Characterization of PCM

In latent thermal storage, phase change materials (PCM ... phase change materials) are used as storage media, whereby the phase change is solid/liquid and thus melting heat is used to store thermal energy.

For the design and development of high performance heat storages, it is of particular importance to have access to reliable information regarding the thermophysical material data of PCMs and their interaction with the container or capsule materials. Important parameters are

• melting temperature and the specific heat of fusion,
• specific heat capacity and thermal conductivity,
• density, as well as
• corrosiveness compared to encapsulation materials.

With our available laboratory equipment, we are able to determine many of these quantities - in part also under cyclical thermal load - for typical PCM classes such as

• organic PCM (paraffins, fatty acids, sugar alcohols, etc.) and
• inorganic PCM (salt hydrates, nitrate and nitrite salts)

More detailed information is provided below.

At Fraunhofer IFAM Dresden, high-quality measurement systems for the determination of different thermophysical material data of PCM or PCM-metal composites are available in our thermo-technical laboratory and thermal analysis technologies:

1. Measurement of the melting temperature and the specific quantities heat capacity and melting heat using Differential Scanning Calorimetry (DSC, small sample quantities - pure PCM only)
2. Alternative: Measurement of the specific fusion heat in a specially developed calorimeter (apparatus is still being set up, for larger samples), heat capacity measurement at room temperature also with the hot-disk measuring system,
3. Measurement of the thermal conductivity of the solid phase of PCM or PCM composites at room temperature with the hot-disk measuring system and at higher temperatures (up to 600 °C) with the stationary plate apparatus.

In addition to thermophysical properties, the interactions between the materials used and the PCM also play a decisive role with regard to the reliable and safe use of latent thermal storage. Corrosion in the storage tank can lead to a change in the PCM and above all to the destruction of the tank.

In our thermo-technical laboratory, cyclic tests of samples of the capsule material in interaction with the PCM can be performed under real phase change conditions. A special apparatus is used to cyclically heat and cool the material sample in the PCM within a defined temperature difference around the melting point:

• sample size in the single-digit cm range,
• from room temperature to 400 °C,
• fully automated thermal cycling.

After the required number of cycles has been completed, established analysis methods (metallographic sections, SEM, EDX, etc.) and a broad materials science know-how are available for evaluation.