Publications

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The project objectives are to (1) Demonstrate the feasibility of significant thermal storage for dish Stirling systems to leverage their existing high performance to greater capacity; (2) Demonstrate key components of a latent storage and transport system enabling on-dish storage with low exergy losses; and (3) Provide a technology path to a 25kW_e system with 6 hours of storage.

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The goals of the project are to: 1) Demonstrate the feasibility of significant thermal storage for dish Stirling systems to leverage their existing high performance to greater capacity, 2) Demonstrate key components of a latent storage and transport system enabling on-dish storage with low exergy losses, and 3) Provide a technology path to a 25kWe system with 6 hours of storage.

The goals of this project are to: 1) Demonstrate the feasibility of significant thermal storage for dish Stirling systems to leverage their existing high performance to greater capacity, 2) Demonstrate key components of a latent storage and transport system enabling on-dish storage with low exergy losses, and 3) Provide a technology path to a 25kWe system with 6 hours of storage. Innovations associated with this project are: 1) Leverage high performance heat pipes to support feasible system layout, 2) Develop and test high temperature, high performance PCM storage, 3) Optimize storage configuration for cost and exergy performance, and 4) Latent storage and transport matches Stirling cycle isothermal input.

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Concentrating solar power receivers are comprised of panels of tubes arranged in a cylindrical or cubical shape on top of a tower. The tubes contain heat-transfer fluid that absorbs energy from the concentrated sunlight incident on the tubes. To increase the solar absorptance, black paint or a solar selective coating is applied to the surface of the tubes. However, these coatings degrade over time and must be reapplied, which reduces the system performance and increases costs. This paper presents an evaluation of novel receiver shapes and geometries that create a light-trapping effect, thereby increasing the effective solar absorptance and efficiency of the solar receiver. Several prototype shapes were fabricated from Inconel 718 and tested in Sandiaas solar furnace at an irradiance of ∼30 W/cm2. Photographic methods were used to capture the irradiance distribution on the receiver surfaces. The irradiance profiles were compared to results from raytracing models. The effective solar absorptance was also evaluated using the ray-tracing models. Results showed that relative to a flat plate, the new geometries could increase the effective solar absorptance from 86% to 92% for an intrinsic material absorptance of 86%, and from 60% to 73% for an intrinsic material absorptance of 60%.

The goals of this project are to demonstrate the feasibility of significant thermal storage for dish stirling systems to leverage their existing high performance to greater capacity; demonstrate key components of a latent storage and transport system enabling on-dish storage with low energy losses; and provide a technology path to a 25kW_e system with 6 hours of storage.

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The project goals are: demonstrate the feasibility of significant thermal storage for dish Stirling systems to leverage their existing high performance to greater capacity; demonstrate key components of a latent storage and transport system enabling on-dish storage with low exergy losses; and provide technology path to a 25kW_e system with 6 hours of storage.

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Sandia Optical Fringe Analysis Slope Tool (SOFAST) is a mirror facet characterization system based on fringe reflection technology that has been applied to dish and heliostat mirror facet development at Sandia National Laboratories and development partner sites. The tool provides a detailed map of mirror facet surface normals as compared to design and fitted surfaces. In addition, the surface fitting process provides insights into systematic facet slope characterization, such as focal lengths, tilts, and twist of the facet. In this paper, an analysis of the sensitivities of the facet characterization outputs to variations of SOFAST input parameters is presented. The results of the sensitivity analysis provided the basis for a linear uncertainty analysis, which is also included here. Input parameters included hardware parameters and SOFAST setup variables. Output parameters included the fitted shape parameters (focal lengths and twist) and the residuals (typically called slope error). The study utilized empirical propagation of input parameter errors through facet characterization calculations to the output parameters, based on the measurement of an Advanced Dish Development System (ADDS) structural gore point-focus facet. Thus, this study is limited to the characterization of sensitivities of the SOFAST embodiment intended for dish facet characterization, using an LCD screen as a target panel. With reasonably careful setup, SOFAST is demonstrated to provide facet focal length characterization within 0.5% of actual. Facet twist is accurate within ±0.03 mrad/m. The local slope deviation measurement is accurate within ±0.05 mrad, while the global slope residual is accurate within ±0.005 mrad. All uncertainties are quoted with 95% confidence. Copyright © 2014 by ASME.

Solar thermal receivers absorb concentrated sunlight and can operate at high temperatures exceeding 600°C for production of heat and electricity. New fractal-like designs employing light-trapping structures and geometries at multiple length scales are proposed to increase the effective solar absorptance and efficiency of these receivers. Radial and linear structures at the micro (surface coatings and depositions), meso (tube shape and geometry), and macro (total receiver geometry and configuration) scales redirect reflected solar radiation toward the interior of the receiver for increased absorptance. Hotter regions within the interior of the receiver also reduce thermal emittance due to reduced local view factors in the interior regions, and higher concentration ratios can be employed with similar surface irradiances to reduce the effective optical aperture and thermal losses. Coupled optical/fluid/thermal models have been developed to evaluate the performance of these designs relative to conventional designs. Results show that fractal-like structures and geometries can reduce total radiative losses by up to 50% and increase the thermal efficiency by up to 10%. The impact was more pronounced for materials with lower inherent solar absorptances (< 0.9). Meso-scale tests were conducted and confirmed model results that showed increased light-trapping from corrugated surfaces relative to flat surfaces.

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