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The solar energy research team at VU is developing a novel process for producing Magnesium
(Mg) that is less energy intensive, less polluting, and cost competitive with commercially deployed
processes. To reduce the energy intensity and CO2 emissions associated with Mg production, the
VU process leverages the high-temperatures attainable with concentrated solar energy from the
solar furnace in the James. S. Markiewicz Solar Energy Research Facility in order to substitute
solar thermal energy for electric energy.
A major challenge of the research effort is to develop an interface that allows the integration of
the solar thermal energy into the electrolytic cell where the Mg is produced. The heat transfer from
the outside of the cell into its hot interior containing a molten salt electrolyte near 1000 °C must
be fast enough to prevent overheating of cell components. In this study, our aim was to measure
the rate of heat transfer into the cell which occurs via natural convection. Experiments are
necessary because of the unique geometry of the internal elements of the electrolytic cells that
precludes the application of existing empirical correlations for natural convection heat transfer.
A direct measurement of the heat transfer in the molten salt electrolyte during MgO electrolysis,
while preferred, presents substantial experimental challenges due to the corrosiveness of the
molten salts, the solid-to-liquid phase change upon heating, operating temperatures near 1200 K,
and convoluting effects from electric currents. In an effort to increase the accuracy of the heat
transfer measurement in the face of these challenges, we developed a low-temperature
experimental model based on non-corrosive fluids. Lower temperatures allowed the application
of readily available copper-constantan thermocouples and electric surface-mounted heaters
capable of supplying heat fluxes at accurate levels. Our model allowed for the measurement of
Nusselt numbers—which provide an indication of the rate of heat transfer—over a range of
Rayleigh numbers from 107 to 7×108 and Prandtl numbers from 2 to 6200 that encompass the
expected operating range of the high-temperature motel salt filled electrolytic cell. The heat
transfer results are collapsed into a single correlation equation that predicts the Nusselt number in
the electrolytic within 16%. Application of the model suggests that for the heat fluxes required in
the electrolytic cell, the heat transfer will lead to a 40 °C temperature difference between the
outside and inside of the cell, well below levels for which overheating becomes a concern.