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 fo rhigh temperature(HT-TCP) has been built and tested. Its design and construction procedure are adapted from the ambient temperature ther- mal conductivity probe(AT-TCP) due to the good performance softhislast.The construction procedure and the preliminary tests are accurately described.The probe contains a PtwireasheaterandatypeKthermocouple(K-TC)as temperature sensor, and its size are so small (diameter0.6mmandlength60mm) to guaranteea length to diameter ratioofabout100.Calibration tests with glycerolfor temperatures between 0C and 60C have shown a good Agreement with literature data,within3%.First tests on aternarysalt(18%inmassof NaNO3, 52% KNO3, and30% LiNO3) at120C and 150C , have given good results:an Agreement was found with the Thermal conduc- tivity of the standard solar salt(60% NaNO3, 40% KNO3), even if the data for this last have been extrapolated,being it solidat those temperatures. Unfortunately, at the higher temperaturetested(200C), the viscosityof the salt highly decreases,and free convection starts, making the measurements unreliable. A numerica linvestigation of the performance of the storage and evolution of the ther- mocline for theOPTSFull scaleconguration and for the OPTSsystem of theEnea Casaccia facility is carriedon.The full scale conguration has a tankheightinthe order of12m,because this choice allow stop operate the systeminnatural convection regime forlow charge fraction softh e storage.In order to obtainnumerical results in a time scalesuitable with computer resources and activities, the adoption of anaxisym- metric simplication of the geometriesis pursuit. The code OpenFOAMversion2.2.0 is used to perform the simulations. Code and model settings together with the adopted computational grids,initia land boundary conditionsare described in the following sec- tions. A summary of the simulation results is then given. A steady-state numerical investigation of the MSHeatExchanger prototype developed in ENEA Casaccia is presented.This component is realized to perform the heat exchange between moltensalts(aternarymixture ) and adiathermicoil and with a moderate tem- peraturegap(38C). In order to optimize the heate xchangereciency and toobtain the greatest contact are a between uids the pipe line series of diathermicoil is designed with anhelical geometry.The moltensaltsside is aconvectionalcy lindricalgeome- try with the Greater diameter in the region where pipe line series are located while the other portion of the heater has a diameter lower than the length of the cylinder.The codeOpenFOAMversion2.2.0 is use d 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.