Publications

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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.

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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 Sandias 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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Improving the thermal performance of a trough plant will lower the LCOE: (1) Improve mirror alignment using the TOPCAT system - Current - increase optical intercept of existing trough solar power plants, Future - allows larger apertures with same receiver size in new trough solar power plants, and Increased concentration ratios/collection efficiencies & economies of scale; and (2) Improve tracking using a closed loop tracking system - Open loop tracking currently used own experience and from industry show need for a improved method. Performance testing of a Trough module and/or receiver on the rotating platform: (1) Installed costs of a trough plant are high. A significant portion of this is the material and assembly cost of the trough module. These costs need to be reduced without sacrificing performance; and (2) New receiver coatings with lower heat loss and higher absorbtivity. TOPCAT system is an optical evaluation tool for parabolic trough solar collectors. Aspects of the TOPCAT system are: (1) Practical, rapid, and cost effective; (2) Inherently aligns mirrors to the receiver of an entire solar collector array (SCA); (3) Can be used for existing installations -no equivalent tool exits; (4) Can be used during production; (5) Currently can be used on LS-2 or LS-3 configurations, but can be easily modified for any configuration; and (6)Generally, one time use.

Several studies predict an economic benefit of using nitrate-based salts instead of the current synthetic oil within a solar parabolic trough field. However, the expected economic benefit can only be realized if the reliability and optical performance of the salt trough system is comparable to today's oil trough. Of primary concern is whether a salt-freeze accident and subsequent thaw will lead to damage of the heat collection elements (HCEs). This topic was investigated by experiments and analytical analysis. Results to date suggest that damage will not occur if the HCEs are not completely filled with salt. However, if the HCE is completely filled at the time of the freeze, the subsequent thaw can lead to plastic deformation and significant bending of the absorber tube.

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Sandia National Laboratories (SNL) is evaluating the potential of an innovative approach for splitting water into hydrogen and oxygen using two-step thermochemical cycles. Thermochemical cycles are heat engines that utilize high-temperature heat to produce chemical work. Like their mechanical work-producing counterparts, their efficiency depends on operating temperature and on the irreversibility of their internal processes. With this in mind, we have invented innovative design concepts for two-step solar-driven thermochemical heat engines based on iron oxide and iron oxide mixed with other metal oxides (ferrites). The design concepts utilize two sets of moving beds of ferrite reactant material in close proximity and moving in opposite directions to overcome a major impediment to achieving high efficiency--thermal recuperation between solids in efficient counter-current arrangements. They also provide inherent separation of the product hydrogen and oxygen and are an excellent match with high-concentration solar flux. However, they also impose unique requirements on the ferrite reactants and materials of construction as well as an understanding of the chemical and cycle thermodynamics. In this report the Counter-Rotating-Ring Receiver/Reactor/Recuperator (CR5) solar thermochemical heat engine and its basic operating principals are described. Preliminary thermal efficiency estimates are presented and discussed. Our ferrite reactant material development activities, thermodynamic studies, test results, and prototype hardware development are also presented.

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The Salt River Project (SRP), in conjunction with Sandia National Laboratories (SNL) and Energy Laboratories, Inc. (ELI), collaborated to develop, test, and evaluate an advanced solar water-heating product for new homes. SRP and SNL collaborated under a Department of Energy Cooperative Research and Development Agreement (CRADA), with ELI as SRP's industry partner. The project has resulted in the design and development of the Roof Integrated Thermal Siphon (RITH) system, an innovative product that features complete roof integration, a storage tank in the back of the collector and below the roofline, easy installation by homebuilders, and a low installed cost. SRP's market research guided the design, and the laboratory tests conducted at SNL provided information used to refine the design of field test units and indicated that the RITH concept is viable. ELI provided design and construction expertise and is currently configured to manufacture the units. This final report for the project provides all of the pertinent and available materials connected to the project including market research studies, the design features and development of the system, and the testing and evaluation conducted at SNL and at a model home test site in Phoenix, Arizona.

Sandia National Laboratories has completed thermal performance testing on the Schott parabolic trough receiver using the LS-2 collector on the Sandia rotating platform at the National Solar Thermal Test Facility in Albuquerque, NM. This testing was funded as part of the US DOE Sun-Lab USA-Trough program. The receiver tested was a new Schott receiver, known as Heat Collector Elements (HCEs). Schott is a new manufacturer of trough HCEs. The Schott HCEs are 4m long; therefore, two were joined and mounted on the LS-2 collector module for the test. The Schott HCE design consists of a 70mm diameter high solar absorptance coated stainless steel (SS) tube encapsulated within a 125mm diameter Pyrex{reg_sign} glass tube with vacuum in the annulus formed between the SS and glass tube to minimize convection heat losses. The Schott HCE design is unique in two regards. First, the bellows used to compensate for the difference in thermal expansion between the metal and glass tube are inside the glass envelope rather than outside. Second, the composition of materials at the glass-to-metal seal has very similar thermal expansion coefficients making the joint less prone to breakage from thermal shock. Sandia National Laboratories provided both the azimuth and elevation collector module tracking systems used during the tests. The test results showed the efficiency of the Schott HCE to be very similar to current HCEs being manufactured by Solel. This testing provided performance verification for the use of Schott tubes with Solargenix trough collector assemblies at currently planned trough power plant projects in Arizona and Nevada.

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The Solar Thermal Program at Sandia supports work developing dish/Stirling systems to convert solar energy into electricity. Heat pipe technology is ideal for transferring the energy of concentrated sunlight from the parabolic dish concentrators to the Stirling engine heat tubes. Heat pipes can absorb the solar energy at non-uniform flux distributions and release this energy to the Stirling engine heater tubes at a very uniform flux distribution thus decoupling the design of the engine heater head from the solar absorber. The most important part of a heat pipe is the wick, which transports the sodium over the heated surface area. Bench scale heat pipes were designed and built to more economically, both in time and money, test different wicks and cleaning procedures. This report covers the building, testing, and post-test analysis of the sixth in a series of bench scale heat pipes. Durability heat pipe No.6 was built and tested to determine the effects of a high temperature bakeout, 950 C, on wick corrosion during long-term operation. Previous tests showed high levels of corrosion with low temperature bakeouts (650-700 C). Durability heat pipe No.5 had a high temperature bakeout and reflux cleaning and showed low levels of wick corrosion after long-term operation. After testing durability heat pipe No.6 for 5,003 hours at an operating temperature of 750 C, it showed low levels of wick corrosion. This test shows a high temperature bakeout alone will significantly reduce wick corrosion without the need for costly and time consuming reflux cleaning.