Modeling of high temperature thermal energy storage in rock beds – Experimental comparison and parametric study

The increasing penetration of renewables in energy systems requires the integration of energy storage units into electrical grids as a crucial aspect. Nevertheless, current ES technologies show, as a common issue, an excessive capital cost, which prevents them from being implemented on a large scale. The combination of high temperature thermal energy storage and bottom steam cycles has recently become an object of interest as a potential cost-effective alternative to traditional ES. In this study, a two-dimensional model of an existing high temperature thermal energy storage rock bed unit with 450 kWh_th of thermal capacity is implemented. A description of the geometry, equations and boundary conditions is provided, as well as a comparison of the model results with the experimental data logged from the reference testing unit. A brief discussion about the solution method and the relative error on the final results is also presented. A study regarding the charge phase was performed, with the main focus on the modification of some parameters of interest such as rock size, air flow rate, rock type and insulation material. Results are presented in terms of two-dimensional temperature profiles, charge efficiencies and heat losses, highlighting the differences between the scenarios considered. Thermal charge efficiency was found in the range 69–96% for the considered simulations, and different improvement aspects are suggested for future studies.