The National Solar Thermal Test Facility (NSTTF) at Sandia National Laboratories is conducting research on a Generation 3 Particle Pilot Plant (G3P3) that uses falling sand-like particles as the heat transfer medium.  The system will include a thermal energy storage (TES) bin with a capacity of 6 MWh~t­~ requiring \~120,000 kg of flowing particles.  The vertically integrated system design allows particles to fall by gravity from the receiver into the insulated TES bin before entering the heat exchanger.  Testing and modeling were conducted to develop a validated modeling tool to understand temporal and spatial temperature distributions within the storage bin as it charges and discharges.  The current G3P3 bin design has a flat bottom with a center hole that induces funnel-flow dynamics in the bulk solids.  Funnel-flow is characterized by a central flow channel surrounded by an annulus of stagnant particles.  Particles flow away from the cooler walls of the bin and into the hotter central flow channel before reaching the outlet. 

Flow and energy transport in funnel-flow was modeled using volume averaged conservation equations (similar to porous flow) coupled with level set interface tracking equations that prescribe the dynamic geometry of particle flow within the storage bin.  A thin layer of particles on top of the particle bed was allowed to flow toward the center and into the flow channel above the outlet.  A stagnant region of particles existed around the flow channel.  The drawdown angles of the top particle layer and flow channel were prescribed in the level-set model based on measured particle properties at various temperatures.  The prescribed geometry of the funnel-flow zone was stationary, but the level of particles within the storage bin was lowered at a rate commensurate with the desired mass flow rate out of the bin.  Similar methods were used to evaluate the inverse process of filling a bin. 

Model results were validated using particle discharge temperatures taken from thermocouples mounted throughout a small steel bin initially filled with \~65 kg of particles heated to 800° C.  The model was then used to predict heat loss during charging, storing, and discharging operational modes at the G3P3 scale.  Comparative results from the modeling and testing of the small bin indicate that the model captures many of the salient features of the transient particle outlet temperature over time.  The model also showed that the funnel-flow design has less heat loss overall than a similarly sized bin designed for uniform mass-flow.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.


Testing and Simulations of Spatial and Temporal Temperature Variations in a Particle-Based Thermal Energy Storage Bin

Division / Sponsor:
Advanced Energy Systems Division (AESD) & Solar Energy Division (SED)

Paper Proceedings: https://asmedigitalcollection.asme.org/ES/ES2020/volume/83631

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