  Phase change energy storage is getting increasing attention as a
representative technology to achieve carbon neutrality. The phase change
process exists typical phenomenon of asymmetry that affects the energy storage
performance. However, the mechanism of asymmetry is currently lack of
elaboration because the half domain model is always used to simplify the
numerical simulation and avoid the appearance of asymmetry. In this study, the
entire domain model and boundary conditions were adopted and numerically
simulated for the melting process of paraffin wax, i.e., the charging process
of energy storage. The nonlinear dynamics method was applied to explain the
asymmetric flow and heat transfer phenomenon. The three-stage characteristic
based on charging speed was proposed for charging process and was explained by
thermal conduction or natural convection. The asymmetric phenomenon was
elaborated on the cause of formation, change mechanism, and effect evaluation.
Results show that natural convection accounts for the multiple solutions
(including the asymmetric ones) of charging process. It is also pointed out
that asymmetric solutions can exist under symmetric geometry structure. To
accurately solve the charging process, it is necessary to use the entire domain
model. Both the charging speed and energy storage capacity have a positive
correlation with asymmetry. Thus, a potential idea for energy storage
application was proposed to increase the charging speed; that is, adding
thermal disturbance during the melting process to destroy flow stability and to
enhance convective heat transfer. The research methods and findings will
support the development of energy storage and numerical investigation in other
related areas.
