Thermal conductivity and thermal linear expansion measurements on molten salts for assessing their behaviour as heat transport fluid in thermodynamics solar systems

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Molten salts (sodium and potassium nitrides) are going to be used in many different plants as heat transferring fluids, e.g. concentration solar plants, nuclear power plants, etc. In fact they present may important advantages: their absolute safety and non toxicity, availability and low cost. But their use, e.g. in the energy receiving pipe in the focus of the parabolic mirror concentrator of the solar thermodynamic plant, requires the accurate knowledge of the thermophysical properties, above all thermal conductivity, viscosity, specific heat and thermal linear expansion, in the temperature range 200°C÷600°C. In the new laboratory by ENEA Casaccia, SolTerm Department all these properties are going to be measured. Thermal conductivity is measured with the standard probe method (linear heat source inserted into the material) manufacturing a special probe suited to the foreseen temperature range (190-550°C). The probe is made of a ceramic quadrifilar pipe containing in different holes the heater (Ni wire) and the thermometer (type J thermocouple). The thermal linear expansion will be measured by a special system designed and built to this end, measuring the sample dilatation by the reflection of a laser beam by the bottom of the meniscus in the liquid solid interface. The viscosity will be evaluated detecting the start of the natural convection in the same experiment as to measure thermal conductivity. In the paper the construction of the devices, the results of preliminary tests and an evaluation of the obtainable accuracy are reported.