Publications

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CSP Dish systems are parabolic mirror structures that track the sun in two axes, focusing the Direct Normal insolation (DNI) to a point or spot on a boom or tripod mounted to the tracking dish structure. This focused light is typically utilized in-situ to operate a heat engine, such as a Stirling cycle, Brayton cycle, or Rankine cycle engine to make electricity. Other dish systems have been used to generate steam for a centralized engine fed by multiple dishes, or to operate thermochemical processes for industrial use, storage, or creation of fuels. Because the dish is always pointing at the sun, a well-designed dish system has a very high concentration ratio, allowing the generation of high temperatures, leading to high thermodynamic efficiencies.

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Parabolic dish technology, for concentrating solar power (CSP) applications, has been continuously modified and improved since the pioneering work in the 1970s. Best practice dishes now have features such as lightweight structure, balanced design, high-quality, low-cost mirror panels, and can be deployed rapidly with little in-field labour. This review focuses on the evolution of dish design, by examining features such as mode of tracking, structure and mirror design, for a wide selection of CSP dish examples. The review includes a brief summary of power generation options – both on-dish and central plant – as well as a discussion about options for storage and hybridisation.

Sandia National Laboratories and Heat Pipes Laboratory of National Technical University of Ukraine «KPI» have developed several methods of improving robustness of the high-temperature heat pipe wick for their application to concentrating solar power systems with Stirling engine. In this case the wick structures must retain high heat pipe performance with robustness for long-term operation. Recent modeling indicates that wicks based on various fiber combinations could provide the robustness combined with sufficient performance. Results of the development, characterization, modeling, and testing of advanced felt metal wicks that addresses durability issues while maintaining sufficient performance are showed in the paper. The project resulted in an ongoing durability bench-scale heat pipe that simulates wick load conditions required for 80kWth throughput solar receiver, including periodic non-destructive evaluation of the wick durability. Two mock-ups of high-temperature heat pipes were made for long life and performance tests: the first sample with wick based on 12 μm fibers; and the second with hybrid wick based on 30 μm fibers and 6.5 μm fibers. The second heat pipe operation has continued unattended at 775 °C vapor temperature for nearly 13,600 h, with no observable loss of performance or change in startup characteristics.

Sodium as a heat transfer fluid (HTF) can achieve temperatures above 700°C to improve power cycle performance for reducing large infrastructure costs of high-temperature systems. Current concentrating solar power (CSP) sensible HTF’s (e.g. air, salts) have poor thermal conductivity, and thus low heat transfer capabilities, requiring a large receiver. The high thermal conductivity of sodium has demonstrated high heat transfer rates on dish and towers systems, which allow a reduction in receiver area by a factor of two to four, reducing re-radiation and convection losses and cost by a similar factor. Sodium produces saturated vapor at pressures suitable for transport starting at 600°C and reaches one atmosphere at 870°C, providing a wide range of suitable latent operating conditions that match proposed high temperature, isothermal input power cycles. This advantage could increase the receiver and system efficiency while lowering the cost of CSP tower systems. Although there are a number of desirable thermal performance advantages associated with sodium, its propensity to rapidly oxidize presents safety challenges. This investigation presents a literature review that captures historical operations/handling lessons for advanced sodium systems, and the current state-of-knowledge related to sodium combustion behavior. Technical and operational solutions addressing sodium safety and applications in CSP will be discussed, including unique safety hazards and advantages using latent sodium. Operation and maintenance experience from the nuclear industry with sensible and latent systems will also be discussed in the context of safety challenges and risk mitigation solutions.

Novel designs to increase light trapping and thermal efficiency of concentrating solar receivers at multiple length scales have been conceived, designed, and tested. The fractal-like geometries and features are introduced at both macro (meters) and meso (millimeters to centimeters) scales. Advantages include increased solar absorptance, reduced thermal emittance, and increased thermal efficiency. Radial and linear structures at the 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 can reduce thermal emittance due to reduced local view factors to the environment, and higher concentration ratios can be employed with similar surface irradiances to reduce the effective optical aperture, footprint, and thermal losses. Coupled optical/fluid/thermal models have been developed to evaluate the performance of these designs relative to conventional designs. Modeling results showed that fractal-like structures and geometries can increase the effective solar absorptance by 5 – 20% and the thermal efficiency by several percentage points at both the meso and macro scales, depending on factors such as intrinsic absorptance. Meso-scale prototypes were fabricated using additive manufacturing techniques, and a macro-scale bladed receiver design was fabricated using Inconel 625 tubes. On-sun tests were performed using the solar furnace and solar tower at the National Solar Thermal Test facility. The test results demonstrated enhanced solar absorptance and thermal efficiency of the fractal-like designs.

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Dish Stirling energy systems have been developed for distributed and large-scale utility deployment. This report summarizes the state of the technology in a joint project between Stirling Energy Systems, Sandia National Laboratories, and the Department of Energy in 2011. It then lays out a feasible path to large scale deployment, including development needs and anticipated cost reduction paths that will make a viable deployment product.

The systematic development of microstructure, solidification characteristics, and heat of solidification with composition in copper-silicon-magnesium alloys for thermal energy storage is presented. Differential scanning calorimetry was used to relate the thermal characteristics to microstructural development in the investigated alloys and clarifies the location of one of the terminal three-phase eutectics. Repeated thermal cycling highlights the thermal storage stability of the transformation through multiple melting events. In conclusion, two near-terminal eutectic alloys display high enthalpies of solidification, relatively narrow melting ranges, and stable transformation hysteresis behaviors suited to thermal energy storage.

Sodium heat pipes have been identified as a potentially effective heat transport approach for CSP systems that require near-isothermal input to power cycles or storage, such as dish Stirling and highly recuperated reheat-cycle supercritical CO2 turbines. Heat pipes offer high heat flux capabilities, leading to small receivers, as well as low exergetic losses through isothermal coupling with the engine. Sandia developed a felt metal wick approach in the 1990's, and demonstrated very high performance1. However, multiple durability issues arose, primarily the structural collapse of the wick at temperature over short time periods. NTUU developed several methods of improving robustness of the wick2, but the resulting wick had limited performance capabilities. For application to CSP systems, the wick structures must retain high heat pipe performance with robustness for long term operation. In this paper we present our findings in developing an optimal balance between performance and ruggedness, including operation of a laboratory-scale heat pipe for over 5500 hours so far. Application of heat pipes to dish-Stirling systems has been shown to increase performance as much as 20%3, and application to supercritical CO2 systems has been proposed.

Sodium Heat transfer fluids (HTF) such as sodium, can achieve temperatures above 700°C to obtain power cycle performance improvements for reducing large infrastructure costs of high-temperature systems. Current concentrating solar power (CSP) sensible HTF's (e.g. air, salts) have poor thermal conductivity, and thus low heat transfer capabilities, requiring a large receiver. The high thermal conductivity of sodium has demonstrated high heat transfer rates on dish and towers systems, which allow a reduction in receiver area by a factor of two to four, reducing re-radiation and convection losses and cost by a similar factor. Sodium produces saturated vapor at pressures suitable for transport starting at 600°C and reaches one atmosphere at 870°C, providing a wide range of suitable operating conditions that match proposed high temperature, isothermal power cycles. This advantage could increase the efficiency while lowering the cost of CSP tower systems. Although there are a number of desirable thermal performance advantages associated with sensible sodium, its propensity to rapidly oxidize presents safety challenges. This investigation presents a literature review that captures historical operations/handling lessons for advanced sodium receiver designs, and the current state-of-knowledge related to sodium combustion behavior. Technical and operational solutions addressing sodium safety and applications in CSP will be discussed, including unique safety hazards and advantages using latent sodium. Lessons obtained from the nuclear industry with sensible and latent systems will also be discussed in the context of safety challenges and risk mitigation solutions.

Novel designs to increase light trapping and thermal efficiency of concentrating solar receivers at multiple length scales have been conceived and tested. The fractal-like geometries and features are introduced at both macro (meters) and meso (millimeters to centimeters) scales. Advantages include increased solar absorptance, reduced thermal emittance, and increased thermal efficiency. Radial and linear structures at the 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 can reduce thermal emittance due to reduced local view factors to the environment, and higher concentration ratios can be employed with similar surface irradiances to reduce the effective optical aperture, footprint, and thermal losses. Coupled optical/fluid/thermal models have been developed to evaluate the performance of these designs relative to conventional designs, and meso-scale tests have been performed. Results show that fractal-like structures and geometries can increase the thermal efficiency by several percentage points at both the meso and macro scales, depending on factors such as intrinsic absorptance. The impact was more pronounced for materials with lower intrinsic solar absorptances (<0.9). The goal of this work is to increase the effective solar absorptance of oxidized substrate materials from ∼0.9 to 0.95 or greater using these fractal-like geometries without the need for coatings.

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Direct solar power receivers consist of tubular arrays, or panels, which are typically tubes arranged side by side and connected to an inlet and outlet manifold. The tubes absorb the heat incident on the surface and transfer it to the fluid contained inside them. To increase the solar absorptance, high temperature black paint or a solar selective coating is applied to the surface of the tubes. However, current solar selective coatings degrade over the lifetime of the receiver and must be reapplied, which reduces the receiver thermal efficiency and increases the maintenance costs. This work presents an evaluation of several novel receiver shapes which have been denominated as fractal like geometries (FLGs). The FLGs are geometries that create a light-trapping effect, thus, increasing the effective solar absorptance and potentially increasing the thermal efficiency of the receiver. Five FLG prototypes were fabricated out of Inconel 718 and tested in Sandia's solar furnace at two irradiance levels of ∼15 and 30 W/cm2 and two fluid flow rates. Photographic methods were used to capture the irradiance distribution on the receiver surfaces and compared to results from ray-tracing models. This methods provided the irradiance distribution and the thermal input on the FLGs. Air at nearly atmospheric pressure was used as heat transfer fluid. The air inlet and outlet temperatures were recorded, using a data acquisition system, until steady state was achieved. Computational fluid dynamics (CFD) models, using the Discrete Ordinates (DO) radiation and the k-? Shear Stress Transport (SST) equations, were developed and calibrated, using the test data, to predict the performance of the five FLGs at different air flow rates and irradiance levels. The results showed that relative to a flat plate (base case), the new FLGs exhibited an increase in the effective solar absorptance from 0.86 to 0.92 for an intrinsic material absorptance of 0.86. Peak surface temperatures of ∼1000°C and maximum air temperature increases of ∼200°C were observed. Compared to the base case, the new FLGs showed a clear air outlet temperature increase. Thermal efficiency increases of ∼15%, with respect to the base case, were observed. Several tests, in different days, were performed to assess the repeatability of the results. The results obtained, so far, are very encouraging and display a very strong potential for incorporation in future solar power receivers.

In this study, the performance of Y₂O₃-stabilized ZrO₂ (YSZ), Y₂O₃, and Al₂O₃ plasma sprayed coatings are investigated for their ability to prevent attack of Haynes 230 by a near-eutectic Cu-Mg-Si metallic phase change material (PCM) in a closed environment at 820 °C. Areas where coatings failed were identified with optical and scanning electron microscopy, while chemical interactions were clarified through elemental mapping using electron microprobe analysis. Despite its susceptibility to reduction by Mg, the Al₂O₃ coating performed well while the YSZ and Y₂O₃ coating showed clear areas of attack. These results are attributed to the evolution of gaseous Mg at 820 °C leading to the formation of MgO and MgAl₂O₄.

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This is a summary of the performance objectives and outcomes for fiscal year 2015, quarter 4.

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 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.

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.

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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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Reflective mirror facets for concentrating solar power (CSP) systems have stringent requirements on the surface slope accuracy in order to provide adequate system performance. This paper presents a tool that can fully characterize facets quickly enough for 100% inspection on a production line. A facet for a CSP system, specifically a dish concentrator has a parabolic design shape. This shape will concentrate near-parallel rays from the sun to a point (or a line for trough systems). Deviations of surface slope from the design shape impact the performance of the system, either losing power that misses the target or increasing peak fluxes to undesirable levels. During development or production, accurate knowledge of facet defects can lead to improvements to lower cost or improve performance. The reported characterization system, SOFAST (Sandia Optical Fringe Analysis Slope Tool), has a computer-connected camera that images the reflective surface, which is positioned so that it reflects an active target, such as an LCD screen, to the camera. A series of fringe patterns are displayed on the screen while images are captured. Using the captured information, the reflected target location of each pixel of mirror viewed can be determined, and thus through a mathematical transformation, a surface normal map can be developed. This is then fitted to the selected model equation, and the errors from design are characterized. While similar approaches have been explored, several key developments are presented here. The combination of the display, capture, and data reduction in one system allows rapid characterization. An “electronic boresight” approach is utilized to accommodate physical equipment positioning deviations, making the system insensitive to setup errors. Up to 1.5×10⁶ points are characterized on each facet. Finally, while prior automotive industry commercial systems resolve the data to shape determination, SOFAST concentrates on slope characterization and reporting, which is tailored to solar applications. SOFAST can be used for facet analysis during development. However, the real payoff is in production, where complete analysis is performed in about 10s. With optimized coding, this could be further reduced.

The Concentrating Solar Technologies Organization at Sandia National Laboratories has a long history of performing important research, development, and testing that has enabled the Concentrating Solar Power Industry to deploy full-scale power plants. Sandia continues to pursue innovative CSP concepts with the goal of reducing the cost of CSP while improving efficiency and performance. In this pursuit, Sandia has developed many tools for the analysis of CSP performance. The following capabilities document highlights Sandia's extensive experience in the design, construction, and utilization of large-scale testing facilities for CSP and the tools that Sandia has created for the full characterization of heliostats. Sandia has extensive experience in using these tools to evaluate the performance of novel heliostat designs.

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Dish concentrators can produce highly concentrated flux for the operation of an engine, a chemical process, or other energy converter. The high concentration allows a small aperture to control thermal losses, and permits high temperature processes at the focal point. A variety of optical errors can influence the flux pattern both at the aperture and at the absorber surface. Impacts of these errors can be lost energy (intercept losses), aperture compromise (increased size to accommodate flux), high peak fluxes (leading to part failure or life reduction), and improperly positioned flux also leading to component failure. Optical errors can include small scale facet errors ("waviness"), facet shape errors, alignment (facet pointing) errors, structural deflections, and tracking errors. The errors may be random in nature, or may be systematic. The various sources of errors are often combined in a "root-mean-squared" process to present a single number as an "error budget". However, this approach ignores the fact that various errors can influence the performance in different ways, and can mislead the designer, leading to component damage in a system or poor system performance. In this paper, we model a hypothetical radial gore dish system using Sandia's CIRCE2 optical code. We evaluate the peak flux and incident power through the aperture and onto various parts of the receiver cavity. We explore the impact of different error sources on the character of the flux pattern, and demonstrate the limitations of lumping all of the errors into a single error budget. © 2010 by ASME.

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Charles Andraka from Sandia Labs was given the Hoyt Clarke Hottel Award for his lifelong work in the concentrating solar power industry.

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Concentrating Solar Power (CSP) dish systems use a parabolic dish to concentrate sunlight, providing heat for a thermodynamic cycle to generate shaft power and ultimately, electricity. Currently, leading contenders use a Stirling cycle engine with a heat absorber surface at about 800°C. The concentrated light passes through an aperture, which controls the thermal losses of the receiver system. Similar systems may use the concentrated light to heat a thermochemical process. The concentrator system, typically steel and glass, provides a source of fuel over the service life of the system, but this source of fuel manifests as a capital cost up front. Therefore, it is imperative that the cost of the reflector assembly is minimized. However, dish systems typically concentrate light to a peak of as much as 13,000 suns, with an average geometric concentration ratio of over 3000 suns. Several recent dish-Stirling systems have incorporated reflector facets with a normally-distributed surface slope error (local distributed waviness) of 0.8 mrad RMS (1-sigma error). As systems move toward commercialization, the cost of these highly accurate facets must be assessed. However, when considering lower-cost options, any decrease in the performance of the facets must be considered in the evaluation of such facets. In this paper, I investigate the impact of randomly-distributed slope errors on the performance, and therefore the value, of a typical dish-Stirling system. There are many potential sources of error in a concentrating system. When considering facet options, the surface waviness, characterized as a normally-distributed slope error, has the greatest impact on the aperture size and therefore the thermal losses. I develop an optical model and a thermal model for the performance of a baseline system. I then analyze the impact on system performance for a range of mirror quality, and evaluate the impact of such performance changes on the economic value of the system. This approach can be used to guide the evaluation of low-cost facets that differ in performance and cost. The methodology and results are applicable to other point- and line-focus thermal systems including dish-Brayton, dish-Thermochemical, tower systems, and troughs. Copyright © 2008 by ASME.

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A Dish Stirling parabolic concentrator typically consists of a number of mirror facets that must be aligned to focus the concentrated sunlight on the engine receiver. An alignment strategy must be developed to deliver the energy uniformly to the receiver while maximizing system performance. Several criteria must be met in order to maximize the performance and lifetime of the system. The peak flux should be minimized at the receiver to extend life. This is accomplished by locally optimizing the mirror aimpoints, minimizing overlap of facet images. The energy delivered to each cylinder of a multi-cylinder engine should be balanced to maximize the power production capability of the engine. This is accomplished through globally optimizing the mirror aimpoints. Depending on dish geometry, both of these constraints will be met by moving the aimpoints of certain facets away from a single point at the center of the aperture. However, this often results in a larger aperture or more flux spillage. The larger aperture results in greater thermal and reflective losses from the receiver cavity. This paper proposes and demonstrates a novel approach to optimizing the alignment strategy while obeying these constraints. The method uses an approach similar to molecular dynamics to globally and locally distribute the power on the receiver, while imposing movement constraints at the aperture to limit the focal plane spot size. The method can also impose additional geometric constraints at the receiver plane to accommodate un-cooled surfaces. The method is explored and demonstrated on the Stirling Energy Systems 25kW dish Stirling system at Sandia National Laboratories. The approach provides a receiver flux distribution and power balance equal to the strategy developed by McDonnell Douglas in the early 1980's, but with an aperture size equal to that of the single aimpoint strategy. This should result in about a 1kW increase in power generated at rated conditions, with no additional cost, due to reduced thermal losses from the receiver. The method can be extended to other point-focus concentrating solar technologies. On a tower, the heliostat aiming strategy could be dynamically updated to accommodate flux profile needs, sun position, or maintenance in the field. Copyright © 2007 by US Government.

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The integration and approaches utilized in the various stages of the Advanced Dish Development System (ADDS) project are presented and described. Insights gained from integration of the ADDS are also discussed. The ADDS project focuses on development of a product that meets the needs of the remote power market and helps to identify key technology development needs that resulted in a system that is closer to commercialization. A pursuance of solving problems, a lack of fear of breaking things, and hands-on involvement by design engineers are the key components leading to rapid improvement of the project.

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The development of liquid metal heat-pipes for use in solar powered Stirling engines has led to an in-depth analysis of heat-pipe wick properties. To model the flow of liquid sodium through the wick its two-phase permeability measurement is of interest. The permeability will be measured by constructing a test cell made up of a wick sample sintered to a manifold. Measuring the volumetric flow rate through the wick will allow for a determination of the wick's permeability as a function of pressure. Currently, simple estimates of permeability as a function of vapor fraction of a porous media are being used as a model to calculate the two-phase permeability. The above mentioned experiment will be used to test the existing formulas validity. The plan is to make use of a known procedure for testing permeability and apply those techniques to a felt-metal wick. The results will be used to verify and/or modify the two-phase permeability estimates. With the increasing desire to replace directly illuminated engines with the much more efficient heat-pipe apparatus it is inherently clear that the usefulness of known wick properties will make wick permeability design a simpler process.

Stirling-cycle engines have been identified as a promising technology for the conversion of concentrated solar energy into usable electrical power. In previous experimented work, we have demonstrated that a heat pipe receiver can significantly improve system performance-over a directly-illuminated heater head. The design and operating conditions of a heat pipe receiver differ significantly from typical laboratory heat pipes. New wick structures have been developed to exploit the characteristics of the solar generation system. Typically, these wick structures allow vapor generation within the wick. Conventional heat pipe models do not handle this enhancement yet it can more than double the performance of the wick. In this study, I develop a steady-state model of a boiling-enhanced wick for a solar heat pipe receiver. The model is used for design-point calculations and is written in FORTRAN90. Some limited comparisons have been made with actual test data.

The thermal conduction of a portion of an enhanced surface heat exchanger for a gas fired heat pipe solar receiver was modeled using the boundary element and finite element methods (BEM and FEM) to determine the effect of weld fillet size on performance of a stud welded pin fin. A process that could be utilized by others for designing the surface mesh on an object of interest, performing a conversion from the mesh into the input format utilized by the BEM code, obtaining output on the surface of the object, and displaying visual results was developed. It was determined that the weld fillet on the pin fin significantly enhanced the heat performance, improving the operating margin of the heat exchanger. The performance of the BEM program on the pin fin was measured (as computational time) and used as a performance comparison with the FEM model. Given similar surface element densities, the BEM method took longer to get a solution than the FEM method. The FEM method creates a sparse matrix that scales in storage and computation as the number of nodes (N), whereas the BEM method scales as N{sup 2} in storage and N{sup 3} in computation.

Stirling-cycle engines have been identified as a promising technology for the conversion of concentrated solar energy into usable electrical power. A 25kW electric system takes advantage of existing Stirling-cycle engines and existing parabolic concentrator designs. In previous work, the concentrated sunlight impinged directly on the heater head tubes of the Stirling Thermal Motors (STM) 4-120 engine. A Sandia-designed felt-metal-wick heat pipe receiver was fitted to the STM 4-120 engine for on-sun testing on Sandia`s Test Bed Solar Concentrator. The heat pipe uses sodium metal as an intermediate two-phase heat transfer fluid. The receiver replaces the directly-illuminated heater head previously tested. The heat pipe receiver provides heat isothermally to the engine, and the heater head tube length is reduced, both resulting in improved engine performance. The receiver also has less thermal losses than the tube receiver. The heat pipe receiver design is based on Sandia`s second-generation felt-wick heat pipe receiver. This paper presents the interface design, and compares the heat pipe/engine test results to those of the directly-illuminated receiver/engine package.

Reflux heat-pipe receivers have been identified as a desirable interface to couple a Stirling-cycle engine with a parabolic dish solar concentrator. The reflux receiver provides power nearly isothermally to the engine heater heads while decoupling the heater head design from the solar absorber surface design. The independent design of the receiver and engine heater head leads to higher system efficiency. Heat pipe reflux receivers have been demonstrated at approximately 65 kW{sub t} power throughput. Several 25 to 30-kW{sub e} Stirling-cycle engines are under development, and will soon be incorporated in commercial dish-Stirling systems. These engines will require reflux receivers with power throughput limits reaching 90-kW{sub t}. The extension of heat pipe technology from 60 kW{sub t} to 100 kW{sub t} is not trivial. Current heat pipe wick technology is pushed to its limits. It is necessary to develop and test advanced wick structure technologies to perform this task. Sandia has developed and begun testing a Bekaert Corporation felt metal wick structure fabricated by Porous Metal Products Inc. This wick is about 95% porous, and has liquid permeability a factor of 2 to 8 times higher than conventional technologies for a given maximum pore radius. The wick has been successfully demonstrated in a bench-scale heat pipe, and a full-scale on-sun receiver has been fabricated. This report details the wick design, characterization and installation into a heat pipe receiver, and the results of the bench-scale tests are presented. The wick performance is modeled, and the model results are compared to test results.

Pool-boiler reflux receivers have been considered as an alternative to heat pipes for the input of concentrated solar energy to Stirling-cycle engines in dish-Stirling electric generation systems. Pool boilers offer simplicity in design and fabrication. The operation of a full-scale pool-boiler receiver has been demonstrated for short periods of time. However, to generate cost-effective electricity, the receiver must operate without significant maintenance for the entire system life, as much as 20 to 30 years. Long-term liquid-metal boiling stability and materials compatibility with refluxing NaK-78 is not known and must be determined for the pool boiler receiver. No boiling system has been demonstrated for a significant duration with the current porous boiling enhancement surface and materials. At least one theory explaining lncipientboiling behavior of alkali metals indicates that favorable start-up behavior should deteriorate over time. Many factors affect the stability and startup behavior of the boiling system. Therefore, it is necessary to simulate the full-scale pool boiler design as much as possible, including flux levels, materials, and operating cycles. On-sun testing is impractical because of the limited test time available. A test vessel was constructed with a Friction Coatings Inc, porous boiling enhancement surface. The boiling surface consisted of a brazed stainless steel powder with about 50% porosity. The vessel was heated with a quartz lamp array providing about 90 Wlcm2 peak incident thermal flux. The vessel was charged with NaK-78, which is liquid at room temperature. This allows the elimination of costly electric preheating, both on this test and on fullscale receivers. The vessel was fabricated from Haynes 230 alloy, selected for its high temperature strength and oxidation resistance. The vessel operated at 750°C around the clock, with a 112-hour shutdown cycle to ambient every 8 hours. Temperature data was continually collected. The test completed 7500 hours of lamp-on operation time, and over 1000 startups from ambient. The test was terminated when a small leak in an lnconel 600 thermowell was detected. The test design and data are presented here. Metallurgical analysis of virgin and tested materials has begun, and initial results are also presented.

Alkali metal heat-pipe receivers have been identified as a desirable interface to couple a Stirling-cycle engine with a parabolic dish solar concentrator. The reflux receiver provides power nearly isothermally to the engine heater heads while de-coupling the heater head design from the solar absorber surface design. The independent design of the receiver and engine heater head leads to high system efficiency. Heat pipe reflux receivers have been demonstrated at approximately 30 kW{sub t} power throughput by others. This size is suitable fm engine output powers up to 10 kW{sub e}. Several 25-kW{sub e}, Stirling-cycle engines exist, as well as designs for 75-kW{sub t} parabolic dish solar concentrators. The extension of heat pipe technology from 30 kW{sub t} to 75 kW{sub t} is not trivial. Heat pipe designs are pushed to their limits, and it is critical to understand the flux profiles expected from the dish, and the local performance of the wick structure. Sandia has developed instrumentation to monitor and control the operation of heat pipe reflux receivers to test their throughput limits, and analytical models to evaluate receiver designs. In the past 1.5 years, several heat pipe receivers have been tested on Sandia`s test bed concentrators (TBC`s) and 60-kW{sub t} solar furnace. A screen-wick heat pipe developed by Dynatherm was tested to 27.5 kW{sub t} throughput. A Cummins Power Generation (CPG)/Thermacore 30-kW{sub t} heat pipe was pushed to a throughput of 41 kW{sub t} to verify design models. A Sandia-design screen-wick and artery 75-kW{sub t} heat pipe and a CPG/Thermacore 75-kW{sub t} sintered-wick heat pipe were also limit tested on the TBC. This report reviews the design of these receivers, and compares test results with model predictions.

Pool-boiler reflux receivers have been considered as an alternative to heat pipes for the input of concentrated solar energy to Stirling-cycle engines in dish-Stirling electric generation systems. Fool boilers offer simplicity in desip and fabrication. Pool-boiler solar receiver operation has been demonstrated for short periods of time. However, in order to generate cost-effective electricity, the receiver must operate without significant maintenance for the entire system life. At least one theory explaining incipient-boiling behavior of alkali metals indicates that favorable start-up behavior should deteriorate over time. Many factors affect the stability and startup behavior of the boiling system. Therefore, it is necessary to simulate the full-scale design in every detail as much as possible, including flux levels materials, and operating cycles. On-sun testing is impractical due to the limited test time available. No boiling system has been demonstrated with the current porous boiling enhancement surface and materials for a significant period of time. A test vessel was constructed with a Friction Coatings Inc. porous boiling enhancement surface. The vessel is heated with a quartz lamp array providing about 92 W/Cm{sup 2} peak incident thermal flux. The vessel is charged with NaK-78, which is liquid at room temperature. This allows the elimination of costly electric preheating, both on this test and on full-scale receivers. The vessel is fabricated from Haynes 230 alloy, selected for its high temperature strength and oxidation resistance. The vessel operates at 750{degrees}C around the clock, with a 1/2-hour shutdown cycle to ambient every 8 hours. Temperature data is continually collected. The test design and initial (first 2500 hours and 300 start-ups) test data are presented here. The test is designed to operate for 10,000 hours, and will be complete in the spring of 1994.

Heat-Pipe reflux receivers have been identified as a desirable interface to couple a Stirling engine with a parabolic dish solar concentrator. The reflux receiver provides power uniformly and nearly isothermally to the engine heater heads while de-coupling the heater head design from the solar absorber surface design. Therefore, the heat pipe reflux receiver allows the receiver and heater head to be independently thermally optimized, leading to high receiver thermal transport efficiency. Dynatherm Corporation designed and fabricated a screen-wick heat-pipe receiver for possible application to the Cummins Power Generation, Inc. first-generation 4 kW{sub e} free-piston dish-Stirling system, which required up to 30 kW{sub t}. The receiver features a composite absorber wick and a homogeneous sponge-wick on the aft dome to provide sodium to the absorber during hot restarts. The screen wick is attached to the absorber dome by spot welds. Refluxing troughs collect the condensate in a cylindrical condenser and return it directly to the absorber surface. The receiver was fabricated and lamp tested to 16 kW{sub t} throughput by Dynatherm. The receiver has been tested on Sandia`s 60 kW{sub t} solar furnace to a throughput power of 27.5 kW{sub t} and vapor space temperature up to 780{degrees}C. Infrared thermography was used to monitor the entire absorber dome for impending dryout while the receiver was tested. The receiver was started using solar input, without the assistance of electrical pre-heaters. The power was extracted with a gas-gap cold-water calorimeter to simulate the operation of a Stirling engine. The receiver design, thermal performance analysis, flux distribution analysis, test results, and post-test analysis are presented. 11 refs.

The efficient operation of a Stirling engine requires the application of a high heat flux to the relatively small area occupied by the heater head tubes. Previous attempts to couple solar energy to Stirling engines generally involved directly illuminating the heater head tubes with concentrated sunlight. In this study, operation of a 75-kW{sub t} sodium reflux pool-boiler solar receiver has been demonstrated and its performance characterized on Sandia's nominal 75-kW{sub t} parabolic-dish concentrator, using a cold-water gas-gap calorimeter to simulate Stirling engine operation. The pool boiler (and more generally liquid-metal reflux receivers) supplies heat to the engine in the form of latent heat released from condensation of the metal vapor on the heater head tubes. The advantages of the pool boiler include uniform tube temperature, leading to longer life and higher temperature available to the engine, and decoupling of the design of the solar absorber from the engine heater head. The two-phase system allows high input thermal flux, reducing the receiver size and losses, therefore improving system efficiency. The receiver thermal efficiency was about 90% when operated at full power and 800{degree}C. Stable sodium boiling was promoted by the addition of 35 equally spaced artificial cavities in the wetted absorber surface. High incipient boiling superheats following cloud transients were suppressed passively by the addition of small amounts of xenon gas to the receiver volume. Stable boiling without excessive incipient boiling superheats was observed under all operating conditions. The receiver developed a leak during performance evaluation, terminating the testing after accumulating about 50 hours on sun. The receiver design is reported here along with test results including transient operations, steady-state performance evaluation, operation at various temperatures, infrared thermography, x-ray studies of the boiling behavior, and a postmortem analysis.

The feasibility of competitive, modular bulk electric power from the sun may be greatly enhanced by the use of a reflux heat pipe receiver to combine a heat engine such as Stirling with a paraboloidal dish concentrator. This combination represents a potential improvement over previous successful demonstrations of dish-electric technology in terms of enhanced performance, lower cost, longer life, and greater flexibility in engine design. There are, however, important issues and unknowns which must be addressed to determine engineering feasibility of these devices. In the pool boiler reflux receiver, concentrated solar radiation causes liquid metal (sodium or potassium) to boil. The vapor flows to the engine heater heads, where it condenses and releases the latent heat. The condensate is returned to the receiver absorber pool by gravity (refluxing). This is essentially an adaptation of heat pipe technology to the peculiar requirements of concentrated solar flux, and provides many advantages over conventional heated tube receiver technology. Boiling theory indicates that long-term stable boiling of liquid metal may be difficult to achieve. Laboratory scale experiments have been performed. Initial tests confirmed that boiling is unstable in a baseline boiler. Boiling stability was established after the addition of ''artificial cavities'' to the heated surface, and successful boiling of sodium was demonstrated for 100 hours. Other stabilizing influences may have been present, and will be discussed. The flux and geometry closely simulated a real receiver. The results of these tests are presented, along with the design of a full scale receiver for on-sun testing and considerations for long term operation. 15 refs., 10 figs.