Phase transition enthalpy

The phase transition enthalpy is defined as the difference between the (extrapolated) solid and liquid enthalpies.
Two different cases can be distinguished:

• Solid and liquid heat capacities of the PCM are equal

  The phase transition enthalpy is constant. This is the case for the user defined PCM_01 in the next figures. Within (and outside) the phase transition range the phase transition enthalpy is always 180.0 J/g (left figure). Moreover, the 'peak area' of the apparent heat capacity is also 180.0 J/g (shown in the right figure by the area between apparent heat capacity and baseline).

       

• Solid and liquid heat capacities of the PCM are NOT equal

  The phase transition enthalpy depends on temperature. This is the case for the user defined PCM_02 in the next figures. Evaluated at 52 °C the phase transition enthalpy is 180.0 J/g (left figure). However, this value is smaller for lower temperatures. Moreover, the 'peak area' of the apparent heat capacity is also lower (shown in the right figure by the area between apparent heat capacity and baseline).

       

Heat capacity of PCM's is often charaterized either by the value of the 'peak area' (area between apparent heat capacity and baseline, see above) or by the 'total area' (area between apparent heat capacity and zero). As discussed above, these values do not necessarily coincide with the value of the (temperature-dependent) phase transition enthalpy evaluated at a certain temperature.

In slPCMlib the following definitions are used:

• In the database in the table Material's specification the 'peak area' obtained from the data for heating is given.
• In the internal calculations and in the computer code generated by slPCMlib the phase transition enthalpy is the difference between the (extrapolated) solid and liquid enthalpies.
• For the definition of user defined PCM the value evaluated at the upper limit of the phase transition temperature range is used as a reference value.

References

• Barz, T., Buruzs, A., & Sommer, A. (2023).
  Major and minor hysteresis loops in the enthalpy-temperature and phase fraction-temperature diagrams of solid/liquid phase change materials.
  International Journal of Engineering Science, 191, 103913. DOI: https://doi.org/10.1016/j.ijengsci.2023.103913