Publications

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A set of on-sun experiments was performed on a 1 MWth cavity-type falling particle receiver at Sandia National Laboratories. A computational model of the receiver was developed to evaluate its ability to predict the receiver performance during these experiments and to quantify the thermal losses from different mechanisms. Mean particle outlet temperatures and the experimental receiver thermal efficiencies were compared against values computed in the computational model. External winds during the experiments were found to significantly affect the receiver thermal efficiency, and advective losses from hot air escaping the receiver domain were found to be the most significant contribution to losses from the receiver. Losses from all other mechanisms including radiative losses amounted to less than 10% of the total incident thermal power.

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Falling particle receivers are an emerging technology for use in concentrating solar power systems. In this work, quartz tubes cut in half to form tube shells (referred to as quartz half-shells) are investigated for use as a full or partial aperture cover to reduce radiative and advective losses from the receiver. A receiver subdomain and surrounding air volume are modeled using ANSYS® Fluent®. The model is used to simulate fluid dynamics and heat transfer for the following cases: (1) open aperture, (2) aperture fully covered by quartz half-shells, and (3) aperture partially covered by quartz half-shells. 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 out of the aperture. Contrary to expected outcomes, simulation results using the simplified receiver subdomain show that quartz aperture covers can increase radiative losses and, in the partially covered case, also increase advective losses. These increased heat losses are driven by elevated quartz half-shell temperatures and have the potential to be mitigated by active cooling and/or material selection.

A computational fluid dynamics model of a 50 MWe falling particle receiver has been developed to evaluate the ability of the receiver concept to scale to intermediate sized systems while maintaining high thermal efficiencies. A compatible heliostat field for the receiver was generated using NREL's SolarPILOT, and this field was used to calculate the irradiance on the receiver at seventeen different dates and times throughout the year. The thermal efficiency of the receiver was evaluated at these seventeen different samples using the CFD model and found to vary from 83.0 - 86.8%. An annualized thermal efficiency was calculated from the samples to be 85.7%. A table was also generated that summarized this study along with other similar CFD studies on falling particle receivers over a wide ranges of scales.

A computational fluid dynamics model of a 50 MWe falling particle receiver has been developed to evaluate the ability of the receiver concept to scale to intermediate sized systems while maintaining high thermal efficiencies. A compatible heliostat field for the receiver was generated using NREL's SolarPILOT, and this field was used to calculate the irradiance on the receiver at seventeen different dates and times throughout the year. The thermal efficiency of the receiver was evaluated at these seventeen different samples using the CFD model and found to vary from 83.0 - 86.8%. An annualized thermal efficiency was calculated from the samples to be 85.7%. A table was also generated that summarized this study along with other similar CFD studies on falling particle receivers over a wide ranges of scales.

A thorough code verification effort has been performed on a reduced order, finite element model for one-dimensional (1D) fluid flow convectively coupled with a three-dimensional (3D) solid, referred to as the “advective bar” model. The purpose of this effort was to provide confidence in the proper implementation of this model within the sierra/aria thermal response code at Sandia National Laboratories. Furthermore the method of manufactured solutions (MMS) is applied so that the order of convergence in error norms for successively refined meshes and timesteps is investigated. Potential pitfalls that can lead to a premature evaluation of the model's implementation are described for this verification approach when applied to this unique model. Through observation of the expected order of convergence, these verification tests provide evidence of proper implementation of the model within the codebase.

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This report summarizes the results of a two-year project funded by the U.S. Department of Energy's Solar Energy Technologies Office (SuNLaMP 1506) to evaluate the performance of high-temperature (>700 °C) particle receivers for concentrating solar power (see Appendix A for project information). In the first year, novel particle release patterns were designed and tested to increase the effective solar absorptance of the particle curtain. Modeling results showed that increasing the magnitude and frequency of different wave-like patterns increased the effective absorptance and thermal efficiency by several percentage points, depending on the mass flow rate. Tests showed that triangular-wave, square-wave, and parallel-curtain particle release patterns could be implemented and maintained at flow rates of ~10 kg/s/m. The second year of the project focused on the development and testing of particle mass-flow control and measurement methods. An automated slide gate controlled by the outlet temperature of the particles was designed and tested. Testing demonstrated that the resolution accuracy of the slide-gate positioning was less than ~1 mm, and the speed of the slide gate enabled rapid adjustments to accommodate changes in the irradiance to maintain a desired outlet temperature range. Different in-situ particle mass-flow measurement techniques were investigated, and two were tested. The in-situ microwave sensor was found to be unreliable and sensitive to variations in particle flow patterns. However, the in-situ weigh hopper using load cells was found to provide reliable and repeatable measurements of real-time in-situ particle mass flow. On-sun tests were performed to determine the thermal efficiency of the receiver as a function of mass flow rate, particle temperature, and irradiance. Models of the tests were also developed and compared to the tests.

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Novel particle release patterns have been proposed as a means to increase the thermal efficiency of high-temperature falling particle receivers. Innovative release patterns offer the ability to utilize light-trapping and volumetric heating effects as a means to increase particle temperatures over a conventional straight-line particle release pattern. The particle release patterns explored in this work include wave-like patterns and a series of parallel curtains normal to the incident irradiation that have shown favorable results in previous numerical studies at lower particle temperatures. A numerical model has recently been developed of an existing falling particle receiver at the National Solar Thermal Test Facility at Sandia National Laboratories to evaluate these patterns at elevated temperatures necessary to evaluate radiative and convective losses. This model has demonstrated that thermal efficiency gains of 2.5-4.6% could be realized using these patterns compared to the conventional planar release depending on the particle mass flow rate. Increasing the number of parallel curtains, increasing the spacing between curtains, and shifting the particle mass flow rate deeper in the receiver cavity was also found to increase the thermal efficiency. These effects became less significant as the particle mass flow rate increased.

A 10 MWe north-facing falling particle receiver (FPR) is proposed in this document to support performance comparisons of this design when compared with a direct s-CO₂ solar receiver concept. This document describes the modeling and simulation effort for the proposed FPR to evaluate its thermal performance. A description of the modeling strategy is provided in the following section including details on the receiver and heliostat field. Then, this model is used to evaluate the performance of the receiver at various times of throughout the year. Finally, the results of this analysis are summarized. Direct comparisons with a similarly sized s-CO₂ solar receiver concept are not discussed here.

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A verification effort was undertaken to evaluate the implementation of the new advective bar capability in the Aria thermal response code. Several approaches to the verification process were taken : a mesh refinement study to demonstrate solution convergence in the fluid and the solid, visually examining the mapping of the advective bar element nodes to the surrounding surfaces, and a comparison of solutions produced using the advective bars for simple geometries with solutions from commercial CFD software . The mesh refinement study has shown solution convergence for simple pipe flow in both temperature and velocity . Guidelines were provided to achieve appropriate meshes between the advective bar elements and the surrounding volume. Simulations of pipe flow using advective bars elements in Aria have been compared to simulations using the commercial CFD software ANSYS Fluent (r) and provided comparable solutions in temperature and velocity supporting proper implementation of the new capability. Verification of Advective Bar Elements iv Acknowledgements A special thanks goes to Dean Dobranich for his guidance and expertise through all stages of this effort . His advice and feedback was instrumental to its completion. Thanks also goes to Sam Subia and Tolu Okusanya for helping to plan many of the verification activities performed in this document. Thank you to Sam, Justin Lamb and Victor Brunini for their assistance in resolving issues encountered with running the advective bar element model. Finally, thanks goes to Dean, Sam, and Adam Hetzler for reviewing the document and providing very valuable comments.

This paper evaluates novel particle release patterns for high-temperature falling particle receivers. Spatial release patterns resembling triangular and square waves are investigated and compared to the conventional straight-line particle release. A design of experiments was developed, and a simulation matrix was developed that investigated three twolevel factors: amplitude, wavelength, and wave type. Results show that the wave-like patterns increased both the particle temperature rise and thermal efficiency of the receiver relative to the straight-line particle release. Larger amplitudes and smaller wavelengths increased the performance by creating a volumetric heating effect that increased light absorption and reduced heat loss. Experiments are also being designed to investigate the hydraulic and thermal performance of these new particle release configurations.