Experimental testing and computational modeling of a radial packed bed for thermal energy storage

To de-risk commercial applications, this paper describes the experimental testing and computational simulation of a 100 kWhth radial packed bed for thermal energy storage (TES). Air is utilized as the heat transfer fluid and the thermal storage medium is pea gravel. Air flows radially from and to the injection well through the gravel for charging and discharging. The packed bed Reynolds number varies from approximately 25 at the inner radius to 10 at the outer radius, implying laminar flow throughout the packed bed. A special-purpose one-dimensional radial MATLAB model is employed for simulating the behavior of the packed bed storage system. The experimental results and the computational results are compared. It is determined that a modified version of the 1929 Schuman model reasonably captures the physics of the radial packed bed, despite the fact that the heat transfer fluid is a gas with thermally-varying density and viscosity, rather than a constant-density liquid as assumed by Schumann. To accurately simulate long-duration storage, in addition to volumetric convective heat transport, it is also essential to include models representing thermal diffusion within the packed bed, as well as models for heat loss to the surrounding environment. The main purpose of this paper is to gain sufficient technical understanding of air/gravel radial packed beds to design and de-risk larger TES systems for commercial purposes. The first key outcome is that the exergetic round-trip efficiency of the repository for two diurnal heat storage use cases is defined, demonstrated, and simulated. A second outcome is that the exergetic efficiency of a much larger TES system is predicted. A scaled-up 100 MWhth TES repository is simulated over a 16-day period of long duration energy storage (LDES) thermal cycling. The predicted exergetic efficiency of the larger facility is found to be much higher than that of the small-scale 100 kWhth experiment. Another key outcome of this work is that a high-temperature radial air/gravel TES system is shown to be an efficacious way to store thermal energy, and this paper increases confidence in the prospects of developing larger facilities with round-trip exergetic efficiencies around 90%, suitable for LDES for power generation and process heat applications.