Experimental measurement of the molten salts (MS) thermal conductivity and verification of the thermocline stability in thermal energy storage (TES) system

A thermal conductivity probe for high temperature (HT-TCP) has been built and tested. Its design and construction procedure are adapted from the ambient temperature thermal conductivity probe (AT-TCP) due to the good performances of this last. The construction procedure and the preliminary tests are accurately described. The probe contains a Pt wire as heater and a type K thermocouple (K-TC) as temperature sensor, and its size are so small (diameter 0.6 mm and length 60 mm) to guarantee a length to diameter ratio of about 100. Calibration tests with glycerol for temperatures between 0 ◦C and 60◦C have shown a good agreement with literature data, within 3%. First tests on a ternary salt (18% in mass of N aNO3, 52% KNO3, and 30% LiNO3) at 120◦C and 150◦C , have given good results: an agreement was found with the thermal conductivity of the standard solar salt (60% N aNO3, 40% KNO3), even if the data for this last have been extrapolated, being it solid at those temperatures. Unfortunately, at the higher temperature tested (200◦C), the viscosity of the salt highly decreases, and free convection starts, making the measurements unreliable. A numerical investigation of the performance of the storage and evolution of the thermocline for the OPTS Full scale configuration and for the OPTS system of the Enea Casaccia facility is carried on. The full scale configuration has a tank height in the order of 12 m, because this choice allows to operate the system in natural convection regime for low charge fractions of the storage. In order to obtain numerical results in a timescale suitable with computer resources and activities, the adoption of an axisymmetric simplification of the geometries is pursuit. The code OpenFOAM version 2.2.0 is used to perform the simulations. Code and model settings together with the adopted computational grids, initial and boundary conditions are described in the following sections. A summary of the simulation results is then given. A steady-state numerical investigation of the MS Heat Exchanger prototype developed in ENEA Casaccia is presented. This component is realized to perform the heat exchange between molten salts (a ternary mixture)and a diathermic oil and with a moderate temperature gap (38◦C). In order to optimize the heat exchanger efficiency and to obtain the greatest contact area between fluids the pipeline series of diathermic oil is designed with an helical geometry. The molten salts side is a convectional cylindrical geometry with the greater diameter in the region where pipeline series are located while the other portion of the heater has a diameter lower than the length of the cylinder. The code OpenFOAM version 2.2.0 is used to perform the simulations for the discharging phase. Code and model settings together with the adopted computational grids, initial and boundary conditions are described in the following sections and summary of the simulation results is then given.