Phase-change material


A phase-change material is a substance which releases/absorbs sufficient energy at phase transition to provide useful heat or cooling. Generally the transition will be from one of the first two fundamental states of matter - solid and liquid - to the other. The phase transition may also be between non-classical states of matter, such as the conformity of crystals, where the material goes from conforming to one crystalline structure to conforming to another, which may be a higher or lower energy state.
The energy required to change matter from a solid phase to a liquid phase is known as the enthalpy of fusion. The enthalpy of fusion does not contribute to a rise in temperature. As such, any heat energy added while the matter is undergoing a phase change will not produce a rise in temperature. The enthalpy of fusion is generally much larger than the specific heat capacity, meaning that a large amount of heat energy can be absorbed while the matter remains isothermic. Ice, for example, requires 333.55 J/g to melt, but water will rise one degree further with the addition of just 4.18 J/g. Water/ice is therefore a very effective phase change material and has been used to store winter cold to cool buildings in summer since at least the time of the Achaemenid Empire.
By melting and solidifying at the phase-change temperature, a PCM is capable of storing and releasing large amounts of energy compared to sensible heat storage. Heat is absorbed or released when the material changes from solid to liquid and vice versa or when the internal structure of the material changes; PCMs are accordingly referred to as latent heat storage materials.
There are two principal classes of phase-change material: organic materials derived either from petroleum, from plants or from animals; and salt hydrates, which generally either use natural salts from the sea or from mineral deposits or are by-products of other processes. A third class is solid to solid phase change.
PCMs are used in many different commercial applications where energy storage and/or stable temperatures are required, including, among others, heating pads, cooling for telephone switching boxes, and clothing.
By far the biggest potential market is for building heating and cooling. In this application, PCMs hold potential in light of the progressive reduction in the cost of renewable electricity, coupled with the intermittent nature of such electricity. This can result in a mismatch between peak demand and availability of supply. In North America, China, Japan, Australia, Southern Europe and other developed countries with hot summers, peak supply is at midday while peak demand is from around 17:00 to 20:00. This creates opportunities for thermal storage media.
There are two common ways that PCMs may be used: first, they can be used passively, where the PCM is located so as to absorb and then release heat due to temperature difference, which is thereby moderated. In such applications, the PCM may be encapsulated, and integrated into the structure of the object or space that is to be conditioned. In some applications, especially when incorporation to textiles is required, phase change materials are micro-encapsulated. Micro-encapsulation allows the material to remain solid, in the form of small bubbles, when the PCM core has melted.
Alternatively, the PCM can be contained in a vessel, and heat flow to and from the PCM can be controlled by pumping a heat transfer fluid through a heat exchanger, generally immersed in the PCM within the vessel. In this case the system is a sub-category of "thermal battery" or "TES", thermal energy storage, which encompasses sensible heat storage as well.

Classification of phase-change materials

Phase-change materials used for thermal energy storage are commonly classified according to their chemical composition and phase transition behavior. Most reviews distinguish three broad groups – organic, inorganic and eutectic PCMs – and, more recently, composite and microencapsulated PCMs are considered as separate subclasses because they are specifically engineered to overcome drawbacks such as low thermal conductivity, leakage and phase segregation.

Organic PCMs

Organic PCMs are mainly based on paraffin waxes and non-paraffin organics such as fatty acids, fatty alcohols and polyols. They undergo a solid–liquid phase transition over a relatively narrow temperature range and typically exhibit latent heat values of roughly 150–250 kJ·kg⁻¹ in the building-relevant temperature range. Organic PCMs are chemically stable, exhibit little or no supercooling and show good cycling stability, which makes them attractive for long-term operation. They are also non-corrosive towards most container materials and can be produced from petrochemical or bio-based feedstocks.
However, organic PCMs generally suffer from low thermal conductivity, which limits the rate of heat storage and release unless conductive fillers or fins are added. Paraffins are also flammable, and some fatty-acid based PCMs may emit odors or interact with polymer matrices in composite systems. Their volumetric energy density is lower than that of many inorganic salt hydrates because of their lower density.

Inorganic salt hydrates and other inorganic PCMs

Inorganic PCMs include salt hydrates, anhydrous salts, oxides and metallic alloys. Salt hydrates are widely studied for low- and medium-temperature thermal energy storage because they combine relatively high latent heat with higher thermal conductivity and higher volumetric storage density than common organic PCMs. Inorganic PCMs are non-flammable and many compositions are inexpensive, which makes them attractive for large-scale systems such as building envelopes, heat pumps and industrial waste-heat recovery.
The main drawbacks of salt hydrates are their tendency to suffer from supercooling, phase segregation and incongruent melting, which can lead to a gradual loss of storage capacity over repeated cycles if not mitigated by nucleating agents, thickeners or encapsulation strategies. Some inorganic PCMs can also be corrosive to metals, requiring careful selection of container and heat-exchanger materials.
At higher operating temperatures, metallic alloys and molten salts based on nitrates, chlorides or fluorides are considered as high-temperature PCMs for concentrating solar power and industrial process heat. These materials offer high thermal stability and good thermal conductivity, but issues such as corrosiveness, oxidation and the need for high-temperature containment remain important design challenges.

Eutectic PCMs

Eutectic PCMs are mixtures of two or more components that melt and solidify congruently at a fixed composition and a single, sharply defined temperature that is lower than the melting point of any individual component. Eutectic systems can be formulated from organic–organic, inorganic–inorganic or organic–inorganic combinations, providing great flexibility to tune the phase change temperature to a specific application. For example, eutectic mixtures of fatty acids or paraffins are often tailored for human comfort temperatures in building envelopes, while salt–salt eutectics are explored for medium- and high-temperature thermal storage.
Because eutectic PCMs melt congruently, they usually avoid the phase segregation problems observed in some salt hydrates. Their thermophysical properties, however, depend strongly on the choice of constituents, and experimental characterization is often required to confirm long-term cycling stability and compatibility with container materials.

Composite PCMs

Composite PCMs are formulated by combining a base PCM with a supporting matrix or high-conductivity fillers to improve properties such as thermal conductivity, shape stability and mechanical strength. Common strategies include embedding organic or inorganic PCMs into porous metals, carbon foams, expanded graphite, silica aerogels or polymer networks, which physically confine the liquid phase and prevent leakage during melting. Graphite- and carbon-based composites in particular can increase the effective thermal conductivity by one to two orders of magnitude while maintaining a high latent heat.
Recent studies also explore composites with nano-structured additives such as carbon nanotubes, graphene nanoplatelets or metal nanoparticles to enhance heat transfer and tailor the rheological behavior. Composite PCMs are widely used in applications where high power density, mechanical integrity or shape stability are required.

Microencapsulated PCMs

Microencapsulated phase-change materials consist of a PCM core surrounded by a thin polymeric or inorganic shell, typically in the micrometre size range. Microencapsulation physically isolates the PCM from the external environment, preventing leakage, reducing reactivity with the matrix and increasing the heat transfer area due to the large number of small particles.
MicroPCMs can be dispersed in water, polymer binders, mortars or textile fibers, enabling their integration into building materials, coatings and functional fabrics. Shell materials include melamine–formaldehyde resins, polyurethanes, poly and, more recently, inorganic shells based on silica or titania. Key design parameters are the encapsulation efficiency, shell thickness, mechanical robustness and long-term thermal cycling stability.
Microencapsulation is also applied to inorganic PCMs such as salt hydrates to mitigate supercooling and phase segregation while keeping the material compatible with water-based slurries and cementitious matrices. As a result, microencapsulated PCMs are increasingly used in building envelopes, lightweight plasters, and coatings to passively regulate indoor temperature and reduce peak cooling loads.

Selection criteria

The phase change material should possess the following thermodynamic properties:
Kinetic properties
  • High nucleation rate to avoid supercooling of the liquid phase
  • High rate of crystal growth, so that the system can meet demands of heat recovery from the storage system
Chemical properties
  • Chemical stability
  • Complete reversible freeze/melt cycle
  • No degradation after a large number of freeze/melt cycle
  • Non-corrosiveness, non-toxic, non-flammable and non-explosive materials
Economic properties
  • Low cost
  • Availability