning methods have proven their superiority over conventional image processing techniques in many domains, their use in airborne heliostat monitoring remains limited. Our aim is to bridge this gap by developing models to improve and extend existing image-based measurement methods in this field. We use Blender and BlenderProc to generate synthetic image data, which grants us access to vast amounts of training data essential for developing effective deep learning models. The exemplary model we train can potentially solve the following tasks related to airborne heliostat field monitoring: detection of heliostats and detection of mirror facet corners. Our promising preliminary results demonstrate the applicability of our approach to use synthetic training data for the development of the intended deep learning models.

The heliostat field at the Juelich solar towers meanwhile has a history of 15 years of operation experience. This paper gives a short overview over the problems and improvements since the first erection of the field. Reflectors had to be replaced after just 5 years of operation and have been refitted with a high quality four facet reflector system with individual canting. Results of deflectometric measurements will be demonstrated. The poor tracking performance of the initial heliostat design led to a retrofit of the tracking system in the years 2019 to 2023. With the current system precise tracking could be achieved since mid-2021. This paper will show different technological approaches to reach the desired quality improvements. Tracking results from the first days of operation to the current day will be presented. The whole field is operated by the heliostat field control system HeliOS. The control system has some specific functionalities which are uncommon to standard single tower systems but are a requirement for the research focused field in Juelich and for the simultaneous operation of multiple receivers in two adjacent towers by multiple operators. We will demonstrate some features of the system in this paper.

A method is proposed to keep all heliostats in a Concentrated-Solar-Power (CSP) facility under closed-loop pointing control while also providing feedback on the detailed alignment of the segment mirrors of each heliostat. The method uses the sunlight reflected by the heliostats towards the receiver, and thus works under full operational conditions and without the need of secondary optical alignments. It is based on retro-reflectors (‘retros’) to simultaneously return samples of the sunlight reflected by each mirror back to that same mirror. It goes beyond previous efforts at using retros by placing them into the concentrated sunlight, instead of in its periphery. Quartz glass will be used for its heat tolerance, and reflectivity modulation for visibility is achieved by rotating the retros. The technology can be retrofitted into existing CSP facilities to improve operational efficiency, and it can be used to relax the stability requirements of heliostats, and thus their cost, in the planning of new ones.

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.

The ongoing global energy transition towards sustainable and climate-neutral power generation has led to the increasing adoption of concentrated solar tower power plants, relying on heliostats for precise solar tracking. Heliostat calibration, vital for maintaining accurate alignment, traditionally assumes a decrease in accuracy over time due to various factors. However, the impact of data set sampling on reported tracking accuracy has been overlooked. This paper utilizes a kNN (k-Nearest Neighbors) data set sampling approach to investigate data set distribution's impact on model accuracy. Results indicate that conventional time-dependent sampling can lead to an overestimation of reported accuracies. In contrast, the kNN sampling approach demonstrates a strong correlation between model performance and the proximity of test data to training data. Simulations reveal that reported accuracy scores are influenced by the similarity between training and test data sets. The study highlights the critical importance of considering data set distribution when interpreting accuracy scores. The proposed method improves tracking accuracy and offers a dependable metric for evaluating calibration results. It provides valuable insights to enhance heliostat calibration models, advancing precise solar tracking in concentrated solar tower power plants and supporting the global transition towards sustainable energy solutions.

Shape-adjustable heliostats can maintain a focused image of the solar disc on a receiver target throughout the day, as the angle of incidence (AOI) changes on the reflector. This requires the heliostat reflector to be deformed into a unique biconic concave shape dependent on AOI. The reflector comprises a single rectangular sheet of silvered float glass mounted to a steel frame. Its shape is set initially, by adjusting the height of mounting points, to the biconic shape required for imaging at 60 degrees AOI. Shapes for other AOIs are obtained by twisting the frame from its four corners and center. A finite element model was made of a heliostat designed to form a disc image on a 130 m distant receiver using a single sheet glass reflector, 1/8” thick x 130” x 96”, supported by 58 points on a rectangular tube frame structure weighing 120 kg. Analysis shows an overall RMS slope error <1 mrad for all AOI from 0 to 70 degrees. Without twisting, the RMS slope error would be ~3.5 mrad at 0° and 70° AOI. Preliminary results from analysis of slope error maps generated from the FE model indicate encircled energies within the ideal solar disc radius of >85% are achievable. Models of fields of closely packed heliostats of this type, on target axis mounts, demonstrate a geometrical throughput into the receiver of >73% of the total mirror area, after accounting for blocking, shadowing, and cosine loss. In one model, with 450 heliostats powering five compound parabolic concentrators at a receiver, a concentration of > 3,000 was obtained at powers > 1 MW, through much of the day.

Nowadays, the development of optical systems applied to the generation of clean and renewable energy is one of the great challenges of the 21st century. This work presents a theoretical and experimental study of shrinkage in multiplexed holographic lenses (MHL) stored in a low-toxicity photopolymer named Biophotopol. For this purpose, a study has been carried out using k-space tools. In addition, a study has been performed to optimize the number of MHLs, the hologram thickness, and the angular distance between peaks. To evaluate the efficiency, the short-circuit current has been evaluated under solar illumination with different incident reconstruction angles.

Concentration factor of an optical concentrator is defined as the ratio of photon fluxes at its incident and exiting surfaces. It is equal to the product of geometric gain and optical efficiency. Scaling up a luminescent solar concentrator (LSC) increases its geometric gain, but its optical efficiency decreases due to the loss of photoluminescence (PL) photons during the concentration process. In a leaf LSC, a luminescent fiber is coupled to the side surface of a luminescent plate such that an incident photon goes through two-stage photoconversion. Its geometric gain increases drastically but the second photoconversion process decreases its optical efficiency. In experiment, a 1.5 mm-diameter fiber was placed between two 2 mm-thick luminescent plates. The plates emitted green PL photons and the fiber converted them to red PL photons. Position-dependent optical efficiency was measured by exciting a single spot at various positions on the plate with a 1 mm-diameter laser light at 450 nm. The optical efficiency averaged over the incident area increased from 0.004 to 0.007 by decreasing the lateral size of the plate from 50 mm to 10 mm. Ray tracing simulations reproduced the measurement. A clear lightguide with arc-bend couplers can guide the PL photons from multiple luminescent fibers to a photovoltaic cell. By connecting N devices, N-fold increase in geometric gain is expected while the optical efficiency remains the same. Hence, this configuration provides a solution to the trade-off between geometric gain and optical efficiency.

We have designed a Linear Fresnel-type Reflector (LFR) to reduce the area of light concentration based on the caustic surfaces produced by reflection. The LFR is designed by a set of planar mirrors, which appropriately have slopes in such a way that input energy can be focused at predefined absorber area. Also, losses due to riser steps were obtained from a geometrical point of view, to reduce and reconfigure the LFR shape in order to facilitate its manufacture. Finally, a LFR prototype will be fabricated on a single aluminum sheet where their grooves will be molded through CNC machine.

We design two different Hartmann type null screens based on an exact ray trace for testing a fast plano-convex aspherical condenser. The first null screen is designed for testing the external convex surface or periphery area for the condenser by reflection. We have implemented an exact ray trace assuming a point source placed along the optical axis, emitting a bundle of rays, which are reflected by the surface under test, to obtain a non-uniform array of spots, which are printed on paper sheet and wrapped on a plastic cylinder fabricated by using additive manufacture. Subsequently, by reversibility Principle’s after by reflection we obtain a uniform array pattern displayed at the detection plane. Alternatively, to evaluate the whole area for the condenser, the second null screen is designed for testing the central convex area for the condenser by refraction. Thus, we have implemented an exact ray trace assuming an incident plane wavefront, these rays are refracted through the lens under test, to obtain a non-uniform array of drop spots, which are printed on plastic sheet and placed in front of the lens under test. Finally, assuming the reversibility Principle’s after by refraction we obtain a uniform array pattern displayed at the detection plane. For this method, we have called Hartmann type hybrid null screens.