The following information are not exhaustive and the only aim here is to introduce the vocabulary related to the world of PCM.
What is a Phase Change Material (PCM) ?
It is said about any material which have the ability to change their state, however the term is more specific to a material which have a solid-liquid or a solid-solid change of state. It allows to absorb a large amount of energy and power by changing to the reverse state (ie liquid-solid) and thus, it can store and destock energy when it changes of state. (KAPLAN ENERGY favors PCM with a latent heat generally above 180 kJ.kg-1).
How they work
Types of PCM
There are three main classes of PCM :
- Inorganic metal salt hydrates
- Paraffinic – petrochemically derived
- Other organic materials – depending on the source these can be manufactured from renewable and bio-based materials
What is the Latent Heat Storage ?
PCM work by storing energy as latent heat during their phase transition.
Most materials store heat as sensible heat and as the temperature increases so does the material, PCM follow a similar profile to most materials until they reach their phase transition temperature.
Once in the phase transition zone, they start to absorb energy and store this energy as latent heat, the higher the latent heat capacity of a compound, the more energy it can absorb/release during this phase transition. This means that PCM with high latent heat capacities can maintain constant temperatures for longer periods of time.
Once all of the stored latent heat capacity has been utilised and the PCM is in its solid/liquid phase, the PCM will return to a sensible heat profile.
What is the Differential Scanning Calorimetry (DSC) ?
Differential scanning calorimetry is a thermal analysis technique which using a calorimeter to measure the heat flow of a PCM sample when heated, cooled or maintained at a constant temperature.
What is the subcooling (or supercooling) ?
Matter in metastable state where the PCM stays liquid at a temperature below its crystallization temperature Tc.
What is the nucleation ?
The nucleation is the initial step of crystallization. It is characterized by the apparition of a tree structure (dendrites) around a nucleus which is a neat little group of molecules.
What is a nucleating agent ?
Agent which promotes the nucleation by acting as a nuclei. In the nature, nucleating agents are often played by impurities.
How choose your PCM
The energetic density, or latent heat, is a key factor in the PCM choice.
For choosing a PCM, KAPLAN ENERGY recommends to take a material with a latent heat superior or equal to 180KJ/Kg
The PCM may be subject to subcooling.
Some non exaustifs factors that increase the supercooling : the use of salt as PCM, the PCM purity, the scanning rate of PCM (expressed in ° C / min), ...
To best minimize the supercooled, KAPLAN ENERGY chose to work with biobased organic phase change materials, which have less supercooling even with a high purity.
The choice of the DSC curve* must be made according to the targeted applications.
A DSC chart can be very different based on the scan rate! KAPLAN ENERGY presents his DSC graphics with a standard value of 10 ° C / min.
*DSC melting curve (Differential Scanning Calorimeter)