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Research Papers

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

[+] Author and Article Information
Judith C. Gomez

National Renewable Energy Laboratory,
1617 Cole Boulevard, Golden, CO 80401
e-mail: Judith.Gomez@nrel.gov

Nicolas Calvet

National Renewable Energy Laboratory,
1617 Cole Boulevard,
Golden, CO 80401; CIC Energigune,
Albert Einstein 48,
01510 Miñano (Álava), Spain

Greg C. Glatzmaier

National Renewable Energy Laboratory,
1617 Cole Boulevard,
Golden, CO 80401

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the Journal of Solar Energy Engineering. Manuscript received March 2, 2012; final manuscript received November 8, 2012; published online January 25, 2013. Assoc. Editor: Rainer Tamme.

J. Sol. Energy Eng 135(2), 021016 (Jan 25, 2013) (8 pages) Paper No: SOL-12-1063; doi: 10.1115/1.4023182 History: Received March 02, 2012; Revised November 08, 2012

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 undergo 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.

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Figures

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Fig. 1

Ternary phase diagram of Ca(NO3)2—NaNO3—KNO3 showing isothermal liquidus lines [24-25]. Binary phase diagrams: upper left: KNO3—Ca(NO3)2 [26]; upper right: Ca(NO3)2—NaNO3 [27]; and bottom: KNO3—NaNO3 [28].

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Fig. 2

Typical XRD pattern of the premelted product of Ca(NO3)2—NaNO3—KNO3 for compositions higher than 42 wt. % Ca(NO3)2, showing a glassy (amorphous) phase

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Fig. 3

DSC plots for formulation 1 (48 wt. % Ca(NO3)2—7 wt. % NaNO3—45 wt. % KNO3) for three heating/cooling cycles from 35 to 200 °C at 10 K/min. The lower plot corresponds to the heating cycle and the upper plot corresponds to the cooling cycle.

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Fig. 4

DSC plots for formulation 2 (45 wt. % Ca(NO3)2—11 wt. % NaNO3—44 wt. % KNO3) for three heating/cooling cycles from 35 to 200 °C at 10 K/min. The lower plot corresponds to the heating cycle and the upper plot corresponds to the cooling cycle.

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Fig. 5

DSC plots for formulation 3 (44 wt. % Ca(NO3)2—12 wt. % NaNO3—44 wt. % KNO3) for three heating/cooling cycles from 35 to 200 °C at 10 K/min. The lower plot corresponds to the heating cycle and the upper plot corresponds to the cooling cycle.

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Fig. 6

DSC plots for formulation 4 (42 wt. % Ca(NO3)2—15 wt. % NaNO3—43 wt. % KNO3) for three heating/cooling cycles from 35 to 200 °C at 10 K/min. The lower plot corresponds to the heating cycle and the upper plot corresponds to the cooling cycle.

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Fig. 7

DSC plots for formulation 5 (36 wt. % Ca(NO3)2—16 wt. % NaNO3—48 wt. % KNO3) for three heating/cooling cycles from 35 to 200 °C at 10 K/min. The lower plot corresponds to the heating cycle and the upper plot corresponds to the cooling cycle.

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Fig. 8

DSC plots for formulation 6 (30 wt. % Ca(NO3)2—24 wt. % NaNO3—46 wt. % KNO3) for three heating/cooling cycles from 35 to 200 °C at 10 K/min. The lower plot corresponds to the heating cycle and the upper plot corresponds to the cooling cycle.

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Fig. 9

TGA results for the relative thermal stability of anhydrous formulation 3 (44 wt. % Ca(NO3)2—12 wt. % NaNO3—44 wt. % KNO3) under nitrogen and zero air

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Fig. 10

Maximum stability temperature calculated using the T3 method [33] for 3% of mass loss from the anhydrous weight defined at 400 °C as a function of the Ca+2/Na+ molar ratio

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Fig. 11

Viscosity of formulation 3 (44 wt. % Ca(NO3)2—12 wt. % NaNO3—44 wt. % KNO3) and formulation 6 (30 wt. % Ca(NO3)2—24 wt. % NaNO3—46 wt. % KNO3) compared with Bradshaw results [17] for formulation 5 (36 wt. % Ca(NO3)2—16 wt. % NaNO3—48 wt. % KNO3)

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