Enabling high conductance and high energy density in supercritical fluids for thermal storage applications
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(2020) 42:529
TECHNICAL PAPER
Enabling high conductance and high energy density in supercritical fluids for thermal storage applications Gustavo M. Hobold1,2 · Alexandre K. da Silva1 Received: 23 April 2020 / Accepted: 12 August 2020 © The Brazilian Society of Mechanical Sciences and Engineering 2020
Abstract Supercritical fluids have recently been proposed as a sensible media for thermal energy storage. Given the importance of natural convection to sensible storage, the present work explores the existence of optimal operating pressures in supercritical fluids relying on natural convection as the heat transfer mechanism. A theory-based formulation for estimating the optimal operating conditions is presented and validated with experimental data reported in the literature. The heat transfer coefficient is shown to be related to the thermodynamic state of the fluid, which strongly affects its thermophysical properties. In particular, the optimal operating conditions are shown to lie in a thermodynamic region where properties present large variations within small temperature and pressure intervals. Finally, the relationship between the optimal operating pressures for maximum natural convection heat transfer, resulting in maximal global conductance, and maximum energy density in supercritical fluids is explored. Here, it is demonstrated that these desirable properties are strongly correlated, suggesting that a supercritical thermal energy storage system can take advantage of both enhanced heat transfer, as well as high energy density. Keywords Supercritical heat transfer · Natural convection · Thermal energy storage · Supercritical working fluid List of symbols A Area [m²] c Specific heat [J/(kg K)] D Diameter [m] E Energy [J] g Acceleration due to gravity [m/s²] h Heat transfer coefficient [W/(m² K)] i Specific enthalpy [J/kg] k Thermal conductivity [W/(m K)] L Characteristic length [m] M Mass [kg] n Power law coefficient [–] p Pressure [Pa] P Power [W] Q Energy density [J/m³] Technical Editor: Francis HR Franca, Ph.D. * Alexandre K. da Silva [email protected] 1
Department of Mechanical Engineering, Federal University of Santa Catarina, Florianópolis, SC 88040‑900, Brazil
Present Address: Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
2
t Time [s] T Temperature [K] Greek symbols β Coefficient of thermal expansion [1/K] µ Dynamic viscosity [Pa s] ρ Density [kg/m³] Dimensionless numbers Nu Nusselt number Ra Rayleigh number Subscript b Bulk c Critical L Characteristic length l Unloaded max Maximum min Mininum p At constant pressure u Loaded w Wall Superscript – Mean ~ Dimensionless
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Journal of the Brazilian Society of Mechanical Sciences and Engineering
1 Introduction The use of supercritical fluids in heat transfer and energy applications has been under investigation since the 1960s and 1970s, which first demonstrated that supercritical fluids may offer high heat transfer coeffic
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