Evaluation of thermal physical properties of molten nitrate salts with low melting temperature

https://doi.org/10.1016/j.solmat.2017.11.011Get rights and content

Highlights

  • A novel molten nitrate salt with melting temperature of 368 K is found.

  • Thermophysical properties of six nitrate salts are comprehensively determined.

  • The heat storage and transfer capacity and cost are evaluated based the measured parameters.

  • Multicomponent molten salt has lower melting temperature and larger energy storage capacity.

  • Solar salt has the best heat transfer performance and low cost.

Abstract

NaNO3-KNO3 (60–40 wt%, Solar salt) has been used as medium for TES and HTF in the CSP system. One of the key challenges using Solar salt is its high melting temperature, which may freeze and block the pipeline. In this work, a novel eutectic nitrate molten salt of the LiNO3-NaNO3-KNO3-CsNO3 system with low melting temperature of 368 K is designed using Calphad method. Its thermal physical properties, as well as that of Solar salt, LiNO3-NaNO3-KNO3, NaNO3-KNO3-CsNO3, NaNO3-KNO3-Ca(NO3)2, and LiNO3-NaNO3-KNO3-CsNO3-Ca(NO3)2 molten salts are comprehensively determined and evaluated for better understanding their thermal storage and heat transfer performances, such as the melting temperature, thermal stability, specific heat capacity, thermal diffusivity, density and viscosity. The energy storage capacities and figures of merit of the six molten nitrate salts are calculated based on their thermophysical properties to evaluate their TES and HTF performances. This work not only provides the basic engineering data for CSP system, but is useful for choosing media of TES and HTF.

Introduction

Solar energy as a clean and sustainable energy has drawn increasing attention worldwide, but its intermittent energy production has been a challenge for a long time. Concentrated solar power (CSP) plant coupled with storage tank enables continuous energy production even at night or during cloudy days, moreover, it is a reliable electrical power option and carbon free energy production [1]. Medium for thermal energy storage (TES) and heat transfer fluid (HTF) is one of the key techniques because it affects not only the energy production efficiency but also the cost of operation and maintenance of CSP plant [2]. A potential method is to look for a medium with lower melting point and appropriate thermophyscial properties, such as viscosity, density, thermal diffusivity and specific heat capacity [1].

The thermal physical properties of NaNO3-KNO3, LiNO3-NaNO3-KNO3 (LiNaKNO3), and NaNO3-KNO3-Ca(NO3)2 (NaKCaNO3) systems have been reported by several researchers, especially for NaNO3-KNO3 system, the mixture of 40 wt% KNO3-60 wt% NaNO3 (Solar salt), was commercialized as the thermal storage and heat transfer medium in 1937 [3]. Its relatively high melting temperature (~ 493 K) represents an important risk of local solidification and thus block the pipeline during absence of sunshine [4]. Moreover, the investment and operation costs produced by auxiliary facilities of thermal insulation due to high melting temperature are so high that searching for new medium with low melting temperature (lower than 373 K) is necessary. Adding LiNO3, CsNO3, and Ca(NO3)2 to Solar salt is intended to reduce the melting temperature [5]. A molten salt in the LiNO3-NaNO3-KNO3-CsNO3-Ca(NO3)2 (LiNaKCsCaNO3) system was reported to melt at 338 K with a thermal stability over 773 K [6]. We studied the LiNO3-NaNO3-KNO3-CsNO3 (LiNaKCsNO3) system using the Calphad method and found a novel molten nitrate salt with low melting temperature of 368 K.

Well known, melting temperature is not the only criterion to evaluate TES and HTF medium. Thermal physical properties are also important, such as viscosity, density, specific heat capacity, and thermal diffusivity, because they are the basic and essential engineering data. However, the thermal physical properties are not comprehensively studied in a same work, and the results from different work show discrepancy from each other [7]. For example, the melting temperature of Solar salt is reported to be from 493 to 500 K [8]. The thermal physical properties of LiNaKNO3 system were studied by several researchers, including thermal stability, density, viscosity and specific heat capacity [9], [10], [11], [12], [13], [14], [15]. For the NaKCaNO3 system, its thermal physical properties were also reported in Refs. [16], [17]. The melting point, stability limit, specific heat capacity, viscosity, density, and thermal conductivity of the nitrate molten salts were summarized in Refs. [10], [18]. Although the thermophysical properties of Solar salt, LiNaKNO3 and NaKCaNO3 molten nitrate salts were studied by several researchers, the experimental data are scattered. More important, the thermal diffusivity/conductivity in the liquid state is sparsely available in literatures.

In this work, a novel eutectic molten salt of LiNO3-NaNO3-KNO3-CsNO3 system with the melting temperature of 368 K is found. The thermophysical properties of Solar salt, LiNaKNO3, NaNO3-KNO3-CsNO3 (NaKCsNO3), NaKCaNO3, LiNaKCsNO3 and LiNaKCsCaNO3 molten salt systems are systematically and comprehensively determined and evaluated, including melting point, thermal stability, density, specific heat capacity, thermal conductivity, and viscosity. Their thermal energy capacity and figures of merits to evaluate heat transfer capacity are subsequently studied. This work is significant for selection and development of TES and HTF media in the CSP plant.

Section snippets

Sample preparation

The compositions of each molten salts are listed in Table 1. Raw materials of LiNO3, NaNO3, KNO3 and CsNO3 with purity of 99.9% were separately dried at 473 K under argon atmosphere for 24 h. The mixture of Solar salt was kept at573 K, and the mixture of LiNaKNO3, NaKCsNO3, and LiNaKCsNO3 salt was kept at 473 K for 48 h to make it homogeneous, and then cool down. For NaKCaNO3 and LiNaKCsCaNO3 molten salts, Ca(NO3)2·4H2O was used, the raw materials were mixed and transferred in a furnace and heated

Results

For testing the melting temperatures of Solar salt, LiNaKNO3, NaKCsNO3, NaKCaNO3, LiNaKCsNO3, and LiNaKCsCaNO3 molten salts, about 10 mg nitrate salt sample was put in a graphite crucible. The molten salts were heated from room temperature to 523 K for Solar salt, and 473 K for the other five nitrate salts at 5 K/min under the argon atmosphere. The DSC curves of the six nitrate salts are shown in Fig. 1, and the melting peaks are marked as A, B, C, D, E, and F, respectively. For Solar salt, it

Conclusion

In this work, a novel quaternary molten nitrate salt with the melting temperature of 368 K is found using Calphad method. Its thermal physical properties, such as melting temperature, specific heat capacity, thermal diffusivity, density, and viscosity, together with those of Solar salt, LiNaKNO3, NaKCsNO3, NaKCaNO3, and LiNaKCsCaNO3 molten salts are comprehensively studied in this work. The thermophysical properties are the basic input data to calculate thermal energy storage and heat transfer

Acknowledgement

The authors are grateful to acknowledge the financial supported by the National Natural Science Foundation of China (21406256), the “Youth Innovation Promotion Association” (2015214) and “Strategic Priority Research Program” (XD02002400) of Chinese Academy of Sciences.

References (29)

  • Y. Jin et al.

    Accurate viscosity measurement of nitrates/nitrites salts for concentrated solar power

    Sol. Energy

    (2016)
  • W. Kirst et al.

    A new heat transfer medium for high temperatures

    Trans. Am. Inst. Chem. Eng.

    (1940)
  • J.W. Raade et al.

    Development of molten salt heat transfer fluid with low melting point and high thermal stability

    J. Sol. Energy Eng.

    (2011)
  • Y. Jin et al.

    Research on thermo-physical properties of several typical molten salt coolants

    Nucl. Tech.

    (2016)
  • Cited by (55)

    View all citing articles on Scopus
    View full text