  Thermal Energy Storage (TES) devices, which leverage the constant-temperature
thermal capacity of the latent heat of a Phase Change Material (PCM), provide
benefits to a variety of thermal management systems by decoupling the
absorption and rejection of thermal energy. While performing a role similar to
a battery in an electrical system, it is critical to know when to charge
(freeze) and discharge (melt) the TES to maximize the capabilities and
efficiency of the overall system. Therefore, control-oriented models of TES are
needed to predict the behavior of the TES and make informed control decisions.
While existing modeling approaches divide the TES in to multiple sections using
a Fixed Grid (FG) approach, this paper proposes a switched Moving Boundary (MB)
model that captures the key dynamics of the TES with significantly fewer
dynamic states. Specifically, a graph-based modeling approach is used to model
the heat flow through the TES and a MB approach is used to model the
time-varying liquid and solid regions of the TES. Additionally, a Finite State
Machine (FSM) is used to switch between four different modes of operation based
on the State-of-Charge (SOC) of the TES. Numerical simulations comparing the
proposed approach with a more traditional FG approach show that the MB model is
capable of accurately modeling the behavior of the FG model while using far
fewer states, leading to five times faster simulations.
