Publications

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The goal of the DuraMAT Predictive Simulation Capability Area is to develop a suite of modeling and simulation tools to enhance understanding of module-level thermo-mechanical-electrical effects contributing to degradation of solar photovoltaic modules. Since these effects invoke multiple physical mechanisms and take place over greatly varied time- and length- scales, developing a module-level model of sufficient resolution to capture all geometries and physics of interest was expected to be near code capability and computational resource limits. A series of sub-models and workflows were developed to assess physics and material model maturity and the necessary computational capacity for resolving degradation mechanisms of interest. Knowledge gained from these activities help to better scope future development efforts, in line with available computational and code capabilities. This memo serves as the DuraMAT Quarterly Progress Indicator (QPI) for Quarter 3 of Fiscal Year 2017 (Project Quarter 1 / Month 3), and documents completion of Milestone Subtask 2.2.1.

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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).

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Cavity receivers used in solar power towers and dish concentrators may lose considerable energy by natural convection, which reduces the overall system efficiency. A validated numerical receiver model is desired to better understand convection processes and aid in heat loss minimization efforts. The purpose of this investigation was to evaluate heat loss predictions using the commercial computational fluid dynamics software packages FLUENT 13.0 and SolidWorks Flow Simulation 2011 against experimentally measured heat losses for a heated cubical cavity model[1] and a cylindrical dish receiver model [2]. Agreement within 10% was found between software packages across most simulations. However, simulated convective heat loss was under predicted by 45% for the cubical cavity when experimental wall temperatures were implemented on cavity walls, and 32% when implementing the experimental heat flux from the cavity walls. Convective heat loss from the cylindrical dish receiver model was accurately predicted within experimental uncertainties by both simulation codes using both isothermal and constant heat flux wall boundary conditions except at inclination angles below 15° and above 75°, where losses were under- and over-predicted by FLUENT and SolidWorks, respectively. Comparison with empirical correlations for convective heat loss from heated cavities showed that correlations by Siebers and Kraabel [1] and for an assembly of heated flat plates oriented to the cavity geometry [3] predicted heat losses from the cubical cavity within experimental uncertainties, while correlations by Clausing [4] and Paitoonsurikarn et al. [8] were able to do the same for the cylindrical dish receiver. No single correlation was valid for both receiver models. Different turbulence and air-property models within FLUENT were also investigated and compared in this study. Copyright © 2012 by ASME.

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