National Solar Thermal Test Facility Research Articles

Demonstration of a multi-channel fluidized bed particle–supercritical carbon dioxide heat exchanger for concentrating solar applications

High-temperature thermal energy storage in oxide particles at temperatures above 600°C can couple concentrated solar energy with high-efficiency thermal power cycles to provide dispatchable solar-driven electricity. Challenges remain in developing cost-effective primary heat exchangers, which require expensive alloys, to extract the high-temperature thermal energy from the particles to power cycle fluids, such as supercritical CO2 (sCO2) in recuperated Brayton cycles. To explore one pathway for cost-effective, high-temperature particle heat exchangers, the current study demonstrates a shell-and-plate, particle–sCO2 heat exchanger with narrow-channel fluidized beds coupled with micro-channel sCO2 flows in the heat exchanger walls. This study evaluates the feasibility of multiple parallel, narrow-channel fluidized beds in shell-and-plate particle–sCO2 HXs, to achieve high bed-wall heat fluxes at elevated temperatures. A reduced-order model simulates the narrow-channel, fluidized-bed particle–sCO2 heat exchanger to design the fluidized bed geometry, in terms of depth, height, and number of channels,for a nominal 40-kWth heat exchanger at particle and sCO2 inlet temperatures up to 600°C and 400°C respectively. The resulting shell-and-plate heat exchanger design operates with bubbling fluidization of the downward-flowing oxide particles to enhance bed-wall heat transfer. The heat exchanger core is fabricated with etched sCO2 micro-channels in thin wall plates that are diffusion bonded to spacer frames to form the shell-and-plate structure with 12 parallel, fluidized bed channels, 10.4 mm deep. The heat exchanger is tested at the National Solar Thermal Test Facility at Sandia National Laboratories with CARBOBEAD HSP particles at design particle flow rates of 0.20 kg s−1 and inlet temperatures up to 525°C. Results show that fluidization across multiple parallel channel beds can maintain uniform particle inventory with a common freeboard zone above the heat exchanger core. Bubbling fluidization improves particle–wall heat transfer coefficients but also increases axial dispersion of particle thermal energy, which lowers the log-mean temperature difference such that total heat transfer remains relatively constant to within ±10% over a broad range of fluidization gas velocities. The axial dispersion required particle and sCO2 flow rates to be increased by 25% over model-designed conditions to achieve the targeted 40 kWth, which indicates the importance of incorporating axial dispersion into heat exchanger design models and of deploying bed structures to suppress it. This study demonstrates the feasibility and preferred fluidizing gas conditions for particle heat exchangers for releasing high-temperature thermal energy storage systems.

Development of the Generation 3 Particle Pilot Plant Falling Particle Receiver

A falling particle receiver (FPR) has been designed to integrate with the G3P3-USA pilot plant currently being constructed at the National Solar Thermal Test Facility (NSTTF) at Sandia National Laboratories. This receiver integrates several innovative design features including a converging tunnel (SNOUT), an optimized cavity geometry, and a multistage “catch-and-release” trough. Details about the integration of these features and the final G3P3-USA FPR design and construction are described. Ray-tracing models of the FPR utilizing the NSTTF heliostat field are developed leveraging previous modeling efforts using SolTrace. Models demonstrate that at least 1.5 MWth of incident radiative energy can be provided to the FPR on a clear day throughout a typical year in Albuquerque, NM. Spillage fluxes around the periphery of the aperture are within acceptable bounds for the majority of the year. Intercept factors are computed for each utilized heliostat at the Vernal equinox to provide guidance to heliostat operators during operation of the system.

Testing of a 40-kWth Counterflow Particle-Supercritical Carbon Dioxide Narrow-Channel, Fluidized Bed Heat Exchanger

Particle-based primary heat exchangers (HXs) must deliver sCO2 fluid temperatures above 700°C to couple particle-based concentrating solar receivers and thermal energy storage (TES) sub-systems with efficient sCO2 power cycles. Particle-sCO2 HX designs have struggled to meet DOE cost targets (≤ $150/kWth) due to the amount of expensive nickel alloys necessary for manufacturing full-scale, particle-sCO2 HXs. Our team has demonstrated that mild bubbling fluidization of falling particles in a counterflow narrow-channel fluidized bed can reduce required HX surface area and thus, costs by increasing particle-wall heat transfer coefficients hT,w > 800 W m-2 K-1. This paper reports on the fabrication and testing of a stainless steel, particle-sCO2 HX with 12 fluidized-bed channels approximately 10.5 mm deep spaced between diffusion-bonded, micro-channel sCO2 plates. The HX with a core length of ≈0.56 m is fed with CARBOBEAD HSP particles through a short, fluidized freeboard zone just above the core. Testing to date in the National Solar Thermal Test Facility (NSTTF) at Sandia National Laboratories has shown that parallel bed fluidization maintains uniform particle inventory across the instrumented channels. Heat transfer thermal duty between the particle and sCO2 flows exceeds 30 kWth with sCO2 inlet temperatures of 200ºC and particle inlet temperatures up to 440ºC and mass flow rates of 0.2 kg s-1 fluidized by counterflowing gas flow rates of 0.005 kg s-1. Tests at higher particle and sCO2 inlet temperatures (600ºC and 400ºC respectively) are targeted to achieve > 40 kWth with model-predicted overall heat transfer coefficients U > 400 W m-2 K-1.

Toward Autonomous Field Inspection of CSP Collectors With a Polarimetric Imaging Drone

We developed a polarimetric imaging drone to perform field inspections of heliostats and carried out field tests at Sandia’s National Solar Thermal Test Facility (NSTTF). The preliminary results show that Degree of Linear Polarization (DOLP) and Angle of Polarization (AOP) images greatly enhanced the edge detection results compared with the conventional visible images, supporting fast and accurate detection of heliostat mirror edges and cracks. The system holds the promise to enable future automated detection of heliostats optical errors and mirror defects.

Preface: SolarPACES 2022, 28th International Conference on Concentrating Solar Power and Chemical Energy Systems

The SolarPACES 2022 International Conference was held from September 27 to September 30, 2022 in Albuquerque, New Mexico. As the premier international conference and network for advancing commercial deployment and research and development of concentrating solar-thermal power (CSP) and related technologies, the community was excited to convene in person after two years of virtual collaboration. The conference brought together more than 400 participants from 29 countries. The technical presentations and plenary sessions highlighted exciting advances in state-of-the-art commercial deployment and next-generation research and development. There was much focus on emerging commercial markets, including the value of thermal energy storage for cost-effective long duration energy storage – charged either with electricity or solar heat – and the use of solar thermal energy for industrial and chemical processes, especially to decarbonize the large portion of energy that’s currently used for industrial process heating. By holding the conference in the U.S., we were able to highlight particular technical and strategic objectives being advanced by the U.S. Department of Energy (DOE) and the US Government, including the progress in advancing the Generation 3 (Gen3) CSP test facility [1] at Sandia National Laboratories. This megawatt-scale system will demonstrate a fully integrated thermal transport and storage system that can collect, store, and deliver heat above 700° Celsius, using solid particle heat transfer media. Attendees to the conference were able to tour the globally renowned CSP research center at Sandia National Laboratories and the National Solar Thermal Test Facility (NSTTF). The NSTTF opened in 1978 as the world’s first tower research facility for CSP and has been operating for more than 40 years. The site has been a center of excellence in the development of molten salt technologies for CSP and has played a key role in the demonstration of technologies used in some of the largest solar power plants in the world. We also highlighted the recent announcements from the U.S. Department of Energy of the Energy Earthshot Initiatives to accelerate breakthroughs of abundant, affordable, and reliable clean energy solutions by the end of this decade. Specifically, the Long Duration Storage Shot [2] established a target to reduce the cost of grid-scale energy storage by 90% for systems that deliver 10 or more hours of duration, to $0.05 per kilowatt-hour of electricity delivered from storage. The Industrial Heat Shot [3] aims to develop cost-competitive industrial heat decarbonization technologies with at least 85% lower greenhouse gas emissions by 2035. Hydrogen Shot [4] seeks to reduce the cost of clean hydrogen by 80%, to $1 per 1 kilogram. CSP technologies have promising potential to contribute to all of these important goals. The 2022 conference covered important contributions in all of these areas, and more, across 15 technical tracks, coordinated and chaired by international experts in each field. We had high quality contributions across the board, including 211 oral presentations and 111 posters. Additionally, the plenary sessions brought together leading experts on energy storage, US and global markets for CSP technologies, and a specific focus on industrial decarbonization. On behalf of SolarPACES, I thank all of the conference participants, authors, presenters, track chairs, and the conference organizing committee for an extremely successful meeting! Avi Shultz Chairman of the 28th SolarPACES conference

Effect of Quartz Aperture Covers on the Fluid Dynamics and Thermal Efficiency of Falling Particle Receivers

Abstract Falling particle receivers are an emerging technology for use in concentrating solar power systems. In this study, quartz half-shells are investigated for use as full or partial aperture covers to reduce receiver thermal losses. Quartz half-shell aperture covers offer the ability to minimally interfere with incoming solar radiation from the heliostat field while obstructing thermal radiation and advection from leaving the receiver cavity. The fluid dynamics and heat transfer of a receiver subdomain and surrounding air are modeled using ansys®fluent. We compare the percentage of total incident solar power lost due to conduction through the receiver walls, advective losses through the aperture, and radiation exiting the aperture. Contrary to expected outcomes, results show that quartz aperture covers can increase radiative losses and result in modest to nonexistent reductions in advective losses. The increased radiative losses are driven by elevated quartz half-shell temperatures and have the potential to be mitigated by active cooling and/or material selection. Quartz half-shell total transmissivity was measured experimentally using a radiometer and the National Solar Thermal Test Facility heliostat field with values up to 0.97 ± 0.01. Quartz half-shell aperture covers did not yield expected efficiency gains in numerical results due to increased radiative losses, but efficiency improvement in some numerical results and the performance of quartz half-shells subject to concentrated solar radiation suggest that quartz half-shell aperture covers should be investigated further.

Design Evaluation of a Next-Generation High-Temperature Particle Receiver for Concentrating Solar Thermal Applications

High-temperature particle receivers are being developed to achieve temperatures in excess of 700 °C for advanced power cycles and solar thermochemical processes. This paper describes designs and features of a falling particle receiver system that has been evaluated and tested at the National Solar Thermal Test Facility at Sandia National Laboratories. These advanced designs are intended to reduce heat losses and increase the thermal efficiency. Novel features include aperture covers, active air flow, particle flow obstructions, and optimized receiver shapes that minimize advective heat losses, increase particle curtain opacity and uniformity, and reduce cavity wall temperatures. Control systems are implemented in recent on-sun tests to maintain a desired particle outlet temperature using an automated closed-loop proportional–integral–derivative controller. These tests demonstrate the ability to achieve and maintain particle outlet temperatures approaching 800 °C with efficiencies between 60 and 90%, depending on incident power, mass flow, and environmental conditions. Lessons learned regarding the testing of design features and overall receiver operation are also presented.

Reflections between heliostats: Model to detect alignment errors

This paper introduces a novel technique to assess the alignment quality of heliostats in operation. This technique combines theoretical imaging with aerial vision of an object heliostat reflecting the back of a neighbor target heliostat. An accurate optical model is developed to generate the theoretical images that allows to detect alignment errors by superposition on actual photographs. The resulting code is grounded in geometric transformations between systems of coordinates, and makes use of the pinhole camera model and the geometric determination of reflection points in concave mirrors. The optical model is experimentally validated with real heliostats at the National Solar Thermal Test Facility (NSTTF). In terms of pixel shifts in the target-reflected image, a sensitivity analysis of the technique is performed. The influence of several geometric factors is researched: camera position, camera focal length, heliostat-to-camera distance, and curvature of the facets. Preliminary analysis shows that canting errors as low as 0.25 mrad can be detected by facet framing.

System Design of a 1 MW North-facing, Solid Particle Receiver

Abstract Falling solid particle receivers (SPR) utilize small particles as a heat collecting medium within a cavity receiver structure. The components required to operate an SPR include the receiver (to heat the particles), bottom hopper (to catch the falling particles), particle lift elevator (to lift particles back to the top of the receiver), top hopper (to store particles before being dropped through the receiver), and ducting. In addition to the required components, there are additional features needed for an experimental system. These features include: a support structure to house all components, calibration panel to measure incident radiation, cooling loops, and sensors (flux gages, thermocouples, pressure gages). Each of these components had to be designed to withstand temperatures ranging from ambient to 700 °C. Thermal stresses from thermal expansion become a key factor in these types of high temperature systems. The SPR will be housing ∼3000 kg of solid particles. The final system will be tested at the National Solar Thermal Test Facility in Albuquerque, NM.

Compensation of Gravity Induced Heliostat Deflections for Improved Optical Performance

Heliostat optical performance can be affected by both wind and gravity induced deflections in the mirror support structure. These effects can result in decreased energy collection efficiency, depending on the magnitude of structural deflections, heliostat orientation and field position, and sun position. This paper presents a coupled modeling approach to evaluate the effects of gravity loading on heliostat optical performance, considering two heliostat designs: The National Solar Thermal Test Facility (NSTTF) heliostat and the Advanced Thermal Systems (ATS) heliostat. Deflections under gravitational loading were determined using finite element analysis (FEA) in Ansys Mechanical, and the resulting deformed heliostat geometry was analyzed using Breault Apex optical engineering software to evaluate changes in beam size and shape. Optical results were validated against images of actual beams produced by each respective heliostat, measured using the Beam Characterization System (BCS) at Sandia National Laboratories. Simulated structural deflections in both heliostats were found to have visible impacts on beam shape, with small but quantifiable changes in beam power distribution. In this paper, the combined FEA and optical analysis method is described and validated, as well as a method for modeling heliostats subjected to gravitational deflection and canted in-field, for which mirror positions may not be known rigorously. Furthermore, a modified, generalized construction method is proposed and analyzed for the ATS heliostat, which was found to give consistent improvements in beam shape and up to a 4.1% increase in annual incident power weighted intercept (AIPWI).

Wind Patterns over a Heliostat Field

Heliostats constitute a major portion of the direct costs of a concentrating solar power tower plant. As a result, a significant amount of effort is being focused on designing and developing cheaper heliostats. The optical and structural performance of these new and existing heliostats under dynamic wind loads must be characterized and understood in order to meet both cost and performance objectives. This paper presents the second phase of a U.S. DOE-sponsored program at Sandia National Laboratories’ National Solar Thermal Test Facility (NSTTF) that includes dynamic testing and analysis of multiple full-scale heliostats. The objectives of these tests and analyses are to characterize and understand some differences in the impacts of dynamic wind loads on heliostat strain and cyclic fatigue between perimeter and inner-field heliostats.A weather tower with three tri-axial ultrasonic anemometers has been erected just outside the field to measure the approaching boundary winds, while a portable tower was set up to characterize wind velocities and turbulence between subsequent rows within the field. Anemometers have also been mounted to some heliostats to gather close range measurement of turbulence and wind frequencies and to provide a point of comparison for computational fluid dynamics models of wind flow over the field.This paper presents mean wind speeds and wind loads on heliostats as a function of field position. The calculated mean wind loads were used to assess the mean wind-load reduction correlation of Peterka [1].

Field Demonstration of an Automated Heliostat Tracking Correction Method

This study provides a demonstration of an eight-parameter heliostat tracking-error correction method applied to heliostats at the National Solar Thermal Test Facility at Sandia National Laboratories. This method models the non-random physical error sources that arise from imperfect mount fabrication and installation processes. Previous work demonstrated this method for a single heliostat over a one-month period [1]. This study extends the previous work by expanding the number of heliostats modeled and tested to over 200. The duration over which errors were collected and modeled spanned 7 months. Smaller subsets of heliostats were observed and tested over an even longer, 10 month period. Error data was obtained by bringing heliostat beams (one by one) to a surveyed target on the Solar Tower and recording the measured elevation and azimuthal offsets. Beam position was determined by the centroid of the intensity image. Using automated sequencing, we were able to limit the time to process a heliostat to ∼32seconds. This allowed us to process the entire 200 heliostat field in just 2 full days (weather permitting). These 2- day collections were performed about once every 2-3 months, to cover the full annual range of motion. All error data were automatically collected, time tagged and synchronized, in real-time. The error data were then converted, by a post-processor, to updated model parameters. The updated model data were subsequently uploaded to the heliostat field for model performance testing. In this way, almost 50,000 error observations were collected for 200 heliostats during the course of the study. In addition, 19,000 model performance observations were also recorded. Initial results indicate this method was able to halve the RMS average pointing error from 1.5 mrad to 0.78 mrad across the field of 200 heliostats.

Lab-scale Experimentation and CFD Modeling of a Small Particle Heat Exchange Receiver

Concentrating solar power currently relies on high temperature central receivers that utilize liquid cooling and operate in power steam cycles. However, highly efficient central receivers are being designed to operate at higher temperatures in a gas turbine power cycle. To address this, San Diego State University's (SDSU) Combustion and Solar Energy Laboratory is experimenting with a lab-scale Small Particle Heat Exchange Receiver (SPHER) in order to understand performance and develop experience for designing and operating a full-scale 5 MW design. The full-scale design will be tested at the National Solar Thermal Test Facility at Sandia National Laboratories as part of the Department of Energy SunShot Initiative grant. The SPHER relies on carbon nanoparticles as an absorption medium and air as a working fluid. The carbon particles are generated onsite by the Carbon Particle Generator (CPG) and are mixed with dilution air prior to entering the SPHER. Lab scale on-sun testing is carried out with a 15kWe solar simulator. The lab scale experimental goal is to achieve an outlet flow of 650°C at 5bar absolute operating pressure. To model the performance of the SPHER, CFD analysis is being used for comparison to lab scale testing. The lab scale SPHER is being modeled in ANSYS Fluent with coupled codes for oxidation and radiation input. In this paper, we present results of testing the lab-scale receiver and compare the measured outlet temperatures to predictions from the computer model. Finally, correlations are drawn for future experimenation and feasibility.

Long-distance flux mapping using low-cost collimated pyranometers

Concentrating solar thermal power tower plants with capacities of 100MWe or greater require large heliostat fields with heliostats over 1500m (nearly a mile) away from the tower. The accuracy and performance of these heliostats must be evaluated and understood as new heliostat designs emerge to reduce costs. Conventional beam characterization systems that use photographs of the reflected beam on a tower-mounted target are typically not large enough to capture the beam at long distances, and the magnitude of the irradiance for long-distance heliostats is quite low (only a fraction of a sun), which can make the beam image difficult to discern from the ambient lighting on the target. The Long-Range Heliostat Target (LRHT) implements a technique for mapping low density flux images from heliostats and reflectors at slant ranges up to approximately 1700m.The LRHT is a vertical array of collimated pyranometers deployed to a test site via flat-bed trailer and quickly erected on an aluminum truss tower. Once the sensors have been aimed at the heliostat, the heliostat beam is swept azimuthally across the array whereupon the data is stitched into a flux map indicating horizontal and vertical beam dimensions and flux intensities. The LRHT was used to evaluate and compare beam shape, peak flux, and total power of heliostats and single facet reflectors at distances from 300 to 1700m. Results were compared to theoretically rendered flux maps created by computational ray tracing algorithms, and to photographs taken on the beam characterization system (BCS) at the National Solar Thermal Test Facility at Sandia National Laboratories.

A Photographic Flux Mapping Method for Concentrating Solar Collectors and Receivers

A new method is described to determine irradiance distributions on receivers and targets from heliostats or other collectors for concentrating solar power applications. The method uses a digital camera, and, unlike previous beam characterization systems, it does not require additional sensors, calorimeters, or flux gauges on the receiver or target. In addition, spillage can exist and can also be measured (the beam does not need to be contained within the target). The only additional information required besides the images recorded from the digital camera is the direct normal irradiance and the reflectivity of the receiver. Methods are described to calculate either an average reflectivity or a reflectivity distribution for the receiver using the digital camera. The novel feature of this new photographic flux (PHLUX) mapping method is the use of recorded images of the sun to scale both the magnitude of each pixel value and the subtended angle of each pixel. A test was performed to evaluate the PHLUX method using a heliostat beam on the central receiver tower at the National Solar Thermal Test Facility in Albuquerque, NM. Results showed that the PHLUX method was capable of producing an accurate flux map of the heliostat beam on a Lambertian surface with a relative error in the peak flux of ∼2% when the filter attenuation factors and effective receiver reflectivity were well characterized. Total relative errors associated with the measured irradiance using the PHLUX method can be up to 20%–40%, depending on various error sources identified in the paper, namely, uncertainty in receiver reflectivity and filter attenuation.

Practical Field Alignment of Parabolic Trough Solar Concentrators

In this paper a new technique for parabolic trough mirror alignment based on the use of an innovative theoretical overlay photographic (TOP) approach is described. The technique is a variation on methods used to align mirrors on parabolic dish systems. It involves overlaying theoretical images of the heat collection element (HCE) in the mirrors onto carefully surveyed photographic images and adjustment of mirror alignment until they match. From basic geometric principles, for any given viewer location the theoretical shape and location of the reflected HCE image in the aligned mirrors can be predicted. The TOP approach promises to be practical and straightforward, and inherently aligns the mirrors to the HCE. Alignment of an LS-2 mirror module on the rotating platform at the National Solar Thermal Test Facility (NSTTF) with the TOP technique along with how it might be implemented in a large solar field is described. Comparison of the TOP alignment to the distant observer approach on the NSTTF LS-2 is presented and the governing equations used to draw the theoretical overlays are developed. Alignment uncertainty associated with this technique is predicted to be less than the mirror slope error.

Testing of Thermocline Filler Materials and Molten-Salt Heat Transfer Fluids for Thermal Energy Storage Systems in Parabolic Trough Power Plants

Parabolic trough power systems that utilize concentrated solar energy to generate electricity are a proven technology. Industry and laboratory research efforts are now focusing on integration of thermal energy storage as a viable means to enhance dispatchability of concentrated solar energy. One option to significantly reduce costs is to use thermocline storage systems, low-cost filler materials as the primary thermal storage medium, and molten nitrate salts as the direct heat transfer fluid. Prior thermocline evaluations and thermal cycling tests at the Sandia National Laboratories’ National Solar Thermal Test Facility identified quartzite rock and silica sand as potential filler materials. An expanded series of isothermal and thermal cycling experiments were planned and implemented to extend those studies in order to demonstrate the durability of these filler materials in molten nitrate salts over a range of operating temperatures for extended time frames. Upon test completion, careful analyses of filler material samples, as well as the molten salt, were conducted to assess long-term durability and degradation mechanisms in these test conditions. Analysis results demonstrate that the quartzite rock and silica sand appear able to withstand the molten salt environment quite well. No significant deterioration that would impact the performance or operability of a thermocline thermal energy storage system was evident. Therefore, additional studies of the thermocline concept can continue armed with confidence that appropriate filler materials have been identified for the intended application.

Operation of Large-Scale Pumps and Valves in Molten Salt

The molten salt pump and valve (P&V) test loops at Sandia National Laboratories (SNL) National Solar Thermal Test Facility (NSTTF) operated between Jan. 1988 and Oct. 1990. The purpose of the P&V test was to demonstrate the performance, reliability, and service life of full-scale hot and cold salt pumps and valves for use in commercial central receiver solar power plants. The P&V test hardware consists of two pumped loops; the “Hot Loop” to simulate the hot (565°C) side of the receiver and the “Cold Loop” to simulate the receiver’s cold (285°C) side. Each loop contains a pump and five valves sized to be representative of a conceptual 60-MWe commercial solar power plant design. The hot loop accumulated over 6700 hours of operation and the cold loop over 2500 hours of operation. This project has demonstrated that standard commercial scale pump and valve designs will work in molten salt. The test also exposed some pitfalls that must be avoided in specifying such equipment. Although certainly not all of the pitfalls were discovered, careful design and specification should result in reliable or at least workable equipment.

The development of stretched-membrane heliostats in the United States

For more than 10 years, the US Department of Energy's Solar Thermal Program has pioneered the development of heliostats, mirrors that track the sun, for solar central-receiver power plants. The field of heliostats is the single most expensive part of such plants, so their cost must be as low as possible for the technology to be commercially successful. In the last six years, Sandia National Laboratories has been developing and transferring to private industry a technology that uses stretched-membrane reflectors in place of the more familiar glass mirrors. Because of their simplicity and lighter weight, stretched-membrane heliostats have the potential to cost significantly less than current glass-mirror designs. Two generations of 50 m 2 prototype stretched-membrane mirror modules were evaluated at Sandia's National Solar Thermal Test Facility in Albuquerque, NM. They demonstrated an optical performance rivaling that of the more familiar glass-mirror heliostats. Sandia recently initiated the final phase of development: the design of fully integrated, market-ready heliostats. Field tests are planned for late 1990.