1. S. Pistacchio2 , G. Bovesecchi1 , P. Coppa1
Thermal conductivity, viscosity and specific heat of Molten Salts (MS) to be used as heat transfer and storage fluids in the solar thermodynamic systems (parabolic trough)
S. Pistacchio2 , G. Bovesecchi1 , P. Coppa1
1 University of Rome “Tor Vergata” – Department of Industrial Engineering
2 Research Center ENEA Casaccia - Technical Unit for Renewable Energy Sources (UTRINN)
PhD Program in Industrial Engineering for Health, Environment and Energy

2. Contents
Overview of the experimental methods taken into account to the characterization of the thermophysical Molten Salt properties:
Viscosity (momentum transfer method);
Specific Heat (DSC);
Hot Wire Method;
Probe Method;
Preliminary calibration case: A glycerine test.
Thermal Energy Storage (TES) tank optimization:
Geometry details;
Obtained results and comparison with experimental data;
Conclusion.
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3. Molten Salts (MS) Standard binary mixture (Solar Salt)
Sodium nitrate (NaNO3 ) 60%
Potassium nitrate (KNO3 ) %
Standard binary mixture
(Solar Salt)
High Thermal stability ( ≈ 600 ° );
Low cost and low toxicity for the environment;
High Thermal capacity and low viscosity at operating temperatures in CSP systems;
Possibility to use molten salts both heat transfer fluid (HTF) and heat storage material (HSM).
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4. Viscosity (momentum transfer method)
Base of the method: Friction between the fluid and the moving boundaries causes the fluid to shear. The force required for this action is a measure of the fluid's viscosity.
For a newtonian fluid, the gradient of velocity γ (shear-rate) should be considered uniform between the boundary layers and defined as:
With: ux = velocity [m/s]
y = distance [m]
F = force[N]
A= surface [m²]
shear-rate
shear-stress
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5. Viscosity - Reometer
Instrument: rotational reometer TA Instruments AR2000EX
Principle of working:
Statore
Rotore
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6. Viscosity - Reometer Analisys procedure
Sample quantity used : 1600 mg.
Viscosimetry shear-rate range is between (1/sec) for every single measurement.
Each measurement was realized with operating temperatures of concentrating solar power plant (CSP). Temperature range between 260°C and 500°C.
13 experiments for each temperature analyzed.
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7. Shear stress – Shear rate
Viscosity
For the newtonian fluids the shear-stress trend in function of the shear-rate is linear.
Shear stress – Shear rate
Ternary Mixture
Binary Mixture
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8. Viscosity Ternary Mixture Binary Mixture
For the newtonian fluids viscosity is not dependent to the shear-rate used for the measurement.
Ternary Mixture
Binary Mixture
Discarded values in the average calculation
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9. Viscosity - Results Viscosity Temperature Binary Ternary
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10. Specific Heat
Differential scanning calorimeter (DSC), base of the method:
Two samples in two different sample holders, the first the test sample and the second the reference (generally Al2O3) are heated in a furnace at constant rate;
The temperature difference between the two samples is measured (DTA) or heat supplied to maintain the same temperature between the samples (DSC);
Advantages
Accurate and standard measurement;
Liquids, powders, with very small quantity can be measured (few mgs);
Drawbacks:
Small sizes of samples require accuracy in sampling;
Measurement accuracy is dependent on the reference purity;
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11. Specific Heat Differential Scanner Calorimetry Blank
Specific Heat calculation
Sapphire
3 steps
Salt
High Cp =
High thermal storage capacity
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12. Specific Heat - Results
Binary Salt
Ternary Salt
Specific Heat
Temperature
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13. Hot wire method
Base of the method: a metal wire is heated by an electric current. Detected quantities:
Wire temperature;
Voltage and current of the wire, and hence thermal power diffused in the sample per unit length;
From the analytical relationship between the temperature rise of the wire and the time
Temperature trend of the wire as a function of log of time is linear for high times (>10÷50 s), and slope is inversely proportional to thermal conductivity.
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14. Probe method
Similar to the previous method, requires a probe with a thermometer and a heater built inside.
Requirements:
l/d ratio >50, better 100;
Ratio rsample /rprobe>100 (better, but if it is lower test times must be reduced, according to twall);
Advantages:
Compact
portable, can also be used in field;
Drawbacks:
Requires an accurate construction;
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15. Probe method
Probe built by the lab. «thermophysical properties» of the Univ. of Rome «Tor Vergata»
Specifics of the probe:
d=0,6 mm;
L= 60 mm
Thermocouple type T;
Pt wire heater (d=50 µm);
Accuracy 5% at about ambient temperature;
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16. Special probe for high temperature (till to 600°C) for molten salts
Probe method
Special probe for high temperature (till to 600°C) for molten salts
Thermal conductivity between 250°C and 600°C
At high temperature only metals and ceramics can be used;
Thermal contact resistance between wire and case must be avoided (case must be filled with MgO or Al2O3 powder);
Accuracy results lower (5÷10%);
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17. Experimental measurements
An example of calibration of the HW method using glycerine at ambient temperature
tests
λ [W/mK]
0,8V
0,372
1,6V
0,371
2,0V
0,378
2,4V
0,385
3,2V
0,400
4,0V
0,369
4,4V
0,395
6,0V
0,360
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18. Thermal Energy Storage (TES) tank
Plant scheme
Solar Collectors
Storage tank and SG
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19. Thermal Energy Storage (TES) tank
Geometry details:
Sketch of the TES tank in the ENEA CSP facility
Sketch of TES tank with axisymmetric SG configuration
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20. Thermal Energy Storage (TES) tank
Computational grids investigated
(Produced by SnappyHexMesh grid generator)
(Produced by BlockMesh grid generator)
Approx cells
Approx cells
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21. Thermal Energy Storage (TES) tank
Boundary conditions
Velocity:
No-slip condition was imposed at all solid surface walls;
Imposed time dependent volumetric flow at inlet;
Pressure:
Zerogradient everywhere with exception of the outlet where a fixedvalue (P-rgh=0) has been imposed;
Temperature:
Adiabatic thermal condition was applied for the walls;
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22. Thermal Energy Storage (TES) tank
Obtained Results
Temperature fields:
t = 0s
t = 100s
t = 500s
t = 900s
t = 1250s
After 100s the cold jet coming from the SG has mainly mixed up the lower layers of the temperature stratification.
Thermocline zone moves with time from the bottom to the top of the tank.
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23. Thermal Energy Storage (TES) tank
Obtained Results
Velocity fields:
t = 0s
t = 100s
t = 500s
t = 900s
t = 1250s
The highest velocity values are located at the inlet port and impinging zone.
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24. Thermal Energy Storage (TES) tank
Obtained Results
Temperature fields:
t = 0s
t = 1000s
t = 5000s
t = 10000s
t = 12000s
t = 14400s
From the temperature field it can be seen how the stratification is stable. No relevant differences in the temperature field at different radial positions appear.
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25. Thermal Energy Storage (TES) tank
Obtained Results
Velocity fields:
t = 0s
t = 1000s
t = 5000s
t = 10000s
t = 12000s
t = 14400s
The velocity field shows both the evolution of the recirculation zones close to the diffuser and the extension of the downcoming flow at walls due to thermal losses.
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26. Thermal Energy Storage (TES) tank
Comparison with experimental data
Velocity fields:
The initial conditions at the bottom are not measured, which includes some uncertainty.
Excessive diffusion between the experimental data and the numerical data.
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27. Conclusion and perspectives
Viscosity values after 450°C are similar between binary and ternary mixtures.
Need to improve the experimental setup used for the HWM to make a proper calibration and subsequent measurement campaigns.
Possibility to realize an alternative setup using a four terminals hot wire.
Realize a new study case of the TES tank simulation using a different solver (chtMultiRegion) in order to reduce the diffusion effect between the numerical and experimental trends.
Thanks for your attention!
Ph D Program in Industrial Engineering - Research activity of interest for Energy