L. McLaughlin, N. Schroeder, W. Gerstle
Sandia National Laboratories,
Keywords: thermal energy storage, packed bed, long duration energy storage, low cost
Summary:Long duration energy storage (LDES) technologies are essential to maintain a resilient, reliable, and 24/7 energy grid that is powered by intermittent renewable sources. Thermal energy storage (TES) systems designed for ≥8 hours of storage and ≥100 MWe power capacity present economic advantages over similarly sized battery technologies and a higher degree of geological independence compared to pumped hydro storage and compressed air energy storage in caverns. This work describes the design, testing, and model validation of a low-cost radial packed bed TES system to be used for long duration energy storage (LDES). The work aims to develop, validate, and de-risk the LDES TES technology at a 100-kWh scale to accelerate the technology to commercial deployment at utility scale. The system in this work utilizes low-cost gravel as a storage medium and air as a heat transfer fluid (HTF). Experimental and numerical results are presented for a 4-hour charge, 16-hour hold, and 4-hour discharge of the packed bed system. Results show that a thermocline formed in the packed bed during the 4 charging hours and that >80% of the stored thermal energy was retained during 16 hours of holding. Effects of buoyancy within the bed were found to be minimal during charging, holding, and discharging, suggesting that forced convection is the dominant heat transfer mechanism within the packed bed. Results also show that the radial bed technology heated uniformly, suggesting that the proposed design mitigates preferential flow. Numerical model results well captured the flow characteristics of the experimental system, such as pressure drop through the bed, whereas the modelled thermocline showed a larger gradient compared to the experimental thermocline. These findings suggest the model captures the flow dynamics of the packed bed system, where parameters within the modelled Schumann equation should be refined to better capture the thermal interaction between the HTF and storage medium. This study demonstrates the potential of the radial packed bed as a LDES technology and has prompted continued development of the system with the inclusion of electric charging, heat output for process heat applications, and electric power generation via an organic steam cycle.