PCM Components in a Hot Water Tank: Load & Discharge

PCM Components in Hot Water Tank Thermal Analysis (Load & Discharge), CFD Simulation Training

Description

A Phase Change Material (PCM) is a substance capable of releasing or absorbing substantial energy during a phase transition, making it useful for heating or cooling applications. This transition typically occurs between the solid and liquid states, though it can also occur between non-classical states of matter — such as a crystal shifting between different crystalline structures representing higher or lower energy states.

By melting and solidifying at a defined Phase Change Temperature (PCT), a PCM can store and release far more energy than conventional sensible heat storage, since heat is absorbed or released specifically as the material's phase or internal structure changes — a mechanism that gives PCMs their designation as Latent Heat Storage (LHS) materials.

This project simulates a system of 16 PCM capsules positioned within a water tank. Hot water enters the domain at 0.5 m/s and 335 K, driving the PCM capsules through a complete melting cycle; once fully liquid, cold water is introduced to cool the system, initiating the reverse solidification process as the PCMs release their stored thermal energy.

Geometry & Mesh

The 3D domain was designed in Design Modeler, featuring an inlet, an outlet, and dedicated wall boundaries representing the PCM capsules. The domain was meshed in ANSYS Meshing using an unstructured grid totaling 339,466 elements.

Methodology

Several assumptions were applied to the simulation: a pressure-based solver was used, gravitational effects were excluded, and the simulation was run as unsteady.

Key simulation settings included:

• Viscous model: SST k-omega, with the energy equation enabled

• Boundary conditions: Velocity inlet at 0.5 m/s, with hot water entering at 333 K and cold water at 272 K; pressure outlet at 0 Pa gauge pressure; stationary tank walls with no thermal condition applied; stationary PCM capsule walls with a coupled thermal condition

Conclusion

The average temperature trends for both the surrounding environment and the PCM capsules reveal a clear two-stage thermal cycle. As hot water enters the domain, the environment's temperature rises, correspondingly driving up the PCMs' average temperature and initiating melting at around 343 seconds; by 1500 seconds, all PCM capsules have fully melted.

Once cold water is introduced, the domain's average temperature begins to decline, though the PCMs release their stored latent heat more gradually — causing their own average temperature to decrease at a notably slower rate than the surrounding water. Solidification begins roughly 500 seconds after the cold water enters, with the PCM capsules fully solidified by 4500 seconds — confirming the system's ability to store and later release thermal energy across a complete charge-discharge cycle.