Influence of the operational conditions on the transient charging performance of a thermal storage system using supercritical carbon dioxide

Abstract

Building upon the available literature, the present study investigates how the operational conditions of supercritical carbon dioxide (s-CO₂) acting as a storage medium influence the performance of a thermal energy storage (TES) system. The geometry is represented by a rigid-wall cylindrical reservoir filled with s-CO₂ and surrounded by an axially flowing heat transfer fluid (HTF) (not modeled in the present analysis). A 2D axisymmetric, transient formulation is developed for the reservoir and s-CO₂ assembly while relying on the SST k – ω turbulence model. The numerical solution uses finite volume-based commercial software to solve the conservation equations for both domains. The heat transfer mechanism within the s-CO₂ is exclusively based on natural convection and mimics the thermal charging process of the supercritical TES medium. The parametric analysis considers the s-CO₂ pressure and the charging temperature of the HTF as independent variables. A total of three discrete values were assumed for the charging temperature of the HTF (i.e., 325 K, 335 K, and 345 K) and four discrete initial pressure values for the CO₂ for each temperature, which ranged between 9 MPa and 19 MPa, totaling 12 different initial combinations. The results show that, for a given temperature, there is a pressure that maximizes the heat transfer coefficient. However, the volume-specific energy stored (i.e., kWh/m³) within the supercritical TES medium increases with the pressure since higher pressures imply more s-CO₂ within the storage system as the domain encapsulating the fluid has a fixed volume. Conversely, the results show that the mass-specific energy stored (i.e., kWh/kg) presents an optimal pressure value for a given temperature.

Keywords:

• Operational conditions
• supercritical carbon dioxide
• thermal energy storage
• thermodynamic states

Additional information

Funding

The authors appreciate the financial support received from Petrobras/ANEEL10.13039/501100007133 through grant #PD-00553-0042/2016. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001 and CNPq. F.G. Battisti thanks the financial support of ANID/CONICYT through FONDECYT Postdoctorado 2022 #3220792.