PCM – Phase Change Material
Phase Change Material
The term “phase change material” (PCM) usually describes materials that are used as so-called latent heat storage. This makes use of the effect that the temperature of a material does not change during a phase change (e.g. from solid to liquid) until the phase change is complete. The phase change itself can thus be used as an energy store or energy supplier, so to speak. When heat is available, for example, one phase of a PCM is converted into another. In most cases, these are solid-liquid transitions or solid-solid transitions. If heat is required, the reverse phase transition is used. Ideally, in a technical process, the required heat can be obtained by waste heat recovery.
Melting of a material is an endothermic transition that requires heat. This heat of fusion is stored in the liquid phase as so-called latent heat (latent-heat-storage – LHS) and can be released during solidification/crystallization. This heat can then be used to heat buildings or, on a smaller scale, for heating pads, as shown in Fig. 1.
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.
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.
PCMs can be divided into two groups: organic and inorganic 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. However, the disadvantages of organic materials compared to inorganic ones are flammability, relatively low heat storage capacity, and low thermal conductivity. Inorganic PCMs are mainly salt hydrates and salts. Most of them have high operating temperatures and are available at low cost. Disadvantages are that they can be corrosive and often undergo a large volume change.
Different applications require different operating temperatures that correspond to the melting point of the PCM. Operating temperatures can range from near room temperature (for most organic PCMs, but also for some inorganic ones such as hydrated lithium nitrate (LiNO3*3 H2O)) to several hundred degrees Celsius (for inorganic ones such as alkali metal salts).
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.