Thermal conductivity viscosity and specific heat of Molten

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) 1 University S. Pistacchio 2 , G. Bovesecchi 1 , P. Coppa 1 of Rome “Tor Vergata” – Department of Industrial Engineering 2 Research Center ENEA Casaccia - Technical Unit for Renewable Energy Sources (UTRINN) Ph. D Program in Industrial Engineering for Health, Environment and Energy

Contents Overview of the experimental methods taken into account to the characterization of thermophysical Molten Salt properties: v. Viscosity (momentum transfer method); v. Specific Heat (DSC); v. Hot Wire Method; v. Probe Method; Ø Preliminary calibration case: A glycerine test. Ø Thermal Energy Storage (TES) tank optimization: v. Geometry details; v. Obtained results and comparison with experimental data; Ø Conclusion. Ø Ph D Program in Industrial Engineering - Research activity of interest for Energy

Molten Salts (MS) Standard binary mixture (Solar Salt) § Sodium nitrate (Na. NO 3 ) 60% § Potassium nitrate (KNO 3 ) 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) Ø 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 AR 2000 EX 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 Al 2 O 3) 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 Ø Base of the method: a metal wire is heated by an electric current. Detected quantities: v. Wire temperature; v. 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. 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. Ø Requirements: vl/d ratio >50, better 100; v. Ratio rsample /rprobe>100 (better, but if it is lower test times must be reduced, according to twall); Ø Advantages: v. Compact vportable, can also be used in field; Ø Drawbacks: v. 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» 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 Mg. O or Al 2 O 3 powder); • Accuracy results lower (5÷ 10%); Ph D Program in Industrial Engineering - Research activity of interest for Energy

∆T_wire [°C] Experimental measurements An example of calibration of the HW method using glycerine at ambient temperature 19 0. 8 V 14 1. 6 V 2. 0 V 9 2. 4 V 3. 2 V 4. 0 V 4 4. 4 V 6. 0 V -1 2 3 4 5 log(t) 6 7 λ tests [W/m. K] 0, 8 V 0, 372 1, 6 V 0, 371 2, 0 V 0, 378 2, 4 V 0, 385 3, 2 V 0, 400 4, 0 V 0, 369 4, 4 V 0, 395 6, 0 V 0, 360 8 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 Snappy. Hex. Mesh grid generator) Approx. 720000 cells (Produced by Block. Mesh grid generator) Approx. 210000 cells Ph D Program in Industrial Engineering - Research activity of interest for Energy

Thermal Energy Storage (TES) tank Boundary conditions Ø Velocity: v. No-slip condition was imposed at all solid surface walls; v. Imposed time dependent volumetric flow at inlet; Ø Pressure: v. Zerogradient everywhere with exception of the outlet where a fixedvalue (P-rgh=0) has been imposed; Ø Temperature: v. 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 = 0 s t = 100 s t = 500 s t = 900 s t = 1250 s v After 100 s the cold jet coming from the SG has mainly mixed up the lower layers of the temperature stratification. v 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 = 0 s t = 100 s t = 500 s t = 900 s t = 1250 s v 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 = 0 s t = 1000 s t = 5000 s t = 10000 s t = 12000 s t = 14400 s v 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. Ph D Program in Industrial Engineering - Research activity of interest for Energy

Thermal Energy Storage (TES) tank Obtained Results Velocity fields: t = 0 s t = 1000 s t = 5000 s t = 10000 s t = 12000 s t = 14400 s v 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: v The initial conditions at the bottom are not measured, which includes some uncertainty. v 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 Ø Viscosity values after 450°C are similar between binary and ternary mixtures. Ø Need to improve the experimental setup used for the HWM to make a proper calibration and subsequent measurement campaigns. Ø Possibility to realize an alternative setup using a four terminals hot wire. Ø Realize a new study case of the TES tank simulation using a different solver (cht. Multi. Region) 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