Publications

Solar Two was a demonstration of the viability of molten salt power towers. The power tower was designed to produce enough thermal power to run a 10-MWe conventional Rankine cycle turbine. A critical component of this process was the solar tower receiver. The receiver was designed for an applied average heat flux of 430 kW/m2 with an outlet temperature of 565°C (838.15 K). The mass flow rate could be varied in the system to control the outlet temperature of the heat transfer fluid, which was high temperature molten salt. The heat loss in the actual system was calculated by using the power-on method which compares how much power is absorbed by the molten salt when using half of the heliostat field and then the full heliostat field. However, the total heat loss in the system was lumped into a single value comprised of radiation, convection, and conduction heat transfer losses. In this study, ANSYS FLUENT was used to evaluate and characterize the radiative and convective heat losses from this receiver system assuming two boundary conditions: (1) a uniform heat flux on the receiver and (2) a distributed heat flux generated from the code DELSOL. The results show that the distributed-flux models resulted in radiative heat losses that were ∼14% higher than the uniform-flux models, and convective losses that were ∼5-10% higher due to the resulting non-uniform temperature distributions. Comparing the simulations to known convective heat loss correlations demonstrated that surface roughness should be accounted for in the simulations. This study provides a model which can be used for further receiver design and demonstrates whether current convective correlations are appropriate for analytical evaluation of external solar tower receivers. Copyright © 2012 by ASME.

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.

Predicting the structural and optical performance of concentrating solar power (CSP) collectors is critical to the design and performance of CSP systems. This paper presents a performance analysis which utilizes finite-element models and ray-tracing of a parabolic trough collector. The finite-element models were used to determine the impact of gravity loads on displacements and rotations of the facet surfaces, resulting in slope error distributions across the reflective surfaces. The geometry of the LUZ LS-2 parabolic trough collector was modeled in SolidWorks, and the effects of gravity on the reflective surfaces are analyzed using SolidWorks Simulation. The ideal mirror shape, along with the 90° and 0° positions (with gravity deformation) were evaluated for the LS-2. The ray-tracing programs APEX and ASAP are used to assess the impact of gravity deformations on optical performance. In the first part of the analysis, a comprehensive study is performed for the parabolic trough to evaluate a random slope error threshold (i.e., induced by manufacturing errors and assembly processes) above which additional slope errors caused by gravity sag decrease the intercept factor of the system. The optical performance of the deformed shape of the collector (in both positions) is analyzed with additional induced slope errors ranging from zero up to 1° (17.44 mrad). The intercept factor for different solar incident angles found from ray-tracing is then compared to empirical data to demonstrate if the simulations provide consistent answers with experimental data. Copyright © 2011 by ASME.

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Understanding the effects of gravity and wind loads on concentrating solar power (CSP) collectors is critical for performance calculations and developing more accurate alignment procedures and techniques. This paper presents a rigorous finite-element model of a parabolic trough collector that is used to determine the impact of gravity loads on bending and displacements of the mirror facets and support structure. The geometry of the LUZ LS-2 parabolic trough collector was modeled using SolidWorks, and gravity-induced loading and displacements were simulated in SolidWorks Simulation. The model of the trough collector was evaluated in two positions: the 90° position (mirrors facing upward) and the 0° position (mirrors facing horizontally). The slope errors of the mirror facet reflective surfaces were found by evaluating simulated angular displacements of node-connected segments along the mirror surface. The ideal (undeformed) shape of the mirror was compared to the shape of the deformed mirror after gravity loading. Also, slope errors were obtained by comparing the deformed shapes between the 90° and 0° positions. The slope errors resulting from comparison between the deformed vs. undeformed shape were as high as ∼2 mrad, depending on the location of the mirror facet on the collector. The slope errors resulting from a change in orientation of the trough from the 90° position to the 0° position with gravity loading were as high as ∼3 mrad, depending on the location of the facet. © 2010 by ASME.

Understanding the effects of gravity and wind loads on concentrating solar power (CSP) collectors is critical for performance calculations and developing more accurate alignment procedures and techniques. This paper presents a rigorous finite-element model of a parabolic trough collector that is used to determine the impact of gravity loads on bending and displacements of the mirror facets and support structure. The geometry of the LUZ LS-2 parabolic trough collector was modeled using SolidWorks, and gravity-induced loading and displacements were simulated in SolidWorks Simulation. The model of the trough collector was evaluated in two positions: the 90{sup o} position (mirrors facing upward) and the 0{sup o} position (mirrors facing horizontally). The slope errors of the mirror facet reflective surfaces were found by evaluating simulated angular displacements of node-connected segments along the mirror surface. The ideal (undeformed) shape of the mirror was compared to the shape of the deformed mirror after gravity loading. Also, slope errors were obtained by comparing the deformed shapes between the 90{sup o} and 0{sup o} positions. The slope errors resulting from comparison between the deformed vs. undeformed shape were as high as {approx}2 mrad, depending on the location of the mirror facet on the collector. The slope errors resulting from a change in orientation of the trough from the 90{sup o} position to the 0{sup o} position with gravity loading were as high as {approx}3 mrad, depending on the location of the facet.