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


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.

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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.
    Solar Energy Materials and Solar Cells 01/2015; 132:172–177. DOI:10.1016/j.solmat.2014.08.020 · 5.34 Impact Factor
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    • "Thus, while lower melting salt mixtures or mixtures involving less expensive components than LiNO 3 are available for use as a solar heat transfer fluid, their thermal stabilities are reduced by the addition of the Ca(NO 3 ) 2 or nitrite component. Halotechnics, Inc. has determined a five-component salt system with a melting point of 65 C and thermal stability to at least 500 C [13]. The salt composition consists of 44 wt. "
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    ABSTRACT: One of the major challenges preventing the concentrated solar power (CSP) industry from occupying a greater portion of the world's energy portfolio are unattractive start up and operating costs for developers and investors. In order to overcome these reservations, plant designers must be able to achieve greater efficiencies of power production. Molten salt nitrates are ideal candidates for CSP heat transfer fluids and have been proposed to offer significant performance advantages over current silicone based oil heat transfer fluids. Ternary molten salt nitrates offer high operating temperatures while maintaining low freezing temperatures. However, a shortage of important thermophysical property data exists for these salts. Previous work has shown the ternary compositions of LiNO3-NaNO3-KNO3 salts offer the widest possible temperature range for use in a CSP system. The present work contains data for the viscosity, specific heat, and latent heat of some mixtures of these salts at various temperatures, providing vital information for plant designers to optimize power generation and attract future investment to CSP systems.
    Journal of Solar Energy Engineering 08/2013; 135(3). DOI:10.1115/1.4024069 · 1.61 Impact Factor
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    • "TGA alone has been used by others [23] [24] to evaluate thermal stability of molten salts for TES. Raade and Padowitz [24] defined the high temperature limit of the salt as that where the salt is observed to rapidly begin to lose weight beyond a maximum acceptable Table 3 Repeatability of melting and solidification point measurement. "
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    ABSTRACT: The thermal stability of the eutectic LiNO3-NaNO3-KNO3 salt was investigated by simultaneous differential scanning calorimetry, thermogravimetry and mass spectrometry (DSC/TG-MS). The work was carried out between room temperature and 1000 degrees C in blanket gas atmospheres of argon, nitrogen, oxygen and air. The stability of the salt, as measured by the gases evolving from the melt, was influenced by the atmosphere. Evolution of the main gaseous species NO was detected at 325 degrees C in an atmosphere of argon, at 425 degrees C in an atmosphere of nitrogen, at 475 degrees C in an atmosphere of air and at 540 degrees C in an atmosphere of oxygen. Prior to melting, the eutectic underwent endothermic (alpha/beta) solid-solid type transformation at 87 degrees C. The melting point was 121 degrees C, and the solidification point 98 degrees C. Under-cooling of the salt coincided with the onset of the (alpha/beta) solid-solid transformation upon heating. At a temperature of 500 degrees C in air, TG analysis showed that the long-term stability of the salt was limited and this was confirmed by DSC. Uncertainty analysis indicated that the measurement of temperature is accurate to +/- 2.7 degrees C.
    Thermochimica Acta 04/2013; 560:34-42. DOI:10.1016/j.tca.2013.02.029 · 2.18 Impact Factor
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