Judith C. Gomez

Nanofluid heat capacities

Significant increases in the heat capacity of heat transfer fluids are needed not only to reduce the costs of liquid heating and cooling processes, but also to bring clean energy producing technologies like concentrating solar power (CSP) to price parity with conventional energy generation. It has been postulated that nanofluids could have higher heat capacities than conventional fluids. In this work, nano- and micron-sized particles were added to five base fluids (poly-α olefin, mineral oil, ethylene glycol, a mixture of water and ethylene glycol, and calcium nitrate tetrahydrate), and the resulting heat capacities were measured and compared with those of the neat base fluids and the weighted average of the heat capacities of the components. The particles used were inert metals and metal oxides that did not undergo any phase transitions over the temperature range studied. In the nanofluids studied here, we found no increase in heat capacity upon the addition of the particles larger than the experimental ...

Ca(NO3)2—NaNO3—KNO3 Molten Salt Mixtures for Direct Thermal Energy Storage Systems in Parabolic Trough Plants

Molten salts are currently the only thermal energy storage media operating with multiple hours of energy capacity in commercial concentrated solar power (CSP) plants. Thermal energy is stored by sensible heat in the liquid phase. A lower melting point in the range of 60–120 C and a decomposition temperature above 500 _C are desired because such a fluid would enhance the overall efficiency of the plants by utilizing less energy to keep the salt in the liquid state and by producing superheated steam at higher temperatures in the Rankine cycle. One promising candidate is a multicomponent NaNO3—KNO3—Ca(NO3)2 molten salt. Different compositions have been reported in literature as the best formulation for CSP plants based on melting temperature. In this paper, the National Renewable Energy Laboratory (NREL) presents the handling, preparation, thermal properties, and characterization of different compositions for this ternary nitrate salt, and comparisons are drawn accordingly. This system has a high tendency to form supercooled liquids with high viscosity that undergoes glass formation during cooling. When the proportion of Ca(NO3)2 decreases, the formulations become more thermally stable, the viscosity goes down, and the system increases its degree of crystalline solidification. Differential scanning calorimetry (DSC) tests showed the presence of a ternary eutectoid solid–solid invariant reaction at around 100 _C. The eutectic invariant reaction was resolved between 120 and 133 _C as reported in the literature. Based on DSC and viscosity results, the best composition would seem to be 36 wt. % Ca(NO3)2—16 wt. % NaNO3—48 wt% KNO3, which showed a low solidification point. [DOI: 10.1115/1.4023182]

Can particle-stabilized inorganic dispersions be high-temperature heat-transfer and thermal energy storage fluids?

Particle-stabilized dispersions are considered as potential high-temperature, high-energy–density heat transfer fluids as well as thermal energy storage materials. To be useful practically, these dispersions need to be stable against coalescence and have low viscosity. We present indirect experimental evidence of particle stabilization of Al–Si in NaCl–NaF dispersions with graphite as the stabilizer. We found no evidence of particle stabilization in the same system with boron carbide, silicon carbide, silica, or zirconia as the stabilizer. We also present indirect experimental evidence of particle stabilization in Al/B2O3/C and Al/NaCl–KCl/Al2O3 dispersed phase/dispersion media/stabilizer systems.

Determining the Cost Benefit of High-Temperature Coatings for Concentrating Solar Power Thermal Storage Using Probabilistic Cost Analysis

Probabilistic cost analysis determined the cost benefit for applying a protective coating to the wetted surfaces of stainless steel tank walls for concentrating solar power (CSP) thermal storage applications. The model estimated the total material cost of coated 347 or 310 stainless steel (347/310) and the cost of uncoated Inconel 625, which served as the reference tank wall cost. Model results showed that the cost of the coated 347/310 stainless steel was always statistically less than the cost of the bare Inconel 625 when these materials are used for tank walls at representative tank diameters and temperatures for CSP storage applications.

One-Pot Shear Synthesis of Gallium, Indium, and Indium–Bismuth Nanofluids: An Experimental and Computational Study

Nanofluids are often proposed as advanced heat transfer fluids. In this work, using a one-step nanoemulsification method, we synthesize gallium, indium, and indium–bismuth nanofluids in poly-alpha-olefin (PAO). The size distributions of the resulting nanoparticles are analyzed using transmission electron microscopy (TEM). X-ray diffraction (XRD) analysis of the alloy nanoparticles indicates that their composition is the same as that of the bulk alloy. It was found that oleylamine stabilizes both gallium and indium particles in PAO, while oleic acid is effective for gallium particles only. The microscopic adsorption mechanism of surfactants on gallium and indium surfaces is investigated using density functional theory (DFT) to understand why oleylamine is effective for both metals while oleic acid is effective for gallium only.

Hot Corrosion Studies Using Electrochemical Techniques of Alloys in a Chloride Molten Salt (NaCl-LiCl) at 650°C

Next-generation solar power conversion systems in concentrating solar power (CSP) applications require high-temperature advanced fluids in the range of 600° to 900°C. Molten salts are good candidates for CSP applications, but they are generally very corrosive to common alloys used in vessels, heat exchangers, and piping at these elevated temperatures. The majority of the molten-salt corrosion evaluations for sulfates with chlorides and some vanadium compounds have been performed for waste incinerators, gas turbine engines, and electric power generation (steam-generating equipment) applications for different materials and molten-salt systems. The majority of the molten-salt corrosion kinetic models under isothermal and thermal cyclic conditions have been established using the weight-loss method and metallographic cross-section analyses. Electrochemical techniques for molten salts have not been employed for CSP applications in the past. Recently, these techniques have been used for a better understanding of the fundamentals behind the hot corrosion mechanisms for thin-film molten salts in gas turbine engines and electric power generation. The chemical (or electrochemical) reactions and transport modes are complex for hot corrosion in systems involving multi-component alloys and salts; but some insight can be gained through thermochemical models to identify major reactions. Electrochemical evaluations were performed on 310SS and In800H in the molten eutectic NaCl-LiCl at 650°C using an open current potential followed by a potentiodynamic polarization sweep. Corrosion rates were determined using Tafel slopes and the Faraday law. The corrosion current density and the corrosion potentials using Pt wire as the reference electrode are reported.Copyright

Numerical analysis of total energy storage of nanofluidized heat transfer fluid in thermocline thermal energy storage system

An important factor to enhance the concentrating solar power plant’s efficiency relies on the thermal energy storage (TES) system. A single-tank thermocline storage in which both, the hot and cold, fluids are contained in the same tank separated by density difference is investigated. In theory, the overall efficiency and storage capacity of a TES system depends on the thermodynamic properties of heat transfer fluid (HTF). In this study, Solar Salt® is used as both the HTF and TES medium. The operating temperatures used in the system analysis range from 300°C to 500°C. The working temperatures will determine the efficiency of storage and the heat storage capacity. To potentially increase the heat capacity and thermal conductivity values, the molten salt has been doped with SiO2, Al2O3, CuO, Ag and ZnO nanoparticles at different concentrations. The variation of the thermodynamic properties of Solar Salt® along with the performance of single-tank TES system were evaluated and compared to obtain the best working composition. The goal of this study is to comprehend and adapt the behavior of the molten salt doped with nanoparticles. In this study, the effective heat capacity, thermal conductivity, density, and viscosity of Nano-fluid are calculated and used in the models to approximate the total energy storage capacity. The models were evaluated by numerical analysis using finite volume computational fluid dynamics software. The numerical results are compared with experimental results in literature.Copyright

Post-Industrial Ceramics Compatibility With Heat Transfer Fluids for Low-Cost Thermal Energy Storage Applications in CSP

This paper investigates the possibility of using a post-industrial ceramic commercially called Cofalit as a promising, sustainable, and inexpensive (

Compatibility of a post-industrial ceramic with nitrate molten salts for use as filler material in a thermocline storage system

10/ton) thermal energy storage material. This ceramic presents relevant properties to store thermal energy by means of sensible heat in the temperature range of concentrated solar power (CSP) plants from ambient temperature up to 1100 °C. In the present study, the compatibility of this ceramic was studied with two conventional heat transfer fluids: nitrate molten salts for medium-temperature applications (200 to 500 °C) and air for high-temperature applications (500 to 900 °C). The use of this ceramic in direct contact with the heat transfer fluid should significantly reduce the cost of thermal energy storage systems in CSP applications and help to achieve the U.S. Department of Energy’s SunShot Initiative cost targets.Copyright