Alan Michael Kruizenga

Thermophysical Property Measurement of Nitrate Salt Heat Transfer Fluids

Nitrate salts have been used for decades in the concentrating solar power industry as heat transfer fluids and thermal storage media. For most of this time these inorganic fluids have been restricted to use in central receiver platforms due to the useful working temperature range of the most widely researched formulation, a near eutectic mixture of sodium and potassium nitrate, which melts at 220°C and is stable in air to nearly 580°C. Recent research efforts have led to the development of nitrate salt mixtures that melt at lower temperatures and are suitable for use in parabolic trough systems. These mixtures include three or more components and generally have melting points in the range of 100°C, with stability in air up to 500°C. The design of parabolic trough systems that utilize molten salt heat transfer fluids is complicated by the fact that the properties of these fluids are considerably different from the organic heat transfer fluids that they may replace. In this paper we present measured thermophysical property data for several commercial and non-commercial molten salt mixtures that can be used in the system level design of parabolic trough and central receiver power plants. The data presented include heat capacity, density, thermal conductivity, viscosity, all as a function of temperature, along with melting point and thermal stability limits. Some properties, such as density, can be predicted by simple mixing rules. The dependence of viscosity was strongly influenced by the composition of the molten salts and, particularly, the proportion of calcium nitrate.Copyright

Supercritical Carbon Dioxide Heat Transfer in Horizontal Semicircular Channels

Brayton cycle: thecomplexity of heat exchanger design due to the vast change in thermophysical propertiesnear a fluid’s critical point. Turbulent heat transfer experiments using carbon dioxide,with Reynolds numbers up to 100 K, were performed at pressures of 7.5–10.1 MPa, attemperatures spanning the pseudocritical temperature. The geometry employed ninesemicircular, parallel channels to aide in the understanding of current printed circuitheat exchanger designs. Computational fluid dynamics was performed using

Thermal Property Testing of Nitrate Thermal Storage Salts in the Solid-Phase

Implementation of molten salt compounds as the heat transfer fluid and energy storage medium provides specific benefits to energy collection and conversion. Nitrate salts have been identified as a strong candidate for energy transfer and storage and have been demonstrated for use in these applications over time. As nitrate salts have solidification temperatures above ambient, concern for recovery from salt freezing events has instigated efforts to understand and predict this behavior. Accurate information of salt property behavior in the solid-phase is necessary for understanding recovery from a freeze event as well as for phase change thermal energy storage applications. Thermal properties for three representative salts (that span the range of melting temperatures from approximately 90–221 °C), have been obtained. These properties include specific heat, coefficient of thermal expansion, and thermal conductivity. Specific heat and thermal conductivity were measured using differential scanning calorimetry.Copyright

Corrosion and Erosion behavior in Supercritical CO2 power cycles.

Supercritical Carbon Dioxide (S-CO2) is emerging as a potential working fluid in power-production Brayton cycles. As a result, concerns have been raised regarding fluid purity within the power cycle loops. Additionally, investigations into the longevity of the S-CO2 power cycle materials are being conducted to quantify the advantages of using S-CO2 versus other fluids, since S-CO2 promises substantially higher efficiencies. One potential issue with S-CO2 systems is intergranular corrosion [1]. At this time, Sandia National Laboratories (SNL) is establishing a materials baseline through the analysis of 1) “as received” stainless steel piping, and 2) piping exposed to S-CO2 under typical operating conditions with SNL’s Brayton systems. Results from ongoing investigations are presented.A second issue that SNL has discovered involves substantial erosion in the turbine blade and inlet nozzle. It is believed that this is caused by small particulates that originate from different materials around the loop that are entrained by the S-CO2 to the nozzle, where they impact the inlet nozzle vanes, causing erosion. We believe that, in some way, this is linked to the purity of the S-CO2, the corrosion contaminants, and the metal particulates that are present in the loop and its components.Copyright

Materials Compatibility In Dish-Stirling Solar Generators Using Cu-Si-Mg Eutectic for Latent Heat Storage.

Dish-Stirling systems are a strong candidate to meet cost production goals for solar thermal power production. Thermal energy storage improves the capacity factor of thermal power systems; copper-silicon-magnesium eutectic alloys have been investigated as potential latent heat storage materials. This work examines the ability of commercially available plasma spray coatings to serve as protective barriers with these alloys, while highlighting mechanistic insights into materials for latent heat storage systems. Computed tomography was leveraged as a rapid screening tool to assess the presence of localized attack in tested coatings.

Coupling a Supercritical Carbon Dioxide Brayton Cycle to a Helium-Cooled Reactor.

This report outlines the thermodynamics of a supercritical carbon dioxide (sCO2) recompression closed Brayton cycle (RCBC) coupled to a Helium-cooled nuclear reactor. The baseline reactor design for the study is the AREVA High Temperature Gas-Cooled Reactor (HTGR). Using the AREVA HTGR nominal operating parameters, an initial thermodynamic study was performed using Sandias deterministic RCBC analysis program. Utilizing the output of the RCBC thermodynamic analysis, preliminary values of reactor power and of Helium flow rate through the reactor were calculated in Sandias HelCO2 code. Some research regarding materials requirements was then conducted to determine aspects of corrosion related to both Helium and to sCO2 , as well as some mechanical considerations for pressures and temperatures that will be seen by the piping and other components. This analysis resulted in a list of materials-related research items that need to be conducted in the future. A short assessment of dry heat rejection advantages of sCO2> Brayton cycles was also included. This assessment lists some items that should be investigated in the future to better understand how sCO2 Brayton cycles and nuclear can maximally contribute to optimizing the water efficiency of carbon free power generation

Materials corrosion of high temperature alloys immersed in 600C binary nitrate salt.

Thirteen high temperature alloys were immersion tested in a 60/40 binary nitrate salt. Samples were interval tested up to 3000 hours at 600%C2%B0C with air as the ullage gas. Chemical analysis of the molten salt indicated lower nitrite concentrations present in the salt, as predicted by the equilibrium equation. Corrosion rates were generally low for all alloys. Corrosion products were identified using x-ray diffraction and electron microprobe analysis. Fe-Cr based alloys tended to form mixtures of sodium and iron oxides, while Fe-Ni/Cr alloys had similar corrosion products plus oxides of nickel and chromium. Nickel based alloys primarily formed NiO, with chromium oxides near the oxide/base alloy interface. In625 exhibited similar corrosion performance in relation to previous tests, lending confidence in comparisons between past and present experiments. HA230 exhibited internal oxidation that consisted of a nickel/chromium oxide. Alloys with significant aluminum alloying tended to exhibit superior performance, due formation of a thin alumina layer. Soluble corrosion products of chromium, molybdenum, and tungsten were also formed and are thought to be a significant factor in alloy performance.

Materials Corrosion Concerns for Supercritical Carbon Dioxide Heat Exchangers

Supercritical Carbon Dioxide (S-CO2) is an efficient and flexible working fluid for power production. Research to interface S-CO2 systems with nuclear, thermal solar, and fossil energy sources are currently underway. To proceed, we must address concerns regarding high temperature compatibility of materials and compatibility between significantly different heat transfer fluids.Dry, pure S-CO2 is thought to be relatively inert [1], while ppm levels of water and oxygen result in formation of a protective chromia layer and iron oxide [2]. Thin oxides are favorable as diffusion barriers, and for their minimal impact on heat transfer. Chromia, however, is soluble in molten salt systems (nitrate, chloride, and fluoride based salts) [3–8]. Fluoride anion based systems required the development of the alloy INOR-8 (Hastelloy N, base nickel, 17%Mo) [9] to ensure that chromium diffusion is minimized, thereby maximizing the life of containment vessels.This paper reviews the thermodynamic and kinetic considerations for promising, industrially available materials for both salt and S-CO2 systems.Copyright

Corrosion of Austenitic Alloys in Binary 60/40 Nitrate Salt at 600°C

Industrial power utilities are using molten binary nitrate salt as a heat transfer fluid and thermal storage media for solar energy generation. Currently, the maximum bulk temperature is 565°C, due to concerns of salt degradation and materials compatibility with containment vessels.To increase overall cycle efficiency, one must increase the upper temperature of the nitrate salt, thereby lowering the levelized cost of electricity (LCoE) through higher power cycle efficiency. The corrosion performance of 316 stainless steel and Inconel 625 is currently characterized at 600°C. However, the 316SS has exhibited stress corrosion cracking (thought due to aqueous flush in Solar Two [1]), and while In625 performs well, its cost is prohibitive. Therefore, current research seeks to evaluate heat-resistant austenitic alloys for use with nitrate salts, ascertaining if they have superior performance characteristics, as well as assessing their mechanisms of corrosion.Sandia National Laboratory is researching four alloys (S35140, ATI332Mo, RA330, and HA556) for corrosion performance at 600°C for 3000 hours, under a cover gas of air. Air is used to simulate the chemistry conditions expected in a power plant.This work details the corrosion rate and the oxide structure for each alloy. Research indicates all alloys are very corrosion-resistant, with metal loss rates projected to be less than 21μm/year after 3000 hours. Though all alloys performed well, corrosion rate data for RA330 (Fe-19Cr-35Ni + minor elements) currently appears to exhibit a linear loss mechanism. In conclusion, this paper will explore the differences in oxide formation between these similar alloys.Copyright

Thermal characterization and model free kinetics of aged epoxies and foams using TGA and DSC methods.

Two classes of materials, poly(methylene diphenyl diisocyanate) or PMDI foam, and cross-linked epoxy resins, were characterized using thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC), to help understand the effects of aging and %E2%80%9Cbake-out%E2%80%9D. The materials were evaluated for mass loss and the onset of decomposition. In some experiments, volatile materials released during heating were analyzed via mass spectroscopy. In all, over twenty materials were evaluated to compare the mass loss and onset temperature for decomposition. Model free kinetic (MFK) measurements, acquired using variable heating rate TGA experiments, were used to calculate the apparent activation energy of thermal decomposition. From these compiled data the effects of aging, bake-out, and sample history on the thermal stability of materials were compared. No significant differences between aged and unaged materials were detected. Bake-out did slightly affect the onset temperature of decomposition but only at the highest bake-out temperatures. Finally, some recommendations for future handling are made.