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A second heat reservoir is the simple increase of temperature without phase change. For such applications, the specific heat capacity of the storage material as well as its density should be high in order to store a maximum of heat on a minimum of material/space (storage capacity). This effect is used for air conditioning of buildings and their thermal comfort. In order to transfer energy well from the PCM to the environment, it should also have a high thermal conductivity.
Properties of PCM
Phase change materials should therefore have the following properties:
- a high storage capacity or latent heat per volume. This is achieved when the molar heat of fusion is high and, at the same time, there is a high density and a high specific heat capacity.
- a high thermal conductivity for rapid heat exchange between the PCM and the environment
- a high nucleation rate to avoid supercooling and to achieve phase change at the operating temperature.
- a low volume change during the phase transition to avoid mechanical stress on the vessels and cracks in the solid phase.
- low cost per stored energy and good availability
- high chemical stability without decomposition, allowing many melting/freezing cycles.
Advantages of PCM materials
Organic materials (mainly hydrocarbons, oils and fats, but also carbohydrates) have lower operating temperatures than inorganic materials and some other advantages such as their thermal and chemical stability.
Disadvantages of PCM materials
Thermal analysis of PCM
Thermal analysis is a very powerful tool for the development and characterization of PCMs:
- Thermogravimetry (TGA) is used to study thermal stability or decomposition.
- Differential scanning calorimetry (DSC) is used not only to measure melting temperature (operating temperature) and enthalpy of fusion, which provide useful information about storage capacity, but also to measure specific heat capacity (Cp).
There are also numerous known techniques for measuring thermal conductivity, with the hot-wire-method being the most powerful for PCM applications. Other methods include heat flow techniques and the laser flash method. In all methods of thermal conductivity measurement, the particular challenge is to obtain reliable data exactly during the phase change of a PCM, which is why the fast hot-wire-method has decisive advantages here.
