Adam C. Moya

CFD Simulation and Performance Analysis of Alternative Designs for High-Temperature Solid Particle Receivers

Direct-absorption solid particle receivers are theoretically capable of yielding temperatures in excess of 1000°C, which enables higher efficiency power cycles and lower thermal storage costs. This paper presents rigorous CFD simulations of alternative solid particle receiver designs with recirculation to help identify optimal configurations that maximize the receiver thermal efficiency. The alternative receiver designs considered are a north-facing cavity receiver and a face-down surround-field cavity receiver. The CFD simulations model incident solar radiation from a heliostat field as a boundary condition on the model domain. The CFD simulations also couple convective flow with the thermal and discrete-phase (particle) solutions, which in turn affects absorption of incident solar radiation and thermal re-radiation within the receiver. The receivers are optimized to yield comparable particle temperatures at the outlets of 750–850°C, heated from an injection temperature of 300°C, and are compared on the basis of thermal efficiency. The CFD simulations yielded thermal efficiencies of the north-facing receiver at 72.3% (losses were 6.5% radiative and 20.9% convective) and the face-down receiver at 78.9% (losses were 11.4% radiative and 9.6% convective) at solar noon on March 22. Ongoing efforts are focused on reducing convective and radiative losses from both receiver configurations.Copyright

Modeling and Validation of Heliostat Deformation due to Static Loading

Accurate and reliable models are necessary to predict the performance and efficiencies of concentrating solar power plant components and systems such as heliostats and central receiver systems. Heliostat performance is impacted from effects such as wind and gravity, and understanding the impact of these loads on the optical performance can yield heliostat designs that are potentially cheaper, while maintaining required structural stability. Finite element models of heliostats at the National Solar Thermal Test Facility (NSTTF) at Sandia National Laboratories in Albuquerque, NM, were developed to simulate displacements under different loading scenarios. Solidworks was used to develop the three-dimensional model of the NSTTF heliostat, and Solidworks Simulation was used to perform the finite element analysis with simulated loads along different points of the heliostat. Static displacement tests were performed on the NSTTF heliostat in order to validate these FEA models. The static test results provide us with a data set in which to properly calibrate the FEA model to better represent the NSTTF heliostat for future simulations of optical performance with impacts of wind and gravity sag. In addition to a single model validation, this real world test provides a method to validate and understand the structural stability of a heliostat under static loads.Copyright

Structural Dynamics Testing and Analysis for Design Evaluation and Monitoring of Heliostats

Heliostat vibrations can degrade optical pointing accuracy while fatiguing the structural components. This paper reports the use of structural dynamic measurements for design evaluation and monitoring of heliostat vibrations. A heliostat located at the National Solar Thermal Testing Facility (NSTTF) at Sandia Labs in Albuquerque, New Mexico, has been instrumented to measure its modes of vibration, strain and displacements under wind loading. The information gained from these tests will be used to evaluate and improve structural models that predict the motions/deformations of the heliostat due to gravitational and dynamic wind loadings. These deformations can cause optical errors and motions that degrade the performance of the heliostat. The main contributions of this work include: (1) demonstration of the role of structural dynamic tests (also known as modal tests) to provide a characterization of the important dynamics of the heliostat structure as they relate to durability and optical accuracy, (2) the use of structural dynamic tests to provide data to evaluate and improve the accuracy of computer-based design models, and (3) the selection of sensors and data-processing techniques that are appropriate for long-term monitoring of heliostat motions.Copyright

Experimental and Numerical Studies of Air Curtains for Falling Particle Receivers

The use of an air curtain blowing across the aperture of a falling-particle receiver has been proposed to mitigate convective heat losses and to protect the flow of particles from external winds. This paper presents experimental and numerical studies that evaluate the impact of an air curtain on the performance of a falling particle receiver. Unheated experimental studies were performed to evaluate the impact of various factors (particle size, particle mass flow rate, particle release location, air-curtain flow rate, and external wind) on particle flow, stability, and loss through the aperture. Numerical simulations were performed to evaluate the impact of an air curtain on the thermal efficiency of a falling particle receiver at different operating temperatures. Results showed that the air curtain reduced particle loss when particles were released near the aperture in the presence of external wind, but the presence of the air curtain did not generally improve the flow characteristics and loss of the particles for other scenarios. Numerical results showed that the presence of an air curtain could reduce the convective heat losses, but only at higher temperatures (>600°C) when buoyant hot air leaving the aperture was significant.Copyright

Modal Analysis of a Heliostat for Concentrating Solar Power

A heliostat is a structure whose function is to reflect sunlight to a target collector. Heliostat vibrations can degrade optical pointing accuracy and fatigue the structural components. This paper reports on an experimental and analytical program with a goal to improve understanding of the response to wind loading on heliostats. A modal test was performed on a heliostat located at the National Solar Thermal Testing Facility (NSTTF) at Sandia Labs in Albuquerque, New Mexico. Modal tests were performed with artificial and natural wind excitation. Strain and displacements were also measured under wind loading. The information gained from these tests has been used to evaluate and improve structural models that predict the deformations of the heliostat due to gravitational and dynamic wind loadings. The paper will provide an up-to-date summary of model validation work, evaluation of suitable sensors, and development of data-processing methods for long-term deformation monitoring.

Comparison of FRF Correlation Techniques.

Mode shape correlation techniques have proven to be an excellent method for assessing the degree of similarity between a finite element model and a set of test data. Because mode shapes inherently contain spatial information on the mass and stiffness matrices of the model and are easily extracted from test data, this allows an overall similarity assessment as well as providing indications of localized areas of discrepancy. Although frequency response function (FRF) correlation contains the same information as used in mode shape correlation (both contain a collection of measurement points), FRF correlation has been found to be more difficult to use successfully. This difficulty is primarily because of the high sensitivity of frequency response functions to slight perturbations both in the boundary condition and in the excitation and response. As a result, the globally extracted mode shapes minimize the contamination due to testing perturbations, while frequency response functions maintain this sensitivity This sensitivity makes it possible to measure similarity, or lack thereof, in so-called identical components or assemblies.

Modal Analysis and Dynamic Monitoring of a Concentrating Solar Heliostat.

Heliostats are structures that track the sun and reflect sunlight to a centrally located receiver on top of a tower to produce heat for electricity generation. Commercial power towers can consist of thousands of heliostats that are subject to wind-induced loads, vibration, and gravity-induced sag. This paper presents modal tests of a heliostat located at the National Solar Thermal Test Facility (NSTTF) at Sandia National Labs in Albuquerque, New Mexico. The heliostat was instrumented with 22 accelerometers, 4 strain gauges, and 3 wind anemometers to examine manually and wind-induced vibrations of the structure. Data acquisition software was developed to provide real-time monitoring of the wind velocity, heliostat strain, mode shapes, and natural frequencies which will be used to validate finite element models of the heliostat. The ability to test and monitor full-scale heliostats under dynamic wind loads will provide a new level of characterization and understanding compared to previous tests that utilized scaled models in wind-tunnel tests. Also, the development of validated structural dynamics models will enable improved designs to mitigate the impacts of dynamic wind loads on structural fatigue and optical performance.

The Challenge of Dynamic Similarity Assessment

Throughout the development cycle of structural components or assemblies that require new and unproven manufacturing techniques, the issue of unit to unit variability inevitably arises. The challenge of defining dynamic similarity between units is a problem that is often overlooked or forgotten, but can be very important depending on the functional criteria of the final product. This work aims to provide some guidance on the approach to such a problem, utilizing different methodologies from the modal and vibration testing community. Expanding on previous efforts, a non-intrusive dynamic characterization test is defined to assess similarity on an assembly that is currently being developed. As the assembly is qualified through various test units, the same data sets are taken to build a database of “similarity” data. The work presented here will describe the challenges observed with defining similarity metrics on a multi-body structure with a limited quantity of test units. Also, two statistical characterizations of dynamic FRFs are presented from which one may choose criterion based on some judgment to establish whether units are in or out of family. The methods may be used when the “intended purpose” or “functional criteria” are unknown.