Modeling and Design of Net ZEBs as Integrated Energy Systems - Modeling, Design, and Optimization of Net-Zero Energy Buildings - page 73

Depending on the volume of the storage tank, its level of insulation, the
ambient temperature, and the level of fluid movement inside the tank,
temperature stratification can be observed.
2.2.2.4 Modeling of Thermal Storage Tanks
Several models of storage tank have been developed with different levels of
detail to account for thermal stratification.
The simplest approach considers the storage tank as a single node ( Figure
2.21 ) . Performing an energy and mass balance (with or without heat
exchanger) for that node allows the calculation of the node temperature
evolution over time. This approach can be acceptable for small storage
tanks. The following are the equations for a single node storage tank with
one heat exchanger: