s. pistacchio 2, g. bovesecchi 1, p. coppa 1 1 university of rome “tor vergata” – department...

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

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)

Ph D Program in Industrial Engineering - Research activity of interest for Energy

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.

Molten Salts (MS)

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).

Standard binary mixture (Solar Salt)

Sodium nitrate (NaNO3 ) 60% Potassium nitrate (KNO3 ) 40 %


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:

shear-stress

With: ux = velocity [m/s] y = distance [m] F = force[N] A= surface [m²]

shear-rate


Viscosity - Reometer

Statore

Rotore

Instrument: rotational reometer TA Instruments AR2000EX

Principle of working:


Viscosity - Reometer

Analisys procedure

• Each measurement was realized with operating temperatures of concentrating solar power plant (CSP). Temperature range between 260°C and 500°C.

•Viscosimetry shear-rate range is between 20-500 (1/sec) for every single measurement.

• Sample quantity used : 1600 mg.

• 13 experiments for each temperature analyzed.


Viscosity

Shear stress – Shear rate

Binary MixtureTernary Mixture

For the newtonian fluids the shear-stress trend in function of the shear-rate is linear.


Viscosity

Discarded values in the average calculation Discarded values in the average calculation

Ternary Mixture Binary Mixture

For the newtonian fluids viscosity is not dependent to the shear-rate used for the

measurement.


Viscosity - Results

Temperature

Vis

cosi

ty

Ternary

Binary


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;


Specific Heat

Differential Scanner Calorimetry

Specific Heat calculation

3 steps

Blank

Sapphire

Salt

High Cp=

High thermal storage capacity


Specific Heat - Results

Temperature

Sp

ecifi

c H

ea

t

Ternary Salt

Binary Salt


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

log4

qt const

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.


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;


Probe method

Probe built by the lab. «thermophysical properties» of the Univ. of Rome «Tor Vergata»

20mm 50÷60 mm

stainless steel tube

Pt wires (heater)

epoxy handle termocouple

wires

termocouplewires

Pt wires (heater)

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;


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%);


Experimental measurementsAn example of calibration of the HW method using glycerine at

ambient temperature

2 3 4 5 6 7 8-1

4

9

14

19

0.8V1.6V2.0V2.4V3.2V4.0V4.4V6.0V

log(t)

∆T_w

ire [°

C]

testsλ

[W/mK]0,8V 0,3721,6V 0,3712,0V 0,3782,4V 0,3853,2V 0,4004,0V 0,3694,4V 0,3956,0V 0,360


Thermal Energy Storage (TES) tank

18

Solar Collectors

Storage tank and SG

Plant scheme



19

Sketch of the TES tank in the ENEA CSP facility

Sketch of TES tank with axisymmetric SG

configuration

Geometry details:



Computational grids investigated

(Produced by SnappyHexMesh grid generator) (Produced by BlockMesh grid generator)

Approx. 720000 cells Approx. 210000 cells



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;



Obtained Results

t = 0s t = 100s t = 500s t = 900s t = 1250s

Temperature fields:

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.



Obtained Results

t = 0s t = 100s t = 500s t = 900s t = 1250s

Velocity fields:

The highest velocity values are located at the inlet port and impinging zone.



Obtained Results

Temperature fields:

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.

t = 0s t = 1000s t = 5000s t = 10000s t = 12000s t = 14400s



Obtained Results

Velocity fields:

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.

t = 0s t = 1000s t = 5000s t = 10000s t = 12000s t = 14400s



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.


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!


s. pistacchio 2, g. bovesecchi 1, p. coppa 1 1 university of rome “tor vergata” – department...

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