Article

Development of Molten Salt Heat Transfer Fluid With Low Melting Point and High Thermal Stability

Journal of Solar Energy Engineering (Impact Factor: 1.61). 08/2011; 133(3). DOI: 10.1115/1.4004243

ABSTRACT

This paper describes an advanced heat transfer fluid (HTF) consisting of a novel mixture of inorganic salts with a low melting point and high thermal stability. These properties produce a broad operating range molten salt and enable effective thermal storage for parabolic trough concentrating solar power plants. Previous commercially available molten salt heat transfer fluids have a high melting point, typically 140 °C or higher, which limits their commercial use due to the risk of freezing. The advanced HTF exploits eutectic behavior with a novel composition of materials, resulting in a low melting point of 65 °C and a thermal stability limit over 500 °C. The advanced HTF described in this work was developed using advanced experiment design and data analysis methods combined with a powerful high throughput experimental workflow. Over 5000 unique mixtures of inorganic salt were tested during the development process. Additional work is ongoing to fully characterize the relevant thermophysical properties of the HTF and to assess its long term performance in realistic operating conditions for concentrating solar power applications or other high temperature processes.

    • "kJ kg K and it is suitable for operating temperatures up to 1300 °C [9]. Salt melting temperatures are usually between 150 °C and 350 °C, however, this temperature can be reduced below 100 °C with the addition of lithium nitrates [39]; even some mixtures of nitrate salts of lithium, sodium, potassium, cesium and cadmium result in a melting point as low as 65 °C [40]. A low overall heat transfer coefficient between the HTES tank and the ambient can be achieved using a design with multiple insulation layers [41]. "
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    ABSTRACT: In this study, the dynamic behavior of a concentrated solar power (CSP) supercritical CO2 cycle is studied under different seasonal conditions. The system analyzed is composed of a central receiver, hot and cold thermal energy storage units, a heat exchanger, a recuperator, and multi-stage compression-expansion subsystems with intercoolers and reheaters between compressors and turbines respectively. Energy models for each component of the system are developed in order to optimize operating and design parameters such as mass flow rate, intermediate pressures and the effective area of the recuperator to lead to maximum efficiency. The results show that the parametric optimization leads the system to a process efficiency of about 21 % and a maximum power output close to 1.5 MW. The thermal energy storage allows the system to operate for several hours after sunset. This operating time is approximately increased from 220 to 480 minutes after optimization. The hot and cold thermal energy storage also lessens the temperature fluctuations by providing smooth changes of temperatures at the turbines and compressors inlets. The results obtained in this paper indicate that concentrated solar systems using supercritical CO2 could be a viable alternative to satisfying energy needs in desert areas with scarce water and fossil fuel resources.
    No preview · Article · Oct 2015 · Applied Thermal Engineering
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    • "Zaversky et al. [13] developed a transient performance simulation model of the parabolic trough collector using the molten salt as a HTF, which successfully validated against measurement data obtained at the SOLTERM facility of Italy. Additionally, Raade et al. [14] experimentally invesigated the thermal properties of the mixture of molten salts, and obainted a unique mixture exploits eutectic behavior resulting in a low melting point of 338 K and a thermal stability limit over 773 K. Accompanied with the technic efforts like Archimede company [15] [16], the technology of using the molten salt as a HTF in the parabolic trough will be more mature. Moreover, the molten salt can be used directly as a heat storage medium, the heat storage efficiency can be enhanced and the capital investment can be reduced. "
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    ABSTRACT: In this paper, a new parabolic trough solar power system that incorporates a dual-solar field with oil and molten salt as heat transfer fluids (HTFs) is proposed to effectively utilize the solar energy. The oil is chosen as a HTF in the low temperature solar field to heat the feeding water, and the high temperature solar field uses molten salt to superheat the steam that the temperature is higher than 773 K. The produced superheated steam enters a steam turbine to generate power. Energy analysis and exergy analysis of the system are implemented to evaluate the feasibility of the proposed system. Under considerations of variations of solar irradiation, the on-design and off-design thermodynamic performances of the system and the characteristics are investigated. The annual average solar-to-electric efficiency and the nominal efficiency under the given condition for the proposed solar thermal power generation system reach to 15.86% and 22.80%, which are higher than the reference system with a single HTF. The exergy losses within the solar heat transfer process of the proposed system are reduced by 7.8% and 45.23% compared with the solar power thermal systems using oil and molten salt as HTFs, respectively. The integrated approach with oil and molten salt as HTFs can make full use of the different physical properties of the HTFs, and optimize the heat transfer process between the HTFs and the water/steam. The exergy loss in the water evaporation and superheated process are reduced, the system efficiency and the economic performance are improved. The research findings provide a new approach for the improvement of the performances of solar thermal power plants.
    Full-text · Article · Oct 2015 · Applied Thermal Engineering
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    • "The potential for improving the salt resides in optimising its physiochemical properties, mainly its melting point, thermal stability and heat capacity, by developing new quaternary mixtures or by incorporating novel components. Raade et al. [4] presented mixtures with up to five components and diverse compositions and introduced caesium nitrate to mixtures reported in the literature, succeeding in reducing the melting point to 70 1C. However, the cost of CsNO 3 (which is even greater than that of LiNO 3 ) makes these mixtures commercially unviable. "
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    ABSTRACT: Enhancements to energy storage systems developed for solar thermoelectric technologies can yield considerable increases in efficiency for this type of renewable energy. Important improvements include the design of innovative storage fluids, such as molten salts possessing low melting points and high thermal stabilities. This research examines the design of an innovative quaternary molten nitrate mixture, with the goal of improving the solar salt used currently as an energy storage fluid in CSP plants. This quaternary salt, which contains different weight percentages of NaNO3, KNO3, LiNO3 and Ca(NO3)2, exhibits better physical and chemical properties than the binary solar salt (60% NaNO3+40% KNO3) currently used. The melting points, heat capacities and thermal stability of the quaternary mixtures were studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). In addition to DSC and TGA tests, viscosity and electrical conductivity measurements were carried out for the quaternary mixtures at different temperatures. The new salt was designed by taking into consideration the risk of solid species formation at high temperatures when calcium nitrate is present (which requires that the wt% does not exceed 20%) and the costs of LiNO3. These boundaries set the maximum wt% of LiNO3 to values below 15%. Finally it was determined that the proposed quaternary mixture, when used as a heat transfer fluid (HTF) in parabolic trough solar power plants, is able to expand plants׳ operating range to temperatures between 132 and 580 °C.
    Full-text · Article · Jan 2015 · Solar Energy Materials and Solar Cells
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