Latent Heat Thermal Energy Storage System Simulation With A Supercritical Carbon Dioxide Working Fluid in a Concentric Borehole Tube for Solar Plants

The shift to intermittent renewable energy, such as wind and solar power, requires the use of energy storage systems to maintain a reliable flow of power at all times. Thermal energy storage, when coupled to a heat engine, is being considered as an alternative to batteries for electrical generation, and can be used directly for industrial heat needs. This paper investigates one section of a Metal Alloy Thermal Energy Storage System (MATESS) using supercritical carbon dioxide (sCO2) as the heat transfer fluid and aluminum silicide (AlSi) as the phase change material (PCM) for energy storage. The geometry considered is a concentric borehole heat exchange pipe carrying the sCO2 into a vessel containing the AlSi. In an actual system there would be many such heat exchange pipes in the vessel, and here we model one and the surrounding PCM. Numerical simulations are used to evaluate the thermal performance during both charging (melting) and discharging (solidifying) phases, though due to run-time limitations this paper focusses on the discharging mode. The model incorporates the full 9 m long borehole pipe geometry, including a dome-shaped end and a coupled inner wall, to enhance accuracy in heat transfer and flow dynamics. Simulations were performed using ANSYS Fluent with a pressure-based solver and solidification/melting models. The PCM domain was modeled using the k-ω turbulence model to capture buoyancy-driven convection, while the sCO2 domain was treated as turbulent based on the Reynolds number. A 2D axisymmetric model with a fine mesh and inflation layers, along with the wall model in Fluent, handled turbulent boundary layer effects in the sCO2. The dome-shaped end influenced natural convection patterns in the PCM, while treating the inner wall as coupled reduced sCO2 outlet temperatures compared to adiabatic conditions as was done in previous work. New simulations under no-gravity conditions highlighted significant differences in phase-change dynamics, emphasizing the importance of gravitational forces on flow and heat transfer behavior. The mean outlet temperature of the sCO2 was found as a function of time, showing that the system could directly power a sCO2 Brayton cycle for over 15 hours before a noticeable decrease in efficiency occurs. Understanding the behavior of a single heat transfer tube can help in the design of a full-scale system to store energy for utilities or industry for periods when solar or wind power is not available.