Question

can you cite an industrial process where phase-change issues are a concern? What is common practice in the industry to deal with this issue?

Answer 1

There are six changes of phase that substances go through:

• Freezing: liquid to solid.
• Melting: solid to liquid.
• Condensation: gas to liquid.
• Vaporization: liquid to gas.
• Sublimation: solid to gas.
• Deposition: gas to solid. These are the phase change processes that are used in industries.

In the thermal energy storage system, the thermal properties of phase change materials (PCM) have a great influence on the system performance. In this paper, paraffin-based composite phase change material with different graphite additive (expanded graphite, EG; graphene, GR; and graphene oxide, GO) and different concentrations (0.5 to 2.0%) are manufactured by a two-step method combining mechanical agitation and ultrasonic vibration. Physical principles of thermal energy storage
When a thermal storage need occurs, there are three main physical principles to provide a
thermal energy function:
• Sensible heat
The storage is based on the temperature change in the material and the unit storage
capacity [J/g] is equal to heat capacitance × temperature change.
• Phase-change
If the material changes its phase at a certain temperature while heating the substance
then heat is stored in the phase change. Reversing, heat is dissipated when at the phasechange temperature it is cooled back. The storage capacity of the phase change
materials is equal to the phase change enthalpy at the phase change temperature +
sensible heat stored over the whole temperature range of the storage.
• Chemical reactions
The sorption or thermo chemical reactions provide thermal storage capacity. The basic
principle is: AB + heat ⇔ A+B; using heat a compound AB is broken into
components A and B which can be stored separately; bringing A and B together AB is
formed and heat is released. The storage capacity is the heat of reaction or free energy
of the reaction.
Figure 1 illustrates the change of storage capacity Q for the three different thermal storage
types as a function of temperature or fraction of compound (X=B).

The storage systems based on chemical reactions have negligible losses whereas a sensible
heat storage dissipates the stored heat to the environment and need to be isolated.