(14j) Surface Tension Measurements of Solar Salt at High Temperature

For energy storage for solar thermal and many nuclear engineering practices, molten salts are used as a heat storage medium or heat transfer fluids due to its unique properties such as low vapor pressure and high heat capacity. However, their properties are not very well measured since experiments should be done at high temperatures and, sometimes, high pressure. In addition, many kinds of molten salts are expensive, yet many different methods require a large amount (> 30 g) to measure properties. In this report, we present a method that uses only small amounts (~ 50 mg) of molten salt in measuring surface tension with ADSA (Axisymmetric Drop Shape Analysis) method. Though ADSA method is very well known and used for many years, the implementation of the method inside an oven at high temperature limits the maneuver that is required for aligning the liquid droplets to a camera. Since the droplets cannot be placed as a liquid, but as solid, the formation of an axisymmetric droplet is not guaranteed. Therefore, this report presents the challenges of using the ADSA method at high temperatures with molten salt inside a closed oven. The goal was to measure the surface tension of solar salt (eutectic mixture of 60 wt. % sodium nitrate and 40 wt. % potassium nitrate) from 250 to 450°C. First, the method and the computer code written for the method were verified by capturing and analyzing water droplet images inside an oven at room temperature. This yielded the water surface tension value within a reasonable error (< 2.8 %). Subsequently, solar salt was used to measure its surface tension. Small discs of frozen solar salt were placed on boron-nitride-coated microscope slides at room temperature. These slides were placed inside an oven and the temperature was increased to the desired temperature which melted the frozen solar salt. The droplets were images with a long-range microscope installed outside the oven. The analysis with the ADSA method yielded the surface tension of solar salt from 109.9 to 115.6 mN/m (depending on temperature) which is in good accordance with the literature.