Molten Salt as a Heat Transfer Fluid
in Solar Thermal Power Plant
Applications
Stewart Wyatt
MANE6970
Fall 2015
Solar Thermal Power Plants
A Solar Thermal Power Plant (STPP)
utilizing molten salt as a heat
transfer fluid and thermal energy
storage medium, may continue to
operate during solar disturbances.
Figure from Medrano, M., A. Gil, I. Martorell, X. Potau, L. F. Cabeza. 2010. “State of the Art on High-Temperature Thermal Energy
Storage for Power Generation. Part 2 – Case Studies.” In Renewable and Sustainable Energy Reviews, Volume 14, Pages 56-72.
Molten Salt as a Heat Transfer Fluid
The use of molten nitrate salt (60%w NaNO3, 40%w KNO3) is investigated with respect
to its heat transfer characteristics at the high Reynolds and Nusselt numbers found in
the receiver of a practical solar thermal power plant.
A review of the literature has provided data for this and other molten salts but only
below the required Reynolds and Nusselt number ranges.
Molten Salt as a Heat Transfer Fluid
The differential temperature between the tube wall and heat transfer fluid of the
STPP central receiver is investigated with concentrated solar flux of up to 1.0 MW.m-2.
Available means to deal with the heat transfer predictions for turbulent flow with
variable physical properties are reviewed.
Nusselt number correlations reviewed include:
• Dittus-Boelter
• Sieder-Tate
• Petukhov
• Gnielinski
Molten Salt as a Heat Transfer Fluid
The influence of the variable properties with temperature of molten salt, especially
dynamic viscosity, are considered.
Dynamic Viscosity versus Temperature for liquid 60%w NaNO3/40%w KNO3:
Molten Salt as a Heat Transfer Fluid
The influence of surface roughness in the calculation of the Nusselt number is
considered. The wide range of friction factor data suggests it may be of greater
significance than the Nusselt number correlation.
Kolb, G. J. 2011. An Evaluation of Possible Next-Generation High-Temperature Molten-Salt Power Towers. Albuquerque,
NM: Sandia National Laboratories.
Norris, R. H. 1970. “Some Simple Approximate Heat-Transfer Correlations for Turbulent Flow in Ducts with Rough
Surfaces.” In Augmentation of Convective Heat and Mass Transfer, Edited by A. E. Bergles, R. L. Webb., The Winter Annual
Meeting of the American Society of Mechanical Engineers. New York: ASME.
Molten Salt as a Heat Transfer Fluid
Various operating scenarios are considered with the resulting temperature
differential between the inside tube wall and the bulk fluid temperature determined.
A circular tube exposed to external collimated radiation model is used [Mackowski
2015]. The resulting deviation from the constant fluid property assumption is
investigated.
Mackowski, D. W. 2015. Conduction Heat Transfer: Notes for MECH 7210. Mechanical Engineering Department, Auburn
University. Accessed 11 September. www.eng.auburn.edu/~dmckwski/mech7210/condbook.pdf