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
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.

Ph D Program in Industrial Engineering - Research activity of interest for Energy
Molten Salts (MS)
Standard binary mixture
(Solar Salt)
 Sodium nitrate
(NaNO3 ) 60%
 Potassium nitrate (KNO3 ) 40 %
 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).
Ph D Program in Industrial Engineering - Research activity of interest for Energy
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 (shearrate) should be considered uniform between the
boundary layers and defined as:
With:
ux = velocity [m/s]
shear-rate
shear-stress
Ph D Program in Industrial Engineering - Research activity of interest for Energy
y = distance [m]
F = force[N]
A= surface [m²]
Viscosity - Reometer
Instrument: rotational reometer TA Instruments AR2000EX
Principle of working:
Rotore
Statore
Ph D Program in Industrial Engineering - Research activity of interest for Energy
Viscosity - Reometer
Analisys procedure
• Sample quantity used : 1600 mg.
•Viscosimetry shear-rate range is
between 20-500 (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.
Ph D Program in Industrial Engineering - Research activity of interest for Energy
Viscosity
For the newtonian fluids the shearstress trend in function of the
shear-rate is linear.
Shear stress – Shear rate
Ternary Mixture
Binary Mixture
Ph D Program in Industrial Engineering - Research activity of interest for Energy
Viscosity
For the newtonian fluids viscosity is not dependent to
the shear-rate used for the measurement.
Ternary Mixture
Discarded values in the average calculation
Binary Mixture
Discarded values in the average calculation
Ph D Program in Industrial Engineering - Research activity of interest for Energy
Viscosity - Results
Binary
Viscosity
Ternary
Temperature
Ph D Program in Industrial Engineering - Research activity of interest for Energy
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;
Ph D Program in Industrial Engineering - Research activity of interest for Energy
Specific Heat
Differential Scanner Calorimetry
Blank
3 steps
Specific Heat calculation
Sapphire
Salt
High Cp
=
High thermal
storage capacity
Ph D Program in Industrial Engineering - Research activity of interest for Energy
Specific Heat - Results
Specific Heat
Binary
Salt
Ternary
Salt
Temperature
Ph D Program in Industrial Engineering - Research activity of interest for Energy
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
t
const 
q
4
log 
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.
Ph D Program in Industrial Engineering - Research activity of interest for Energy
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;
Ph D Program in Industrial Engineering - Research activity of interest for Energy
Probe method
Probe built by the lab. «thermophysical properties» of the
Univ. of Rome «Tor Vergata»
stainless
steel tube
epoxy
handle
termocouple
wires
0.6 mm
termocouple
wires
Pt wires (heater)
20mm
50÷60 mm
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;
Ph D Program in Industrial Engineering - Research activity of interest for Energy
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%);
Ph D Program in Industrial Engineering - Research activity of interest for Energy
Experimental measurements
An example of calibration of the HW method using glycerine at
ambient temperature
19
0.8V
∆T_wire [°C]
14
1.6V
2.0V
9
2.4V
3.2V
4.0V
4
4.4V
6.0V
-1
2
4
6
λ
tests [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
8
log(t)
Ph D Program in Industrial Engineering - Research activity of interest for Energy
Thermal Energy Storage (TES) tank
Plant scheme
Solar Collectors
Storage tank and
SG
18
Ph D Program in Industrial Engineering - Research activity of interest for Energy
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
19
Ph D Program in Industrial Engineering - Research activity of interest for Energy
Thermal Energy Storage (TES) tank
Computational grids investigated
(Produced by SnappyHexMesh grid generator)
Approx. 720000 cells
(Produced by BlockMesh grid generator)
Approx. 210000 cells
Ph D Program in Industrial Engineering - Research activity of interest for Energy
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;
Ph D Program in Industrial Engineering - Research activity of interest for Energy
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.
Ph D Program in Industrial Engineering - Research activity of interest for Energy
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.
Ph D Program in Industrial Engineering - Research activity of interest for Energy
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
stratification is stable. No relevant differences in
temperature field at different radial positions appear.
Ph D Program in Industrial Engineering - Research activity of interest for Energy
the
the
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.
Ph D Program in Industrial Engineering - Research activity of interest for Energy
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.
Ph D Program in Industrial Engineering - Research activity of interest for Energy
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