  Molten salts are being used or explored for thermal energy storage and
conversion systems in concentrating solar power and nuclear power plants.
Thermal conductivity of molten salts is an important thermophysical property
dictating the performance and cost of these systems, but its accurate
measurement has been challenging, as evidenced by wide scattering of existing
data in literature. The corrosive and conducting nature of these fluids also
leads to time consuming sample preparation processes of many contact-based
measurements. Here, we report the measurement of thermal conductivity of molten
salts using a modulated photothermal radiometry (MPR) technique, which is a
laser-based, non-contact, frequency-domain method adopted for molten salts for
the first time. By unitizing the advantages of front side sensing of
frequency-domain measurements and the vertical holder orientation, the
technique can minimize the natural convection and salt creeping effects, thus
yielding accurate molten salt thermal conductivity. The MPR technique is first
calibrated using standard molten materials including paraffin wax and sulfur.
It is then applied on measuring pure nitrate salts ($NaNO_3$ and $KNO_3$),
solar salt ($NaNO_3-KNO_3$ mixture), and chloride salt ($NaCl-KCl-MgCl_2$). The
measurement results are compared with data from literature, especially those
obtained from laser flash analysis (LFA). Our results demonstrate that the MPR
is a convenient and reliable technique of measuring thermal conductivity of
molten salts. Accurate thermal conductivity data of molten salts will be
valuable in developing the next-generation high-temperature thermal energy
storage and conversion systems.
