Skip to main content
en

Thermal storage with phase change materials

Latent heat provides a large buffer, but as the melt front advances the resistance grows. The critical quantity is not stored energy but how fast it can be taken out.

A phase change material (PCM) stores a large amount of latent heat within a narrow temperature range around its melting point. Damping electronics peak loads, buffering a battery pack and building thermal inertia are typical applications.

The real constraint is power, not capacity

The thermal conductivity of PCMs is low — for typical organic PCMs it is below that of water. As melting progresses the liquid layer thickens and an ever-growing resistance appears between the heat source and the still solid region.

The consequence: the material can store enough energy but cannot take it in fast enough. The design work is therefore about raising the conductivity — fins, metal foam, graphite matrix or encapsulation.

Do not neglect natural convection

In the molten region the liquid circulates under buoyancy, and that circulation markedly increases the melting rate. A conduction-only model systematically overpredicts the melting time. See the natural convection note.

Modelling decisions

Enthalpy-porosity approach. The interface is not tracked explicitly; each cell is given a liquid fraction and the solid region is immobilised with a high resistance. This is the practical route for industrial problems.

Melting range. Outside pure substances, melting occurs over a range rather than at a single temperature. Setting the range near zero destabilises the solution.

Hysteresis. The melting and solidification curves are not identical, and in a cyclically operating system that difference accumulates.

Volume change. Melting typically involves a few percent expansion; in a sealed capsule that means pressure.

Validation

PCM analyses are extremely sensitive to material data. Without stating the source of the latent heat, melting range and conductivity, the result cannot be argued about. See the significant digits note.

References

  1. Shah, R. K. & Sekulić, D. P. (2003). Fundamentals of Heat Exchanger Design. John Wiley & Sons. DOI: 10.1002/9780470172605
  2. Churchill, S. W. & Chu, H. H. S. (1975). Correlating equations for laminar and turbulent free convection from a vertical plate. International Journal of Heat and Mass Transfer, 18(11), 1323–1329. DOI: 10.1016/0017-9310(75)90243-4

Let's discuss your project

Let us pin down together the system to be analysed and the question that needs answering.

Let's discuss your project