  Thermal energy storage using sensible heating of a solid storage medium is a
potential low-cost technology for long-duration energy storage. To effectively
get heat in and out of the solid material, channels of heat transfer fluid can
be embedded within the storage material. Here we present design principles to
improve performance of channel-embedded thermal energy storage systems, and we
apply these principles to a high-temperature system using graphite as the
storage material and liquid tin as the heat transfer fluid. We first analyze
the impact of geometry and material properties on the performance of the
system, determining the ideal channel spacing and length to achieve high
(dis)charge temperature uniformity. We then analyze how controlling the fluid
flowrate, heating infrastructure, and heat engine can increase discharge power
uniformity and accelerate charging. Finally, we model 100 high-temperature
graphite storage blocks using a porous media approximation and implement the
developed design principles to demonstrate significant improvement in
performance for both discharging (constant discharge power for >90% of rated
duration) and charging (>90% charged within 4 hours). Overall, the hierarchical
design procedure presented here enables the design of cheap yet high-performing
solid thermal energy storage systems.
